Polymer Synthesis, Volume 3, Second Edition (Organic Chemistry, a Series of Monographs)

PREFACE TO SECOND EDITION This volume continues in the same format as the first edition with updates on the syntheses o...

Author: Stanley R. Sandler | Wolf Karo

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PREFACE TO SECOND EDITION

This volume continues in the same format as the first edition with updates on the syntheses of various types of polymers, including olefin-sulfur dioxide copolymers, polythioesters, sulfide polymers, polyisocyanates, polyoxyalkylhydroxy compounds, polyvinyl carbazole, polyvinyl acetate, polyallyl esters, polyvinyl fluoride, and miscellaneous polymer preparations. The book should be useful to academic and industrial chemists who desire typical synthetic procedures for preparing the polymers described herein. In addition to reviewing the latest journals, we survey the patent literature and give numerous additional references. Safety in carrying out the preparations is essential to experimenters and their co-workers. The Material Safety Data Sheets (MSDS) for all chemicals to be used must be reviewed for hazards and safe handling procedures. When in doubt about any matter in the MSDS, please consult the manufacturer or supplier. Also, seek the advice of an experienced chemist on any questions that you may have concerning safety. Be sure to follow the recommendations for personal protective equipment and always operate in a well-ventilated hood with proper shielding. We again express our appreciation to our wives for their understanding and encouragement during the preparation of the manuscript for this second edition. We also thank the staff of Academic Press for guiding the publication of the manuscript to its final book form.

Stanley R. Sandler Wolf Karo

ix

PREFACE TO FIRST EDITION

In a manner similar to Volumes I and II, detailed laboratory instructions are presented for the preparation of various types of polymers such as olefin-sulfur dioxide copolymers, polythioesters, sulfide polymers, polyisocyanates, polyoxyalkylhydroxy compounds, polyvinyl carbazole, polyvinyl acetate, polyallyl esters, polyvinyl fluoride, and miscellaneous polymer preparations. The latest journal articles and the patent literature have been reviewed and tabulated in each chapter. In some cases, the major literature sources used were patents. As in our earlier volumes, the procedures were selected both on the basis of safety and as representative of the general polymer preparation in question. Each chapter should be considered a good preparative introduction to the subject but not a final, definitive work. This book should be especially useful to both industrial chemists and students of polymer chemistry by providing a ready source of preparative procedures for various polymer syntheses. We have omitted many details of the mechanisms and kinetics of these polymerizations and have concentrated only on the synthetic details. Safety hazards and precautions are stressed in all chapters and the reader is urged not only to observe these but constantly to seek up-to-date information on a given monomer from both the literature and the chemical manufacturer. This book is only designed to provide useful polymer synthesis information and not to override the question of patentability or to suggest allowable industrial use. The toxicological properties of the reagents have in most cases not been completely evaluated, and the reader is urged to exercise care in their use and professional judgments before undertaking a procedure. We assume no liability for injuries, damages, or penalties resulting from the use of the chemical procedures described. We express our appreciation to our wives and children for their understanding and encouragement during the preparation of this manuscript. Special thanks are xi

xii

Preface to First Edition

due to Miss Emma Moesta for typing our manuscript in a most professional fashion. Finally, we thank the staff of Academic Press for guiding the publication of the manuscript to its final book form.

Stanley R. Sandler Wolf Karo

CONTENTS OF VOLUME 1, SECOND EDITION Chapter 1. Polymerization of Olefinic and Diolefinic Hydrocarbons Chapter 2. Polyesters Chapter 3. Polycarbonates Chapter 4. Polyamides Chapter 5. Polymerization of Aldehydes Chapter 6. Polymerization of Epoxides and Cyclic Ethers Chapter 7. Polyureas Chapter 8. Polyurethanes Chapter 9. Thermally Stable Polymers Chapter 10. Polymerization of Acrylate and Methacrylate Esters Chapter 11. Polymerization of Nitrile Monomers Chapter 12. Polyacrylamide and Related Amides Chapter 13. Organophosphorus Polymers Chapter 14. Free-Radical Initiation of Vinyl and Related Monomers Appendix Subject Index

CONTENTS OF VOLUME II, SECOND EDITION Chapter 1.

Urea,Melamine, Benzoguanamine-Aldehyde Resins (Amino Resins or Aminoplasts) Chapter 2. Phenol-Aldehyde Condensations Chapter 3. Epoxy Resins Chapter 4. Silicone Resins (Polyorganosiloxanes or Silicones) Chapter 5. Alkyd Resins Chapter 6. Polyacetals and Poly(vinyl acetals) Chapter 7. Poly(vinyl ethers) Chapter 8. Poly (N-vinylpyrrolidone) Chapter 9. Polymerization of Acrylic Acids and Related Compounds Chapter 10. Poly(vinyl chloride) Subject Index

xiii

Chapter 1

Olefin-Sulfur Dioxide Copolymers 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Conditions of Copolymerization of Sulfur Dioxide with Unsaturated Compounds . . . . . . . 3. Polysulfones by the Reaction of Olefins (Linear, Cyclic, and Branched) with Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1. General Procedures for Copolymerization of Olefins and Sulfur Dioxide . . . . . . . . . 3-2. Sulfur Dioxide-Butene Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3. Preparation of a Copolymer of Ethylene and Sulfur Dioxide . . . . . . . . . . . . . . . . . 3-4. Preparation of Propylene-Sulfur Dioxide Copolymer . . . . . . . . . . . . . . . . . . . . . . 3-5. Preparation of Octene-Sulfur Dioxide Copolymer . . . . . . . . . . . . . . . . . . . . . . . . 3-6. Preparation of Cyclohexene-Sulfur Dioxide Copolymer . . . . . . . . . . . . . . . . . . . . . 3-7. Emulsion Copolymerization of 1-Butene and Sulfur Dioxide . . . . . . . . . . . . . . . . . . 4. Polysulfones from the Reaction of Dienes (Conjugated and Unconjugated) with Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1. Copolymerization of 1,3-Butadiene with Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . 4-2. Terpolymerization of 1,3-Butadiene, Cyclopentadiene, and Sulfur Dioxide . . . . . . . . 4-3. Copolymerization of Sulfur Dioxide with 1,5-Cyclooctadiene--General Method . . . . 4-4. Terpolymerization of Butadiene, trans-Piperylene with Sulfur Dioxide . . . . . . . . . . . 5. Polysulfones by the Reaction of Acetylene with Sulfur Dioxide . . . . . . . . . . . . . . . . . . . 5-1. General Procedure for the Preparation of Polysulfones from Acetylenes . . . . . . . . . 6. Polysulfones by the Reaction of Vinyl Monomers (CH2 -- CH -- R) with Sulfur D i o x i d e . . 6-1. Preparation of a Propylene-Methyl Methacrylate-Sulfur Dioxide Terpolymer . . . . . 6-2. Preparation of Propylene-Methyl Acrylate-Sulfur Dioxide Terpolymer . . . . . . . . . . 6-3. Preparation of Bicyclo[2.2.1]hept-2-ene-Ethyl Acrylate-Sulfur Dioxide Terpolymer 7. Polysulfones by the Reaction of Allylic Compounds with Sulfur Dioxide . . . . . . . . . . . . 7-1. Preparation of Allyl Chloride-Sulfur Dioxide Copolymer . . . . . . . . . . . . . . . . . . . 7-2. Terpolymerization of Allyl Alcohol, Acrylic Acid, and Sulfur Dioxide . . . . . . . . . . . 8. Miscellaneous Preparations and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 3 6 10 16 17 18 19 19 19 22 23 25 25 27 27 28 31 32 34 34 37 37 39 41 42

2

1.

1. Olefln--Sulfur Dioxide Copolymers

INTRODUCTION

It was early recognized that amorphous products were formed by the reaction of olefins and sulfur dioxide [1-1 d]. The elucidation of the products formed was first reported by Marvel [2] and Staudinger [3, 3a] in the early 1930s. Marvel reported that propylene and cyclohexene react with sulfur dioxide to form alternating copolymers of olefin and sulfur dioxide in a head-to-tail arrangement [2, 4]. Staudinger reported that 1,3-butadiene reacts with sulfur dioxide to form a cyclic sulfone and an amorphouse linear polysulfone [3, 3a, 5]. C--C

/

+ 802

-~

\

- - S O 2 - - C - - I -I

CH2 = CH-- C H = CH2 + SO2

,

so

- cI-I - ci-i= c i - i - cii

(1)

/

-), + t.

/

J

(2)

SO2 Sulfolene m.p. 65~

Fitch [6] at Phillips Petrolium Co. was issued a patent in 1936 that was followed by many others on the formation of polymeric sulfones from the reaction of olefins and sulfur dioxide. A patent to Frey and Snow of Phillips Petroleum Co. also describes some of the prior literature [7]. The olefin-sulfur dioxide polysulfone compositons are usually light-colored, thermoplastic, amorphous products that are moldable and extrudable. The polymers are not very resistant to alkaline solutions. The only olefin-sulfur dioxide product sold commercially is sulfolene by Phillips Petroleum Co. The linear polymers remain to be commercially produced. It is imeresting to note that a polysulfone, i.e., poly(phenylene sulfone), has been sold commercially since 1966 [8]. For more detailed discussions and historical background several earlier reviews are worth consulting [9, 9a]. The ceiling temperature of polymerization (T~) of the SO2/olefin polymers are between 25 to 100~ NMR analysis has indicated [9b] that the olefin used does not isomerize during copolymerization. Various vinyl monomers copolymerize with SO2 such as vinyl chloride, styrene, acrylamide, and chloroprene. However, methyl methacrylate is reported [9b] to homopolymerize in SO2 when used as a solvent (cationially or radically) but not to form polysulfones (sulfur dioxide copolymers). Acetylene and disubstituted acetylene (such as 2-butyne) do not form copolymers with SO2. However, n-alkyl acetylenes and phenylacetylene reaction to give a copolymer.

3

2. Conditionsof Copolymerization of Sulfur Dioxide

Polysulfones have found use as resists in the area of microlithography since the polysulfones are unstable toward radiation and heat below their melting points and the SO2 and olefin degradation products are volatile. It is interesting to note that some olefins undergo isomerization during the radiation induced degradation step [9c]. The structure of the olefin SO2 polymers has been investigated by NMR [9d] and a helical structure has been reported for these polymers via transmission electron microscopy.

0

CONDITIONS OF COPOLYMERIZATION OF SULFUR DIOXIDE WITH UNSATURATED COMPOUNDS

Sulfur dioxide does not homopolymerize, but on reaction with olefins it yields copolymers.Terminal olefins react more readily than those with an internal double bond. The presence of various substituents affects the rate of polymerization. Conjugated dienes copolymerize with sulfur dioxide to give linear polymers comaining residual double bonds. The copolymerization reaction is free radical in nature and is catalyzed by such initiators as peroxides [10], oxygen, azo compounds [11, 1 l a], and light [9a, 12]. Styrene dissolved in liquid sulfur dioxide and catalyzed by stannic chloride (cationic catalyst) gives only polystyrene whereas the use of azobisisobutyronitrile gives poly(styrene sulfone). Sulfur dioxide is a gas at ordinary temperatures and has the following physical properties. m.p.

- 75.46~

b.p.

- 10.02~

vapor pressure

( d e n s i t y o f liquid at - 10~

-

1.46 g r n / m l )

28.5 c m H g at - 3 0 ~ 53.1 c m H g at - 2 0 ~

115 171 2456

cmHgat0~ cmHgat

10~

c m H g at 2 0 ~

Reactions with sulfur dioxide are carried out in pressure vessels constructed of glass or stainless steel. Caution should be used and all reactions should be done under a hood and behind a barrier for protection. Copoiymerization is usually carried out in solution or emulsion [13-13b], but not in the vapor phase [14]. The reaction in the gaseous phase usually gives sulfinic acids. A solid state copolymerization has been reported for vinyl acetate with sulfur dioxide initiated by radiation [15].

1. Olefin-Sulfur Dioxide Copolymers

4

\

Liquid phase"

S O 2 -~-

/

\

/ C=C

9~

\ \ /

Gas phase"

802

-+-

C=C

\

/

\

/

EC--CSO2--1n \

/ C--C

\

/

502

\

/

/ "C--C--S02

- -~

/

/ C=C

(4)

\ SO2H

A ceiling temperature [16] (temperature at which rate of propagation and depropagation are equal) is independent of the kind of initiator, but depends on the given olefin. The polymerization is not always accelerated by heat and in fact in some cases slows as it approaches the ceiling temperature. The ceiling temperature for the copolymerization of sulfur dioxide with various vinyl compounds is shown in Table I [17] and is said to follow the relationship: T~ = A H I S

where AH is the heat change and S is the entropy change. Olefins with electron withdrawing substituents (--CN, --COOH, --COOR) cannot enter into copolymerization with sulfur dioxide. The reactivities of the olefins with sulfur dioxide is obtained by using cyclohexene as a standard at -20~ far below the ceiling temperature where depropagation is negligible [18]. Charge transfer complexes of the monomers were studied in the terpolymerization of neutral monomers (N) with electron-donor (D) and electron-acceptor (A) monomers [18a]. For example, norbornene as (D) monomer, SO2 as (A) monomer, and acrylonitrile as (N) molecules were studied. Thus acrylonitrile may not be effective in copolymerization but can be terpolymerized with SO2 [18a]. The NMR spectra have been reported for a variety of polyalkylene sulfones [ 18b]. All the polymers were prepared by radical copolymerization of a mixture of sulfur dioxide and alkene (mw 104-107). The NMR spectra were determined in DMSO or CDC13 solutions. Acrylamide has also been reported to be effective in producing copolymer using AIBN or ),-ray irradiation, as described by Bell Labs (R. E. Cais and G. J. Stute [ 18c]). Other catalysts have been reported by Shell [18d] to effect SO2/olefin polymerization, which involves Group VIII metal compounds, a bidentate ligand of P, As, or Sb, and an anion of nonhydrohalogenic acid having pK of approximately 6 (for example, the catalyst Pd(OAc)2"4-MeC6HaSO3H, and Ph2P(CHz)3PPh2).

2.

Conditions o f Copolymerization o f Sulfur Dioxide

TABLE I

Ceiling Temperatures for Polysulfone Formation a

Olefin Straight-chain 1-olefins Ethylene Propylene 1-Butene 1-Pentene 1-Hexene 1-Hexadecene Branched 1-olefins Isobutene

5

Structural skeleton

C=C C--C--C C2--C=C C3--C--C C4--C--C C 14 ~ C - - C

T~, ~

85 46 45

66___4

Trace of gummy material formed after 3 hr irradiation at - 8 0 ~ No polymer formed after 3 hr irradiation at - 80~

a Reprinted in part from R. E. Cook, F. S. Dainton, and K. J. Ivin, J. Polym. Sci. 26, 351 (1957). Copyright 1957 by the Journal of Polymer Science. Reprinted by permission of the copyright owner.

0

POLYSULFONES BY THE REACTION OF OLEFINS (LINEAR, CYCLIC, AND BRANCHED) WITH SULFUR DIOXIDE In 1910 B a d i s c h e A n i l i n e u n d S o d a - F a b r i k A k t i e n - G e s e l l s c h a f t ( B A S F ) [ 1 a]

o b t a i n e d a p a t e n t on the p r o d u c t ( p o l y s u l f o n e ) o b t a i n e d b y the a c t i o n o f s u n l i g h t o n a s o l u t i o n o f e t h y l e n e in s u l f u r d i o x i d e in a s e a l e d tube. T h e s a m e p a t e n t

3. Polysulfones by the Reaction of Olefins

7

noted that the polymerization was slower than that obtained with 2-butene or propylene with sulfur dioxide. In 1914, Matthews and Elder [1 c] described the sunlight or ultraviolet light initiated copolymerization of either 2-butene, propylene, or amylene with sulfur dioxide. Matthews and Elder [lc] also noted that the butene sulfone polymer was soluble in chloroform and tetrachloroethane. They reported that the polymer softens without dissolving in acetone and that it may be useful as an additive in films, varnishes, and ornamental articles. Staudinger and Ritzenthaler [3] reinvestigated the copolymerization of ethylene or propylene with sulfur dioxide but described very little of the polymer characteristics. Dainton and co-workers [19] described the kinetics and thermodynamics of the polymerization of butene [20]. In 1955 Dainton and Bristow [20] found that cis-2-butene and trans-2-butene gave the same polymeric product. In 1957 Skell and co-workers [21] confirmed the nonstereospecificity of the copolymerization reaction. Copolymers have also been prepared using mixtures of olefins with sulfur dioxide. Olefin pairs studied were butene with propylene [22-22b], butene with pentene [13], butene with isobutene [13b], butene with acrylonitrile [13,23], butene with vinyl acetate [24], butene with methacrylate esters [25], butene with acrylic esters [25], and butene with butadiene [24]. Sulfur dioxide reacts with pentenes [26, 26a], higher olefins (1-hexene [22], 1-nonene [27]), cyclopentene [17], and cyclohexene [2, 14]. Marvel and Weil have presented evidence to support a head-to-tail structure in the propylene sulfur dioxide copolymer [4] [see Eq. (5)].

CH3CH--CH2 + SO2

-~ -- CH -- CH2 -- SO2 -- ,,

(5)

Some examples of olefin-sulfur dioxide copolymers are shown in Table II. Initiation of the olefin-sulfur dioxide copolymerization is radical in nature and a variety of radical initiators are shown in Table III. The use of certain metal salts to initiate copolymerization has also been reported and the mechanism of initiation is the subject of some controversy. For example, Ivin [28] has suggested that metal nitrate reacts with sulfur dioxide to yield nitro radicals and sulfite anion radicals. M+NO~ - +

SO2

~

M + + NO2 ~ +

SO3 ~

(6)

The effect of various metal salt catalysts [29-33] on the production of olefinsulfur dioxide copolymers was studied earlier by Frey, Snow, and Schulze [33]. It was found that the soluble catalysts (silver nitrate, lithium nitrate, ammonium nitrate, and dilute alcoholic nitric acid) are much more effective than insoluble salts (barium nitrate, zirconium nitrate, titanium nitrate, strontium nitrate, and mercuric nitrate) which usually have long induction periods for reactions as shown in Table III [33].

8

1.

TABLE II

Olefin--Sulfur Dioxide Copolymers

Sulfur Dioxide-Olefin Copolymers Polymer properties

Initiator

Olefin Ethylene Pyropylene 1 -Butene

2-Butene (cis, trans) 1 -Pentene 1-Octene

Cyclopentene Cyclohexene Bicyclo[2.2.1 ] Hept-2-ene cis-cis- l ,5Cyclooctadiene

Peroxide Azobisisobutyronitrile Peroxide Azobisisobutyronitrile Peroxide Azobisisobutyronitrile Benzoyl peroxide Azobisisobutyronitrile light H202-paraldehyde light H202-paraldehyde H202 H202 None Peroxide

m.p. (~

Solubility

300-310 d

H2SO4(conc)

250-270 d

H2SO4(conc) HNO3 (conc) DMSO THF, CHC13, SO2 Acetone, MEK, DMSO THF, CHC13, SO2 or DMSO

---

340 175-200 d -200 d

CHCI3, DMSO, SO2 Toluene, Benzene, CHC13, SO2

-250 d

Ref. a b a b c d c b e d f g h i j

DMSO, Tetramethylene sulfone

k

H. Staudinger and B. Ritzenthalen, Ber. Dtsch. Chem. Ges. B. 68, 455 (1935). b M. A. Naylor, Jr. and A. W. Anderson, J. Am. Chem. Soc. 76, 3962 (1954). r Z. Kuri and M. Ito, Kogyu Kaqaku Zasshi 69, 1066 (1966). aM. A. Jobald, J. Polym. Sci. 29, 275 (1958). ep. S. Skell, R. C. Woodworth, and J. H. McNamara, J. Am. Chem. Soc. 79, 1253 (1959); J. Harmon, U.S. Patent 2,190,836 (1940). fC. S. Marvel and W. H. Sharkey, J. Am. Chem. Soc. 61, 1603 (1939). gF. J. Glavis, L. L. Ryden, and C. S. Marvel, J. Am. Chem. Soc. 59, 707 (1937). h O. Pipik, Bull. Acad. Sci. URSS, CI. Sci. Math., Ser. Chim. 1097 (1938). i D. S. Frederick, H. D. Cogan, and C. S. Marvel, J. Am. Chem. Soc. 56, 1815 (1934). iN. L. Zutty, C. W. Wilson, III, G. H. Potter, D. C. Priest, and C. J. Whitmore, J. Polym. Sci., Part A 3, 2781 (1965). kA. H. Frazer and W. P. O'Neill, d. Am. Chem. Soc. 85, 2613 (1963); A. H. Frazer, J. Polym. Sci., Part A 2, 4031 (1964); U.S. Patent 3,133,903 (1964); Chem. Abstr. 61, 12168 (1964). a

Some characteristic infrared absorption frequencies in the 7.5-9.5 micron range are shown for several olefin polysulfones in Fig. 1. Two strong absorption bands, one at 7.7 microns and the other at 8.75-9.0 microns, are characteristic stretching vibrations of the sulfone group as shown in Fig. 1. The olefin-sulfur dioxide copolymers are thermoplastic and colorless; transparent coherent moldings can be prepared. The properties of the polymers vary from that of ethylene-sulfur dioxide, which is very interactable, to 1-decenesulfur dioxide, which is soft, rubbery, and soluble in many organic solvents. The

FABLE III

Percentage Conversion of 2-Butene and Sulfur Dioxide to Polysulfone [29] a Time

Catalyst b ~0.5-1% by wt)

4 (hr)

Dilute nitric acid Lithium nitrate Ammonium nitrate Ethyl nitrite Beryllium nitrate Postassium nitrate Magnesium perchlorate Perchloric acid Thallium nitrate Calcium nitrate Sodium nitroprusside Phenyl mercuric nitrate Mercuric nitrate. Triphenyl bismuthine Tetraethyllead Mercury diethyl Mercury di-n-butyl Zirconium nitrate Titanium nitrate Sodium nitrite Uranyl acetate Barium nitrate Strontium nitrate Lead nitrate Cobalt nitrate Isoamyl nitrite Sodium chlorate

100

6 (hr)

1 (day)

2 (day)

3 (day)

4 (day)

5 (day)

7 (day)

9 (day)

15 (day)

21 (day)

26 (day)

35 (day)

45 (day)

r 100 100

1

a

10 10

100 60 20

e~

100 80 95 75 45

5 0 10 12

5

100 100 90 55 65

10 20

90 90

50 2 30

5

0

50

m

15 75 50

70 20 25

75 30 30

10

15 --

25

0

0

20

30 0 0 0 0 30

a Equal volumes of liquid sulfur dioxide and 2-butene. bData from F. E. Frey, R. D. Snow, and W. A. Schulze, U.S. Patent 2,280,818 (1942).

-~ 20

95 80 40 40 17 10

0 0 0 0 -35

100 100 100 100 95 99 80 70 60 75 50 10 60 3 15 0 15 40

r~

100 80 100 90 95

12 25 10

65

100 100 80 65 20 40 20 20 85 65

I I I I

100 70 40 40 40 40

75 50 45

,D

10

1. Olefln--SulfurDioxide Copolymers '

'

I

'

I

'

I

'

I

'

I

'9

pBUntte1en: r~. 2.Methyl_~l /~!~ Hexene'l~~f~%~P ~ ~ penetene t I~J~rl~ IHeec;1/~ ne."~

"~--

~~("~ ,

3-MethI'y t /

' 8 ' 19 • -- JJ/~/

ether

j~II

/ ~ butene"l l ' / j J K,,,~ thy~l~V~q ! 4~4ntDinm.~

L-Cvc,o- ^I

hexene/~ Pentene-2~ Heptene-2 ~

;ii i e

Diisobut~ N ~/~D'~et"~ A (liq.) ?~ sulfoneDi~/// ethyl~ / \ A/k sulfone

,

8I

,

9 sulfone 87 ~

,

,

acid

9~

8

i~

,

I ,

,

8

,

9#ml

Fig. 1 Infrared absorption spectra of some olefin polysulfones and some aliphatic sulfones. Absorption increasing upward. [Reprinted from R. E. Cook, F. S. Dainton, and K. J. Ivin, J. Polymer Sci. 26, 351 (1957). Copyright 1957by The J. Polymer Sci. Reprintedby permission of the copyright owner.]

resins prepared from the olefin have higher softening points, are less soluble in organic solvents, and are more resistant to alkali. Although all the olefin-sulfur dioxide resins decompose by heating well above their softening points, they can be satisfactorily molded within a fairly wide temperature range [22a]. For example, propylene-sulfur dioxide copolymers can be molded at 180~176 Some properties of the molded resins are shown in Table IV [9a]. Some other examples of olefin-sulfur dioxide copolymers and their polymerization conditions are shown in Tables V-IX.

3-1.

General Procedures for Copolymerization of Olefins and Sulfur Dioxide [9a]*

C--C -~- SO2 ~ C - - C - - SO2-(7) / \ \ n * Reprinted from R. D. Snow and F. E. Frey, Ind. Eng. Chem. 30, 176 (1938). Copyright 1938 by the American Chemical Society. Reprinted by permission of the copyright owner.

3. Polysulfones by the Reaction of Olefins

11

Most of the small-scale, resin-forming reactions shown in Table IV are carried out in Pyrex glass tubes sealed at the bottom and beating a tube with a 3-5 mm-diameter neck at the top, which could be readily sealed by a flame after filling. The tubes are 15-30 cm long to facilitate cooling in a Dewar flask filled with liquid nitrogen or solid carbon dioxide, and vary from a few millimeters to 6 cm in diameter, depending upon the quantity of product desired and the vapor pressure to be withstood. When a catalyst is used, it is introduced first. The tube is then connected by a manifold-beating connection to a vacuum pump, closed manometer, and cylinders of olefins and of sulfur dioxide. Usually the tube is cooled in liquid nitrogen while being evacuated. Approximately equal liquid volumes of olefin and sulfur dioxide are then condensed in the tube, which is again evacuated and sealed with a torch. Equal volumes of solidified olefin and sulfur dioxide* correspond to approximately 2 moles of sulfur dioxide to 1 mole of a simple olefin. This excess of sulfur dioxide is found desirable because most of the resins that are insoluble in the reaction mixture tend to carry down dissolved sulfur dioxide, thereby depleting the olefin-sulfur dioxide liquid phase in which the reaction takes place. In the case of the more soluble resins, excess of sulfur dioxide maintains a fluid condition throughout the reaction. After being sealed, the tubes are allowed to warm in the dark. As soon as the contents of the tube have melted, they are thoroughly mixed and are then brought to the desired conditions of the experiment. At the end of the reaction the tubes are cooled to approximately - 10~ opened, and allowed to warm to room temperature so that the unreacted olefin and sulfur dioxide can escape (use a hood). Experiments on a larger scale are carried out in cylindrical stainless steel bombs holding 1-4 gal. To facilitate the removal of solid resin, both ends of the bomb are closed with blind flanges lined with stainless steel plates drawn against lead gaskets. The top flange is fitted with one or two filling tubes closed by Hoke valves. The bomb is evacuated and cooled in an ice water bath to facilitate filling. The olefins and sulfur dioxide are then passed in as liquids and thoroughly mixed by tumbling the bomb on a shaft in the manner of an oldfashioned chum. The required amount of catalyst, usually in the form of an alcoholic solution, is then forced into the bomb, and the contents are again mixed by tumbling. For the rapid propylene reaction which is quite exothermic, it is advantageous to keep the bomb in the cooling bath for approximately an hour after filling. In many cases higher conversions can be obtained by adding the catalyst in two or more portions at intervals than can be obtained by adding the same total quantity at the start of the reaction. At the end of the experiment the valves are opened, and unreacted olefin and sulfur dioxide are allowed to escape before dissembling the bomb to remove resin (use a hood). * When filling at liquid nitrogen temperature, at least 30% of the volume of the tube is left to provide for expansion of the charge in coming to room temperature.

TABLE IV

Specific gravity

Tensile strength

Compressive strength

Transverse strength

(lb.~in. 2)

(lb.~in. z)

(lb./in. z)

20,400-23,650

2000-5000 4100-6720 5285 5380

Propylene Propylene Propylene Propylene 2 parts SO2 to 1 propylene 5 parts SO2 to 1 propylene 10 parts SO2 to 1 propylene 92% Propylene, 8% 1-butene 84% Propylene, 16% 1-butene 75% Propylene, 25% 1-butene

1.49-1.51 2000-3200 3200-3685 1800-3950 1800-2575 3488 3733 3343 3780-4300 3820-4260 5615

50% Propylene, 50% 1-butene

5929 4330--4415 3820--4080 4070--4200

87.5% Propylene, 12.5% 2-butene 75% Propylene, 25% 2-butene 50% Propylene, 50% 2-butene

Properties of Molded Resins a

Impact strength energy

(ft. lb for an in. 2) 1.43-1.72 1.84-1.96

Dielectric strength (instantaneous) per mil thickness

Hardness

Moisture absorption in 48 hr

(volts)

(Brinell No.)

(%)

338-381

40

0.262-0.285

I

23,400-24,250 21,250-21,850 22,150-23,100 {14'850-18'350l 11,850-17,350J 22,400-22,650 21,850-22,400 19,350-19,650

5330-5474 4080-6195 6000-7150 7100-7980 6570-8450

k

h

1.42-1.76 1.50-1.92 1.42-1.83

389-393

0.279-0.297

7210-8050

1.68-1.79

347-381

0.347-0.378

5910-7000 4900-5290 4800-6900

1.74-1.98 1.77-1.82 1.73-1.77

372-417 361-401

0.357-0.372 0.477-0490

I

75% Propylene, 25% isobutylene 75% Propylene, 25% 1-pentene 50% Propylene, 50% l-pentene Mixed preformed resin (25% butene, 75% propylene) 2-Butene 2-Butene 2-Butene 50% 2-Butene, 50% 1-butene 1-Butene 1-Butene

1-Butene from n-butyl chloride Allyl alcohol Allyl alcohol 1-Pentene 1-Pentene

3360-4370 2792-3715 2800-3820 2465

22,850-24,350 12,350-12,850 12,350-18,350

1.30-1.36 2800--4200 3100-4000 2300--4460 3820 1.35-1.40 3200--4100 3113 4298 2400-4100 2355 1.31 2100-2350 2100-3190

16,350-20,550

6530-6720 3650-4960 4235-5700

t~

5135-5520 4730-6025 4845-5150 4730 4800-6000 4100--4250

1.47-1.73 1.69-1.83

3745-7050

1.16-1.26

12,350-13,350 12,350-15,350

3600-4300 3190-4295

12,350-15,100

4600-5280 3550-4375

0.665-1.26 0.95-1.32 1.12-1.20 1.08-1.11

15,600 12,050-16,350

1-Pentene

Treated refinery butene Acid-regenerated refinery butene 1-Pentene (with paper pulp filler) Mixed butene (with paper pulp filler)

3200-3450 3330-3955 6408 5550

1.49-1.81 1.37-1.54 1.78-1.88

1.85 1.14-1.18

346-358

25-30

0.695-0.748

329-360

20-23

0.371-0.398 t~

35-40 355-369

18-19

0.315-0.345

3.94 4.57

~Reprinted from R. D. Snow and F. E. Frey, Ind. Eng. Chem. 30, 176 (1938). Copyright 1938 by the American Chemical Society. Reprinted by permission of the copyright owner.

ta~

TABLE V

Representative Polysulfone Preparations Reaction conditions m.p.

Sulfur dioxide

Olefin 10ml

10 ml

10.5 gm 2.8 gm

24 gm

1-Butene

36gin

1O0 g m

2-Pentene

36 gm 36 gm 36 gm 36 gm 36grn 20 ml

1O0 1O0 1O0 1O0 1O0

Methyl propene 1 Propylene 1- and 2-Butenes

1-Pentene 1-Nonene

3-Cyclohexylpropene 3-Methylcyclohexene Cyclohexene

gm gm gm gm gm

20 ml

Catalyst 1 ml Paraldehyde 1.0 ml 3% H202 50 gm Abs ethanol 0.1 gm Benzoyl peroxide 0.1 grn Ascaridole 0.32 gm Benzoyl peroxide 10 gm Abs ethanol 10 gm Abs ethanol 10 gm Abs ethanol 10 gm Abs ethanol 10 gm Abs ethanol 10 grn Abs ethanol 5.0 ml 95% CzHsOH 3.0 ml 2% aq H202

Time (hr)

(oc)

Temp (~

Yield

(decomp.)

12

25

75-90%

340

120

25

25.2 gm

72

25-30

49grn

72 72 72 72 72 24

25-30 25-30 25-30 25-30 25-30 25-30

75% 80-90% 78-80% 75% 5-10% 93-100%

290-300 340 300 330 270

Ref.

Pentene Styrene

5 ml 10 ml

5 ml 10 ml

Octene

10 ml

10 ml

Octene C6 --C 18 olefins C9 ----C14 olefins

C20

150 ml 28 ml

300 ml 20 ml

UV lamp 2 ml Ethyl alcohol 2 drops 30% H202 5 ml paraldehyde (old) 2 ml Ethyl alcohol 2 ml 3.0% H202 5 ml Paraldehyde (old) Co 60 y-rays 1% tert-butyl hydroperoxide based on olefin 7_rays Co60 MEK hydroperoxide

24 12-18

25 25

5-5.2 gm 0.4-2.5 gm

-185-190

d e

12-18

25

2-4 gm

175-200

e f

20 1 1

25 25 0-20

g

31 gm

h i

aL. L. Ryden and C. S. Marvel, J. Am. Chem. Soc. 57, 2311 (1935). b j. Harmon, U.S. Patent 2,190,836 (1940). CD. S. Frederick, H. D. Cogan, and C. S. Marvel, J. Am. Chem. Soc. 56, 1815 (1934); C. S. Marvel and D. S. Frederick, U.S. Patent 2,136,389 (1938). d C. S. Marvel and W. H. Sharkey, J. Am. Chem. Soc. 61, 1603 (1939). e F. J. Glavis, L. L. Ryden, and C. S. Marvel, J. Am. Chem. Soc. 59, 707 (1937). fM. A. Jobard, J. Polym. Sci. 29, 275 (1958). g J. E. Crawford and D. N. Gray, J. Appl. Polym. Sci. 15, 1881 (1971). h B. G. Harper and C. F. Smith, U.S. Patent 3,409,548 (1968). ill. G. Burkard, R. O. Henselman, H. N. Miller, and N. Tunkel, U.S. Patent 3,442,790 (1969).

~~

16

1.

TABLE VI

Olefln--Sulfur Dioxide Copolymers

Preparation of Olefin-Sulfur Dioxide Copolymers Using cqt~'-azobisisobutyronitrile Initiator (0.2 gm) a Conditions

Olefin

(gm)

Ethylene Propylene Propylene Propylene Propylene 2-Butene Isobutene Isobutene Butadiene Propylene 2-Butene Propylene 2-Butene Propylene Ethylene

26 120 33 110 40 120 80 120 50 42 56 42 56 42 28

Sulfur dioxide (gm) 32 32 126 40 150 40 35 30 40 40 40 30

Solvent (gm) Benzene 80 Propylene Sulfur dioxide Toluene 100 Sulfur dioxide 2-Butene Toluene 130 Isobutene Toluene 130 Propylene 2-Butene Propylene 2-Butene Benzene 90

Synthesis temp (~ 70 70 70 -40 - 50 - 25 - 20 - 20 - 20 70 - 40 70

Time (h) 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

Yield (gin)

Type Thermal Thermal UV (3500-3600 UV (3500-3600 UV (3500-3600 UV (3500-3600 UV (3500-3600 UV (3500-3600 UV (3500-3600 Thermal Thermal UV UV Thermal Thermal

A) ,~) A) A) A) ,~) ,~)

41 40-50 83 10 37 10 8.6 9.5 34 31

34

a Reprinted in part from M. A. Naylor and A. W. Anderson, J. Am. Chem. Soc. 76, 3962 (1954). Copyright 1954 by the American Chemical Society. Reprinted by permission of the copyright owner.

3-2.

Sulfur Dioxide-Butene Copolymers [21]* CH3CH--CHCH3

+ S02

~

( - - C H ( C H 3 ) - - C H ( C H 3 ) - - SO2 - - )n

(8)

A nitrogen atmosphere is maintained. Pyrex containers are used in all photopolymerizations. Silver nitrate catalyzed polymerizations are carried out in the dark. Close pairing of experiments is maintained for concentrations of reactants, distance from light source, and procedure for polymer isolation. Samples of unreacted olefins are isolated by pumpting the gases in v a c u o through a tower of diethanolamine. In all experiments the infrared spectrum of the residual olefin (gas, 100-200 mm, 10-cm path) is indistinguishable from the spectrum of the starting olefin. See Table IX for conditions used. For examination of infrared spectra of the copolymers a sample (1 mg) is ground with KBr (0.2 gm) and compressed to a transparent disk at 16,000 lb/in 2. A similar blank is used in the reference beam. Although quantitative work is not feasible in examination of the spectra, particular attention is given to relative * Reprinted from P. S. Skell, R. C. Woodworth, and J. H. McNamara, J. Am. Chem. Soc. 29, 1253 (1957). Copyright 1957 by the American Chemical Society. Reprinted by permission of the copyright owner.

3.

17

Polysulfones by the Reaction o f Olefins

TABLE VII

Reaction of Olefins with Sulfur Dioxide

Monoolefins Ethylene Propylene 1-Butene 2-Butenes Isobutene (2-methylpropene) 1-Pentene 2-Pentene Isopropylethylene 1-Hexene 1-Octene 1-Nonene Cyclohexene 2- Methylcyclohexene Styrene 2- Cyclohexylpropene

Yield

Melting point (~

Ref. a, a, a, a,

75-90 80-90 75

340 340 290-300

Small 75-80

175-200

Small 75

o

CHESH CHzS /

H

SCH2

C\

(28) CHzS

Marvel and co-workers [86] described the preparation of linear type polymercaptals and polymercaptols. The reactions were carried out between a wide variety of carbonyl compounds and dimercaptans in the presence of dry hydrogen chloride with or without solvent (dioxane) present. The reactions are summarized in Table IX.

3-8.

General Procedure for the Preparation of Polymercaptols Shown in Table IX [86]

The carbonyl compound and an equivalent amount of the dithiol were placed in 10 ml of freshly distilled dioxane. The solution was cooled in an ice bath

TABLE IX

Dithiol Decamethylene Hexamethylene Decamethylene Hexamethylene Decamethylene Decamethylene Hexamethylene The above polymer after a Hexamethylene The above polymer after a

gm

Properties of Polymercaptals and Polymercaptols a

Carbonyl compound

0.9444 Acetaldehyde 0.9800 Acetaldehyde 0.9370 Butyraldehyde 0.9700 Butyraldehyde 0.9800 Benzaldehyde 0.9800 Benzaldehyde 0.9700 Benzaldehyde month in corked tube 0.9700 Benzaldehyde month in vacuum

gm

Time (min) passing HC1

% Reaction detd by unused -- SH

0.2078 0.3685 0.4587 0.8170 1.0504 1.0504 1.0504

30 1189 17 15 12 (hr) b 15c 7

95.39 --

35 60

m ---

40 135 120 115

1.0504

10

--

110

m.p. (~

Inherent viscosity

MW (Calc)

0.042 0.146 0.102 0.049 0.428 0.452 1.374 0.864 0.98 0.93

1100 5100 3300 1400 18600 19900 76000 43000 50000 47000

Eqv wt by amperometric titration w

3930

25030 29500 480

r t~ t~

Hexamethylene Hexamethylene Hexamethylene Decamethylene Hexamethylene Hexamethylene Hexamethylene Hexamethylene Hexamethylene Hexamethylene Hexamethylene Hexamethylene Decamethylene

1.5329 0.9810 0.9840 1.8847 0.9872 0.9818 0.9823 0.9886 0.9779 0.9920 0.9797 0.9815 0.9502

Vanillin Vanillin Anisaldehyde m-Nitrobenzaldehyde p-Nitrobenzaldehyde p-Bromobenzaldehyde 2-Butanone 3-Methyl-2-pentanone 4-Methyl-2-pentanone Acetophenone Cyclohexanone Cyclohexanone Vanillin

1.0178 0.9950 0.9278 1.3842 1.0005 1.2253 0.5023 0.6653 0.6649 0.8013 0.7345 0.6662 0.7042

25 10 10 60 22 17 188 (hr) 60 60 12 10 10d 10

--99.80 99.75 99.71 99.16 70.87 45.83 48.45 71.40 90.00 95.93 97.18

40 150 130 75 80 e 72 Low Low Low 30

0.065 Insoluble 0.440 0.160 Insoluble 0.067 0.088 0.048 0.053 0.088

75 140

0.340 Insoluble

1900

528

19000 5700 2000 2700 1300 1500 2700

2380 1360

14300

a Reprinted in part from C. S. Marvel, W. H. H. Shen, and R. R. Chambers, J. Am. Chem. Soc. 72, 2106 (1950). Copyright 1950 by the American Chemical Society. Reprinted by permission of the copyright owner. b Hydrogen chloride gas was passed very slowly into the reaction mixture. This reaction was carried out without solvent. r After passing hydrogen chloride into the mixture for 15 min, the reaction mixture was left in contact with air for 6 hr. This reaction was carried out in dioxane solution (10 ml). d The polymer solution was exposed to air for 10 hr before precipitating by methanol. e Calc: N, 4.90; Found: N, 4.87.

3.

102

SulfidePolymers

while dry hydrogen chloride was bubbled imo the mixture for varying lengths of time as indicated. In all cases the reaction was exothermic and usually developed a pink color. The mixture was cooled and the polymer precipitated by the addition of 20 ml of cold methanol. The polymer was filtered, washed with cold methanol, and dried in a desiccator under reduced pressure to give yields as shown in Table IX. E.

P o l y ( a l k y l e n e sulfides) by t h e D e h y d r a t i o n o f Mercaptoalkanols

The acid-catalyzed polycondensation of certain mercaptoalcohols represents a method for the preparation of polyalkylene sulfides [87-90]. HOCH2CHzSH

H--H20

' (-- CHzCH2 -- S -- )n

(29)

Acids which are particularly effective in catalyzing this condensation are p-toluenesulfonic acid, sulfuric acid, zinc chloride, boron trifluoride-ether complexes, and acid clays [88]. The preferred catalyst (0.5 to 10 parts per 100 parts monomer) is p-toluenesulfonic acid and it is important that the catalyst be nonoxidizing relative to the mercaptan group present. The pure alkylene sulfides can also be prepared first by the dehydration of 2-mercaptoethanol using a catalyst consisting of an alkali or alkaline earth with metal along with compounds of A1, B, Si, P, lanthanides, etc., with metals according to a disclosed procedure [86a]. These alkylene sulfides can then be subsequently polymerized.

3-9.

Cocondensation of Mercaptoethanol and Mercaptopropanol [88]

HS--CH2CH2OH + HSCH2 -- CH -- OH

I CH3 H(-- S-- CH2CH2)x(S-- CH2CH--)yOH CH3 I

(30)

To a 5-liter reactor, which is equipped with a mechanical stirrer, thermometer, nitrogen bubbler tube, condenser, and Barrett trap, are added 39.0 gm (0.5 mole) of mercaptoethanol, 46 gm (0.5 mole) of mercaptopropanol, 4 gm p-toluenesulfonic acid (PTSA), 75 ml of benzene (see note of caution), and 75 ml of toluene. Nitrogen is bubbled through the reaction mixture and a small nitrogen pressure is maintained. The reaction mixture is refluxed for 3 hr at 90~176 and 17.5 ml of water is collected. Then 250 ml of toluene is added to dilute the

103

4. Poly(arylene sulfides)

reaction mixture and the mixture is further heated for a short time to insure complete reaction (no more water collected in Barrett trap). The reaction mixture is cooled to 80~ then 5 gm Filtrol No. 13 and a few grams of Celite Super-Cel is added, and stirring is continued. The hot mixture is filtered through a stream-jacketed Buchner funnel and the clear, hot filtrate collected. The filtrate is placed into Pyrex trays and the solvent evaporated first in a hood and then in a vacuum oven at 80~ (also in a hood). The polymer obtained weighs 62.5 gm and is a thermoplastic, rubbery material.

CAUTION: Benzene is known to be toxic and carcinogenic. It can be substituted with toluene in this preparation. 4.

POLY(ARYLENE SULFIDES)

The search for a thermally stable thermoplastic polymer led to the recent developments in poly(phenylene sulfides). The latter polymers are analogous to the poly(phenylene ethers) described in an earlier volume of this series [91]. Poly(p-phenylene sulfide) was first reported in 1897 by Genvresse [92] who reported an insoluble resin prepared by the reaction of benzene with sulfur in the presence of aluminum chloride. A variety of other procedures were reported to yield similar resins. Macallum [93] in 1948 reported a novel procedure that yielded an improved resin. Lenz and co-workers [94-96] modified the procedure and Edmonds and Hill [97] of the Phillips Petroleum Co. developed a commercially successful process. The material is now marketed under the trade name Ryton [98]. The crystallinity of the polymer has recently been reported [99-101]. Earlier suggested syntheses for poly(arylene sulfides) are shown in Scheme 4. Most of these syntheses involve either electrophilic or thermal reactions. ~--OH+SC12

~-OH+S

~

8-/1

HS ~ Scheme

SH

~~

+S+AICI3

~

+S

-SH + A1C13or H2SO4or

8OC12

4 Earlierpreparations of Poly(arylene sulfides) [92, 102-109].

104

3. Sulfide Polymers

Macallum [93, 110] reported a more convenient poly(phenylene sulfide) syntheses by the reaction of p-dichlorobenzene in a dry state using a mixture of sulfur and sodium carbonate at 300~176 Macallum reported that if sodium sulfide were used in place of sodium carbonate and sulfur then a small amount of sulfur was still required to catalyze the reaction (Eq. 31). C I - ~

(31)

C1 + Na2CO3 + S

The reaction of sulfur and sodium carbonate gives the nascent metal sulfide as earlier described by Pearson and Robinson [ 111 ]. 3Na2CO3 + (2n + 2)S

' 2Na2Sn + Na2S203 + 3CO2

(32)

The poly(phenylene sulfides) are light-colored, cream to canary yellow, in the solid state having good thermal stability. The addition of sulfur acts as a plasticizer to give a poly(phenylene sulfide) with rubber-like properties. In the presence of metallic lithium thereaction of $8 with 1,4-dichlorobenzene yields a linear poly(1,4-phenylene polysulfide) in which the sulfur content could be 3 sulfur atoms per phenylene unit (Mn as high as 12,000) [11 l a]. CI-~C1

+ $8 + Li

THF 25~

~ S x

~n + 2LiC1 + Li2S

where x = 3 In more recent developments poly(p-phenylene sulfides) have been reported to have been prepared by the oxidative polymerization of diphenyl disulfides or thiophenol with quinones at room temperature [37b, 11 l b]. C1

C1

n SS- + nO@O C1

,

C1 C1

el

C1

el

The advantage of this process is that no salt is produced as a by-product and the resins are obtained with improved electrical performance and moldability properties as seen in Table IXa.

