Ozonation of Organic & Polymer Compounds
Authors: Gennady E. Zaikov and Slavcho K. Rakovsky
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Ozonation of Organic & Polymer Compounds
Authors: Gennady E. Zaikov and Slavcho K. Rakovsky
Ozonation of Organic and Polymer Compounds
Gennady E. Zaikov and Slavcho K. Rakovsky
iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net
First Published in 2009 by
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C
ontents
Introduction....................................................................................................1 1
Kinetics and Mechanism of Ozone Reactions with Organic and Polymeric Compounds in the Liquid Phase........................................9
1.1
Reaction mechanisms .........................................................9 1.1.1
1.2
2
Thermochemical analysis ..............................................11
Structural-kinetic investigations ........................................12 1.2.1
Normal paraffins and isoparaffins ................................12
1.2.2
Cycloparaffins ..............................................................20
1.2.3
Method for estimation of reaction mechanisms ...........26
1.2.4
Application of ERM .....................................................29
1.3
Cyclohexane .....................................................................36
1.4
Cumene ............................................................................47 1.4.1
Ozonation in the presence of transition metal compounds ...................................................................51
1.4.2
Ozonolysis in the presence of NiO ...............................57
1.4.3
Ozonolysis in the presence of Mo and V oxides ...........60
1.4.4
Cumenehydroperoxide .................................................64
1.5
Polyethylene and polypropylene .......................................68
1.6
Polystyren .........................................................................74
Ozonolysis of oxygen-containing organic compounds ..................93
2.1
Alcohols ...... 932.1.1 Application of Estimation of Reaction Mechanism (ERM) .........................................................100
2.2
Ketones ...........................................................................102 iii
Ozonation of Organic and Polymer Compounds
2.3
Hydrotrioxides ...............................................................110
2.4
Synthesis of oxolanes ......................................................124
2.5
2.4.1
Derivatives of 4-hydroxymethyl-1,3-dioxolane ...........124
2.4.2
Phenyl ethers of 4-hydroxymethyl-1,3-dioxolane .......124
2.4.3
Alkyl ethers of 4-hydroxymethyl-1,3-dioxolane .........125
2.4.4
5-Nonylene-1,2,4-trioxolane ......................................127
Ethers .............................................................................130 2.5.1
2.6
Hydroxybenzenes ...........................................................140 2.6.1
2.7
2.8 3
Application of ERM ..................................................146
Carbohydrates and model compounds ............................149 2.7.1
2,3-Butanediol ...........................................................153
2.7.2
1,2-Cyclohexanediol ...................................................155
2.7.3
Mannitol and its derivatives .......................................156
2.7.4
A-D-Glucose ...............................................................158
2.7.5
A-D-Methyl-glucose....................................................160
2.7.6
B-Cyclodextrine and starch .........................................161
Catalytic ozonolysis and oxidation .................................162
Ozonolysis of alkenes in liquid phase............................................179
3.1
3.2
iv
Application of the ERM ............................................135
Olefins ............................................................................179 3.1.1
Mechanisms ...............................................................179
3.1.2
Kinetics ......................................................................189 3.1.2.1
Gas phase ...................................................189
3.1.2.2
Liquid phase ...............................................191 3.1.2.2.1
Effect of solvent .......................203
3.1.2.2.2
Effect of configuration ..............205
3.1.2.2.3
Effect of structure .....................209
Polydienes .......................................................................216
Contents
4
3.2.1
Polybutadiene .............................................................219
3.2.2
Cis-1,4-polyisoprene ...................................................227
3.2.3
Polychloroprene .........................................................228
3.2.4
Butadienenitrile rubbers .............................................230
3.2.5
Ethylenepropylene rubbers .........................................235
Degradation and Stabilisation of Rubber ......................................251
4.1
Ozonolysis of Elastomers in Elastic State ........................252
4.2
Antiozonants ..................................................................261 4.2.1
Mechanism of Action .................................................263
4.2.2
Synthesis .....................................................................267 4.2.2.1
Paraphenylenediamine ................................267 4.2.2.1.1
Alkylation in the Presence of Hydrogen ......................................267
4.2.2.1.2
Alkylation in the Absence of Hydrogen ......................................273
4.2.2.2
Hydroquinolines.........................................275
4.2.2.3
N,N´-Disubstituted Hexahydropyrimidines 276
4.2.2.4
N-Substituted Dimethylpyrols ....................276
4.2.2.5
Enamines ....................................................276
4.2.2.6
Nitrone Compounds ...................................277
4.2.2.7
Derivatives of 3(5)-Methylpyrazone ...........278
4.2.2.8
Enolethers ..................................................279
4.2.2.9
Ethers .........................................................279
4.2.2.10 Cyclic and Acyclic Acetals and Ketals .........280 4.2.2.11 Other Classes of Compounds ....................280 4.2.2.11.1 Bis-Alkylaminophenoxy Alkanes .... 280 4.2.2.11.2 Derivatives of 2,2,7,7,-Tetramethyl-1,4-Diazocyclopentane ......281 4.2.2.11.3 Bis-Cyclopentadienyl Compounds ..
v
Ozonation of Organic and Polymer Compounds
281 4.2.2.11.4 Alkylnaphthenes .......................281 4.2.2.11.5 Aminomethylene Derivatives of Furane ......................................282 4.2.2.11.6 Lactams ....................................282 4.2.2.12 Sulfur-Containing Compounds ...................282 4.2.2.13 Si-Containing Compounds .........................285 4.2.2.14 P-Containing Compounds ..........................285 4.2.3
Application .................................................................286
4.2.4
High Molecular Antiozonants ....................................296 4.2.4.1
4.3
Apparatus for Ozone Resistance Determination ............301
4.4
Evaluation of Industrial Stabilisers .................................305
4.5
5
vi
Ethylene-Propylene Rubber With Diene Monomer (EPDM) ...............................................299
4.4.1
Antioxidant Action .....................................................306
4.4.2
Atmospheric Ageing ...................................................308
4.4.3
Antiozonant Action ....................................................308
Prediction .......................................................................312 4.5.1
Oxidation ...................................................................312
4.5.2
Ozonolysis ..................................................................312
4.6
Efficiency of Antiozonants Under the Conditions of Various Deformations ..................................................................319
4.7
Effect of Vulcanisate’s Structure ......................................327
Quantum Chemical Calculations of Ozonolysis of Organic Compounds ......................................................................................359
5.1
Alkanes ...........................................................................359
5.2
Oxygen-containing Compounds ....................................364 5.2.1
Water..........................................................................364
5.2.2
Methanol ....................................................................364
Contents
5.3
5.4
5.5
5.2.3
Ethylene Glycol ..........................................................368
5.2.4
Formaldehyde and Acetone ........................................368
5.2.5
Dimethylether .............................................................371
Sulfur-containing Compounds .......................................373 5.3.1
Hydrogen Sulfide ........................................................373
5.3.2
Methylsulfide .............................................................374
5.3.3
Dimethylsulfide ..........................................................375
Nitrogen-containing Compounds ...................................377 5.4.1
Ammonia ...................................................................377
5.4.2
Methylamine ..............................................................380
5.4.3
Dimethylamine ...........................................................381
5.4.4
Trimethylamine ..........................................................384
Phosphorus-containing Compounds ...............................384 5.5.1
Phosphine ...................................................................384
5.5.2
Methylphosphine ........................................................385
5.5.3
Dimethylphosphine ....................................................387
5.5.4
Trimethylphosphine ....................................................388
Abbreviations .............................................................................................395 Index .........................................................................................................399
vii
Ozonation of Organic and Polymer Compounds
viii
I
ntroduction
Introduction The development of ozone chemistry started in 1840 when Schönbein discovered the gas which had a distinctive smell, and which formed near to the electrodes of electrical machines [1, 2]. He called it ‘ozone’, which in Greek means ‘smell’. It is a highly reactive gas - an allotropic modification of oxygen containing three atoms of oxygen [3, 4]. Upon studying the reaction of ozone with ethylene, which appears to be the first ozone reaction ever performed, it was shown that the ozone reacts with the double bonds, and products of their cleavage are formed. At the beginning of the last century Harries showed firstly that natural rubber reacts rapidly with ozone. He applied ozone as ‘chemical scissors’ to degrade the natural rubber and on the basis of the cut-off products he determined the elastomer structure and proposed the mechanism of the ozone reaction with the elastomers [5-12]. At present the mechanism of ozone reaction with double bonds, as proposed by Criegee [13-16], is widely accepted. The development of analytical methods opens possibilities for intensive and expanded research on the reactions of ozone with various polymers, elastomers and chemical compounds [17-25]. Since 1958 to the present day, a number of reviews and monographs describing and discussing the various aspects of ozone chemistry have been published [26-37]. The intensive research on this topic is illustrated convincingly by the huge number of publications, more than 10,000 titles in the patent and scientific literature, covering all aspects of ozone chemistry and physics, ozone preparation, application, storage, decomposition, etc. The study of the kinetics and mechanism of ozone reactions is an important field in modern science. It is closely related to the solution of the problem of ‘ozone holes’, the development of physical, organic, inorganic and polymer chemistry and biochemistry in connection with ozone, chemical kinetics, the theory and utilisation of the reactivity of chemical compounds with ozone, the development of new highly efficient technologies for the chemical industry, electronics, fine organic synthesis, the solution of ecological and medical problems by employing ozone, and the degradation
1
Ozonation of Organic and Polymer Compounds of organic, polymer, elastomer and biological materials, etc., and their stabilisation against ozone’s harmful action [39, 40]. The oxidative, antibacterial and antiviral properties of ozone make it very attractive for the purification of drinking water, the treatment of natural, waste and process water, artificial pools, waste gases and contaminated soils, in human and veterinary medicine, sterilisation, etc. Usually ozone can be found in the atmosphere at an altitude of 25-30 km. Here it is generated photochemically and its concentration reaches values of 10-20 ppm. Ozone absorbs ultraviolet (UV) light in the region of 200-300 nm wavelength and protects life on the earth from its harmful effects. The natural concentration of ozone at the ground level is in the range of 0.005-0.01 ppm, but in some cases it may approach values up to 1 ppm. At ground level, ozone is produced by photochemical reactions in the ‘smog phenomenon’, during the decay of some seaweed, during thunder and lightning storms, near high-voltage conduits, and in the vicinity of radiation sources, UV-applying units, lasers and radar units, electrolysis, and galvanic and welding moulding apparatus. The control of all these sources of ozone generation is rather difficult and in many cases they cause ecological pollution of dwellings and work places with harmful ozone levels. Ozone is highly toxic in concentrations greater than 0.1 mg/m3. The presence of ozone in the lower atmospheric layer leads to the appearance of undesirable phenomena related to the earth’s flora and fauna, and degradation of valuable organic, inorganic, polymeric and biological objects, materials and articles. In this connection the harmful effect of ozone on human health, rubber goods, polymeric materials should be outlined [31, 41-77]. The purposeful application of ozone promotes invention and the development of novel and improvement of well-known methods for its generation and analysis, and the means and methods for its more effective application, etc. [31, 61, 69-79]. A number of laboratory and industrial methods for its synthesis have been proposed. The modern ozonation stations have a capacity of 102-103 kg O3/day with ozone concentrations not higher than 5-7% vol. In some particular cases, solid or liquid ozone, or ozone deposited on inorganic supports such as silica gel, or absorbed in freons, etc. [80-89], is used. The joint research work on ozone chemistry has been intensively developing since 1972 [90-92]. We should mention that several hundred publications, reviews, plenary lectures, oral and poster presentations and patents have been published during this time interval. Serious attention has been paid to applied research work leading to the solving of the stabilisation problem of pneumatic tyres and rubber technical goods.
2
Introduction
References 1. C.F. Schonbein, Poggendorff’s Annalen der Physik und Chemie, 1840, 49, 616. 2. C.F. Schonbein, Comtes Rendus Hebdomadaires des Séances de l’Académie des Sciences, 1840, 10, 706. 3. C. Nebel in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, John Wiley, New York, NY, USA, 1981, 16, 650. 4. M. Horwath, L. Vitzky and G. Huttner, Ozone, Academy Kiado, Budapest, Hungary, 1985. 5. H. Staudinger, Chemische Berichte, 1925, 58, 1088. 6. C.D. Harries, Chemische Berichte, 1904, 37, 2708. 7. C.D. Harries, Chemische Berichte, 1905, 38, 1195. 8. C.D. Harries, Chemische Berichte, 1912, 45, 936; 9. C.D. Harries, Untersuchungen über die Naturlichen und Kunstlichen Kautschukarten, Springer Verlag GmbH, Berlin, Germany, 1919. 10. C.D. Harries, Chemische Berichte, 1907, 40, 4905. 11. C.D. Harries, Chemische Berichte, 1908, 41, 1227. 12. C.D. Harries and V. Weiss, Liebig’s Annalen der Chemie, 1905, 343, 369. 13. R. Criegee, Annalen, 1949, 564, 9. 14. R. Criegee, Annalen, 1953, 583, 1. 15. R. Criegee, Record of Chemical Progress, 1957, 18, 111. 16. R. Criegee, Angewandte Chemie, 1975, 21, 765. 17. S. Hatakeyama, H. Lai, S. Gao and K. Murano, Chemistry Letters, 1993, 8, 1287. 18. W.S. Schutt, M.E. Sigman and Y.Z. Li, Analytica Chimica Acta, 1996, 319, 3, 369. 19. M.J. Lee, H. Arai and T. Miyata, Chemistry Letters, 1994, 6, 1069.
3
Ozonation of Organic and Polymer Compounds 20. Y.S. Hon and J.L. Yan. Tetrahedron Letters, 1994, 35, 11, 1743. 21. Y.S. Hon and L. Lu. Tetrahedron Letters, 1993, 34, 33, 5309. 22. R. Atkinson, E.C. Tuazon, J. Arey and S.M. Aschmann, Atmospheric Environment, 1995, 29, 3423. 23. T.L. Rakitskaya, A.Y. Bandurko, A.A. Ennan and V.V. Litvinskaya, Kinetics and Catalysis, 1994, 35, 5, 705. 24. S.W. Mcelvany, J.H. Callahan, M.M. Ross, L.D. Lamb and D.R. Huffman, Science, 1993, 260, 5114, 1632. 25. R. Keshavaraj and R.W. Tock, Advances in Polymer Technology, 1994, 13, 2, 149. 26. S.D. Razumovskii, S.K. Rakovski, D.M. Shopov, and G.E. Zaikov, Ozone and Its Reactions with Organic Compounds, Publishing House of the Bulgarian Academy of Sciences, Sofia, Bulgaria, 1983. [in Russian] 27. Ozonation in Organic Chemistry, Volumes I and II, Ed., P.S. Bailey, Organic Chemistry Monographs, Volume 39, Academic Press, New York, NY, USA, 1978 and 1982. 28. S.D. Razumovskii and G.E. Zaikov, Ozone and Its Reactions with Organic Compounds, Elsevier, Amsterdam, The Netherlands, 1984. 29. P.S. Bailey, Chemical Reviews, 1958, 58, 925. 30. Ozone Chemistry and Technology, Advances in Chemistry Series, Volume No.21, ACS, Washington, DC, USA, 1959. 31. Ozone Reactions with Organic Compounds, Ed., P.S. Bailey, Advances in Chemistry Series Volume No.112, ACS, Washington, DC, USA, 1972. 32. G.Y. Ishmuratov, R.Y. Kharisov, V.N. Odinokov and G.A. Tolstikov, Uspekhi Khimii, 1995, 64, 6, 580. 33. B. Dhandapani and S.T. Oyama, Applied Catalysis B - Environmental, 1997, 11, 2, 129. 34. S.K. Rakovsky in Polymer Materials Encyclopedia, Ed., J.C. Solomone, CRC Press, Boca Ratan, FL, USA, 1996, p.1878. 35. ACS Division of the Petroleum Chemistry - Preprints, 1972, 16, 2.
4
Introduction 36. Handbook of Ozone Technology and Applications, Volume 2, Eds., R.G. Rice and A. Netzer, Butterworth Publishers, Boston, MA, USA, 1983. 37. Ozone: Science & Engineering, 1978, 1, 1-4; 38. Ozone: Science & Engineering, 1998, 20, 1-4. 39. D.P. Chock, G. Yarwood, A.M. Dunker, R.E. Morris, A.K. Pollack and C.H. Schleyer, Atmospheric Environment, 1995, 29, 21, 3067. 40. R.D. Bojkov and V.E. Fioletov, Journal of Geophysical Research – Atmosphere, 1995, 100, D8, 16537. 41. J.M. Giovannoni, F. Muller, A. Clappier and A.G. Russell, Tropospheric Modelling and Emission Estimation, 1997, 7, 111. 42. S.M. George and F.E. Livingston, Surface Review and Letters, 1997, 4, 4, 771. 43. M.T. Benjamin and A.M. Winer, Atmospheric Environment, 1998, 32, 1, 53. 44. B. Frakes and B. Yarnal, International Journal of Climatology, 1997, 17, 13, 1381. 45. L. Coy and R. Swinbank, Journal of Geophysical Research - Atmosphere, 1997, 102, D22, 25763. 46. W.R. Stockwell, F. Kirchner, M. Kuhn and S. Seefeld, Journal of Geophysical Research - Atmosphere, 1997, 102, D22, 25847. 47. B. Ramacher, J. Rudolph and R. Koppmann, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 466. 48. R. Sander, R. Vogt, G.W. Harris and P.J. Crutzen, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 522. 49. P.A. Ariya, V. Catoire, R. Sander, H. Niki and G.W. Harris, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 583. 50. J. Rudolph, B. Ramacher, C. Plassdulmer, K.P. Muller and R. Koppmann, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 592. 51. Bulletin of the American Meteorological Society, 1997, 78, 11, 2681. 52. P.A. Newman, J.F. Gleason, R.D. McPeters and R.S. Stolarski, Geophysical Research Letters, 1997, 24, 22, 2689.
5
Ozonation of Organic and Polymer Compounds 53. R.B. Pierce, T.D. Fairlie, E.E. Remsberg, J.M. Russell and W.L. Grose, Geophysical Research Letters, 1997, 24, 22, 2701. 54. J.P. McCormack, L.L. Hood, R. Nagatani, A.J. Miller, W.G. Planet and R.D. McPeters, Geophysical Research Letters, 1997, 24, 22, 2729. 55. M.K. Dubey, G.P. Smith, W.S. Hartley, D.E. Kinnison and P.S. Connell, Geophysical Research Letters, 1997, 24, 22, 2737. 56. S. Solberg, T. Krognes, F. Stordal, O. Hov, H.J. Beine, D.A. Jaffe, K.C. Clemitshaw and S.A. Penkett, Journal of Atmospheric Chemistry, 1997, 28, 1-3, 209. 57. R. Schmitt and A. Volzthomas, Journal of Atmospheric Chemistry, 1997, 28, 1-3, 245. 58. C. Nevison and E. Holland, Journal of Geophysical Research - Atmospheres, 1997, 102, D21, 25519. 59. Proceedings of the World Environmental Conference, Rio de Janeiro, Brazil, 1992. 60. Proceedings of the World Environmental Conference, Kyoto, Japan, 1998. 61. M.K.W. Ko, N.D. Sze and M.J. Prather, Nature, 1994, 367, 6463, 505. 62. V.A. Basiuk, Uspekhi Khimii, 1995, 64, 11, 1073. 63. V. Cataldo and O. Ori, Polymer Degradation and Stability, 1995, 48, 2, 291. 64. R.W. Murray in Techniques and Methods of Organic and Organometallic Chemistry, Ed., D.B. Denney, Marcel Dekker, New York, NY, USA, 1969, p.1-32. 65. Proceedings of the Second International Symposium on Ozone Technology, Eds., R.G. Rice, P. Pichet, M-A. Vincent, Montreal, Quebec, Canada, 1975. 66. M.J.S. Dewar, J.C. Hwang and D.R. Kuhn, Journal of the American Chemical Society, 1991, 113, 3, 735. 67. J.S. Belew in Oxidation: Techniques and Applications in Organic Synthesis, Volume 1, Ed., R.L. Augustine, Marcel Dekker, New York, NY, USA, 1969. 68. R.L. Kuczkowski, Chemical Society Reviews, 1992, 21, 79. 69. J. Prousek and A. Klcova, Chemicke Listy, 1997, 91, 8, 575. 6
Introduction 70. C.A. Zaror, Journal of Chemical Technology and Biotechnology, 1997, 70, 1, 21. 71. B.I. Tarunin, V.N. Tarunina, M.N. Klimova and M.Y. Ratkova, Zhurnal Obshchei Khimii, 1997, 67, 5, 806. 72. E. Otal, D. Mantzavinos, M.V. Delgado, R. Hellenbrand, J. Lebrato, I.S. Metcalfe and A.G Livingston, Journal of Chemical Technology and Biotechnology, 1997, 70, 2, 147. 73. F.J. Benitez, J. Beltranheredia, J.L. Acero and M.L. Pinilla, Journal of Chemical Technology and Biotechnology, 1997, 70, 3, 253. 74. B.J. Finlaysonpitts and J.N. Pitts, Science, 1997, 276, 5315, 1045. 75. H.K Roscoe and K.C. Clemitshaw, Science, 1997, 276, 5315, 1065. 76. F.H. Tiefenbrunner, H.G. Moll, A. Grohmann, D. Eihelsdorfer, K. Seidel and G. Golderer, Ozone: Science Engineering, 1990, 12, 393. 77. F. Hegeler, and H. Akiyama, Japanese Journal of Applied Physics, Part 1, 1997, 36, 8, 5335. 78. S. Rakovski, V. Podmasterev, S. Razumovskii and G. Zaikov, International Journal of Polymeric Materials, 1991, 15, 2, 123. 79. R.A. Kerr, Science, 1996, 271, 5245, 32. 80. D.P. Ufimkin, V.V. Lunin, L.V. Sabitova, L.N. Burenkova, V.A. Voblikova and S.N. Tkachenko, Zhurnal Fizicheskoi Khimii, 1995, 69, 11, 1964. 81. B Dhandapani and S.T. Oyama, Chemistry Letters, 1995, 6, 413. 82. A. Naydenov, R. Stoyanova and D. Mehandjiev, Journal of Molecular Catalysis A - Chemical, 1995, 98, 1, 9. 83. I.V. Martynov, V.I. Demidyuk, S.N. Tkachenko and M.P. Popovich, Zhurnal Fizicheskoi Khimii, 1994, 68, 11, 1972. 84. E. Merz and F. Gaia, Ozone: Science and Engineering, 1990, 12, 401. 85. J. Kitayama and M. Kuzumoto, Journal of Physics D - Applied Physics, 1997, 30, 17, 2453. 86. V.A. Basiuk, Uspekhi Khimii, 1995, 64, 11, 1073.