105

4. Poly(arylene sulfides) TABLE IXa Metal Content of PPS Produced by Various Methodsa

Method

Monomer

Na (ppm)

C1 (ppm)

Cu (wt%)

Br (wt%)

Oxidative polym. Oxidative polym. Polycondensation Polycondensation

Diphenyldisulfide (3,5-Dimethylphenyl)disulfide Sodium sulfide + p-Dichlorobenzene Copper p-Bromothiophenoxide

7 5 1500 4700

125 85 4700 3500

0 0 0 >1

0 0 0 1.7

a

Data taken from [37b].

Diphenyldisulfide can also be polymerized to poly(p-phenylene sulfide)s using Lewis acids such as SbC15 at room temperature [11 l c]. However, these resins may be slightly contaminated by residual metal catalyst impurities. Poly(phenylene sulfide)s are highly stable and have a high melting point. The presence of sulfur atoms between the aromatic tings provide flexibility and the resins can be used for fibers, and thermoplastics. Polyphenylene sulfides doped with AsFs, I2, or SO3 could also provide electrically conductive materials [111d].

4-1. Preparation of Poly(phenylene disulfide) [93] C1 ~~

S + S + Na2CO3

,

+ NaC1 +

Na2SO4

+

CO2 (33)

C1 To a heavy-walled, glass-polymer tube is added a prepulverized mixture of 3.0 gm of sulfur and 3.9 gm of low-density sodium carbonate (prepared by heating sodium bicarbonate at 200~176 Then 3.3 gm ofp-dichlorobenzene is added and the tube is evacuated and sealed. The tube is now heated for 20 hr at 300~176 in a nearly horizontal position. On cooling the tube is cautiously opened and vented under a hood and the contents are pulverized. The product is extracted with water and the residue dried to give 4.1 gm of a sulfur-colored resin containing 56.2% total S (modified Liebig method) and 9-10% labile S (by Parker's method). The crude product is purified by continuous extraction with acetone and then toluene. It is dried at 140~176 under reduced pressure to give 2.0 gm of product. The product is straw-colored, brittle when cold, resilient at 80~176 and plastic from 180~ to above 350~ Analyses indicate it corresponds to (C6H4S2.3).

o~

TABLE X

Acid-Catalyzed Polycondensation of Mercaptoethanol and Mercaptopropanol a Reaction conditions

Mercaptoethanol (moles)

Mercaptopropanol (moles)

Solvent (ml)

0.5

0.5

Toluene 75 Benzene 200 Toluene 150 Benzene 800 Benzene 2000 Toluene:Benzene 150:150 Benzene: Toluene 125:125 Benzene: Toluene 125:125

10

10

0.5

0.5

5

5

10

10

1.9

0.1

1.0

1.02

1.02

1.0

PTSA catalyst (gm)

Temp (~

Time (hr)

Yield

4

90-95

3-4

62.5 grn

32

80

18

4

100-110

1.5-2

16

90

10-11

32

80

15

8

90-95

7.5

6

90-95

4.5

120 gm

Elastomer

6

90-95

4

119 gm

Rubbery thermoplastic

aData taken from M. B. Berenbaum, E. Broderick, and R. C. Christena, U.S. Patent 3,317,486 (1967).

m.p.

(oc)

60.g gm 91% 678 gm 100%

169-174

107


4-2. Preparation of Poly(phenylene sulfide) [93] C1 + S + Na2CO3 C1

s_] +

aCl + a,_SO4 +

Using a procedure similar to Preparation 4-1 a mixture of 1.2 gm of sulfur. 4.0 gm of anhydrous sodium carbonate, and 3.3 gm of p-dichlorobenzene was heated in a similar manner to give 2.4 gm of a pale-yellow, crude resin which is purified in a similar fashion. After drying the recovery amounts to 87-88% of the crude which is a white powder, brittle when cold, and fuses sharply at 255~ The polymer corresponds to the empirical composition of (C6H4S1.2) with MW 35,000-70,000. Macallum observed that the addition of 1,2,4-trichlorobenzene to the monomer mixture gave a polymer which was moldable at 280~176 under low pressure to yield a tough plastic [11 le] (Eq. 35). C1 x

C1 + y

C1

+ S + Na2CO3

'

C1

, S

S

+ NaC1 + Na2SO4 + CO2

(35)

Y

x

where x >>y. Lenz and Carrington [ 11 l f] reinvestigated the Macallum polymerization and extended our polymerization data on these systems. Some typical polymerizations are shown in Table XI. The infrared spectra of phenylene sulfide polymers are chosen in Fig. 2. Linear poly(phenylene sulfides) prepared by the condensation polymerization of alkali metal salts ofp-halothiophenols have been reported [112]. The reaction can be carried out in the absence of solvent at 10~176 below the melting point or in pyridine at 250~ The rate constants for the reaction in pyridine is shown

oo

TABLE XI Conditions Polymer No.

Time (hr)

Temp. (~

I II III IV V VI

20 20 20 24 20 6 48 48 6 48 6 96

350 300 300 300 300 200 350 350 200 350 200 350

VII VIII IX

Conversions and Yields in the Macallum Polymerizationa Mole ratio of reactants

p-C6H4CI2

1,2,4-C6H3C13

Yields (gm)

S

Na2CO3

% cony b

Dol C polymer

Insol polymer

Total

1 1 1 1 1

0.1 0.042 0.1

1.5 1.53 1.63 1.84 1.73

1.5 1.53 1.56 1.84 1.73

98.4 80.5 93.0 99.0 99.0

24.8 m

45.4 m

70.2 77.5 100.4 94.9 94.2

83.3 73.6

11.6 20.6

1 1

-0.15

1.5 2.0

1.5 2.0

90.7 93.4

62 49

25 46

87 95

1

0.15

2.0

2.0

93.6

58

28

86

1

0.15

2.0

2.0

92.7

36

56

92

aReprinted from R. W. Lenz and W. K. Carrington, J. Polym. Sci. 41, 333 (1959). Copyright 1959 by Joumal of Polymer Science. Reprinted by permission of the copyright owner. b Conversion based on chloride ion. c Soluble in boiling toluene and/or boiling diphenyl ether, insoluble in methanol.

109


? A

t

1

I

1

I

I

1

I

1

I

1

I

1600 1500 1400 1300 1200

1

I

1

I

I

1

1

i

I

I

1100

1000

900

800

1

I 700 600

Wave number (cm -l)

Fig. 2 Infrared spectra of phenylene sulfide polymers: (A) linear polymer, (B) Macallum homopolymer, (C) Macallum copolymer (Nujol mulls). [Reprinted from R. W. Lenz and Handlovits, J. Polymer Sci. 43, 167 (1960). Copyright 1960 by the Journal of Polymer Science. Reprinted by permission of the copyright owner.]

in Table XII for the reaction:

110

3. Sulfide Polymers

TABLE XII OverallRate Constants for the Polymerizationof a Series of Alkali p-Halothiophenoxides in Pyridine at 2 5 ~ a

x- sM X

M

Second-order rate constants k (liter mole- 1min- 1)

F C1 Br I Br Br Br

Na Na Na Na Li Na K

0.026 0.011 0.13 0.86 2.6 0.13 0.096

a Reprinted from R. W. Lenz, C. E. Handlovits, and H. A. Smith, J. Polym. Sci. 58, 351 (1962). Copyright 1962 by the Journal of Polymer Science. Reprinted by permission of the copyright owner. where M = Na, Li, K; X = F, C1, Br, I. The percent conversions are 91.5% when X = F, M = Na; 75.76% when X = C1, M = Na; 82-93% when X = Br, M = K; and 74-77% when X = I, M = Na. Handlovits [113] reported that poly(phenylene sulfide) can also be prepared by the polymerization of copper p-bromothiophenoxide in the solid state or in pyridine

CuS

r

37,

Recently, Price [114] reported that poly(phenylene sulfides) can be obtained with the use of the analogous silver salts. Poly(phenylene sulfides) have also been prepared by metal salts of m-halothiophenoxides [ 115] as well as by a two-component system using metal salts of dithiohydroquinone and dihalobenzene [116]. Poly(phenylene sulfide) may also be prepared starting with mercaptobenzenediazonium salts [ 117]. Edmonds and Hill of Phillips Petroleum Co. reported that poly(phenylene sulfides) may be prepared by the reaction of p-dihalobenzene with sodium sulfide in N-methylpyrrolidone or dimethylformamide [118, 119]. Copper metal or cuprous chloride was also shown by Edmonds and Hill to give improved yields [118]. Table XIII lists examples of various poly(phenylene sulfides) and the conditions used for their preparation. Philllips Petroleum Co. is now producing poly(phenylene sulfides) under the name Ryton (trademark of Phillips Petroleum Co.) [98].

TABLE XIII

Na2S'9H20 (gm) 73.4 73.4 240.2 240.2 240.2 240.2 240.2 240.2 240.2 240.2 240.2 240.2 27.75 (anhydrous) 73.2

p-Dichlorobenzene (or other, as specified) (gm) 45 45 147 147 147 147 147 147 147 147 147 73.5 80.5 50.7 100

Preparation of Polyphenylsulfides Using Procedure 3-3 a Reaction

Additive (gm)

CuE1 (2) CuE1 (4) Cu (tubing) CuE1 (8) p-dichlorobenzene, 2,4-dichlorotoluene p-dichlorobenzene bis(p-bromophenyl)-ether

Volume of solvent (ml)

Temp (~

Time (hr)

Polymer (gm)

Yield

m.p.

(%)

(oc)

200 b 200 b 1,000 b 1,200 c 1,000 b 1,000 b 1,0006 1,000 b 1,000 b 1,000 b 1,000 b 100 b

260-265 260 250 250 250 250 250 250 250 250 250 250

41.5 91 17 16 17 17 17 17 17 17 17 17

26 20 96.3 59.9 91.0 90.7 91.2 92.8 92.6 98.8 89.6 100.3

79 60.5

89.5

275 248.5-252 275 240 282.5-285 28i5-291 284-287 282.5-286.5 285.5-286.5 276-280 282.5-288.5 1 x 1014 (25~ 2.2 • l0 ll (200~

a Reprinted from T. W. Campbell and K. C. Smeltz, J. Org. Chem. 27, 2069 (1963). Copyright 1963 by the American Chemical Society. Reprinted by permission of the copyright owner. b Film was molded at 300~ and drawn 4.5 times at 115~

136

4. Polymerization Reactions of Mono- and Diisocyanates

TABLE VI

Physical Test Data on Poly(toluene carbodiimide) Film Stripsa

Temp. ~

Tenacity g.p.d.

Elongation, %

Initial modulus, g.p.d.

Denier

25 110

3.2 0.3

52 121

38 0.19

467 449

a Reprinted from T. W. Campbell and K. C. Smeltz, J. Arg. Chem. 27, 2069 (1963). Copyright 1963 by the American Chemical Society. Reprinted by permission of the copyright owner.

0

POLY-2-OXAZOLIDONE BY THE REACTION OF ISOCYANATES WITH EPOXIDES +-

(CH3)4NI OCN--R--NCO

+ CH2--CH--CH20--R'--OCH2CH--CH2 \ / \ /

0

DMF

0

/CH2--CH

~ c

I

O--CH2

~ "

N--R--N~

I

CH-- CH20--R'--

II o TABLE VII Equivalents N = C - - N per Mole for Polycarbodiimides Derived from Toluene 2,4-Diisocyanate a

Alcohol used

CO2 (gm)

MW calc. from CO 2 data

Ethanol Ethanol Isopropyl alcohol Isopropyl alcohol Benzyl alcohol Ethanol Ethanol

7.3 6.7 7.0 7.8 6.9 8.2 7.3

902 670 794 1334 855 2114 1332

Total wt

Equiv N--C=N/mole 4.89 3.11 3.85 8.00 3.58 14.22 4.86 b

a Reprinted from T. W. Campbell and K. C. Smeltz, J. Org. Chem. 27, 2069 (1963). Copyright 1963 by the American Chemical Society. Reprinted by permission of the copyright owner. b Polycarbodiimide derived from methylenebis(4-phenyl isocyanate).

OCH2--

4. Poly-2-oxazolidone by the Reaction of Isocyanates with Epoxides

137

TABLE VIII Comparisonof Molecular Weight for Stabilized Polycarbodiimides Using Titration Data and CO2 Dataa

Diisocyanate u s e d 2,4-TDIb 2,4-TDI MDIc 2,6-TDIa MDI

Alcoholused

Total wt CO2 (grn)

MW calcd from CO2 data

MW from titration data

Ethanol Isopropyl alcohol Ethanol Ethanol Ethanol

6.7 7.0 7.3 6.6 8.1

670 794 1332 642 2752

664 803 3700 1176 2771

a Reprinted from T. W. Campbell and K. C. Smeltz, J. Org. Chem. 27, 2069 (1963). Copyright 1963 by the American Chemical Society. Reprinted by permission of the copyright owner. bToluene 2,4-diisocyanate. cMethylenebis(4-phenyl isocyanate). a Toluene 2,6-diisocyanate.

Sandier, Berg, and Kitazawa [9] reported that 2,4-toluene diisocyanate reacts with diglycidyl ethers to give poly-2-oxazolidones. A similar reaction was also reported for monoisocyanates reacting with monoepoxides [9, 10]. The reaction takes place in DMF as a solvent and is catalyzed by tetramethyl-ammonium iodide at 160~ for 6 hr. A recent review of this reaction is worth consulting for additional background material [ 10a]. Since the earlier work by Sandier, Berg, and Kitazawa [9] numerous studies [10b-f] and patents [10g-i] have appeared and a few are cited here. This area continues to be an active area for research and applications. Among catalyst studies [ 10b, c] it has been reported that 2-ethyl-4-methylimdazole (EMI) is preferred since it is soluble in the reaction medium and was very effective. The use of N-methyl-pyrrolidone (NMP) in these studies gave a polyoxazolidones with little or no isocyanurate trimer seen in the infrared spectrum of the product (reaction of MDI with DGEBA using 0.17 EMI at 180-185~ in NMP) [ 10C]. Other studies [ 10d-f] report on numerous other catalysts and in some cases on the use of aliphatic isocyanates. Most of the structural evidence is based on infrared spectral data (IR) and no reports appear using nuclear magnetic resonance (NMR) to confirm structures. These polyoxazolidones represent a new generation of isocyanate derived polymers which have characteristic thermal stability properties. They are useful in adhesives, elastomers, coatings, foams, and electrical applications. Numerous other applications will no doubt be found for them.

138

4. Polymerization Reactions of Mono- and Diisocyanates

4-1. Preparation of the Poly-2-oxazolidone from the Reaction of 2,4-Toluene Diisocyanate and the Diglycidyl Ether of 2,2-Bis(4-hydroxyphenyl)propane [9] CH3

+-

I

CH2 - C H - CH2 - C6I-L- C - C6H4- OCH2CH - CH2

N /

\ /

/

O

CH3 H2-CH

I N

(CH3)4NI

DMF

O

/O-CH2 H3C,~~] O=C, I

CH3,

N - CH- CH20- C6H40- C-I C6FL- OCH2

,

CH3

_

To a flask is added 0.1 mole of 2,4-toluene diisocyanate, 50 ml DMF, and 0.1 mole of the diglycidyl ether of 2,2-bis(4-hydroxyphenyl)propane. Then 0.2 gm tetramethylammonium iodide is added and the reaction mixture heated with stirring for 6 hr to give a 91% yield of the polymer, m.p. range 175~176 IR strong absorptions at 5.65 and 7.10 microns, with no absorption at 4.25-4.60 microns for the free NCO groups. Sandler [11] also reported that poly-2-oxazolidones can be prepared by the reaction oftrimerized isocyanates, i.e., isocyanurates on reaction with diepoxides. A variety ofpoly-2-oxazolidones have been reported (see Table IX). The reactions can be used to cure diisocyanate prepolymers as an alternate cure method to the diamines. Some typical preparations are reported in Table IX. Sandler [ 12] and later Ashida and Frisch [ 13] reported that poly-2-oxazolidones can be converted to polyisocyanurates.

0

POLYMERIZATION OF POLYISOCYANATES TO POLYISOCYANURATES

Hofmann [ 14] in 1858 was the first to report the trimerization of isocyanates to isocyanurates. Hofmann used triethylphosphine as a catalyst to trimerize phenylisocyanate to triphenylisocyanurate. NCO 3

O

C6H5~ N"~ N/C6H5 (C2Hs'3P O~... N ~ O I

C6H5

(9)

TABLE IX

Preparation of Monomeric and Polymeric Substituted 2-Oxazolidones from 1,2-Epoxides and Isocyanates a Reaction conditions

Experiment No.

Epoxide (moles)

Isocyanates (moles)

Catalyst (gm)

Solvent

Time (hr)

Temp (~

2-Oxazolidone

m.p. (~

Yield (%)

Characteristic infrared spectral bands and commentsd

TMAI b 0.2

DMF

6

160

3-Phenyl5-phenoxymethyl

137-8

28

0.10 Phenyl

Pyridine

DMF

6

160

136-7

144

0.10 Phenyl

0.2 ZnBr2

DMF

6

160

136-7

33

Same as above

0.10 Phenyl

0.2 None

DMF

6

160

136-7

15

Same as above

0.10 2,4-Toluenedi-

TMAI

DMF

6

160

0.10

0.20

3-Phenyl5-phenoxymethyl 3-Phenyl5-phenoxymethyl 3-Phenyl5-phenoxymethyl 2,4-di[3-(5phenoxymethyl-2-oxazolidonyl)] toluene

5.60 (w) 5.65 (w) 5.68 (mw) 5.70 (m) 5.72 (m) 5.72 (ms) 5.75 (m) 5.78 (ms) 5.80 (m) 5.81 (ms) 7.08 (s) Same as above

3-Phenoxy1,2-propylene

Phenyl

0.10 3-Phenoxy1,2-propylene 0.10 3-Phenoxy1,2-propylene 0.10 3-Phenoxy1,2-propylene 0.10 2-Phenoxy1,2-propylene

0.20

60-63

78

5.50 (w) 5.57 (m) 5.62-5.72 5.75 (s) 5.90 (m)

5.55 (mw) 5.60 (s) (vs) 5.85 (s) 7.10 (vs)

(continues)

k~

TABLE IX

(continued)

Reaction conditions Experiment No.

Epoxide (moles)

Isocyanates (moles)

Catalyst (gm)

Solvent

Time (hr)

Temp (~

DGBA C

Phenyl

TMAI

DMF

6

160

0.050 DGBA

0.10 2,4-Toluenedi-

0.20 TMAI

None

2

120

0.025

0.50

0.20

DGBA

2,4-Toluenedi0.050 2,4-Toluenedi-

None

None

2

120

TMAI

DMF

20

160

0.050

0.050

0.20

DGBA

1,6-Hexa-

TMAI

0.050

methylenedi0.050

0.20

2-Oxazolidone 2,2-bis(3-phenyl-5phenoxymethyl 2-oxaolidone)propane Polymere (brittle)

0.025 DGBA

Polymer MW

m.p.

Yield

Characteristic infrared spectral bands and comments d

(oc)

(%)

(u)

40

Softens 70-80 m.p. >300

Softens 175-185

93

5.55 5.70 5.77 7.10

(w) (s) (s) (s)

100

5.60 5.68 5.72 5.80 7.10

(w) 5.65 (w) (mw) 5.70 (m) (ms) 5.75 (s) (vs) 5.85 (vs) (vs) No reaction

91

2880

10

DMF

6

160

Polymer gelled

79

5.65 (s) 5.75 (s) 5.85 (s)

5.52 (w) 5.75 (m) 5.60 (ms) 5.63 (ms) 5.65 (vs) 5.84 (ms) 7.10 (vs) (No absorption at 4.25-4.60/t) 5.68 (m) 5.90 (vs) 6.80 (m) 12.0 (m)

8.05 (m) (No absorption at 4.25-4.60/~)

i:l r~

11

DGBA

12

0.050 Epon 828

13

0.050 Epon 828

14

0.050 DGBA

15

0.050 0.050

16

DGBA

17

0.050 DGBA (3.4 g)

Softens 170 190-5

120

Polymer MW g 2870 m.p. Polymer (soft)

6

160

Polymer (hard)

Softens 90-100

None

2

120

Polymer (hard)

0.4 0.4

None

6

160

Polymer (hard)

None

None

6

160

Liquid (viscous)

TMAI

None

2

120

Polymer

4,4'-diphenylmethane-di0.050 2,4-Toluenedi0.050 2,4-Toluenedi0.050 2,4-Toluenedi0.050 0.050

TMAI

None

6

160

0.20 TMAI

None

2

0.4 TMA

None

0.4 TMAI

2,4-Toluenedi0.050 Adiprene L-315f (17.8 g)

(0.06)

100

5.55 (mw) 5.70 (s) 5.80(m) 7.10(s)

100

100

4.38 (m) 5.75 (s) 5.82 (vs) 7.10

100

Softens 170-180 m.p. ~ 0

< ~

0

0

0

I I I I I I

r

0

0

.,-

r 0

0

,-'~

-~ o ~ = =

""

0

7. Polymerization of Vinyl Acetate and Other Vinyl Esters

I I I I I~~l

? •

I I

I I I I I I I I I I I I

I I t I I ~ - ~

I~l

•

7 •

~b

~Z

~176176

I I I I I I I I~176

I ~

2.

Bulk Polymerizations

225

It should also be noted that at least one group of investigators [77] observed an induction period for the process. Few if any of the other workers in the field have mentioned this. The induction period is said to be variable, decreasing as the polymerization temperature increases. The cause for this induction period was not further explored. It could not reasonably be attributed to residual inhibitor since the experimemal description specifically indicated that an effort was made to remove the inhibitor first; then the monomer was partially polymerized. Only then was the unpolymerized monomer distilled under reduced pressure in a nitrogen atmosphere. When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of "dead" polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the free-radical polymerization of other types of monomers. In devising experimental procedures for the polymerization of vinyl esters, the elimination of oxygen is extremely important. Joshi [ 17] has shown that the bulk polymerization of vinyl acetate and vinyl propionate exhibited autoacceleration from the start and proceeded nearly to completion with only 2-4% unreacted monomer within 200 min in one case. When, however, the monomer had come in contact with air, inhibiting impurities developed and even after hours of heating, "dead-end polymerization" had taken place with 30--40% of unreacted monomer remaining. As we have memioned in previous volumes in this series, we consider bulk polymerizations in sealed ampoules or even sealed heavy-walled tubes not merely unsafe, but dangerous. The procedure, in some respects is so trivial, that it is rarely described in any detail. Yet, so much polymer chemistry has been studied by sealed-tube polymerization that the procedure has to be described. Procedure 2-1 is a composite of those described elsewhere [28, 34, 35, 75, 76]. Safety procedures will have to be designed to conform to OSHA regulations.

2-1.

Sealed-Tube, Bulk Polymerization of Vinyl Acetate

In a heavy-walled Pyrex ampoule with a constriction near its upper opening is placed a dispersion of 10 ml of purified vinyl acetate and 0.0004 gm of 2,2'-azobisisobutyronitrile. The content is sparged with oxygen-free nitrogen or argon and attached to high-vacuum line. The monomer is chilled with a dry iceacetone mixture. The tube is evacuated on the high-vacuum line while the

226

7.

Polymerization of Vinyl Acetate and Other Vinyl Esters

monomer remains frozen. By conventional techniques, the monomer is then degassed repeatedly. Finally the tube is sealed under reduced pressure. After the seal has cooled, the tube is placed in a suitable, protective sleeve and slowly tumbled, end-over-end, in a constant temperature bath at 50 ___0.2~ with suitable safety precautions. Conversion, after 3 hr, is approximately 35.6%. To isolate the product, the ampoule is cooled in dry ice-acetone, appropriately wrapped, and with suitable safety precautions, the ampoule is opened. The polymer may be dissolved in such solvents as acetone, benzene, or tetrahydrofuran (THF) and precipitated with petroleum ether or with redistilled hexane. The polymer may also be isolated from benzene solution by freezedrying. Bulk polymerizations have also been carried out in a conventional reflux apparatus. While, in principle, this procedure permits the preparation of larger quantities of polymer than is possible in sealed tubes, the control of the reaction temperature presents problems. The polymerization is autoaccelerating. As the viscosity of the medium increases, as the molecular weight increases, heat transfer from the interior of the reacting mass becomes increasingly difficult. The heat of polymerization is quite high so that great care is required. Naturally, a means of removing the polymer mass from the equipment also needs to be provided. The pneumatic jack and temperature controller mentioned in Ref. [44] will be useful for bulk processes. Procedure 2-2 is an adaptation of the procedure of Braun et al. [79].

2-2.

Bulk Polymerization of Vinyl Acetate (Reflux Apparatus) [79]

A 500-ml glass resin kettle is fitted with a reflux condenser, addition funnel, a nitrogen inlet tube extending to the bottom of the reactor, a means of evacuating the apparatus, and a thermometer. The gas exhaust, through the reflux condenser, is fitted with a Bunsen valve which prevents oxygen and moisture from entering the equipment. The equipment is flamed out under oxygen-free nitrogen and then is evacuated and flushed with oxygen-free nitrogen three times. While a slow stream of oxygen-free nitrogen passes through the equipment, 20 gm of a solution of highly purified vinyl acetate containing 0.14 gm of recrystallized dibenzoyl peroxide is added to the apparatus. The mixture is warmed with a water bath held at 80~ Once the polymerization has been initiated, the temperature of the water bath is lowered to maintain a gentle reflux. The reaction is maintained at a gentle reflux by the dropwise addition of further quantities of vinyl acetate solution containing 0.7% of dibenzoyl peroxide. After the desired quantity of monomer has been added, the reaction

2.

227


temperature is continued at 80~ for 30 min, followed by heating for 60 min at 90~ The residual monomer is removed by distilling it out of the equipment at 90~ under reduced pressure. Air is now allowed to enter the apparatus. The overhead equipment (condenser, thermometer, gas inlet tubes, etc.) is removed. The resin kettle itself is heated to 170~ and the viscous vinyl acetate is poured out of the kettle. A porcelain spatula may be useful in removing the polymer from the equipment. The equipment may be cleaned by refluxing methanol in the kettle (approximately 20 ml of methanol are required to dissolve 1 gm of polymer). The polymer may be recovered from its methanol solution by precipitation with an eight-fold excess of water. This portion of the product may be dried under reduced pressure at 50~ Unreacted vinyl acetate may be separated from the polymer, which has been precipitated in an aqueous system, by steam distillation. This procedure is also thought to decompose excess initiators [80]. The method used in Procedure 2-2, in a sense, is an adaptation of an old patented process for the commercial preparation of poly(vinyl acetate). In that procedure, as a stream of monomer is added to a polymerizing reaction mixture, an equal volume of the partially polymerized mixture is withdrawn to a finishing vessel where the reaction is allowed to go to completion at reflux [81]. On a commercial scale, a variation of this procedure, involves the extrusion of the finished product onto a moving belt. The molecular weight may be maintained within fairly narrow limits by this procedure, particularly when propionaldehyde is used as a chain-transfer agent. In a typical formulation, 3140 lb of vinyl acetate, 27 lb of dibenzoyl peroxide, and 11.8 lb of propionaldehyde give rise to a product with an average molecular weight of about 40,000 [82]. Pinacols are unique polymerization initiators. At temperatures below 100~ where the usual free-radical initiators such as the peroxides, are effective, pinacols are only of limited value. However, at temperatures above 100~ diaryl hydroxymethyl radicals form that initiate polymerization by hydrogen transfer [Eqs. (3, 4, 5)]. Ar Ar I I Ar--C--C--Ar

lOOOC

Ar I , 2. C - - O H

I I OH OH ar

I C H 2 = C - - R + .C--OH I I X Ar

(3)

I Ar ar

I } C--O + CH3--C--R I I Ar X

(4)

228


C H 3 - - C - - R + monomer

polymeric free radicals, etc.

(5)

I X Note that it is hydrogen abstraction which brings about initiation with pinacols rather than the addition of initiator radicals, as is the case with dibenzoyl peroxide, for example. Such high-temperature initiation may find specialized applications [73]. The bulk polymerization of vinyl acetate in dilatometers has been described [31, 72, 74, inter alia]. As the carboxylate moieties of vinyl esters increase in length, the degree of branching due to chain transfer to both monomer and polymer increases. The relationship between the chain-transfer constant (C.T.) and the number of carbon atoms in the carboxylate portion of a fatty acid vinyl ester in the presence of vinyl acetate is given by the expression C.T. = 4.0 X 10 -3 + n X 0.7 • 10 -3

(6)

where n is the number of non-a-methylene groups in the carboxylic acid chain beyond the propionate group. While the polymerization kinetics of vinyl propionate is normal in the sense that the rate is first order in monomer and 0.5 order in initiator, the rate of polymerization is much lower than that of vinyl acetate and the chain-transfer constant to monomer is greater than expected (C.T. = 4.9 X 10 -3) [83]. Table X lists chain-transfer constants of a number of ethyl esters reacting with vinyl acetate. According to Buselli et al. [83], the chain-transfer behavior of vinyl esters toward vinyl acetate is similar, except, of course, for the inexplicably anomalous vinyl propionate. The heat of polymerization of vinyl esters exhibits essentially no dependence on the length of the carboxylic acid. Generally the range of the heats of polymerization is between 20.5 and 21 kcal/mole. Steric strain is thought to reduce this value in the case of vinyl benzoate and the plasticizing effect of the side chains is believed to raise the value slightly in the case of vinyl 2-ethylhexanoate [ 17, 18]. In the bulk polymerization of vinyl propionate, up to a conversion of 25%, the average degree of polymerization remains constant. Beyond this point, the molecular weight increases as the conversion increases [78]. Table XI illustrates the effect of the increase in molecular weight with increasing conversion of vinyl butyrate [84]. As has been observed before, as the length of the acyl group increases from the n-butyrate to the n-caproate, the chain transfer to monomer tends to increase. Also, as the molecular weight of the polymer increases the polymer tends to assume a coiled configuration [84].

2.

229


TABLE X

Representative Chain-Transfer Constants for Vinyl Acetate [83] ~

Chain-transfer agent

Chain-transfer constant b (XlO 2)

Ethyl stearate Ethyl laurate Ethyl pelargonate Ethyl octanoate Ethyl butyrate Ethyl propionate Ethyl acetate Ethyl formate Ethyl 2-ethylhexanoate Ethyl isobutyrate Ethyl trifluoroacetate Toluene (BPO initiated) Toluene (AIBN initiated) Vinyl propionate

1.40 1.05 0.80 0.70 0.45 0.40 0.12 0.22 0.65 1.60 0.30 1.00 1.23 0.49

a Polymerization conditions: The bulk polymerization was conducted using 0.24 gm of dibenzoyl peroxide per 100 gm of monomer at 50~ to 10% conversion. b Constants for the ethyl ester are said to be comparable to those of the vinyl esters [83].

The reactivities of various vinyl esters in copolymerizations are generally very similar. The sampling of reactivity ratios given in Table XII indicates this quite clearly. The somewhat unique behavior of vinyl benzoate is indicated in the Table XII. The copolymerization of various substituted vinyl benzoates has been studied. It was found, first of all, that there was no polymerization with vinyl pnitrobenzoate. A plot of the relative reaction rates of vinyl benzoates with TABLE XI

The Effect of Conversion on the Intrinsic Viscosity of Poly(vinyl butyrate) [84] a

Benzoyl peroxide used (moles/liter x 103)

Reaction time (min)

Conversion (%)

Intrinsic viscosity (dl/gm)

2.611 4.975 2.611 4.975

19 20 50 240

20.5 45.5 61.5 94.5

0.288 0.307 0.381 0.840

a Polymerization temperature was 79.6~

230


TABLE XII

Reactivity Ratios of Selected Vinyl Esters

M2

rl

r2

Ref.

Copolymers of various monomers (M2) with vinyl acetate (M~) Vinyl formate Vinyl propionate Vinyl butyrate Vinyl trimethylacetate ("Vinyl pivalate") Vinyl 2-ethylhexanoate Vinyl neonanoate Vinyl neodecanoate Vinyl phenylacetate Vinyl benzoate Vinyl ethyl oxalate Vinyl thioformate Acrylic acid Behenyl acrylate Butyl acrylate Crotonic acid Ethyl acrylate Ethylene Maleic anhydride Methyl methacrylate Phenyl acrylate Vinyl chloride

0.94 0.98, 1.06 1.00 0.79

0.95 0.98, 0.76 0.97 0.96

85 85, 3a 85 3

1.19 0.93 0.99 0.96 0.70 0.30 0.05 0.02 0.021 0.05, 0.05 0.21 0.62 0.02 1.00 0.01 0.03, 0.03 0.22 0.60

1.90 0.90 0.92 0.92 1.13 3.00 5.50 20.64 1.76 5.89, 5.50 3.3 0.27 7.20 1.00 0.01 26.0, 22.21 2.48 1.40

3b 3 3 85 85 86 87 c 113 c. 170 171 c c 3 c 85, 3 114 c

Copolymers of vinyl esters (M2) with methyl methacrylate (M~) Vinyl Vinyl Vinyl Vinyl Vinyl Vinyl Vinyl

formate acetate propionate butyrate phenylacetate benzoate ethyl oxalate

28.6 26.0, 22.21 24.0 25.0 26.4 20.3 6.0

0.05 0.03, 0.03 0.03 0.03 0.03 0.07 0.10

85 85, 3 85 85 85 85 86

a All the data from Ref. [3] are said to be calculated values (see c below). b private communication of E. L. Kitzmiller, Lehigh University to Union Carbide Chemicals and Plastics Co., South Charleston, WV, in Ref. [3]. c Calculated data from "Vynate, Vinyl Ester Monomers," Technical Bulletin F-60848 10/93-3M, Union Carbide Corporation, Danbury, CT, 1993, and D. Lee, Am. Paint Coatings J. Oct. 18, 1993, reprinted in Technical Bulletin F-60889 11/93-1M, Union Carbide Corp.

2.


231

para-substituents such as hydrogen, methoxy, methyl, chloro, bromo, and cyano groups against HammeR's a-values gave a straight line. A small Hammer p-value indicated that, in this case, a small polar effect operates on the vinyl group [88]. The radical copolymerization of vinyl thioacetate and vinyl thiobenzoate has also been investigated. Overall, the polymerization rate of vinyl thioacetate was smaller than that of vinyl acetate, but its reactivity in copolymers was larger. This has been attributed to the participation of a d - n interaction of the sulfur groupings with the vinyl moiety [87]. Dialkyl vinylphosphonates, where the alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, and n-pentyl have been studied. The polymerization kinetics of these monomers indicate that chain termination may be by chain transfer through the alkyl groups of the monomer and that this chain transfer takes place in the later stages of polymerization [89]. The tacticity of the polymers of vinyl acetate and other vinyl esters has been studied for some time. Increasing stereoregularity of polymers was noted as esters of halogenated acids with increasing amounts of chlorine were polymerized. The sequence of increasing degree of stereoregularity was in the order: vinyl acetate < monochloroacetate < polychloroacetate < trifluoroacetate. Even in the most stereoregular poly(vinyl trifluoroacetate) ester, the sequence of regularly oriented units was relatively short [75]. Fordham and co-workers [75] observed that syndiotactic propagation may be preferred for free-radical polymerization in general. In other words, it was proposed that free-radical polymers may be significantly more regular in conformation than had previously been realized. They believed that the polymerization process was characterized by syndiotactic propagation, with frequent interruptions by isotactic propagation steps. It has been stated that at a conversion of 15%, the free-radical polymerization of vinyl acetate with AIBN and 2.4% triethylamine at 27~ produced an atactic poly(vinyl acetate) with a number-average molecular weight of 166,000. When chloroform was added to this initiator system, chain-transfer activity was sufficient to reduce the molecular weight of an atactic poly(vinyl acetate) to 1894 [90]. The vinyl esters of perfluorinated acids give rise to polymers which tend to be unstable to moisture. In fact, both the monomers and the polymers hydrolyze readily [91]. Interestingly enough, unlike the corresponding vinyl acetate system, poly(vinyl trifluoroacetate) is insoluble in its monomer. Substitution of fluorines for the hydrogens of the acetate portion of the monomer is thought to reduce chain transfer and branching. The poly(vinyl alcohols) derived from the hydrolysis of stretched poly(vinyl trifluoroacetate) were found to be highly ordered and birefringent [92]. Even in the presence of such aldehydes as acetaldehyde, propionaldehyde, butyraldehyde, and heptanol (all conventional chain-transfer agents for vinyl

232

7. Polymerization of Vinyl Acetate and Other Vinyl Esters TABLE XlII BulkPolymerization of Vinyl 1-Adamantanecarboxylateand Vinyl Trialkylacetates [94]a Polymerization Monomer

Vinyl 1-adamantane' carboxylate Vinyl trimethylacetate Vinyl tri-ethylacetate Vinyl tri-n-propylacetate Vinyl tri-n-butylacetate

Temp (~

Time (hr)

Yield (%)

Softening point (~

0.2

60

5

89

220-225

53

0.2 0.1 0.05 0.1

60 45 60 60

22 24 6 24

80 96 87 84

73-78 80-83 51-55 68-70

56 61 62 64

Initiator (moles)

Syndiotacticity (%)

a polymerization conditions: The monomer concentration was 20 moles. Polymerization was conducted using 2.2'-azobisisobutyronitrileas the initiator in degassed sealed ampoules. Times and temperatures are given for each monomer in the body of the table. esters) in the temperature range from - 4 0 ~ to 60~ the free-radical polymerization of vinyl trifluoroacetate gives rise to a polymer in which syndiotactic and isotactic diads are found in up to 56% concentration [93]. Bulky substituents on the acid portion of vinyl ester evidently affect the stereoregularity of the polymer obtained either by free-radical or cationic mechanisms. To study this matter further, vinyl 1-adamantanecarboxylate vinyl 1-adamentyl ether, and, as open-chain analogs of the adamentyl group, various vinyl trialkylacetates and vinyl tri-n-propylcarbinyl ethers were prepared and polymerized. The admantanecarboxylate ester exhibited a syndiotactic propagation stage much like the trialkylacetates. Syndiotacticity of the polymers seems to increase as the molecular weight of the trialkylacetate moeity increases up to a limiting value of approximately 65% syndiotacticity for vinyl tri-n-butylacetate as is indicated in Table XIII. Table XIII also lists the general conditions for the preparation of these polymers by bulk polymerization in degassed, sealed ampoules. The final purification of the polymers was by reprecipitation from benzene solutions with methanol. These polymers have also been prepared by irradiation with a low-pressure mercury lamp and with a tri-n-butylborane-air initiation system [94]. The exceptionally high softening point of poly(vinyl 1-adamantanecarboxylate) as compared with those of the vinyl trialkylacetates is noteworthy. 3.

SOLUTION

POLYMERIZATION

The polymerization of vinyl acetate in solution may be carried out to produce lacquers, chewing gum bases, and adhesives. Usually the polymers are used

3. Solution Polymerization

233

directly in the solvents in which they are formed. A few applications may call for the recovery of the solid polymer from solution. This may be accomplished by addition of a nonsolvent of the polymer or, where applicable, by removal of the solvent by steam distillation [95]. Most of the solvents that have been studied act as chain-transfer agents in the polymerization of vinyl acetate. The fact that the monomer itself acts as a chaintransfer agent for its own radicals has already been discussed, tert-Butyl alcohol is exceptional in that it is one of the few common solvents with minimal, if any, chain-transfer activity [96, 97]. Among the solvents which have been used in the polymerization of vinyl acetate are ethyl acetate [62, 82, 95, 96, 98, 99], butyl acetate [95], dimethylcarbitol [62], acetic acid [95], acetic acid-water mixtures [100], dimethylformamide [ 101 ], acetone [95], benzene [25.95, 98, 99, 102], various alcohols [25, 67, 95, 96, 99, 103, 104], toluene [95, 104], dichloromethane (methylene chloride) [ 104], and 1,2-dichloroethane (ethylene dichloride) [ 105]. An azeotropic mixture of methyl acetate and methanol is a common coproduct from the manufacture of poly(vinyl alcohol) from poly(vinyl acetate) in the presence of methanol and sodium methoxide. The polymerization of vinyl acetate in this azeotropic solution, with the total removal of oxygen from the system, and using conventional free radical initiators is said to afford highmolecular weight poly(vinyl acetate) [ 106]. The solution polymerization may be carried out conveniently in a typical reflux apparatus. If samples are to be taken during the course of the reaction, a sampling tube leading downward through a short condenser to a receiving flask may be incorporated. Although the reactions are usually carried out under a nitrogen atmosphere, the careful degassing of the reactants, which is possible in sealed ampoules or enclosed dilatometer polymerizations is normally not achieved. Consequently, induction periods must be anticipated. Procedure 3-1 is based on directions given in a study of the retarding and inhibiting effects of low levels of acrylonitrile on the polymerization of vinyl acetate [ 105].

3-1.

Polymerization of Vinyl Acetate in Ethylene Dichloride [105]

In a 3-liter, three-necked flask fitted with a propeller-type agitator, thermometer, reflux condenser, and inlet and outlet tubes for purified nitrogen which are fitted with bubble counters, a solution of 600 gm of purified vinyl acetate and 900 gm of ethylene dichloride is heated to reflux with stirring while a nitrogen flow of 7 ml/min is maintained. The heat is adjusted to maintain a reaction temperature of 70~ At that temperature, 1.5 gm of dibenzoyl peroxide is added and the nitrogen flow is reduced to 5 ml/min. Within approximately 5.5 hr, approximately 85% of the monomer is converted to polymer.

234


A sample of the polymer may be isolated by evaporating an aliquot of the product solution under reduced pressure, dissolving the residue in approximately 20 volumes of methanol, and precipitating the polymer with water. The filtered product is then dried under reduced pressure at a modest temperature. The polymerization of vinyl acetate in various alcohols has been suggested by many workers. Certainly when methanol is used as a solvent, the solvent concentration has a profound influence on the molecular weight of the polymer [67, 99, 103]. This effect has been attributed to the formation of acetaldehyde, a well-known chain-transfer agent, by a transesterification reaction involving the monomer [103]: O II CH2---CHOC--CH3 + CH3OH

O O II II ~ CH3COCH3 + CH3CH

(7)

Table XIV indicates the effect of methanol and dibenzoyl peroxide concentration on the percent conversion of the monomer and on the degree of polymerization of the polymer. Of t h e common solvents, tert-butyl alcohol because of its very low chaintransfer constant, may be used to produce polymers of relatively high molecular weight. If we concede that single-point measurements of specific viscosity and inherent viscosity may be considered indications of the general trend of molecular weights, then the effect of various solvents on the molecular weights of the poly(vinyl acetate) produced may be seen in Table XV. Procedure 3-2 outlines the method for polymerizing vinyl acetate in tert-butyl alcohol. Particular attention is directed to the method for removing excess monomer and solvent. TABLE XIV The Polymerization of Vinyl Acetate in Methanol Solution [103] Concentration of methanol (%)

Dibenzoyl peroxide (% on monomer)

15 15 30a 33 33 33 33 50 50

0.025 0.050 0.10 0.017 0.066 0.23 0.46 0.23 0.92

a Based on data from Horn [99].

Polymerization time (hr) 18 16 -46 45 17 15 26 26

Conversion (%) 64 96 92 96 97 97 98 98

Degree of polymerization of poly(vinyl acetate) 1800 1900 850 640 760 670 600 360 300

3.

235

Solution Polymerization

TABLE XV

The Effect of Solvents on the Polymerization of Vinyl Acetate [96] ~

Diluent

Yield of polymer (%)

Specific viscosity

Inherent viscosity (dl/gm)

(None) Isopropyl alcohol n-Propyl alcohol Acetic acid Ethyl acetate tert-Butyl alcohol

92.4 75.4 50.0 34.0 73.2 84.2

0.210 0.023 0.031 0.047 0.080 0.186

1.89 0.22 0.29 0.44 0.75 1.69

a polymerization conditions: Polymerization was conducted using 50 gm of vinyl acetate, 30 ml of diluent, and 0.05 gm of dibenzoyl peroxides. The solution was heated for 22 hr at reflux.

3-2.

Polymerization of Vinyl Acetate in tert-Butyl Alcohol [96]

In reflux equipment similar to that described in Procedure 3-1, but scaled down to a 500-ml flask, 50 gm of freshly distilled vinyl acetate is dissolved in 30 ml of tert-butyl alcohol. To this solution is added 0.05 gm of dibenzoyl peroxide. The mixture is heated under nitrogen on a steam bath for 22 hr. The warm viscous solution is diluted with 200 ml of acetone. The solution is transferred to a 3-liter flask containing l-liter of hot, distilled water to precipitate the polymer. The equipment is set up for steam distillation, steam is passed through the suspension to distill out unreacted vinyl acetate and tert-butyl alcohol. After the steam distillation is completed, the polymer is filtered off and dried to constant weight at 80~ and 8 mm Hg pressure (yield, 42.1 gm or 84.2%; specific viscosity, 0.186; inherent viscosity in benzene, 1.69 dl/gm). By using greater levels of tert-butyl alcohol, polymers with somewhat lower molecular weights are produced. As indicated in Table XV, the chain-transfer activity of this solvent is substantially less than that of many other common solvents. This solvent has also been suggested for continuous polymerization or copolymerization processes [97]. Benzene, in small quantities, reduces the rate of polymerization of vinyl acetate. In effect it acts as an inhibitor for this polymerization [86]. Actually, the function of benzene in the polymerization of vinyl acetate is quite complex and has been discussed above. Benzene may form a complex with the monomer, it may act as a chain-transfer agent, or it may actually copolymerize [cf. 37-43]. A solution of 70% vinyl acetate in benzene with 0.1% dibenzoyl peroxide produces a polymer with a degree of polymerization of 850 [99]. The procedure for the polymerization in benzene given by Sorensen and Campbell [102] is

236


completed in approximately 2 hr and yields a polymer with an inherent viscosity greater than 1 (concentration in chloroform: 0.5% at 25~ One of the advantages of the use of benzene as a reaction solvent is the ease of its separation from the polymer by steam distillation. This procedure also removes unreacted monomer, of course. In a suitable solvent, solution polymerization involving the delayed addition of the monomer may be carried out. In this procedure, all of the solvent and 10-25% of the monomer are heated to reflux in the presence of an initiator. As the polymerization proceeds, additional monomer is gradually added. To complete the polymerization, the reaction solution is heated for a prolonged period after the monomer addition has been completed. Even so, the residual monomer content may be as high as 2% [95]. The determinination of the rate of polymerization of vinyl acetate in solution has been carried out in a mercury recording dilatometer. In this procedure, the solution of the monomer and 2,2'-azobis(2,4-dimethylvaleronitrile) is placed in the dilatometer, degassed, and sealed at a pressure of approximately 1 0 - 4 m m Hg. The dilatometer is maintained at 50.0 ___0.02~ The total shrinkage is calculated using 0.892 gm/ml as the density of the monomer and 1.166 gm/ml as that of the polymer. The rate of polymerization is usually determined from the rate of shrinkage in the conversion range of 5-7% [62]. A continuous solution-polymerization process using ethyl acetate as solvent and a range of propionaldehyde as chain-transfer agent has been described. In this process, a solution of dibenzoyl peroxide in ethyl acetate is gradually added to a reactor along with the solution of propionaldehyde in vinyl acetate at 80~ The ratio of the monomer solution to the initiator solution is given as 7 : 3. The solution leaving the reactor contains 5-8% of residual monomer. Its level is reduced by reacting the product solution for several hours in a separate reactor at

80oc [52]. In the polymerization of vinyl esters of perfluorinated acids, ordinary organic solvents are not suitable because of the low solubility, particularly of the higher esters. Therefore, fluorinated solvents have been suggested. Methyl perfluorobutyrate is considered satisfactory. While benzotrifluoride may be used, the polymers produced in this solvent are low molecular weight [91 ]. The polymerization of vinyl formate is of considerable interest since its polymer is particularly readily hydrolyzed to poly(vinyl alcohol). For example, according to one patent, a solution of 70% vinyl formate in methyl formate containing 0.2% of 2.2'-azobisisobutyronitrile was polymerized in a sealed tube under nitrogen for 10 hr at 30~ The reaction product was hydrolyzed to give a poly(vinyl alcohol) with a degree of polymerization of 2110 [107]. In a 1955 patent, a polymerization procedure is discussed which may be considered a transition stage between solution and suspension polymerizations, i.e., the processes which will be discussed in Section 4. The polymerization is

3.


237

carried out in a water and ketone solution. The product, because of its low molecular weight and the presence of a solvent separates as an oil rather than as beads usually associated with suspension polymers. In this procedure, the molecular weight of the polymer is controlled by the use of the ketone. Generally ketones are not considered as effective as chaintransfer agents for the polymerization of vinyl acetate as aldehydes of approximately the same molecular weight. However, the products are thought to be heat resistant and odor free. In the example cited from the patent, the product is said to be useful as a chewing gum base. Considering the fact that methyl ethyl ketone is used in the procedure, current views on toxicity raise serious questions about the suitability of this product for use in chewing gums [ 108].

3-3. Formation of Low-Molecular-Weight Poly(vinyl acetate) (Gradual Addition Procedure) [108] In a 2-liter reaction kettle equipped with a mechanical stirrer, addition funnel, reflux condenser, and a thermometer are placed 100 gm of vinyl acetate, 200 gm of methyl ethyl ketone, 20 gm of water, 0.50 gm of sodium bicarbonate, and 0.62 gm of 30% hydrogen peroxide (CAUTION: Strong oxidizing agent, must be handled by personnel using protective gloves, face shield, and other appropriate safety clothing). The mixture is heated at the reflux temperature with agitation. After the polymerization has been initiated and requires no external heating, 300 gm of vinyl acetate is added gradually over a 3-hr period while moderate refluxing is maintained. After the addition has been completed, heating is continued at the reflux temperature for an additional 4 hr. At this point approximately 75% of the monomer has been converted. The polymer mixture may be separated and dried at reduced pressure for 5 hr at 60~ Alternatively, the reaction temperature may be raised gradually while more water is added to the reaction mixture. During this process, the polymer separates as a soft semisolid from the aqueous system. The excess monomer and the methyl ethyl ketone is then separated by steam distillation. The water is separated from the polymer by decantation. The product is then dried under reduced pressure at 60~ A molar solution of the product in benzene has a viscosity of 1.5 cp at 20~ In this process, with increasing levels of hydrogen peroxide, the molecular weight of the product decreases. According to the patent, at least 0.01 mole of sodium bicarbonate per mole of hydrogen peroxide is required for the process to be operative. In organic solutions, polyethylene oxides of molecular weight less than 1000 or copolymers of PEO may act as molecular weight regulators of the polymerization of vinyl acetate. For 100 gm of the monomer, from 0.5 to 20 gm of the

238


molecular weight regulator have been used along with conventional initiators [109]. Trialkyl phosphites have been used in the presence of dibenzoyl peroxide (BPO) for the radical polymerization of vinyl acetate. In the presence of tris(2,2,2-trifluoroethyl)phosphite, there appears to be no significant change in molecular weight and polydispersity with increasing conversion. Lower molecular weights are formed with higher concentrations of BPO, regardless of the ratio of initiator to phosphite. There is an effect of the structure of the alkyl groups of the phosphite. With increased size, the rate is reduced. Yet the fastest rate was observed with the trifluoroethyl substituent. Pentavalent phosphates lead to increased rates of polymerization, greater polydispersity, and molecular weight results similar to those obtained with BPO alone. With initial conditions of a ratio of [P(trifluoroethoxy)]3 to BPO of 1.5 to 1; a concentration of BPO of 0.025 mol/liter of benzene; and 7 moles/liter of vinyl acetate in benzene, a polymerization was carried out at 60~ for 6 hr. At a conversion of 86%, the number average molecular weight of the polymer was 63,000, and the polydispersity was 1.39 [110]. Control of the molecular weight of poly(vinyl acetate) has also been accomplished in one recent example by an extension of what appears to be a "redox" polymerization in nonaqueous solutions. In this work, the reducing component is an organochromium(II) complex, the oxidizing agent is dibenzoyl peroxide (BPO), and the solvent is tetrahydrofuran (THF). Specifically, 1 liter of a THF solution containing 5 mole of vinyl acetate, 0.25 mole of BPO, and a composition of 0.25 mole of Cr 2+ ions and 0.25 mole of 1,4,7,10,13,16-hexaazacyclooctadecane trisulfate was maintained for 30 hr at 20~ After this period, the conversion to polymer was up to 90%. There was evidence that the concentration of the active species was constant throughout the experiment. The number average molecular weight of the isolated product was reported to be 15,000. In general, with increasing conversion, the molecular weight increased from approximately 5,000 at 18% conversion to 15,000 at 90% conversion. The polydispersity ranged from 1.54 to 1.63 [111 ]. This procedure is of considerable theoretical interest. The use of heavy metal ions, however, will present problems. Chromium ions are thought to be carcinogenic. The removal of such species to below the one part per billion level from the final product will be problematical. Consequently, a polymer produced with a heavy metal ion as part of the initiator system probably will have difficulties as a commercially viable product. A variety of copolymers of vinyl esters have been prepared in solution. For example, vinyl 2-ethylhexanoate and 2-ethylhexyl acrylate have been copolymerized in feed ratios ranging from 1 to 1 to 1 to 3 (on a molar basis). The acrylic monomer was said to have a greater reactivity than the vinyl ester. The product was studied as a possible viscosity index improver [112].

4. Suspension Polymerization

239

The reactivity ratios for the copolymerization of vinyl acetate and behenyl acrylate were determined by polymerization with BPO in toluene solution at 70~ They were found to be 0.021 for vinyl acetate and 1.76 for behenyl acrylate [113]. The copolymerization of vinyl acetate (ml) and phenyl acrylate (m2) led to a copolymer with a greater tendency to forming an alternating product than a copolymer of vinyl acetate and vinyl benzoate. The reactivity ratio for vinyl acetate was 0.22, that of phenyl acrylate was 2.48 [ 114].

4.

SUSPENSION POLYMERIZATION

In general, in previous discussions of free-radical polymerizations, we have attempted to draw a sharp distinction between suspension- and emulsionpolymerization processes. This distinction is quite readily apparent in the case of monomers which are quite insoluble in water, such as styrene. In that case, by use of monomer-soluble initiators and a variety of suspending agents, the suspension-polymerization process leads to the formation of spherical particles which can be separated by filtration. When water-soluble initiators and surface-active agents are used, relatively stable latices are formed from which the polymer cannot be separated by filtration. In the case of vinyl acetate, the distinctions are more blurred. Our description of Procedure 3-3 above represents a transitional situation between a solution and a suspension process since the product separated from the reaction medium. Between the true suspension and the true emulsion polymerization, we find, according to Bartl [4], the processes for formation of reasonably stable dispersion of fine particles of poly(vinyl acetate) using reagents which are normally associated with suspension polymerization. The product is described as "creme-like." The well-known white, poly(vinyl acetate), household adhesives may very well be examples of these creamy dispersions. The true latices are characterized by low viscosities and particles of 0.005-1 ~tm diameter. The creme-like dispersions exhibit higher viscosities and particle diameters of 0.5-15/tm. Probably most industrial homopolymerizations of vinyl acetate are carried out by suspension processes, since the resulting beads are readily converted to poly(vinyl alcohol). This is the major reason for producing the homopolymer. Surprisingly little has been published about the suspension polymerization of vinyl acetate, despite the fact that Lindemann [ 1] cites nearly 1100 references on the subject of vinyl acetate and approximately 200 references on the higher vinyl esters. While there have been many patents issued which deal with peripheral matters such as suspending agents and initiators, much of the published technology has advanced a little beyond that known by the end of World War II. As in the case of poly(vinyl chloride) technology, the chemistry of poly(vinyl

240


acetate) consists of seemingly closely guarded industrial know-how. However, enough information is available to develop useful laboratory procedures. As has been discussed in other cases of free-radical suspension polymerizations, the process may be considered as consisting of bulk polymerizations of small droplets cooled by the surrounding aqueous media. This similarity of the kinetics of suspension and bulk polymerizations of vinyl acetate has been demonstrated [115]. The fact that vinyl acetate is moderately soluble in water may be a cause for the tendency for coalescence of the suspended particles unless care is taken in the proper selection of initiator concentration, suspending agents, and stirring rate. Table XVI indicates that at a low stirring rate, particles tend to coalesce. The reactions tabulated were carried out in air. Under nitrogen a somewhat faster rate and polymers with reduced branching were observed. In Table XVI, we have included data from Wagner [61] to show that the suspension process may be carried out for prolonged reaction periods, at a low rate, and still produce products of high molecular weight. From the preparative standpoint several points should be considered. The need for an inert atmosphere over the reaction system appears to be secondary. Sakurada et al. [ 115] indicate that the polymerization is somewhat more rapid under nitrogen than under air. The difference in the molecular weight of poly(vinyl acetate) produced under these two conditions is not great. However, when the polymers are hydrolyzed to poly(vinyl alcohol), the polymer produced under nitrogen gives rise to a substantially higher degree of polymerization of the poly(vinyl alcohol) than the polymer formed in air. This seems to indicate a

TABLE XVI Effectof the Stirring Rate on the Suspension Polymerization of Vinyl Acetate [ 115]a Initiator concentration (% on the monomer)

Stirring rate (rpm)

0.1 0.3 1.0 0.3 0.4 (25~ 50~ b (water to monomer ratio not given)

500 900 500 350 not given

Polymerization State time Conversion of Degree of (hr) (%) particles polymerization 4.0 2.5 4.0 1.5 22

85 82 92 67 not given

good good good poor presumed good

4650 1650 6100C

a Polymerization conditions: The water to monomer ratio was 2 to 1 by volume. Polymerization was conducted at 65~ using 1 gm poly(vinyl alcohol) per 100 ml water with dibenzoyl peroxide as the initiator. Considering the high conversion within relatively short reaction times, we suspect that the data refers to the use of 2,2'-azobisisobutyronitrile at 65~ rather than dibenzoyl peroxide. bFor comparison. c Data from Wagner [61].


241

substantial difference in the degree and nature of the branches in the poly(vinyl acetate). The polymerization of vinyl acetate is best carried out in a pH range in which the hydrolysis of the monomer is minimized. Since this hydrolysis leads to the formation of acetaldehyde, a notorious chain-transfer agent, careful control of the pH is important, albeit the available literature rarely considers this. The pH range of 4-5 is considered optimum for minimizing vinyl ester hydrolysis. Formic acid, at a level of 0.15-0.25% of the monomer has been suggested for this purpose [4]. In connection with this it should be kept in mind that formic acid does have an aldehydic structure and may, therefore act as a chain-transfer agent. It is also a potent reducing agent and may create a redox system with BPO or other oxidizing initiators. In this connection, two points should be investigated further. 1. To what extent are the modifications of polymerization characteristics the result of formic acid acting as the reducing agent in a redox system which is being compared with a simple case of thermal initiation? 2. Since formic acid is toxic, can this material be removed sufficiently well from the final product to permit use of poly(vinyl acetate) or poly(vinyl alcohol) in situations where toxicity factors come into play (e.g., adhesives for food packaging, household adhesives)? Factors which influence the particle size formed during suspension polymerization are the ratio of monomer to water, the viscosity of the aqueous medium as modified by suspending agents or protective colloids, the rate of agitation, the diameter and shape of the reaction vessel, and the relationship of agitation rate to agitator shape and diameter, to mention only a few [ 116]. The reproducibility of suspension polymers is thought to be improved if, instead of allowing the monomer reflux to drop back to the surface of the reaction medium, the reflux is returned to the bottom of the reactor. The suspension polymer has a tendency to be soft and tacky. Several techniques have been proposed to overcome this problem. These include coating the polymer beads prior to filtration or hardening the beads by cooling the reaction mixture to below 10~ prior to filtration. The polymer isolated may contain substantial quantities of water and must, therefore, be carefully dried [95]. Typical suspending agents for the vinyl acetate polymerization are poly(vinyl alcohol) [particularly a grade represented as approximately 88% hydrolyzed poly(vinyl acetate)], gum arabic, hydroxyethyl cellulose, methyl cellulose, starches, sodium polyacrylate or sodium polymethacrylate, gelatin, and an equimolar copolymer of styrene and maleic anhydride neutralized with either sodium hydroxide or aqueous ammonia. Water-insoluble dispersing agents or

242


high concentrations of electrolyte to reduce water solubility are not usually used in the suspension polymerization of vinyl acetate. The usual initiators are monomer-soluble ones such as dibenzoyl peroxide, lauroyl peroxide, and di-o-toluyl peroxide. Hydrogen peroxide and a few other water-soluble initiators usually associated with emulsion polymerizations have also been used. The molecular weight of the polymer may be controlled by variations in the concentration of the initiator. This effect is illustrated in Procedure 4-1. It is interesting to note that this procedure which goes back to FIAT Final Report 1102, i.e., a report compiled toward the end of World War II [4], was still used in 1974 according to Bravar et al. [117]. We have adapted these procedures to a laboratory scale.

4-1.

Suspension Polymerization of Vinyl Acetate (Control of Molecular Weight by Variation in Initiator Level) ( B a s e d o n B a r t l [4] a n d B r a v a r et al. [117])

a. Preparation of Low-Molecular-Weight Poly(vinyl acetate) In a 3-liter reaction kettle fitted with a mechanical stirrer, reflux condenser, thermometer, and an addition funnel, 800 ml of distilled water and 0.8 gm of the sodium salt of an equimolar copolymer of styrene and maleic anhydride (German trade name Styromal) are heated to 80~ with agitation. Meanwhile a solution of 600 gm of vinyl acetate, 5.4 gm of dibenzoyl peroxide, and 3 gm of ethyl acetate is prepared. To the warm aqueous suspending agent is added 100 gm of the vinyl acetate solution. The stirred mixture is brought up to 80~ by external heating. Once the polymerization has started, heating and cooling is applied as required while the remainder of the monomer solution is added over a 5-hr period. After the addition has been completed, heating is continued for an additional 3-hr period. The residual monomer is removed by steam distillation with agitation. The aqueous dispersion is cooled with agitation to 4~ The polymer beads are filtered off or centrifuged and washed repeatedly with water at 5~ to remove the suspending agent. The polymer is then dried under reduced pressure at 30~ The dry product is glass clear. The product is reported to have MW 110,000.

b. Preparation of an Intermediate-Molecular-Weight Poly(vinyl acetate) For this preparation, the procedure used is the same as that given in Section a except that the reactants used are 800 ml of distilled water and 0.8 gm of Styromal (sodium salt) to which is added by the described gradual addition

4. SuspensionPolymerization

243

technique a solution of 600 gm of vinyl acetate and 1.2 gm of dibenzoyl peroxide. The final product has MW on the order of 1,000,000.

c. Preparation of a High-Molecular-Weight Poly(vinyl acetate) For this preparation, the procedure used is the same as that given in Section a except that the reactants used are 800 ml of distilled water and 1.2 gm of Styromal (sodium salt) to which is added by the described gradual addition technique a solution of 600 gm of vinyl acetate and 0.18 gm of di-o-toluyl peroxide. The final product has MW on the order of 1,500,000. A patented procedure for the suspension polymerization of vinyl acetate which is claimed to produce a nonsticky bead polymer [118] uses an aqueous phase consisting of 537 gm of distilled water, 0.25 gm gum tragacanth, and 0.10 gm sodium dioctylsulfosuccinate (Aerosol OT). The monomer charged consists of 690 gm of vinyl acetate and 0.69 gm of dibenzoyl peroxide. In a complex apparatus, Gunesch and Schneider [30, 119] studied the suspension polymerization of vinyl acetate. Their procedure involved equipment which automatically added tempered water to the reacting system as heat was evolved as a result of the polymerization process. Thus they maintained isothermal reaction conditions. The rate of reaction could be followed by recording the water uptake of the equipment with time. The heat of polymerization was also determined (found to be 23 kcal/mole which was considered a satisfactory check of the literature value which is scattered around 21.4 kcal/mole). From this work, a somewhat different mechanism of the suspension polymerization process emerges than the widely accepted concept of the "water-cooled bulk polymerization of small particles." It was noted that with an increase in the initiator concentration, there was the expected increase in polymerization rate. With increasing stirring rate, the rate of polymerization decreased. Along with the suspension polymerization, there was always a certain amount of undesirable emulsion polymerization. It was postulated that in the process, free radicals, formed in a monomer drop may be extracted into the aqueous phase where they may act on dissolved vinyl acetate by kinetic processes Unique to this system and different from the conventional mechanism of suspension polymerization. Sodium thiosulfate interfered with the polymerization in the aqueous phase. This manifested itself in a decrease in the overall rate of polymerization (cf. Table XVII). In this work, substantial additions of sodium chloride were made to the aqueous phase to reduce further the unwanted emulsion polymerization. The suspending agent used in these experiments was a partially saponified poly(vinyl acetate) whose trade designation is Rhodoviol HS 100. This poly(vinyl alcohol) was normally added to the reaction system as a 6% aqueous

244


solution. Table XVII primarily shows the effect of the rate of agitation on the bead size and the yield of polymer. The amount of water added during the polymerization was 1430 ml of 20~ water. Unfortunately the report did not define very precisely the quantities of initiators used. The polymerization of vinyl pivalate by the suspension procedure gives a product of very high molecular weight. The molecular weight seems to be independent of the extent of the conversion. This has been attributed to the fact that the pivalate moiety has no a-hydrogens and hence cannot act as a chaintransfer agent in a manner analogous to an acetate. It is interesting to note that the suspension polymer of vinyl pivalate has a higher molecular weight than its emulsion polymer [116]. Procedure 4-2 details the suspension process.

4-2.

Suspension Polymerization of Vinyl Pivalate [116]

In a 2-liter reaction kettle equipped with a mechanical stirrer, thermometer, nitrogen inlet and outlet tubes, reflux condenser, and an addition funnel, under nitrogen a solution of 500 ml of distilled water containing 7.5 gm of Mowiol 70/88 [a grade of poly(vinyl alcohol), probably consisting of 88% hydrolyzed TABLE XVlI The Effect of the Agitation Rate on Yield and Bead Size in the Suspension Polymerization of Vinyl Acetate [30] a Rate of agitation (rprn/min)

Yield (%)

Average diameter of beads (mm)

Lauroyl peroxide (only)

120 240 480

98 95 93

0.9 0.5 0.2

Lauroyl peroxide with sodium thiosulfate

120

89

0.8

240 480

85 76

0.4 0.2

120

77

0.6

240 480

61 58

0.2 0.1

Initiator

Dibenzoyl peroxide with sodium thiosulfate

a Polymerization conditions: Polymerization was conducted using 250 ml vinyl acetate (233 gm, carefully purified), 500 ml distilled water containing 50 gm sodium thiosulfate (when indicated), and 6 ml of 6% solution of poly(vinyl alcohol) (Rhodoviol HS 100). To control the reaction temperature at 63.5~ under nitrogen, water was gradually added. The water temperature was 20~ During the preparation a total of 1430 ml of water was added. The initiator used was lauroyl peroxide or AIBN. On 250 ml of vinyl acetate, 0.5-3 gm of initiator was used.


245

poly(vinyl acetate) of high molecular weight] is stirred at 60~ while a solution of 0.118 gm of dibenzoyl peroxide in 250 gm of vinyl pivalate is added all at once. The reaction mixture is heated for 5.5 hr. Then the reaction temperature is raised to 80~ and maintained at that temperature for 22 hr. The reaction mixture is then cooled to 4~ The polymer is filtered off and washed repeatedly with water at 5~ The product is dried under reduced pressure at 30~ (yield 236 gm or 94.3%; MW, 3.58 • 106). It should be noted that if the reaction temperature is raised to 80~ earlier than indicated in this procedure, the degree of polymerization will be reduced. To return to the matter of increased molecular weights of suspension polymers, it seems that the incorporation of a small amount of hydrogen peroxide along with an organic peroxide in a suspension-polymerization procedure increases the molecular weight of the polymer substantially. It also reduces the induction period [ 120]. It is interesting to speculate whether the small amount of hydrogen peroxide used is sufficient to initiate polymerization of a seed polymer in the aqueous phase about which the suspension polymer forms. Procedure 4-3 illustrates the use of hydrogen peroxide to form a relatively high-molecularweight polymer.

4-3.

Suspension Polymerization of Vinyl Acetate in the Presence of Hydrogen Peroxide [120]

In equipment similar to that indicated in Procedure 4-2, into 500 ml of a 2% solution of poly(vinyl alcohol) (viscosity and degree of hydrolysis not specified in the original patent) is added a solution of 500 gm of vinyl acetate containing 0.25 gm of dibenzoyl peroxide along with 5 ml of a 1.0% aqueous solution of hydrogen peroxide. With agitation, the mixture is heated for 6 hr at 70~ The reaction mixture is then cooled to 4~ filtered, and washed repeatedly with water at 5~ The polymer is dried under reduced pressure at moderate temperatures (yield, 85% of theory). The viscosity of a 1 "molar" solution of this polymer in benzene is reported to be 150 cP. When the polymerization is carried out without hydrogen peroxide, the viscosity of a solution of comparable concentration is only 92 cP. The technique described above may be modified by use of gradual addition of the monomer. Methyl cellulose may be substituted for poly(vinyl alcohol). A novel method of producing suspension polymers of poly(vinyl acetate) with a closely controlled molecular weight distribution involves the solution polymerization of vinyl acetate in methanol. By varying the ratio of monomer to methanol, a variety of molecular weight distributions may be prepared. The solution polymer is then added with agitation to an aqueous system containing poly(vinyl alcohol). The methanol is then distilled off to give a bead polymer [121].

246


One interesting aspect of this method of producing a suspension polymer is the fact that this method produces beads of poly(vinyl acetate) which may be loosely coated with poly(vinyl alcohol). This coating may be removed much more readily than the poly(vinyl alcohol) used in a conventional polymerization procedure. In the conventional process, a fair amount of the suspending agent probably forms a graft copolymer with vinyl acetate and is, therefore, permanently bound to the polymer bead. The properties of the two types of bead polymers are expected to be somewhat different since the graft copolymer with poly(vinyl alcohol) is present only in one and not the other polymer. When substantial quantities of a suspending agent are used in a typical suspension polymerization (i.e., 2-5% by weight of suspending agent based on the weight of the monomer) instead of beads which settle out of the aqueous medium, a stable dispersion of creme-like consistency is formed. The product consists of particles with average diameters in the range of 0.5-15 pm, which are somewhat larger than the diameters associated with latex particles (0.005 to 1/zm) dispersed in the aqueous medium. Poly(vinyl alcohol) and the partially hydrolyzed poly(vinyl acetates) are particularly suitable suspensing agents for these creme-like dispersions. Just as in the case of latex polymers of vinyl acetate, these dispersions tend to exhibit better freeze-thaw stability when partially hydrolyzed poly(vinyl acetate) is used rather than the fully hydrolyzed poly(vinyl acetate), i.e., 100% poly(vinyl alcohol). When ammonium persulfate is used as an initiator, the rate of polymerization is said to be increased if a mild alkali such as sodium carbonate is added. In fact, the high pH thus achieved may have an additional effect, since in the absence of the buffer, substantial coagulation takes place. The addition of a neutral salt such as sodium chloride (at a level of 0.05-1% of the reaction charge) is said to increase the molecular weight to very high levels [122]. Procedure 4-4 is an example of the preparation of a stable dispersion by this procedure.

4-4.

Preparation of a Stable Poly(vinyl acetate)

Dispersion [122] In a l-liter reaction kettle fitted with two addition funnels, reflux condenser, mechanical stirrer, and a thermometer, 14 gm of a 87% hydrolyzed poly(vinyl acetate) and 1.5 gm of sodium carbonate are dispersed in 190 ml of water. The aqueous phase is heated to 68~ with agitation. Over a 5.5-hr period, with agitation, 274 gm of vinyl acetate and a solution of 1 gm of potassium persulfate in 20 ml of water are added simultaneously from separate addition funnels. The rates of addition are controlled so that the ratio of monomer to initiator is maintained constant in the reaction mixture. During the addition stage, the reaction temperature is permitted to rise to reflux at 80~ After the addition has been completed, the reaction temperature is raised over a 1-hr period to 90~

247


Then, if necessary, the residual monomer may be stripped out by steam distillation. The resulting product is a smooth, creamy dispersion with particle diameters between 2 and 6 r The nonvolatile content of the dispersion is approximately 58%. Ordinarily the use of hydrogen peroxide as the only initiator is not satisfactory for the suspension polymerization of vinyl acetate. Incomplete conversion of the monomer is usual. By introducing additional hydrogen peroxide after a substantial amount of polymer has formed, this problem may be overcome. A typical reaction charge would be Reagent

Amount (gm)

Water Gum arabic Sodium dioctylsulfosuccinate Ferric chloride hexahydrate Vinyl acetate Hydrogen peroxide (as a 0.3% aqueous dispersion)

440 25 1.50 0.02 530 0.20

In the preparation, 10% of the monomer and 25% of the hydrogen peroxide are heated in the aqueous phase containing the gum arabic, sodium dioctylsulfosuccinate, and ferric chloride hexahydrate at 75~176 with agitation. The remaining monomer and the hydrogen peroxide are added gradually over a 2- to 3-hour period [ 123]. The control of particle size from 0.5 to 15/zm is possible by adding varying amounts of a protective colloid such as hydroxyethyl cellulose, an alkali salt of a maleate half ester, sodium phenyl phenolate, and/or a phosphate buffer at pH 6.9 [124]. It should be noted again that in the procedure attributed to Wilson [ 123], as in many other suspension polymerization procedures mentioned above and in many procedures for emulsion polymerizations to be described later, reaction temperatures are given which are above the boiling point of the monomer (72.7~ at 760 mm Hg), not to mention, above the boiling point of the vinyl acetate-water azeotrope (66~ (composition, 92.7% vinyl acetate, 7.3% water, cf. Table I). For reactions carried out in sealed ampoules or closed bottles, this reaction temperature is readily explained. How such reaction temperatures are reached in a reflux apparatus open to the atmosphere is in question. It is hardly likely that the rate of polymerization is so rapid that no free monomer exists when it is added with conventional initiators to hot water. We presume that most of the polymerizations reported to proceed at about 66~ in an aqueous medium are simply run at reflux. At such a temperature, initiation by dibenzoyl peroxide is rather slow. If the suspension polymerization is to be forced at higher temperatures, provisions will have to be made to force the monomer into the

248


reaction system. Simply to depend on condensing vinyl acetate dripping from a condenser to the surface of the water is not a very satisfactory procedure. It is best to introduce the refluxing monomer well under the surface of the reaction system or to operate in an appropriate autoclave. Mixtures of methyl acrylate and vinyl acetate, in various proportions, have been grafted to starch. To a stirred dispersion of 20 gm of starch, under nitrogen, was added 30 grn of the monomer composition. After 5 min of stirring, a solution of 0.676 gm of ceric ammonium nitrate in 6 ml on 0.1 N nitric acid was added. The mixture was stirred at room temperature for 3 hr. The crude product was filtered off, resuspended in water. The aqueous supematent layer was neutralized with 0.1 N sodium hydroxide. The graft copolymer was again separated by filtration, washed with methanol, and then air dried. To separate homopolymers from this product, 5 gm of this polymer was dispersed in 100 ml of acetone containing 2% of water. The solid graft copolymer was isolated by centrifugation and dried under reduced pressure. The product could be extruded into ribbons [125]. 5.

EMULSION

POLYMERIZATION

From the industrial standpoint, the suspension polymerization of vinyl acetate is of primary interest for the production of poly(vinyl acetate) homopolymer beads. Most of these beads are converted to poly(vinyl alcohol) with a variety of degrees of hydrolysis and in a number of different molecular weight ranges. On a laboratory scale, suspension copolymerizations of vinyl acetate with other monomers may have an advantage in terms of ease of handling and ease of developing a suitable suspension medium. Industrially emulsion polymerizations, both homo- and copolymerizations, are of great importance, particularly in the development of adhesives, paints, paper coatings, and textile finishes. The production of vinyl acetate monomer by the current top ~four U.S. producers (H6chst-Celanese, Union Carbide, DuPont, and Quantum Chemical) in 1989 was about 1.25 x 109 kg [8], compared to 1.14 x 109 kg reported for 1980 [7] and 0.89 x 109 kg for 1977 [126]. Since there are major producers of poly(vinyl acetate) not only in the U.S. but also in Japan, the United Kingdom, Germany, Switzerland, Canada, France, and Italy, one may assume that the total annual production of the monomer is well above the figure given here for the U.S. production. Although more than one third of all the monomer is used to produce latices in the form of paints and adhesives, published information on the emulsion polymerization of vinyl acetate is limited. References [1, 4, 7, 8] are general references. Bacon [127] reviews the redox initiation of the polymerization. Shapiro [128] deals with the applications of vinyl acetate to the paper industry.

5. Emulsion Polymerization

249

The emulsion polymerization of vinyl acetate may be unique among polymerization processes in that true latices have been formed with anionic surfactants, cationic surfactants, nonionic surfactants, or protective colloids, and with combinations of two or more such reagents, as well as without any added emulsifier. According to O'Donnell et al. [130], the emulsion polymerization of vinyl acetate follows the Smith-Ewart theory of emulsion polymerization [131] because the rate of polymerization is independent of the total amount of monomer present, the rate is a function of the 0.6th power of the emulsifer concentration, and the rate of emulsion polymerization is a function of the 0.7th power of the initiator concentration instead of the expected 0.4th power. In this work poly(vinyl alcohol), 88% hydrolyzed with a medium molecular weight (i.e., Du Pont's Elvanol 52-22), was used as the only externally added emulsifier. Light-scattering studies indicated that this emulsifier formed no aggregates in the aqueous solution. These latter observations may, however, have been made at room temperature and not at the reaction temperature [ 1]. The conversion versus time curve was essentially linear up to 80% conversion. Since the solubility of vinyl acetate is 2.1% at 50~ and 3.5% at 70~ [15], deviations from the Smith-Ewart treatment are not entirely surprising. The water solubility of vinyl acetate was one of the significant factors in the deviation from the conventional theory of emulsion polymerization. Another factor is the reactivity of the vinyl acetate radicals toward other materials present in the system such as the surfactants. Okamura and Motoyama [132] showed that the emulsion polymerization of vinyl caproate, a monomer of low water-solubility, followed the same pattern as styrene did as far as the Smith-Ewart theory is concerned. During the emulsion polymerization of vinyl acetate, unlike the case of the styrene polymerization, emulsion particles form up to a conversion of 80%. The reaction also appears to be dependent on the rate of agitation; the more vigorous the stirring, the slower the rate. It is also reported that the effect of the initiator concentration (i.e., the concentration of a persulfate) is complicated by the formation of free sulfuric acid during the reaction. This leads to the hydrolysis of some of the monomer to acetaldehyde which, aside from its chain-transfer activity, also retards the rate [132]. These observations again point up the importance of pH control during the polymerization of vinyl acetatewa matter already mentioned in connection with suspension polymerizations. As a matter of fact, the rate of emulsion polymerization of vinyl acetate is said to be at a maximum at a pH of 7 [ 133]. This is higher than the range of pH 4-5 which is the one most desirable from the standpoint of minimal hydrolysis of vinyl esters [4]. An additional factor which may not have been considered adequately in the theoretical treatment of the emulsion polymerization of vinyl acetate arises from the work of Dunn and Taylor [134]. These researchers noted that in their

250


dilatometric study of the emulsion polymerizations, the contraction of the monomer in an aqueous system was only 15.7 ___0.4%, whereas the bulk polymerization contraction was reported to be 26-28% [135]. The difference was attributed to the solvation of the monomer in the aqueous medium. If this is indeed so, it is not inconceivable that under the conditions of both suspension and emulsion processes, not the monomer vinyl acetate, but new monomers, of variable composition and conceivably of distinctly different chemical properties, are involved; vinyl acetate hydrates. Among the observable facts it was found that there is no significant effect of the concentration of emulsifier on this system. Therefore, the implication is that the polymerization initially takes place exclusively in the aqueous phase [ 136]. The resulting polymer particle "precipitates" as it forms [ 134]. In this case we may assume, that only a microscopic phase-separation takes place. The polymer particles which form adsorb emulsifier from the aqueous environment and remain dispersed. Then the particles may absorb more monomer somewhat in the manner called for by the Smith-Ewart theory. Of course, other dissolved vinyl acetate monomer molecules may continue to be polymerized in aqueous solution, thus accounting for the increase in the number of particles as the polymerization proceeds to high conversion. The classical Smith-Ewart treatment states that the number of particles is determined by the surfactant to monomer ratio and, in effect remains constant throughout the process. When neutral salts are added to an emulsion-polymerization system for vinyl acetate, the monomer, in effect, is salted out. The process then takes place at a much higher rate in the monomer-polymer particles than before. This is generally expected for ordinary emulsion polymerizations [ 136]. When reviewing the published literature on the emulsion polymerization of vinyl acetate, one is struck with seemingly contradictory data presented by many reputable research teams. Some of these results published may not be strictly comparable because of variations in the polymerization recipes used. For example, the effect of the emulsifiers on the rate of polymerization may have a profound effect on the course of the reaction. In a persulfate-initiated system using no other surfactant, it has been postulated that the free radicals formed from the decomposition of the initiator combine with the monomer in solution. As polymer forms, aggregates develop which absorb more monomer and the number of particles increases up to a constant value (at about 5% conversion). Then, while the number of particles remains constant at 1.7 • 1012 per ml, the reaction rate increases. Ultimately, as a last stage of the reaction, the rate begins to drop off. The latex formed in this process is said to consist of particles of great uniformity with a diameter of 0.26/~m [137]. In the case of polymerizations in the presence of seed latices, there is evidence that polymerization of additional monomer occurs exclusively in polymer particles which have been swollen by the available monomer [138].

251


This observation seems to be in line with the Smith-Ewart concepts. The adsorption of surfactants on the surfaces of latex particles influences the capture by the particles of low-molecular-weight polymers formed in the aqueous solution. This in turn affects the reaction kinetics and the formation of new particles. The number of free radicals per particle, which is usually considered to be constant during the major phases of an emulsion polymerization, seems to vary considerably during the polymerization of vinyl acetate [139]. The effect of the ionic strength in the aqueous system on the emulsion polymerization has been investigated in some detail [140, 141]. According to this work, the rate of polymerization of vinyl acetate at low ionic strength is directly proportional to the first power of the initiator concentration and is independent of the surfactant concentration. In seeded polymerization procedures with media of low ionic concentration, the rate of polymerization is given by Rp oc [I]~176

(8)

0"33

where Rp is the rate of emulsion polymerization of a seeded system; [I], the concentration of initiator (potassium persulfate); [N], the concentration of polymer particles; [ V], the monomer volume. In all cases, the rate is reported to be almost independent of the monomer concentration in the particles up to 85-90% conversion [ 140]. With increasing ionic strength, the solubility of the monomer increases. At constant temperature, this is attributed entirely to a decrease in interfacial tension. The temperature effect is on both the monomer-water interaction and on the interfacial tension [ 141 ]. Stannett and co-workers [ 190] postulate that in aqueous media, water soluble polymers of the structure indicated in Eq. (9) are formed. O II (n + 1) C H 2 = C H - - O - - C - - C H 3 / H2C

I C~ // O

CH2 \ CH

I 0

(9)

CH2--CH

J O--C--CH3 II O

n

where 0 < n < 10. In the proposed mechanism, the butyrolactonyl group is formed by chain transfer to the acetyl group of the monomer followed by cyclization [Eq. (10)].

252


O II CHz--CH--O--C--CH2"

CH2 \ ~ HzC CH" I I C~O II O /

(10)

The butyrolactonyl free-radical, then reinitiates polymerization in aqueous solution. In general initiation is postulated to take place in the aqueous phase followed by stabilization due to the adsorption of surfactants on the growing polymer chain. Under the conditions of preparation, when the growing polymer reaches a degree of polymerization between 50 and 200, it is captured by the seed particles. Growth continues in these particles. The main termination step is the reaction of a butyrolactonyl radical with a growing aqueous polymer radical. At low ionic strength, a secondary termination step consists of the capture of an aqueous radical by a particle which already contains a radical. At high ionic strength, this may be the major termination step. It was observed in this work that the rate of polymerization was increased somewhat when potassium sulfate was added to the system. However, the rate was greatly increased when a phosphate buffer was added. This buffer adjusts the pH of the system to 7. Evidently Stannett and co-workers [ 140] overlooked the pH effect noted already by Naidus [ 133] as operating here. Also to be noted is that although the rate increased with increasing ionic strength of the medium, the dependence of the rate on the initiator concentration decreased. At high salt concentrations, the particle diameter increased. The effect of a fourfold increase in the surfactant concentration had no significant affects on the rate of polymerization, nor had a doubling of the ratio of monomer to water [ 140]. The average number of free radicals per emulsion particle has been estimated to be between 0.01 and 0.5. This represents an unusually low concentration of active free radicals. One proposal to explain this phenomenon is that monomeric radicals are rapidly lost by chain-transfer reactions in the polymer particles [142]. For the emulsion polymerization of vinyl acetate without seed polymer under purified nitrogen, using sodium lauryl sulfate as surfactant and potassium persulfate as initiator, at 50~ with agitation at 400 rpm in a resin kettle of 500 ml capacity, the same research group made the following observations [143]. 1. When the initial emulsifier concentration is low, the number of particles is constant regardless of initiator concentration. As the initial emulsifier concentration is increased, the percent conversion of the monomer at which the number of particles becomes constant also rises.

5.

Emulsion Polymerization

253

2. The rate of polymerization is essentially linear between 15 and 80% conversion and increases with emulsifier concentration, there is an abrupt change in the slope near the critical micelle concentration (CMC). 3. The initial monomer concentration has no effect on the number of particles. The reaction order with respect to the monomer concentration is about 0.36. 4. The number of polymer particles is independent of the initiator concentration. The order of the reaction with respect to the initiator concentration is approximately 0.5. It was also concluded that since there is a sharp break in the plot of the number of particles versus the emulsifier concentration and since this discontinuity is near the CMC of sodium lauryl sulfate, it seems reasonable to conclude that the polymer particles are generated from the emulsifier micelles [143]. The following additional observations were also made about the same time [144]. 5. The reaction order with respect to the number of particles is small, between 0.05 and 0.2. 6. The average number of free radicals in each particle is small, usually between 0.001 and 0.01. 7. The molecular weight of the polymer produced is independent of the number of particles, the particle size, and the initiator concentration. With increasing conversion, the molecular weight distribution broadens and the average molecular weight increases. The branching reactions contribute to the broadening of the molecular weight distibution with conversion. At high conversion, the emulsion exhibits a broader range in the molecular weight distribution than that obtained in bulk polymerizations [ 144]. That there is no generally acceptable model for the emulsion polymerization was emphasized by Min and Ray in their extensive discussion of mathematical modeling of emulsion polymerizations [145]. They list five deviations of the emulsion polymerization of vinyl acetate from the Smith-Ewart theory, which may be a bit different from the points made by Nomura and co-workers [ 143] and by Friis and Hamielec [144]. These points are as follows: 1. The rate of polymerization is not linearly related to the number of particles. It shows a 0.14-0.12 power dependence. 2. The rate of polymerization is independent of the emulsifier concentration while the number of particles increases with emulsifier concentration. 3. The rate of polymerization is directly proportional to the monomer concentration dissolved in the aqueous phase.

254


4. The number of particles increases to a maximum at about 10% conversion and then decreases continuously during the period of constant reaction rate. A large number of particles is formed even in the absence of micelles. 5. The conversion versus time curve is sigmoidal in shape [145]. To explain some of these phenomena, it is postulated that radicals may grow in the aqueous phase to a degree of polymerization between 50-200. These oligomers form new particles by precipitation from the aqueous phase or in micelles. Small particles may coalesce readily to form larger particles. There is an appreciable migration, or desorption, of free radicals out of the particles back into the aqueous phase. Termination reactions may take place in the aqueous phase as well as within the polymer particles. The number of end-groups increases on the surface of the polymer particle as the conversion proceeds. These end-groups, in effect, augment the quantity of available emulsifier in the system [145]. Despite the difficulties in developing a universal theory for the emulsion polymerization of vinyl acetates, large quantities of usable latices are being produced throughout the world every day. Considerable quantities of poly(vinyl acetate) emulsions are produced with anionic surfactants along with various colloidal materials which are thought to prevent the coagulation of the latex (hence the name "protective colloid"). The nature of the surfactants and of the protective colloid and their concentrations have profound effects on the properties and applications of a latex. For example, if poly(vinyl alcohol) is used as the sole protective colloid, emulsions of large particle size (2-10/lm) may be formed. Films produced from such latices tend to be hazy. They are usually sufficiently sensitive to water to be redispersible. The latices may be coagulated with electrolytes such as borax. On the other hand, they are highly stable to agitation and mechanical shear, the addition of solvents, and to freeze-thaw cycling. In the presence of surfactant along with poly(vinyl alcohol) finer particlesized latices are formed. The emulsions form clear, glossy films. On addition of borax, such latices may be coagulated. Combinations of synthetic colloids with surfactants may produce fine-particle emulsions stable to borax and other additives such as starches, dextrines, and salt. Many natural gums have been used as protective colloids along with surfactants to produce fine-particle, waterresistant, borax-stable emulsions [128]. The degree of hydrolysis of poly(vinyl alcohol) influences the emulsion polymerization of vinyl acetate. The lower the degree of hydrolysis, the better the emulsifying action of the protective colloid and the faster the rate of polymerization [146]. In connection with this statement we must point out that the terminology "a poly(vinyl alcohol) of low degree of hydrolysis" is widely

5.