7
Ozonation of Organic and Polymer Compounds 87. S. Rimmer, J.R. Ebdon and M.J. Shepherd, Reactive & Functional Polymers, 1995, 26, 1-3, 145. 88. L.V. Ruban, S.K. Rakovsky and A.A. Popov, Izvestiya AN SSSR Seriya Khimicheskaya, 1976, 40, 9, 1950. 89. D.P. Ufimkin, V.V. Lunin, L.V. Sabitova, L.N. Burenkova, V.A. Voblikova and S.N. Tkachenko, Zhurnal Fizicheskoi Khimii, 1995, 69, 11, 1964. 90. S.K. Rakovsky, Kinetics and Mechanism of the Ozone Reactions with Paraffins in Liquid Phase, Institue of Chemical Physics, Moscow, 1975. [PhD Thesis] 91. M.P. Anachkov, Kinetics and Mechanism of the Ozone Reaction with Diene Rubbers in Solution, Institute Neftekhimii, Moscow, 1982. [PhD Thesis] 92. Proceedings of the 1st International Micro Symposium on Ozone Degradation of Polymers, Sofia, Bulgaria, 1984.
8
1
Kinetics and Mechanism of Ozone Reactions with Organic and Polymeric Compounds in the Liquid Phase
The oxidation of low molecular weight hydrocarbons by ozone is one of their few reactions that takes place both at low and ambient temperatures and transforms them into oxygen-containing products [1-12]. This reaction has been investigated by a number of authors [13-15]. The action of ozone as an initiator of oxidation processes [16-18], a modifying polymer agent [19-21] and an oxidiser in the preparation of alcohols, ketones and some other compounds is of special interest [22-24]. The kinetics and mechanism of this reaction have been intensively discussed [25-46]. Our investigations [47-64] are also dedicated to these problems.
1.1 Reaction mechanisms The first studies of ozone reactions with paraffins were carried out in the last century using the examples of methane, ethane, propane and butane in the gas phase [2-8]. It was established that upon the absorption of 1 mole of ozone 1 mole of final product is formed. These authors proposed two different mechanisms of ozone action: ozone is decomposed into atomic oxygen, which initiates the oxidation process [2] (M.1.1) and ozone interacts directly with the alkane [3-5] (M.1.2). O 3 = O2 + O RH + O m Rv + vOH
(M.1.1)
RH + O3 m ROv + vO2H
(M.1.2)
Later it was shown that mechanism M.1.1 can be realised only in the gas phase at temperatures >>50-60 oC [28, 29]. Thus this reaction is of no importance in the liquid phase whereby the reactions are usually carried out at ≤50-60 oC. On the other hand mechanism M.1.2 assume the formation of H2O2 in quantities proportional to the absorbed amount of ozone, but really in the liquid phase only a small amount of hydroperoxide is formed [1] which means that the role of the mechanism in M.1.2 in ozonation of alkanes is negligible.
9
Ozonation of Organic and Polymer Compounds The ozonolysis of alkanes in the gas phase leads to the formation of excited species, which determines the specific character of these reactions [65]. The first studies in the liquid phase were carried out by Azinger and co-workers [16] on ozonation of octadecane. They came to the conclusion that ozone directly attacks the CH2 groups. Upon ozonolysis of decalin and adamantane the tert-CH groups are attacked, leading to the formation of the respective tert-alcohols. In cyclohexane, ozone reacts with the CH2 groups giving rise to cyclohexanol and cyclohexanone formation [16]. The kinetics of intermediate and end-product formation has been studied and the mechanism of ozone reaction with tetradecane is suggested to be [66]: RH + O3 m RO2v + vOH
(M.1.3)
Although the basic reaction in M.1.3 is exothermal its occurrence in two steps appears to be more entropy favourable than in one as assumed by this mechanism in regard to entropy. Hamilton and co-workers [6, 67] consider the most probable mechanism to be the ozone attack on the CH bonds via a 1,3-transition state (M.1.4): RH + O3 TS m [Rv + HO3v Rv + HO3 ] m ROOOH (1) (M.1.4) n ROH + O2 (s) (2);
cage
n
Rvl+ lvOH + O2 (t) (3)
n RO2v + O2, which is followed by H-atom abstraction and hydrotrioxide (1), alcohol, singlet oxygen (2), peroxy radical and triplet oxygen (3) leaving the kinetic cage in the bulk liquid phase. They postulate that the products in the cage have a radical character, while according to Myurei [68] they are more probably ionic pairs. Nangiya and Benson [28] assume the mechanism M.1.4 to be more probable, but the interaction between ozone and CH bonds is not synchronised.
10
Ozonolysis of low and high molecular weight saturated hydrocarbons Denissov and co-workers [26] considered the reaction of ozone with C-H bonds within the framework of the parabolic model of the potential surface and they also adhere to mechanism M.1.4. Another view of the mechanism of alkane ozonolysis is that Rv, HOv radicals are kinetically responsible for the reaction proceeding and that ROH, Rv, HOv and O2 leave the kinetic cage into the solution (M.1.5) [51]: RH + O3 m [Rv + HOv + O2]
(M.1.5)
However, one cannot exclude the possibility of RO2v radical formation from ROOOH as a precursor via the mechanism of ozone insertion into the CH bonds (M.1.6): RH + O3 m ROOOH
(M.1.6)
ROOOH could act as a precursor in all the mechanisms discussed in the literature [9, 67]. This reaction is probably entropically disadvantageous, because it goes through a cyclic transition state and it is necessary to assume a five-fold coordinated C atom.
1.1.1 Thermochemical analysis The heat of ozone reactions with paraffins via different mechanisms are calculated using the example of methane ozonation. Experimental and calculated (?) values of the bond energies and heat of formation [69, 70] have been used: CH3-H = 104 kcal/ mol; OO-O = 24; CH3O-H = 102; CH3OO-H = 77?; CH3OOO-H = 70?; CH3OOOH = 51?; CH3O-OOH = 47?; CH3-OH = 91; CH3-OOH = 69?; CH3-OOOH = 61?; H-Ov = 102; H-OOv = 77; H-OOOv = 62; C-Ov = 78; C-OOv = 59; and C-OOOv = 48 kcal/mol and the heat of the reactions (Q) in the various mechanisms are as shown in Table 1.1.
Table 1.1 Heats of ozone reactions (Q) Mechanism Q (kcal/mol)
M.1.1 26
M.1.2 -27
M.1.3 -33
M.1.4 66
M.1.5 26
M.1.6 -5
For alkanes with secondary and tertiary C-H bonds with lower bond energies, the Q values for each reaction will be higher by 10-15 kcal.
11
Ozonation of Organic and Polymer Compounds
1.2 Structural-kinetic investigations As we have shown above there are two approaches concerning the interaction of ozone with paraffins: ozone insertion in the CH bonds and H-atom abstraction. We should note, however, that the effect of paraffin composition in this reaction has not been studied systematically, which limits the possibilities for analysis and elaboration of the mechanism of these reactions [25, 59]. The variety of the proposed mechanisms and the insufficient kinetic data means a systematic investigation of the reaction will need to cover a wide spectrum of conditions and theoretical and experimental research methods.
1.2.1 Normal paraffins and isoparaffins We have studied the kinetics of ozone reactions with a series of paraffins of different structure [47, 59]. The rates of the reactions (W) have been determined at stationary and dynamic conditions. By following the change of [O3] with time (t) under static conditions we have determined the rate constant of the pseudomonomolecular reaction ka = k × [RH] and the bimolecular constant – k: W = d [O3]/dt = k [RH]0.[O3] = ka [O3]
(1.1)
The constant ka has been determined during the half-decomposition time interval: ka= ln2/T1/2
(1.2)
or based on the slope of the curve in coordinates ln([O3]o/[O3])/t. The reaction in an open system has been carried out by bubbling ozone through the paraffin phase and following spectrophotometrically its concentration in the ultraviolet (UV) region (254-300 nm). The course of the concentration change has the following pattern: first it drops down abruptly from [O3]o, the concentration at the reactor inlet, to [O3]g, the concentration at the reactor outlet, after which for a long time it remains parallel to the x-axis, at a distance of $[O3] = {[O3]o – [O3]g}. The analysis of the kinetic curves has been done on the basis of its stationary part, when the rate of the chemical reaction becomes equal to the rate of ozone consumption W = WO3, or: k.[RH].[O3]l = W.$[O3]
(1.3)
from which it follows that: k = W.$[O3]/([RH].[O3]l) 12
(1.4)
Ozonolysis of low and high molecular weight saturated hydrocarbons where W = v/V is the relative rate, v is the rate of the gas flow, V is the volume of the liquid phase; in one of the models used [O 3 ] l = A × [O 3 ] 9 the ozone concentration in the liquid, A is Henry’s coefficient [31]. This model is valid in all cases when the rate of ozone absorption is greater than the rate of the chemical reactions. Several criteria could by applied if Henry’s law is observed in the course of absorption and there proceeds an irreversible first-order chemical reaction (k1): k1.T [O3]o are higher than those found by the bubbling method (Table 2.6). This is not difficult to explain because in the former case the values obtained represent the total effective constant of interaction of the enol and the keto form. The results from the calculation of the equilibrium constants of keto-enol tautomerism for some aliphatic ketones in CCl4 are given in Table 2.7. In this table the data for acetylacetone and 2-naphthylmethylketone are not presented because in the former case the rate of reaching the equilibrium is commensurable with the rate of ozone interaction and in the latter case the ozone reacts with the double bonds in the naphthyl ring. The equilibrium constants do not differ from those found within the temperature range of 21 oC to 3 oC and agree with data from the literature [47-50]. The kinetics of ozone reactions with ketones are also determined by gas chromatography. The relative rate constants shown in Table 2.6, column 4 demonstrate that only acetone and methylketone possess lower reactivity than the standard. It is seen that the rate constants calculated from the relative values and the value of the standard constant correspond to those found by the bubbling method. The main products of ozone interaction with methylethylketone are 2-hydroxymethylethylketone, diacetyl, peroxides - alkyl and hydro, acetaldehyde and AcAc.
107
108
Ke × 10
4
Ketone
1.2
CH3COCH3 1.2
CH3COC2H5 46
CH3COC5H11 29
CH3COC6H13 96
CH3COC7H15
130
CH3COC10H21
Table 2.7 Equilibrium constants of keto-enol tautomerism of some ketones in CCl4 solution at 21 oC determined by titration with ozone 190
c-C7H12O
Ozonation of Organic and Polymer Compounds
Ozonolysis of oxygen-containing organic compounds
0.06 4
lg([O3]0/[O3]t)
3 0.04
2
0.02
1
0.00 0
20
40
60
80
100
Time, s
Figure 2.6 Semilogarithmic anamorphosis of the kinetic curve of ozonolysis of: 1 - methylethylketone; 2 - methylpentylketone; 3 – ethylpentylketone; and 4 – cycloheptylketone; 21 oC, ketone concentration 28 mM
On the basis of the kinetic results obtained and the product analysis we suggest Scheme 2.3 for the proceeding of ketone ozonolysis:
R1CHCOR2 + O3
H...O3 # [R1CHCOR2]
a. R1CH(OH)COR2
R1CHO + R2CHO
H
[R1CHCOR2 + OH + O2 , (HO3)]
a, b, c
+b R1COCOR2 + R1COCH(OH)R2 + O2 b. R1CH(OO)COR2
+b +RH
c. R1CH(OOOH)COR2
-O2
2R1CO + 2R2CHO
2 R1CH(O)COR2 + O2 R1CH(OOH)COR2
R1CHO + R2C(OH)O
R1CH(OH)COR2
Scheme 2.3
109
Ozonation of Organic and Polymer Compounds Ozone attacks the A-H atom forming a LC which further undergoes decomposition to a radical (or ion) pair in one kinetic cage, or leads to the formation of hydroperoxy and alkoxy radicals. The latter can further undergo monomolecular decomposition. The intermediate formation of LC is assumed in the first stage, followed by breaking of A-C-H leading to the formation of a radical (or ion) pair. Then A-hydroxyketone, A-peroxyketone radicals and A-hydroxytrioxyketone through C-C bond breaking can decompose to two aldehydes. The A-peroxyketone radical reacting with the initial ketone can be transformed into A-hydroxyperoxyketone which is decomposed through breaking of the C-C bond to aldehyde and acid or the two radicals recombine giving rise to diketone and A-hydroxyketone or two A-oxyketone radicals. The latter reacting with the initial ketone are transformed into A-hydroxyketone or through a monomolecular decomposition and breaking of the C-C bond to an alkoxy radical and aldehyde. A-Hydrotrioxyketone is decomposed to A-hydroxyketone and it in its turn to two aldehydes.
2.3 Hydrotrioxides Hydrotrioxides (HTO) are defined as compounds containing an HOOO group. HTO of oxygen-containing compounds have been studied and described in the literature in detail. They include those of ethers, acetals, alcohols and aldehydes [51-63]. They are obtained by ozonation of the corresponding organic compounds, for example: benzaldehyde, 2-methyltetrahydrofuran, methylisopropyl ether, di-iso-propyl ether, etc., at low temperatures (3 × 105 M-1.s-1, respectively, whereas the rate constants of benzene, toluene and anisole ozonation in organic media are 2, 14 and 2.9 × 102 M-1.s-1 [129]. Gurol and co-workers [130] found that the relative rates of pyrocatechol/phenol and resorcinol/phenol ozonation in water medium are 220 and 70, respectively. Provided that the rate constant of phenol ozonation is known [129], the calculated values of the rate constants of pyrocatechol and resorcinol ozonation are 2.86 × 105 M-1.s-1 and 9.1 × 104 M-1.s-1, respectively. However, if the reaction is carried out in organic solvents the values are quite different. For example, in CCl4 and at room temperature the following values have been obtained for: benzene - 0.06, ethylbenzene - 0.2, anisole - 1.1, phenol - 230 and pyrocatechol - 3.2 × 103 M-1.s-1 [136-139]. One of the possible explanations for the different values of the rate constants of hydroxyphenol ozonation obtained by the various researchers could be the great influence of the water on this reaction, for example for phenol they vary in the range 100-180 M-1.s-1, 500 M-1.s-1 for pyrocatechol and 300 M-1.s-1 for 3,6-di-tert-butylpyrocatechol. Pryor and co-workers [128] have reported that the rate constants of ozone reaction with A-tocopherol in CCl4 and water are 5.5 × 103 M-1.s-1 and 1 × 106 M-1.s-1, with A-tocopherol acetate - 1.45 × 102 M-1.s-1 and that with A-tocopherolquinone it is 1.15 × 104 M-1.s-1. Product analysis of pyrocatechol ozonation in aqueous medium shows that 3 moles of ozone are readily absorbed to give CO2 (24.8%), CO (6%), formic acid (32.5%) and glyoxal (4.2%). As CO2 and formic acid are the main reaction products, it seems very likely that most of the pyrocatechol undergoes anomalous ozonolysis [135]. Radical formation was observed during 2,6-di-tert-butylphenol ozonolysis. When the ratio of absorbed ozone to phenol is 0.8, 50% viscous yellow oil and 3,5, 2,6-di-tert-butyl-o-quinone were identified in the products [138]. In 3,6-di-tertbutyl-pyrocatechol ozonation the corresponding quinone was found to be the main product after complete consumption of the initial substrate [139]. Side products like 2,5-di-tert-butyl-, 3-hydroxy-p-quinone and 3,6-di-tert-butyl-1,2-phenylacetal5-hydroxy-3,6-di-tert-butyl-p-benzophenone were also found. The rate constant of this reaction amounts to 3 × 102 M-1.s-1. The same constant with the corresponding pyrocatechol with O-hydroxy-acetylated groups is estimated to be a two orders of magnitude lower value. A mechanism has been suggested to involve the formation of either a 1,3-cyclic activated complex between two hydrogen atoms from the OH
140
Ozonolysis of oxygen-containing organic compounds groups and one ozone molecule or P- or S-complexes with the benzene ring. The different values of the literature constants and the various mechanisms proposed for this reaction testify the necessity of further research of its kinetics and reaction pathway. In this connection we have studied the ozonolysis of the following hydroxybenzenes (Scheme 2.6): t-Bu t-Bu
OH
t-Bu
(II)
OH t-Bu
(III)
t-Bu O
O
O
O
t-Bu
t-Bu
(V)
S=O
O
O
O
O CHPh
CH2
C(CH3)
t-Bu
(IV)
t-Bu
t-Bu
t-Bu
OH
OH
OH
OH (I)
t-Bu
OH
OH
(VI)
t-Bu
(VII)
(VIII)
t-Bu
t-Bu O
O P O
HO t-Bu
t-Bu (IX)
Scheme 2.6 The probable mechanisms of ozone interaction with dihydroxybenzenes are presented in Scheme 2.7: OH R
TS
+ O3
PRODUCTS
OH H
TS
OH O +
R
O
#
H OH
O
H O
CC OOOH OH
C
R
R
OH
OH
O
A
H O O
LC-I O3
O R OH
LC-II
R
LC-I, CC
#
O
R
OH
TS
O
O
O
O-
OH A
B R
LC-II
O
O
O
O OH
Ozonide
O3 O R O
#
O CX 2 + O 3
R
CX2
D
Ozonides
O Sor P-complex
Scheme 2.7
141
Ozonation of Organic and Polymer Compounds Mechanism A with a cyclic complex (CC) formation in the transition state was suggested by Konstantinova and co-workers [139]. Mechanism B with linear complex LC-II in the transition state was suggested and discussed by Bailey [2]. The interaction of ozone with C-H bonds with the formation of trioxide [127] as a possible parallel reaction is indicated in mechanism C. The acetylated forms of dihydroxybenzenes can react only via attack on the benzene ring according to mechanism D or C. Formally, mechanism D can be regarded as an extended version of mechanism B, involving the formation of TS similar to P- or S-complexes. We suggest a new mechanism, an extended version of the Razumovskii mechanism whereby the transition state is linear with LC-I structure. Upon ozonation of any of the investigated catechols directly in the electron spin resonance cell or after freezing the reaction products in liquid nitrogen no signals were detected. The kinetic curve of the change of ozone concentration at the bubbling reactor outlet (Figure 2.10, curve 1) is characterised by three different regions: AB - fast ozone consumption after the addition of pyrocatechol, BC - steady-state part when the rate of the chemical reaction becomes equal to the rate of ozone supply, and CD - ozone concentration begins to rise due to the pyrocatechol consumption. The BC part of the curve allows calculation of the rate constant, and from the area below the curve ABCD - the stoichiometry of the reaction. The straight line designated {O3}0 is the ozone concentration at the reactor inlet. Curve 2 presents the o-quinone formation with the reaction time. Its profile suggests the intermediate formation of o-quinone. The kinetic curves of the product formation in pyrocatechol ozonolysis, its consumption and o-quinone consumption during its ozonation in a separate experiment are given in Figure 2.11. Calculated from the kinetic curves in Figure 2.11, rate constants of pyrocatechol and o-quinone consumption were 3.2 × 104 M-1.s-1 and 7.1 × 104 M-1.s-1, respectively. The initial rate of o-quinone formation had almost the same value as that of pyrocatechol consumption. The small variation of the constants is due to the participation of pyrocatechol in parallel reactions. Actually, during the reaction small amounts of open-chain products were identified. Pyrocatechol ozonation at 15% conversion gave the following yields: o-quinone 85%, pyrogallol - 3%, ozonide - 10%, muconic acid -2%, maleic acid and fumaric acids and polymer products - 1%. The ratio between the amount of absorbed ozone and the consumed pyrocatechol was calculated as 6. Similar ratio values were obtained for the other hydroxybenzenes with free hydroxy groups.
142
Ozonolysis of oxygen-containing organic compounds
12
[O3]0 10
D
A
1
8 6 4 2
B
C
0
2 0
2
4
6
8
10
12
14
Time, min
Figure 2.10 Kinetic curves of ozone absorption at reactor outlet (1) and o-quinone accumulation (2), at ambient temperature, [PC] = 0.227 mM in CCl4 (10 ml) during bubbling of ozone with 0.1 l/min flow rate
1. 0
1
0. 8 0. 6
3
0. 4
2
0. 2 0. 0 0
50
100
150
200
Time, s
Figure 2.11 Kinetics of pyrocatechol consumption (1), o-quinone accumulation (2) and o-quinine consumption (3), ambient temperature, [O3] = 1 × 10-5 M
143
Ozonation of Organic and Polymer Compounds It was found that with the increase of the conversion from 0 to 100% the amount of open-chain products continuously increases while that of the remaining products passes through a maximum. The individual compounds identified by 13C-NMR are: muconic semialdehyde and acid - 20%, maleic and fumaric semialdehyde and acid - 40%, glyoxal, formic acid, oxalic acid, carbon dioxide and polymer products - 40%. The reaction rate of dihydroxybenzene ozonation follows first-order kinetics in relation to each reagent (Figure 2.12, A and B). Table 2.25 presents experimental kinetic data on the ozonation of the investigated dihydroxybenzenes at various temperatures.
Table 2.25 Kinetic data of dihydroxybenzene ozonolysis in CCl4, 20 oC Substrate
k (M-1.s-1)
log A
Ea (kcal/mol)
I
3230
5.35
2.5
II
3100
4.91
1.9
III
3100
5.07
2.1
IV
3200
4.80
1.7
V
500
7.14
6.0
VI
749
7.17
5.8
VII
828
7.06
5.6
VIII
598
6.84
5.5
IX
111
6.81
6.4
Benzene
0.06
8.32
12.8
Toluene
0.40
7.05
10.3
Anisole
10
-
-
Phenol
160
7.31
7.0
A drastic difference between the values of the kinetic parameters of catechols I-IV and V-IX in which the OH groups are acetylated is observed. The rate constants of catechols I-IV manifested 4-28 times higher values than those of catechols of V-IX. On the other hand the pre-exponentials demonstrated by about two orders of magnitude lower values with the former compounds. The activation energies are 2.05 ± 0.5 and 5.9 ± 0.5 kcal/mol, respectively, or the acetylation of the HO groups leads to an increase in EA by about 4 kcal. Probably, ozone reacts predominantly with the hydrogen atoms of the HO groups and, to a very small extent, with the benzene
144
Ozonolysis of oxygen-containing organic compounds ring. The ratio of these ozone interactions varies from 94:4 to 80:20 depending on the hydroxybenzene nature. The lower activation energies of catechols I-IV with respect to those of V-IX are more consistent with the formation of an activated complex of contact type in the transition state, i.e., with structure LC-I (Scheme 2.7). This assumption could also be confirmed by analysis of the kinetic parameters of the ozonation of anisole, benzenes, phenol and toluene. The reaction of ozone with benzene proceeds with a low specific rate of 0.06 M-1.s-1 and relatively high activation energy equal to 12.8 kcal/mol. The methyl derivatives of benzene, i.e., toluene, reacts 6.7 times faster and the Ea is decreased by 2.5 kcal/mol, and the rate of anizol ozonation is even higher - 10 M-1.s-1. The exchange of ketyl group by a hydroxyl one leads to significant acceleration of the ozonation rate and approaches values of 160 M-1.s-1, and the activation energy is reduced by 5.8 kcal/mol. Probably, in this case the mechanism of the reaction is changed and ozone interacts predominantly with the hydrogen atoms of the OH groups. The decrease in the reactivity of anisole compared with that of phenol (^16 times) supports this assumption.