255

used but confusing. The term refers to a poly(vinyl acetate) of which only a few acetate moieties have been saponified to alcohol units. All poly(vinyl alcohols) may be considered to be block copolymers of vinyl acetate and the hypothetical vinyl alcohol. A poly(vinyl alcohol) of low degree of hydrolysis is a block copolymer consisting primarily of poly(vinyl acetate) blockswith only a few poly(vinyl alcohol) units. The distribution of the blocks of poly(vinyl acetate) and poly(vinyl alcohol) in a particular poly(vinyl alcohol) may effect emulsion properties. For example, a poly(vinyl alcohol) with a "blocky" intramolecular distribution of residual acetate groups produces latices of greater viscosity and stability toward electrolytes than a more randomly distributed structure [147]. The blocky nature of these products vary considerably with the manufacturer of the polymer. Therefore it is quite important that the description of a grade of poly(vinyl alcohol) used in a process specify not only the degree of hydrolysis of the base poly(vinyl acetate) and the viscosity of a 4% solution in water at 20~ (an indication of the molecular weight), but also the actual manufacturer of the product. By the way, it is interesting to note that the type of poly(vinyl alcohol) most conveniently dissolvable in cold water contains 10-30% residual poly(vinyl acetate). The compatibility of the poly(vinyl acetate) blocks with latex particles may explain the suitability of these grades of poly(vinyl alcohol) in vinyl acetate emulsion polymerizations [ 145]. In their review of the effects of poly(vinyl alcohol) on the polymerization of vinyl acetate. Dunn and co-workers [145] point out that because of the variations in the distribution of vinyl acetate blocks in poly(vinyl alcohol), similar grades from different manufacturers differ in effect on emulsion polymerizations. For example Elvanol (product of Du Pont Co.) has a retarding effect when compared to Gehsenol (product of Nippon Gosei Co.). The difference in structure may also affect the ease with which micelles may form in poly(vinyl alcohol) solutions. Among the effects that poly(vinyl alcohol) may have in emulsion polymerization systems are adsorption on the poly(vinyl acetate) latex, chain-transfer reactions, enhancement of the initiation rate since it increases the rate of decomposition of potassium persulfate, the oxidation of poly(vinyl alcohol) by the initiator, solubilization of monomer and polymer [ 148]. An interesting interaction of poly(vinyl alcohol) on the initiator system was reported by Hayashi and co-workers [149]. Using poly(vinyl alcohol) NO-05 from Nippon Synthetic Chemical Industry Co., Ltd., freed of low-molecularweight polymer and of sodium acetate by dialysis, they found that stable latices could be formed using a hydrogen peroxide-ascorbic acid redox initiator. When this initiator was used in the absence of the protective colloid, the latex coagulated. On the other hand, even without any protective colloid, a very stable poly(vinyl acetate) latex was formed when the initiator consisted of potassium

256


persulfate and ascorbic acid. It is also interesting to note that the rate of polymerization was virtually constant over a six-fold increase in the concentration of poly(vinyl alcohol). In the presence of a poly(vinyl alcohol) emulsifier, the emulsion polymerization of vinyl acetate initiated by potassium persulfate is said to be seriously inhibited by dissolved air. The rate of polymerization is enhanced by the addition of triethanolamine. At a concentration of 5 x 10 -3 moles/liter of potassium persulfate up to 0.03 moles/liter of triethanolamine enhanced the polymerization rate. Beyond this concentration of the amine, the rate decreases but the induction period in air is eliminated [150]. Nonionic surfactants such as polyoxyethylated fatty alcohols (such as Emulphor ON-870 from GAF), alkyl phenyl polyethylene glycol ethers (such as the Tergitols from Union Carbide) and polyoxyethylated octylphenol may be used as protective colloids along with anionic surfactants or, in some cases, as emulsifiers in their own fight. The block copolymers of polyoxyethylene and polyoxypropylene (Pluronics) solubilized vinyl acetate. Polymerization takes place at the interface of the surfactant-monomer droplet and the aqueous phase [151]. In a study of the use of Pluronic F68 in the emulsion polymerization of vinyl acetate, it was found that the particle surface area per unit weight of emulsion was directly proportional to the ratio of the surfactant to the monomer. The viscosity of the latex was also directly proportional to the ratio of the surfactant to the monomer. The number of particles per unit volume, at a constant ratio of the weight of the polymer to the emulsion weight, was directly proportional to the cube of the surfactant concentration [152]. Most commonly, in the emulsion polymerization of vinyl acetate, anionic surfactants are used either alone or in combination with a protective colloid. Typical examples of surfactants which have found application are Aerosol OT (sodium dioctylsulfosuccinate), alkyl aryl sulfonate salts (e.g., Santomerse-3), sodium lauryl sulfate, etc. A study of the kinetics of the vinyl acetate polymerization in the presence of sodium lauryl sulfate indicated that the rate of polymerization was proportional to the square root of the initiator concentration and the 0.25th power of the number of particles. The number of particles were proportional to the 0.5th_ 0.05 power of the surfactant concentration but independent of the level of potassium persulfate. The intrinsic viscosity of the final polymer was said to be independent of the initiator concentration and of the number of polymer particles. These observations were said to suggest that the mechanism of the vinyl acetate polymerization in emulsion resembles that of vinyl chloride [153]. Cationic emulsifiers have been used to prepare positively charged poly(vinyl acetate) emulsions. The method of preparation is said to be similar to that used in conventional anionic latex preparations except that a cationic surfactant is


257

used. Unfortunately details were not published since they were subject to patent applications. It is unfortunate that reputable journals permit the publication of such incomplete data [154]. The rate of polymer formation in the presence of cationic surfactants (cetyltrimethylammonium or dedecyltrimethylammonium bromide) is slower than the rate in the absence of any surfactant, at least until the 20% conversion level. Thereafter it is comparable to the rate found for systems with no surfactants or with non-ionic surfactants. With anionic surfactants, the rate of conversion is fastest. The square root of the percent conversion is linear with time up to approximately 50% conversion [155]. To be noted here is that these observations contradict the observations of Patsiga [ 136], who stated that there was no dependence of the system on the emulsifier concentration. Cationic surfactants, in contrast to anionic surfactants, usually reduce both the number of particles involved in the polymerization and the rate of polymerization. The nature of the stabilizing emulsifier has a marked effect on the polymerization kinetics. For example, addition of a non-ionic stabilizer [e.g., poly(vinyl alcohol), a block copolymer of carbowax 6000 and vinyl acetate, or ethylene oxide-alkyl phenol condensates] to a seed polymer stabilized by an anionic surfactant decreased the rate of polymerization to 25% of the original rate. The effect was as if the nonionic stabilizer (or protective colloid) acted as a barrier around the seed particles to alter the over-all kinetics. It may be that the viscosity of the medium in the neighborhood of the nonionic surfactant coating of the polymer particle is sufficiently different from that of an anionic layer to interfere with the diffusion of monomer or free radicals. There may also be a change in the chain-transfer characteristics of the system [ 156]. In the preparation of emulsion polymers, particularly when copolymer systems are involved, several methods of adding the monomer to the reacting system are available. Obviously, one may add all of the monomer at once to the aqueous phase. Alternatively, the monomers may be added gradually stepwide. Finally, the monomers may be added gradually and continuously. The first two methods lead to more or less heterogeneous copolymers while the continuous addition method affords homogeneous copolymers. The best adhesive properties are achieved from homogeneous copolymer systems; other properties may vary considerably with the degree of heterogeneity of the polymer [157]. There are, of course, procedures which also call for the gradual addition of initiator solutions, additional surfactant, plasticizer, etc. The manipulation of various emulsion polymerization techniques assists in developing poly(vinyl acetate) latices with specific characteristics needed for certain end-uses. Some of these are summarized in Table XVIII. In preparing emulsion polymers, some procedures have been developed which are carried out under an atmosphere of nitrogen while others are carried out in

258


TABLE XVIII Effect of Polymerization Technique Modification on Latex Characteristics Technique modification Increasing the concentration of surfactant and protective colloid Decreasing the concentration of surfactant and protective colloid Delayed addition of surfactant and protective colloid Delayed addition of monomer Increasing the initial monomer charge 15-20% Reducing the rate of delayed monomer addition Use of water-soluble initiators Use of monomer-soluble initiators Increasing initiator concentration Decreasing initiator concentration Delayed addition of initiator Use of a redox initiator Increasing the temperature of polymerization Decreasing the temperature of polymerization Increasing the potential solids contraction of the latex Decreasing or eliminating materials with chain-transfer activity

Effect Latex viscosity increases: particle size decreases Particle size increases Increases particle size Reduces particle size Increases MW Reduces particle size Reduces particle size Increases particle size Reduces particle size: effect on MW is minimal Increases particle size Increases particle size May increase MW Increases latex viscosity: reduces particle size Increases particle size: increases MW Increases latex viscosity Increases MW

air. Atmospheric oxygen does cause some induction of the polymerization. Low levels of triethanolamine are said to reduce this induction period [150]. Presumably many redox systems are not seriously affected by air. In reducing the air in the system, many processes call for bubbling purified nitrogen through the aqueous solutions. Obviously, in the case when anionic surfactants are present, a serious foaming problem will result. We therefore suggest that freshly boiled deionized water is flushed with the nitrogen before adding emulsifiers. Then, while the surfactant is being added, nitrogen is bled in over the liquid surface. During the polymerization itself, the nitrogen is also kept just over the surface of the liquid to minimize foaming.

CAUTION" The simplest emulsion polymerizations have been carried out in closed bottles or sealed ampoules [130, 138, 139]. We generally do not consider these procedures safe. They are also of questionable value if the nature of the latex particles is to be studied. Two examples are given here only to illustrate the techniques used.

5.


5-1.

259

Emulsion Bottle Polymerization of Vinyl Acetate at 70~

[130]

In a 4-oz bottle equipped with a cap lined with Mylar are placed 25 gm of distilled water. 0.0050 gm of potassium persulfate, 0.51 gm of poly(vinyl alcohol) [Elvanol 52-22, a Du Pont product, 86-89% hydrolyzed poly(vinyl acetate) of medium molecular weight with a viscosity of a 4% aqueous solution at 20~ of 20-25 cP], and 5.00 gm of vinyl acetate. Prior to capping the bottle, oxygen-free nitrogen is bubbled through the contents. The bottle is closed, placed in a steel protective sleeve, and placed on a suitable rack which permits end-over-end rotation of several such bottles in a constant temperature bath at 70 ~ + 0.lOG. Bottles are withdrawn periodically to study the progress of the reaction. After an induction period of 30 min, the polymerization proceeds rapidly. After another 30 min, conversion is approximately 80%. With decreasing levels of poly(vinyl alcohol) the rate decreases. Upon removal of each bottle and cooling of it, a small quantity of styrene is added to the contents to inhibit the polymerization. The conversion is evaluated by determining the total solids of the latex. Procedure 5-2 illustrates the formation of a latex from an aqueous solution of vinyl acetate.

5-2.

Emulsion Bottle Polymerization of Vinyl Acetate from Aqueous Solution at 40~ [138, 139]

With suitable safety precautions in a 100-ml volumetric flask is placed 47 ml of distilled water, 3.75 mmoles of potassium persulfate, 0.5 mmoles of sodium hexadecyl suflate, and 3 gm of vinyl acetate. The volumetric flask is stoppered and placed in a constant temperature bath at 40~ After seven days conversion is 100%. The resulting latex may be freed of initiator and surfactant by dialysis. If the undialyzed latex is to be coagulated, the addition of 0.5 mg of a cationic surfactant such as hexadecyltrimethylammonium bromide is suggested. For kinetic studies, emulsion polymerizations have been carried out in a variety of dilatometers [134, 139, 148, 149]. Perhaps, most interesting is the work of Litt et al. [139], who used a large stirred dilatometer. Typically, they used a recipe consisting of 180.0 gm of distilled water, 60.0 gm of distilled vinyl acetate, 1.2 gm of sodium lauryl sulfate, and 0.04 gm of potassium persulfate. The polymerization was carried out under oxygen-free nitrogen with suitable nitrogen purging prior to the filling of the dilatometer. Agitation was at 150-200 rpm with a reaction temperature of 60 +__0.03~ In general, approximately 85% conversion had taken place after approximately 1 hr with an

260


induction period of 5-10 min, depending on the concentration of potassium persulfate and the extent of the nitrogen purge prior to the ran. The total volume contraction anticipated in these experiments ranged from 8 to 10 ml. In discussing experimental techniques, Dunn and Taylor [134] mention that while dilatometric studies with degassed materials eliminated the induction period, the start of the reaction was difficult to observe since the initiation of the polymerization takes place during the thermal expansion period as the equipment was thermostated. Most emulsion polymerizations are, of course, carried out in conventional stirred reactors. There are procedures in which all of the monomer is charged at once to the reactor; procedures wherein the monomer is gradually added, possibly with gradual addition of surfactant solutions, initiator solutions, and other modifying agents; procedures using redox initiation or thermal initiation; and procedures using preemulsified monomers. Procedure 5-3 uses poly(vinyl alcohol) only as an emulsifier. Its level is rather high (7.5% on the monomer). The percent solids of the resulting latex is moderately high. To be noted is the use of formic acid both to adjust the pH of the system and to act as a reducing agent.

5-3.

Emulsion Polymerization of Vinyl Acetate with Poly(vinyl alcohol) as Emulsifier [158]

In a l-liter, four-necked resin kettle fitted with a mechanical stirrer, a reflux condenser, a pressure-equalizing addition funnel, and a thermometer is heated (using a water bath which has been arranged for convenient and safe raising and lowering) 285 ml of deionized water with agitation at 60-70~ Into this water is sifted 15 gm of poly(vinyl alcohol) [Lemol 42-88, a Borden product, 88% hydrolyzed poly(vinyl acetate), high-molecular-weight product with a viscosity of a 4% aqueous solution at 20~ of 42 cP]. Warming and stirring is continued until the emulsifying agent is completely dissolved. This may require 1 hr or more. Then 300 gm of vinyl acetate is added from the addition funnel. Through the condenser, are added, in turn, 0.2 gm of 30% hydrogen peroxide [CAUTION: strong oxidizing agent; face mask, gloves, and protective clothing must be worn when handling 30% hydrogen peroxide; severe skin burns may be caused upon contact] and 0.6 gm of formic acid [CAUTION: toxic reagent]. The mixture is stirred and heated to 70~ and maintained at that temperature for 5 hr by raising or lowering the water bath as required. Then the latex is cooled to room temperature. This latex contains approximately 43% nonvolatiles. The study of Nomura et al. [143] is perhaps one of the most detailed investigations on the kinetics and mechanism of the emulsion polymerization of vinyl acetate published. The work presents a detailed evaluation of the interactions of monomer, surfactant, and initiator concentration in the course of the


261

reaction. Procedure 5-4 illustrates the use of potassium persulfate and sodium lauryl sulfate in one emulsion procedure developed for these data.

5-4.

Emulsion Polymerization of Vinyl AcetatePotassium Persulfate-Sodium Lauryl Sulfate System [143]

To a 500-ml, resin kettle with a bottom stopcock, equipped with a pressureequalizing addition funnel, reflux condenser (topped with a pressure regulator permitting exhausting of nitrogen but preventing air from entering the system), a thermometer, a 4-bladed paddle, 3.75 cm in diameter with blades set at 90 ~ to each other in the reaction chamber with inside diameter of 7.5 cm, 4 baffle plates approximately 0.75-cm wide, and a nitrogen inlet, were added 250ml of distilled water, 50 gm of vinyl acetate, and 0.25 gm of sodium lauryl sulfate. The mixture was freed of oxygen by bubbling nitrogen through it for at least 0.5 hr. Then 0.3125 gm of potassium persulfate was added (possibly in aqueous solution which had been deoxygenated separately). The agitator was operated at 400 rpm and the reaction is maintained at 50 ___0.5~ Within 40 min, nearly 100% conversion was observed. Many variations on the "delayed addition" or gradual addition procedures are possible. Procedure 5-5 is a relatively simple example of the technique which produces a latex of high solids content with a low level of anionic surfactant and a modest level of protective colloid.

5-5.

High Percent Solids Continuous Addition Emulsion Polymerization of Vinyl Acetate: Potassium Persulfate Initiated [ 159]

In equipment similar to that used in Procedure 5-3 with provisions for maintaining a nitrogen atmosphere in the system, 210.75 gm of deionized water, 0.75 gm of Tergitol-7 (sodium heptadecyl sulfate, an anionic surfactant from Union Carbide), 12.5 gm of Cellosize WP-09 (a protective colloid from Union Carbide) and 1 gm of potassium persulfate is blended under nitrogen with agitation. Then 25 gm of vinyl acetate is added and the mixture is heated to 70~ When the polymerization has started, with stirring, over a 2- to 3-hr period, 250 gm of vinyl acetate is added. The temperature is maintained at 70~176 After the addition has been completed, heating at 70~176 is continued for an additional 30 min. The residual monomer content of this latex is less than 1%. More complex gradual addition procedures have been reported. For example, in Vona et al. [95], a procedure for the gradual addition of monomer and

262

7. Polymerizationof VinylAcetate and OtherVinylEsters

initiator is given. In Gulberkian [152], three ingredients are added simultaneously at a steady rate: a monomer-surfactant solution, the persulfate initiator solution, and a sodium hydroxide solution. Procedure 5-6 is an example of a redox-initiated system involving the use of three separate streams to add monomer, oxidizer, and reducer.

5-6.

Continuous Addition Emulsion Polymerization o f Vinyl Acetate: Hydrogen Peroxide-Tartaric Acid Initiation [160]

In a reactor equipped much like that used in Procedure 5-3 except that provision is made for additions from two burets, 30 gm of poly(vinyl alcohol) (such as Lemol 42-88) is dissolved in 260 ml of water at 70~ Once the emulsifier has been dissolved completely, 26 gm of vinyl acetate, 4 ml of 2% aqueous hydrogen peroxide, and 4 ml of a 3% aqueous tartaric acid solution are added. The mixture is heated, with agitation at 70~ for 1 hr. Then over a 3-hr period, there are added continuously from separate streams 240 gm of vinyl acetate, 4 ml of 2% aqueous hydrogen peroxide, and 8 ml of a 3% aqueous tartaric acid solution. After the addition has been completed, the reaction mixture is heated at 80~ for 1 hr. Bouchard [161] developed a poly(vinyl acetate) adhesive formulation using hydrogen peroxide-zinc formaldehyde sulfoxylate as the initiator system, poly(vinyl alcohol) and sodium decylbenzene sulfonate as the emulsification system, and lauryl peroxide dissolved in the seed monomer to reduce the viscosity of the final latex (see Table XVIII). To this, from separate streams, a hydrogen peroxide solution, a sodium bicarbonate solution, and vinyl acetat e are added. The final latex had a pH of 4.5 and a viscosity of 7 cP at 25~ Many different redox systems have been used in the emulsion polymerization of vinyl acetate. Further investigations on the use of persulfate-bisulfite, hydrogen peroxide-ascorbic acid, tert-butyl hydroperoxide with various water-soluble as well as monomer-soluble reducing agents, etc., should be carried out. In an effort to develop "high solids" latices, one early approach has been to preemulsify the monomer in a distinctly separate step from the polymerization step. While the example in Procedure 5-7 goes back to a patent of 1951 [162], a more recent example of a somewhat complex copolymer system will be found in Vegter [ 163].

5-7.

Emulsion Polymerization o f Vinyl Acetate with Preemulsification [162]

An aqueous phase is prepared by dissolving 0.5 gm of sodium dioctylsulfosuccinate (Aerosol OT), 0.6 gm of sodium cetyl sulfate, and 1.4 gm of ethyl

5.


263

alcohol in 200 ml of distilled water. After dissolution is complete, 1.5 gm of ammonium persulfate and 0.01 gm of sodium bicarbonate are added. This aqueous phase, along with 200 ml of vinyl acetate, is emulsified in a hand operated emulsifying machine. The monomer emulsion is placed in a l-liter flask fitted with a mechanical stirrer and a reflux condenser. The reaction mixture is heated with agitation at 60~ for 6 hr. After this time, the reaction is judged to be complete. A freeflowing latex with pH 4.8 and 47.8% nonvolatiles is isolated. In a study of the polymerization of vinyl compounds initiated by sulfite radicals, Sully [164] showed that in styrene polymerizations in emulsion with radicals generated by the oxidation of sodium sulfite, the polymer molecules formed have sulfonic acid end-groups. It is, therefore, not entirely unanticipated that monomers polymerized in the presence of persulfate have end-groups characteristic of anionic surfactants. Indeed, mention has been made of stable latices of poly(vinyl acetate) prepared without external surfactants [137]. More recently, it was pointed out that with persulfate initiated polymerizations, both hydroxyl and sulfate groups become part of the polymer systems at the particle surface. Whereas a surfactant like sodium lauryl sulfate can be removed from latex particles by dialysis and slight hydrolysis of poly(vinyl acetate) favors the desorption of anionic surfactants, sulfate groups attached to the polymer, obviously are not removable. Therefore, stable latices are formed [ 165]. Before this explanation was offered, a patent had been issued for the preparation of stable latices of high solids content which were free of emulsifier. To achieve additional stability, a protective electrolyte was added. These electrolytes are salts of organic or inorganic acids containing two or more oxygens. The nature of the action of these salts is obscure, but it should be pointed out that with the exception of bisulfates (which were not cited in the examples), all of them have a buffeting action. The patent in question goes into considerable detail about the ratio of electrolyte to initiator, reaction temperatures, and other factors. A detailed discussion and critique of this is beyond the scope of this chapter. Table XIX gives typical reaction conditions. Procedure 5-8 is an adaptation of an example found in the patent [ 166].

5-8.

Emulsion Polymerization of Vinyl Acetate without Surfactant [166]

In a 3-liter reaction vessel equipped with pressure-equalizing addition funnel, reflux condenser, thermometer, and mechanical stirrer, 3.5 gm of potassium persulfate and 5.5 gm of potassium citrate monohydrate are dissolved in 80 gm of water maintained at 82~ To this solution, with agitation, 100 gm of vinyl acetate is added continuously over a 3-hr 20-min period. During the addition period the reaction temperature is maintained between 82 and 85~ After the

TABLE xIX

Reactions Conditions for the Polymerization of Vinyl Ester Homo- and Copolymers without Surfactant [ 166] Reaction temperatures

Initiator (gm)

Protective electrolyte (gm)

Water gm (Solution temperature (~

Monomer(s) (gm)

K citrate monohydrate (0.55) Na acetate trihydrate (0.178) Na bicarbonate (0.89) Disodium phosphate tetrahydrate (0.48) K citrate monohydrate (0.42) Na succinate hexahydrate (0.5)

80 (82) 71 (-)

Vinyl acetate (100) Vinyl acetate (100)

80 (82) 80 (-) 86 (-)

(0.5)

K citrate monohydrate (0.35)

80 (-)

K2S208 (0.4)

K citrate monohydrate (0.42)

80 (-)

Vinyl propionate (100) Vinyl butyrate (100) Vinyl acetate/(35) Vinyl pelargonate (65) Vinyl acetate/(35) Vinyl 2-ethylhexanoate (65) Vinyl acetate/(33) Vinyl butyrate (67)

K2S208 (0.35) K2S208 (0.444) K2S208 (0.625) K2S208 (0.4) Na2S208 (0.44) K25208

Particle size 0.3 ___0.1. b Reported in Reference 166.

a

Addition period (hr)

Addition stage (~

Finished stage (~

3.3

82-85

5.75

79.5-8.5

1 hr 55 min

82-85

Yield (%)

Nonvolatiles

90 (20 min) 89 (0.25 hr)

91.6

53a

94.1

59.6

95

58.4

88.8

82

89.6

53.4

(%f

.',,I

5.75

74-85

95 (20 min) 77 (1 hr) (1.3 hr)

6.5

76-85

(1.5 hr)

95

54.3

4.3

69-78

(10 min)

90.6

52.0

15

70-75

5.

265


addition is complete, the temperature is allowed to rise and is maintained at 90~ for 20 min. Upon cooling, the yield of latex is 91.6%, the solids content 53%, and the particle size 0.3 ___0.1/tm with remarkable uniformity of size. By conventional procedures, the expected particle size distribution would have been between 0.5 and 2/tm. In Section 4 on the preparation of poly(vinyl acetate) suspension polymers, mention was made of the preparation of polymer beads by adding a methanol solution of that polymer to an aqueous solution of poly(vinyl alcohol). By varying the concentration of emulsifying agent and the rate at which solvents are distilled off, it is possible to develop conditions for the generation of emulsions with polymer particles of diameter between 1 and 3/zm [ 121 ]. The emulsion polymerization of vinyl esters of the high carboxylic acids is somewhat difficult since stable emulsification before and during polymerization is difficult to achieve with the common soaps of the alkylaryl sulfonate salts. Best results are said to be achieved when a mixture of a nonionic surfactant such as esters of anhydrosorbitol or anhydromannitol (e.g., Span 20) and a branchedchain alcohol sulfate [e.g., Tergitol Paste 4 (50%)] are used. These emulsifers do not produce emulsion polymers with vinyl acetate but do so with vinyl palmitate. The usual procedure for polymerization consists of weighing monomer and emulsifiers into a flask through which oxygen-free nitrogen is passed. With swirling and warming, a uniform mixture is produced which is transferred to a mechanical blender containing oxygen-free distilled water. In the blender, during agitation, nitrogen is continuously passed through. The resultant emulsion is then polymerized in conventional equipment after 7 hr at 55~176 A typical recipe consists of Reagent

Amount

Vinyl palmitate Span 20 Tergitol Paste 4 (50%) Water, distilled Potassium persulfate

0.20 moles 2.8 gm 5.7 gm 113 gm 0.27 gm (0.001 moles)

,,

Within 7 hr at 55~176 a conversion greater than 80% was achieved. To isolate the polymer, the latex may be coagulated by pouting it into a warm aqueous sodium chloride solution. The polymer may be purified by dissolving it in benzene and then, at - 5 ~ pouting the solution while swirling by hand into acetone. The poly(vinyl palmitate) could be isolated as discrete particles [167]. The monomer vinyl pivalate polymerizes readily. Unlike most common monomers, this monomer yields polymers of high molecular weight by the suspension procedures and modest molecular weights in an emulsion polymerization. This has been attributed to its lower capability of diffusing through the

266


aqueous system as compared to vinyl acetate. This monomer is quite resistant to saponification and exhibits little chain-transfer activity [116]. Procedure 5-9 illustrates a redox procedure using a persulfate-bisulfite redox system.

5-9.

Emulsion Polymerization of Vinyl Pivalate [116]

In a 2-liter, reaction kettle provided with a reflux condenser, mechanical stirrer, thermometer, pressure-equalizing addition funnel, and provision for passing oxygen-free nitrogen through the system, to a solution of 0.52 gm of ammonium persulfate, 50 ml of a 15% solution of sodium hydroxyoctadecylsulfonate (Amphoseife-18) in 450 ml of distilled water is added 1.05 gm of potassium bisulfite followed by 250 gm of vinyl pivalate. The mixture is agitated under nitrogen at 40~ for 24 hr. During this time, the pH of the latex is adjusted to 3.5 by additions of formic acid. If desired, the polymer may be isolated by coagulation with a 10% aqueous aluminum sulfate solution. Procedure 5-10 is an adaption of a ter-polymerization that starts with the formation of a seed latex followed by the gradual addition of both a monomer composition and an initiator solution at separate rates. The resulting latex has a high percentage of non-volatiles. It is said to be suitable for formulating good emulsion paints. In connection with this preparation, care must be taken that the initiating ammonium persulfate is indeed active.

5-10.

Emulsion Ter-polymerization of Vinyl Acetate, Butyl Acrylate, and Vinyl Neodecanoate (Seeded Process with Gradual Monomer and Initiator Additions) [ 168]

To a l-liter resin kettle equipped with a reflux condenser, an explosion-proof stirrer, a thermometer and temperature controller, and a nitrogen inlet is charged 204.00 grn of deionized water. Then 6.00 gm of Cellosize hydroxyethyl cellulose WP-300, 3.00 gm of Tergitol NP-40, 3.90 gm of Tergitol NP-15 (two nonionic surfactants), 3.3 gm of Siponate DS-4 (an anionic sufactant), and 0.6 gm of ammonium bicarbonate are added and the stirred mixture is blanketed with nitrogen and warmed to 55~ This temperature is maintained for 20 min. A mixture of 18.00 gm of vinyl acetate, 4.50 gm of butyl acrylate, and 7.5 gm of vinyl neodecanoate is added, followed by 0.24 gm of ammonium persulfate. The reaction mixture is heated to 75~ and maintained at that temperature for 15 min to form the seed latex. The reaction temperature is then raised to 78~ and the gradual addition of monomers and initiator solution is begun. The monomer solution, consisting of 162 gm of vinyl acetate, 41.4 gm of butyl acrylate, and 66.6 grn of vinyl neodecanoate, is added over a 2-hr period. The initiator solution of 0.60 gm of

5.


267

ammonium persulfate dissolved in 60.00 gm of deionized water is added over a period of 2.5 hr. All the while, the reaction temperature is maintained at 78 ~. After the last of the initiator solution has been added, heating and good stirring is continued for another hour. The latex is cooled and filtered though a 200 mesh stainless steel screen. The % nonvolatiles of the latex is 53.2%. Coagulum is 0.01%. Vinyl acetate is fairly water soluble and somewhat deficient in hydrolytic stability. The incorporation of a vinyl ester of the higher branched carboxylic acids into a copolymer system improves the resistance of the product to hydrolysis. Thus the latex formed in Preparation 5-10 has been used in formulating exterior paints. Copolymers of vinyl acetate with increasing concentrations of vinyl 2-ethylhexanoate (VEH) were prepared by a procedure similar to that of Preparation 5-10, except that the initiator system consisted of tert-butyl hydroperoxide and sodium formaldehyde sulfoxylate, and the buffer was sodium acetate. As the level of VEH increased, the hydrolytic stability of the copolymer increased significantly [ 169]. The emulsion copolymerization of vinyl acetate and butyl acrylate has received considerable attention. The butyl acrylate confers improved film forming characteristics to the polymer. The disparities in their water solubilities and of their individual polymerization rates may help to explain the variations in reactivity ratios that have been reported [ 170, 171]. The variation in reactivity ratios may also by related to the following observations: The reaction method has an effect on the morphology of the polymer particles. In a batch emulsion process, a butyl acrylate--rich core is formed which is surrounded by a vinyl acetate-rich shell, in a process in which the monomers are fed into the reactor in a semicontinuous manner, particles form with a more uniform distribution of the monomers [172]. The kinetics for a batch process indicates that the initially formed polymer is indeed high in butyl acrylate. As this monomer is used up, eventually a copolymer high in vinyl acetate develops. It is this latter polymer which forms the final shell around the particles. When a copolymer seed is preformed and then a preemulsified mixture of vinyl acetate and butyl acrylate in water (using sodium dodecylbenzenesulfonate, Siponate DS-10, as surfactant) is added slowly, the composition of the product approaches that calculated from the reactivity ratios. The particle size and the size distribution is related to the ionic strength of the medium. With low levels of initiator and buffer, the particle size tends to be small and the size distribution narrow. As ionic strength increases, the particle size increases. At low monomer addition rates, the particle size increases. Typically, particle sizes range from 0.206 to 1.05/tm with coefficients of variation ranging from 10.5 (for the smaller particles) to 5.9 (for the larger ones) [173, 174]. To a latex of poly(vinyl acetate) that had been freed of surfactant by prolonged dialysis, more monomer and more potassium persulfate, but no

268


further surfactant, were added. The rate of polymerization was found to be [I]~176176176 where [I] is the initiator concentration. With the addition of fresh monomer, a significant increase in the diameter of the particles took place provided the weight ratio of monomer to initial particles was kept below approximately 10 to 1. When the monomer level was raised above this, new particles formed, albeit without affecting the polymerization rate [175]. 6.

NONAQUEOUS DISPERSION

POLYMERIZATION

The preparation of polymers as dispersions in nonaqueous systems has not been discussed very extensively. Even so, an interesting book has been published [ 176]. Stable dispersions in organic liquids are usually formed by using graft copolymers to stabilize the system. According to one patent, the copolymer "emulsifier" is selected so that its polymer chains contain groups of differing polarities; some of which may be solvated by the organic solvent and other groups which may become associated with the particles of the dispersed polymer [177]. These stable dispersions have found application in baked-on finishes. In Procedure 6-1, a remarkably high solids dispersion of poly(vinyl acetate) in cyclohexane is prepared. Since this procedure has been patented, it is given here only to illustrate the techniques used in preparing nonaqueous polymer dispersions.

6-1. Preparation of Poly(vinyl acetate) in Nonaqueous Dispersion [178] In a resin kettle equipped with a mechanical stirrer, reflux condenser, thermometer, and provisions for passing oxygen-free nitrogen through the system, with agitation, under a nitrogen atmosphere, 240 gm of vinyl acetate, 160 gm of cyclohexane, and 1.4007 gm of an ethylene-vinyl acetate copolymer (DQDA 3267, a copolymer containing 28 wt% vinyl acetate and having a melt index of 23.8 dg/min) are heated to 70~ for approximately 1 hr to bring about complete solution. Then 0.1514 gm of dibenzoyl peroxide is rapidly added. After stirring and heating for about 1.5 hr a milky dispersion begins to form. The polymerization is continued for 22 hr. The product is a dispersion with solids content of 58% at 97% conversion of the monomer to polymer. The poly(vinyl acetate) exhibited an inherent viscosity (0.2% solution in cyclohexane at 30~ of 45. The particle size of the polymer is between 0.3 and 5/zm. This dispersion can be used to produce a dispersion of poly(vinyl alcohol) [albeit only about 75% hydrolyzed poly(vinyl acetate)] by hydrolyzing it with a solution of sodium methylate in methanol at 30~

7.

Radiation-Initiated

269

Polymerizations

Dispersions of poly(vinyl acetate) in n-alkanes have been prepared using a diblock copolymer of poly(styrene-b-[ethylene-co-propylene]) as the stabilizer for the colloidal PVAc. The dispersing agent contained 38.5% styrene and was used at a concentration of 1-5 wt%. The monomer concentration ranged from 10 to 30 wt% in the various experiments. A typical initiator was 2,2'-azobisisobutyronitrile. The particle diameters were in the range of 0.10 to 0.31/lm [179]. 7.

RADIATION-INITIATED

POLYMERIZATIONS

Many details of the complex steps in the formation of polymers of high molecular weight beginning with the initiation step have been studied by UV-radiation-initiated polymerization. By a technique which interposed a rotating sector between a source of UV radiation and a dilatometer beating a monomer, variations in the rotational speed of the sector and size of the opening, controlled bursts of radiation strike the monomer and induce polymerization. From the frequency of exposure and the effect on the polymerization many of the kinetic constants were evaluated. In this connection it should be noted that vinyl acetate exhibits virtually no absorption of UV radiation at 290-300 nm. On the other hand, acetaldehyde has an extinction coefficient of 14 at 290 nm and an extinction coefficient of 15 at 300 nm. Therefore acetaldehyde can act as a photo-sensitizer for the polymerization of vinyl acetate at wavelengths above 299.8 nm [23]. At 366 nm, 2,2'-azobisisobutyronitrile has been used as a sensitizer [29, 180]. Azobicyclohexane carbonitrile is a UV sensitizer suitable for use with a 124 watt mercury arc at 25~ which does not produce a dark reaction in rotating-sector experiments. Its absorption peak is at 350 nm with an extinction coefficient of 16 [181]. The UV-initiated polymerization of vinyl acetate, sensitized with various quantities of AIBN at - 19~ gives polymeric products which, upon saponification and reacetylation, produce polymers with substantially the same molecular weight as that initially observed. These observations contribute to the postulate that at low temperatures, polymerizations proceed essentially with the formation of branched chains [26]. A composition of vinyl acetate, ethylene dimethacrylate, benzil (a UV sensitizer), and finely ground vinyl acetate-ethylene dimethacrylate popcom copolymer on exposure to radiation at 365 nm gave rise to a proliferating polymerization which continued after the source of radiation had been tumed off. It has been postulated that in this case most of the growing radicals are formed by chain scission during polymerization [182]. Vinyl acetate, frozen to a glassy state or a crystallized form, has been subjected to solid-state polymerization with UV radiation. The rate of polymerization was found to vary with the physical state [183]. On the other hand at -195~ vinyl acetate did not polymerize when exposed to a dosage rate of

270

7, Polymerization of Vinyl Acetate and Other Vinyl Esters

7.8 • 105rad (),-radiation). It has been postulated that solid-state vinyl polymerizations proceed by an anionic mechanism [184]. At 20~ the rate of polymerization of vinyl acetate initiated by ),-radiation is substantially greater than that of many common monomers [ 1]. Emulsion polymerizations initiated by ),-radiation have been carried out [141,185]. For example, compositions of 10 ml of a 3.75 gm per 100 ml aqueous solution of sodium dioctylsulfosuccinate (Aerosol OT or Manoxol OT) and 1.25 ml of vinyl acetate with small additions of sodium dihydrogen phosphate, after exclusion of oxygen by at least four degassings, were sealed in glass ampoules. The samples were exposed, at 15~ to a 6~ source (100 curie): Conversion was nearly 75% after 30 rain. Polymers of MW as high as 106 were formed. The polymers contained trapped free radicals capable of initiating graft polymerization with methyl methacrylate after removal from the radiation source [ 185]. By ),-radiation a latex was also prepared from a composition of 151.4 gm of vinyl acetate and 10.9 gm of sodium lauryl sulfate in 300 gm of distilled water (irradiation was 26.5 hr at 6 • 104 rad/hr) [142].

go

POLYMERIZATIONS INITIATED BY IONIC, COORDINATION COMPLEXES, AND OTHER MECHANISMS

The copolymerization of styrene and vinyl acetate affords an example of the significant differences in the effects of the initiation system on the monomer composition. The copolymerization ratios for the three processes may be used as a guide to the effect under study. For M1 as styrene and M2 as vinyl acetate, in the cationic copolymerization, rl is 8.25 and 1"2 is 0.1, while for the free-radical polymerization, r~ is 55 and r2 is 0.01. The cationic copolymerization was carried out in air at 25~ in a nitrobenzene solution using stannic bromide as catalyst, while the anionic polymerization was studied in air with sodium dissolved in ammonia [ 186]. Although many monomers may be polymerized by quaternary ammonium salts, vinyl acetate does not produce any polymer on treatment with dimethylphenylbenzylammonium chloride [ 187]. The polymerization of vinyl esters in the presence of alkali metals, particularly lithium, was characterized by lengthy induction periods. It was not possible to verify whether a nonfree-radical mechanism was involved in the polymerization process [ 188]. Ziegler-Natta type polymerizations of vinyl esters with trialkyl aluminum and metal halides have been discussed [189-193]. The "living" radical polymerization of vinyl acetate and subsequent use of the "living" polymer to form block copolymers, makes use of an initiating system

9.

Miscellaneous

271

consisting of triisobutyl aluminum, 2,2'dipyridyl, and the 2,2,6,6-tetramethyl1-piperidinyloxy free radical ("TEMPO"). The molar ratio of the trialkyl aluminum to Tempo was 3 to 2; that of the trialky aluminum to dipyridyl was 1 to 1. At room temperature, 90% conversion of the monomer was achieved in 1 day. The molecular weight of the product, Mn, was 50,000, with polydispersity ranging from 1.25 to 1.5. As more monomer was added to the system, the molecular weight increased. When other monomers, such as methyl methacrylate or styrene, were added, block copolymers formed. This was considered a characteristic of "living" polymerization. The authors claim that this work represented the first cases of block-copolymer formation with poly(vinyl acetate) as one of the blocks [194]. The use of trialkylboron and related initiators is discussed in references [195-197]. The polymerization of vinyl esters with trialkylaluminum and organic peroxide was patented in 1974 [ 198]. Zinc chloride has been used in a variety of polymerization systems. References to its use and to charge transfer complex polymerizations are Nikolayev et al. [29], Imoto et al. [34], Semiuk and Thomas [199], and Seymour et al. [200].

9.

MISCELLANEOUS

Hydrolysis of PVAc, Hydrogels, Poly(vinyl alcohol) [PVA] 1. Poly(vinyl acetals) from polyvinyl alcohol and 2,6-dichlorobenzaldehyde [201]. 2. Poly(pyrrole)-poly(vinyl alcohol) composite films which are conducting and transparent [202]. 3. Preparation of concentrated PVA solutions [203]. 4. Stretched PVA-methyl acrylate graft copolymers for moisture-shrinkable films [204]. 5. Properties of PVA-hydroxypropylmethylcellulose-water systems as dispersants in suspension polymerizations [205]. 6. Hydrogel preparation by grafting 2-hydroxyethyl acrylate onto PVA [206]. 7. Prediction of concentration, conversion, particle size for the emulsion polymerization of PVAc stabilized by PVA using mathematical modeling [207]. 8. A "smart" material based on PVA-borate gelation [ 13]. 9. Protein deposition on PVAc-poly(methyl methacrylate) contact lens type hydrogels [208]. 10. pH-sensitive hydrogels based on block-copolymers of poly(hydroxethyl methacrylate) with PVA esterified with maleic anhydride or acryloyl chloride [209].

272


11. Isotactic poly(propylene)-PVA graft copolymers [210]. 12. Hydrolysis of methyl tributyltin maleate-vinyl acetate copolymers [211]. 13. Biodegradability of poly(vinyl alcohol) [212]. Complexes and Polyblends 14. Formation of binary complexes of PVAc with poly(acrylonitrile), poly(methyl methacrylate), or poly(methacrylic acid) [213]. 15. Polyblends of cellulose with PVA [214]. 16. Interpenetrating polymer networks of epoxy novolac-based vinyl ester in polyurethane systems [215]. 17. Blends of [poly(vinyl acetate)-co-poly(vinyl alcohol)] with poly(4-vinylpyridine) [216]. 18. Synthesis and polymerization of metal-vinyl acetate colloids [217]. General 19. Thermal and dilute solution properties of low molecular weight poly(vinyl acetate) [218]. 20. Emulsion polymerization of vinyl acetate with a polymerizable surfactant, sodium dodecyl allylsulfosuccinate [219]. 21. Fluorescence of pyrene derivatives in low molecular weight copolymers of vinyl acetate and sodium methallylsulfonate [220]. 22. Microemulsions of vinyl acetate polymerization [221]. 23. Method of fractionation of PVAc [222]. 24. Effect of retardation by ethyl formate on the latex polymerization of vinyl acetate [223]. 25. Use of ultrasound in latex polymerization of vinyl acetate copolymers [224]. 26. Polyisobutylene-polyoxyethylene-polyisobutylene triblock copolymers as stabilizers in the aqueous polymerization of vinyl acetate [225]. 27. Organic sulfide-terminated poly(styrene) as photoinitiator for the polymerization of vinyl acetate [226]. 28. Cyclopolymerization of vinyl cinnamate [227]. 29. Cyclopolymerization of vinyl methacrylate [228]. 30. Precipitation copolymerization of vinyl esters [229]. 31. Terpolymers of vinyl chloride, vinyl acetate, and vinyl alcohol [230]. 32. Polymerization at subzero temperatures initiated by methyl chlorosulfite and hydroperoxide [231 ]. 33. Polymerization in DMF with 2,2'-azobisisobutyronitrile and dissolved salts [ 101 ]. 34. Solution copolymerization of vinyl acetate and ethylene [232].

References

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35. Preparation of syndiotactic polymers of vinyl esters of fluorinated acids [233]. 36. Kinetics of emulsion polymerization of vinyl stearate [234]. 37. Redox emulsion copolymerization of vinyl acetate and vinyl propionate

[235]. 38. 39. 40. 41.

Continuous emulsion polymerization of vinyl acetate [236]. Polymerization of vinyl acetate initiated by metal chelate [237, 238]. Carbanionic copolymerizations with vinyl butyl sulfone [239]. Preparation of monomeric vinyl alcohol from acetals, by rearrangement of allyl alcohols (to 1-propene-l-ols) and polymerization thereof

[240, 241]. 42. Free radical copolymerization of vinyl acetate with N-substituted maleimides and bismaleimides [242]. 43. Vinyl acetate-co-vinyl pyrrolidone: A study of the solution behavior of various copolymer compositions in aqueous and in alcoholic systems [243].

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171. M. A. Abd E1-Ghaffar, A. S. Badran, and S. M. M. Shendy, J. Elastomers Plast. 24(3), 192 (1992). 172. C. Pichot, X. Z. Kong, J. Guillot, and C. J. Cavaille, Proc. Am. Chem. Soc., Polym. Mater. Sci. Eng. 64, 276 (1991). 173. G. A. Vandezande and A. Rudin, Proc. Am. Chem. Soc., Polym. Mater. Sci. Eng. 64, 274 (1991). 174. G. A. Vandezande and A. Rudin, "Polymer Latexes," ACS Symposium, Vol. 492, pp. 114, 492. Am. Chem. Soc. (1992). 175. B. W. Brooks and J. Wang, Polymer 34, 119 (1993). 176. K. E. J. Barrett and M. W. Thomson, in "Dispersion Polymerization in Organic Media," (K. E. J. Barrett, ed.). Wiley (Interscience), New York, 1975. 177. D. W. J. Osmond, British Patent 1,052,241 (1966); German Patent 1,520,119; Chem. Abstr. 69, 10929y (1967). 178. L. A. Pilato and E. R. Wagner, French Patent 1,531,022, 1968; Chem. Abstr. 71, 13770s (1969). 179. J. V. Dawkins and S. A. Shakir, Proc. Am. Chem. Soc., Polym. Mater. Sci. Eng. 64, 357 (1991). 180. M. S. Matheson, E. E. Auer, E. B. Bevilacqua, and E. J. Hart, J. Am. Chem. Soc. 71, 2610 (1949). 181. W. I. Bengough and H. W. Melville, Proc. R. Soc. London, Ser. A 225, 330 (1954). 182. J. W. Breitenbach and H. F. Kauffmann, Makromol. Chem. 162, 295 (1972). 183. I. M. Barkalov, V. I. Goldanskii, N. S. Enikolopyan, S. F. Terekhova, and G. M. Trofimova, J. Polym. Sci., Part C 4, 909 (1963). 184. Y. Tsuda, J. Polym. Sci. 49, 369 (1961). 185. P. E. M. Allen, G. M. Burnett, J. M. Downer, and Sir H. Melville, MakromoL Chem. 38, 72 (1960). 186. Y. Landler, C. R. Hebd. Seances Acad. Sci. 230, 539 (1950). 187. T. Fueno, H. Okamoto, T. Tsuruta, and J. Furukawa, J. Polym. Sci. 36, 407 (1959). 188. D. J. Kelley, J. Polym. Sci. 59(167), $6 (1962). 189. E. I. du Pont de Nemours and Co., British Patent 840,910 (1960); Chem. Abstr. 54, 26007e (1960). 190. W. Kawai and S. Tsutsumi, J. Polym. Sci. 46, 273 (1960). 191. J. C. Mackenzie and A. Orzechowski, U.S. Patent 3,285,895 (1966); Chem. Abstr. 66, 29,392c (1967). 192. T. Otsu, S. Aoki, M. Nishimura, M. Yamaguchi, and Y. Kusuki, J. Polym. Sci., Part B 5, 835 (1967). 193. T. Saegusa, T. Yatsu, S. Miyaji, and H. Fujii, Polym. J. (Japan) 1(1), 7 (1970); H. Wexler and J. A. Manson, J. Polym. Sci., Part A 3, 2903 (1965). 194. D. Mardare and K. Matyjaszewski, Am. Chem. Soc., Polym. Prepr. 34(2), 566 (1993). 195. N. Ashikari, J. Polym. Sci. 28, 250 (1958). 196. K. Noro and H. Kawazura, J. Polym. Sci. 45, 264 (1960). 197. S.-I. Nozakura, M. Sumi, M. Uoi, T. Okamoto, and S. Murahashi, J. Polym. Chem., Part A-1 11,279 (1973). 198. T. Yatsu and H. Maki, Japanese Patent 74 32,669 (1974); Chem. Abstr. 82, P73690c (1975). 199. G. E. Serniuk and R. M. Thomas, U.S. Patent 3,183,217 (1965); British Patent 946,052 (1964). 200. R. B. Seymour, D. P. Garner, G. A. Stahl, and L. J. Sanders, Polym. Prepr. Am. Chem. Soc., Div. Polym. Chem. 17(2), 660 (1976). 201. E. Schacht, G. Desmarets, E. Goethals, and T. St. Pierre, Macromolecules 16, 291 (1983). 202. T. Ojio and S. Miyata, Polym. J. 18(1), 95 (1986).

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Chapter 8

Polymerization of Allyl Esters 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chain Transfer in Allyl Polymerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyclopolymerization of Diallyl Esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymerization of Allyl Acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-1. 4-2.

Bulk Polymerization of Allyl Acetate in Sealed Tubes . . . . . . . . . . . . . . . . . . Solution Polymerization of Allyl Acetate . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Polymerization of Allyl Esters of Higher Monoearboxylic Acids . . . . . . . . . . . . . . . . .

5-1.

Copolymerization of Vinyl Chloride and Allyl 1 O,11-Dibromo-undecanoate and Fractionation of the Copolymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

6. Polymerization of Allyl Acrylate and Methacrylate . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-1. 6-2. 6-3.

Solution Polymerization of Diethylene Glycol Bis(allyl carbonate) . . . . . . . . . Preparation of Diethylene Glycol Bis(allyl carbonate) Cast Sheet . . . . . . . . .

8. Polymerization of Diallyl Esters of Phthalic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . .

8-1. 8-2. 8-3. 8-4. 8-5.

Bulk Polymerization of Diallyl o-Phthalate with High-Temperature Initiator.. Sealed-Tube Bulk Polymerization of Diallyl o-Phthalate . . . . . . . . . . . . . . . . Bulk Polymerization of Diallyl o-Phthalate at 200~ . . . . . . . . . . . . . . . . . . Solution Polymerization of Diallyl o-Phthalate . . . . . . . . . . . . . . . . . . . . . . "'Suspension Polymerization" of Diallyl o-Phthalate . . . . . . . . . . . . . . . . . .

9. Polymerization of Allyl Esters of Other Polyfunctional Acids . . . . . . . . . . . . . . . . . . .

9-1. 9-2.

305

Solution Polymerization of Allyl Methacrylate . . . . . . . . . . . . . . . . . . . . . . . 306 Ultraviolet-Initiated Polymerization of Allyl Methacrylate with Benzoyl Peroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 Ultraviolet-Initiated Polymerization of Allyl Methacrylate with Biacetyl . . . . . 309

7. Polymerization of Diallyl Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-1. 7-2.

282 283 287 295 298 299 301

Preparation of Diallyl Brassylate Prepolymer in a Stirred Reactor . . . . . . . . Copolymerization of an Unsaturated Polyester and Diallyl Adipate . . . . . . . .

10. Miscellaneous Preparations and Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

311 312 313 314

316 316 317 320 321 322 323 324 326 327 281

282

1.