12
A
W0.105, M.s -1
10 8 6 4 2 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
[O3]t.105, M 3.0
B
2.5
W0 / [O3]t, s -1
2.0 1.5 1.0 0.5 0.0 0
2
4
6
8
[P].104, M
Figure 2.12 Dependence of the rate of pyrocatechol ozonation on the concentration of ozone (a) and pyrocatechol (b)
145
Ozonation of Organic and Polymer Compounds Analysis of the kinetic parameters of catechols in acetylated form and those of benzene, toluene, anisole and phenol shows that the rate constants increase and the activation energies go down with increasing numbers of electron-donating substituents (Table 2.25). The high values of the pre-exponential factors could be associated with the high values of the energy of free rotation in the activated complex as a result of the steric hindrances caused by the presence of the bulky tert-butyl groups. This means that when the hydroxyl groups are acetylated, the activated complex should have a LC-II like structure and the ozonation would lead only to the formation of ozonides and open ring products. The kinetic parameters of some dihydroxybenzenes were determined in ozonation in aqueous medium (for those which are water soluble - I-IV, VIII and IX). However, we found that because of diffusion limitations in the bubbling reactor we were not able to measure constants higher than 1 × 104 M-1.s-1. In fact, the literature values for some of these constants are of this order, but they have been measured by a modified stop-flow technique [27]. The higher reactivity of dihydroxybenzenes in aqueous medium is connected with the change of the ozonation mechanism from a hidden radical to an ion one. However, this question requires special study and discussion. The analyses made so far show that the reaction path of the ozone reaction with dihydroxybenzenes depends strongly on their nature and it proceeds via a transition state with activated complex - LC-I or LC-II
2.6.1 Application of ERM For detailed analysis of the reaction mechanism of the ozone reaction with catechols we have used the ERM method. The necessary parameters for calculations are presented in Table 2.26. In addition, we calculated the heats of formation of pyrogallol - –117.6 kcal/mol (–129 kcal/mol [140]), 3-trihydroxy pyrocatechol - –59 kcal/mol, H2O3 - –21.2 kcal/ mol (–17.7 kcal/mol [140]) and ozone - 34.1 kcal/mol. Thus, the heats of the ozonation reaction according to the different mechanisms were calculated and are as follows: A - 33.1; B and D - 46.1 and C - 32.1 kcal/mol. All the mechanisms are exothermic and, therefore thermodynamically favourable. In this case only the magnitude of the activation energy and the entropy benefits will determine the reactivity of these compounds and the reaction pathway. Calculated pre-exponentials (Table 2.27) were compared with those obtained experimentally (Table 2.25) and it is seen that A have lower values (^10 times) for the cyclic form of
146
Ozonolysis of oxygen-containing organic compounds the activated complex if compared with the experimental values. At the same time the values of A for LC are about 200 times higher. The values of A for CC are the highest as predicted by the theory and they are lower than the experimental values. This suggests that the reaction takes place via a cyclic complex. The values of A calculated for LC in Table 2.27 were obtained without taking account of the energy of free rotation. The latter was calculated as a sum of the rotation around H-O and O-O axes (by MOPAC6) and amounts to 3.1 kcal. This means that the real values in column 4 are about 200 times smaller. The comparison between the A values corrected in this way and the experimental data for the compounds I-IV reveals full agreement.
Table 2.26 Symmetry numbers (S),van der Waals radii (r), heats of formation ($H) of dihydroxybenzenes and their corresponding quinones and ozonides Substrate
S
r (Å)
$H (kcal/mol)
$Hquinone (kcal/mol)
$Hozonide (kcal/mol)
I
2
3.02
–62 (–85)
–39 (–43)
–73
II
1
3.82
–88
–66
–99
III
2
4.46
–109
–86
–125
IV
1
4.39
–110
–90
–123
V
2
5.07
–116
–125
VI
2
4.56
–90
–103
VII
1
5.37
–63
–86
VIII
1
4.77
–73
–77
IX
1
5.30
–135
–140
Benzene
6
2.86
19 (13)
21
Toluene
1
3.37
12 (3)
11
Anisole
1
3.45
–16 (–26)
–20
Phenol
1
2.96
–21 (–38)
–24
Note: The experimental values are in brackets [140].
147
Ozonation of Organic and Polymer Compounds The reaction pathway of the ozonation reaction of compounds V-IX is quite different as the transition state includes the formation of a S or P-complex. For these compounds the formation of AC in the transition state is impeded due to the breaking of their aromatic character but simultaneously the free rotation is facilitated, which may be even zero, due to the action of the principle of the lowest energy. If the energy of free rotation is assumed to be very low then the values of A calculated for LC-II would coincide with the experimentally found ones. Such a coincidence is observed for compound V in column 5 (Table 2.27).
Table 2.27 Calculated pre-exponential values for ozone reaction with dihydroxybenzenes Substrate
log A
log A,
log A,
log A,
Acalc/
log p
TC
CC
LC
calc.
Aobs
1
2
3
4
5
6
7
I
11.318
4.346
7.69
5.39
1.10
5.928
II
11.422
3.905
7.25
4.95
1.10
6.472
III
11.500
4.070
7.42
5.12
1.12
6.380
IV
11.490
3.780
7.12
4.82
1.05
6.670
V
11.572
4.017
7.36
7.36
1.66
4.212
VI
11.511
4.056
7.40
7.40
1.70
4.102
VII
11.801
3.934
7.28
7.28
1.66
4.521
VIII
11.533
3.717
7.06
7.06
1.66
4.473
IX
11.575
3.689
6.88
6.88
1.10
4.695
Benzene
11.319
5.124
8.47
8.47-
1.41
2.849
Phenol
11.322
4.196
7.54
7.54
1.70
3.782
Note: p is the collision factor.
The basic conclusion for the analysis of the results obtained for this reaction is that the kinetics and mechanism of the ozonation reaction of dihydroxybenzenes depend strongly on their structure and the type of the reaction medium.
148
Ozonolysis of oxygen-containing organic compounds
2.7 Carbohydrates and model compounds The selective oxidation of carbohydrates has received increasing attention in recent years [141-151]. This stems from interest in the transformation of carbohydrates to new valuable and ecologically friendly oxygen-containing compounds from available starting materials. Hence, a detailed clarification of the processes related to oxidation, the right choice of the catalysts and the definition of optimum conditions are of great importance [143, 145, 146, 148]. These reactions are invaluable for structure determination and transformation of polyhydrates and other natural products. The formation of two carboxylic groups alters significantly the carbohydrate (polymer) properties making it water soluble and imparting to it ion-acceptor properties. This polymer can be used for preparing new classes of detergents and ion-capture agents. The most interesting reaction is oxidative cleavage of C2-C3 bonds [147]:
OH
OH
O
O
-OOC
HO
-OOC
OH O
n
O
n
Formation of two carboxylic groups dramatically changes the properties of carbohydrates making them water soluble and they become ion capturers. If combined with hydroxylation of the double bond, the oxidation can be used as an alternative method for direct cleavage of double bonds by ozone [1, 2]. The oxidation of C2-C3 bond is accomplished by applying stoichiometric oxidants such as Pb(AcO)4 in organic solvents and HIO4 in aqueous media. Occasionally, other oxidising agents have been used such as ceric ammonium nitrate, sodium bismuthate, chromium trioxide and trivalent or pentavalent [152-161] iodine compounds. The mechanism of the C-C bond cleavage of vicinal diols with periodine acid can be represented as follows:
149
Ozonation of Organic and Polymer Compounds _
H
O
OH +
O I
O
_
O
O
I O
OH
O
O
OH
O A
_ + IO3 + H2O
O O
_
_
O O
O O
O
I O
O
OH I
+ H2O
O
OH
O
O O
_ + IO3
The cleavage of C-C bonds between two adjacent OH groups in the linear diols is a good source for aldehydes [162-164]. The efficiency and selectivity of the ozone reaction with C=C bonds in organic compounds is well known [1, 2]. Rate constants are of the order of 14-106 M-1.s-1, in contrast to 10-2-10 M-1.s-1 for the ozone reaction with C-H bonds. The rate determining step of the alcohol ozonolysis is the H-atom abstraction in the A position in relation to the OH group. The relative reactivity of the primary:secondary:tertiary alcohols with respect to ozone lies in the following sequence: 1:15:138 and corresponds to the C-H bond energies. The main products of alcohol ozonation are the corresponding aldehydes or ketones. The carbonyl compounds are in equilibrium with their enol forms which brings about their further decomposition via an ozone reaction with the C=C bond of the enol form according to Scheme 2.8. C
OH
O3
C
O
H+
C
OH
C
OH
-H2O; -O2
C
OH
HO-
C
OH
OH C
OH
O3
C
O
C
OH
H2O
C
O
OH
Scheme 2.8
150
+ H2O2
Ozonolysis of oxygen-containing organic compounds The monomer glucose unit contains the following A-hydrogen atoms: tert-C-H bond at C2 and C3 adjacent to the OH group; at C4 - adjacent to acetal group; at C1 - aldehyde hydrogen atom; at C5 -with alkyl substituents; and sec-C-H bond at C6. The energy of the aldehyde hydrogen atom is slightly lower than that of the tert-C-H bonds (difficult to determine the difference) and considerably lower than that of the sec-C-H bond. Obviously, the hydrogen atom in the aldehyde group will be the most reactive. Among the tert-C-H bonds those with higher electron density will be more reactive. They will be presented in the following order of decreasing reactivity: C5>C4>C3=C2. The sec-C-H bond at C6 appears to be the most inert in the ozonolysis reaction. The preliminary analysis reveals that ozone will attack predominantly the H atom at the C1 atom in starch with the formation of carboxylic groups and opening of the glucoside ring, and to a lesser extent, give rise to carbonyl compounds at the C5, C4, C3 and C2 atoms which can be easily converted to acids. However, a selective cleavage of the C2-C3 bond could not be expected to occur. Conventionally, the oxidation reactions of carbohydrates are carried out in an aqueous medium. It is known that the solubility of ozone in water is low and requires very good stirring. Moreover, at a pH above 7, ozone begins to react rapidly with the hydroxyl anions in the solution yielding HO radicals [129, 165-167]. The latter reduces the selectivity of the oxidation process (Scheme 2.9): HO2 O2 + H+
(1)
O3 + HO m HO2v (O2 ) + O2
(2)
O3 + (H+)O2 m (H+)O3 + O2
(3)
O3 + HOv m HO2v (O2 ) + O2
(4)
Initiation:
Propagation:
Termination: Any recombination between HOv, HO2v, O2
(t)
where (1) pKa= 4.8; (2) k2 = 48; 70 M-1.s-1. O3 + O2 m O3 + O2
(5)
151
Ozonation of Organic and Polymer Compounds O3 + H+ HO3v
(6)
HO3v m HOv + O2 ,
(7)
where k5 = 1.6 × 109 M-1.s-1; k6 = 5.2 × 1010 M-1.s-1, k-6 = 3.7 × 104 s-1; k7 = 1.1 × 105 s-1 Scheme 2.9 The study of this reaction is impeded by the low solubility of polymeric carbohydrates (starch) in water and in this connection we have used model compounds for elucidation of the reaction mechanism. The ozonolysis of methanol, ethanol, isopropanol and other alcohols proceeds via A-H-atom abstraction by ozone leading to the formation of a carbonyl group which is subsequently oxidised to a carboxylic one. It has been found that the ethanol ozonolysis is promoted by adding small amounts of hydroperoxide [168] because the steady concentration of the HO radicals is increased. Upon ozonolysis of isopropanol in an aqueous medium at pH 8, controlled by titration with NaOH, ambient temperature and 36% conversion, the following products in the high-performance liquid chromatography (HPLC) chromatogram were identified: acetone - 25%, AcAc - 6% and formic acid - 5% (Figure 2.13).
iso-Propanol
10
Ozone
[P], mM
8
6
4 Acetone 2 NaOH 0 0
20
40
60
80
Time, min
Figure 2.13 Kinetics of product formation during the ozone reaction with isopropanol at pH 8. The products were identified by means of HPLC and 13 C-NMR
152
Ozonolysis of oxygen-containing organic compounds The mechanism of the reaction could be expressed as shown in Scheme 2.10: (CH3)2C'OH + HO3' (HO' + O2)
(CH3)2CHOH + O3
-O2 (CH3)2C(OOOH)OH
(CH3)2C(OH)OH
-H2O
(CH3)2C(O')OH
-HO2'
(CH3)2CO CH3COOH + CH3'
Scheme 2.10
2.7.1 2,3-Butanediol Butanediol was oxidised in aqueous medium without pH control and at constant pH 10 and the main kinetic results are given in Figure 2.14.
25 Ozone
pH=10 20
[P], mM
15 NaOH Butandiol 10
5 Acetoin AcOH
0 0
20
40
60
80
100
120
Time, min
Figure 2.14 Kinetics of product formation during ozonolysis of 2,3-butanediol at pH 10. The products were identified by means of HPLC In the HPLC chromatogram of the starting material two peaks were recorded from the two stereoisomers. New peaks were found after ozonation: keto alcohol, acetoin,
153
Ozonation of Organic and Polymer Compounds AcAc, acetaldehyde, formaldehyde and formic acid. At higher conversions the peaks of the keto alcohol and acetoin disappeared whereas the peak from AcAc increased its intensity. The observed rate of butanediol consumption is higher than the sum of rates of product formation determined by HPLC. Probably at higher conversions of ozone the HO radicals oxidised the reaction products to carbon dioxide and water. In this connection calcium carbonate was found in the trap. The stoichiometry of the ozonation reaction with respect to ozone is unity and the rate constant of the reaction is 0.2 M-1.s-1. At low conversions up to 10-120% the main products of the reaction are acetoin and AcAc. It has been found that at pH 8 the rate of acetoin formation is lower than that without pH control. The scheme proposed for the ozonation reaction is shown in Scheme 2.11:
HO CH3
OH
C
C
H
H
HO
+ O3
HO C
CH3
CH3
HO
H
CH3
C
OH
O
OH
C
C
C
C
CH3
CH3
CH3 C O'
D
OH C
-H2O;-O2
O
O
C
C
CH3
C
H
CH3
+
'C
CH3
H
+RH F
B
F
G
G
Scheme 2.11
154
C
OH
O
E
C
E
CH3
CH3
OH
HOO
CH3
CH3
HO
-HO'
D
CH3
HO
H
-H2O2
+O3
C
H
D
HO
E
H
CH3
+ RH
O
A
C
'OO B
-H2O
-HO2'
CH3
OH
C
CH3
H
-H2O
H
HO
OH
A -O2
CH3 + HO3' + (HO', O2)
H
C
HOOO
OH C
C .
CH3
OH
C
CH3
HO CH3
+O3 CO2, HCHO, CH3CHO...
Ozonolysis of oxygen-containing organic compounds
2.7.2 1,2-Cyclohexanediol A portion (100 ml) of 10 mM 1,2-cyclohexanediol was ozonised with stirring at ambient temperature. The pH of the medium was kept constant at 7, 8, 9 or 11. In the experiments without pH control its change was measured during the reaction duration. In all cases formation of keto alcohol, diketone and open-chain products was observed (Figure 2.15).
Figure 2.15 Initial rates of product consumption and formation upon 1,2cyclohexanediol ozonation (Sub) at various pH and without pH control (NC). The products cyclohexanol-2-on (KetAlc) and adipic acid (AdAc) were identified by HPLC and 13 C-NMR, and the total acid number by NaOH
Upon ozonolysis without pH control the main products were KetAlc and AdAc. The value of pH decreases rapidly at the beginning from 7 to 2.2. In the 13C-NMR spectra of the reaction mixture peaks were recorded at 179.5, 180.1 and 179.4 ppm which could be ascribed to adipic acid semialdehyde (179.5 and 179.4 ppm) and AdAc (180.1 ppm). Moreover, at conversions up to 20% other products with open chains like glutaric, succinic, malonic, AcAc and formic acid were also identified. The rate
155
Ozonation of Organic and Polymer Compounds of cyclohexanediol is found to be 2-3 times higher than the rate of ozone uptake, the latter corresponding to the sum of the rates of KetAlc and AdAc formation. This fact suggests that its decomposition occurs via another route. The ratio between the rate of formation of cyclic and open-chain products decreased from 2 to 0.5 with increasing pH from 4 to 11. Simultaneously the rate of alcohol consumption increases 4 times. When the pH was controlled from 7 to 11, the rate of consumption of NaOH and ozone corresponds to the rate of cyclohexanediol decomposition. In the NMR spectra the diversity of peaks corresponding to carboxylic carbons reaches its maximum at pH 11 and peaks at 167.8, 174.7, 184, 178.3 and 184.8 ppm were recorded. All of the above implies that the mechanism of cyclohexanediol ozonolysis in aqueous media is changed from stoichiometric A-C-H-atom abstraction in acidneutral media to a chain hydroxy-radical oxidation in alkaline media.
2.7.3 Mannitol and its derivatives The kinetics of product formation during the ozonolysis of mannitol and 1,2,5,6-diO-iso-propyl-mannitol:
HO
O
HO
O
HO
HO OH
OH
OH
O
OH
O
is shown in Figures 2.16 and 2.17. In the ozonation of mannitol at pH 7 the inner OH groups were converted to carbonyl groups or the C-C bonds broke leading to the formation of products with a reduced number of carbon atoms compared with that of the starting compound. For example, carbonyl and carboxylic compounds, CO2, H2O were obtained. The rate of ozone absorption and that of mannitol consumption coincide; the sum of the rate of acid formation and keto compounds is also close to these values.
156
Ozonolysis of oxygen-containing organic compounds
3.0 pH=7
Mannitol 2.5
Ozone
2.0 1.5
[P], mM
3,4-C=O
1.0
NaOH
0.5 0.0 -20
0
20
40
60
80
100
120
140
160
Time, min
Figure 2.16 Kinetics of product formation during the ozone reaction with mannitol at pH 7. Products were identified by HPLC and 13C-NMR
7
NaOH
pH=4
6
[P], mM
5
Ozone
4 3 PM 2 Mannitol
1 0 -1 0
50
100
150
200
250
300
350
Time, min
Figure 2.17 Kinetics of product accumulation during 1,2,5,6-O-di-isopropylmannitol ozonolysis. The products were identified by HPLC and 13C-NMR
The acetylation of the 1,2,5,6-OH groups at pH up to 7 causes hydrolysation of the compound thus obtaining mannitol and acetone (Figure 2.17). At pH above these values the selectivity of ozone interaction with the third and fourth A-H atom is increased.
157
Ozonation of Organic and Polymer Compounds If the ozonation (Scheme 2.12) was carried out without pH control the pH changes from 7 at the beginning to 2.8 at 20% conversion. A good correlation has been found between the rate of PM decomposition, ozone consumption and the rates of product (acetone, ketones and acids) formation. Ozonation at constant pH 4, kept constant through titration with 0.1 N NaOH, shows similar kinetic characteristics, i.e., monoketone at the third and fourth position is formed which is further converted into 3-4-diketone. It has been established that the rate of the reaction was accelerated by 4-5 times in an alkaline medium.
O
O
O O
O
O HO HO
+
O3
OOOH
-H2O
OH
-O2
OH
O OH
O O
O
O
O
O
A
.
O A
-HO2
OH
O HO
B
+ O
O O
;
A
-HO
B
+RH; -H2O2
;
A
+RH -H2O
B
Scheme 2.12 Ozonation of 1,2,5,6-O-di-iso-propylmannitol
2.7.4 A-D-Glucose In the ozonation of A-D-glucose it was found that firstly ozone attacks the H atom of the aldehyde group at the first C atom, and after that the H atoms of the tert-C-H bonds at C2, C3, C4 and C5. Upon ozonation without pH control, the values of pH changes from 7 to 2 in the course of the reaction (Figure 2.18). The yield of gluconic acid was 60% at 5% conversion (C1 signal at 177.2 ppm). The rest of products at this moment was about 40%: D-arabino-hexulose (C2 - 178.4 ppm), D-ribo-hexos3-ulose (C3 - 177.8 ppm), D-xylo-hexos-4-ulose (C4 - 176.9 ppm), D-xylo-hexos-5ulose (C5 - 176 ppm), D-gluco-hexodialdose (C6 - 175.9 ppm), oxalic (167.2 ppm) and formic acids. The pattern of product formation during glucose ozonolysis at pH from 7 to 9 was similar to that in an acid medium, but with a different ratio of product distribution. In that case the main product was gluconic acid reaching values up to 90-95% (Figure 2.19).
158
Ozonolysis of oxygen-containing organic compounds
13
C NMR - 177.2, 167.2, 178.4, 177.8 ppm
10
10
D-glucose
8 pH
6
6 NaOH
4
2
pH
[P], mM
8
4
2 Gluconic acid
0
0 0
20
40
60
80
100
120
140
Time, min
Figure 2.18 Kinetics of product formation during ozonolysis of D-glucose without pH control. Products were identified using HPLC and 13C-NMR 13
C NMR - 180.1, 173.0
pH=7
D-glucose
10
Ozone
[P], mM
8
6
NaOH
4
Gluconic acid
2
0 0
50
100
150
200
250
Time, min
Figure 2.19 Kinetics of product formation during the reaction of ozone with D-glucose at pH 7. Products were identified by means of HPLC and 13C-NMR
In an alkali medium sodium salts were formed and the peaks of the carboxylic carbon atoms were slightly shifted. The rate of the reaction rises 1.5-2 times compared with that in an acid medium. Among the other products, products were identified from the ozone attack on C2, C3, C4 and C5 - 177.4, 176.2, 174.7 and 172.6 ppm. Thus 159
Ozonation of Organic and Polymer Compounds analysing the results it can be assumed that ozone attacks all tert-C-H bonds in the glucose molecule with almost equal probability but the rate of cleavage depends on the energy bond and the electron density. The variations in pH change the rate of ozone interaction with the hydroxyl anions and thus changes the oxidation carrier species from ozone to hydroxyl radical.
2.7.5 A-D-Methyl-glucose The change of H in the hydroxyl group at C1 for a methyl group is used for protection of this group and for modelling of a part of the polymeric carbohydrate chain. From this point of view A-D-methyl-glucoside (MG) can be regarded as a suitable low molecular model for elaborating the reactivity and mechanism of ozonolysis. Figure 2.20 demonstrates a pattern of the ozone reaction with MG at pH 8. The reaction was carried out in an alkaline medium to prevent the hydrolysation of MG in acid medium and the transformation of the methoxy group to a hydroxyl one. It is seen that MG is almost quantitatively converted into glucose although the ozonation was carried out in an alkaline medium. This could be attributed to the occurrence of oxidative hydrolysation under the action of ozone. After this ozone and HO-radicals react with it yielding acids, identified by the 13C-NMR signals (Figure 2.20). We determined that the rate of MG decomposition is 3.4 × 10-7 M/s, the rate of acid groups accumulation as 3.12 × 10-7 M/s, the rate of ozone absorption 2.43 × 10-7 M/s and the initial rate of glucose formation as 1.9 × 107 M/s. 13
C NMR - 180.1, 172.5, 174.6 ppm
pH=8
6
NaOH
5 Ozone
[P], mM
4 3 -Me-D-glucoside
2 1
D-glucose
0 -50
0
50
100
150
200
250
300
350
Time, min
Figure 2.20 Kinetics of product accumulation during A-D-methyl-glucoside at pH 8. The products were identified by HPLC and 13C-NMR 160
Ozonolysis of oxygen-containing organic compounds
2.7.6 B-Cyclodextrine and starch The ozonolysis of B-cyclodextrine and starch was carried out in solution and suspension and the kinetic results are presented in Figures 2.21 and 2.22.