8. Polymerization of Allyl Esters

INTRODUCTION

The 2-propene group is commonly referred to as the "allyl group." The term is derived from the Latin allium meaning garlic. Many natural and artificial flavoring agents contain allyl derivatives. For example, oil of garlic contains diallyl sulfide, other allyl sulfur derivatives are components of oil of mustard, dill, wasabi, horseradish, and onion. Allyl esters are found in many perfumes [1]. Since allyl compounds are capable of free-radical polymerization (albeit sluggishly) they are used in a number of copolymer systems. Only four allyl esters are of significant industrial importance as monomers for the production of homopolymers: diethylene glycol bis(allyl carbonate), which, as "CR-39," is widely used as a plastic spectacle lens material of exceptional clarity, scratch, abrasion, and impact resistance, and diallyl o- and m-phthalates, supplied commercially as soluble low-molecular-weight prepolymers. These prepolymers may be molded and subsequently cross-linked to give rigid plastic objects. The prepolymer of o-diallyl phthalate (DAP) copolymerized with unsaturated polyesters and reinforced with either glass fibers or glass roving find application in high impact resistant molding compounds for electronic molded parts, decorative panels, and nonwoven glass fabrics, to mention only a few applications. The prepolymer of meta-isomer (DAIP) copolymerizes more rapidly than DAP and produces materials with higher heat resistance than the analogous DAP derivative. It is used in molting powders. Because of its high refractive index, DAIP is of interest in the manufacture of lenses. Other monomers which find large-scale applications are diallyl chlorendate (I), diallyl maleate, diallyl fumarate, allyl methacrylate, and allyl acrylate. C1

I

i ~ C H - - oCII -- O-- CH2-- C H = CH2 Ilel-Celf.

C1-- C / C1-- C

| _ _ C H - - C -- O-- CH2-- CH-- CH2

II

I

O

C1 Diallyl Chlorendate (I) The high level of chlorine in diallyl chlorendate contributes to the flame retarding properties of polymers containing this monomer. The free-radical polymerization process with allyl esters resembles that of vinyl and acrylic esters in that there are similar initiation, propagation, and

2.

Chain Transfer in Allyl Polymerizations

283

termination steps. Of particular importance in the case of allyl compounds is the chain-transfer process. It is this chain-transfer reaction which constitutes the predominant characteristic differences between allylic and vinyl polymerization reactions. In the case of allyl esters both "effective" and "degradative" chaintransfer seem to occur side by side. Some investigations indicate that free radicals generated from allylic compounds may be either "head radicals" or "tail radicals." The polymerization of allyl methacrylate and acrylate is of more than academic interest. The reactivities of allyl and acrylic moieties are so different that the acrylic portion of the monomer essentially may homopolymerize while the allylic groups are unaffected. At a latter stage, perhaps by increasing the reaction temperature or by incorporating additional initiator, the allyl groups may be used to crosslink the polymer. Such a two-stage system permits ready molding or extrusion of an acrylic copolymer which can be rendered thermosetting at a later time. The polymerization of diallyl o-phthalate as well as of other a-, r esters is characterized by the formation of cyclic structures along the lowmolecular-weight chains. At approximately 25% conversion to a low-molecularweight polymer, the mass rapidly gels. The ease of ring formation of diallyl esters decreases with increases in the separation between the two allyl groups. Interestingly enough, the cyclic reagents involved in diallyl o-phthalate prepolymers form quite readily even though the tings probably consist of 11- or 12-member tings, sizes thought to be difficult to establish, at least in the case of cyclic polymethylene derivatives. In this chapter, procedures for the polymerization of allyl acetate and related monocarboxylates of diallyl carbonate types, of diallyl phthalates, and of diallyl esters of other dicarboxylic acids are discussed. References [1-13] are a selection of review articles and books dealing with the chemistry of allyl compounds.

2.

CHAIN TRANSFER

IN ALLYL

POLYMERIZATIONS The free-radical polymerization of allyl esters exhibits several unique characteristics. In this respect they differ materially from the polymerization characteristics of vinyl esters. Litt and Eirich [14] note the following unique factors for the peroxide initiated process: 1. In any one experiment, the rate of decomposition of the peroxide is first order. 2. Yet there is a concentration dependence of the rate constant.

284

& Polymerization of Allyl Esters

3. The rate of monomer disappearance with respect to the initiator concentration is constant during any one experiment. 4. With increasing peroxide concentration, the rate of disappearance of monomer with respect to peroxide, d[M]/d[P], decreases. 5. Over a fivefold variation of the peroxide concentration, at a constant polymerization temperature, the MW of poly(allyl acetate) remains virtually constant at 1300 (i.e., DP = 13) (Litt and Eirich found the degree of polymerization to be 20-25 in their work). 6. Upon substituting deuterium for the two allylic hydrogens in allyl acetate (cf. Structure II) as the polymerizing monomer, both the molecular weight and the disappearance of the monomer with respect to peroxide, d[M]/d[P], increased [ 15]. O II

CH2--CH--CD2--O--C--CH3 Allyl-1, 1-dideutero Acetate or Allyl-l-d2 Acetate (II) The rates of polymerization of the deuterated analog of allyl acetate were from 1.93 to 2.09 times those of the undeuterated compound while the average MW of deuterium-containing polymer was 2.38 times greater than that produced from ordinary allyl acetate. The kinetic chain length of a free-radical polymerization process is the ratio of kp/kr, where kp is the rate of propagation and kr is the rate of termination. Since allylic hydrogens are not involved in the propagation step, substitution of deuterium for these hydrogens was expected to affect the termination process. When this indeed was found to be the case, as evinced by the increase in the kinetic chain length and the MW of the polymer, it was considered reasonable evidence that the allylic hydrogens were the cause of the characteristic behavior of allyl esters [ 15]. In their pioneering studies of the free-radical polymerization of allyl acetate, Bartlett and Altschul [ 16, 17] found that the average MW of poly(allyl acetate) produced by bulk polymerization with benzoyl peroxide or di(p-chlorobenzoyl)peroxide was generally about 1300 (DP = 13). While oxygen is capable of retarding the polymerization, the low molecular weight of the polymer had to be attributed to other causes. The work with di(p-chlorobenzoyl)peroxide showed that 72.5% of the peroxide fragments were bound to the polymers and 16.8% were converted to free p-chlorobenzoic acid (an additional 10.7% was not accounted for). The peroxide fragments which were not effective in initiating polymerization must have abstracted hydrogen from the monomer to form p-chlorobenzoic acid. At least 72% (but not all) of the polymer molecules produced had a p-chlorobenzoate end group; the remainder did not. This

2.

285

Chain Transfer in Allyl Polymerizations

observation indicates that while most of the polymer was formed from free radicals generated from the peroxides, a substantial quantity was initiated by a chain-transfer process. The abstraction of an allylic hydrogen from allyl acetate, either by benzoyloxy radicals or by other free radicals, such as growing, polymeric free radicals, yields a radical capable of resonance stabilization (Structure III): O II CH2=CH--CHOCCH3

-~

O II -~-CH2--CH--CHOCCH3

(III) Since such radicals react less rapidly than radicals not stabilized by resonance, the observable polymerization rate decreases. For this reason, the process is termed "degradative chain-transfer." In degradative chain-transfer, growing, polymeric free radicals collide with a monomer molecule to form a new, stable free radical which propagates only with difficulty. The allylic radical may terminate growing radicals, dimerize, cause the decomposition of peroxidic initiators, or initiate the formation of new polymerizing species. The first of these possible processes has been termed "cross termination" [14]. Along with degradative chain-transfer, there is evidence, as already mentioned above, that some new chains are initiated by allylic radicals. This has been termed "effective chain-transfer." In the polymerization of vinyl acetate, chain transfer to the acetate portion of the monomer is an important aspect of the process (see Chapter 7 of this volume). In the case of the allyl acetate polymerization, chain transfer to the acetate moiety is considered negligible as compared to the degradative chaintransfer process [ 14, 15]. Very much as in the case of vinyl acetate, the solution polymerization of allyl acetate in benzene leads to the addition of the growing radical chains to benzene to produce stable aromatic adducts with a tendency to terminate by combination. This explains the retention of the degree of polymerization in the expected range ( D P = 14.3-14.9) and the fact that a polymer chain may contain more than one aromatic group [ 14]. The preceding indicates that in bulk and in benzene solution, the polymerization of allyl acetate leads to polymers of relatively low molecular weight. It had been expected that in emulsion processes, a higher-molecular-weight polymer would be formed. However, with a persulfate initiator, the emulsion polymerization of this monomer led to a product with the same molecular weight as was found upon bulk polymerization [18]. The ratio of the rate constant for degradative chain-transfer to that of effective chain-transfer has been derived as a function of the ratio of the rate of monomer consumption, the rate of initiator consumption, and the degree of polymerization

286


[19]. This ratio may also be estimated from a determination of the amount of initiator residues found in the monomer. From such data the significance of degradative chain-transfer in an allyl ester polymerization can be judged. For example, it was found that in the allyl ethyl carbonate polymerization, degradative chain-transfer predominates, whereas in the polymerization of allyl laurate and allyl benzoate, effective chain-transfer predominates [ 19]. The order of decreasing degradative chain-transfer constants for a series of allyl esters has been given as allyl ethyl carbonate > allyl acetate > allyl propionate > allyl laurate > allyl trimethylacetate [20]. Effective chain-transfer also predominates for allyl benzoate, allyl chloroacetate, and allyl chloride [20]. It is interesting to note that measurement of the rate of decomposition of dibenzoyl peroxide in various allyl esters showed that the decomposition rate decreased in the same order as the degradative chaintransfer constants decreased [21 ]. This indicates that the radicals formed during the initiation stage not only propagate and attack the allylic hydrogens as expected, but also the acyl portion of the ester group. The degree of polymerization of the isolated polymers increases as the degradative chain-transfer decreases [22]. However, a generalization that degradative chain-transfer decreases with increasing chain length of the carboxylic acid portion of the ester is a gross oversimplification. Thus, the allyl esters of undecanoic; 10- and 11-phenylundecanoic; 10,11-dibromoundecanoic; 11-iodounde-canoic; 12-hydroxystearic; and 12-ketostearic acid did not homopolymerize at all in either bulk or emulsion. Copolymers were not formed readily with either styrene or methyl methacrylate. However, with vinyl chloride, fairly homogeneous copolymers were formed. For M1 as allyl iodoundecanoate and M2 as vinyl chloride, at 60~ in benzene, rl was estimated to be 0.42 and r2 was 1.64 [23]. Chow and Marvel [23] attribute the fact that allyl esters copolymerized readily with vinyl chloride and not with liquid monomers to the pressure under which the vinyl chloride polymerizations were carried out (approximately 8 atm). They pointed out that Walling and Pelion [24, 25] had shown that a high pressure (e.g., at 8500 atm [85 MPa]) the overall rate of polymerization of allyl acetate had increased 50-fold yet the MW of the polymer had not changed materially over that formed at atmospheric pressure. This indicates that chain transfer is no longer degradative. Therefore, Chow and Marvel seem to reason, even at a pressure of 8 atm the pressure is sufficiently high that degradative chain-transfer can no longer interfere with copolymerization. The addition of zinc chloride to allyl acetate in equivalent amounts lead to increased polymerization rates upon exposure to 6~ radiation. The viscosity MW of the poly(allyl acetate) produced increases with increased levels of zinc chloride [24]. The zinc chloride may assist in activating allylic-free radicals and in developing an organized array of monomers for effective polymerization. In the presence of zinc chloride it has also been proposed that the probability of

287

3. Cyclopolymerization of Diallyl Esters

degradative chain-transfer taking place has been reduced while the initiation rate has increased. [27]. Even though monomers like allyl acetate do not polymerize rapidly nor produce products of high molecular weight, with active chain-transfer agents good yields of addition products may be isolated [28]. Among the chain-transfer agents proposed for such systems are chloroform and carbon tetrachloride. Sakurada and Takahashi [29], formulated the reaction as follows: "~CH2--CH" + CC14

I

; 'w'CH2--CHC1 + "CC13

I

CH2

CH2

O

O

I

I

I

I

C=O

(1)

C=O

I

I

CH3

CH3

Equation (1) represents a termination step for one particular chain with the simultaneous formation of a new free radical with considerable reactivity. This trichloromethyl radical is capable of initiating a new chain leading to further propagation if the initial concentration of carbon tetrachloride is low. This step is shown as 9CC13 + CH2=CH

} C13C--CH2--CH.

I

I

CH2

CH2

I

I

O

O

I

I

C=O

(2)

C--O

I

I

CH3

CH3

Thus, the chain length of polymers is shortened while the reaction itself is not terminated. Experimentally it was found that each polymer molecule contained between two and three chlorine atoms. With decreasing concentrations of allyl acetate in carbon tetrachloride, the MW of the polymer decreased [28, 29]. Another factor contributing to the formation of low-molecular-weight polymers of allyl ethers of dibasic acids is the tendency of some of these to cyclize. This process is discussed in greater detail in the next section.

3.

CYCLOPOLYMERIZATION

OF DIALLYL

ESTERS

The polymerization of allyl esters may suffer from the standpoint of practical applications because the resins formed usually have a low molecular weight, and are slow to form even in the presence of excesses of initiator at advanced temperatures. On the other hand, these same shortcomings make allyl ester polymers uniquely suitable for the study of the fine structure of the polymer. The

288


slow rates of polymerization permit studies involving changes that take place with increasing conversion. The requirement for high levels of initiator has been used to evaluate the effects and nature of end groups by conventional wetanalytical techniques [ 16]. The molecular weights of allylic polymers tend to be low (frequently their degree of polymerization is in the range of 20 mer units). This readily permits the use of colligative property measurements to establish molecular weights with a reasonable degree of assurance. Such determinations may then be correlated with molecular weight determinations based on viscosity measurements. The study of unconjugated diolefin polymerizations is usually considered quite difficult because complex, three-dimensional networks are set up at very low conversion. Such cross-linked materials are generally intractable. The situation is quite different in the case of diallyl esters, especially in the case of diallyl phthalates. Both diallyl o-phthalate and diallyl m-phthalate may be converted to "prepolymers," which are soluble in a variety of solvents. These two prepolymers are commercially availabe. When dissolved in their respective monomers and heated in the presence of typical initiators, such solutions are converted to cross-linked resins. Most of the shrinkage related to the conversion of monomers to polymers has taken place when the pre-polymer was formed originally. Therefore solutions containing relatively high levels of diallyl o-phthalate prepolymer shrink little on polymerization. It was observed that up to about 25% conversion, the polymerization of diallyl o-phthalate is linear with time and initiator concentration. As the process continues, a cross-linked gel forms. The polymer formed up to 25% conversion has a melting point of about 90~ is soluble, and is less unsaturated than would be expected for a linear polymer. Therefore, it was presumed that the prepolymers consisted of a main chain with a number of short branches [30]. In 1950, Haward [31 ] postulated that in these polymerizations the concentration of free radicals normally is low. In the reaction with diallyl o-phthalate, for example, intramolecular crosslinking is more likely than intermolecular reactions. The collisions between segments of the same molecule would lead to cyclic structural features. Regardless of conversion, up to the gel point, the soluble prepolymer (or "fl "-polymer to use the nomenclature of Simpson and co-workers) exhibited a degree of unsaturation corresponding to 25% of the unsaturation of monomeric diallyl o-phthalate. This observation, along with measurements of the degree of polymerization of poly(diallyl o-phthalate) at various conversions and the degree of polymerization determinations of poly(allyl acetate) produced by saponifying the diallyl o-phthalate prepolymer followed by reactylation of the isolated poly(allyl alcohol) by conventional methods, permitted Simpson and co-workers to develop considerable detail about the fine structure of this polymer [32].

3.

Cyclopolymerization of Diailyl Esters

289

It should be noted that in this work, Simpson and co-workers [32] report their viscosities in units of ml/gm (i.e., cm3/gm) rather than in units of dl/gm (i.e., dm3/gm) which we have used conventionally in this series. For their molecular weight-viscosity relationship, they make use of the expression LVN = 2.35 X 10-3M where LVN is the limiting viscosity number in cm3/gm and M is the molecular weight. To be noted is that the exponent of M is 1. In Table 1 this bulk polymerization data and certain calculated values are presented. The experiments used covered a range of conversions, including one preparation carried to a highly crosslinked state. If we consider the results given in Table I for Experiment No. 1, we find that the polymer molecule consists of chains with a DP of 19.4, joined by o-phthalate units. Each of these chains bears 5.8 tings per chain, leaving 13.6 monomer units as a linear array. It has been suggested that the formation of cyclic units is, in part, responsible for the difficulty in predicting the degree of conversion at which gelation takes place. In the case of diallyl o-phthalate, gelation takes place at a much higher degree of conversion than can be calculated on the basis of the modified Stockmayer equation [33]. The probability of cyclopolymerization by diallyl o-phthalate by various possible ring closures was calculated by Haward [34]. He predicted that 31% of the monomer units in the soluble polymer were cyclized. Experimentally, Simpson and co-workers [32] found that 41% were involved. This was considered excellent agreement between calculated predictions and empirical observations. Haward based his calculations on the assumption that the cyclic structures formed contained 11 or 13 members. Ziegler [35], Stoll and Rouve [36], and Prelog and co-workers [37], among others, had concluded long ago that 10- to 13-membered carbon tings have a low probability of forming stable structures, compared to 15- to 20-membered tings. Haward, however, indicated that this only had the effect of overestimating the contribution of the ring formation of this size compared to the probability of large tings forming. He implied that various other limitations of this proposal would affect his estimates in reverse order. In sum, all the simplifying assumptions would tend to balance each other. The fact is that experimental data confirmed his predictions. Considering various possible ring-closure reactions involving the action of one allyl radical on the other of a single diallyl o-phthalate monomer, Haward proposes four possible structures. Figures 1, 2, 3, and 4 give the skeletons of these cyclic structures with the points of attachment to a polymer chain (indicated in the figures by AN') and the location of the odd electron of the propagating free radical (indicated by ~). The structural designation x, y, z, and w are those used by Haward.

t~

TABLE I

Exp No 1 2 3 4

Reaction time (min) 65 155 245 2880 (48 hr)

Conversion (%)

No avg degree of polymerization of diallyl o-phthalate b

6.7 15.6 22.2 86

15.9 32.0 55.2 d

Polymerization of Diallyl o-Phthalate [32] a

Residual unsaturation (%) 28.01 26.64 26.02

No avg degree of polymerization of derived acetate C 19.4 19.6 19.8 (20.8)

n No of branches in polymer

R No of rings per polymer molecule

R/n No of rings per chain

1.17 2.40 4.12

6.8 13.6 23.3

5.8 5.6 5.8

a Polymerization conditions: Polymerization using benzoyl peroxide (11,200 grn/dm 3 of monomer, i.e., 1 wt%, 0.0463 moles dm 3) was conducted at 80 __+0.25~ with a variable reaction time under reduced pressure in sealed ampoules. b It is interesting to note that the increase in molecular weight per unit time for the first three polymers is essentially constant, 60, 50.7 and 55.4 Daltons/ min, with a mean of 55.4 Daltons/min. The values in this column represent the DP's for the total polymer. c See Procedure 8-2, p. 316, as to method of preparation. The DP's show the chain lengths of poly(allylic segments) which had been joined originally by the phthalate moieties. d Cross-linked polymer.


*--C

C I C --C --C --C - - O - - C O

C

f

291

C O - - O - - C --C = C

I C

C

I

I

C

c

I

I

co

co

Fig. 1

f

Structure x.

C I C --C--C --O--CO

C

C *-C

C O - - O - - C --C = C

C

I

I

CO

CO

Fig. 2

IC\

Structure y.

t C --C--C--C--O--CO

'w'C - - C

I

I

c

c

I

I

o

0

I

I

co

co

Fig. 3

Structure z.

~ccc-o-co "" C --C

I C

CO--O--C --C--C

C O - - O - - C --C = C

C

I C-*

I

I

0

0

I

I

CO

CO

Fig. 4

Structure w.

292

s. Polymerization of AUyl Esters TABLE II Proportion of Cyclic Structures Found in Diallyl o-Phthalate Prepolymers [34]

Structure

Figure

Cyclic structures (%)

x y z w

1 2 3 4

53.9 16.2 22.7 7.2

The proportions of these structures in the cyclized prepolymers were computed as shown in Table II. Using only one of these cyclic configurations, for purposes of illustration an approximate structure of a diallyl o-phthalate prepolymer near the gel point is given in Fig. 5. The cyclic units may be one or more of those shown in Figs. 1-4. The various monomeric units are probably more randomly distributed. It will be noted that in Fig. 5, the low-molecular-weight polymer chains are based, substantially, on the polymerization of only one of the two ally groups of each monomer with one allyl group of an adjacent monomer molecule. This mechanism of the polymer formation has been treated mathematically by employing "cascade formalism" [38, 39]. Hence, we may term the process a "cascade polymerization." The pendant allyl groups may occasionally form tings if they are at the free-radical end of a propagating chain ("incestuous polymerization" [40]) or participate in crosslinking reactions, making use of"spanning tree

t

~ C H

CO I O \ CH2 I CH

CO I O \ CHv I CH

CH2

CH2

//

I CH21

CH / I ~H2

O I CO

O I CO

CH I CH2 / O I CO

CO I O \ CH2 I

ctt //

//

/t/

CH2

Fig. 5 Approximate structure of diallyl o-phthalate prepolymer near the gel point. At the gel point: m, approximately 7-8; and m + n, approximately 17-18.


293

approximations" [38, 39]. These picturesque mixed metaphors are useful in visualizing the course of the cross-linking polymerization. Of course, also to be included in this list of reactions is degradative chain-transfer along with the usual initiation, termination, and more ordinary propagation steps. It will be noted, on examining Fig. 5, that the cyclic portions of the polymer are incapable of cross-linking. In other words, approximately 40% of the polymer cannot undergo this reaction. This fact contributes to the deviation from the theoretical gel point calculation in the direction of gelation taking place at an unusually high degree of conversion [40]. When the conversion at gel-formation was studied for the diallyl esters of isophthalic, terephthalic, oxalic, and sebacic acid, it was found that all systems gelled at approximately 25% conversion of the monomer to the polymer. All polymers, on saponification and reacetylation of the resultant poly(allyl alcohol) gave rise to chains with DPn in the range 18.6 to 20.6. These chain lengths appear to be independent of the degree of conversion of the polymer under investigation. Whereas the extent of residual unsaturation of poly(diallyl o-phthalate) was approximately 26%, for the other four polymers in this series, the unsaturation value was approximately 40%. With increasing conversion, the unsaturation of poly(diallyl terephthalate) decreased considerably while that of poly(diallyl o-phthalate) and of the other three monomers decreased only slightly as the conversion increased [41 ]. From these observations it was concluded that, of these diallyl esters, only diallyl o-phthalate gave rise to highly cyclized structures, although the extent of cyclization in the other systems is not negligible. The chain length, when expressed in terms of the number of reacted double bonds is significantly less for poly(diallyl o-phthalate) than the other polymers. All systems gel at the same degree of conversion regardless of monomer. Cyclization does not seem to affect gelation as expected from theory. The satisfactory correlation to the gelation of diallyl o-phthalate is considered fortuitous [41 ]. It should be noted here, that a prepolymer of diallyl isophthalate is commercially available. Its use is similar to that of the ortho-isomer although the properties of the final crosslinked resin are somewhat different. This may be the result of the fact that only the poly(o-phthalate) is markedly cyclized [42]. The terephthalate has not been studied very extensively. However, it has been found that diallyl terephthalate initially does not cyclize, as the conversion increases, more than one crosslink per chain is formed [42, 43]. The termination step of the polymerization of diallyl o-phthalate involves degradative chain-transfer to fresh monomer, yet a significant fraction of the radicals so produced are capable of reinitiating chain-propagation steps [42]. By studying the changes in the infrared absorption band at 1645 cm-1 (an olefinic double bond stretching band) with time, the rate, order of initial polymerization reaction, activation energy, and the extent of the reaction could

294

8. Polymerization of Allyl Esters TABLE III Initial Degrees of Polymerization of Diallyl Esters of Aliphatic Dicarboxylic Acids [46] a Diallyl ester of Oxalic acid Malonic acid Succinic acid Adipic acid Sebacic acid

DP~ 85 70 71 61 54

a Polymerization conditions: bulk polymerization using dibenzoyl peroxide as the initiator. The polymerization was conducted under reduced pressure in sealed ampoules.

be studied well beyond the gelation [44]. It has been stated that the gel point of diallyl o-phthalate could be predicted with a reasonable degree of certainty by taking into account the relative rates of entry of the doubly unsaturated and the singly unsaturated monomer units along with those units in which two reacted allyl groups exist in the same polymer chain [44]. In dilute solution, it was found that even diallyl terephthalate was capable of undergoing intramolecular cyclization. In general, with decreasing monomer concentration in a solvent, such as benzene, the possibility of cyclopolymerization increases [45]. Because of the ease of synthesis and industrial importance of diallyl esters much of the research has dealt with the behavior of the isomeric phthalates. Some other dicarboxylic acid esters have been studied by Simpson and Holt [41 ]. The kinetics of the polymerization of the diallyl esters of oxalic, malonic, succinic, adipic, and sebacic acid have also been considered. In previous kinetic studies, no differentiation was made between the behavior of the uncyclized monomer (or its free radical) and of the cyclic free-radicals. A priori, differences should have been presumed, but evidently Matsumoto and Oiwa [46] were the first seriously to attempt a kinetic analysis based on the concept that the linear and the cyclic units are two different species. In effect, these two species copolymerize with each other. However, the analysis has not been carried so far as to determine reactivity ratios. In the bulk polymerization of the diallyl esters of the aliphatic dicarboxylic acids, the initial degrees of polymerization are quite large (Table III). In these data, it should be emphasized that the degree of polymerization___was determined by osmometry and refers to the total polymer and not to the DPn of chains obtained upon saponification of such polymers. The ratio of the unimolecular rate-constant of the cyclization to that of the bimolecular propagation-reaction of the linear free-radical has been given the

295

4. Polymerization of Allyl Acetate TABLE IV Ratio of Rate of Cyclization to Rate of Bimolecular Propagation, Kc, and Overall Activation Energies of Polymerization of Diallyl Esters of Aliphatic Dicarboxylic Acids [46]a

Diallyl ester of Oxalic acid Malonic acid Succinic acid Adipic acid Sebacic acid

Kc (moles/liter)

Overall activation energy of polymerization (kcal/mole)

3.6 3.2 2.8 2.5 1.2

21.1 24.2 21.7 22.0 22.2

a Polymerization conditions: bulk polymerization using dibenzoyl peroxide as initiator. The polymerization was conducted under reduced pressure in sealed ampoules.

symbol Kc. This term is a measure of the tendency of the molecule to cyclize. It was found (cf., Table IV) that with increasing separation between the carboxylic acid groups of the dicarboxylic acid, Kc decrease and, therefore, the tendency to cyclize decreases. Table IV also lists the overall activation energies of polymerization [46]. In attempts to copolymerize various diallyl esters with styrene it was found that the reactivity was quite low. Further, the rate and degree of copolymerization was roughly inversely related to the concentration of diallyl ester. With increasing styrene levels, the tendency toward cyclopolymerization decreased [47]. In effect, the diallyl esters act as chain-transfer agents for the polymerization of styrene. Table V gives the chain-transfer constants and the reactivity ratios for the copolymerization with styrene. It is noteworthy that the formation of cyclic structures is not confined to diallyl esters. For example, Haward and Simpson found evidence for cyclopolymerizations at low conversion of solutions of divinylbenzene in styrene [48]. Diallyl ammonium halides were among early examples of cyclopolymerizations [49]. Two reviews of interest are Marvel [50] and Butler [51]. In their highly mathematical treatment of cyclization in cross-linking polymerizations, it is interesting to note that Du~ek and Ilavsk~ [38, 39] found that the data of Simpson and Holt [41], gathered 20 years earlier, were quite consistent with their analysis.

4.

POLYMERIZATION OF ALLYL ACETATE

The polymerization of acrylic and vinyl esters generally proceeds quite rapidly, with relatively low levels of initiators, to form products of considerable molecular weight. By contrast, allyl esters polymerize slowly with high levels of

296

8. Polymerization of AUyl Esters

TABLE V Chain-TransferConstants, Cs, and Copolymerization Reactivity Ratios of Diallyl Esters with Styrene (M1) [7] Reactivity ratiosa Diallyl ester of (M2) Carbonic acid Oxalic acid Malonic acid Succinic acid Adipic acid Sebacic acid o-Phthalic acid Isophthalic acid Terephthalic acid

G (X 104)

rI

r2

6.2 4.2 5.2 5.4 6.0 4.8 6.3 3.5 4.5

55.0 59.6 71.8 71.0 70.0 65.6 53.4 53.0

0.052 0.050 0.053 0.044 0.052 0.029 0.046 0.034

a Determined for only one functional group of the diallyl ester. initiators to give polymers of low molecular weight. When one considers the difficulties involved in the synthesis of vinyl esters as compared to the ease with which allyl esters may be prepared, this is a rather unfortunate circumstance. As a result, very few allyl esters have any industrial uses as polymers or copolymers. The polymerization of allyl acetate has been studied reasonably extensively in the effort to elucidate the mechanisms of the polymerization process. The utility of poly(allyl acetate) as a resin is believed to be negligible. Nevertheless, this compound has been polymerized in bulk, in solution, and in emulsion. There is mention of a cationic polymerization process and of radiation-induced chargetransfer processes. The ordinary, free-radical induced bulk polymerization of this monomer appears to be only modestly affected by the presence of atmospheric oxygen (cf. Table VI). Despite these observations, Bartlett and Altschul [ 16] routinely used degassing (Procedure 4-2) for their experiments. Additives such as water, dilute hydrochloric acid, or a trace of pyridine appear to have no effect on the rate of polymerization of allyl acetate [16]. As expected, at a constant polymerization temperature and constant time, with increasing percentages of initiator, the conversion of monomer to polymer increases. In Table VII this effect of the concentration of benzoyl peroxide on the polymer formation is shown. The reaction times, given in this table vary slightly. The data are taken from a more extensive study [ 17]. In connection with Table VII it should be noted that at 80~ 48 hr represents approximately 12 half-life periods for dibenzoyl peroxide and 45 hr is approximately 9 half-life periods for di(p-chlorobenzoyl)peroxide. This means that in

4. Polymerization of Allyl Acetate

TABLE VI

297

Effect of Supernatent Oxygen on the Formation of Poly(allyl acetate) [16] a Conversion (after 12 hours)

(%)

Degassing procedure 1. Air in ampule, no agitation 2. Ice-cooled monomer was evacuated at the water pump five times at approximately 20 mm Hg; with purified nitrogen flushing between evacuation. Monomer finally sealed under purified nitrogen 3. Procedure similar to 2 except that cooling was with dry ice-ethanol and evacuation was with an oil pump at approximately 2 mm Hg 4. Procedure similar to 3 except that thawing and refreezing step followed each evacuation 5. Procedure similar to 4 except that monomer was cooled with liquid nitrogen and evacuation was with a diffusion pump to less than 1 0 - 4 m m Hg

11.2

12.5 12.2 12.4 12.5

~ Polymerization conditions: Polymerization using dibenzoyl peroxide as initiator (1.12% of the monomer system) was conducted at 80.0 ~ • 0.2~ in sealed ampoules for 12 hr with degassing procedure as indicated.

both cases, the initiator has been essentially completely destroyed at the indicated time. The bulk polymerization of allyl acetate using 2,2'-azobisisobutyro-nitrile proceeds in the same manner at 80~ as similar polymerizations involving the use of diacyl peroxides [52]. CAUTION" care.

Allyl compounds are toxic and should be handled with great

TABLE VII

Effect of Peroxide Concentration on the Conversion of Allyl Acetate to Polymer [ 17] a

Concentration of dibenzoyl peroxide (%)

Polymerization time (hr)

1.01 2.14 6.10 9.95 Avg

5.906

tpol =

Polymer formed (%)

49.0 47.5 48.0 45.1

12.0 25.2 49.0 70.9

47.4 46.5

50.3

a Polymerization conditions: Polymerization using dibenzoyl peroxide (concentration given in first column) was conducted at 80.0 ~ _+ 0.3~ in sealed tubes degassed by Procedure 2 of Table VI. b Di(p-chlorobenzoyl)peroxide instead of dibenzoyl peroxide.

298


In the example of bulk polymerizations cited here, the polymer isolation procedure follows that of Litt and Eirich [14] since they maintain that the original procedures of Bartlett and co-workers did not adequately remove volatile, low-molecular-weight components from the polymers. Consequently, the MW determinations by Bartlett and co-workers are thought to be low.

4-1.

Bulk Polymerization of Allyl Acetate in Sealed Tubes [16, 17]

In a constricted test tube is placed a solution of 0.603 gm of di(p-chlorobenzoyl)peroxide in 9.61 gm of allyl acetate. The sample is degassed five times at ice temperature with a water aspirator at 15-20 mm Hg. The tube is flushed with highly purifed nitrogen between each degassing step. The tube is sealed under nitrogen pressure. After placing the sealed tube in a protective metal sleeve, the assembly is heated in an oil bath at 80 ~ for 46.5 hr. After this period, the tube is cooled to room temperature, then chilled in an ice bath, and cautiously opened. The reaction product is dissolved in 40 ml of reagent-grade benzene and extracted with four portions of a 35% ice-cold aqueous potassium carbonate solution. The last aqueous extract gives no precipitate upon acidification. The benzene layer is neutral. The organic layer is separated and, if necessary, filtered. The benzene is evaporated off in a slow stream of clean dry air at room temperature. The residue is dried for at least three days at 60~ at a pressure of 0.3 mm Hg to constant weight. To follow the polymerization of allyl acetate dilatometrically, Bartlett and Tate [15] made use of Adams brand Wintrobe hemacrit tubes which are fitted into a vapor thermostat. The hemacrit tube was calibrated in millimeters and had a total capacity of 1 ml. Degassed monomer solutions were transferred to the Wintrobe hemacrit tube (which had been degassed and prepared with a nitrogen atmosphere) by means of syringes. The apparatus permitted observation of the volume shrinkage and hence the progress of a polymerization. Use of a hemacrit tube as a low-capacity dilatometer may be of interest in other studies involving the polymerization of many monomer systems. Presumably hermacrit tubes are relatively inexpensive compared to precision-bored glass capillary tubes frequently used in the construction of dilatometers. The solution polymerization of allyl acetate was studied in an effort to determine the effect of monomer concentrations on the reaction kinetics [14]. These studies were limited to the use of benzene. The growing allyl acetate radicals formed stable adducts with the solvent much as vinyl acetate does. The stabilized adduct is terminated by combination with a growing radical. The twinning reaction is said to account for the relatively high molecular weight of the polymer despite the fact that the reaction with benzene is a chain-transfer reaction. Ethyl acetate, on the other hand, would have been a preferable solvent

4. Polymerization of Allyl Acetate

299

since chain transfer occurs only slowly and the growing chains are re-initiated relatively rapidly in this solvent. Procedure 4-2 outlines one technique used in the solution polymerization of allyl acetate.

4-2.

Solution Polymerization of Allyl Acetate [14]

In a stirred apparatus which allowed material to be degassed and polymerized (as well as permitting the removal of samples from time to time without exposure to the atmosphere) is placed a solution of 1.17 gm (0.00826 mole) of dibenzoyl peroxide and 50 grn (0.5 mole) of allyl acetate in 48.8 gm of benzene. The solution is cooled to - 8 0 ~ with dry ice-acetone and degassed to 0.3 mm Hg. The monomer solution is allowed to thaw. The pressure is then brought up to atmospheric pressure with nitrogen which had been prepurified by passage through Fieser's Solution. A mercury safety valve is used to regulate the gas pressure. The monomer solution is then refrozen and degassed three more times. The apparatus containing the degassed solution, at atmospheric pressure, under nitrogen, is then placed in a water bath and stirred. For kinetic measurements, the starting time for the reaction is considered to be 3 min after placement in the bath at 65~ 4 min at 80~ and 5 min at 90~ Samples of the reaction mixture are withdrawn at appropriate intervals. The reaction solution is heated for at least 6 half-life periods of the initiator. The polymer solution is diluted to a workable viscosity with benzene and extracted three times with 5% aqueous potassium hydroxide solution, so that a sample of the next to the last aqueous extract gives no precipitate upon acidification. The benzene solution is then repeatedly washed with deionized water until the washings have the same (near neutral) pH as the fresh, deionized water being used. The benzene solution is carefully separated and filtered into a tared beaker. The solvent and monomer is allowed to evaporate at room temperature. The residue is dried to constant weight for 3 days at 60~ and 0.3 mm Hg. In the polymerization of typical vinyl and acrylic esters, the molecular weights of polymers produced by emulsion-polymerization processes are generally substantially greater than those produced in bulk or in solution. The bulk polymers produced by treating allyl acetate with peroxides or with 2,2'-azobisisobutyronitrile have molecular weights in the range of only 1400-3000. It was therefore important to study emulsion polymerization of this monomer in an effort to produce a substantially higher-molecular-weight resin. The results show that even by this procedure, the degree of polymerization remained in the range of 13.6-14.3 (i.e., MW 1360-1430). The polymers contained sulfate end-groups, presumably from the persulfate initiator used [18]. Unfortunately, the experimental details for this work are not very clear. The work was done by shaking

300

8.

Polymerization of Allyl Esters

the emulsion recipe in sealed tubes in an oil thermostat at 80~ Bartlett and Nozaki [18] tabulated data for two experiments. In Experiment 1, 9.28 moles of allyl acetate per liter of latex was emulsified with 0.087 mole of sodium lauryl sulfate, buffered with 0.45 mole of sodium pyrophosphate, and initiated with 0.0920 mole/liter of potassium persulfate. In experiment 2, again 9.28 moles of allyl acetate per liter of latex was polymerized in the presence of 0.087 mole of sodium lauryl sulfate, 0.45 mole of sodium pyrophosphate, and 0.366 mole/liter of potassium persulfate. Since the MW of allyl acetate is 100, the above information implies that the basic monomer to water ratio is an unlikely 928 gm of monomer to approximately 70 gm of water. If indeed these are the experimental facts, then the fact that the polymers produced resembled those produced in bulk or in solution is not surprising. A reaction mixture consisting of nearly 93% pure monomer, naturally would be expected to produce a polymer similar to one produced from a pure (i.e., bulk) monomer and not one similar to an emulsion polymer. Compositions of less than 60% monomer in water would ordinarily be expected to produce latices. Perhaps the data in question refer to a ratio of 9.28 moles of monomer to one liter of water. By the way, it should be noted that Barlett and Nozaki included sodium pyrophosphate in their formulation to counteract the decrease in pH as a persulfate-initiated polymerization proceeds. At a low pH, the monomer is said to be susceptible to hydrolysis [18]. Work by R~.nby and co-workers has indicated that the nature of the addition of free radicals such as HEN- and OH. is profoundly influenced by changes in the pH of the medium from 1.4 to approximately 7.8 [53-55]. The radicals formed from persulfate in emulsion systems may be expected to be influenced by pH variations also. One patent reports the use of a "seed"-polymerization technique to produce an allyl acetate-ethyl acrylate copolymer [56]. In a typical example, to 305 gm of a 3.28% solution of poly(vinyl alcohol) (DP 1000) is added 10 gm of allyl acetate and 0.8 gm of potassium persulfate. The mixture is heated to 75~ followed by the addition of 190 gm of ethyl acrylate with heating for 2.25 hr at 80~176 After stirring for an additional hour and cooling to room temperature, the resultant latex is claimed to exhibit 99.6% conversion of all of the monomer to polymer. The pH of the latex is said to be 2.5. Cationic polymerizations of allyl esters require further investigation. Schildknecht [7] mentions the polymerization of allyl acetate and of allyl formate in benzene solution upon heating the solution with 6% of benzenediazonium fluoroborate under nitrogen at 200~ in sealed tubes. The solid products melt at about 190~ (d) and are soluble in benzene and insoluble in methanol. A 10% solution of allyl acetate in petroleum ether treated in a dry ice-acetone bath with gaseous boron trifluoride for several hours gave rise to a solid polymer with m.p. approxiamtely 145~ The product exhibited a certain amount of

5. Polymerization of Allyl Esters of Higher Monocarboxylic Acids

301

crystallinity. A poly(allyl trichloroacetate) formed under similar conditions had m.p. 330~ without decomposition [57]. The slowness of these polymerizations is rather surprising for cationically initiated processes. Zubov and co-workers observed that photo-induced polymerizations of a number of monomers were accelerated by certain metallic compounds. Allyl acetate, for example, exhibited an increased rate of polymerization in the presence of zinc chloride. It was postulated that a mobile, organized assemblage of monomers was involved in activating the allyl radicals. This may well be an example of a charge-transfer process [58]. In another paper, Zubov and co-workers [59] observed that ordinarily, radiation-initiated polymerizations of allyl acetate proceed sluggishly to low-molecular-weight products. However, upon the addition of phosphoric acid to the system, the rate of polymerization increases and solid polymers can be isolated. The proposed mechanism for this process involves the postulation of the formation of oligomers of allyl acetate with residual double bonds to which the phosphoric acid adds. This is thought to activate the polymerization of the oligomer. Other possible hypotheses for the activation by phosphoric acid involves chain-transfer reactions or the formation of complexes of phosphoric acid with allyl acetate monomer and allyl acetate free radicals [59]. 5.

POLYMERIZATION

OF ALLYL ESTERS OF

HIGHER MONOCARBOXYLIC A C I D S The synthesis of vinyl esters of the higher monocarboxylic acids is troublesome and costly. The preparation may involve transvinylation with vinyl acetate and the higher carboxylic acid with a costly mercuric salt as a catalyst. In this procedure, the equilibrium situation is unfavorable and yields of product usually are low. Altematively, a similar catalyst may be used for the addition of acetylene to the carboxylic acid. This procedure requires pressure equipment not usually available in a synthesis laboratory. The allyl esters, on the other hand are readily prepared by conventional esterification procedures. Therefore, they are of potential interest as plasticizers which might copolymerize with resins such as vinyl acetate that normally give rise to hard resins. Unfortunately, allyl esters of more ordinary carboxylic acids homopolymerize sluggishly. Their ability to enter into copolymer systems also seems to be marginal. Swem and Jordan [60] prepared the allyl esters of caproic, caprylic, pelargonic, capric, lauric, myristic, palmitic, and stearic acids. Attempts to bulk polymerize the compounds with 0.5% of dibenzoyl peroxide resulted in only a slight lowering of the monomers' iodine numbers, indicating very limited polymer formation. Copolymers with diallyl o-phthalate seemed to form with 1-20% of allyl esters. These copolymers were insoluble in acetone, amyl acetate, benzene, and acetic acid.

302

S. Polymerization of Allyl Esters

If the initiator concentration is reasonably high and heating is carried out for a sufficiently long time, allyl esters of fairly long-chain carboxylic acids have produced polymers. For example, after 24 hr at 80~ with 2% dibenzoyl peroxide, a 52% conversion of allyl stearate to its polymer is obtained [61]. Under similar conditions, conversion of allyl oleate is only 19%, allyl linoleate 5%, and allyl 10,12-octadecadienoate 3%. When allyl linoleate was heated with di-tert-butyl peroxide at 130~ for 24 hr, 21% of the monomer was converted to a polymer which was found to be partially soluble in methanol. It was postulated that under the reaction conditions, copolymerization of the allyl group of one molecule with a double bond of the acid portion of another molecule has taken place [61 ]. With a high-temperature initiator such as di-tert-butyl peroxide, copolymers of 1-octene with allyl propionate or allyl butyrate have been prepared in sealed tubes. The reaction conditions included methyl ethyl ketone as a solvent, 0.05% di-tert-butyl peroxide heated in sealed ampoules at 200~ for 4 hr. The molecular weights of the product were in the range of 600 _ 200 [62]. Allyl esters of long-chain carboxylic acids such as undecanoic: 10- and 11-phenylundecanoic; 10,11-dibromoundecanoic; 11-iodoundecanoic; 12-hydroxystearic; and 12-ketostearic acids do not copolymerize significantly with either styrene or methyl methacrylate. On the other hand, reasonably high conversions with significant concentrations of the ester in the copolymer are formed when these allyl esters are copolymerized with vinyl chloride [23]. Since the pressure inside the copolymerization apparatus is estimated to reach 8 atm or more at 60~ when vinyl chloride is the co-monomer, it is postulated that the effect observed by Walling and Pellon [24, 25] (that degradative chain-transfer is reduced at high pressure) comes into play here. While we believe that the pressure generated may be rather low for this Walling effect, the observed copolymerization is an experimental fact of significance. Generally, as expected, the rates of copolymerization are slow and conversions are low. Procedure 5-1 details the preparation of a copolymer of vinyl chloride and allyl 10,11-dibromoundecanoate. It should be noted that the directions are based on Chow and Marvel [23] which goes back to 1968, before the hazards of vinyl chloride monomer were appreciated. Therefore, no safety precautions which would be meaningful today are given. In fact, the whole concept of heating a sealed bottle containing vinyl chloride under pressure ~should be considered unsafe. On the positive side, the procedure does give details on a fractionation procedure for the copolymer. It is interesting to note that the fractions isolated by this procedure varies in composition~approximately 70% of the polymer (i.e., the high-molecular-weight fractions) was reasonably constant in composition while the low-molecular-weight fractions were richer in vinyl chloride.

5. Polymerization of Allyl Esters of Higher Monocarboxylic Acids

5-1.

303

Copolymerization of Vinyl Chloride and AUyl lO,11-Dibromo-undecanoate and Fractionation of the Copolymer [23]

NOTE: The directions given here are for reference only and require modification to conform with applicable safety regulations for the handling of the known carcinogens, vinyl chloride monomer and benzene. Read the MSDS for each chemical. In a tared pressure bottle of approximately l l0-ml capacity (i.d. 1.5 in. x length 7 in.) are placed 5 ml of benzene, 5.0 gm of allyl 10,11-dibromoundecanoate, and 0.10 gm of 2,2'-azobisisobutyronitrile. With proper safety precautions about 6 gm of vinyl chloride is charged to the bottle. This mixture is degassed three times to approximately 1 mm Hg and then filled with prepurified nitrogen. Then the excess of vinyl chloride is distilled off to leave 5.0 gm of vinyl chloride in the composition. The bottle is capped and placed in a protective metal sleeve. The polymerization bottle is tumbled end-over-end in a water bath at 60~ for 44 hr (approximately 2 half-life periods). The bottle is then cooled in a dry ice-acetone bath, opened, and carefully vented as the content warms to room temperature (precautions against exposure of personnel and the environment to the carcinogen vinyl chloride must be taken). The product is precipitated by the addition of methanol. The polymer is dried to constant weight under reduced pressure. Conversion is 57%, inherent viscosity 0.24 (in 0.2% solution in THF at 30~ 47.3% ester in polymer.