13
C NMR - 172.4, 181.1, 179.8, 174.5, 177.1 ppm
16
pH=7
14
Ozone
12
[P], mM
10 8 6
NaOH
4 2 -cyclodextrine 0 -2 0
50
100
150
200
Time, min
Figure 2.21 Kinetics of product formation during ozone reaction with B-cyclodextrine at pH 7. Products were identified by HPLC and 13C-NMR
B-Cyclodextrine is an excellent model of polysaccharides that contains 10 glucoside units arranged in one cycle, in which the OH groups in the 1 and 4 position are blocked (acetylated). The OH groups at C2, C3 and C6 remain free. Upon ozonolysis of aqueous B-cyclodextrine solution at pH 7 its consumption rate amounts to 1.67 × 10-7 M/s. This value is almost equal to the rate of ozone consumption while the rate of titration by NaOH is found to be by about 25% of these rates. Ozone converts the B-cyclodextrine molecule into a linear one with formation of carboxylic groups. The latter were identified by the appearance of chemical shifts at 172.4, 181.1, 179.8, 174.5 and 177.1 ppm in their 13C-NMR spectra. The underlined values can be assigned to the carboxylic atoms at C2-C3 atoms. At ozonation of starch suspension (Figure 2.22), firstly it is oxidised and only after that does it dissolve. A series of peaks which could be ascribed to the carboxylic acid formation has been observed in the NMR spectra. A very good agreement was obtained between the rate of starch consumption, ozone uptake and rate of titration by NaOH.
161
Ozonation of Organic and Polymer Compounds
13
C NMR - 178.2, 177.4, 174.9, 172.6, 162.6 ppm
3.0 Starch
2.5
[P], mM
2.0 1.5 1.0 Ozone 0.5 NaOH 0.0 0
10
20
30
40
Time, min
Figure 2.22 Kinetics of product accumulation during starch ozonolysis at pH 7. Product identification was carried out using HPLC and 13C-NMR
The observation that the suspension is dissolved and the coincidence between the amount of absorbed ozone and the acid number of the oxidate suggest that the ozonation method can be used for transformation of natural polyhydrates to new valuable products. The mechanism of the reaction of ozone with model compounds (vicinal alcohols), as shown above, is based on the interaction between ozone and the A-H atom in the alcohol molecule, the cleavage of C-C bonds being a very rare and random process. The latter was observed during the monomolecular decomposition of oxy-radicals generated in the course of some of the conversion steps of intermediate products. The possibility of improving the selectivity of the C-C bond cleavage could be based on oxidation of carbohydrates using catalytic systems and specific reagents.
2.8 Catalytic ozonolysis and oxidation The selectivity of the C-C bond cleavage at vicinal OH groups is improved by applying specific reagents like RuO4 [29]. Usually Ru(VIII) is obtained in situ via Ru(III) oxidation by suitable oxidants, for example NaOCl in alkaline medium. After that RuO4 interacts with the alcohol forming a Ru-diol complex according to the following scheme (Scheme 2.13):
162
Ozonolysis of oxygen-containing organic compounds RuCl3 + 3NaOH Ru+3
+O3
Ru+3 + 3OH- + 3Na+ + 3Cl-
Ru+4
-O3-
+O3 -O2
Ru+6
+O3 -O2
Ru+8
O OH OH
-H2O + RuO4
H
O
H
O
Ru O
-RuO3
HO
O
+O3
HO
O
O
Scheme 2.13 Our idea was to use ozone instead of hypochloride. In this connection we have measured the rate constant of Ru(III) oxidation by ozone which at ambient temperature at pH 7 has the value of 15 M-1.s-1. Unfortunately, from a kinetic point of view this value is extremely low to compensate the rate of RuO4 interaction with the alcohol or carbohydrate and to ensure high Ru(VIII) concentration. Because of this, the ozonolysis was carried out in the following manner: the black solution of 1 wt%. RuCl3.H2O in 50 ml water was ozonised at pH 7 until it turned yellow (RuCl4). After that the substrate solution was added drop-wise. In this way all alcohols and carbohydrates mentioned above, were ozonised. AcAc was the main product of 2,3-butanediol ozonolysis (Figure 2.23).
NaOH
pH=8
20
15
Butanediol
Ozone
10
AcAc 5
0 0
75
150
225
300
Time, min
Figure 2.23 Kinetics of 2,3-butanediol ozonolysis (50 ml water), in the presence of 0.1 g RuCl3.H2O, ozone flow rate - 0.1 l/min and pH 8. Product identification was performed by HPLC and 13C-NMR 163
Ozonation of Organic and Polymer Compounds It is seen that (Figure 2.23) the initial rates (up to 90 minutes) of butanediol consumption, ozone absorption, AcAc formation and titration by NaOH are almost the same as shown by the equal slopes of the curves. However, after 90 minutes other products were also formed and the acidity of the reaction mixture begins to rise as shown by the sharp increase in the titration curve (by NaOH). Upon ozonolysis of 1,2-cyclohexanediol, mannitol and its derivatives via the method mentioned previously, a selective C-C bond cleavage between the vicinal HO groups with carboxylic group formation was observed only to 20-30% conversions. At higher conversions the number of products is increased, namely because of the occurrence of secondary processes. In the cases with straight-chain alcohols two monoacids are formed from one alcohol molecule and with cyclic alcohols - one dicarboxylic acid, for example adipic acid at 1,2-cyclohexanediol ozonolysis or 2,3-dicarboxylic acid at glucose (its derivatives) ozonolysis. When the oxidation was carried out at lower pH up to 6.5, a hydrolysis of the protected HO-groups takes place. Such an example is the ozonolysis of 1,2,5,6-di-O-mannitol at pH 6.5 (Figure 2.24).
13
C NMR - 174.7 ppm pH=6.5
5
NaOH 4 GlicAc
[P], mM
3
2
1
PM
0 0
20
40
60
80
Time, min
Figure 2.24 Kinetics of product formation during ozonolysis of 1,2,5,6-di-Omannitol in the presence of 1 wt%. RuCl3.H2O
The rate of glyceric acid formation (174.7 ppm) is almost the same as that of PM consumption but is found to be lower than the rate of NaOH titration. This could be associated only with the partial hydrolysation of PM and the formation of low molecular acids such as formic and acetic acids.
164
Ozonolysis of oxygen-containing organic compounds However, it should be noted that the carbohydrate oxidation by ozone and ruthenium has some shortcomings, such as the removal of ruthenium from the reaction mixture, its volatility, high toxicity and price. It is known that NaIO4, similar to Ru(VIII), can cause the selective C-C bond cleavage at the vicinal hydroxy groups with two carbonyl compound formation. Simultaneously it is reduced to NaIO3. The idea was to use ozone for reoxidation of iodate to periodate. Unfortunately, the rate of this process compared with that of reduction: IO4 + Substrate = IO3 + Products (very fast) IO3 + O3 = IO4 + O2 (slow) Products + O3 = Acids + O2 (fast) is not sufficiently high. This requires special conditions for performing ozonolysis: maximum iodate concentration and the absence of organic products in the ozonolysis medium. Such a situation could be achieved if the products of NaIO4 interaction with the substrate are oxidised and removed, and the ozonolysis proceeds further for the NaIO3 oxidation to NaIO4. The oxidation of 1,2-cyclohexanediol (10 ml) by an equimolar amount of NaIO4 under continuous stirring leads to the quantitative formation of adipic aldehyde which is further oxidised by ozone to semialdehyde and adipic acid. No oxidation of iodate to periodate was observed. The application of a number of heterogeneous catalysts Ag2O, Co(AcO)2, FeCl3, Pt/C, Pd/C, Adams catalyst, zeolite catalysts TS-1, microporous titanosilicate (ETAS) and catalysts synthesised by us on the basis of ion exchange: Ce with silicalite - Ce-Silic, Ce and Ru with alumophosphates - Ce-APO and Ru-APO and phthalocyanine (Pc) complexes: PtAu6Pc, PdAu6Pc and (manganese phthalocyanine) MnPc for the selective ozonation of alcohols, model compounds of saccharides and natural carbohydrates, in all cases accelerate the process but did not make the process selective with respect to C-C bond cleavage.
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165
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177
Ozonation of Organic and Polymer Compounds
178
3
Ozonolysis of alkenes in liquid phase
3.1. Olefins The reactions of ozone with alkenes are the best known of all reactions of ozone. These reactions take place with high rates, low activation energy and are a basic source for ozonide preparation, i.e., cyclic 1,2,4-trioxalanes [1-7]. The interaction of ozone with the C=C bonds is a powerful method for structural studies of polydienes, functionalising of complex organic compounds at low temperatures and synthesis of valuable oxygen-containing compounds [8-22]. This reaction plays an important role in the atmosphere [23-30] during degradation of rubber materials [31, 32]. All this makes the study of the theoretical and practical aspects of these reactions attractive and relevant for many researchers [1-13]. The ozonation of alkene provoke a great interest both for elucidating the mechanism and reactivity of alkenes and for ozone application in organic synthesis, protection of rubber products against ozone degradation and creation of novel efficient and economical technologies [1, 2, 8, 13, 15, 25-34]. We have carried out detailed and systematic studies on the ozonolysis of olefins and their polymeric analogues [35-51].
3.1.1 Mechanisms The reactions of ozone with alkenes are a subject of interest and study since the discovery of ozone in 1840 [52-84]. The first experimental research and conclusions concerning these reactions dated from the beginning of the 20th century [52-55]. Harries observed that certain olefinic compounds reacted with ozone to give peroxide oils to which he firstly assigned structure (1) and later he changed the structural assignment to structure (2) as shown in Scheme 3.1 [56-63]: C O
C
C
O
O
C O
O
O
1
2
179
Ozonation of Organic and Polymer Compounds H2O C=O + H2O2 + O=C
2
H
2
O C
+ O=C O
O
H
C
C-OH O
O
Scheme 3.1
Harries proposed that the compound with structure (2) decomposed to hydrogen peroxide and carbonyl compounds. However, the possibility for self-decomposition of (2) to 1,2-dioxalane rearranging further to a carboxylic acid and a carbonyl compound could not be excluded. Staudinger [64] suggested a more complicated mechanism of olefin ozonolysis (Scheme 3.2) than Harries: O
O R 2C
R 2C=CR 2+ O 3
CR 2
O
O
R 2C
O
O R 2C
CR 2
;
O
+OOCR2 R 2C=O + O=O=CR 2
O
R=H
O Polymer
R-C-OH
O
CR 2 O
CR 2
R 2C O
O
O
Scheme 3.2
He assumed that ozone reacts with olefin producing an intermediate adduct with a four-member ring, which undergoes rearrangement to 1,2,3-trioxalane, diperoxide,
180
Ozonolysis of alkenes in liquid phase carbonyl compound or an acid or polymerises. Later it was established that this scheme failed to explain a number of experimental observations [66]. In 1949-1951 Criegee proposed another mechanism of ozonolysis which can be considered as classical [66-71]. This mechanism is outlined in Scheme 3.3. R2C=CR2
R2C
O3 R2C
CR2
O
+O
O
O
CR2 O_
O primary ozonide O _ + R2C=O + O-O-CR2 zwitterion
R2C
O CR2 + polymeric ozonides
O ozonide
HG O R2C O
O CR 2 +
OOH polymeric peroxides
;
R2C
G
;
rearrangement products
O
diperoxide _ G=OH, OR, RCOO
Scheme 3.3 According to Scheme 3.3, ozone reacts with C=C bonds forming a primary ozonide (PO) - 1,2,3-trioxalane. The reaction is exothermic producing more than 50 kcal/mol. PO is very unstable and rapidly decomposes to a zwitterion and a carbonyl compound. At least four ways have been suggested by which the zwitterion can stabilise itself: (1) reaction with its own ketone or aldehyde to give ozonide (1,2,4-trioxalane), (2) reaction with the wall of the solvent cage, composed mainly of solvent molecules thus producing hydroperoxides, (3) reaction with other zwitterions outside the cage yielding diperoxide - 1,2,4,5-tetraoxalane and polymeric peroxides, -(-O-O-C-O-O-) n-, reactions with carbonyl compounds resulting from the decomposition of another PO and formation of ‘cross’ ozonides, reactions with preliminary added aldehydes, ketones, alcohols, water, and so on leading to the formation of ‘foreign’ ozonides or peroxide compounds, and (4) undergoes monomolecular rearrangement giving 1,2dioxalane which isomerises further to acid. The proposed mechanism has been confirmed by much experimental data from various researchers [1, 2]. The low temperature reduction of olefin ozonolysis products,
181
Ozonation of Organic and Polymer Compounds obtained at low temperatures, gives 1,2-alcohol with 1,2,3-trioxalane as a precursor [87-89]. The formation of a zwitterion is demonstrated by the formation of: (1) ‘foreign’ ozonide [84], (2) methoxy hydroperoxides in methanol solution [91-94], (3) ‘cross’ ozonides [95-98], and (4) ozonide during the photochemical decomposition of diazo compound solutions in the presence of oxygen and an aldehyde [99]: hN
(C6H5)2CN2
(C6H5)2C:
O2
(C6H5)2C-O-O
radical reactions (C6H5)2C-O-O (C6H5)2C=O+-O-
RCHO (C6H5)2C=O+-O-
O
C6H5
O R
C6H5
H
O
Rieche [73, 74] confirmed the ozonide structure through a counter synthesis, and later it was established by different spectral and theoretical methods [74-83]. The Criegee mechanism is very close to the contemporary concepts for the mechanism of olefin ozonolysis. The additional studies carried out by Criegee accounting for the effect of the number, size, spatial arrangement, electronic properties of the substituents, conformational state and electronic structure of the zwitterion [84-86] on the alkene ozonolysis throw light on the mechanism of the ‘anomalous’ ozonolysis of some olefins with more particular structure and substituents [87-101]. The ozonide stereochemistry can be predicted if the configuration of the zwitterion - anti or syn is known, which is strongly dependent on the nature and size of the substituents and the experimental conditions [100-102]: _
_
O
O +O
+O
C
C R anti
H
H
R syn
The studies on olefin ozonolysis up to 1958 were summarised by Bailey [3]. Many other authors have carried out research on separate problems regarding these reactions [4-16]. These works stimulated a number of new studies on the mechanism and stereochemistry of the reaction, on the ozonolysis of new types of olefins, the
182
Ozonolysis of alkenes in liquid phase preparation of new products and the application of novel methods of analysis [5457, 71-76]. Concurrently with the accumulation of the convincing evidence favouring the Criegee mechanism, other evidence began to appear which could not be explained by the classical Criegee mechanism. For example, upon ozonolysis of cis- and trans-1arylpropenes, the ratios of ozonide to polymer peroxides and/or free aldehydes are quite different for the two isomers [77-79]. It was also reported that the cis:trans ratios of cross ozonides [80-88] obtained from cis and trans unsymmetrical olefins, often differ whereas according to the simple Criegee mechanism cis-olefin should yield cis-ozonide and trans-olefin trans-ozonide. For this reason, Loan and co-workers [87] proposed a scheme which explains the formation of trans- and cis-ozonide from cis- and trans-olefin, respectively (Scheme 3.4).
o + O3
o o
o o
; o
o
o
o
o
o
Scheme 3.4 The mechanism indicated above includes the formation of a seven-member ring intermediate with conformation responsible for the final ozonide conformation. Upon ozonolysis of trans-olefins the cis:trans-ozonide ratio should be close to 1, and higher than 1 upon cis-olefin ozonolysis. In case of hindered olefins, the formation of a S-complex rearranging further to ozonide, is considered as most probable. This mechanism lost its importance very quickly as many new experimental results emerged which could not be completely explained by it. Thus the ozonolysis of trans-di-tert-butylethylene yields trans-ozonide [102] while according to the Story mechanism it should give cis-ozonides. Moreover, the Story mechanism could not explain the formation of epoxides which are the major products in ozonolysis of highly hindered olefins. It was found that the foreign aldehyde with deuterated 18O atom is incorporated into the zwitterion in preference to the epoxy orientation of the ozonide cycle [103-112]. This observation is in accord with the classical concept of the Criegee mechanism whereas the Story mechanism supposes the preference of the peroxy-bridge orientation [100, 101]. Bauld, Bailey and co-workers [102] suggested refinements for the Criegee mechanism in order to account for the strong and varied experimental facts on the ozone
183
Ozonation of Organic and Polymer Compounds reaction with olefins. For this purpose, they accepted that: the PO exists in C-C half-chair conformation; the zwitterion (carbonyl oxide, CO) exists in syn and anti configurations depending on the geometry of the reagent and the experimental conditions; the preferred ozonide conformation is C-O half-chair; the addition of ozone to the C=C bond is 1,3-cis-stereoselective yielding a five-member ring - 1,2,3trioxolane; the concerted cleavage of PO gives zwitterion and carbonyl compounds, which via 1,3-bipolar cycloaddition results in ozonide formation in the cage, and in the solution volume, the products suggested previously by Criegee; and the reaction of the zwitterion and the carbonyl compound occurs stereoselectively depending on the zwitterion configuration and the preferred conformation of the end ozonide. Thus, Bauld and Bailey formulated the following stereochemical rules: (1) equatorial (trans) substituents in PO are preferentially converted into anti, and axial (cis) substituents into syn carbonyl oxide, (2) the equatorial substituent in the PO is incorporated into a carbonyl oxide zwitterion in preference to an axial substituent, (3) aldehydes interact preferentially with anti-carbonyl oxides so as to orient bulky substituents into a,e-conformation, i.e., cis-ozonide formation, and with syn-carbonyl oxides so as to orient bulky substituents into a,a or e,e-conformation (trans). Thus Bailey succeeded in explaining the cis:trans ratios of normal and cross ozonides, particularly those obtained from olefins with bulky substituents, and trans-olefins yield trans-ozonides and cis-olefins yield cis-olefins, predominantly. These suggestions, however, are true only for olefins with bulky substituents such as, iso-C3H7 or tert-C4H9, but not for olefins with smaller substituents, i.e., CH3, C2H5 [113-116]. On the basis of microwave spectra Lattimer and co-workers [112] showed that ethylene, propylene and trans-2-buteneozonide have an oxygen-oxygen half-chair rather than carbon-oxygen chair conformation as suggested by the Bailey mechanism. This means that the e,e- and a,s-substituents are located in the trans-position and the a,e-substituents in the cis-position [117-123]. The studies on the conformation of PO, ozonide and zwitterions reveal that Bailey’s rule 2 is not always valid and should be refined. It was shown that during the decomposition of cis-2-butene PO the anticarbonyl oxide yields cis-ozonide and the syn-carbonyl oxide trans-ozonide, regardless of the ozonide conformation. The small substituents - CH3, C2H5 and C3H7 in the PO of 1,2-disubstituted ethylenes are oriented in the e,e-position and the bulky ones, iso-C3H7 and tert-C4H9, in the a,a-position [124-129]. According to reference [112] the ozonolysis of olefins takes place as a supra-supracycloaddition of 4n-2n-systems via a five-member ring formation. The cycloreversible decomposition of PO, depending on the configuration, results in: (1) syn-carbonyl oxide with a-orientation of the R substituent, and (2) anti-carbonyl oxide in the case of e-orientation of the substituent. The stereochemistry of the olefins ozonolysis is shown in Table 3.1.
184
Ozonolysis of alkenes in liquid phase
Table 3.1 Stereochemical course of ozonide formation and decomposition of PO after the mechanism of Bauld-Bailey (before the slash) and Kuczkowski (after the slash) Olefin
PO, C-C half chair/ O-envelope
COX
OZ, O-O-half chair/-
OZ
e/e
anti/anti
e,a/-
cis/cis3
Trans1
a,a/e
syn/syn
a,a/-
trans/trans
2
e,a/e
anti/anti
e,a/-
cis/cis
-
-/trans
1-Alkene
Cis
Cis3
-/a 1
-/syn 2
3
Note: - bulky substituents; - bulky substituents; - small substituents.
Later, Criegee summarised the available data and modified the mechanism of olefin ozonolysis taking account of the different conformations of the carbonyl oxide [125127]. Murray, Hagen and Bailey established that the cis:trans-ozonide ratios vary at: (1) addition of various complex-forming reagents such as toluene, o-xylene, isodurene, 1-mesithyl-1-phenylmethane, hexamethylbenzene, and (2) change of the heating rate during PO decomposition (Scheme 3.5) [128-130]: _ O
O C H
+
H C
CH3 H - bond
_O
O
+ C
R H
_O
O
+
H
C
R
CH3 anti-complex
syn-complex
Scheme 3.5 The more stable transition state of PO would give predominantly anticarbonyl oxide, being dependent on the size of the substituents. Thus, trans-1,2-di-isopropylethylene in its reaction with ozone in the presence of complexing agents, gives a cis:transozonide ratio >1. It has been found that this ratio remains the same in the absence of a complexation agent and at slow heating from (–155 oC) which, however, is against the stereochemical predictions. This fact could be explained by the occurrence of an equilibrium between the syn- and anti-isomers of CO, which takes place at high rate even at –155 oC passing through a carbonyl oxide formation (Scheme 3.6).
185
Ozonation of Organic and Polymer Compounds
_
O
O
O+
+O
O C+
C R
_
_
O
H
R
C H
R
H syn
anti
Scheme 3.6 This hinders the complexation between the CO and the complexing agent. These results have been observed during the study on the low-temperature ozonolysis of cis-olefins. On the basis of conformational considerations, the anti-isomer of CO appears to be more stable than the syn-isomer although in some cases the latter form can be stabilised on account of H-bond formation (Scheme 3.5). On the other hand, it can be assumed that CO preferentially leads to anti-CO-complex formation. Its stability will increase with the increase of the substituent size and then the equilibrium syn-anti will be shifted to the latter. This will result in rise of the cis-ozonide yield as predicted by the third rule of Bailey [129]. On the basis of experimental and theoretical data [130-134], and applying the rule of least motion whereby the PO decomposition occurs via a minimum change of atom coordinates, Bailey [130] demonstrated that the CO conformation depends rather on the rate of PO decomposition than on the thermodynamic stability of the stereoisomers. On this basis he proposed some additional refinements to the mechanism of olefins ozonolysis. The cis-trans-ozonide ratio depends on: (1) the decomposition of the thermodynamically preferable PO conformer, (2) the decomposition of the kinetically favourable PO conformer, (3) the competition in regard to the second and probably the third rule of Bailey for olefins with small substituents, and (4) the equilibrium of syn-/anti-CO conformers prior to their recombination with the carbonyl moiety. The slow heating of the e,e-conformer, which most likely has a lower activation energy of decomposition than the a,a-conformer, would produce preferentially anti-CO and the cis-ozonide in a smaller amount. In the case of rapid heating the decomposition would occur so quickly that it would not affect the equilibrium of the two conformers as it is shown below: anti-COX k e,e-PO j a,a-PO m syn-COX In references [129, 130] it is reported that rule 3 should be altered so as to conform to the O-O half-chair conformation of the end ozonide [111, 112, 134]:
186
Ozonolysis of alkenes in liquid phase Thus the revised rule 3 should be: upon the interaction of the aldehydes with anti-CO the bulky substituents are oriented equatorially-axially (e,a) producing cis-ozonide, while aldehydes react with syn-CO to orient them diaxially (a,a) producing transozonide. Bulky substituents in PO are oriented diaxially (a,a) resulting in trans-ozonide formation. The mechanism of ozone reaction with olefins has been studied by means of thermochemical and quantum chemical methods in a series of contributions [135145]. Harding and Goddard [137] have found that the exothermicity of the reaction between ozone and ethylene amounts to 92-96 kcal/mol, while it is 101.7 kcal/mol according to reference [11]. Goddard states that the ozonolysis of cis-trans olefins takes place through a biradical pathway (Scheme 3.7): .
R R
. C
R
+ O3
H
O
O
+ RCHO
syn (I)
R
. C
H
H . O
O
R (I)
R
O
C
R
. C
. O
O
trans
B
R
R
O
R
1,5-biradical
1,5-biradical A
R C
C
H
H
O
O
O
C
+ O3
. O
. C
H
O
+ RCHO
R anti R
H
. C
. O
O (II)
H
H
H
(II)
R . O
O O
C
O
C
. C
bulky R
O
1,5-biradical
1,5-biradical
C
D
(II)
C
small R
R C
C
R
R
O
O
R
cis
trans-ozonide
Scheme 3.7 In the transition states A, B, C and D, the orientation of the carbonyl oxygen adjacent to the carbon atom of the peroxymethylene allows the stabilisation of the lone electron pair on the oxygen p-orbital through exchange with the C-O bond. The arrows around the biradical C-O bond denote the most favourable orientations for rotation leading
187
Ozonation of Organic and Polymer Compounds to cyclisation (the bulky substituents should be kept away from their neighbours in their movement). The orbital phase consideration predicts supra-surface addition whereas the rotation around the biradical bond should be clockwise in the transition states B and D, and counter-clockwise in A and C. The pathway of ozonide formation from olefins with bulky substituents (tert-butyl) is denoted by arrows. In the case of olefins with small substituents, i.e., CH3, the transition state C might be assumed to follow the orbital permissible suprasurface addition even when the other transition states (A, B and D) follow the steric directions. In the light of the biradical mechanism the authors interpret the Baileys rules in the following manner: (1) an equilibrium between the syn- and anti-peroxymethylenes (not carbonyloxides) in the absence of a complexing agent, does not occur or take place to a very small extent, because of the high conversion barrier of 29 kcal/mol, (2) the complexing agents accelerate the rate of equilibration which leads to an increase of the anti-peroxymethylene concentration of the cis-ozonide, which is in contrast with the data from reference [131], (3) at higher temperature the cyclisation reaction is less stereoselective which would favour the formation of the more stable trans product, a fact which is experimentally confirmed, and (4) the increase of the solvent polarity increases the lifetime of the 1,5-biradical species and thus reduces the stereoselectivity of the reaction. On the basis of quantum chemical calculations Cremer [126-129] proposed a revised rule for determination of the stereoselective pathways of olefins ozonolysis (Table 3.2).
Table 3.2 Stereochemical courses of ozonide formation depending on the olefin conformation at ozone 1,3-cycloaddition and 1,3-cycloconversion of PO to CO at early (x0.5) transition state Olefin
Size of R1 & R2
COX x0.5
Ozone x0.5
cis-
small
anti-
syn-
cis-
trans-
trans
small
anti-
syn-
cis-
trans-
cis
bulk
anti-
anti-
cis-
cis-
cis
bulk
syn-
syn-
trans-
trans-
It should be noted that the studies discussed above and the conclusions drawn in them are based mainly on the analysis of the products obtained and on the activation energy calculations of the intermediate and stable products. However, the kinetic methods could also provide essential evidence for the elucidation of the mechanism of this reaction.
188
Ozonolysis of alkenes in liquid phase
3.1.2 Kinetics The reaction of ozone with olefins usually proceeds with high rate and low activation energy at temperatures ranging from –100 oC to 300 oC [95, 96, 143, 146].
3.1.2.1 Gas phase Cadle and Schadt were the first to determine the values of k for the ozonolysis of ethylene and hexene-2 in the gas phase [147]. Later, the ozonolysis kinetics of various olefins has been studied by many investigators [146-161]. The kinetics have had second-order with k values (20 oC) in the range of 0.8-450 × 103 M-1.s-1 (Table 3.3).
Table 3.3 Arrhenius parameters for the ozone reaction with olefins in the gas phase No.
Olefin
k × 10-3 (20 oC) (M-1.s-1)
log A
Ea (kcal/mol)
1.
Ethene
1.6 p 0.2 1.6; 1.8; 0.8; 1
2.2; 6.2; 6.73; 5.5
4.2 p 0.4; 4.2; 2.6; 4.9
2.
Propene
7.1
5.46
3.9
3.
Butene-1
6.2; 3.9
6.2
1.7
4.
trans-Butene-2
29; 260; 13
5.55; 5.41; 6.93
0.2 p 0.3; 2.3; 1.1
5.
cis-Butene-2
200; 17
6.28
0.96
6.
iso-Butene
14 p 2
6.15; 6.28
2.8 p 0.4; 1.7
7.
2,3-Dimethylbutene
-
6.23
0.82
8.
Pentene-1
3.9; 3.2
-
-
9.
Hexene-1
4.5
-
-
10.
2-Methylbutene
450
-
-
11
Trimethylethene
12
-
-
12.
Cyclopentene
6.1; 4.6
-
-
13.
1,1-Dichloroethene
2.2
-
-
14.
1,2-Difluoroethene
0.24
-
-
15.
Tetrafluoroethene
81
-
-
16.
Hexafluoropropene
13
-
-
17.
Octafluorobutene
1.1
-
-
189
Ozonation of Organic and Polymer Compounds The various investigators have reported similar values for k for the reaction of ozone with ethene, but the values of A differ more than four-fold [147, 148]. It appears that the values of A should be similar if one takes into account the structure of trans-butene-2 and iso-butene with a lower Ea for iso-butene. In fact, the values of A and Ea differ essentially and the magnitude of Ea for iso-butene is higher (Table 3.3). It has been found that the yield of ozonides, the major products in the liquid phase, is very low. The reaction mixture contains both lower and higher molecular compounds than the initial olefin, such as hydrocarbons, acids and aldehydes, etc. These products are obtained from the consequent reactions of the C=C bond cleavage products [153, 154]. Compounds with mass numbers up to 5 times order of magnitude higher than the initial compounds have been identified by means of mass spectrometry (for butenes with a molecular mass >200) [154]. It has been reported that epoxides are the major products of halogenated olefin ozonolysis [155]. The ozonolysis in the gas phase is accompanied by chemiluminescence, a fact which has found practical application for the production of analytical equipment for ozone analysis in the atmosphere. The light emission is due to the electronic transfer of various excited species such as: H2COX (1Aa) and OH (X2Pi; v a b9; A23+). In the reaction schemes outlined below the composition of the various products and the formation of the intermediate excited species are associated with the occurrence of radical steps, due to the breakdown of the O-O bond of the primary ozone [153] (Scheme 3.8) and further isomerisation of the bipolar ion (Scheme 3.9). Scheme 3.8
O O
O O
R C
C
H
R H
O R
R
C H
_ C+ O O
.
O C
H
O R
R-C-CHR H
*
C
. . R + CO2 + H RH + CO2
OH
Scheme 3.9
190
O OOH R
RCOOH
*
Ozonolysis of alkenes in liquid phase The stoichiometry of the olefin-ozone reaction varies in the range from 1:1.4 to 1:2 depending on the reaction conditions. The stoichiometry is exactly 1:1 in the absence of side reactions. The kinetics of the ozonolysis in the gas phase is strongly dependent on the absence or presence of molecular oxygen [153, 156]. Many workers underline that the ozonolysis kinetics is much more complex in its absence, i.e., in an inert atmosphere. Thus the kinetics of the ozone reaction with tetrafluoroethylene is described by the following relationship (Equations 3.1-3.6): d[O3]/dt = k1[O3][C2F4] + k2 [O3]2.[C2F4],
(3.1)
logk1 = (8.2 (0.5) – [(9.5 ± 0.7)/2.3.RT]
(3.2)
logk2 = (14.6 (0.5) – [(10.1 ± 0.6)/2.3.RT
(3.3)
and the reaction of ozone with allene [150]: d[O3]/dt = k1[O3][C3H4] + k2 [C3H4].([O3]2/[O3]0)
(3.4)
logk1 = (6.0 (0.7) – [(5.5 ± 1)/2.3.RT]
(3.5)
logk2 = (6.9 (0.7) – [(6.2 ± 0.8)/2.3.RT]
(3.6)
The foregoing discussion shows that the analysis of the kinetic data in the gas phase requires particular attention. Thus one should bear in mind that probably some of the reported constants for the gas phase ozonolysis are actually products, summations or partial quotients of parallel reaction constants.
3.1.2.2 Liquid phase The study of the kinetics of olefin ozonolysis in the liquid phase appears to be a more complicated task than in the gas phase because of the high reaction rate and more complex analysing equipment [145, 146]. The first studies in this respect were devoted to determination only of the relative rates in regard to a standard olefin-cyclohexene [162]. The first values of k were reported in 1969-1970. It has been found that olefin ozonolysis in the liquid phase follows also second-order kinetics [163-165]. We have established that the outlet ozone concentration is proportional to the change of the olefin concentration during acrylic acid ozonolysis (Figure 3.1).
191
Ozonation of Organic and Polymer Compounds
1.6
[O3]0
CH3COOH
1.2
0.8
[O3]g 0.4
0.0
0
100
200
300
400
500
600
Time, s
Figure 3.1 Kinetics of ozone concentration at reactor outlet upon ozonolysis of 0.02 M acrylic acid in acetic acid medium at 200 oC. The arrows denote the points of stoichiometry and rate constant estimation
The outlet ozone concentration drops abruptly after the start of the reaction and it remains low, close to zero, almost up to the complete olefin depletion (up to 4 minutes). Then it begins to rise rapidly and approaches the ozone concentration at the reactor inlet. The pattern of the kinetic curve depends on k - it becomes rectangular at high values of k and at low values of k the ozone concentration in the stationary region rises, the time necessary for reaching the inlet concentration becomes longer and the curve form is changed dramatically. A 1:1 stoichiometry has been found. After the addition of 1 mol ozone to 1 mol olefin the reaction rate is reduced by a factor of about 6-8. This fact suggests that the bipolar ion and the carbonyl compounds hardly interact with ozone. The major part of the products (>95%) are generated via one single reaction pathway at low and moderate temperatures. The values of A are found to amount to 106 M-1.s-1 which are typical for simple liquid phase reactions. The methods for kinetic studies are based on determination of the consumption rate of one of the reagents. In the absence of intermediate equilibrium steps, the rate of the first reaction step can be directly found as follows: The absence of a reverse reaction is testified by the following two facts: (1) the PO formation is an exothermic process –38 kcal/mol [143], and (2) the accumulation
192
Ozonolysis of alkenes in liquid phase of significant amounts of PO at temperatures 110-120 oC. If an inert gas is then blown through the reaction mixture only the dissolved ozone is liberated. If a reverse reaction occurs: O O R H
C
O C
H
R-CH=CH-R + O3
R
at blowing through, the equilibrium will be shifted to the starting compounds and the ozone concentration will be equal to the whole amount absorbed by the system. In this case an olefin rather than PO should be identified. Upon repeated ozonation the olefin will absorb again an equimolar amount of ozone. The experiments, however, show that only PO is identified in the reaction mixture, practically no olefin is found and ozone is not consumed which demonstrates the absence of a reverse reaction. The rate constants were measured: (1) through ozonation of a mixture composed of unknown olefin and a reference, for example cylcohexene [163, 164] as discussed above, (2) through ozone bubbling through the olefin solution in an inert solvent (in regard to ozone) and registration of the change of outlet ozone concentration [165], and (3) through fast mixing of ozone and olefin solutions by means of stopped-flow techniques [166]. All these methods provide similar results for the k values but the bubbling methods appears to be more convenient and experimentally available. We have successfully applied this method throughout our studies in all cases when it is possible. The rate constants of ozonolysis of some olefins in liquid phase are presented in Table 3.4. The comparison of the data in Tables 3.3 and 3.4 reveals that, mostly, the rate constants measured in the liquid phase are an order of magnitude several times higher than those found in the gas phase which is in accordance with the collision theory [166]. The study of the rate dependence on the concentration of the reagents shows that in most cases it is first order with respect to ozone and slightly greater than one in regard to olefin. However, the kinetics approximate first order with respect to olefin at low or sufficiently low olefin concentrations. The increase of the concentration results in an increase of the reaction rate more than that predicted by the second-order kinetics. This could be explained by the fact that the PO formation is preceded by the formation of an ozone-olefin complex (Scheme 3.10).
193
Ozonation of Organic and Polymer Compounds
Table 3.4 Rate constants (k × 10-5 M-1.s-1) of olefin ozonolysis in CCl4 at 20 oC No.
Olefin
[153]*
[155]
[156]
1.
Ethene
-
-
0.4
2.
Butene-1
-
-
1.3
3.
trans-Butene-2
0.4-1.6
-
-
4.
cis-Butene-2
0.3-1.0
-
-
5.
Isobutene
0.2
0.97
-
6.
Pentene-1
2.4
-
5.0
7.
2-Methylbutene-2
8.
Cyclopentene
4.5
2.0
4.0
9.
Hexene-1
0.27
0.76
1.4
10.
Hexene-2
-
1.48
5.0
11.
Tert-methylethene
-
2.0
-
12.
Styrene
0.30
1.0
3.0
13.
p-Methylstyrene
0.31
-
-
14.
m-Methylstyrene
Note: *measured in CHCl3 with respect to cyclohexane.
O O R-CH=CH-R + O3
1 Olefin 3
R-CH
CH-R O3
2
R H
C
O C
H R
Scheme 3.10
This complex is thermodynamically stable and at collision with a new olefin molecule is broken down to the initial products. Then the dependence of the observed rate constant (kobs) on the olefin will be described as shown in Equations 3.7 and 3.8:
194
kobs = k1.k2/(k2 + k3 .[Olefin])
(3.7)
1/kobs = 1/k1 + k3/(k1 .k2.[Olefin])
(3.8)
Ozonolysis of alkenes in liquid phase Upon ozonolysis of acrylic acid the values of k1 and k3/k2 amount to 4.2 × 103 M-1.s-1 and 8 × 106 M-1.s-1, respectively. It should be pointed out that these values are higher than those reported in reference [163, 164] as the olefin concentrations are lower at the points of rate constants determination (denoted by arrows). In this case the rate constant was calculated by applying the real olefin concentration which is equal to the difference between the initial and the consumed concentration up to the moment of calculation. Generally, this difference is within the limits of the experimental error and thus the reported constants could be used for kinetic characteristics of the process. However, when possible we have used the values of k1. The temperature dependencies of the rate of hexene-1 and maleic anhydride ozonolysis, obtained by us, are presented in Table 3.5.
Table 3.5 Dependence on temperature of the rate constant k1 of the ozone reaction with some olefins Hexene -1
17 oC
0 oC
–20 oC
–40 oC
–60 oC
–78 oC
E (kcal)
k1 (M-1.s-1)
1.4 × 105
1.5 × 105
1.2 × 105
0.7 × 105
1.0 × 105
0.6 × 105
0.5 p 0.5
Maleic anhydride
43 oC
35 oC
26 oC
17 oC
9 oC
E (kcal)
k1 (M-1.s-1)
68.3
61.5
36.1
25.8
18.4
6.1 p 0.5
As seen, the rate constant of hexene-1 does not change with temperature while the activation energy of maleic anhydride reaches the value of 6.1 kcal/mol. The dependence of the rate constant on the temperature for maleic anhydride, according to our data [32, 33], is described by the following expression (Equation 3.9): k1 = (1.1 ± 0.2).106.exp(-6100 ± 500/RT)
(3.9)
The approximate value of A for hexene-2 is 106-107 M-1.s-1. Generally the olefin ozonolysis is characterised by very low values of Ea = 1-10 kcal/mol. However the inaccurate determination of the rate constants often does not allow the precise definition of the Arrhenius parameters which in turn could lead to misleading interpretation of the experimental results. The values of Ea are commensurable with a number of other energy factors, influencing the reactivity, which impedes the evaluation of their contribution [167]. Among them are the reorientation of the surrounding solvent, the thermal energy of the molecules, the temperature
195
Ozonation of Organic and Polymer Compounds dependence of the diffusion coefficients, etc. Consequently, the dependence of the rate constant on temperature cannot be outlined as according to definition (Equation 3.10): Ea = E0 + 0.5 RT
(3.10)
where Eo is the height of the potential surface at -273 oC. The rate constants for olefins with various structures are given in Table 3.6. The rate constants of the reaction of ozone with olefins with terminal double bonds (hexene-1, octene-1 and decene-1) are high and similar. The isomeric olefins with more internal double bonds (hexene-2 and methyloleate) react much more readily that those mentioned above. The rate of the ozone attack increases as the number of alkyl substituents on the double bond increases (hexene-1 and 2-methylpentene-2, acrylic acid and methacrylic acid, polybutadiene and polyisoprene). Cycloolefins C5-C12 react with relatively similar rate constants but more slowly than those with an open chain (cyclohexene k1 = 4 × 105 and k1 for methyloleate = 1 × 106 M-1.s-1). The presence of phenyl groups results in acceleration of the reaction (styrene) while the introduction of second, third and fourth groups (stilbene, 1,1,4,4-tetraphenylbutadiene) has no further influence on the reaction rate. The olefin ozonolysis can be regarded as an electrophilic 1.3-addition and similarly to the reactions of Prilezhaev and Diels-Alder which have close electronic multiplicity, one can expect that electron-accepting substituents in the molecule will decrease the reaction rate. Actually, Br, Cl, -COOH and NO2-substituted olefins react much more slowly with ozone [32-35, 168-170]. The comparison of the rate constants allows disclosure of the effects depending on the structure of the olefins under study. Thus, the values of k for high molecular compounds (polybutadiene, polyisoprene and squalene) are lower than those found for their low molecular analogues (methyloleate and tri-trans-cyclododecatriene and 2-methylpentene-2) [171, 172]. It has been also found that the reactivity of cis- and trans-isomers (maleic and fumaric acids, cylcohexene and tri-trans-cyclododecatriene1,5,9, gutta-percha and natural rubber) varies as the trans-isomers are more active in their reaction with ozone [141, 168, 169]. The rate constant of cis-diphenylethene is 8.9 × 104 M-1.s-1 while for trans-diphenylethene it is 1.8 × 105 M-1.s-1. Usually these facts are attributed to the occurrence of the compensation effect. Huisgen [173, 174], however, explained this fact by the repulsion of the cis-substituents in the transition state. The hybridisation of the carbon atoms of the double bond changes from sp2 to sp3 in the activated complex which results in overlapping of the van der Waals radii of the cis-substituents. Thus the rate of the ozone attack is retarded whereas overlapping is impossible in the case of trans-isomers and thus the reaction is faster. Similar differences in the reactivity of cis- and trans-isomers have been also observed in Prilezhaev reactions.
196
Ozonolysis of alkenes in liquid phase
Table 3.6 Rate constants of olefin ozonolysis in CCl4 at 20 oC No.
Olefin
k1 × 10-5 (M-1.s-1)
1.
Ethene
0.4
2.
Butene-1
1.3
3.
cis-Butene-2
1.6
4.
Isobutene
1.0
5.
1,3-Butadiene
0.74
6.
Pentene-1
1.4
7.
2-Me-Butene
5.0
8.
Hexene-1
1.4
9.
Hexene-2
5.0
10.
2-Me-Pentene-1
5.0
11.
Octene-1
1.3
12.
Octadecene-1
1.8
13.
Cyclopentene
4.0
14.
Cyclohexene
4.0
15.
Cyclodecene
4.0
16.
Tri-trans-cyclo-dodecatriene-1,5,9
3.5
17.
Norbornene
5.0
18.
Styrene
3.0
19.
Stilbene
1.8
20.
1,4-Diphenybutadiene
0.9
21.
1,1,4,4,-Tetraphenyl butadiene
0.8
22.
Acenaphthylene
2.2
23.
Acrylic acid
0.042
24.
Methacrylic acid
0.078
25
Methylmetacrylate
0.078
26.
Maleic acid
1.42.10-3
27.
Fumaric acid
0.084
28.
Maleic anhydride
3.6.10-4
29.
2,6-Dimethylhept-2-en-2-carbonic acid
0.026
30.
Oleic acid
10.0
31.
Methyloleate
10.0
32.
Vinylchloride*
0.022
33.
Allylchloride*
0.085
34.
cis-1,2-Dichloethane*
3.57 × 10-4
197
Ozonation of Organic and Polymer Compounds
35.
Vinylidene chloride*
2.11 × 10-4
36.
Trichloroethene* Tetrachloroethene*
3.6 × 10-5 1.0 × 10-5
37.
2-Bromopropene
0.028
38.
Acrylonitrile
1.0 × 10-3
39.
Tetracyanoethene
0.4 × 10-5
40.
1,1,2-Triphenyl-2,2-dinitroethene
3.6 × 10-5
41.
1,1-Diphenyl-2,2-dinitroethene
0.7 × 10-5
42.
Polybutadiene
0.6
43.
Squalene
7.5
44.
Natural rubber
4.4
45.
Gutta percha
1.7
46.
Polychloroprene
0.042
Note: *the values are taken from [154, 158].