Polymer Fractionation A 4.465-gm sample of the polymer is dissolved in 70 ml of THF in a 200-ml beaker. To the solution is added 70 ml of methanol and 2 drops of concentrated sulfuric acid. After allowing the mixture to settle for 24 hr at room temperature, the first fraction is isolated by decanting the solvent into another beaker. The polymer in the first beaker is dried to constant weight. The solvent in the second beaker is treated with 30 ml of methanol and again allowed to settle for 24 hr to produce a second fraction of the polymer. This polymer fraction is isolated as the previous fraction and dried. To the filtrate is added 70 ml of methanol to yield a third fraction. The last fraction is obtained by adding 100 ml of methanol to the filtrate. Thus, 4.465 gm of polymer is dissolved in 70 ml of THF and precipitated with a total of 270 ml of methanol. Table VIII gives details of the properties of these fractions. With vinyl chloride (caution: known carcinogen) as the major component, copolymers of allyl laurate and vinyl chloride have been prepared by a suspension-polymerization procedure. In a typical formulation, 2.5 gm of allyl laurate,

8. Polymerizationof Allyl Esters

304

TABLE VIII Fractionation of a Vinyl Chloride-Allyl 10,11-Dibromoundecanoate Copolymer [23]

Fraction No.

Cumulative volumes (ml) of methanol added to 4,465 gm of Copolymer in 70 ml of THF

1 2 3 4

70 100 170 270

a

Polymer recovered (%)

Inherent viscosity of fraction (at 30~

Allyl ester content (%)

0.26 0.21 0.13 0.07

54.4 53.8 45.3 44.9

11.4 60.1 16.8 5.0a

93.3% recovery of polymer from the fractionation.

200 gm of water containing 0.05 gm of poly(vinyl alcohol), and 97.5 gm of vinyl chloride are heated with 0.07 gm of lauryl peroxide at 65~ [63]. In copolymerizations involving allyl esters, chain transfer between the allylic portion of the comonomer and the growing chain leads to termination of the growing polymer and, consequently, to low molecular weight products. To reduced the possibility of chain transfer by abstraction of the allylic hydrogen during copolymerization with vinyl acetate, the effect of varying the electron density around the alpha carbon atom was studied. This involved the copolymerization of vinyl acetate with a variety of allyl esters. In Table IXa the esters are listed with their reactivity ratios and the Alfrey-Price Q - e parameters for the copolymerization of these monomers with vinyl acetate. The polymerizations were carded out by a bulk process in sealed, heat resistant bottles at monomer ratios ranging from equi-molar to 9 mol of the allyl monomer to one of vinyl acetate with 0.1% by weight of dibenzoyl peroxide at 60~ for up to 168 hr in some cases. Even so, the polymer yields were less than 5% [64]. The molecular weights of the copolymers ranged from 10,000 to 150,000. It is to be noted that in every case, the product of rl r2 is less than unity. Therefore each monomer pair can copolymerize easily. However, at mole percentages of TABLE IXa ReactivityRatios and Q-e Parameters for Copolymerization of Allyl Esters (MI) with Vinyl Acetate (M2) [64] Allyl monomer

rl

r2

rl r2

Allyl acetate Allyl propionate Allyl butyrate Allyl isobutyrate Allyl trimethylacetate Allyl valerate

0.70 0.42 0.64 0.51 0.34 0.58

1.00 1.29 0.97 1.04 1.15 1.07

0.70 0.54 0.62 0.53 0.39 0.62

e -

1.48 1.66 1.57 1.68 1.85 1.57

Q 0.044 0.040 0.049 0.050 0.053 0.045

305

6. Polymerization of Allyl Acrylate and Methacrylate TABLE IXb Reactivity Ratios Allyl Acetate (M~) with Other Monomers (M2) [65] M2

rl

r2

El r2

Methyl methacrylate

0.024 +_0.009 0.04 +__0.04 0.021 _0.001

41 6 11.7 1 66 4

0.98

Butyl acrylate Styrene

0.47

75 or more of allyl valerate, little copolymerization took place. In the case of the allyl trimethylacetate-vinyl acetate copolymerization, deviations from ideal random copolymerization were noted. Some recent NMR studies on the copolymerization of allyl acetate with methyl methacrylate, butyl acrylate, and styrene reported their reactivity ratios (cf. Table IXb) [65]. The reported error terms, if we assume them to be standard deviations, are quite large with respect to the rl term. Therefore it is problematic whether these numbers are really meaningful. It would seem to us that the large r2 terms imply that substantially only homopolymers of these three "vinyl" monomers form. This situation is modified in the case of allyl methacrylate or allyl acrylate copolymers, as will be mentioned below. With these acrylic derivatives, copolymerization depends on the acrylic bonds primarily with modifications due to the allylic hydrogen. Subsequently, the allylic units in a copolymer of allyl methacrylate with butyl methacrylate, for example, will be the sites for crosslinking. 6.

POLYMERIZATION METHACRYLATE

OF ALLYL ACRYLATE AND

Allyl acrylate and methacrylate are reasonably simple to synthesize. In fact, allyl methacrylate is manufactured in bulk quantities. Since each of these monomers contains two double bonds of distinctly different susceptibility to polymerization when considered individually, these monomers are interesting both from the practical as well as the theoretical standpoint. Early interest in allyl methacrylate arose from the concept that it was possible that this monomer would copolymerize with methyl methacrylate through its methacrylate bonds. The resulting resin could then be shaped, for example by injection molding, since it was still thermoplastic. After the molding had been completed, a separate heating cycle would bring about crosslinking through the allyl bonds to produce hard thermoset materials. Early applications in the production of plastic dental prostheses were visualized.

306

& Polymerization of AIlyl Esters

The use of these monomers for radiation cross-linking of polyethylene has been suggested [66]. With benzophenone as a photosensitizer, atactic as well as isotactic polypropylene is crosslinked with allyl acrylate by UV radiation. In this process both types of double bonds react [67]. Elastomers such as ethylenevinyl acetate copolymer have been cross-linked with this monomer on a roller mill at 150~ using dicumyl peroxide as the initiator. Such cross-linked elastomers exhibit little or no swelling with aromatic solvents or chloroform after 24 hr at 30~ conditions under which the uncured elastomers ordinarily dissolve [68]. Despite these interesting applications for such monomers, the bulk of the commercially produced allyl methacrylate finds application as a synthetic intermediate rather than as a monomer. One of the early theoretical studies of the polymerization of allyl acrylate considered the "homopolymerization" of this monomer to be an "intramolecular copolymerization" of the allyl and the acrylic double bonds. In this process it was calculated that only about 3% of the allyl groups participated in the process. Upon extended heating, the residual allyl groups served as cross-linking sites [69]. An examination of the earlier allyl methacrylate literature by Butler [51] indicated to him that the results reported by earlier investigators may be explained, in part, by assuming that cyclization of allyl acrylate or methacrylate takes place to some extent before gelation takes over. Blout and co-workers [70, 71] studied the initiation of allyl methacrylate using biacetyl and UV radiation at low temperatures and using benzoyl peroxide at higher temperatures. At I~ gelation occured only after 39% conversion, whereas gelation at higher temperatures took place at 6% conversion. Data on the unsaturation of the soluble prepolymers might be used to postulate cyclization polymerizations. At the time of this work, however, this concept had not yet been established, hence this factor was not considered. The bulk polymerization of allyl acrylate with benzoyl peroxide as initiator to 10% conversion gave a brittle, glassy polymer which was considered partially cyclized with a Kc (ratio of rate of cyclization to the rate of bimolecular propagation, cf., Table IV) of 0.41 moles/liter [72]. The polymer was described as soluble in both toluene and in carbon tetrachloride. Two possible structure of the cyclic radicals which may be involved in the polymerization of allyl methacrylate are given in Fig. 6. Procedure 6-1 is an example of the solution polymerization of allyl methacrylate using benzoyl peroxide as initiator. The polymer is said to be thermoplastic.

6-1. Solution Polymerization of Allyl Methacrylate

[74]

In a 100-ml, round-bottom flask fitted with a reflux condenser, a solution of 10 gm of allyl methacrylate and 0.7 gm of recrystallized benzoyl peroxide in

307

6. Polymerization of Allyl Acrylate and Methacrylate

Ha/cH_.2 R --CH2-- C CHI I O --C ~ O / C H 2

CH3 I R --CH2-- C ~ C H --CH2" I I O - - C ~O ./CH2

(a)

(b)

Fig. 6 Structures of cyclic allyl methacrylate free radicals [73]. (a) cLlactone unit with carbonyl stretch frequency at 1740 cm-l and 1230 cm-i. (b) y-lactone unit with carbonyl stretch frequency at 1775 cm -1 and 1275 cm - l

56 gm of dry acetone is heated at reflux for 1.5 hr. The solution is then cooled and carefully poured with vigorous stirring into 1 liter of methanol. The precipitated polymer is collected on a filter, dried under reduced pressure at room temperature. The product is fusible and soluble in acetone. Upon heating under a slight pressure at 90~ the polymer fuses and converts to an insoluble, infusible material. By conventional emulsion polymerization procedures, a crosslinked copolymer of butyl acrylate and 1% allyl methacrylate was formed in the presence of methyl methacrylate. Then additional methyl methacrylate is polymerized in the system. The resultant product was a poly(methyl methacrylate) with improved impact resistance [75]. In toluene solution, syndiotactic block copolymers of methyl methacrylate (PMMA) and allyl methacrylate were formed using triphenylphosphine and triethylaluminum as initiator [76]. In acetone solution, syndiotactic poly(methyl methacrylate) forms a stereocomplex with other syndiotactic polymers. The complex formed with syndiotactic poly(allyl methacrylate), upon separation from the reaction mixture and drying had a melting point of 141.5~ by DSC thermogram. From X-ray powder patterns of this and related complexes of PMMA with other polymethacrylates, the authors postulate that a doublestranded helix may represent a model of the structure of these complexes [77]. Oligomerization of allyl methacrylate apparently takes place when the initiator is either triethylaluminum or chlorodiethylaluminum. If dichloroethylaluminum was used as a catalyst, only Friedel-Crafts type alkylations took place [78]. Copolymers containing allyl methacrylate have found application as an additive to other resin to enhance the properties of the system. For example, in one patent disclosure, an aqueous emulsion polymer was formed in water using 0.03 gm of sodium carbonate, 50 gm of methyl methacrylate, 2.0 gm of ethyl acrylate, and 0.1 gm of allyl methacrylate, and 0.40 gm of the sodium salt of an allyl C13-alkyl ester of sulfosuccinic acid. The polymerization was initiated with sodium persulfate and heated at 83~ for 1 hr. To this latex, 40 gm of butyl acrylate, 10 gm of styrene, 1.0 gm of allyl methacrylate, and another 0.40 gm of the above surfactant were added while polymerization continued. In a third

308

& Polymerizationof Allyl Esters

stage, more methyl methacrylate and ethyl acrylate were added. The resultant latex was dried, ground to a fine powder and with melting and kneading combined with a methyl methacrylate-co-ethyl acrylate resin. The final composition was said to exhibit improved physical properties [79]. While this patent is somewhat overly complex, our main point is that evidently allyl methacrylate may be incorporated readily in a conventional latex copolymerization process. For the manufacture of a resin that is to be used for soft, water-containing contact lenses with high oxygen permeability, good mechanical strength, and heat resistance, a copolymer of 2-hydroxypropyl isopropyl fumarate and allyl methacrylate has been the subject of a Japanese patent disclosure [80]. Presumably these polymers were formed by a bulk polymerization process. Allyl a-(hydroxymethyl)acrylate and ethyl tx-[(allyloxy)methyl]acrylate have been synthesized. Their polymerization behaviors were studied in bulk and in benzene solution. The bulk polymerizations were carried out with 0.5-1% AIBN at 50~176 The solution polymerization of the (allyloxymethyl)acrylate was carried out for five days at 70~176 using as initiator 2-[(carbamoyl)azo]isobutyronitrile. The typical yield was 90%, the intrinsic viscosity taken in chloroform was 0.18 dl/gm. The allyl tx-(hydroxymethyl)acrylate polymerized to a product that was insoluble but slightly swellable. It exhibited considerable residual unsaturation. Presumably, a typical acrylic polymerization had taken place. The product formed from ethyl ~x-[(allyloxy)methyl]acrylate was more complex. After initiation, during the propagation step, 5-membered, tetrahydrofuran-structures form by an intramolecular addition step. Figure 7 shows the proposed structure of this polymer [80a]. With UV radiation, it is possible to interrupt the polymerization process before gelation takes place, at 25-27% conversion. In Procedure 6-2, it should be noted that the reaction mixture must be cooled to prevent premature gelation. Details about the apparatus to be used are given in Procedure 6-3.

6-2.

Ultraviolet-Initiated Polymerization of Ailyl Methacrylate with Benzoyl Peroxide ['/0]

In a 500-ml Vycor flask is placed 200 grn of allyl methacrylate and 0.6 grn of benzoyl peroxide. While stirring vigorously and while continuously maintaining a nitrogen atmosphere over the monomer, the flask is cooled by running water to 10~ • 5~ The mixture is exposed to a Hanovia Luxor L UV arc with all due safety precautions. The polymerization is continued for 1 to 2 hr. Before the mixture gels, it is removed from the source of radiation (25-27% conversion). This point may also be determined by following the change in refractive index of the reacting mixture or by precipitating the polymer.

6. Polymerization of Allyl Acrylate and Methacrylate

309

I

Et Fig. 7 Proposedstructure of poly{ethyl a-[(allyloxy)methyl]acrylate}[80a]. When a similar reaction mixture is polymerized thermally at 75~ gel forms after only 6% conversion [71]. Below 25~ visible light (photoflood lamps) or UV radiation may induce the decomposition of benzoyl peroxide or of biacetyl. These decompositions, in turn, bring about polymerization of the monomer. Biacetyl is activated by visible radiation in the range of 400--460 nm. Benzoyl peroxide is activated in the long UV wavelength range of 350-380 nm. The increase in the extent of conversion at the gel point at lower temperatures is accompanied by a reduction of the product's molecular weight. Since the rate of initiation under radiation remains high and the rate of termination is not affected because of its low activation energy, the decrease in molecular weight has been attributed to a decreased rate of chain growth. This may also be attributed to cyclization taking place under these conditions. The extent of polymerization at the gel point is more dependent on the rate of polymerization when biacetyl is used to initiate the process than when benzoyl peroxide is used. Procedure 6-3 illustrates the use of biacetyl as an initiator for the polymerization of allyl methacrylate at a low temperature. It should be noted that after 200 min the gel point is reached at 30% conversion.

6-3.

Ultraviolet-Initiated Polymerization of Allyl Methacrylate with Biacetyl [71]

a. E q u i p m e n t

The UV-initiated polymerization of allyl methacrylate is carried out in a 500-ml Pyrex or Vycor Erlenmeyer flask with a side arm. The front of the flask is painted black except for an area of 4 x 11 cm which assures that a constant area is exposed to the radiation source. The back and bottom of the flask is covered with aluminum foil. The side arm is used for withdrawing samples periodically. The charged flask is kept in a stirred, water thermostat controlled to +__1~ Preparations are run under a slight positive pressure of purified nitrogen. An ordinary R-2 photoflood lamp is used as a source of visible radiation. As a source of UV radiation, a quartz Hanovia Luxor L arc is used. This source had an aluminum reflector. It is placed approximately 2.5 cm from the nearest part of

310


the clear area of the Vycor flask. The source requires a minimum of 12 min to reach maximum intensity. It is the usual practice to shield the reaction mixture for a full 20 min before exposure to the radiation source. This assures constancy of the radiation.

b. Polymerization To the equipment described in Section a, with suitable protection of personnel against radiation, is charged 200 ml of allyl methacrylate and 0.6 gm of biacetyl. The monomer solution is cooled to I~ and, after the radiation source has reached constant intensity (about 20 min), the monomer mixture is exposed to the radiation at 1~ Samples of the mixture may be withdrawn from time to time and studied. The samples are treated with a small quantity of pyrogallol and then added to an excess of methanol. The methanol mixture is cooled to -50~ The polymer is isolated by centrifugation. The polymer may be dissolved in a minimum amount of acetone, reprecipitated with methanol, and dried under reduced pressure at room temperature. After 200 min of exposure, approximately 30% conversion is achieved. Before gelation sets in, the irradiation is terminated. Table X outlines the results of a number of polymerizations of allyl methacrylate carried out using Procedure 6-3. The reduced viscosities of the isolated polymers are given in Table X. In several cases the reduced viscosities are given for polymer samples taken as the preparation proceeded but before the gel point had been reached. Printing plates have been prepared which depend on the photopolymerization of allyl esters to generate an image [81 ]. As an etch resist for printed circuits, a solution of a prepolymer of allyl methacrylate, benzil, and 4,4'-bis(dimethylamino)benzophenone in methyl isobutyl ketone-cellosolve acetate was coated on a substrate. After exposure to UV, the image was developed with methyl ethyl ketone [81 ]. Among the UV sensitizers, some of which may accelerate the crosslinking rate by 100-200 times that of ordinary free-radical initiated processes, are ethers of benzoin, benzophenones, anthrone, benzil, and Michler's ketone [81]. Anion-initiated polymerizations of allyl methacrylate have been studied. For example, Walling and Snyder produced a polymer using finely divided sodium in benzene, under nitrogen, at 25~ for 22 hr [82]. The widely quoted article by Donati and Farina [83] detailed the use of either butyl lithium or phenylmagnesium bromide to produce solution polymers of allyl acrylate at 50~ in toluene without gel formation even at conversions greater than 70%. The resulting polymers are noncyclic, stereoregular, crystalline polyacrylates. Evidently only the acrylate group polymerizes in their process [83].

7. Polymerization of Diallyl Carbonates

311

TABLE X Polymerization of Allyl Methacrylate: Rates, Gel Points, Viscosities [71] Gel point Initiator (gm/1O0 ml)

Activator

Temp (~

Reaction (%)

Time (min)

0.3

Biacetyl

UV

1

36

200

3.0

Biacetyl

UV

1

39

420

0.3 0.3

Biacetyl Biacetyl

UV UV

15 15

33 33

95 78

0.3

Biacetyl

UV

25

31

41

0.3

Biacetyl

Photoflood

15

29

70

0.3 2.0 0.3 0.3

Biacetyl Bz202 Bz202 Bz2026 + 0.1 Biacetyl Bz202

Photoflooda UV UV UV

15 15 15 15

19 22 19 29

95 53 90 82

Heat

75

6

8

0.3

Reduced viscosity of polymer,

[Y/]sp/C 0.12 0.32 0.25 0.33 0.30 0.21 0.37 0.22 0.41 0.22 0.70

0.79

2.0

a Light source 15 cm from the reaction. Normally the source of radiation was kept at 2.5 cm from the reactor. bAn early preparation run under a stream of nitrogen in an unmasked flask.

Lithium dispersions have also been used to prepare soluble p o l y m e r s high conversions [84]. The pendant allyl groups m a y be used to cross-link the p o l y m e r with sulfur and sulfur m o n o c h l o r i d e [84]. A brief review o f the radical and anionic polymerization o f allyl acrylate and allyl acrylamides has appeared in Spanish [85].

7.

POLYMERIZATION OF DIALLYL CARBONATES The m o n o m e r diethylene glycol bis(allyl carbonate) (Structure IV) 0 0 II II CH2-- CH-- CH2-- O-- C-- O-- CH2-- CH2-- O-- CH2-- CH2-- O-- C-- O-- CH2-- CH =CH2 (w)

has considerable commercial value as a highly transparent, hard, scratchresistant resin for use in high-quality, plastic lenses. Both the m o n o m e r and p o l y m e r have been designated by the tradename "CR-39." The m o n o m e r is also

312

8. PolymerizationofAUylEsters

referred to as "allyl diglycol carbonate." While other diallyl carbonate derivatives have been studied [86], the primary interest has been in CR-39-monomer and its polymerization. Like most allyl esters, diethylene glycol bis(allyl carbonate) requires high concentrations of initiators for conversion. Even then, to complete the process, extensive postcures are required. Since the monomer is reasonably nonreactive, solutions of up to 5% of benzoyl peroxide in CR-39-monomer may be stored at 10~ or lower. If diisopropyl peroxydicarbonate is used, monomer solutions of this material must be stored below - 5 ~ The polymerization is air inhibited. With care a noncross-linked syrup can be prepared which finds application as an optical cement. Whether this material is a cyclic prepolymer seems not to have been considered. Such structures would have tings with 16 members. Procedure 7-1 is an early example of the preparation of a thermoplastic resin in solution.

7-1.

Solution Polymerization o f Diethylene Glycol Bis(atty! carbonate) [86]

In a four-necked, 500-ml flask fitted with a reflux condenser, mechanical stirrer, thermometer, and a means of maintaining a nitrogen atmosphere, are placed 100 gm of diethylene glycol bis(allyl carbonate), 100 gm of dioxane, and 4 gm of dibenzoyl peroxide. The mixture is stirred and the air is displaced with nitrogen. Then the reaction solution is heated between 80 ~ and 85~ until a noticeable increase in viscosity is observed. The reaction mixture is then cooled to room temperature. Methanol is added until the solution becomes slightly turbid. Then the turbid mixture is added with vigorous stirring to five times its volume of methanol. The polymer is filtered off and dried under reduced pressure. This granular, white product, on mixing with 5 wt% of dibenzoyl peroxide, at 145~ and with the application of a pressure of 13.79 MPa (2000 psi), produces a transparent, infusible sheet. To prepare glazing, transparent sheets are cast in cells made of plate glass with flexible gaskets used as spacers (cf., Sandier and Karo [87], Procedure 4-2 for a detailed description of a glass casting cell). Control of the polymerization temperature used with such cells varies considerably with the thickness of the polymer sheet to be produced. For example, a sheet 3- to 5-mm thick can be prepared at 70~ with a monomer solution containing 3% of dibenzoyl peroxide over a period of 60-72 hr in a circulating air oven. After the sheet is removed from the cell, it is postcured at 115~ for an additional 2 hr. With thicker sheets, lower initial temperatures are required to permit proper dissipation of the heat of polymerization. If diisopropyl peroxydicarbonate (also known as isopropyl percarbonate or DIPP) is used in a single temperature process, the polymerization may be carried

313

7. Polymerization of Diallyl Carbonates

out at 45~ for about the same length of time (60-70 hr) [88] . At a constant initial polymerization temperature, the process starts rapidly. As the initiator level decreases with time, the rate of polymerization also levels off. Therefore, the prolonged heating cycles indicated above are necessary to produce good castings by a single temperature process. The process can be substantially sped up by increasing the reaction temperature as the process progresses. Dial et al. [88] go into considerable detail about the method of determining a heating schedule for a CR-39 casting process in which the rising processing temperature approximately maintains a constant rate of initiator decomposition along with a constant rate of polymerization. It was found that the kinetics of the polymerization of diethylene glycol bis(allyl carbonate) differed substantially from that of allyl acetate. This was attributed to the early establishment of a three-dimensional, crosslinked network in the case of the diallyl ester. 1 9 Procedure 7-2 outlines a procedure for the preparation of a 3-1n. cast sheet using a temperature schedule based on data found in Dial et al. [88].

7-2.

Preparation o f Diethylene Glycol Bis(allyl carbonate) Cast Sheet [88] 1

9

In a glass casting cell [cf. 76a] prepared from ~-m. plate glass with a spacer 1 9 sufficient to give a ~-ln. thick final sheet, cooled to 20~ is placed a cool (20~ solution of diethylene glycol bis(allyl carbonate) containing 4 wt% based on the monomer of diisopropyl peroxydicarbonate (DIPP). With appropriate safety precautions the cell is placed vertically in an explosion-proof, air oven and heated according to the following heating cycle. Time (hr) Initial 1

2 3 4 5

Temperature (~ 56 58 64 72 90 90 (end of cycle, cool to room temperatures)

The cell is then disassembled. By the use of temperature schedules of this sort, satisfactory, clear sheets can be prepared in between one-eighth to one-fourth the time normally required for a single temperature process. The effect of sheet thickness on the required temperature cycles is given in detail in Dial et al. [88]. Here we give the range of temperatures and times for comparative purposes.


314

Sheet thickness (in.) 1

l-g 1 1 16-8 1 3 8-8 1

Total process time (hr)

Temperature (~ Initial

Final

12 15 17 24

45 46 44 39

90 105 105 105

It will be recalled that monoallyl esters undergo degradative chain-transfer during polymerization. This process results from the abstraction of a hydrogen atom adjacent to a double bond by one free radical to form a stabilized allylic radical. Beyond the gel point of the polymerization of diallyl esters, however, degradative chain-transfer is less significant. Thus, during the polymerization process there is a gradual shift from degradative chain-transfer to effective chaintransfer. In fact, even before the gel point is reached, effective chain-transfer is said to play a significant role in the polymerization of diallyl esters, presumably because more than a single, physical chain is involved per kinetic chain. To visualize the process, it should be considered that early in the polymerization, allylic radicals are mobile and combine rapidly with each other. As the process proceeds, the allylic radicals formed by degradative chain-transfer are confined by their attachment to the crosslinked polymer network. Their ability to combine with each other is reduced whereas their ability to add to double bonds becomes somewhat more favored. The local rate of polymerization will increase as the degree of crosslinking increases. This could lead to uneven contraction of the resin, particularly if the initiator concentration is low. Greater uniformity of the polymer is achieved with higher levels of initiator [89]. Incidentally, it was discovered that heating the monomer for prolonged periods prior to adding initiator, increases the rate of polymerization without changing the time required to reach the gel point [89]. The preparation of poly(allyl carbonates) from preformed poly(allyl alcohol) has been discussed. In this preparation, a poly(allyl alcohol) of MW approximately 13,000 is treated in a solution of dimethyl sulfoxide, triethylamine, and dioxane with ethyl chloroformate. The resultant polymer is insoluble, contains a high proportion of carbonate units and some eight-membered ring structures [90].

8.

POLYMERIZATION OF DIALLYL ESTERS OF PHTHALIC ACIDS

The diallyl esters of o-phthalic and of isophthalic acids are commercially available both as monomers and as low-molecular-weight prepolymers. Diallyl terephthalate has been studied only occasionally. Diallyl chlorendate is of

8. Polymerization of Diallyl Esters of Phthalic Acids

315

interest in copolymer systems since its high chlorine content is thought to contribute to the flame resistance of organic materials. The most extensive research and development activity has involved diallyl o-phthalate. The studies on the cyclopolymerization of this monomer have already been discussed at length in Section 3 of this chapter. Diallyl isophthalate polymerizes more rapidly than the the ortho-isomer. It cyclizes less during the early stages of polymerization. Consequently the prepolymer of the isophthalate has more reactive double bonds available for further reaction than the o-phthalate and the final resin produced from it is more highly cross-linked [91]. The methods of polymerization of the diallyl phthalates deal with control of the process to permit isolation of the thermoplastic prepolymer before the fully cured, crosslinked resin is formed. Simply heating diallyl o-phthalate with benzoyl peroxide at 115~176 produced a highly cross-linked thermoset resin [92]. The usual methods of producing "prepolymers" involve the interruption of the polymerization process before gelation sets in (at about 25% conversion). One novel method of polymerization without the use of any peroxide initiator involves heating diallyl o-phthalate under nitrogen in the presence of metallic copper. Below 205~ copper acts as a retarder of the polymerization. However, above 225~ it accelerates the process (at about 215~ it neither inhibits nor accelerates) [93]. Shokal and Bent [93] pointed out that the course of the polymerization may be monitored by taking the refractive index of the solution of the prepolymer in the monomer. The refractive index of the pure monomer is n2o5 1.5185. For each 1% of soluble polymer formed, the refractive index increases by 0.0005 units. Thus, at 25% conversion the refractive index of the solution is approximately n2D5 1.531. The isolation of the prepolymer from the reaction mixture usually involves its precipitation with an alcohol or some other nonsolvent. Two Japanese patents are based on this well-known phenomenon. In one, 100 gm of a prepolymer solution containing 25% of the prepolymer is treated at 40~ with 200 gm of ethanol in an extractor operating at 200 rpm for 5 min. After a second extraction at 65~ for 5 min with 240 gm of ethanol, 25 gm of the white prepolymer is isolated. Propanol, isobutanol, and isopropanol may also be used. The process is also applicable to poly(diallyl isophthalate) [94]. For poly(diallyl terephthalate) the use of methanol at 40~ followed by a second extraction with methanol at 65~ is patented [95]. Using high-temperature initiators such as dicumyl peroxide or tert-butyl perbenzoate at high temperature, the expected oxygen inhibition from the environment is substantially reduced and the polymerization proceeds at a reasonable rate [96].

316

&

Polymerizationof Allyl Esters

The use of a high-temperature initiator actually goes back to a 1947 patent. In that patent, di(tert-butyl)peroxideis used as an initiator at 65~ This is curious since the half life of this peroxide at 100~ is over one week! We wonder whether this may not be a typographical error. At 135~ for example, the half life would be about 4 hr, which would be much more reasonable. However, in Procedure 8-1 we follow the original directions [97].

8-1.

Bulk Polymerization of Diallyl o-Phthalate with High-Temperature Initiator [97]

In a l-liter flask fitted with mechanical stirrer, a nitrogen bleed, a means of withdrawing samples, and a thermometer is placed 100 gm of diallyl o-phthalate and 2 gm of di(tert-butyl)peroxide. The mixture is heated with stirring under nitrogen at 65~ (see note in preceding text) until the refractive index of the solution reached n2D5 1.5313. Then the reaction mixture is slowly poured, with stirring, into 600 ml of methanol. The semisolid prepolymer is filtered off and dried under reduced pressure. The product is soluble in a mixture of 3 parts of toluene and 1 part of xylene. This solution may be used as a bake-on coating on steel at 150~ for 1 hr. A hard, flexible, transparent, water-white film forms. The prepolymer has also been prepared in two stages at two different temperatures using two initiators which operate at two distinctly different temperatures like tert-butyl hydroperoxide and di-tert-butyl peroxide [98]. In a strictly thermal process, diallyl o-phthalate has been polymerized at 200~176 The conversion of monomer to polymer was followed by checking the change in refractive index with time. The process was "short stopped" before the gel point was reached by adding a solvent which separated unreacted monomer from the polymer [99]. The classical work by Simpson and co-workers on the cyclization of diallyl o-phthalate during polymerization was carried out in sealed ampoules [32, 41 ]. Procedure 8-2 outlines their method.

8-2.

Sealed-Tube Bulk Polymerization of Diallyl o-Phthalate [32, 41]

A solution of 11.200 gm of recrystallized dibenzoyl peroxide in 1 liter of redistilled diallyl o-phthalate at 20~ is prepared. This solution contains 1 wt% of initiator [0.0463 mole/liter]. Twenty-gram portions of this solution are placed in Pyrex ampoules which are degassed several times and sealed under reduced pressure. The ampoules are places in protective sleeves and heated in a bath thermostated at 80 ~ +__0.25~ At intervals ampoules are removed to establish kinetic data points (cf. Table I).

8. Polymerization of Diailyl Esters of Phthalic Acids

317

The ampoules are cooled and opened. The contents of the ampoules are precipitated by addition to redistilled ethanol which has been denatured with methanol. The polymer is then dissolved in a minimal quantity of acetaone and again precipitated with denatured ethanol. This process is repeated another time. Then the polymer is dried to constant weight under reduced pressure. The filtrates are checked for dissolved low-molecular-weight products by dilution with water. In no case is such material found to be present. For molecular weight determinations, approximately 5 gm of the polymer is dissolved in benzene. The benzene solution is cooled to 0~ and, while maintaining a temperature of 0~ the benzene is sublimed off under reduced pressure for 24 hr. The polymer is finally maintained at 2 mm Hg at room temperature for 14 days. The residual benzene in no case was more than 1.5%. The polymer treated in this manner is used for molecular weight determinations and to determine the degree of unsaturation. The conversion of the monomer to polymer may be determined simply by precipitating aliquots of the reacting solution with methanol, washing the polymer with a 1% solution of hydroquinone in methanol, and finally drying the polymer to constant weight at 100~ In connection with bulk polymerization experiments of diallyl o-phthalate, it should be pointed out that this monomer behaves quite differently from vinyltype monomers in that its rate of polymerization increases linearly with initiator concentration. This phenomenon is conveniently studied by use of a gel-time meter [ 100]. This instrument also is used to demonstrate that hydroquinone is a more effective inhibitor of the polymerization of a 1% solution of dibenzoyl peroxide in diallyl o-phthalate at 100~ than t-butyl catechol [ 100]. While Procedure 8-1 purports to illustrate the use of a high-temperature initiator to bring about polymerization at the relatively modest temperature of 65~ a more recent patent details the use of a high-temperature initiator at a high temperature (200~ At that temperature the half life of the initiator tertbutyl hydroperoxide is approximately 60 min in an aromatic solvent. The polymerization is carried out for 780 min (13 hr). It is noted that the process is quite rapid during the first 20 min. Thereafter the process is said to be quite slow. Since the initial initiator concentration is deliberately kept quite low, it is possible that the reduction of the polymerization rate is related to the destruction of a substantial portion of the hydroperoxide relatively early in the process [101]. Procedure 8-3 is given here only for purposes of illustration since the procedure is patented.

8-3.

Bulk Polymerization of Diallyl o-Phthalate at 200~ [ 101 ].

In a 22-liter, four-necked flask fitted with mechanical stirrer, a sampling and addition line, a thermometer and means of maintaining a nitrogen atmosphere

318

8. Polymerization of AIlyl Esters

over the monomer is placed 11.6 kg of diallyl o-phthalate (haD5 1.5181). The equipment is flushed with nitrogen and a slow stream of nitrogen is maintained in the flask throughout the preparation. The monomer is heated with stirring to 200~ and is maintained at 200~ throughout the process. The time required to reach 200~ is approximately 75 min. Then a solution of 150 mg of tert-butyl hydroperoxide (i.e., 13 ppm on the monomer) is added rapidly. Stirring and heating is continued for 13 hr. From time to time, samples are withdrawn and refractive index and viscosity determinations are made. These observations are as follows: Time (min) sample was taken (after initiator addition)

Refractive index of cooled sample, n2D5

Viscosity of sample cPs at 25~

0 20 40 120 190 250 280 310 370 430 490 550 610 650 690 720 750 770 780

1.5185 1.5209 1.5224 1.5235 1.5248 1.5255 1.5260 1.5265 1.5275 1.5282 1.5290 1.5300 1.5310 1.5316 1.5324 1.5329 1.5332 1.5335 1.5339

50 65 75 100 145 180 255 285 340 400 450

The product solution from several runs is then fed into a wiped still at a rate of 6 kg/hr at 195~176 at 80 p. With a residence time of 2 min, 72 parts of distilled monomer and 28 parts of polymer are isolated. A variety of techniques have been used to monitor the course of the polymerization of diallyl o-phthalate. The traditional methods of precipitating the polymer from the polymerizing solutions lend themselves only to that portion of the process in which gelation has not yet taken place. Methods of studying postgelation kinetics are few. Generally such studies are made on bulk polymers. Starkweather and Eirich [89] determined the refractive index of the crosslinked polymer by floating cylindrical samples on solvent mixtures which were adjusted until the composition had the same refractive index as the polymer. Then the refractive index of solution was measured with an Abbe


319

refractometer. For resins of diallyl o-phthalate, mixtures of bromobenzene and carbon disulfide were used. We presume that today with chemical microscopic technique more widely known than in 1955, the refractive indices of polymer samples would conveniently be taken under an optical microscope using standardized, commercially available refractive index fluids. In any case, using the refractive indices of polymer samples isolated at given time intervals after initiation and the densities of each polymer sample, the unit refraction R was calculated with the Lorenz-Lorentz equation:

[R]

=

2

+ 2

where [R], unit refraction; n, refractive index of a given polymer sample; M, MW of the monomer; p, density of the polymer sample under consideration. The unit refraction may be predicted from the sum of the molar refraction equivalents for the atomic units and for structural features such as double bonds. The unit refraction may be correlated to percent conversion. In the case of diallyl o-phthalate, it was found that the ultimate conversion reached about 60% regardless of initiator concentration. In another procedure, a special cell was constructed to fit an infrared spectrophotometer. This absorption cell was electrically heated. As the polymerization in this apparatus proceeded, it was scanned by the spectrophotometer. The absorption band at 1598 cm -1 was attributed to the aromatic double bond vibration which did not change in intensity during the process. On the other hand the allylic double bond stretching at 1645 cm -~ changed with time. Therefore, the ratio of the percent transmission at 1645 cm -1 to that at 1598 cm-1 was considered a measure of the disappearance of the allylic double bond concentration even beyond the gel point [44]. The isothermal bulk polymerization of diallyl o-phthalate was also measured by following the change in electrical resistivity as the polymerization took place. With increasing conversion, the resistivity increased [ 102]. Unfortunately, these methods do not seem to have a means of detecting the gel point. When the individual measurements made by any of these techniques were plotted against time, smooth curves resulted. There were no significant changes in curvature at the gel point. The exception to this generalization was the technique of Cass and Burner [ 100], which depends on a simple stirring device which stops when the viscosity of a polymerizing system reaches a certain high value. Otherwise the gel points were determined visually or by poking at the polymer. The control of the molecular weight of the prepolymer of diallyl o-phthalate has led to several processes which may be considered solution polymerizations in which the solvent serves also as a chain-transfer agent. Procedure 8-4 is an adaptation from a patented process given here to illustrate the process. In this

320


procedure, the initiator has a considerable effect on the conversion and on the molecular weight of the product as indicated by viscosity measurements of 25% solution of the polymer in its monomer [ 103]. In Procedure 8-4, the initiator is 50.4% hydrogen peroxide. This compound is quite hazardous. While its appearance is similar to plain water, spills on the skin cause severe bums. Spills on paper, etc., may cause fires. While the author of the patent, which we present here, ran the polymerization under a nitrogen atmosphere, we question the need of eliminating atmospheric oxygen by this method, or by any method, in view of the fact that the initiator--hydrogen peroxide--is capable of furnishing oxygen to the system in its own fight.

8-4.

Solution Polymerization of Diallyl o-Phthalate

[103]

In a 2-liter, resin kettle equipped with a mechanical stirrer, reflux condenser, thermometer, and a means of maintaining the reaction mixture under a nitrogen atmosphere, a solution of 886 gm of diallyl o-phthalate, 62.2 gm of anhydrous isopropanol, and 7.5 gm of 50.4% hydrogen peroxide is heated with stirring, under nitrogen at reflux (104~176 for 10 hr. At the end of this time, the viscosity of the reaction mixture, at 106~ is 27 cPs. The reaction mixture is cooled to 25~ whereupon the viscosity of the solution is found to be 425 cPs. The reaction mixture is added with cooling to 5 liters of anhydrous isopropanol which has been cooled to 0~ The prepolymer, which precipitates, is filtered off and dried under reduced pressure (yield: 245 gm 27.6%). The unreacted monomer may be recovered from the filtrates. The product had a softening range of 80~176 a specific gravity (ASTM D792-50) of 1.267 at 25~ and an iodine number of 55. It is soluble in ketones, benzene, ethyl acetate. When cured in the presence of 2% tert-butylperbenzoate (15 min at 175~ under 6000 p.s.i, of pressure), a thermset resin is formed with a Rockwell hardness of 114-116 (M-scale), flexural strength of 9000 p.s.i., and a heat distortion point at 264 p.s.i, of 155~ [103]. Table XI indicates the effect of hydrogen peroxide on the conversion and the viscosity of the propolymer solution. When chloroalkanes or ketones are used as regulators of the molecular weight of diallyl o-phthalate polymers, telemerization has been observed [ 104]. According to another patent, heating 100 parts of diallyl o-phthalate with 2 parts of methanol, 2 parts of carbon tetrachloride, and 0.4 parts of dibenzoyl peroxide for 3.5 hr at 110~ results in the isolation of 27 gm of a prepolymer by precipitation with an excess of methanol [105]. After heating 30 gm of the monomer with 10 gm of hexachloroethane for 18 hr with 0.3 gm of dibenzoyl peroxide at 80~ 12.3 gm of a prepolymer is isolated which could readily be molded. This process affords a greater than 30% yield of prepolymer [106]. Evidently by using a rather large quantity of chain-transfer agent, substantial

321


TABLE XI

Effect of Hydrogen Peroxide Level on the Viscosity of Prepolymer [ 103]a

H202

Conversion

Viscosity of a 25% prepolymer solution in monomer at 25~

(%)

(%)

(cPs)

0.11 0.27 0.43 0.51 0.54

24.5 25.8 27.6 28.0 29.3

550 390 354 220 179

Polymerization conditions: As given in Procedure 8-4 except for indicated changes in percentage of H202.

yields of the prepolymer are readily formed. The patents are not clear as to the composition of these prepolymers. It may be presumed that a reaction mixture containing 25% of a perhalogenated chain-transfer agent such as hexachloroethane will produce a low-molecular-weight polymer replete with highly halogenated segments. Usually, suspension polymerizations consist of a process in which droplets of a monomer (or monomers) containing an initiator in solution are polymerized while being dispersed in an aqueous medium containing a suspending agent. Attempts are usually made to carry the process to high conversion and to isolate the product as rigid beads. The foregoing discussion of the polymerization of the diallyl phthalates has indicated that any process which leads to essentially complete conversion will result in cross-linked beads which will have virtually no uses. Therefore a suspension process for the present monomers would have to be carried out to a modest conversion and isolation of the prepolymer would have to be from the solution of the polymer in its monomer. With dimethylbenzyl alcohol as a polymerization regulator which prevents cross-linking and using animal glue as a suspending agent, a "suspension" polymer has been prepared. This material gave clear solutions in acetone and could be molded as 85~ and 5000 psi within 10 min. Procedure 8-5 illustrates the polymerization process of this patented process [ 107].

8-5.

"Suspension Polymerization" of Diallyl o-Phthalate [107]

In a 2-liter resin kettle fitted with an addition funnel, reflux condenser, mechanical stirrer, thermometer, and a means of maintaining an inert atmosphere, to 600 gm of water heated, with stirring, under a nitrogen atmosphere at 80~ are added 2 gm of animal glue and 35 gm of dimethylbenzyl alcohol. While maintaining a temperature of 80~ a solution of 15 gm of recrystallized

322


dibenzoyl peroxide in 150 gm of diallyl o-phthalate is added from the addition funnel while stirring vigorously. After heating for 21 hr, the mixture is cooled to 30~ and transferred to a separatory funnel. The lower organic layer is added dropwise with vigorous stirring to 500 ml of methanol. The precipitated prepolymer is separated, resuspended in fresh methanol, filtered, and dried under reduced pressure. The yield is 76 gm with a saponification number of 428, an iodine number of 58, and a viscosity, in a 5% solution in benzene, of 0.82 cPs. The use of magnesium carbonate at levels of 2-5% based on diallyl o-phthalate has been suggested as a suspending agent. Since the abstract indicates that polymer conversions of greater than 94% are achieved, we presume that this suspending agent was only of use when rigid polymer beads were to be isolated. At that level of conversion, the polymer may be expected to be thoroughly crosslinked. The same suspending agent had also been suggested for the polymerization of tiallyl citrate [108]. Aqueous emulsions containing between 30% and 70% of the prepolymer or of mixtures of monomer and prepolymer have been suggested for application in textiles and paper making. The emulsion is said to be formulated with 1% of tert-butyl perbenzoate. The composition is cured under pressure at 140~176 Details on the preparation of a true latex are not available [ 109]. There appears to have been little interest in ionic polymerization of the diallyl phthalates. In one patent, it is implied that the initiation of diallyl o-phthalate by boron alkyls in the presence of oxygen probably involves a free-radical process [110]. The soluble prepolymers of diallyl isophthalate may be cured not only by conventional thermal methods but also by photocross-linking techniques. The use of aryl diazides as photoinitiator of solutions of poly(diallyl isophthalate) in aryl ketones has been suggested. With synergistically active sensitizers such as benzil or benzophenone with Michler's ketone curing is possible at 300--400 nm [111]. Studies of the pre-gel stage of free radical polymerizing diallyl isophthalate as well as of diallyl sebacate have been shown to involve significant intramolecular cyclization. On the average six to eight monomer units appear to be involved in each cycle [ 112, 113].

11

POLYMERIZATION OF ALLYL ESTERS OF OTHER POLYFUNCTIONAL ACIDS

Many esters of polyfunctional acids have been prepared. For example, triallyl citrate was mentioned in Chia and Chao [108]. Simpson and Holt [41, 42] studied the oxalate and sebacate. Matsumoto and Oiwa [46, 47] worked with the carbonate, oxalate, malonate, succinate, adipate, and sebacate. Early works with

9. Polymerization of Allyl Esters of Other Polyfunctional Acids

323

the suberate, the fumarate, and the maleate were mentioned in Kardashev et aL [ 114]. The preparation and polymerization of diallyl tartrate along with several other esters was described in Araki and Iida [115]. The formation of prepolymers from diallyl brassylate, a novel monomer, and from diallyl azelate were described in [116]. The bulk-copolymerization procedure for the diallyl esters of the dicarboxylic acids from the carbonate to the sebacate in sealed ampoules uses Procedure 8-2 with substitution of the appropriate diallyl ester for the diallyl phthalate indicated in that procedure. Gelation generally takes place at approximately 25% conversion. Hence the process is generally stopped before cross-linking begins. Diallyl oxalate and diallyl sebacate frequently form semisolids which are difficult to filter. Therefore they may be isolated by centrifugation from methanolic dispersions [41, 42]. The degree of polymerization of the bulk polymers produced are quite large: the initial degree of polymerization of poly(diallyl oxalate) is 85, of poly(diallyl malonate) 70, of poly(diallyl succinate) 71, of poly(diallyl adipate) 61, and of poly(diallyl sebacate) 54 [46]. The tendency of these monomers to cyclize decreases as the separation between the two allyl groups on a given molecule increases (but cf. [112, 113]). The polymerizations of diallyl brassylate and of diallyl azelate in a stirred reactor to soluble prepolymers are given in Procedure 9-1.

9-1.

Preparation o f Diallyl Brassylate Prepolymer in a Stirred Reactor [116]

In a l-liter, three-necked flask fitted with a mechanical stirrer, thermometer, and a means of introducing nitrogen and maintaining a nitrogen atmosphere, 240 gm of a solution consisting of 98 mol% of distilled diallyl brassylate (MW 324) and 2 mol% of dibenzoyl peroxide is heated at 76~ until the refractive index reaches n 25 1.4630. The reaction mixture is cooled in a refrigerator to stop the polymerization. The reaction mixture is then warmed to 25~ and equally divided among four 250-ml centrifuge bottles. To each bottle, 150 ml of methanol is added and stirred for 0.5 hr. The supernatant liquids are discarded. The residues are worked up with methanol four times to remove unreacted monomer. The combined residue is dried under reduced pressure. The polymer is dissolved in acetone. The acetone solution is filtered to separate the cross-linked polymer. The prepolymer is recovered by evaporating most of the acetone on a rotating evaporator at ambient temperature. The remaining solvent is removed by freezedrying at - 2 5 ~ at 0.02 mm Hg pressure. The mother liquors from the work-ups were found to contain oligomers along with unreacted comonomer. Table XII gives properties of the prepolymers of diallyl brassylate and of diallyl azelate prepared by this procedure.

324

8.