The small sizes of the ozone molecule predetermine the insignificant role of steric effects in the reaction. Only one of the examples in Table 3.6 shows an anomaly in the rate which could be ascribed to steric influence (k of acrylic, methacrylic and 2,6-dimethyl-hept-2-ene-2-carboxylic acids are 2.3 × 103, 7.8 × 103 and 2.8 × 103 M-1.s-1, respectively). If one takes into account only the inductive effects of the substituents, then the rate constant of 2,6-dimethyl-hept-2-2-carboxylic acid would be 7.8 × 103 M-1.s-1. In fact the latter is 3 times greater than the experimental value which demonstrates the effect of steric factors on the reactivity. The rate of olefin ozonolysis depends significantly on the olefin structure (k for oleic acid is six-fold higher than that for tetracyanoethylene) (Table 3.6). The quantitative relationship between the electronic and geometric structure of olefins and their reactivity is a very important stage in studying ozone chemistry. The first attempts in this respect were the studies of Cvetanovic and co-workers [163, 164] who found a good linear relationship between log of the rate constant and the ionisation potential of chlorinated ethenes. It has been found that the linear dependence of the reactivity on the free energy change in the system is valid for many organic reactions. The Hammett relationship is well known and widely applied [170] (Equation 3.11):
lgki /k0 = R.S
198
(3.11)
Ozonolysis of alkenes in liquid phase where kj are the rate constants of the compounds studied; k0 is the rate constant of the reference; R is coefficient accounting for the reaction character; S is inductive constant of the substituents. Some other relationships, like that of Taft, have also been used for evaluation of the contributions of inductive, steric and mesomeric effects of various substituents on the rate constant. A dependence of Hammett type has been found upon decomposition of PO and ozonides [171, 172, 175]. It is quite natural that a similar relationship might be observed during ozonolysis of olefins.
Table 3.7 Logarithms of the relative values of the rate constants and the Hammett inductive constants (σpara) of the substituents on the double bonds No.
Olefin
Substituent
3Spara
log ki /k0
1.
Methyloleate
2CnH2n+2
–0.302
1.40
2.
2-Me-pentene-2
2CH3, C2H5
–0.491
1.097
3.
Hexene-2
CH3, C3H7
–0.321
1.097
4.
Styrene
Ph
–0.01
0.875
5.
Styibene
2Ph
–0.02
0.65
6.
Hexene-1
C4H9
–0.151
0.544
7.
Octadecene-1
C16H33
–0.151
0.65
8.
Ethene
H
0
0
9.
Methylmethacrylate
CH3, COOCH3
0.22
–0.7
10.
Methacrylate acid
CH3, COOH
0.28
–0.71
11.
3-Chloropropene-1
CH2Cl
0.18
–0.67
12.
2-Bromopropene
CH3, Br
0.062
–1.15
13.
Acrylic acid
COOH
0.45
–0.98
14.
Acrylonitryle
CN
0.66
–2.6
15.
Maleic acid
2COOH
0.9
–2.45
16.
Fumaric acid
2COOH
0.9
–1.68
17.
1,1,2-Triphenyl-2nitroethene
3Ph, NO2
0.748
–4.04
199
Ozonation of Organic and Polymer Compounds The rate constant of the reaction of ozone with ethene, equal to 4 × 104 M-1.s-1, is accepted as a standard in studying the kinetics of ozonolysis of various olefins. It is calculated on the basis of the k value in the gas phase multiplied by a coefficient of 25 which accounts for the heat of dissolution, 1.9 kcal/mol. The Hammett relationship in Figure 3.2 is plotted on the basis of the data of Table 3.7. As seen the data fall on a straight line (Figure 3.2) thus indicating a linear dependence.
3
= -3.2 R = 0.89883
2 1
2
1 3
7 6
0
4 5 8 10
-1
9 11
13
12 16
-2
-4 -0.8
15
14
-3
17 -0.4
0.0
0.4
0.8
para
Figure 3.2 Hammett relationship of ozone reaction with olefins
The negative value of R = –3.2 indicates the electrophilic nature of the ozone reaction with olefins. The low value of the correlation coefficient R = 0.89883 could attain much higher values if the steric effects of the substituents and the electrostatic interactions are considered. Regardless of this the dependence in Figure 3.2 is quite acceptable for evaluating the rate constants of reactions with new olefins. Another very important question related to the reactivity of olefins, which has not been sufficiently studied, is the relationship between the reactivity of C=C bonds and the deformation and steric factors responsible for the olefin conformation. These factors depend on the inter, intra, supra, micro and macro effects. Thus the intramolecular strains arising in the cyclic olefins are due to the deformation of valent, dihedral angles, interatomic distances and transanular interactions [175-183]. The values of the different deformational and steric components responsible for cyclohexene conformation, as calculated by us, are presented in Figure 3.3.
200
Ozonolysis of alkenes in liquid phase
Energy, kcal 60
50
40
30 MMXE 20 SE 10
TOR
0
BND
2
4
6
8
10
12
Cycloolefins
Figure 3.3 The values of C2-C12 cylcoolefin energies calculated according to the molecular mechanics theory. MMXE - the Alinger energy; SE - the strain energy; TOR - the torsion energy and BND - the bond energy
It is seen that the dependencies have specific character with a minimum in case of cyclohexene. Among the olefins, given in Table 3.6, there are a number of olefins with highly strained molecules, such as cyclododecene, cyclododecatriene, norbornene, ethylidenenorbornene, acenaphthylene and 2,6-dimethylhept-2-2-carbonic acid. It has been found that these olefins react with diazomethane in contrast to those with strainfree molecules. The reduction of strain in the sequence of C4, C5 and C6 cycloolefins correlates with the reduction of the corresponding yield of pyrazolines [183]. The reactivity and size relations of cyclo-olefins are demonstrated in Figure 3.4. It is seen that cyclohexene, the olefin with the lowest strain energy of the initial molecule, exerts the lowest reactivity towards ozone. Simultaneously, its activated complex (AC) has the highest conformation energy. Thus Ea - the difference between the energy of the starting olefin and that of the AC - would be the highest which accounts for the lower reactivity of cyclohexene with respect to the other cycloolefins [1]. Olefins may contain several double bonds separated by three simple C-C bonds. In these cases the C=C bonds react as independent kinetic units. The ozone addition to one of them hardly affects the neighbouring ones. The substituent inductive effect is reduced by a factor of about 2.7 through each C-C bond and thus its influence on the other carbon atoms is significantly reduced [183, 184].
201
Ozonation of Organic and Polymer Compounds
10 SEn/SE6
Relat. values
8 6 4 kn/k6 2 0 4
6
8
10
12
Cycloolefin
Figure 3.4 Dependency of the relative rate constants and strain energies of cycloolefins on the cycle size
Upon ozonolysis of compounds with conjugated bonds (1,3-diene, p-divinylbenzenes, etc.), the different reactivity is determined by the structure of the starting diene, the nature of the olefin after the first ozone addition and the effect of the ozonide inductive factor on the second C=C bond. This is illustrated by the ozonolysis of divinylbenzene (Scheme 3.11).
Scheme 3.11 The proceeding of the reaction in two steps results in the appearance of two stationary regions on the curve [O3]g = f(T) thus allowing the determination of the two rate constants k and ka (Table 3.8).
202
Ozonolysis of alkenes in liquid phase
Table 3.8 Rate constants of ozone reaction with dienes and trienes in CCl4 at 20 oC k × 10-5 (M-1.s-1)
ka × 10-5 (M-1.s-1)
Divinylbenzene
2.0
0.2
Ethylidenenorbornene
3.6
2.0
Vinylnorbornene
3.3
0.8
Vinylcyclohexene
1.4
0.8
Tetrahydroindene
1.4
1.4
Cyclododecatriene-1,5,9*
3.5
3.5 (3.5)
Compounds
Note: *The third double bond reacts with the same rate as the second.
In the example of divinylbenzene the change in the rate constants after ozone addition could be followed. Its correlation with the general principles of the theory can be also evaluated. The rate constant of the ozone reaction with the first C=C bond should be slightly smaller than that of styrene because of the higher conjugation in divinylbenzene. After the ozone attack on the first C=C bond the conjugation degree is reduced and ozone interaction with the second C=C bond should take place with a rate similar to that of styrene (3 × 105 M-1.s-1). However, taking into account the electron-acceptor effect of the ozonide cycle, the rate of the second step should be smaller than that of the first step, which has been also confirmed by experimental data. The comparison of k for ethylidene- and vinylnorbornene, allows the assumption that ozone attacks firstly the double bond in the side chain, as the changes in its structure affect the value of k while the values of ka are equal for the two isomers. The C=C bonds in cyclododecatriene-1,5,9 are equivalent due to the long distance between them and the equal configuration of the neighbouring environment. This is demonstrated by the equal rate constants for the three bonds and by the value of the hydrogenation rate of cyclododecatriene-1,5,9 on palladium [185].
3.1.2.2.1 Effect of solvent It is known that the rate of many reactions depends strongly on the solvent polarity [181, 182]. There are a number of examples whereby the reaction mechanism changes with solvent variation and the ozonolysis of olefins is among them. It has been found that alkoxy- and aryloxy-hydroperoxides [186, 187] instead of ozonides, are the major ozonolysis products of olefin ozonolysis in solvents such as alcohols, acids and other compounds.
203
Ozonation of Organic and Polymer Compounds In order to assess the influence of the solvent on the rate constant we have followed the kinetics of acrylic acid ozonolysis in various solvents. The choice of acrylic acid was mainly for two reasons: (1) its good solubility in the solvents used, and (2) its relatively low rate of interaction with ozone which simplifies the registration of the reaction progress. The following solvents which were used for this study - CCl4, decane, acetic acid and water - differ essentially in regard to their dielectric constants (ε) and their ability to dissolve ozone. Decane and CCl4 are inert towards the intermediate and end products but acetic acid and water react with the dipolar ion. We have found that the reaction obeys first-order kinetics with respect to each reagent.
Table 3.9 Rate constants of acrylic acid ozonolysis in various solvents at 20 o C Solvent
Dielectric constant (ε)
Henry’s constant (α)
k × 10-3 (M-1.s-1)
CCl4
2.24
1.8
2.9
n-Decane
2.05
1.5
2.8
99
6.2
1.70
2.6
84
18.2
1.26
2.6
67
30.9
0.95
2.6
50
43.6
0.77
2.6
16
69.0
0.65
2.8
100
80.2
0.42
2.7
Acetic acid/H2O (%)
The data in Table 3.9 show that the value of k does not depend significantly on the solvent. Thus two basic conclusions emerge: (1) the solvent does not affect the first step of the reaction or, if participates, it is in fact during some of the next reaction steps, as: RHC+OO- + RaCOXOH (RH(HOO)C-O(O)CRa RHC+OC(OO-)CHR + RaC(O)OH (RHC[O(O)CRa]-O-C(OOH)HR
and (2) formation of polar compounds or polar transition states [188-192] is not observed:
204
Ozonolysis of alkenes in liquid phase
C
O
C
O+
C
O
+C
_O
C
_ O O
O C
O +
O_
The polar intermediate compounds should solvate or react with ‘the active solvents’. This should change the k value with the increase of the solvent polarity, and lead to the formation of reaction products with C-C bonds instead of C=C bonds. However, similar compounds have not been experimentally identified. The constancy of k values in various solvents testifies that the reaction occurs without formation of kinetically active polar compounds due to the high rate of electron delocalisation in the AC. The latter is much slower than the relaxation time of the solvent shell (^10-13 s).
3.1.2.2.2 Effect of configuration We have studied the ozonolysis of structural derivatives of one and the same chemical compound: butane dicarbonic acid and its anhydride, maleic anhydride (I) cyclic form, maleic acid (II) cis-form and fumaric acid (III) trans-form: CH
HC
COOH
HOOC C
O=C
C=O
H
C
II
I
C
H
H
O
H
HOOC
C
COOH III
These compounds, similarly to acrylic acid, react relatively slowly with ozone which makes them suitable objects for investigating the reaction kinetics [32, 33]. The concentrations of the reagents vary from 6 × 10-5 to 0.9 M. Water, glacial acetic acid and chloroform were used as solvent media. The products of the ozone reaction with the double bond in the ethers of maleic and fumaric acids have been identified using chemical methods and infrared (IR) spectroscopy. The low values of the rate constants suggest that at definite conditions ozone may also attack the C-H bond (1b) in addition to the basic interaction with the C=C bond (1a): HOOC
HOOC
H
H
+ O3
O3
C HOOC
èëè
C H
HOOC 1a
O3
C
C H
H
HOOC
C
C
HOOC
H 1b
205
Ozonation of Organic and Polymer Compounds In this connection we have determined the reaction stoichiometry and analysed the IR spectra of the solutions during the reaction course. The stoichiometry we found to be 1:1. The iodometrically estimated content of active oxygen in the samples corresponds to the amount of absorbed ozone. The progress of the reaction was accompanied by an increase in the intensity of the C-O bands in the range of 110-1200 cm-1 and narrowness of the C=O bands in the range of 1710-1850 cm-1. However, at high conversion degrees the C=O bands are moved from their initial range due to the inductive effect of the ozonide cycle. With the increase of ozonation duration the bands of the C=C valent vibrations in the 1610-1680 cm-1 range decrease their intensity and an abrupt decrease of the rate of ozone uptake is observed. At complete consumption of C=C bonds the ozone concentration at the reactor outlet becomes equal to that at the reactor inlet. This fact suggests that ozone interacts mainly with the C=C bonds, while its consumption in parallel and consecutive steps is very low. This occurs regardless of the aldehyde formed in ozonolysis, which may also react with ozone. On the other hand the band intensity of the C-H and H-O valent vibrations at 3020 cm-1 and 3550 cm-1, respectively, do not change significantly in the course of the ozonation. These experimental observations confirm the assumption that ozone does not attack the C-H bonds at the C=C bonds. This is also shown by the low k value for butandicarbonic acid (1 × 10-3 M-1.s-1) at room temperature. Briener and coworkers [193-195] also confirmed the absence of ozone interaction with the hydrogen of the OH group. Upon ozonation of the aforementioned acids and their esters they obtained a similar product composition. Consequently ozone reacts predominantly with the C=C bonds of I, II and III according to pathway 1a. It has been found that the rate of the reaction and the product compositions do not vary substantially with the solvents used, i.e., water (= 80.1), acetic acid (= 6.2) or chloroform (= 10). However, it should be noted that because of the lower solubility of fumaric acid in chloroform most of the comparative kinetic studies were conducted in acetic acid medium (Table 3.10).
Table 3.10 Kinetic parameters of the ozone reaction with maleic anhydride, maleic and fumaric acids in acetic acid at 2-6 × 10-6 M ozone concentration Substrate
k1 (M-1.s-1)
k3/k2 (M-1)
log A1
E1 p 0.5 (kcal/mol)
I
26 (17.30); 36 (25.80C); 51 (35.00C); 64 (42.50C)
63 (17.30C)
6.30
6.5
II
114 (17.00C); 142 (23.00C); 162 (35.00C); 232 (42.00C)
4560 (17.00C)
5.50
4.6
III
765 (17.00C); 840 (22.00C); 919 (28.50C);1050 (42.50C)
1000 (22.00C)
5.13
3.0
206
Ozonolysis of alkenes in liquid phase We established that at relatively low reagent concentrations the rate of ozone absorption is not proportional to the reagent concentrations although the amount of absorbed ozone is equal to that of the olefin consumer. The reaction exhibits first order with respect to ozone and the dependence on the olefin concentration is more complicated. The rate constant decreases with the increase of olefin concentration. The reciprocal magnitude of the observed rate constant depends linearly on the olefin concentration (Figure 3.5).
0.06 0
17.3 C 0.05
0.04
0
25.8 C
0.03
0
42.5 C
0.02
0.01
0.00 0
1
2
3
4
5
3
[I].10 , M
Figure 3.5 Dependence of the reciprocal value of the rate constant on the concentration of maleic anhydride at various temperatures in acetic acid and [O3] = 5 × 10-6 M
These results indicate the occurrence of a side non-efficient reaction, in which the olefin is consumed via a reaction with an intermediate product formed between the ozone and olefin (Scheme 3.12): Olefin + O3 m P1
(1)
P1 m Products
(2)
P1 + Olefin m 2Olefin + O3
(3)
Scheme 3.12
207
Ozonation of Organic and Polymer Compounds On the basis of the kinetic analysis of Scheme 3.12 an expression for the reciprocal value of the observed rate constant, similar to Equation 3.8, is derived: 1/kobs = 1/k1 + (k1.k3/k2) [Olefin]
(3.12)
Actually Figure 3.5 illustrates graphically Equation 3.12. The values of the rate constants as determined on the basis of Scheme 3.12 are given in Table 3.10. The analysis of the data in Table 3.10 reveals two significant facts: (1) the high values of k3/k2 ratios, and (2) the increase in the rate constant k1 in the sequence: maleic anhydride (cyclic form)<maleic acid (cis-isomer)C=C< + O3 m Products Scheme 3.13 However the literature provides data that the olefins ozonolysis probably takes place via the formation of P-complex according to Scheme 3.14: >C=C< + O3 m P-Complex m Products Scheme 3.14
208
Ozonolysis of alkenes in liquid phase UÊ ««V>ÌÊvÊ , In order to propose the most probable mechanism of the ozone reaction with I, II and II we have applied the method based on comparison of the calculated values of the pre-exponential A according to the activated complex method with different forms of the activated complex in the transition state with those obtained experimentally using two forms of TS: R2C
CR2 O
O
R2C
CR2
O
O O
O LC
CC
The calculated and experimental values for A are presented in Table 3.11. It is seen that the values of A calculated with CC of the activated complex are lower than those with LC. The values of A calculated on the assumption that the energy of free rotation is zero are given in column 4 and those taking account of this energy are placed in column 5. Compared to Aexp, the values of A-CC are lower by about 1-2 orders. The conclusion based on these facts is that the transition state cannot have a cyclic form as these values of A are the highest possible predicted by the theory with this form of the activated complex. The values of A calculated with the linear form of the activated complex are similar to those obtained experimentally. Therefore, it can be assumed that the reaction proceeds via the formation of a P-complex according to Equation 3.13: kobs = k1.k2/(k-1 + k2)
(3.13)
at k2 >>k-1 and kobs = k1 The assumption that k2>>k-1 is supported by the good fit between the calculated value for A of the right reaction with that obtained experimentally for I, II and III ozonolysis. This is also confirmed by the absence of any additional bands in the IR spectra of the ozonised solutions of I, II and II in methylene chloride registered for 48 h at –80 oC.
3.1.2.2.3 Effect of structure For detailed analysis of the effect of the chemical structure of olefins on their reactivity, we have synthesised a number of substituted cyanostilbenes:
209
Ozonation of Organic and Polymer Compounds H C R1
C C
R2
N
Earlier, these compounds were investigated as phosphorescent and colouring agents [196, 197]. Their interaction with ozone, however, has not received considerable attention although their specific electronic structure provides a unique possibility for assessing the effect of such an important chemical factor on the reaction ability. Some characteristics of the synthesised and investigated cyanostilbenes are presented in Table 3.12 [35, 36].
Table 3.11 Arrhenius parameters for the ozonolysis of I, II and III Compound
kobs (20 oC)
log ACC
(M-1.s-1)
log ALC at Efr= 0
log ALC, at real Efr
log Aexp
Ea (kcal/ mol)
Maleic anhydride
29.1
4.48
7.48
6.30 (1.60)
6.30
6.50
Maleic acid
128
4.42
7.42
5.50 (2.60)
5.50
4.60
Fumaric acid
810
4.33
7.33
5.13 (3.00)
5.13
3.00
Numbers in brackets are the estimated energies of free rotation in kcal/mol.
Table 3.12 Kinetic and physical constants of synthesised and studied cyanostilbenes in CCl4 at 20 oC No.
Cyanostilbene
k × 10-5
mp
(M-1.s-1)
(oC)
log k
3S
1.
2-Phenyl-3-phenyl
1.3
5.114
0
2.
2-Tolyl-3-(4-chlorophenyl)
1.3
5.114
0.687
3.
2-(4-Chlorophenyl)-3-tolyl
1.2
5.079
0.687
4.
2-(3,4-Dimethoxyphenyl)-3nitrophenyl
-
-
0.625
5.
2-(4-Nitrophenyl)-3-(4chlorophenyl)
0.6
4.778
1.005
6.
2-(4-Nitrophenyl)-3-(4nitrophenyl)
-
-
1.556
210
Ozonolysis of alkenes in liquid phase
7.
2-(4-Nitrophenyl)-3-(4dimethylaminophenyl)
1.6
5.204
–0.052
8.
2-(4-Aminophenyl)-3-(4methoxyphenyl)
3.0
5.477
–0.928
9.
2-(4-Nitrophenyl)-3-(4methoxyphenyl)
1.1
5.041
0.510
10.
2-(4-Methoxyphenyl)-3-(4methoxyphenyl)
2.6
5.415
–0.536
11.
2-(4-Aminophenyl)-3-(4methoxyphenyl)
0.5
4.699
0.608
12.
2-(4-Nitrophenyl)-3-(3methylphenyl)
2.4
5.380
–0.091
13.
2-(3,4-Dimethoxyphenyl)-3-(3fluorphenyl)
-
-
0.074
14.
2-(3,4-Dimethoxyphenyl)-3-(4chlorophenyl)
1.7
5.230
–0.170
15.
2-(4-Cyanophenyl)-3-(4dimethylaminophenyl)
5.9
5.771
–0.200
16.
2-Tolyl-3-(4-aminophenyl)
1.4
184
5.146
0.470
17.
2-(3,4-Dimethoxyphenyl)-3-(3,4dibromphenyl)
-
107
-
–0.163
18.
2-(3,4-Dimethoxyphenyl)-3-(1naphthyl)
-
-
-
0.768
19.
2-Phenyl-3-(4dimethylaminophenyl)
-
-
-
–0.840
20.
2-(3,4-Dimethoxyphenyl)-3piperonyl
-
145
-
-
21.
2-(3,4-Dimethoxyphenyl)-3-(3,4dimethoxyphenyl)
-
-
-
–0.306
22.
2-(3,4-Dimethoxyphenyl)-3aminophenyl
-
85
-
–0.813
23.
2-(4-Nitrophenyl)-3-phenyl
-
-
-
0.768
24.
2-(3,4-Dimethoxyphenyl)-3-(2methoxy-5- bromophenyl)
-
175
-
–0.156
25.
2-(3,4-Dimethoxyphenyl)-3-(3fluorophenyl)
-
-
-
0.184
Note: The constants are measured only for tetrachloromethane-soluble compounds.
211
Ozonation of Organic and Polymer Compounds The values of the melting points are given only for the newly synthesised compounds. The concentration of cyanostilbenes in the kinetic experiments amounts to 1.33 mM. For determining the rate constants upon a high reaction rate we have applied the absorption theory which is enhanced by the proceeding of a first-order chemical reaction (Equation 3.14) [34]: R0 = [O3]*(k1.D)1/2
(3.14)
where R0 is the absorption rate at bubbling liquid in stationary conditions, mol/(cm3); [O3]* = A.[O3]g is the equilibrium ozone concentration on the liquid-gas surface (mol/ cm3), where A = 2 is Henry’s coefficient at 20 oC; D = 7.8 × 10-8(KMB)1/2.T, (MB.VA0.6)-1 = 3.06 ×10-5 cm2/s - is the coefficient of ozone diffusion (A) in CCl4 (B), calculated by the Wilke and Chang equation [198]; K = 1 is the coefficient of association; MB is the molecular weight of CCl4, g; MB = 1.2 x 10-3 Pa-s which is the viscosity of B; VA = 30 cm3/g, mol the molecular volume and k1 is the rate constant. Equation 3.14 is true at high accelerations of the physical absorption resulting from the chemical reaction at (D × k1)1/2/kL>>1. At known values for D = 3.06 × 10-5 cm2/s, kL z 10-2 cm/s and k1 = 106 × 10-3 = 103 s-1, the condition (D × k1)1/2/kL>>1 is fulfilled since the left side of the inequality has a value of 15. The ozone reaction with the cyanostilbenes is a bimolecular process as the rate of the reaction increases linearly with increasing concentrations of both reagents. Consequently ozonolysis is first order with respect to each of the reagents. The quantitative analysis of the band at 1152 cm-1, which is responsible for the ozonide formation, reveals that its accumulation rate is almost equal to the rate of cyanostilbene consumption. It has been found that 4,4a-dinitro-cyanostilbene ozonolysis yields 4-nitrobenzenecyanate and 4-nitro-benzaldehyde as the major products, while the ozonide amount is very low. The scheme of the reaction is given in Scheme 3.15. O H O2N
O
+O3
C
NO2
C
O2N
CN
_ O O O2N
+ C H
+
C
C
H
CN
O2N O
C
NO2
CHO
NO2
CN
Scheme 3.15 212
O
O2N
CNCO
Ozonolysis of alkenes in liquid phase In view of the concepts on the PO decomposition mechanism, with electron-donating substituents, a zwitterion at the carbon atom with a CN group could not be formed due to its destabilising effect. The conjugation between the carbonyl compound formed at this carbon atom and the CN and Ph groups prevents ozonide formation via a zwitterion. For this reason the zwitterion leaves the cage and then it interacts with another zwitterion in the solution volume liberating oxygen. This reaction results from the strong electron-accepting action of the nitro groups in the para-position and provides an example of anomalous ozonolysis. Generally, the cyanostilbenes studied obey the Hammett’s relationship and give a straight line with R = –0.4 (Figure 3.6). This value, although substantially lower than that found for substituents directly attached to the double bond, shows clearly the occurrence of conjugation leading to re-distribution of the substituents inductive effects.
5.8
= -0.4 5.6 8
10
12
5.4 14
5.2
16
7
9 5.0 5
4.8 4.6
11
-1.0
-0.5
0.0
0.5
1.0
Figure 3.6 Dependence of the rate constants on the Hammett inductive constants
The small difference in the rates depending on the inductive constants can be attributed to the low A values, most probably, because of the low activation entropy.
213
Ozonation of Organic and Polymer Compounds The cyanostilbenes studied have substituents in the both benzene rings (A and B). This could be regarded as the reason for the neutralisation of the substituents effect, which contributes to the low value of R. The electronic structure of cyanostilbene is shown as:
The values of the charges are determined by quantum-chemical calculations with AM1 Hamiltonian (MOPAC6). Aiming at more accurate assessment of the substituents effect on the reaction ability, we have synthesised 13 new compounds substituted only in the A- and 14 in the B-benzene ring. The rate constants of the ozone reaction with all these compounds have been evaluated and are outlined in Table 3.13. The correlation of these constants with the Hammett constants (Figure 3.7a, b) gives the following values of R = –0.266 and –0.286, for substitution in the A- and B-benzene ring, respectively. The correlation coefficients are not higher than 0.8. It is evident that the two coefficients (are almost equal since the difference of about 7% is within the limits of the experimental error. Thus the reactivity actually does not depend on the substitution position. The similar slopes of the kinetic curves suggest that the role of the CN group in the re-distribution of the electron density and its effect on the reaction ability of these compounds cannot be differentiated in this reaction.
214
Ozonolysis of alkenes in liquid phase
Table 3.13 Rate constants of cyanostilbene ozonolysis in CCl4 at 20 oC No.
A-Substituents/S
k × 10-5 (M-1.s-1)
1.
p-NH2/-0.66
4.13
2.
m,p-CH3O/–0.153
4.13
3.
m-CH3O/0.115
2.82
4.
m-NO2/0.710
4.36
5.
p-F/0.062
2.08
6.
p-CH3/–0.170
3.06
7.
p-Cl/0.227
2.08
8.
p-CN/0.660
2.08
9.
Naphthyl/–0.01
1.49
10.
Pyrene/–0.01
0.56
11.
Biphenyl/–0.01
2.22
12.
Stilbene
1.80
13.
Cyanostilbene
1.30
14.
p-Br/0.232
1.90
15.
p-Cl/0.227
2.09
16.
p-NHCOXCH3/0.00
2.14
17.
p-NO2/0.778
1.39
18.
m-NO2/0.710
1.50
19.
m-Cl/0.373
1.80
20.
m-CH3O/0.115
1.56
21.
m-Br/0.391
1.56
22.
p-CH3O/–0.268
2.12
23.
p-CH3/–0170
2.60
24.
Pyridine
2.01
25.
Naphthyl
3.05
26.
m,p-CH3O/–0.153
3.13
27.
m-OC2H5,p-CH3O/–0.258
3.08
215
Ozonation of Organic and Polymer Compounds
1
2
5.6
= - 0.266 6
lgk
3 5.4
13 5
5.2 -0.8
-0.6
-0.4
-0.2
0.0
8
7
0.2
0.4
0.6
0.8
5.6
27
26
25 = - 0.286
23 5.4 16
lgk
22
15
24 14
19 21
20
5.2
18 17
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Figure 3.7 Dependence of the rate constants on the Hammett inductive constants
3.2 Polydienes The interest in the ozone reaction with polydienes is closely related to the problem of ozone degradation of rubber products. The reaction kinetics is found to be strongly dependent on the polymeric state, a question which has attracted a special interest in recent years [199]. Flory [200] showed that the reactivity of the functional groups in the polymer molecule does not depend on its length. It is also known that some reactions of the polymers proceed much more slowly compared with their low molecular analogues (catalytic hydrogenation). At the same time many fermentative reactions with polymers take place rapidly while similar reactions with low molecular compounds do not occur
216
Ozonolysis of alkenes in liquid phase at all [201]. The folded or unfolded form of the macromolecules provides various conditions for contact of the reagents with the reacting groups [202]. By using the modified version of this principle [203] it was possible to explain the proceeding of reactions without specific interactions between the adjacent C=C bonds and the absence of diffusion limitations. The study of the mass-molecular distribution (MMD) is in fact a very sensitive method for establishing the correlation between molecular weight and reactivity. The theory predicts that the properties of the system, polymersolvent can be described by the parameter of globe swelling (G) which defines the free energy (F) of the system and thus the rate constant of the reaction. For a reversible reaction, i.e., polymerisation-depolymerisation, the dependence of the rate constant of the chain length increase on the molecular weight is expressed by Equation 3.15: ln kpj/kpd= –const.(5G - 3/G).(dG/dM).M0
(3.15)
where M0 is the molecular weight of the studied sample and kpd is the rate constant for infinitely long macromolecules. A good correlation between the theoretical and experimental data for polystyrene solutions in benzene was found in reference [204]. The study of polymer degradation is complicated by the structural peculiarities at the molecular and supramolecular level and by diffusion reasons. It is difficult to find simple model reactions for clarification of the particular properties and for the express examination of the proposed assumptions. An exception in this respect is the ozone reaction with C=C bonds whose mechanism has been intensively studied and could be successfully applied upon ozonolysis of polymeric materials [178]. This question will be discussed in detail at the end of this Chapter and in Chapter 4. Table 3.14 summarises the rate constants of the ozone reaction with some conventional elastomers and polymers and their low molecular analogues, synthesised by us. It is seen that the reactivity of elastomers and polymers and their corresponding low molecular analogues, as demonstrated by their rate constants, are quite similar, thus suggesting similar mechanisms of their reaction with ozone. This statement is also confirmed by: (1) the dependence of k on the inductive properties of substituents such as k of polychloroprene is higher than that of vinylchloride due to the presence of two donor substituents, and (2) the dependence of k on the configuration of the C=C bond in trans-isomer (gutta percha) and cis-isomer (natural rubber). It has been found that the effects related to the change of the macromolecule length or the folding degree do not affect the ozonolysis in solution. Probably this is due to the fact that the reaction is carried out with thermodynamic elastomer solutions in which the macromolecules can perform free intramolecular movements and do
217
Ozonation of Organic and Polymer Compounds not react with adjacent macromolecules. Moreover, the rate of macromolecule reorganisation is probably higher than the rate of their reaction with ozone as the experiment does not provide evidence for the effects of the change of the parameters mentioned above [205-207].
Table 3.14 Rate constants of ozone with polymers and low molecular analogues in CCl4, 20 oC Compound Polyvinylchloride Vinylchloride 2-Bromopropene Polybutadiene Cyclododecatriene-1,5,9 Polybutadiene-styrene
Mw
k × 10-4 (M-1.s-1)
8 × 105
0.42 p 0.1
62.45
0.18
121
0.28 p 0.05 5
3.3 × 10
6.0 p 1
162
35 p 10
8 × 104
6p1
3 × 10
4
27 p 5
Natural rubber
1 × 10
6
44 p 10
2-Me-pentene-2
85
Squalene
410
Gutta percha
Polystyrene Cumene Polyisobutylene Cyclohexane
5 × 10
35 p 10 74 p 15 5
0.3 × 10-4
120
0.6 × 10-4
1.7 × 105
0.02 × 10-4
84
0.01 × 10-4
However, it should be noted that k of the elastomers are about 2-6 times lower that those of the low molecular analogues. The accuracy of Ea determination does not allow estimation as to which of the two parameters A or Ea one should relate the decrease in k. If we assume that the mechanism of ozone reaction with monomers and elastomers is similar, i.e., the reactions are isokinetic, then Amon = Apol. At kmon/kpol = 2 - 6 the difference in Ea at 20 oC will be 0.5-1.0 kcal/mol. At the low experimental
218
Ozonolysis of alkenes in liquid phase values of Ea, these differences will become commensurable with them and thus the determination of Ea is not accurate enough. In this case two assumptions could be made which can give a reasonable explanation for the lower values of kpol: (1) reorientation of the macromolecules is a slower process that that of olefins which would results in Apol lower than Amon, and (2) the addition of ozone to C=C bonds is accompanied by the rehybridisation of the C atoms from sp2 to sp3 and the movement of the polymer substituents during AC formation will be more restricted than those in olefins mainly because of their greater molecular mass and sizes. This will ultimately result in a decrease of the rate constant. Table 3.14 shows some examples of ozonolysis of saturated polymers - polystyrene and polyisobutylene. These reactions, as mentioned earlier in Chapter 1, take place not via the mechanism of ozone reaction with the double bonds but through a hidden radical mechanism with rate constants of 4-5 orders lower.
3.2.1 Polybutadiene Some industrial samples of rubbers have been the object of investigation (Table 3.15). The polymers were purified by triple extraction with methanol in a Soxhlet extractor. The experiments were conducted in a bubbling reactor containing a polymer solution (0.06-1%) in tetrachloromethane. The progress of the ozonation was followed by viscosimetry, gel chromatography, IR and nuclear magnetic resonance (NMR) spectrometry, iodometry and through triphenylphosphine reduction. Because of the high viscosity and large rate constants the reaction takes place in the diffusion and mixed region. In order to obtain correct kinetic data we have used the theory of boundary surface (Equation 3.16) [34]: [O3] = A[O3]0.exp[– D(k.c.D)1/2]
(3.16)
where [O3] is the ozone concentration at the boundary surface; A is Henry’s coefficient; [O3]0 is the ozone concentration in the gas phase at the reactor inlet; D is the boundary surface depth; k is the rate constant of the ozone reaction with diene; c is the concentration of the monomeric units; D is the diffusion coefficient of ozone in the liquid phase.
219
220 94-98
5
94-97
87-93
1,4-cis (%)
94
2-4
3-8
1,4-trans (%)
-
-
3-5
1,2(%)
-
1-2
-
3,4(%)
180
380
454
Mv × 10-3
1.8
2.0
2.1
n
Note: Mv is the average molecular weight, determined viscosimetrically after equation [H] = k × MvAa, where [H] = (H1/C)(1 + 0.333H1), H1 = Hrel–1; Hrel is the intrinsic viscosity; C is the solution concentration; k = 1.4 × 10-4 is the Staudinger constant and Aa = 0.5-1.5 is the constant depending on the rubber type, being one for natural rubber; Mv z Mw; n = Mw/Mn, where Mw and Mn are the average weight and number average molecular mass, respectively.
-C(Cl)=C-
-C(CH3)=CH-
SKI-3S
Denka M40
95-98
-CH=CH-
SKD 94-98
Unsaturation degree (%)
Monomeric unit
Elastomer
Table 3.15 Some characteristics of polydiene samples
Ozonation of Organic and Polymer Compounds
Ozonolysis of alkenes in liquid phase It was found that the relative viscosity decreases exponentially upon ozonation of SKD and SKI-3S solutions (Figure 3.8).
4.0
3.5
3.0
2.5
3
2
1
2.0
0
5
10
15
20
25
Ozonation time, min
Figure 3.8 Dependence of the relative viscosity (Hrel) of SKD (Russian butadiene rubber) solutions (0.6 g in 100 ml CCl4) on reaction time at ozone concentrations of: 1: 1 × 10-5 M; 2: 4.5 × 10-5 M; 3: 8.25 × 10-5 M
The dotted lines denote the theoretical exponential functions. As the viscosity is proportional to the molecular weight it follows that the polydiene consumption is described by first or pseudo first-order kinetics. The value of F, standing for the number of degraded polymeric molecules per one absorbed ozone molecule can be used to calculate the degradation intensity. The value of this parameter (F) may be estimated using Equation 3.17:
F = 0.5 [(Mvt)-1 – (Mv0)-1].P/G
(3.17)
where Mvt is the molecular weight at time t; Mvo is the initial molecular weight; P is the amount of polymer; G is amount of consumed ozone.
221
Ozonation of Organic and Polymer Compounds The dependence between F and G is a straight line for a given reactor and depends on the hydrodynamic conditions in the reactor.
1.6
1
1.4
2
.102
1.2
3
1.0 0.8
0.6 0.4 0.0
0.5
1.0
1.5
2.0
2.5
G.105, mols
Figure 3.9 Dependence of F on G for SKD (0.6/100) at various ozone concentrations: 1: 1 × 10-5 M; 2: 4.5 × 10-5 M; 3: 8.25 × 10-5 M
It is seen from Figure 3.9 that the F values increase linearly with the reaction time and decrease with increase in ozone concentration. The corresponding F values for SKI-3S (Russian synthetic cis-polyisoprene rubber) and Denka M40 ozonolysis are similar. The F values for Gm0 were used to avoid the effect of hydrodynamic factors on them. The values of F found for SKD, SKI-3S and Denka M40 at [O3] = 1 × 10-5 M amount to 0.7 × 10-2, 0.78 × 10-2 and 0.14, respectively, and the slopes are: –40, –70 and 200 M-1, respectively. Substituting with the known values for the parameters in Equation 3.16 we have obtained D within the range of 1 × 10-3 to 2 × 10-4 cm, which indicates that the reaction takes place in the volume around the bubbles, and hence in the diffusion region. The ozonolysis of polydienes in solutions is described by the Criegee mechanism. The C=C bonds in the macromolecules are isolated as they are separated by three simple C-C bonds. According to the classical concepts, the C=C bond configuration and the electronic properties of the groups bound to them also affect the polymer reactivity, similarly with the low molecular olefins. The only difference is that the polymer substituents at the C=C bonds are less mobile which influences the sp2-sp3 transition
222
Ozonolysis of alkenes in liquid phase and the ozonide formation. In the first stage, when PO are formed, the lower mobility of the polymer substituents requires a higher transition energy, the rate being lower compared with that with low molecular olefins and the arising strain accelerates the PO decomposition to a zwitterion and carbonyl compound. The lower mobility of the polymer parts impedes further ozonide formation and brings about the zwitterion leaving the cage and going into the volume which in its turn accelerates the degradation process. The latter is associated with either its monomolecular decomposition or its interaction with low molecular components in the reaction mixture. The efficiency of degradation is determined by the C=C bond location in the macromolecule, for example, the C=C bond location from the macromolecule centre to its end, is in the range from 2 to 1 (Equation 3.18): M1 = (1/G).M0,
(3.18)
where M2 = M0 – M1; 1≤G≤2 is the coefficient indicating the C=C bond location; M0, M1 and M2 are the molecular weights of the initial macromolecule and the two degraded polymer parts, respectively.
1.6
1.6
1.4
1.4 3
1.2
1.0
F.102
1.0 1
0.8
0.8
2
0.6
0.6
0.4
0.4
0.2
0.2 0
2
F.10
1.2
4
6
8
[O3].105, M
Figure 3.10 Dependence of F on ozone concentration for elastomer solutions: 1: SKD (0.6/100); 2: SKI-3S (0.6/100); 3: Denka M40 (1/100)
At G = 2, i.e., when the broken C=C bond is located in the macromolecule centre, the values of M1 and M2 will be exactly equal to M0/2, at Gm1, i.e., at the terminal C=C bond in the polymer chain, the value of M1 will approximate to M0 and thus
223
Ozonation of Organic and Polymer Compounds the value of M2 will be practically insignificant. For example, M2 may be 50-1,000 which is 3-4 times less than that of the macromolecule and in fact the degradation process will not occur. The viscosimetric determination of the molecular weight which we have applied in our experiments has an accuracy of p 5% and does not allow the differentiation of molecular weights of 22,700, 19,000 and 9,000 for the corresponding rubbers. This suggests that the cleavage of C=C bonds located at a distance of 420, 280 and 100 units from the macromolecule end would not affect the measured molecular weight. As the reaction of elastomer ozonolysis proceeds in the diffusion and diffusion-kinetic region, at low conversions each new gas bubble in the reactor would react with a new volume of the solution. On the other hand, the reaction volume is a sum of the liquid layers surrounding each bubble. It is known that the depth of penetration from the gas into the liquid phase is not proportional to the gas concentration and thus the rise of ozone concentration would increase the reaction volume to a considerably lower extent than the ozone concentration. This leads to the occurrence of the following process: intensive degradation processes take place in the microvolume around the bubble and one macromolecule can be degraded to many fragments while the macromolecules out of this volume, which is much greater, may not be changed at all. Consequently, with an increase in ozone concentration, one may expect a reduction of MMD and an increase of the oligomeric phase content. This will result in an apparent decrease of F at the viscosimetric measurements. The previous discussion allows the correct interpretation of the data in Figure 3.10. On ozonation of the SKD solution (1/100) at 10% conversion an intensive band of C-O in ozonides [207, 208] was observed at 1115 cm-1 in the IR spectrum. A less intensive band at 1735 cm-1, characteristic of C=C vibrations in aldehydes and esters (Figure 3.11, 2), was also registered. As the ozonides did not react quantitatively with HI their reaction with triphenylphosphine (Ph3P) was used for the determination [209]: Ozonide + Ph3P = Ph3PO + 2Aldehyde The amount of ozonide as estimated from the band intensity at 1735 cm-1 (Figure 3.11, 3) was 70 p 5% with respect to the amount of the absorbed ozone. It has been found that the amount of C=O groups as estimated from the band intensity at 1735 cm-1 that appears prior the Ph3P treatment, is 11% of the absorbed ozone (the extinction coefficient is taken from the calibration to >C=O in butyric aldehyde). These functional groups are formed as a result of the proceeding of reactions parallel with those of ozonide formation. Control experiments have demonstrated the relatively high thermal stability of SKD ozonide [210, 211]. Usually it is assumed that the zwitterion isomerisation into acid is responsible for the degradation of macromolecules. However,
224
Ozonolysis of alkenes in liquid phase absorption in the 1710-1716 cm-1 band was not detected, although the extinction coefficient for the >C=O bonds in acids is two to three times higher than that of >C=O in aldehydes and ketones [208, 209]. The acid groups may react with the zwitterions, so that their concentration in solution remains below the threshold sensitivity of the spectroscopic method. This assumption is supported by the fact that butyric acid added to the SKD solution is consumed during the reaction (Figure 3.12).
Figure 3.11 Infrared spectra of SKD solutions (1/100): 1 - non-ozonised; 2 - ozonised to 10% conversion; 3 - solution after Ph3P treatment
The higher band intensity at 1735 cm-1, obtained on ozonolysis in the presence of acid, compared with that of the same band in its absence, reveals overlapping of the >C=O absorption in carbonyl and ester groups [209]. The interaction between the zwitterion and carboxylic group is also confirmed by the appearance of a band at 3480 cm-1 at 50% conversion (Figure 3.12). The latter may be attributed to vibrations of the hydroperoxide OH groups. Thus, it can be concluded that the decay of the excited acid molecule resulting from the zwitterion isomerisation to free radicals [210] is not the only route for their consumption. Degradation may also result from the interactions between zwitterions, leading to di- and polyperoxides which are very unstable and readily decompose into polymer carbonyl compounds with lower molecular weight
225
Ozonation of Organic and Polymer Compounds and an oxygen molecule. It has been found that the most efficient way for reduction of the molecular weight is provided by the interaction of zwitterions formed in the chain centre with low molecular compounds and oligomers.
Figure 3.12 Infrared spectra of SKD solutions (1/100): 1 - nonozonised + butyric acid; 2 - ozonised to 20% conversion; 3 - ozonised to 50% conversion without butyric acid The thermal decomposition data differential scanning calorimetry ((DSC) spectra) of ozonised rubber (SKD) in an inert atmosphere (argon) in the temperature range of 50-180 oC were processed using the equation suggested by Gorbachev and co-workers (Equation 3.19) [211-213], Equations 2.2-24 from Chapter 2 and the expression of Julai and Grinhau (Equation 3.20) [213]: ln h/h1/[ln(1–A)/(1–As)] = {(Ea/RTs) . [(T–Ts)/T] . 1/[ln(1–A)/(1–As)]} + n
(3.19)
where A is the conversion degree; h is the peak height; T is the temperature; s is an index standing for the values of the DSC peak maximum; n is the reaction order:
As = 1 - 1.062n1/(1-n)
where As accounts for the transformation degree in the DSC maximum.
226
(3.20)
Ozonolysis of alkenes in liquid phase The results from the study on the thermal degradation of SKD and Diene 35 NFA rubbers are summarised in Table 3.16.