TABLE XII Properties of Poly(diallyl brassylate) and Poly(diallyl azelate) Prepolymers [116]

a

Prepolymer Property

Diallyl brassylate

Diallyl azelate

Glass transition temperature M.P., C Refractive index, n~5 Density, ml/gm, 25~ MW, number average Degree of polymerization, DPn Apparent MW, weight avgb Apparent polydispersity ratio, Mw/M,,b Viscosity, 25~ cPs Solubilityc Partially soluble in: Completely soluble in:

- 63 21 1.4839 1.0269 28,000 86 716,000 25.6 9.6

- 70 1.4882 1.0767 40,000 149 739,000 18.5 39

acetonitrile dimethyl sulfoxide

diemthyl sulfoxide acetonitrile

a Polymerization conditions are as given in Procedure 9-1. bThe property is designated as apparent because of the great difference in three-dimensional structure between the prepolymers and the polystyrene used as standards in gel permeation chromatography. r Both polymers exhibit these common characteristics: Soluble in acetone, benzene, tetrachloride, chloroform, dioxane, ethyl acetate, diethyl ether, and THF. Insoluble in cyclohexane, dimethyl formamide, methanol, and water. A rapid copolymerization of diallyl adipate with an unsaturated polyester using a small quantity of lauryl mercaptan and stannous chloride along with dibenzoyl peroxide has been described. This patented procedure is of particular interest in the manufacture of flexible coatings or textiles. Procedure 9-2 may be investigated as a general method of accelerating conventional peroxide-initiated polymerizations provided the patent does not impose restrictions on such work.

9-2.

Copolymerization of an Unsaturated Polyester and Diallyl Adipate [ 117]

A mixture of 65 gm of an unsaturated polyester, 35 gm of diallyl adipate, 2 gm of recrystallized dibenzoyl peroxide, and 0.4 ml of lauryl mercaptan (CAUTION: stench), and 0.7 ml of a saturated solution of stannous chloride in DMF is prepared at room temperature. A fibrous substrate is impregnated with this mixture, heated for 5 min at 70~ and cooled to 22~ The resultant impregnated material has slightly tacky faces. By passing the sheet between felt rollers saturated with an accelerator such as dimethylaniline, a smooth, nonsticky skin is formed on the sheet.

9. Polymerization of Allyl Esters of Other Polyfunctional Acids

325

At 70~ this sheet may be stored for 3 weeks. The sheet may be shaped by stamping and may then be cured by heating at 150~ for 15 sec. The diallyl esters of maleic and fumaric acid have found application primarily in copolymer systems. Considering that the distance between the two allyl groups of the maleate ester is similar to those of the o-phthalate, and that of the fumarate is geometrically quite different, it is unfortunate that there seems to have been no study of the cyclopolymerization possibilities of these monomers. In addition there are the problems associated with the copolymerization of an allyl grouping with the double bond of the maleic or fumaric moieties within the same molecule. Highly purified diallyl maleate and fumarate in an inert atmosphere, are said to polymerize very rapidly [118]. However, trace impurities and atmospheric oxygen substantially reduce the polymerization rate under ordinary circumstances. Naturally with the double bond between the two carboxylate groups and the two allylic double bonds, crosslinking takes place at very low conversion. Even so, in copolymer systems such as in poly(vinyl acetate) emulsion copolymers, the cross-linking of a fumarate or maleate within the latex particles appears not to interfere significantly with film formation properties. As a matter of fact these monomers are incorporated in poly(vinyl acetate) latices used in adhesives and in water-based paints. Prepolymers of diallyl maleate have been prepared although not as readily as the prepolymers of the diallyl phthalates. A composition of 80% diallyl maleate prepolymer and 20% monomeric diallyl monomer with 3% dibenzoyl peroxide forms a hard transparent resin when subjected to 90~ at 2000 psi for 20 min [119]. Stereoregular, crystalline polymers are said to form from allyl esters of unsaturated acids in hydrocarbons or ether with such catalysts as butyl lithium, phenyl magnesium bromide, or lithium diethylamide [107]. Typically, these polymerizations were carried out between - 70~ and + 20~ According to the patent, films and fibers are readily formed from these polymers. The properties of these articles can be greatly modified by cross-linking [ 120]. Using differential scanning calorimetry, studies on the isothermal bulk polymerization of diallyl fumarate, diallyl maleate, and diallyl succinate were carried out. At a concentration of 2,2'-azobisisobutyronitrile of 0.0225 mole/liter of diallyl fumarate, the heat of isothermal bulk polymerization at 75~ was found to be - 6 3 . 6 kJ/mole ( - 15.2 kcal/mole). At 96~ it was - 87.9 kJ/mole ( - 2 1 . 0 kcal/mole). Similar decreases in the heat of polymerization were also noted at high levels of initiator over the same temperature range. The overall activation energy for the bulk polymerization of diallyl fumate was determined to be 100.4 kJ/mole (24.0 kcal/mole). At low conversions, the initial rates were a function of the square root of the initiator concentration.

326


At constant initiator concentration and using a programmed heating cycle, the heats of polymerization for diallyl fumarate was found to be -126.4 kJ/mole ( - 30.2 kcal/mole); for diallyl maleate -82.9 kJ/mole ( - 19.8 kcal/mole); and for diallyl succinate - 90.8 kJ/mole ( - 21.7 kcal/mole) [ 121 ]. 10.

MISCELLANEOUS PREPARATIONS INFORMATION

AND

1. Polymerization of diallyl chlorendate (diallyl 1,4,5,6,77-hexachlorobicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) by heating for 4 hr at 80~ with 0.5% dibenzoyl peroxide [122]. 2. Application of diallyl terephthalate as an accelerator for curing fluorinated polymers such as a terpolymer of tetrafluoroethylene, vinylidene fluoride, and perfluoropropylene [ 123]. 3. Reactivities of allyl esters in the radiation-induced telomerization with carbon tetrachloride. Inter alia, it was found that substituted benzoates react more rapidly than substituted phenylacetates. Esters of stronger acids reacted more slowly that those of weaker acids. It was postulated that a C--H"-n interaction prevents allyl esters from interaction with "CC13 radicals [ 124]. 4. Formation of diallyl phthalate prepolymers from preformed poly(monoallyl phthalates) by esterification with allyl alcohol in benzene solution, catalyzed by p-toluenesulfonic acid [125]. 5. Polymerization of bis[p-(allyloxycarbonyl)phenyl] esters of aliphatic and aromatic dicarboxylic acids to produce highly cross-linked polymers with good optico-mechanical properties [126]. This study should be compared with the more recent work of Ref. [80a] on the polymerization of ethyl a-[(allyloxy)methyl]acrylate. 6. Synthesis and graft copolymerization of poly(diallyl dimethylammonium chloride) with acrylamide [127]. 7. Electrochemical reductive dimerization and Claisen rearrangement of allyl acrylate [128]. 8. Anisotropy of side group motion of poly(fluoroalkyl acrylates) crosslinked with allyl methacrylate and ethylene dimethacrylate [129]. 9. Improved impact and heat resistance of an engineering plastic such as ABS by blending with a copolymer of allyl methacrylate and ethylene as well as a EPDM rubber [130]. 10. Highly hydrous contact lenses produced from copolymers of 2-(methacryloxy)ethyl-2'-(trimethylammonio)ethyl phosphate, 2-hydroxyethyl methacrylate, and allyl methacrylate [ 131 ]. 11. Radical branching polymerization of diallyl isophthalate in bulk and in benzene solutions [132].

References

327

12. A study o f the copolymerization o f the three diallyl phthalates with N-vinylpyrrolidone. Reactivity ratios, Q - e values, and intramolecular cyclization factors were evaluated [133]. 13. A study o f compatible simultaneous interpenetrating p o l y m e r networks (SINs) o f diallyl phthalate and (diglycidyl ether o f bisphenol A)-based epoxy resin [134]. 14. Use o f diallyl phthalate and diallyl maleate copolymers with vinyl chloride as rubbery sealams [ 135]. 15. Paper coating latex binder based on the copolymerization o f pre-emulsified vinyl acetate and diallyl maleate with ethylene under pressure [136]. 16. UV-curable coating composition based on diallyl maleate, tetraethylene glycol diacrylate, and a photoinitiator [137].

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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64. Y. Shigetomi, T. Kojima, and N. Ono, J. Polym. Sci., Part A, Polym. Chem. 28, 3317 (1990). 65. F. Heatley, P. A. Lovell, and J. McDonald, Eur. Polym. J. 29(2-3), 255 (1933). 66. G. Odian and B. S. Bernstein, J. Polym. Sci., Part A 2, 2835 (1964). 67. J. R. Hatton, Polymer 8, 41 (1967). 68. H. Von Brachel and F. Engelhardt, British Patent 1,184,764 (1970); Chem. Abstr. 72, 122658c (1970). 69. L. Grindin, S. Medvedev, and E. Fleshier, Zh. Obshch. Khim. 19, 1694 (1949); Chem. Abstr. 44, 1020a (1950). 70. E. R. Blout and B. E. Ostberg, J. Polym. Sci. 1,230 (1946). 71. S. G. Cohen, B. E. Ostberg, D. B. Sparrow, and E. R. Blout, J. Polym. Sci. 3, 264 (1948). 72. M. Raetzsch and L. Stephen, Plaste Kautsch. 18(8), 572 (1971); Chem. Abstr. 76, 46545 (1972). 73. J. P. J. Higgins and K. E. Weale, J. Polym. Sci., Part A-1 6, 3007 (1968). 74. M. A. Pollack, I. E. Muskat, and F. Strain, U.S. Patent 2,273,891 (1942); Chem. Abstr. 36, 38784 (1942). 75. V. I. Kolegov, T. V. Nikolaeva, N. E. Kharitoneva, M. A. Lysova, V. G. Marinin, and A. F. Kulikova, Vysokomol. Soedin. Ser. B 35(5), 3 (1992). 76. T. Kitayama, E. Masuda, M. Yamaguchi, T. Nishiura, and K. Hatada, Polym. J. (Tokyo) 24(8), 817 (1992). 77. T. Kitayama, F. Fujimoto, and K. Hatada, Polym. Bull. (Berlin) 26(6), 629 (1991). 78. K. Yamaguchi and K. Yokota, Polym. J. (Tokyo) 25(6), 639 (1993). 79. Y. Matsumoto, S. Date, and T. Koyama, Japanese Patent JP 05/156,171 [93/156,171] (1991). 80. Y. Koinuma, T. Myazaki, T. Matsumoto, and T. Ootsu, Japanese Patent JP 04/335,007 [92/335,007] (1991). 80a. R. D. Thompson, W. L. Jarrett, and L. J. Mathias, Macromolecules 25, 6455 (1992). 81. F. M. C. Corp., French Patent 1,487,028 (1967); Chem. Abstr. 68, 60636a (1968). 82. C. T. Walling and R. H. Snyder, U.S. Patent 2,500,265 (1952). 83. M. Donati and M. Farina, Makromol. Chem. 60, 233 (1963). 84. H. Kamogawa, S. Furuya, and M. Kato, J. Polym. Sci., Part C 23, 655 (1968). 85. E. L. Madruga, J. Fontan, and J. San Roman, Rev. Plast. Mod. 26(208), 581 (1973); Chem Abstr. 80, 4824v (1974). 86. I. E. Muskat and F. Strain, U.S. Patent 2,592,058 (1952). 87. S. R. Sandier and W. Karo, "Polymer Syntheses," 2nd ed., Vol. 1, p. 333. Academic Press, San Diego, 1992. 88. W. R. Dial, W. E. Bissinger, B. J. DeWitt, and F. Strain, Ind. Eng. Chem. 47, 2447 (1955). 89. H. W. Starkweather, Jr. and F. R. Eirich, Ind. Eng. Chem. 47, 2452 (1955). 90. J. F. Kennedy, S. A. Barker, and A. Rosevear, J. Chem. Soc. C, No. 15, p. 2726 (1971). 91. C. F. Schildknecht, "Allyl Compounds and Their Polymers (Including Polyolefins)," p. 361. Wiley (Interscience), New York, 1973. 92. T. F. Bradley, U.S. Patent 2,311,327 (1943); Chem. Abstr. 37, 4502 (1943). 93. F. C. Shokal and F. A. Bent, U.S. Patents 2,475,296 and 2,475,297 (1949). 94. T. Tanaka and S. Takayama, Japanese Patent 70/14,546 (1970); Chem. Abstr. 73, 110442b (1970). 95. T. Tanaka and S. Takayama, Japanese Patent 70/14,547 (1970); Chem. Abstr. 73, 110443n (1970). 96. C. L. Wright and H. H. Beacham, U.S. Patent 3,527,665 (1970); French Patent 1,551,69? (1970). 97. W. E. Vaughan and F. F. Rust, U.S. Patent 2,426,476 (1947). 98. E. C. Shokal, British Patent 604,544 (1948). 99. J. K. Wagner and E. C. Shokal, U.S. Patent 2,446,314 (1947); Chem. Abstr. 42,852 (1948).

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8.


W. E. Cass and R. E. Bumett, Ind. Eng. Chem. 46, 1619 (1954). S. A. Mednick, L. Seglin, and W. B. Tuemmler, U.S. Patent 3,385,836 (1968). R. W. Warfield and M. C. Petree, J. Polym. Sci. 37, 305 (1959). C. A. Heiberger, U.S. Patent 3,096,310 (1963). D. Porret and L. Leumann, U.S. Patent 3,366,667 (1965). A. Neri and L. Capitano, U.S. Patent 3,474,127 (1966); Chem. Abstr. 65, 9053 (1966). H. Inoue, H. Inukai, and H. Ito, Japan Kokai 72/22,992 (1972); Chem. Abstr. 78, 73083 (1973). C. A. Heiberger and J. L. Thomas, U.S. Patent 2,832,758 (1958). K.-S. Chia and F.-Y. Chao, Chemistry (Taiwan) p. 31 (1956); Chem. Abstr. 51, 1645 (1957). W. Festag, German Patent 1,163,774 (1964); Chem. Abstr. 60, 1345z (1964). Solvay, Belgian Patent 562,701 (1959); Chem. Abstr. 53, 10845 (1959). M. N. Gilano and M. A. Lipson, Tech. Pap., Reg. Tech. Conf., Soc. Plast. Eng., Mid Hudson Sect. p. 30 (1970); Chem. Abstr. 74, 4077f (1971). I. I. Romantsova, Int. Polym. Mater. 19(1-2), 51 (1993). I. I. Romantsova, Polym. Mater. Sci. Eng. 66, 121 (1992). D. A. Kardashev, N. S. Leznov, and V. P. Nuzhdina, Khim. Promst. No. 2, p. 5 (1945); Chem. Abstr. 40, 45594 (1946). T. Araki and H. Iida, Rep. Gov. Chem. Ind. Res. Inst., Tokyo 47, 95 (1952); Chem. Abstr. 47, 108899 (1953). S.-P. Chang, T. K. Miwa, and W. H. Tallent, d. Appl. Polym. Sci. 18, 319 (1974). P. P. W. Varlet, U.S. Patent 3,133,826 (1964). C. E. Schildknecht, "Allyl Compounds and Their Polymers (Including Polyolefins)," p. 638. Wiley (Interscience), New York, 1973. M. A. Pollack, I. E. Muskat, and F. Strain, U.S. Patent 2,370,578 (1945). J. Natta, M. Donati, and M. Farina, Belgian Patents 620,901 and 621,007 (1963); Chem. Abstr. 59, 1777c (1963). F. Cernec, U. Osredker, A. Moze, I. Vizovisek, and S. Lapanje, Makromol. Chem. 178(8), 2197 (1977). C. F. Baranaukas, U.S. Patents 2,810,712 and 2,903,463 (1962). J. F. Smith, U.S. Patent 3,011,995 (1963). M. Okubo, Nippon Kagaku Zasshi 87(11), 1196 (1966); Chem. Abstr. 66, 90113d (1967). Y. Umezu and B. Kobayashi, Japanese Patent 67/26,698 (1967); Chem. Abstr. 68, 9636 (1968). B. M. Zuev, A. P. Filippova, and Z. I. Validov, U.S.S.R. Patent 394,681 (1973); Chem. Abstr. 81, 50460c (1974). Y. Q. Lin, H. Pledger, Jr., and G. B. Butler, d. Macromol. Sci. Chem. A 25(8), 999 (1988). T. Troll and J. Wiedermann, Tetrahedron Lett. 33(27), 3847 (1992). E. R. Basilova, V. A. Shevelev, L. D. Budavskaya, and V. N. Ivanova, Polym. Mater. Sci. Eng. 66, 115 (1992). F. Hirofumi and S. Katsufumi, European Patent Application EP 472,898 (1990). N. Nakabayashi, K. Ishihara, Y. Murata, N. Nakada, T. Matsumoto, and Y. Koinuma, European Patent Application EP 537,972, Oct. 14 (1991). V. I. Kolegov, M. I. Khramushina, and I. N. Yudina, Vysokomol. Soedin. Ser. B 34(11), 9 (1992). A. V. Safronov, L. B. Zubakova, L. Ya. Nikiforova, and I. Yu. Pilina, Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 36(3), 81 (1993). M. S. Lin and C. C. Yeh, J. Polym. Sci., Part A: Polym. Chem. 31(8), 2093 (1993). O. Matsumoto and S. Arima, Japanese Patent Application JP 05/25,346 [93/25,346] (1991). Y. J. Shin, U.S. Patent US 5,219,924 (1993). B. Gupta, U.S. Patent US 5,143,947 (1989).

Chapter 9

Polymerization of Vinyl Fluoride 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

331

2. C h e m i c a l l y Initiated P o l y m e r i z a t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

336

A. Bulk P o l y m e r i z a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-1. Bulk Polymerization of Vinyl Fluoride: Benzoyl Peroxide Initiated. . . . . . . . . . . B. Solution P o l y m e r i z a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-2. Solution Polymerization of Vinyl Fluoride in tert-Butyl Alcohol . . . . . . . . . . . . . 2-3. Solution Polymerization of Vinyl Fluoride in Aqueous Methanol . . . . . . . . . . . . C. S u s p e n s i o n P o l y m e r i z a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-4. Suspension Polymerization of Vinyl Fluoride, Benzoyl Peroxide Initiated . . . . . . D. E m u l s i o n P o l y m e r i z a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-5. Emulsion Polymerization of Vinyl Fluoride . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6. Emulsion Polymerization of Vinyl Fluoride in the Presence of Ammonium Iodide E. O r g a n o m e t a l l i c and Related Initiator Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-7. Solution Polymerization of Vinyl Fluoride with Oxygen-tri-n-Propylborane Initiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Radiation-Initiated P o l y m e r i z a t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-1. Photopolymerization of Vinyl Fluoride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

336 338 338 338 339 339 341 343 344 344 345 347 347 348

4. Health A s p e c t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

349

5. M i s c e l l a n e o u s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

350

References ........................................................

1.

350

INTRODUCTION

The monomer vinyl fluoride has been known since the beginning of the 20th century. The compound is a gas at ordinary temperatures and pressures with a boiling point of - 72.2~ at 1 atm, a critical temperature of 54.7~ and a critical pressure of 760 psi (5.21 MPA) [ 1]. These properties along with the lack of purity 331

332

9. Polymerizationof Vinyl Fluoride

of early samples and the limited understanding of the techniques of flee-radical polymerizations delayed preparation of the polymer well into the 1930s. It was not until after World War II, when the interest in fluoride-containing compounds increased, that developments in the preparation of poly(vinyl fluoride) of high molecular weights were patented, reported, and commercialized. Among early patents dealing with the polymerization of vinyl fluoride are those of Coffman and Ford [2-4]. An early patent that used organic azo initiators was that of Johnston and Pease [5]. Poly(vinyl fluoride) has been formed as a transparent film which is tough, flexible, and capable of orientation by cold drawing. Its primary applications are as weather- and stain-resistant films and coatings. The resin is remarkably transparent to radiation from 230 nm to 1.4/zm. However, below 200 nm light transmission drops off rapidly [6]. Thus it is used in greenhouse windows that block out harmful short-wavelength UV radiation (below 200 nm), but transmit light of wavelengths above 300 nm, which is thought to be beneficial for plant growth [7]. The polymer also exhibits good chemical stability, insolubility in most ordinary solvents, and abrasion resistance. This combination of properties has led to a variety of applications in building materials, packaging, coil coatings, and protective coatings for steel containers. Other applications include the use of films for protective coatings of fiberglass-reinforced polyester panels and preformed plastic parts [8]. Films are also suggested for use as release films in the production of printed circuit laminates and multilayer printed circuit boards [9]. Compositions of aromatic polyether-polyketones, a fluoropolymer, and aluminum borate whiskers have been patented for the production of parts with low coefficients of friction [10]. Another patent uses fluoropolymers in the manufacture of optical fibers constructed of several fibers in a unitary fiber-drawing apparatus [11]. Panels that could be embossed were prepared by laminating poly(vinyl fluoride) over polyaryletherketoneketone with a poly(methyl methacrylate) adhesive over Nomex core paper and in turn adhering such composites to glass fiber reinforced resins [12]. As mentioned before, the monomer has a low boiling point ( - 72.2~ and a relatively low critical temperature (54.7~ For comparison, vinyl chloride has a boiling point of - 13.8~ and a critical temperature of 147~ (cf. this series, Vol. 11, 2nd ed., p. 358). Therefore the polymerization of vinyl fluoride is usually carried out under high-pressure conditions which probably resemble the polymerization of ethylene more than that of the other vinyl halides. The monomer is soluble in a variety of solvents. At room temperature, the homopolymer is insoluble. Therefore the neat subdivision of polymerization techniques into bulk, solution, suspension, and emulsion procedures which we have used in many other sections in this series becomes blurred. Most procedures seem to lead to blocks of polymer or some sort of dispersions from which

1.

Introduction

333

polymeric powders may be isolated. The emulsion polymerization procedures also frequently lead to the isolation of powders. We presume that the high density of the polymer (ranging from 1.37 to 1.72 gm/ml [8], as well as the electronic configuration of the fluorinated monomer and/or polymer, interferes with real latex formation. In one series of experiments on the polymerization of vinyl fluoride in bulk and in solution using tributylborane monoperoxide as the initiator, a difference between the two polymerization techniques was illustrated. In the bulk polymerization, the reaction order was 0.61 with respect to the initiator. In the solution polymerization, the reaction order with respect to the initiator was 1.24. The activation energy for the bulk polymerization of vinyl fluoride was found to be 10.1 kcal/mole [ 13]. The initiation processes used have included various forms of radiation as well as chemical initiators. In connection with chemical initiators, it is interesting to note that among these are water-soluble amidine salts derived from such "oilsoluble" initiators as 2,2'-azobisisobutyronitrile. These water-soluble initiators tend to produce polymer latices in the presence of appropriate surfactants in the way more conventional water-soluble initiators do in the case of other vinyl monomer types. The purification of the monomer is evidently extremely important. Early samples were sufficiently low in quality that satisfactory polymers could not be produced. Consequently the important properties of the resin were not observed until relatively recently. The purification procedure described by Kalb et al. [ 1] includes the following sequence of steps: 1. Distillation to separate the vinyl fluoride from 1,1-difluoroethane, hydrogen fluoride, and other impurities; 2. Removal of last traces of hydrogen fluoride by percolation through soda-lime towers; 3. Separation of acetylene from the monomer by scrubbing with ammoniacal cuprous chloride (CAUTION: This procedure may lead to the formation of metal acetylides among the reaction products. Acetylides of heavy metals are said to be explosive, especially when dry. Partially dried acetylides may present similar hazards); 4. Separation of oxygen by another fractional distillation between - 5 0 ~ and - 2 5 ~ at 40-100 psi (0.25-0.69 MPa) pressure. Monomers with less than 5 ppm of acetylene and less than 20 ppm of oxygen were used for polymerization studies [ 1]. The purification of vinyl fluoride has been monitored by gas-liquid chromatography. Levels of impurities lower than 1 ppm by weight have been observed [14].


334

Vinyl fluoride monomer from a commercial supplier of specialty gases has been purified by passing the gas through silica gel. Then the compound was freed of air by conventional degassing procedures using liquid air to freeze the monomer [ 15]. The homopolymer poly(vinyl fluoride) is not soluble in ordinary solvents at temperatures below 100~176 At more elevated temperatures, dimethylformamide and tetramethylurea are particularly suitable solvents for the polymer. Other solvents are dinitriles, ketones, and tetramethylene sulfone [ 1]. Solution characteristics of the polymer have been studied particularly in dimethylformamide that was made 0.1 N in anhydrous lithium bromide to suppress an apparent polyelectrolyte effect occasionally observed [16]. In this system the Mark-Houwink relation for the intrinsic viscosity at 90~ was found to be [r/] = 6.42 • 10- 5M~176 (1) where Ms is the molecular weight derived from sedimentation studies in the ultracentrifuge and the weight average molecular weight, Mw, is given by

Mw

Ms~(0.90).

(2)

In a series of nine unfractionated polymers, prepared by free-radical polymerizations, the degree of polydispersity, Mw/Mn, ranges from 2.47 to 5.59. The deviation from a normal distribution where Mw/Mn is 2.0, is attributed to chain branching [ 16]. The glass transition temperature of poly(vinyl fluoride) was determined to be 43~ The transition is independent of the molecular weight of the polymer. Polymers, for this study, were prepared in the presence of a variety of initiators such as 2,2'-azobisisobutyronitrile, tributylborane monoperoxide, or diisopropyl diperoxycarbonate [ 17]. Poly(vinyl fluoride) is a somewhat unusual polymer from the structural standpoint. It was found that the telemerization of vinyl fluoride in the presence of trifluoromethyl iodide gave rise to products formed by the addition of trifluoromethyl radical to either the methylene or the CHF end of the monomer. The rate of addition to the methylene group was found to be nearly 11 times greater than the CHF group. The propagating radical CF3(CH2CHF)2 adds to the methylene group only twice as rapidly as it does to a CHF group [ 18]. In other words, the propagation stage has a high probability of forming head-to-headtail-to-tail chain segments along with the more common head-to-tail structures. If the polymerization is carried out at lower temperatures, chains of greater regularity are formed with a more substantial fraction of head-to-tail structures. This leads to greater crystallinity of the polymer [6, 19, 20]. In a flow reactor in which vinyl fluoride is continuously polymerized by photochemical initiation, the polymer formed has a higher proportion of head-to-tail sequences than the commercial product [21 ].

1. Introduction

TABLE I

335 Reactivity Ratios of Vinyl Fluoride (MI) with Co-monomers (M2) at 30~ ,

M2 Vinyl chloride Vinylidene fluoride Hexafluoropropene Ethylene Tetrafluoroethylene 1,1,2-Trifluoro-2-chloroethylene cis- 1,2,3,3,3-Pentafluoropropene Hexafluorocyclobutene Methyl acrylate Acrylonitrile Methyl methacrylate

rl 0.07 + 0.02 0.05 + 0.005 4.2 + 0.4 5.5 + 0.5 1.1 + 0.05 1.01 + 0.01 0.3 + 0.03 0.4 (at 0~ 0.27 + 0.03 0.18 + 0.02 0.09 4- 0.05 3 4- 0.6 0.16 4- 0.01 --~10- 3 0.0030 4- 0.003 (temp not specified)

rE

Ref.

+ 1 + 1 + 0.02 + 0.3 0 0 1.7 + 0.1 3.0 (at 0~ 0.05 + 0.02 0.06 + 0.02 0 0 2.9 4- 0.2 24 4- 2 51.5 + 2 (temp not specified)

19, 20

9 11.0 0.18 0.17

19, 20 19, 20 19, 12 23 19, 20 19, 20 19, 20 19, 20 19, 20 19, 20 22

The matter of the head-to-head, tail-to-tail polymerization of vinyl fluoride, vinylidene fluoride, and trifluoroethylene and the copolymerization of vinyl fluoride with vinylidene chlorofluoride and 1-chloro-2-fluoroethylene has been extensively studied by Cais and Kometani [24-27] and by Bmch, Bovey, and Cais [28]. The synthesis of pure head-to-tail poly(trifluoroethylenes) is described in Ref. [25]. Isomers of poly(vinyl fluoride) with controlled regiosequence microstructure are discussed in Ref. [27]. The copolymerization of vinyl fluoride has been studied systematically only with a few co-monomers. Table I [19, 20, 22, 23] lists some of the reactivity ratios which have been reported. The meaning of the error terms given in the table is, as is so often the case in the chemical literature, not at all clear. It is interesting to note that the same authors, in the same paper, using two different initiator systems now and again report two sets of reactivity ratios which do not always agree with each other even when the error term is added or subtracted. The Q - e values for vinyl fluoride copolymerizations have been reported as Q = 0.010 + 0.005; e = - 0 . 8 +_ 0.2 [10] or Q = 0.0084; e = 0.97 [22]. Generally useful reviews of the polymerization of vinyl fluoride are Kalb et al. [1] and Cohen and Kraft [6]. Kalb et al. [1] is particularly useful for a discussion of the effect of various reaction parameters on the polymerization process when it is chemically initiated. Other reviews are by Trappe and other authors [29-33]. Among more recent reviews are those of Brasure and Ebnesajjad [34] and of Ebnesajjad and Snow [35].


336

2.

CHEMICALLY

INITIATED

POLYMERIZATIONS

For the sake of classifying the methods of polymerization of vinyl fluoride, we use two broad categorizations in this chapter. 1. Chemically initiated processes, i.e., processes in which free radicals are

generated chemically or thermally from typical free-radical initiators such as the organic peroxides, azo compounds, etc. Ziegler-Natta catalysts, metalloorganic systems, and donor-acceptor complex polymerizations are also included in this category. 2. Radiation-initiated processes which include the use of ultraviolet radiation, ?-rays, glow-discharges, etc. Processes, if any, in which chemical initiators are used in conjunction with radiation would also be part of the second category. We believe that such a process may be primarily one in which the radiation initiates the formation of free radicals by the decomposition of the initiator rather than the direct generation of free radicals from the monomer. In either situation, it would be the radiation which actually initiates the polymerization process. A.

Bulk Polymerization

With chemical initiators, examples of true bulk or solution polymerizations are rare. This may simply be a reflection of practical considerations. The monomer is a gas at ordinary temperatures and pressures, therefore work has to be carried out in some sort of pressure vessel. Bulk polymers generally are difficult to remove from reactors. They usually do not exhibit the optimum properties of the polymer. Poly(vinyl fluoride) is quite insoluble in most solvents at temperatures below approximately 100~ It probably is also insoluble in its liquified monomer. Therefore, bulk polymerization processes would probably be strongly influenced by the precipitation of polymer from liquified monomer. A similar difficulty is found in attempts to prepare solution polymers. While the monomer may be soluble in many solvents, the polymer is not. As a result solution processes usually lead to precipitation or dispersion polymers, or some sort of swollen particles or gels. For classification purposes only, we are listing preparations carried out in the presence of a solvent (other than water or monomer) as "solution polymerizations" despite the indicated limitations. In this connection it should also be pointed out that many solvents act as chain-transfer agents for vinyl fluoride polymerizations and are therefore of interest in controlling the molecular weight distribution of the product. Solvents are also significant in the effort to develop processes which may be carried out at lower pressures than those involving nonsolvent systems. Because of the solubility of vinyl fluoride in a given solvent, the weight of monomer which can be compressed into a pressure vessel

2.

Chemically Initiated Polymerizations

337

at any given pressure is greater in the presence of a solvent than in its absence. Suspension and emulsion polymers have been prepared. In the case of emulsion polymers, the choice of surfactant is particularly critical. Salts of perfluorinated heptanoic or octanoic acids have been found suitable [6], although the products frequently are somewhat off-white. Higher yields of polymer and excellent color were obtained when salts of perfluoroalkylpropylamines served as emulsifiers. Even these materials, the patent implies, lead to slumes of polymer in the aqueous medium rather than to reasonably stable latices [36]. In our description of the polymerization processes we do not give detailed descriptions of the equipment and methods necessary for handling gases under pressure. We only mention that careful engineering is mandatory even though the older literature seems to indicate the use of rather haphazardly designed vessels. Procedure 2-1 is an example of a bulk polymerization of vinyl fluoride. It should be noted that the conversion is quite low. At 62~ benzoyl peroxide has a half-life of approximately 35 hr. Consequently heating the reaction mixture for 112 hr represents only a little over 3 half-life periods. From the literature [37] no estimate of the pressure inside the reactor could be made. Table II does indicate that in the presence of acetone or of aqueous acetone the conversion increased substantially. In fact, the procedure of the sixth experiment was found to be highly reproducible. Acetone was thought to enhance the solubility of the initiator in the monomer [37].

C A U T I O N : Vinyl fluoride is very toxic and should be handled with great care (see p. 349). TABLE II Bulk Polymermization Experiments on Vinyl Fluoride (Procedure 2-1)a Exp No.

Initiator (1% concentration of)

Solvent

Temp (~

1 2 3 4 5 6 7 8

Benzoylperoxide Benzoylperoxide Benzoylperoxide Benzoylperoxide Benzoylperoxide Benzoylperoxide Lauroylperoxide Acetyl peroxide

None None None Acetone Acetone 10% H20 in acetone 10% H20 in acetone None

51 62 85 64 72 71 71 43

T i m e Conversion Half life (hr) (%) (hr)b 37 112 89 23.5 23.5 23.5 23.5 117.5

0.5 4.0 1.7 38.4 17.8 35 44 15

150 35 2.3 29 9 10.3 3 >400

Based on Newkirk [37]. bAn estimate of the half life of the indicated initiator at the stated temperature based on the nomograph given in Vol. I of this series. a

338

2-1.

9. Polymerization of Vinyl Fluoride

Bulk Polymerization of Vinyl Fluoride: Benzoyl Peroxide Initiated [37]

With suitable safety precautions, in a small steel bomb is placed liquid vinyl fluoride containing 1% dibenzoyl peroxide. The bomb is sealed and heated at 62~ for 112 hr. Then the bomb is cooled in a dry-ice bath and cautiously opened. The excess monomer is vented off and the polymer is isolated. The yield is 4.0% of the charged monomer. In the case of bulk polymerizations, reaction mechanism seems to depend on the nature of the initiator. Thus, the same research group reports that with respect to the monoperoxide of tributylborane as the initiator, the bulk polymerization is of the 0.61st order with an activation energy of 10.1 kcal/mole [13]. With diisopropyl peroxydicarbonate, while the reaction order was of the 0.58th order with respect to the initiator, the overall activation energy of the bulk polymerization was 16.9 kcal/mole [38]. The conversion with the latter initiator was as high as 90% at a reaction temperature of 40~ [38]. Copolymers of vinyl fluoride with such monomers as vinylidene fluoride or 1-chloro-2-fluoroethylene were prepared in the presence of trichloroacetyl peroxide at 0~ in sealed tubes. The chlorine-containing copolymers were then reductively dechlorinated at 60~ in tetrahydrofuran with tri-n-butyltin hydride in the presence of 2,2'-azobis(isobutyronitrile) for up to 40 hr. This general procedure led to the formation of polymers with a reasonable control of the level of head-to-head, tail-to-tail linkages in the product [27].

B.


Vinyl fluoride monomer exhibits solubility in a variety of organic solvents, e.g., the lower alcohols, ethers, mono- and dinitriles, butyrolactone, liquid amides such as diethylformamide [6], and heptane [38]. As mentioned before, the polymer is generally quite insoluble at ordinary temperatures. At temperatures in the range of 100~ some of these compounds dissolve the polymer. During the polymerization of vinyl fluoride, one would, therefore, expect to find that the polymer precipitates as it forms. Solvents also tend to act as chaintransfer agents which yield products of lower molecular weights than those formed in the absence of solvents. Procedure 2-2 shows the effect of a solvent on the intrinsic viscosity and on the overall conversion. Since the procedure is patented it is given here only to illustrate the procedure.

2-2.

Solution Polymerization of Vinyl Fluoride in tert-Butyl Alcohol [39]

With appropriate safety precautions, a 40-ml autoclave is charged with 9.5 gm of vinyl fluoride, 0.05 gm of 2,2'-azobisisobutyronitrile, and 10 ml of tert-butyl

2.


339

alcohol. The autoclave is heated with agitation at 50~ for 16 hr. The autoclave is then cooled and cautiously vented. The product isolated represented a 76% conversion of monomer to polymer with an intrinsic viscosity of 2.25 dl/gm (DMF, 100~ When 10 ml of water is substituted for the tert-butyl alcohol, conversion drops to 39% while the intrinsic viscosity rises to 5.34 dl/gm (DMF, 100~ The polymerization has been carried out in heptane [38]. The reaction was again 0.60th order with respect to the initiator, diisopropyl peroxydicarbonate. The polymer produced had MW in the range of 44,300 to 358,600. It did not swell in either heptane or chloroform. It swelled more in carbon tetrachloride than in dioxane [38]. Solvents containing water have also been used in solution polymerizations. The resulting products are said to be film-forming polymers. Procedure 2-3, which is based on a patent process, is given here only to illustrate the general procedure used.

2-3.

Solution Polymerization of Vinyl Fluoride in Aqueous Methanol [40]

With suitable safety precautions, in an autoclave is placed at 0~ 150 gm of a deaerated solution consisting of 37.5 gm of water and 112.5 gm of methanol, 0.15 gm of diisopropyl peroxydicarbonate, and 97 gm of vinyl fluoride. The apparatus is sealed and heated at 45~ for 12 hr. During this period the pressure varies between 26.6 and 43.5 kg/cm 2 (0.266 and 0.435 MPa). The reactor is then cooled and cautiously vented. The product is isolated. Conversion is 89.7%. The inherent viscosity of the product is 2.22. When 10% aqueous dimethylformamide is substituted for the 25% aqueous methanol in Procedure 2-3, the conversion is 75% and the inherent viscosity of the product is 1.2.

C.

Suspension Polymerization

Probably a major fraction of commercially-produced poly(vinyl fluoride) is manufactured by suspension polymerization. The various reaction parameters associated with the process have been studied in considerable detail. Below is a brief summary of the paper by Kalb et al. [1 ].

a. Effect of Initiators The nature of the initiators used, despite the low level of initator fragments in the final polymer, affects the final thermal stability and wettability of the product. Furthermore, as generally anticipated, the molecular weight was found

340


to decrease with increasing concentration of initiator. The initiator efficiency also decreases with increasing initiator concentration. Polymers prepared at higher temperatures had, as expected, lower molecular weights than those produced at lower temperatures. The "high-temperature polymers" appeared to be more highly branched and were more easily compression molded. One interesting set of experiments was reported [ 1]. In this case, heavy, glasswalled, liquid level gauges were used as specially designed reactors instead of the usual steel autoclaves. When vinyl fluoride was polymerized in the presence of water while using a water-soluble initiator (2,2'-azobisisobutyramidine hydrochloride or ammonium persulfate), initiation took place in the water phase. The initially clear solution changed appearance from clear, to red, to red-orange, to orange, and finally to opaque. These colors (judging from the description in Kalb et al. [1]) were not due to the formation of chromophoric groups. The colors were those associated with light-dispersion on passing through colloidal particles. The changes in the color are associated with changes in particle size. In this experiment, the polymer was isolated as a dispersion [1 ]. When solvent-soluble initiators were used, polymerization took place above the surface of the water. The polymer chains formed in the gas phase. They were described as "webs." As their sizes increased, they dropped and floated on the surface of the water [ 1]. b. Effect of Temperature

As the temperature is raised, the efficiency of an initiator goes through a maximum in the polymerization of vinyl fluoride. In the case of 2,2'-azobisisobutyronitrile, this temperature of maximum initiator efficiency appears to be independent of pressure, at least in the range from 100 atm (10 MPa) to 250 atm (25 MPa), at approximately 70~ With increasing polymerization temperature, the melt viscosity of the product decreases. The product has a reduced molecular weight and increased chain branching. c. Effect of Pressure

Higher pressures increased the rate of polymerization, favored formation of polymers of higher molecular weight, and enhanced efficiency in the utilization of the initiator. d. Effect of Impurities

In the presence of 0.2% benzoyl peroxide, during the polymerization of vinyl fluoride at 900 atm (90 MPa) and 80~176 the process is slightly inhibited by

2.


341

500 ppm of oxygen. On the other hand, 135 ppm of oxygen seems to promote the process, particularly at the lower temperature ranges. As much as 2% of acetylene strongly inhibits the polymerization to a conversion of 3% of a readily soluble, brittle product. However, when the acetylene concentration is only 1000 ppm, the polymerization is accelerated almost to the point of being uncontrollable. Conversions run to 99%. The product is reported to be insoluble and highly cross-linked. Monomer free of oxygen and acetylene, but containing as much as 2.5% of 1,1-difluoroethane in a 40% methanol-60% water medium, gave rise to a polymer with normal conversion and normal film and molding properties. e. Effect of C h a i n - T r a n s f e r Agents

Among the chain-transfer agents evaluated, are methanol, isopropanol, and 1,3-dioxolane. Of these, methanol has the most modest effect on the molecular weight of the product. A medium containing 30% methanol affords a polymer with an intrinsic viscosity of 1.3 dl/gm whereas the product produced in a methanol-free reaction medium has an intrinsic viscosity of 4.5 dl/gm. Isopropanol and 1,3-dioxolane had much more profound effects on the molecular weight. For example, only 5% of isopropanol in the medium produces a polymer with an inherent viscosity of approximately 0.8, whereas 5% of 1,3-dioxolane in the medium gives a product with an intrinsic viscosity of approximately 0.25 dl/gm. fi Effect of M edi a Other Than W a t e r

Most organic solvents substituted for water in the polymerization of vinyl fluoride exhibit chain-transfer characteristics. Benzene gives rise to lowmolecular-weight products in poor conversion. Procedure 2-4 is a typical suspension polymerization process in which 52% conversion is achieved within 8 hr.

2-4.

Suspension Polymerization of Vinyl Fluoride, Benzoyl Peroxide Initiated [1]

With appropriate safety precautions, 0.4 gm of dibenzoyl peroxide is placed in a 1300-ml stainless steel pressure vessel. The vessel is closed and repeatedly pressured with nitrogen and evacuated until the air has been removed from the reactor. The apparatus is placed on its rocking agitator. Into the evacuated vessel is pumped 400 ml of deoxygenated water and 500 gm of vinyl fluoride. Heating and agitation is begun. During the heating-up period, additional vinyl fluoride is forced into the autoclave so that the 300 atm (30 MPa) of pressure and 85~

TABLE III

Weight, vinyl fluoride (gm)

Weight, water (gm)

Weight, initiator (gm)

120 242 100 15 ml 11.2 and 15.1 Hexafluoropropylene 11.2 and 15.1 Hexafluoropropylene

200 480 275 g 75 66

0.3 ~ 2.42 ~'c 0.75 a 0.0963 a 0.263 ~

100 12.7 22

Suspension Polymerization Conditions for Vinyl Fluoride Weight, suspending agent (gm) 0.4 b 0.48 a

Reaction Pressure (MPa)

Time (hr)

Temp (~

Conversion (%)

0.40

15 18 20

40 40 40 55 50

83.3 86.5 e 100

h 0.06 a 0.2 a

80

Intrinsic viscosity (dl/gm)

Ref.

220 e 1.44 0.94 f 0.34

[42] [43] [44] [45] [46]

0.1 i

3j 65

400 quantity not given 60

0.263 ~

0.1 i

0.151

0.2 a 4* 0.05 m b,o

0.1 ~

0.02 a 3.@

a Diisopropyl peroxydicarbonate. b Poly(vinyl alcohol). c 50 wt% of heptane. d Methyl Cellulose. r Compared to only 23.9% conversion with inherent viscosity of 0.93 when dimethyl phthalate was used instead of heptane. flnherent viscosity. g Containing 25 ml of t e r t - b u t y l acetate. , In a 500 ml autoclave.

8

50

67

7

55

98 72.5

4

40

74

0.31

77"

[47]

[48] [49]

2.0 q

[50]

; Sodium perfluorooctanesulfonate. J FCCI3. * Toluene. 12,2'-azobisisobutyramidine hydrochloride. m t r a n s - 1,2-dichloroethylene. "Fikentscher K value. o 5 ml C2F3C13 or CF3CI, hexane, chloroform, or C2C14F2. P Gaseous ethylene oxide. q Without ethylene oxide, intrinsic viscosity was found to be 2.5 dl/gm.

r ~

2.


343

temperature are reached simultaneously within 1.25 hr. During the polymerization, the pressure is maintained at 30 MPa by repressuring as required. After 8 hr the polymerization comes to a stop. The reactor is cooled, the excess monomer is bled off cautiously and the polymer is isolated (yield: 200 Gm or 52%). Interestingly, when 2,2'-azobisisobutyronitrile is substituted for dibenzoyl peroxide in Procedure 2-4, the reaction can be carried out at 70~ with a pressure of only 70 atm (7.0 MPa) for 19 hr. A 90% conversion of a high molecular-weight-polymer is obtained [1, 5]. There are examples of suspension polymerizations of vinyl fluoride making use of suspending agents. A French patent [41] held by Kureha Chemical Industry Co. reports a reaction that is interesting because of three features: (a) the use of dipropyl peroxydicarbonate as an initiator, (b) the use of methyl cellulose as the suspending agent with sodium phosphate as a buffer, and (c) a two-stage heating cycle at two different temperatures and pressures. This process was said to reduce the over-all processing time. Unfortunately, the Chemical Abstracts write-up of this French patent gives details for the polymerization of vinylidene fluoride rather than of vinyl fluoride. We presume that the examples for the polymerization of vinyl fluoride in the original patent are very similar [41 ]. Table Ill outlines the conditions used for a number of suspension polymerization processes of vinyl fluoride [42-50]. Attention is directed in particular to a number of additives which have been used as chain-transfer reagents to control the molecular weights of the polymers.

D.


The work of Kalb et al. [ 1] indicates that the polymerization of vinyl fluoride in the presence of water with a water-soluble initiator may give rise to a polymer dispersed in the water phase and having the small particle diameter associated with latices. However, there is no particular discussion of the properties of poly(vinyl fluoride) latices in this article. The surfactants usually used in the emulsion polymerizations of nonfluorinecontaining monomers appear to be unsatisfactory in the case of vinyl fluoride polymerizations [6]. Salts of the higher perfluorinated carboxylic acids are more suitable emulsifiers. Cationic surfactants based on perfluoroalkylpropylamines are used in a recent patent for the emulsion polymerization of vinyl fluoride [36]. Procedure 2-5 is a patented process outlined here for reference purposes only. In this preparation, the patentee [51] claims that the incorporation of sodium orthosilicate raised the polymer yield to 95%. Without this component, the yield was only 65%. Even when the emulsifier was doubled, the yield of poly(vinyl fluoride), in the absence of sodium orthosilicate, only rose to 70%. In the

344


procedure, Emulsifier L 1159 is believed to be a perfluorinated carboxylated emulsifier from 3M Corp.

2-5.

Emulsion Polymerization of Vinyl Fluoride [51]

In a suitable autoclave, to a solution of 3 gm of sodium orthosilicate, 0.2 gm of ammonium persulfate, and 0.6 gm of Emulsifier L 1159 (3M Corp.) in 200 gm of distilled water is added 100 gm of vinyl fluoride. The autoclave is sealed and heated at 46~ at 42.5 atm (4.25 MPa) for 8 hr. The conversion to poly(vinyl fluoride) by this procedure is 95%. Without sodium orthosilicate the conversion is 65%. When 1.2 gm of Emulsifier L 1159 is used in the absence of the orthosilicate, conversion is 70%. Redox initiators such as potassium persulfate-sodium metabisulfite or ammonium persulfate-sodium sulfite have been patented for use in poly(vinyl fluoride) emulsion polymerizations [52]. In another patent [53], a seeded emulsion polymerization is described. The "seed" polymer is prepared by emulsion polymerization of vinyl fluoride using tert-butyl peroxypivalate as initiator and ammonium co-hydroperfluorononanoate as emulsifier. The process required 9 hr at 40 kg/cm 2 (0.4 MPa). The resultant particles were described as having an average diameter of 18 pm (which we would consider a a bit large for emulsion particles) and are said to be present in a concentration of 2.26 • 1016 particles/dm 3. In the second stage of the process, this seed latex (400 ml in a total volume of 1.8 dm 3) was subjected to the same reaction conditions a second time. By use of between 0.1% and 1% of a non-ionic surfactant of HLB-value between 8 and 15 and surface tension less than 30 dyne/cm, poly(vinyl fluoride) emulsions with particle diameter less than 5 pm have been prepared at 30~176 [54]. Procedure 2-6 is a patented process given here for reference only. It is of interest because the incorporation of an iodine-containing compound such as ammonium iodide, 2,2'-azobisisobutyramidine hydroiodide, potassium iodide, iodine in isopropanol, isopropyl iodide, tetraiodoethylene in tert-butanol, iodobenzene, 2-iodothiophene, or ethyl iodide give rise to polymers of improved thermal stability and resistance to color deterioration [55]. The process does not seem to involve the use of an emulsifying agent.

2-6.

Emulsion Polymerization of Vinyl Fluoride in the Presence of Ammonium lodide [55]

In a suitable stirred autoclave equipped for the addition of aqueous solutions while under pressure to 1.3 dm 3 of water at 75~ and stirred at 180 rpm is added 350 gm of vinyl fluoride. The free space is purged with some of the monomer. Then the pressure in the autoclave is raised to 200 kg/cm 2 (2 MPa). Under

Z

345


pressure a solution of 2,2'-azobisisobutyramidine hydrochloride in 50 ml of water is added followed in turn by 50 ml of rinse water, 25 ml of an aqueous solution of 25 mg of ammonium iodide, and 25 ml of water. After 150 min, the internal pressure of the autoclave drops to 30 kg/cm 2 (0.3 MPa). The reaction mixture is cooled and stirred at 5000 rpm to coagulate the latex. Conversion is 86%. The polymer is said to have a Fikentscher K-value of 89 (at 120~ in a 1% cyclohexanone solution). By careful removal of oxygen and other impurities from the system by passing the monomer over finely divided copper at 50~ prior to use, another patent claims the production of a 13% nonvolatile latex with particle diameter of 0.36/tm using a water-soluble azo compound as initiator at 60~176 and 40-60 kg/cm 2 (0.4-0.6 MPa) of pressure [56]. With 2,2'-azobisisobutyramidine hydrochloride as an initiator for the emulsion polymerization of vinyl fluoride, the use of an anionic surfactant in the dispersing medium leads to ineffective initiator systems. Nonionic surfactants, while compatible with the initiator, generally lead to low yields and low molecular weights of the polymer. Similarly, ordinary cationic emulsifiers lead to colored products of low degree of polymerization [36]. Uschold's patent [36] makes use of fluorinated amine salts as surfactant. In one example, in a stainless steel shaker tube, to 200 ml of deionized water are added 0.10 gm of 2,2'-azobis(isobutyroamidine) dihydrochloride and 0.400 gm of C6Fla(CH2)aNHaC1.The mixture is frozen. The tube is then evacuated and purged with nitrogen. This procedure is repeated three times. The tube is again evacuated and 100 gm of vinyl fluoride is pumped in. After the apparatus is sealed, it is placed on a shaker and heated at 90~ for 3 hr. Then the reactor is cooled, and under environmentally appropriate conditions, the tube is vented. The product is isolated either by filtration or by use of a high speed centrifuge. The polymer is dried in a circulating air oven at 90~ to 100~ The yield of polymer was said to be 27.5 gm [36].

E.

Organometallic

and Related

Initiator Systems

The usual initiators such as 2,2'-azobisisobutyronitrile, benzoyl peroxide, and lauroyl peroxide have to be decomposed to generate the free radicals which bring about the polymerization process. While there are methods which involve the use of radiation to accomplish this, these are rarely advocated; it is more common to use thermal means to generate the initiating species. In order that the product be formed in a reasonable time interval, it is usual to attempt the use of these initiators at temperatures in the range of 60 ~ to 80~ (with the notable exception of diisopropyl peroxydicarbonate which may be used in the range of 25~ to 50~ Such temperatures are above the critical temperature of vinyl fluoride (54.7~ Consequently, regardless of applied pressure, the monomer cannot be liquified above 54.7~ the temperatures at which the initiators are

346

9.

Polymerization of Vinyl Fluoride

most effective. Processing, therefore, suffers from the fact that only relatively low concentrations of monomer can be reacted even at high pressures. For this reason, low-temperature processes are desirable. Low-pressure procedures are also of interest for other reasons, not the least of which are the enhanced safety of the process and the reduction in the complexity of the technology required when switching from a high- to a low-pressure method. The use of ZieglerNatta catalysts or of boron trialkyl systems was explored since these materials frequently are effective at modest temperatures and at low pressures. The effectiveness of Ziegler-Natta catalysts of the triethylaluminum-titanium tetrachloride type seems to be the subject of some controversy. One patent describes the formation of poly(vinyl fluoride) with such a catalytic system in THF in a bottle polymerization at 30~ and autogenous pressure for 6 hr [57]. A complex of triisobutylaluminum, vanadium oxytrichloride, and THF is said to be particularly effective at 30~ both for the homo- and copolymerizations of vinyl fluoride [58, 59]. The processes are said to resemble typical Ziegler-Natta systems and are independent of the THF concentration when the mole ratio of THF to VOC13 was greater than 2.3:1. The use of triisobutylaluminum with tetraisopropoxytitanium at 30~ for 15 min is said to lead to a process with an ionic-coordination mechanism [60]. On the other hand, Usmanov et al. [17] and Sianesi and Caporiccio [20] imply that Ziegler-Natta catalysts of the trialkylaluminum-titanium (or vanadium) tetrachloride type are ineffective because the rate of polymerization is slow and the degree of polymerization of the product is also low [ 19, 20]. Vanadium compounds such as vanadyl acetylacetonate with aluminum compounds of the type AIR(OR)C1 form effective Ziegler-Natta catalysts [19, 20]. The polymers produced by these catalysts are of rather low molecular weight. However, they have crystalline melting points in the range 220~176 substantially higher than those for poly(vinyl fluoride) produced under high pressure by conventional initiators. The polymers are not stereo-regular. The reaction rates and intrinsic viscosities of polymers produced by a vanadyl tris(acetylacetonate)-RAl(OR')C1 (where R and R' may be either methyl, ethyl, or isobutyl groups) catalysts showed little change with variations in the aluminum to vanadium ratios. Trialkylboranes activated by oxygen, strongly increase the rate of polymerization of vinyl fluoride and bring about high degrees of polymerization. While the oxygen-trialkylborane initiators are the most effective ones of the group, complexes of oxygen with dialkylzinc, oxygen with dialkylcadmium, and oxygen with dialkylberyllium also catalyze the polymerization of vinyl fluoride. Oxygen also enhances the activity of the ammonium addition complex of triethylborane, which is reactive even without oxygen. With these initiators, at conversions above approximately 30%, the rates of polymerization decreased. Maximum conversions achieved were on the order of

3.

347

Radiation-Initiated Polymerizations

70%. This phenomenon has been attributed to catalyst depletion during the initiation of a very rapid reaction and to the heterogeneity of the system brought about by the insolubility of the polymer [20]. Procedure 2-7 is an outline of the method used in the polymerization of vinyl fluoride with oxygen-tri-n-propylborane in ethyl acetate as an indifferent solvent.

2-7.

Solution Polymerization of Vinyl Fluoride with Oxygen-tri-n-Propylborane Initiation [20]

With proper safety precautions, to a 50-ml stainless steel bomb filled with dry nitrogen is charged 2 ml of ethyl acetate and 0.028 gm (0.2 x 10-3 mole) of trin-propylborane. The bomb is cooled at liquid air temperature and 15 gm of vinyl fluoride is distilled into the bomb under reduced pressure. The bomb content is then deaerated by repeated freezing and thawing under reduced pressure. Then, to the frozen and evacuated vessel is added from a small calibrated buret 0.0016 gm (0.1 x 10- 3 mole) of oxygen. The bomb is sealed and heated in a constant temperature bath at 30~ for 5 hr. Then the polymerization is stopped by freezing at liquid-air temperature. The bomb is cautiously vented to permit unreacted monomer to escape and residual polymer is washed repeatedly with boiling methanol and dried at 100~ under reduced pressure (yield, 7.5 gm or 50% conversion; intrinsic viscosity, [r/] = 2.8 dl/gm). In general, these reactions were found to have polymerization rates which are first order in the monomer and 0.5 order in the catalyst and oxygen. Activation energies vary with the catalysts used from 6 kcal/mole in the case of triethylborane initiation to 25 kcal/mole for the case of triethylborane-ammonia complex [61]. These initiators are also effective in the preparation of copolymers of vinyl fluoride [20, 62]. Instead of introducing gaseous oxygen into the system to form the oxygentrialkylborane initiator, a patent relates the use of hydrogen peroxides with trialkylborane in aqueous media [63]. Other organometallic initiators which have been mentioned include tetramethyllead (or tetramethyltin) in tert-butanol dispersed in an aqueous solution of ammonium persulfate and borax [64] and silver compounds such as silver nitrate with organic lead or tin compounds and organic promoters such as acetone, dimethyl sulfoxide, ethanol, or tert-butanol [65]. 3.

RADIATION-INITIATED

POLYMERIZATIONS

Early polymerization experiments on vinyl fluoride which yield a reasonable quantity of the polymer involved the use of ultraviolet radiation [37]. This work was carried out in quartz capillary tubes. Sources which emitted ultraviolet

348

9. Polymerizationof VinylFluoride

radiation at less than 280 nm such as a General Electric Type H-4 lamp, without the outer glass bulb, were used. A 4-watt germicidal lamp whose emission was over 90% at 253.7 nm converted 0.6203 gm of the monomer to 0.22 gm of polymer within 2 days at 27~ Low-pressure polymerizations were initiated by ultraviolet radiation in the presence of di-tert-butyl peroxide in bulk, dimethyl sulfoxide, or tert-butanol solution at - 2 0 ~ to + 30~ While the polymer precipitated out of solution at low conversion, in dimethyl sulfoxide, this precipitate was a gel which was partially transparent to light. At low conversions, the reaction kinetics were treated as pseudohomogeneous processes [15]. The growing vinyl fluoride free radical is not resonance stabilized. Therefore it is quite reactive and capable of abstracting a hydrogen atom from active hydrogen compounds such as isopropanol or acetonitrile with consequent chain termination by a chain-transfer mechanism. In the case of a solvent such as dimethyl sulfoxide, such processes do not take place. As a result, the precipitating polymer has difficulties finding appropriate radicals for termination reactions. As in other examples of the gel or Trommsdorff effect, an acceleration in rate is observed because the free radical propagation is not terminated while the viscosity of the product increases and the rate of the transfer of the released heat to the outside is severely limited. In UV-initiated bulk polymerizations at 25~ the polymerization proceeds to a conversion of 90% in 8 hr. The product was described as a white, soft, porous solid with a density of approximately 0.5 gm/cm 3 from a monomer with a density of 0.6 grn/cm 3. The polymer seems to consist of open pores. When a preparation was carried out in which only the bottom of an ampoule containing the monomer is irradiated, the cellular structure forms near the bottom. The liquid monomer then diffuses into the pores and reacts. The result is the gradual formation of a uniform block of transparent, slightly yellow, solid [75]. Procedure 3-1 outlines the procedure for the photopolymerization of vinyl fluoride.

3-1.

P h o t o p o l y m e r i z a t i o n o f Vinyl F l u o r i d e [15]

With suitable safety precautions, purified vinyl fluoride is frozen in an ampoule by immersion in liquid air. The monomer is degassed several times to remove dissolved air. In the meantime, in a thick-walled glass ampoule (wall thickness, 4 mm, i.d. 12 mm, capacity approximately 12 ml) containing di-tertbutylperoxide (concentration: 0.6 • 10 -2 moles per liter of liquid monomer) and dimethyl sulfoxide (50% by volume of the total liquid monomer composition) is attached to a high-vacuum system so that a measured quantity of vinyl fluoride can be transferred to the ampoule beating the solvent and photoinitiator. The solvent is thoroughly degassed and the monomer is transferred to the

4.

Health Aspects

349

ampoule containing the solvent. The ampoule is sealed off and placed in a constant-temperature bath at 25~ From a distance of 8 cm, the ampoule is irradiated with a Hanau Q81, 1.4 .~ mercury lamp. After 30 min, the ampoule is frozen in liquid air and cautiously opened. The content of the ampoule is allowed to warm up gradually while the unreacted monomer is evaporated off. The product is washed repeatedly with methanol and dried under reduced pressure at 60~ (yield, 23.5%). At comparable concentrations of photoinitiator, reaction temperature, and time, solvents such as dimethyl sulfoxide and tert-butanol enhance the conversion while ethyl acetate, hexane, dimethylformamide, isopropanol, acetone, acetonitrile, and hexamethylphosphoramide reduce the conversion. Sulfalone has no solvent effect. The photochemically-initiated polymerization of vinyl fluoride has also been carried out in a continuous-flow, cylindrical reactor at pressures up to 30 atm (30 Pa). The polymers produced by this method had a higher proportion of a head-to-tail arrangement than commercial polymer [21 ]. Considerable work has been done on the initiation of the vinyl fluoride by ionizing radiation much as ),-radiation from a 6~ source. A selection of references on this research includes Usmanov and other authors [4, 48-62]. Of these, Usmanov et al. [4] deal with the graft copolymerization of vinyl fluoride to some natural and synthetic polymers. Usmanov et al. [53] discuss the formation of branched polymers during radiation-induced polymerization. Gubareva et al. [54] deal with solution polymerizations. Nakamura et al. [58, 59, 61] deal with emulsion polymerizations of vinyl fluoride by radiation initiations. Usmanov et al. [60, 61 ] discuss the effects of chain-transfer agents during radiation-initiated polymerization. Some copolymerization studies are described in Usmanov et al. [55]. The polymerization of vinyl fluoride in a glow-discharge or plasma polymerization is reported in Westwood [63] and Kobayashi et al. [64].

4.

HEALTH ASPECTS

Vinyl chloride was once considered to possess a relatively low toxic character. In fact, at one time it was used as a propellant in spray cans used in the home. Only very recently has the carcenogenic nature of this monomer been discovered. In view of this experience it is difficult to evaluate the toxicity of other compounds such as that of vinyl fluoride. One of the most recent studies is dated 1950 [65]--long before the vinyl chloride toxicity had surfaced. In this study, the maximum allowable concentration for a single short exposure of human beings for vinyl fluoride is given as a concentration of 20% by volume. The effects of long-term exposure or the effect of a single exposure after 20 to 30 yr

350


were not considered then and may very well not have been studied more recently. Therefore we recommend extreme caution in handling vinyl fluoride until such time as reliable medical studies on the toxicology of this monomer have been made.

5.

MISCELLANEOUS

1. 2. 3. 4. 5.

Copolymerization studies [22, 73, 84, 85]. Copolymerization with fluoroketones [86]. Donor-acceptor complex polymerization [87]. Vinyl fluoride polymerization with azobisisobutyramidines [88]. Preparation of head-to-head, tail-to-tail poly(vinyl chloride) and poly(vinyl bromide) by halogenation of 1,4-cis-poly(butadiene) [89]. 6. Synthesis of 3-fluoroacrylic acid by hydrolysis of the reaction product from the reaction of vinyl fluoride and carbon tetrabromide [90]. 7. X-ray diffraction pole figure measurements on a poly(vinyl fluoride) film [91].

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References

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20. D. Sianesi and G. Caporiccio, dr. Polym. Sci., Part A-1 6, 335 (1968). 21. A. Korin, M. Levy, and D. Vefsi, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 20(1), 672 (1979). 22. Kh. U. Usmanov, A. A. Yul'chibaev, A. A. Mat'yakabov, Kh. Kuzieva, and S. Masharipov, Dokl. Akad. Nauk Uzb. SSR 29(12), 24 (1972); Chem. Abstr. 79, 5695 (1973). 23. A. D. Sorokin, E. V. Volkova, and R. A. Naberezhnykh, Radiats. Khim. 2, 295 (1972); from Ref. Zh. Kim. 1973, Abstr. No. 15177 Chem. Abstr. 81, 106152 (1974). 24. R. E. Cais and J. M. Kometani, Macromolecules 17, 1887 (1984). 25. R. E. Cais and J. M. Kometani, Macromolecules 17, 1932 (1984). 26. R. E. Cais and J. M. Kometani, Macromolecules 18, 1354 (1985). 27. R. E. Cais and J. M. Kometani, Polymer 29, 168 (1988). 28. M. D. Bruch, F. A. Bovey, and R. E. Cais, Macromolecules 17, 2547 (1984). 29. G. Trappe, in "Vinyl and Allied Polymers" (P. D. Ritchie, ed.), Vol. 1, p. 260. Iliffe, London, 1968. 30. J. D. Delorme, Rev. Gen. Caoutch. Plast., Ed. Plast. 5(4), 216 (1968). 31. V. A. Korin'ko, M. I. Levinskii, E. A. Chaika, and A. L. Englin, Usp. Khim. 39(1), 94 (1970); Chem. Abstr. 72, 111827 (1970). 32. Kh. U. Usmanov, A. A. Yul'chibaev, G. S. Dorkin, M. K. Asamov, A. Valier, Kh. Kuzieva, and S. G. Yul'chibaev, Nauchn. Tr., Tachk. Gos. Univ. 399, 64 (1970); Chem. Abstr. 77, 152810 (1972). 33. A. A. Yul'chibaev, M. K. Asamov, S. G. Yul'chibaev, A. Valiev, T. Latypov, K. R. Khalikov, Kh. Kuzieva, A. Matyakubov, R. Aslanova et al., Proc. Tihany Syrup. Radiat. Chem., 3rd 1971, Vol. 1, p. 1025 (1972); Chem. Abstr. 78, 137698 (1973). 34. D. Brasure and S. Ebnesajjad, in "Concise Encyclopedia of Polymer Science and Technology" (J. I. Kroschwitz, ed.), pp. 1273ff. Wiley, New York, 1990. 35. S. Ebnesajjad and L. G. Snow, "Kirk-Othmer Encyclopedia of Chemical Technology," 4th ed., Vol. 11, p. 683. Wiley, New York, 1994. 36. R. E. Uschold, U.S. Patent 5,229,480 (1993). 37. A. E. Newkirk, J. Am. Chem. Soc. 68, 2467 (1946). 38. Kh. U. Usamanov, A. A. Yul'chibaev, A. Kh. Gafurov, and V. G. Kolyodin, Vysokomol. Soedin., Ser. B 15(2), 124 (1973); Chem. Abstr. 79, 19188 (1973). 39. T. Nishida and K. Itoi, Japanese Patent 70/18,463 (1970); Chem. Abstr. 73, 99406 (1970). 40. Pittsburgh Plate Glass Co., Netherlands Patent Appl. 6,607,093 (1966); Chem. Abstr. 66, 86116 (1967). 41. Kureha Chemical Industry Co., Ltd., French Patent 1,566,920 (1969); Chem. ,4bstr. 71, 125237 (1969). 42. R. Iwa and Y. Adachi, Japanese Patent 74/27,108 (1974); Chem. Abstr. 82, 86877 (1975). 43. B. Tatsuya, A. Tanaka, and K. Yamashita, Japanese Patent 74/28,670 (1974); Chem. Abstr. 82, 73661 (1975). 44. T. Kawai, M. Ootsuka, and K. Matsuoka, Japan Kokai 74/104,985 (1974); Chem. Abstr. 83, 79961 (1975). 45. Y. Tamura, Y. Shishido, and S. Negishi, Japan Kokai 72/34,785 (1972); Chem. Abstr. 78, 137022 (1973). 46. Y. Tamura and Y. Shishido, Japan Kokai 73/30,787 (1973); Chem. Abstr. 79, 67068 (1973). 47. Y. Tamura and Y. Shishido, Japan Kokai 73/29,883 (1973); Chem. Abstr..79, 67069 (1973). 48. G. Bier, W. Trautvetter, and G. Weisgerber, Fr. Demande 2,004,908 (1969); Chem. Abstr. 72, 122445f (1970). 49. S. Negishi, A. Yonemura, and J. Tamura, Japanese Patent 71/09,261 (1971); Chem. Abstr. 75, 21444 (1971). 50. T. Nishida and Y. Iikubo, Japanese Patent 73/32,579 (1973); Chem. Abstr. 81, 26268 (1974).

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51. Deutsche Solvay-Werke, G.m.b.H. French Patent 1,560,029 (1969); Chem. Abstr. 71, 81896 (1969). 52. L. E. Scoggins, U.S. Patent 3,573,242 (1971); Chem. Abstr. 75, 6639 (1971). 53. M. Tatemoto and S. Sakata, Japanese Patent 74/43,386 (1974); Chem. Abstr. 82, 157053 (1975). 54. S. Yoshida, T. Masui, and Y. Matsunaga, Japanese Patent 74/28,669 (1974); Chem. Abstr. 82, 73874 (1975). 55. Dynamit Nobel A.-G., Fr. Demande 2,004,758 (1969); Chem. Abstr. 72, 112058 (1970). 56. F. Engl~inder and G. Meyer, Ger. Often. 2,321,121 (1974); Chem. Abstr. 82, 86864 (1975). 57. G. F. Helfrich and E. J. Rothermel, Jr., U.S. Patent 3,380,977 (1968); Chem. Abstr. 68, 115212 (1968). 58. R. N. Haszeldine, T. G. Hyde, and P. J. T. Tait, Polymer 14(5), 221 (1973). 59. R. N. Haszeldine, T. G. Hyde, and P. J. T. Tait, Polymer 14(5), 224 (1973). 60. G. Caporiccio and D. Sianesi, Chim. Ind. (Milan) 52(2), 139 (1970). 61. G. Caporiccio, E. Strepparola, and D. Sianesi, Chim. Ind. (Milan) 52(1), 28 (1970). 62. G. Caporiccio and D. Sianesi, Chim. Ind. (Milan) 52(1), 37 (1970). 63. Y. Iikubo, T. Nishida, and Y. Furukawa, U.S. Patent 3,645,998 (1972); Chem. Abstr. 76, 154464 (1972). 64. D. Sianesi and G. Caporiccio, French Patent 1,464,332 (1966); Chem. Abstr. 67, 54590 (1967). 65. A. Damiel, M. Levy, and D. Vovsi, Ger. Often. 2,227,914 (1973); Chem. Abstr. 78, 98285 (1973). 66. L. A. Bulygina and E. V. Volkova, Radiats. Khim. Polym., Mater. Simp. 1964, p. 122 (1966); Chem. Abstr. 66, 95567 (1967). 67. E. V. Volkova, P. V. Zimakov, A. V. Fokin, A. D. Sorokin, V. M. Belikov, L. A. Bulygina, A. I. Skobina, and L. A. Krasnousov, Radiats. Khim. Polym., Mater. Simp., 1964, p. 109 (1966); Chem. Abstr. 67, 22304 (1967). 68. Kh. U. Usmanov, A. A. Yul'chibaev, G. S. Dordzhin, and Kh. Yuldasheva, Proc. Tihany Symp. Radiat. Chem., 2nd, 1966, p. 511 (1967); Chem. Abstr. 68, 30135 (1968). 69. E. V. Volkova, P. V. Zimakov, and A. V. Fokin, At. Energ. 26(3), 240 (1969); Chem. Abstr. 71, 3891 (1969). 70. S. S. Dubov, M. A. Landau, E. V. Volkova, and L. A. Bulygina, Zh. Fiz. Khim. 43(6), 1574 (1969); Chem. Abstr. 71, 102274 (1969). 71. Kh. U. Usmanov, A. A. Yul'chibaev, and T. Sirlibaev, J. Polym. Sci., Part A-1 9, 1779 (1971). 72. L. L. Gubareva, G. S. Dordzin, A. A. Yulchibaev and Kh. U. Usmanov, Izv. Yyssh. Ucheb. Zaved., Khim. Khim. Tekhnol. 14(8), 1252 (1971); Chem. Abstr. 76, 25659 (1972). 73. Kh. U. Usmanov, A. A. Yul'chibaev, A. A. Mat'yakubov, Kh. Kuzieva, and S. Kazakov, Dokl. Akad. Nauk Uzb SSR 29(6), 37 (1972); Chem. Abstr. 79, 44039 (1973). 74. A. M. Zaozerov, A. D. Sorokin, L. A. Bulygina, E. V. Vokova, and A. V. Fokin, Dokl. Akad. Nauk SSSR 210(2), 349 (1973); Chem. Abstr. 79, 79285 (1973). 75. N. Nakamura and M. Yoneya, Japanese Patent 73/01,830 (1973); Chem. Abstr. 80, 27677 (1974). 76. K. Nakamura, M. Ichimura, and Y. Fukushima, Japanese Patent 73/08,755 (1973); Chem. Abstr. 80, 109244 (1974). 77. K. Nakamura, M. Ichimura, and K. Fukushima, Japanese Patent 73/37,754 (1973); Chem. Abstr. 81, 38346 (1974). 78. Kh. Usmanov, A. A. Yul'chibaev, G. S. Dordzhin, and L. L. Gubareva, Deposited Publ. 1973, VINITI 5785; Chem. Abstr. 85, 21954 (1976). 79. T. S. Sirlibaev, A. A. Yul'chibaev, Kh. U. Usmanov, and V. G. Kalyadin, Uzb. Khim. Zh. No. 2, p. 37 (1976); Chem. Abstr. 85, 63586 (1976).

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Chapter 10

Miscellaneous Polymer Preparations 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Preparation of Miscellaneous Organometallic and Metal-Containing Polymers . . . . . . . .

2-1. 2-2. 2-3. 2-4.

Preparation and Polymerization of p-Trimethylleadstyrene . . . . . . . . . . . . . . Preparation of Polyvinylferrocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Polymeric Metal Phthalocyanines . . . . . . . . . . . . . . . . . . . . . Polyaddition of Diphenyltin Dihydride and N,N'-Ethylene Bis-acrylamide . . .

3. Poly(metal phosphinate)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-1. Preparation of Poly[zinc(II)diphenylphosphinate] . . . . . . . . . . . . . . . . . . . . 3-2. Preparation of Poly[zinc(II)methylphenylphosphinate] . . . . . . . . . . . . . . . . 4. Modification of Existing Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Chlorination of Poly(vinyl chloride) (PVC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-1. Preparation of Chlorinated Poly(vinyl chloride) . . . . . . . . . . . . . . . . . . . . . . B. Diels-Alder Reaction of Triene Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-2. Preparation of 1,6-Poly(1,3,5-hexatriene)-Tetracyanoethylene Adduct . . . . . . 4-3. Preparation of Dibromocarbene-Modified Polybutadiene Rubber. i . . . . . . . . 5. Miscellaneous Polymer-Forming Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-1. 5-2. 5-3. 5-4. 5-5. 5-6. 5-7.

Preparation of Poly(diallyldiethylammonium bromide) . . . . . . . . . . . . . . . . . Preparation of Polyimidate Resins from Ethylene Cyanohydrin . . . . . . . . . . . Preparation of Polyamidine from Bisketenimines and Diamine . . . . . . . . . . . Preparation and Polymerization of Vinylene Carbonate . . . . . . . . . . . . . . . . Preparation ofAliphatic Poly-l,3,4-oxadiazoles . . . . . . . . . . . . . . . . . . . . . . Polymerization of m-Diethylnylbenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Poly-p-xylylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Preparation of Polymers from Propargyl-Terminated Monomers . . . . . . . . . . . . . . . . . .

355 356 356

357 358 359 359 360 360 360 361 361 362 362 364 365 366 367 367 370 373 374 376 376

6-1. Preparation of Dipropargyloxy Ether of Sulfonyldiphenol (4,41-Dihydroxydiphenyl Sulfone) and Its Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . 377 6-2. Preparation of the Dipropargyl Ether of Bisphenol A . . . . . . . . . . . . . . . . . . 378 354

1.

Introduction

7. Recent Polymers Based on Fullerenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Polycyanurates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Starburst Dendrimers or Star-Shaped Dendritic Macromolecules . . . . . . . . . . . . . . . . . .

355 380 381 382

9.1. Preparation of a Third Generation Polyamine Dendrimer Starting from Methyl Acrylate, Ammonia, and Ethylenediamine Using Various Reaction Sequences. 383 10. Polymerizations in Supercritical Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10-1 Polymerization of Styrene in Supercritical Carbon Dioxide . . . . . . . . . . . . . . 11. Metal-Containing Polymers as Polymeric Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . .

386 387 388

11-1. Preparation of Anthranilic Acid Anchored to Polystyrene and Complexed with RhC13.3HeO Useful for the Hydrogenation of Olefins . . . . . . . . . . . . . . . . . 391 12. Polymer Particles of Uniform Size Distribution (Monodisperse Latices) . . . . . . . . . . . .

12-1. Preparation of Monodispersed Styrene-co-p-Sodium Styrenesulfonate Latex . . 12-2. Preparation of a Styrene-co-Acrylamide Latex . . . . . . . . . . . . . . . . . . . . . . . 12-3. Chemical Modification of the Functionalities of Poly(styrene-co-Acrylamide) Latices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-4. Monodispersed Crosslinked Microspheres by Dispersion Polymerization . . . . 12-5 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. Optically Active Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Naturally Occurring Chiral Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Introduction of Optical Activity into Inactive Polymers . . . . . . . . . . . . . . . . . . . . .

13-1. Reaction of Chloromethylated Polystyrene with a Tertiary Amine . . . . . . . . . 13-2. Preparation of (+)-Poly-[4-(2-hydroxyethyl)styrene by Asymmetric Reduction C. Polymerization of Optically Active Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13-3. 13-4. 13-5. 13-6.

Low Temperature Cationic Polymerization of ( + )-3-Methyl-l-pentene . . . . . . Polymerization of l-tx-Methylbenzyl Methacrylate . . . . . . . . . . . . . . . . . . . . . Polymerization of o-Vinylbenzyl D-sec-Butyl Sulfide . . . . . . . . . . . . . . . . . . . Polymerization of l-Propylene Oxide with Potassium Hydroxide . . . . . . . . . .

D. Separation and Polymerization of Racemic Monomers . . . . . . . . . . . . . . . . . . . . . . E. Cationically Induced Polymerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

392 396 397 398 401 401 401 402 402 403 404 404 405 406 408 408 409 410

13-7. Summary of the Polymerization Procedure for the Production of a Poly (benzofuran) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 F. Polyisocyanates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13-8. Polymerization of (R)-2, 6-Dimethylheptyl Isocyanate . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.

410

411 411

INTRODUCTION

Several areas of polymer preparations that extend the general scope of the materials covered in Vols. I-III of this series are discussed. The main areas covered are organometallic polymers and modification of existing polymers. In addition a variety of miscellaneous polymers are presented that are of general interest.

356

10. Miscellaneous Polymer Preparations

PREPARATION OF MISCELLANEOUS ORGANOMETALLIC AND METAL-CONTAINING POLYMERS

0

The preparations of several organometallic polymers are given here for reference to some more common types of polymers. They are the vinyl type, complex metal phthalocyanines, organotin hydride-olefin condensations, and the poly(metal phosphinates). More detailed reviews on organometallic polymers should be consulted for additional information [1, 2]. CAUTION: All of these monomers are extremely toxic.

2-1.

Preparation and Polymerization of p-TrimethyUeadstyrene [3]

a. Preparation ofp-Trimethyneadstyrene

CH2 -- CH-~---~?MgCI + (CHa)aPbBr THF CH2-- CH - ~ - - - / ~ Pb(CH3)3

(1)

To a 250-ml, three-necked, round-bottom flask, equipped with a stirrer, dropping funnel, and condenser is added a solution of 0.1 mole of p-vinylphenylmagnesium chloride in 50 ml THF (THF is distilled to remove peroxides before use). A solution of 24.9 gm (0.07 mole) of trimethyllead bromide in 75 ml THF is added and the reaction is exothermic. The temperature of the reaction is kept below 30~ by cooling and when the exothermic reaction ceases the reaction mixture is kept at 35~ for 8 hr. Then 100 mg p-tert-butylcatecholis added and the mixture is added carefully to an ice-cold saturated ammonium chloride solution. The organic layer is separated and the aqueous layer extracted with ether. The combined organic layer is dried over magnesium sulfate. The solvent is removed under reduced pressure and the product distilled to give 22 gm crude product (b.p. 77.5~ at 0.0003 mm Hg; 86~ at 0.01 mm Hg). This fraction is redistilled from a 30-in. Vigreux column to give 17.9 gm (67%) of a slightly yellow oil (b.p. 60~ ~ at 0.0015 mm Hg, n 2~ 1.6070, and d 2~ 1.7278).

b.

Polymerization ofp-Trimethylleadstyrene CH-- CH2

--CH--CH2-(2)

Pb(CH3)3

Pb(CH3)3

~

2. MiscellaneousOrganometallicand Metal-ContainingPolymers

357

One to two grams of 0.1 mol% of 2,2'-azobisisobutyronitrile in p-trimethylleadstyrene is sealed in a glass ampoule. The ampoule is repeatedly evacuated and flushed with oxygen-free nitrogen before sealing under vacuum. The ampoule is heated at 70~ for 24 hr, cooled, and opened. The contents are dissolved in 20 ml benzene and the polymer isolated by precipitation in 400 ml methanol. The polymer is isolated as a white powder after filtration, washing in methanol, and drying at 60~ in a vacuum oven to constant weight. Similarly p-triphenylleadstyrene can be prepared and homo-or copolymerized (with vinyltoluene) [4]. In addition, other [p-(CH3)3M(IV)C6H4CH--CH2] trimethylmetalstyrenes have been prepared and polymerized using a procedure similar to the foregoing one (M = Ge, Sn) [5].

2-2.

P r e p a r a t i o n o f P o l y v i n y l f e r r o c e n e [6] CH2=CH

" --CH2--CH--"

ii:.F,'e:il

(3)

.

To 5 gm vinylferrocene in 5 ml benzene in Fischer-Porter aerosol compatibility tubes equipped with valves is added 2.52 wt% of azobisisobutyronitrile, based on vinylferrocene. (Caution: benzene is extremely toxic.) The tubes are degassed by three alternate freeze-thaw cycles and then filled with nitrogen to slightly above ambient pressure and placed in a 80~ constant-temperature bath. After 2 hr, another portion (2.48 wt%) of azobisisobutyronitrile is added and the heating continued for a total of 20 hr. The tube is cooled, vented, and the polymer is precipitated in methanol. The polymer is purified by two more precipitations and then dried at 70~176 under vacuum to give 3.1 gm (62.4% yield), m.p. 281 ~176 (yellow powder). The MW of the polymer from gel permeation chromatography (GPC) is 4,000-280,000 (37/w). The UV spectrum in methylene chloride exhibits/].max at 440, 323, 260 and 232 nm with extinction coefficients of 109, 4960, 6660, and 6460, respectively. The IR in KBr exhibits bands at 3095, 2860-2990, 1360, 1218, 1103, 1038, 1020, 997, and 670 cm-1 and resembled spectra for monoalkyl ferrocenes. The glass transition temperature was estimated to be 190~ from the broad curves obtained from differential-scanning calorimetry (DSC).

358

10.

Miscellaneous

Polymer Preparations

2-3. Preparation of Polymeric Metal Phthalocyanines [7] O II

,c~c,

O II

O\C~c/O

II O

o II + CuC12 + H2N-- C--NH2

II O

HO

OH

I

I

O=C

o II

o

C=O

N=C.wC--N I II _

HO--C~C II

CuCI2

II

/

0il

u

I

C~"-~C--OH I II

N-c"N'c-N

o

(4) O II HO--C~C II 0

N-- C\N~C-- N

I

II

/ ~

l c~N\c

C'~-~"C--OH I II --N 0

--

O=C

I HO

O II

C=O

I OH

To 20 gm (0.092 mole) of pyromellitic dianhydride, 8.0 gm (0.06 mole) of anhydrous copper chloride, and 108 gm (1.8 mole) of urea is added 0.1-0.4 gm ammonium molybdate catalyst. The reaction mixture is heated at 160~ for 30 min. The final product is washed with 6 N HC1, dissolved in 200 ml conc H2SO4 and reprecipitated by dilution with water. The precipitate is washed with water (24 liter), filtered, and dried to give a product with MW approximately 1500 (dimer). The highest molecular weight obtained is 4,000 (6 repeat units)

359

3. Poly(metal phosphinate)s

using 16.0 gm (0.28 mole) CuC12 and 5.0 gm (0.023 mole) of pyromellitic dianhydride at a reaction temperature of 180~

2-4. Polyaddition of Diphenyltin Dihydride and N,N 'Ethylene Bis-acrylamide [8] O

O

II

II

C H 2 - - C H - - C - - H N C H 2 C H 2 - - N H C - - C H - - C H 2 + (C6Hs)2SnH2 C6H5

I

O

II

Cat

O

II

-

(5)

-- Sn-- CH2-- CH2C--NHCH2NHC--CH2--CH2--

I

C6H5

n

a. Preparation ofN, N'-Ethylene Bis-acrylamide The product was obtained in 83% yield by the reaction of acryoyl chloride and ethylenediamine by an interfacial condensation in a chloroform water system. Crystallization from acetone gave a 64% yield of colorless material (m.p. 142~176

b. Condensation Polymerization with Diphenyltin Dihydride

CAUTION"

Benzene is a known carcinogen and must be handled with great

care.

To a flask containing 5.28 gm (0.0314 mole) of N,N'-ethylene bis-acrylamide in 40 ml benzene is added 8.63 gm (0.0314 mole) of diphenyltin dihydride and the mixture is heated to reflux. After 5-hr refluxing the benzene is removed under reduced pressure and the residue heated at 95~ for 4 hr. The infrared spectrum of the solid indicated no free Sn--H absorption. The product is extracted with acetone for 6 hr to remove low-molecular-weight species and then the residue is dissolved in DMF. The DMF solution is centrifuged and then evaporated to dryness to afford 8.3 gm (60%) of polymer (m.p. 110~176 The related additives of diphenyltin dihydride with ethylene glycol dimethacrylate gave a Mw of 150,000. The addition of diphenyltin dihydride to diacetylenic compounds is also reported to give polymers.

3.

POLY(METAL PHOSPHINATE)S

The work in this field was earlier reviewed by Block who summarized the work through 1968 [9]. The interest in these materials is partly due to their

360

10.

Miscellaneous


thermal stability and the fact that some polymers may also have interesting high-temperature film or coating applications. Several other references to these materials should be consulted for additional details [10-11 ].

3-1.

Preparation of Poly[zinc(ll)diphenyl phosphinate] [11] O II (CH3COO)2Zn + (C6Hs)2P -- OH

,

--Zn~OP--O-\

(6)

C6H5

n

To 20 mmoles of diphenyl phosphinic acid in 300 ml of 95% ethanol is added 10 mmole zinc acetate dihydrate dissolved in 100 ml water. Then 19.7 mmole of aqueous sodium hydroxide is added and a precipitate forms. The white precipitate is filtered, washed several times with water and acetone, and dried in a vacuum oven at 110~ to give 4.2 gm (85%). The polymer is insoluble in water and common organic solvents and is infusible to above 450~

3-2.

Preparation of Poly[zinc(ll)methylphenyl phosphinate] [11]

O

(CH3COO)zZn + 2CH3 -- P -- OH

I

C6H5

~

II

I i OH3 --Zn - - O - - P - - O - -

I

C6H5

(7)

n

To a stirred solution of 11.09 gm (71.0 mmole) of methylphenyl phosphinic acid in 600 ml of ethanol is added 7.72 gm (35.5 mmole) of finely divided zinc acetate dihydrate. A white precipitate results and the mixture is stirred for another 2 hr at room temperature. The precipitate is filtered and washed twice with ethanol to give 11.8 gm (95%) after drying at 60~ in a vacuum oven. The TGA of the polymer indicates it is stable to about 400~ in nitrogen without weight loss. Weight loss starts at 425~ The polymer softens below 100~ and is soluble in benzene and chloroform. The molecular weight of the polymer varies between 2500 and 10,000 (colligative methods).

4.

M O D I F I C A T I O N OF E X I S T I N G P O L Y M E R S

The modification of the polymer backbone by selective reactions (halogenation, esterification, etherification, oxidation, reduction, Diels-Alder reactions, dehydrations, dehalogenations, cyclization, grafting [12], etc.) further extend the usefulness of a particular polymer.

4. Modification of Existing Polymers

361

The chlorination of poly(vinyl chloride) (PVC) and the use of unsaturated polymers for further reaction is presented in this section only as an example of some common techniques used. The use of unsaturated polyesters is described in Vol. II of this series [13]. The reaction of polyvinyl alcohol to give polyacetals or polyketals has been already described in Vol. II of this series. In addition the conversion of polyhydrazides to polyoxadiazoles, and polymeric acids to polyimides is also described in Vol. I of this series [ 14] and in Section 5 of this chapter.

A.

Chlorination of Poly(vinyl chloride) (PVC)

PVC was first chlorinated in Germany to 65% chlorine content to give a polymer more soluble than PVC. The soluble polymer was used as a lacquer for the manufacture of fibers by the wet-spinning process. Renewed interest was started in 1962 by B. F. Goodrich [15], which promoted the resin for hightemperature, water-pipe applications in which PVC was not suitable. Other references to preparations are described in several reviews [ 16]. More thermally stable chlorination products are reported to be obtained by introducing chlorine gas containing chlorinated hydrocarbon vapors to PVC suspended in an aqueous system and irradiating without light in the visible and or short wavelength spectrum as described further on.

4-1.

Preparation of Chlorinated Poly(vinyi chloride) [17]

C12

CH2--CH-+i-I C1

--CH-C1

H-- ----CH2--C-- ----CH2--CH-I I C1

C1

(8)

C1

In a 4-liter, glass-lined vessel equipped with a mechanical stirrer, light source (70 watt UV lamp), and chlorine gas inlet and outlet tubes is added 425 gm of PVC (suspension type have a K value of 68 (see [17a]) of 24% water content (fresh from filtered PVC polymerization). Then 900 ml of conc HC1 and 300 ml water are added. The flask is warmed to 35~ and then 20 liter of chlorine is passed over free space to remove air. The UV lamp is turned on and chlorine is

362

10. Miscellaneous Polymer Preparations

passed through a chloroform saturator at 40~ to the reaction flask. After 56 liter of chlorine is fed into the system over a 85-min period. The UV lamp is turned off and then nitrogen is passed through the mixture to remove free chlorine. A total of 185 gm of chloroform is absorbed by the chlorine gas in circulation. The product is washed with water until neutral and purified by steam distillation. The dried product has a 66.9% chlorine content. The Vicat heat is 119.5~

B.

Diels-Alder Reaction of Triene Polymers

Polytrienes undergo the Diels-Alder reaction With strong dieneophiles and tetracyanoethylene, sulfur dioxide, maleic anhydride, etc., as shown in Eq. (9) and Table I [18]. --CH--CH2

CN

CN

['--CH--CH2--

/ C--C \ CN CN

(9)

n

4.2.

Preparation of 1,6-Poly(1,3,5-hexatriene)Tetracyanoethylene Adduct

CAUTION:

Benzene is carcinogenic and must be handled with great care.

a. Preparation of 1,6-Polyhexatriene (Amorphous) [ 19] To a cooled ( - 25~ 100-ml flask previously flame-dried and blanketed with nitrogen is added via a syringe a solution of 0.17 gm (0.4 mmole) of tetraphenyl titanate and 0.02 ml (0.2 mmole) vanadyl trichloride in 55 ml of chlorobenzene. Then a 3-ml portion (3.0 mmole) of a 1 M solution of di-ethylaluminum chloride in heptane to which a molar equivalent of anisole has been added is injected into the flask. Then 5 ml (0.045 mole) hexatriene is added via a syringe and stirred at - 2 5 ~ for 24 hr. The resulting viscous solution is carefully quenched by pouting into cold, acidified (3% HC1) ethanol to precipitate the polymer. The polymer is washed with cold, acidified ethanol, absolute ethanol, and then dissolved in benzene to remove insolubles. The polymer (2.75 gm 78%) is stable in the benzene solution ([/~]inh 0.5% in benzene at 30~ of 0.61). The polymer on drying becomes crosslinked but is stable in solution. The infrared spectrum of 1,6-polyhexatriene is shown in Fig. 1.

TABLE I

Typical Postreactions of Polytrienes a

Polymer

Time (hr)

Temp (~

Adduct r/inhc

TCNE

4

25

0.52

Benzene d

TCNE

24

25

0.62

0.99

THF

TCNE

24

25

0.64

Anionic polyhexatriene

1.15

THF

TCNE

4

25

0.98

1,6-Polyhexatriene

0.61

THF

CN(CF3)C = C - - CN(CF3)

5

65

0.63

1,6-Polyhexatriene

0.61

Toluene

Maleic anhydride

72

25

1,6-Polyhexatriene

0.61

Toluene

Sulfur dioxide

5

65

1,6-Polyhexatriene

0.61

Benzene d

Dimethyl acetylenedicarboxylate

5.5

80

Base polymer

qinhb

Solvent

1,6-Polyhexatriene

0.61

Benzene d THF

1,6-Polyheptatriene

0.88

2,7-Polyoctatriene

Dienophile

Remarks 45% reaction of trans, trans-diene units 50% reaction of trans, trans-units 43% reaction of trans, trans-dienes 38% reaction of trans, trans- and trans-vinyl dienes 45% reaction of trans, trans-units; C-F bands in infrared 55-60% reaction of trans, trans-units; infrared bands at 1820, 1765, and 1240 - 1 < 10% reaction of trans, trans-units; sulfone bands at 1300 -1 and 1125 cm > 75% reaction of trans, trans-dienes; infrared bands at -1 1750-1700 and 1250 cm

a Reprinted from V. L. Bell, J. Polym. Sci., Part A 2, 5305 (1964). Copyright 1964 by the American Chemical Society. Reprinted by permission of the copyright owner. b 0.5% in benzene at 30~ c 0.5% in THF at 30~ d Caution." Benzene is carcinogenic and must be handled with great care.

ta~ O~

364

10.

Miscellaneous


0.0

0.1

o C

0.2

=. 0.3

a ,12

Polymer Synthesis, Volume 3, Second Edition (Organic Chemistry, a Series of Monographs)

The Organic Chemistry of Drug Synthesis (Organic Chemistry Series of Drug Synthesis) (Volume 7)

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Volume 3, Fiesers' Reagents for Organic Synthesis

Stereoelectronic Effects in Organic Chemistry (Organic Chemistry Series Volume 1)

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