Table 3.16 DSC analysis of products obtained from ozonolysis of polybutadiene rubbers SKD and Diene 35 NFA Heating rates (oC/min)
1
2
5
274/216
338/260
355/276
-/-
370/-
370/-
T (oC)
129/125
136/133
147/144
As
0.65/0.70
0.67/0.70
0.68/0.75
E (kJ/mol)
99.9/91.2
105.5/93.7
121/109.6
n
0.7/0.73
0.73/0.73
0.79/0.73
$H1 (kJ/mol ozonide) $H2 (kJ/mol O3)
3.2.2 Cis-1,4-polyisoprene The positive inductive effect of the methyl group in polyisoprene enhances the rate of ozone addition to the double bonds from 6 × 104 for SKD to 4 × 105 M-1.s-1 for SKI-3S. The asymmetry double bond allows the formation of two zwitterions: one as in butadiene (A) and the corresponding ketone and the other, in which one of the hydrogen atoms is substituted by a methyl group (B) and the corresponding aldehyde. Since the stabilisation of the zwitterion depends on the electron donor properties of the substituents, the ratio of zwitterions A/B< 1 and is equal to –0.56. The interaction between zwitterions and aldehydes leads predominantly to ozonide formation while that with ketone is more difficult although there are conflicting opinions in the literature on the latter [214-219]. The band at 1725 cm-1 observed in the spectrum of the ozonised SKI-3S solutions (Figure 3.12) may be attributed to the absorption of the keto group in the ketone formed together with the zwitterion (A) [220]. By means of a calibration using methylethyl ketone, it was calculated that the amount of ketone is approximately 40% of that of the absorbed ozone. Consequently, the higher degree of SKI-3S degradation compared with that of polybutadiene is due to impeded zwitterion A interaction with the corresponding ketone resulting in its insignificant conversion according to reactions leading to a decrease of the molecular weight. The ozonide formation has been demonstrated by the occurrence of IR bands at 1110 an 1070 cm-1 (Figure 3.13, 2).
227
Ozonation of Organic and Polymer Compounds
Figure 3.13 IR spectra of SKI-3S solutions: 1 - nonozonised; 2 - ozonised to 50% conversion
3.2.3 Polychloroprene The electron-accepting properties of the chlorine atom at the polychloroprene double bond reduces the reactivity of Denka M40 as demonstrated by its relatively low rate constant, i.e., k = 4 × 103 M-1.s-1. In this reaction the ratio between the zwitterion A and B, according to theoretical calculations, is in favour of A, the ratio being A/B = 4.55. The A formation is accompanied by choroanhydryde group formation and that of B with aldehyde. In both cases the ozonide formation is insignificant and the zwitterions react predominantly in the solution volume resulting in enhancement of the degradation process. The intensive band detected at 1795 cm-1 in the IR spectrum of ozonised Denka M40 solutions (Figure 3.14) is characteristic of a chloroanhydride group [220]. This fact correlated well with the conclusion about the direction of the primary ozonide decomposition. It also reveals insignificant conversion of chloroanhydride to monomer and polymer ozonides. It is difficult to ascribe the absorption maximum at 1735 cm-1 to a definite functional group because besides aldehydes and esters, chloroanhydrides also absorb in this region. However, the band at 955 cm-1 is typical of chloroanhydrides. Two other bands - at 1044 and 905 cm-1 - may be attributed to the C-O vibrations. The valent vibrations characteristic of HO-groups are observed in the 3050-3500 cm-1 band. The iodometrical analysis of active oxygen in the ozonised Denka M40 solutions shows that the amount of ozonide is approximately. 43%. It is of interest to note
228
Ozonolysis of alkenes in liquid phase that the HI reaction with ozonised polyisoprene solutions occurs quantitatively for 3-4 hours, while in SKD the same reaction proceeds only to 20% after 24 hours. The above data, however, provide insufficient information for the preferable route of the ozonide formation (via dimerisation, polymerisation of zwitterions or secondary processes).
Figure 3.14 IR spectra of Denka M40 solutions: 1 - nonozonised; 2 - ozonised to 40% conversion
The DSC analysis of the products of Denka M40 ozonolysis together with those of SKD and cumene peroxide as reference, are presented in Table 3.17. The data in Table 3.17 reveal that the chloroprene rubber ozonolysis yields polyperoxide, as the enthalpy of its decomposition is found to be very close to that of dicumylperoxide. The higher value of Ea (approximately two times) testifies to the possible formation of polymer peroxides. Moreover, the SKD ozonolysis leads predominantly to the formation of ozonide (89% yield), its enthalpy being significantly higher than that of Denka M40.
229
Ozonation of Organic and Polymer Compounds
Table 3.17 DSC analysis of Denka M40 ozonolysis Parameter
Denka M40
SKD
Cumene peroxide
Conversion (%)
28
18
-
Weight loss (%)
7-13
6-11
-
$H1 (kJ/mol O3)
239
337
-
$H2 (kJ/mol)
-
-
215
Ea (kJ/mol)
70
128
140
Ti (oC)
51
86
-
Tm ( C)
96
147
-
o
132
175
-
o
Te ( C)
Note: $H1 is the enthalpy related to 1 mol ozone; $H2 is the enthalpy of cumene peroxide, Ea is the decomposition activation energy; Ti, Tm and Te are the temperatures at the beginning, maximum and end of the DSC curves.
3.2.4 Butadienenitrile rubbers The most important characteristics of the nitrile rubbers under study are shown in Table 3.18.
Table 3.18 Characteristics of the butadiene-nitrile rubbers investigated Elastomer
Acrylonitrile (%)
1,4-trans (%)
1,4-cis (%)
1,2(%)
Mv × 10-3
Mw/ Mn
SKN-18
20
64
21
15
380
3.0
SKN-28
28
65
24
11
300
3.0
SKN-40
39
72
22
6
230
2.7
Mv = The molecular mass determined by a viscosimetric method
The results from the NMR study of the nitrile rubbers structure are listed in Table 3.19.
230
Ozonolysis of alkenes in liquid phase
Table 3.19 Distribution of the triad sequence of 1,4-butadiene (B) and acrylonitrile (A) using 1H-NMR data Triads
SKN-18
SKN-26
SKN-40
BBB
73
56
33
BBA
9
14
18
ABA
18
30
49
It was shown that the viscosimetric measurements provide lower values for of the ozone degradation level compared with those obtained by the IR data. This observation may be attributed to the fact that the reaction between ozone and the rubber is localised in a very thin layer around the gas bubbles, while the major part of the macromolecules do not undergo degradation in the solution volume. The intensive progress of the reaction in a very small volume provides conditions for the formation of significant amounts of low molecular products without significant change in the molecular weight. Nevertheless, the viscosimetric determinations of a series of samples confirm some tendencies of the degradation process. The results from the viscosimetric studies of three nitrile rubbers are shown in Table 3.20. From Table 3.20 one can see that F depends on three factors: (1) the ozone concentration, (2) the nature of the solvent, and (3) the rubber structure. Under the experimental conditions of the bubbling method, the influence of ozone concentration is associated with the insignificant increase of the reaction volume and thus to the number of interactions of one macromolecule. The role of the solvent is associated with: UÊ *ÀÛ`}Ê V`ÌÃÊ vÀÊ
}
iÀÊ iµÕLÀÕÊ âiÊ VViÌÀ>ÌÃ]Ê `ÕiÊ ÌÊ Ì
iÊ different values of Henry’s coefficients for different solvents (CCl4 = 2 and CHCl3 = 2.8), UÊ Ài>ÌÊvÊiÜÊVvÀ>ÌÊV`ÌÃÊvÀÊÌ
iÊ>VÀiVÕiÃÆÊivviVÌÊÊÌ
iÊ electronic properties of the reaction centre and the free access of ozone to it and thus on the degradation degree, and UÊ /
iÊ«>ÀÌV«>ÌÊvÊÌÀiÊÀÕLLiÀÊÊÌ
iÊâÞÃÃÊ«ÀViÃÃÊÜ
V
Ê
>ÃÊÌÊLiiÊ experimentally registered).
231
Ozonation of Organic and Polymer Compounds
Table 3.20 Number of chain scissions per molecule of reacted ozone (φ) for ozone degradation of nitrile rubbers at various ozone concentrations in CCl4/CHCl3 solvents Elastomer
Concentration (g/100 ml)
F × 102, 0.0025 mM [O3]0
F × 102, 0.005 mM [O3]0
F × 102, 0.01 mM [O3]0
SKN-18
0.45
2.5/4.8
0.9/2.1
0.6/1.6
SKN-28
0.49
-/7.0
-/5.4
-/1.7
SKN-40
0.40
-/7.5
/6.1
-/2.5
It is seen that the increase in the acrylonitrile content in the elastomers studied reduces the probability of the formation of low molecular fragments and consequently the degradation degree should increase. Indeed, such a dependence on has been experimentally observed (columns 3-5). It should be noted that during the ozonolysis of SKN-18 (for CHCl3 solutions up to 25%) gel formation is hardly observed to 30% conversion. Thus, it can be assumed that under the experimental conditions used the contribution of crosslinking reactions (interaction between the zwitterions and aldehydes with high molecular fragments or other structural reactions between macromolecules resulting in a rise of the molecular weight) to the overall balance of reacted ozone is very small. It was shown that the degradation of macromolecules is a result of the zwitterions reaction with low molecular compounds in the bulk solution, monomolecular isomerisation of the former to acids and di- and polyperoxides decomposition with oxygen evolution [221-223]. Several significant features were found in the 1H-NMR spectra of the ozonised CCl4 solutions of SKN-18: a signal due to the aldehyde proton at 9.75 ppm; a few signals in the 4.97-5.20 ppm region [223-225] ascribed to methine protons and a multiplet at 2.75 ppm with an integral intensity of about 40% of that of the aldehyde peak. The ratio of the aldehyde to the ozonide groups calculated from the integral intensities is about 40:60. As SKN-26 and SKN-40 are not dissolved in CCl4, the ozonolysis and the 1H-NMR spectra were recorded in CHCl3 and CDCl 3 solutions. The insoluble gel after ozonation was separated by filtration and CDCl3 was added. The following peaks were detected in the 1H-NMR spectra: 9.76 ppm (s); 4.97-5.20 ppm (nd); 2.73 ppm (m) and a series of very weak peaks in the 3.12-4.20 ppm range.
232
Ozonolysis of alkenes in liquid phase
Figure 3.15 1H-NMR spectra of SKN solutions (0.75/100); 1 - nonozonised; 2 ozonised to 20% conversion Simultaneously the IR spectra (Figure 3.15) of nitrile rubbers before and after ozonation were registered.
Figure 3.16 IR spectra of SKN-18 solutions (0.75/100): 1 - nonozonised solution; 2 - ozonised to 20% conversion
233
Ozonation of Organic and Polymer Compounds The IR spectra of SKN-26 and SKN-40 are very similar to that of SKN-18 shown in Figure 3.16. The kinetics of ozonide and aldehyde group formation for the three rubbers was studied by monitoring the changes in the optical density at 1110 and 1731 cm-1, respectively. The effect of the reacted ozone and the type of solvent, i.e., CCl4 or CHCl3 was investigated. It may be seen that in all cases the kinetic curves are linear up to 20% conversion. The slopes of the dependence for the solution of SKN-18 depend on the solvent nature and it amounts to 2.4 × 10-2 M aldehyde/2 × 10-4 mol absorbed ozone in CHCl3 and is 1.2 times higher than that in CCl4. The registered optical density of the ozonides does not depend on the solvent but the slope of the curve is found to be 2 times lower than that of the aldehydes in CHCl3. The slopes of the kinetic curve for aldehyde formation for the solutions of SKN-26 and SKN-40 (CHCl3) are identical. However the accumulation of aldehydes takes place at a 2.25 times higher rate. The corresponding slopes are about 8% greater than those of SKN-18. Using literature values for the extinction coefficients for ozonides (ε = 115 [223]) and aldehydes (ε = 230 [223, 226]) it was possible to evaluate the ratio aldehyde/ozonide for the three rubbers studied (SKN-18; SKN-26 and SKN-40) in CHCl3 which are 1.03, 1.12 and 1.12, respectively, at ambient temperature and 20% conversion. For SKN-18 in CCl4 this ratio amounts to 0.85. The difference in the values of this ratio obtained in the two solvents is probably due to the stabilisation of the zwitterion in the more polar solvent, i.e., chloroform, and the differences between the various rubbers is associated with the various amounts and size of the polymer fragments containing 1,2-double bonds. The role of nitrile groups is demonstrated by the electronic effects of the strong electron-accepting CN group and its inductive effect transferred via two S-bonds. The quantitative interpretation of the NMR spectra of SKN-18 ozonised in CHCl3 gives the value of 0.82 for the aldehyde/ozonide ratio which is within the accuracy limits of the extinction coefficient determination, and correlates with the IR data. It has been found that from 1 mol of absorbed ozone, 0.52 moles of aldehyde and 0.72 moles of ozonide are formed in SKN-18 ozonolysis in CCl4. Thus the ratio amounts to 0.72. The results from the two spectral studies support our suggestions for the important role of zwitterion reactions in the ozone degradation of nitrile rubbers. Their reactions out of the cage lead to the formation of di- and polyperoxides which are converted to aldehydes with evolution of oxygen. The effect of temperature on aldehyde/ozonide ratio is illustrated by the kinetic parameters given in Table 3.21. In the ozonolysis of nitrile rubbers the amount of aldehyde groups formed varies within the range of 50-70%, while on ozonolysis of 1,4-cis-polyisoprene it is approximately 11%. The significant difference could be explained by the presence of nitrile groups
234
Ozonolysis of alkenes in liquid phase in the polymer molecules and the difference in the configuration of the C=C bonds: 64-72% trans- for the nitrile rubbers and 94-97% cis- for the isoprene rubber. Data in the literature show that the ozonide yield from trans-olefins is considerably lower than that from cis-olefins [223, 224].
Table 3.21 Activation energy for the formation of aldehydes and ozonides in SKN-18 ozonolysis Solvent
Ea (kJ/mol, ozonides)
Ea (kJ/mol, aldehydes)
CCl4
–0.8
4
CHCl3
–0.4
0.8
3.2.5 Ethylenepropylene rubbers Ethylenepropylene rubbers (EPDM) are copolymers of 65-80% ethylene with 3520% propylene and a third copolymer - diene 1-2% (for example, cyclopentadiene, 1,4-hexadiene, 2-ethylene-norbornene-5). They are characterised by high resistance towards the action of temperature, oxygen and ozone, mainly because of the extremely low content of C=C bonds. The vulcanising properties are due to the presence of the diene copolymer. In most cases the EPDM rubbers are used alone, and only in particular cases, for the preparation of efficient antiozonant and antioxidant materials, are they used in combination with diene elastomers due to their low compatibility with the elastomers. Their application as antiozonant additives will be discussed further in detail. The investigations were carried out on Keltan 778 samples (ethylene:propylene: ethylenenorbornene = 75.5:23:1.5 with Mv = 193,000 and Mv/Mn = 1.84) in CCl4 (0.6/100): H3C
CH
(CH2CH2)n(CH2CH)m CH3 1
CH
(CH2CH2)n(CH2CH)m CH3
CH3 2
The diene copolymer undergoes polymerisation both via the vinyl and norbornene double bond (1 and 2) in 1:4.66 ratio. These structures have been identified by means
235
Ozonation of Organic and Polymer Compounds of 1H-NMR - 250 MHz. The spectra analysis suggests the existence of two preferable conformations for the methyl group, during the polymerisation via the norbornene C=C bond. The first is when the methyl group is oriented towards the polymer chain and the second when it is oriented in the opposite direction. Upon ozonolysis of EPDM the reaction centres are the C=C bonds of the diene copolymer although their concentration is almost 70 times lower compared with that of the C-H bonds, which is due to the substantially higher rate constants (up to 4-6 orders) [227]. Because of the relatively high viscosity of the solutions the rate constants of ozonolysis could not be determined accurately by using Equation 1.4. The value of the activation energy estimated from the dependence of the viscosity change of the ozonised EPDM-rubber solutions on the temperature is found to be Ea = 33 kJ/mol. Ozone attacks these double bonds resulting in the formation of two zwitterions in the volume. The interaction between the latter leads rather to structurisation of the macromolecules and growth of the polymer molecular mass than to its reduction. At low temperatures this process is more efficient and thus the activation energy has a negative value. The presence of peroxide bridges is easily established. The treatment of the ozonised solution with PPh3 gives PPh3O and the molecular weight is restored. The thermal decomposition of ozonised Keltan 778 solutions in an inert atmosphere makes possible the estimation of the Arrhenius parameters for the monomolecular decomposition of the crosslinked molecules. The activation energy was estimated to be 19 kcal/mol. That is in a very good agreement with other data on the decomposition of compounds containing peroxide bridges [228, 229]. In order to determine the type of the bridges the 1H-NMR spectra of the initial rubber have been recorded. It reveals four doublets assigned to the four methine protons: (1) cis-vinylene - 5.08 ppm, (2) trans-vinylene - 5.30 ppm, (3) methine proton in norbornene, located in the direction of the polymer matrix - 5.58 ppm, and (4) methine proton orientated in the opposite direction to the polymer chain - 5.70 ppm. The quantitative analysis shows that trans-vinyl radicals (2) constitute the major portion of the free radicals while the norbornene double bond is found to be the most reactive in the polymerisation process due mainly to the highly strained hydrocarbon structure. The latter could not form ozonide at the rupture of the double bond but easily leads to polyperoxide formation.
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240
Ozonolysis of alkenes in liquid phase 66. R. Criegee and G. Wanner, Annalen, 1949, 564, 9. 67. R. Criegee, Annalen, 1953, 583, 1. 68. R. Criegee, Record of Chemical Progress, 1957, 18, 111. 69. R. Criegee, Angewandte Chemie, 1957, 21, 756. 70. R. Criegee, Angewandte Chemie, 1975, 21, 765. 71. R. Criegee in Peroxide Reaction Mechanisms, Ed., J.O. Edwards, Interscience Publishers, New York, NY, USA, 1960, p.29. 72. D.G.M. Diaper, Journal of Chemical Education, 1967, 44, 354. 73. A. Rieche, Magyar Tudomanyos Akademia Kem Tudomanyok Oszlalynak Kîzlamenyei, 1959, 12, 143. 74. A. Bischoff and A. Rieche, Zeitschrift fur Chemie, 1965, 5, 97. 75. R. Criegee, Chemishe Unserer Zeit, 1973, 7, 75. 76. R. Criegee, Chimia, 1968, 22, 392. 77. F. Gunstone, Education in Chemistry, 1968, 5, 166. 78. R. Murray, Accounts of Chemical Research, 1968, 1, 313. 79. J. Pospisil, Chemie (Prague), 1958, 10, 1030. [In Czechoslovakian] 80. E. Pryde and J. Cowan, Topics in Lipid Chemistry, 1971, 2, 1. 81. B. Visintin and S. Monteriolo, Industrie Sanitarie, 1965, 102. 82. R. Murray, Transactions of the New York Academy of Sciences, 1967, 29, 854. 83. R. Murray, Technical Methods in Organic and Organometallic Chemistry, 1969, 1, 1. 84. M. Schulz and K. Kirchike, Advances in Heterocyclic Chemistry, 1967, 8, 165. 85. G. Schroder, Chemische Berichte, 1962, 95, 733. 86. R. Criegee and H. Korber, Chemische Berichte, 1971, 104, 1807. 87. L. Loan, R. Murray and P. Story, Journal of the American Chemical Society, 1965, 87, 737.
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Ozonolysis of alkenes in liquid phase 106. D.P. Higley and R.W. Murray, Journal of the American Chemical Society, 1976, 98, 4526. 107. K.L. Gallaher and R.L. Kuczkowski, Journal of Organic Chemistry, 1976, 41, 892. 108. S. Fliszar and J. Carles, Journal of the American Chemical Society, 1969, 91, 2637. 109. J. Castonguay, M. Bertrand, J. Carles, S. Fliszar and J. Rousseau, Canadian Journal of Chemistry, 1969, 47, 919. 110. C. Gillies and R. Kuczkowski, Journal of the American Chemical Society, 1972, 94, 7609. 111. C. Gillies and R. Kuczkowski, Journal of the American Chemical Society, 1972, 94, 6337. 112. R. Lattimer, R. Kuczkowski and C. Gillies, Journal of the American Chemical Society, 1974, 96, 348. 113. J. Renard and S. Fliszar, Journal of the American Chemical Society, 1970, 92, 2628. 114. S. Fliszar, J. Renard and D. Simon, Journal of the American Chemical Society, 1951, 93, 6953. 115. S. Fliszar, Journal of the American Chemical Society, 1972, 94, 7386. 116. R. Lattimer, R. Kuczkowski and C. Gillies, Journal of the American Chemical Society, 1973, 95, 1348. 117. P. Hiberty, Journal of the American Chemical Society, 1976, 98, 6088. 118. R. Rouse, Journal of the American Chemical Society, 1973, 95, 3460. 119. S. Fliszar and J. Renard, Canadian Journal of Chemistry, 1970, 48, 3002. 120. S. Fliszar and M. Gaunger, Journal of the American Chemical Society, 1970, 92, 3361. 121. S. Fliszar and M. Gaunger, Journal of the American Chemical Society, 1969, 91, 3330. 122. S. Fliszar and J. Renard, Canadian Journal of Chemistry, 1967, 45, 533.
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Ozonation of Organic and Polymer Compounds 123. S. Fliszar and J. Carles, Canadian Journal of Chemistry, 1969, 47, 3921. 124. P. Bailey, Chemica e l’Industria, 1957, 1148. 125. R. Criegee, Chemische Bericht, 1975, 108, 743. 126. R. Criegee and A. Rustaiyan, Chemische Bericht, 1975, 108, 749. 127. R. Criegee, A. Bancin and H. Reul, Chemische Bericht, 1975, 108, 1642. 128. R.W. Murray and R. Hagen, Journal of Organic Chemistry, 1971, 36, 1098. 129. P.S. Bailey, T.M. Ferrell, A. Rustaiyan, S. Seyhan and L.E. Unruh, Journal of the American Chemical Society, 1978, 100, 894. 130. P.S. Bailey and T.M. Ferrell, Journal of the American Chemical Society, 1978, 100, 899. 131. R.A. Rouse, Journal of the American Chemical Society, 1973, 95, 34600. 132. R.A. Rouse, Journal of the American Chemical Society, 1974, 96, 5095. 133. P.C. Hiberty, Journal of the American Chemical Society, 1976, 98, 6088. 134. R.A. Rouse, International Journal of Quantum Chemistry Symposium, 1973, 7, 289. 135. S.W. Benson, F.R. Cruickshauk, D.M. Golden, G.R. Haugen, H.E. O’Neal, A.S. Budgers, R. Shaw and R. Walsh, Chemical Reviews, 1968, 68, 279. 136. W.A. Goddard, III, T.H. Dunning, Jr., W.J. Hunt and P.J. Hay, Account of Chemical Research, 1973, 6, 368. 137. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1975, 97, 6293. 137b. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1975, 97, 6300. 137c. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1976, 98, 6093. 138. F.M. Bodrowicz and W.A. Goddard, III, in Methods of Electronic Structure Theory, Ed., H.F. Schaefer, III, Modern Theoretical Chemistry, Volume 3, Plenum Press, New York, NY, USA, 1977, 3, p.79.
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Ozonolysis of alkenes in liquid phase 139. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1978, 100, 7180. 140. P.S. Bailey, T.M. Ferrell and A. Rustaiyan, Journal of the American Chemical Society, 1976, 98, 638. 141. D. Cremer, Journal of the American Chemical Society, 1981, 103, 13, 3619. 142. M Olzmann, E. Kraka, D. Cremer, R. Gutbrod and S. Andersson, Journal of Physical Chemistry A, 1997, 101, 49. 143. S.D. Razumovskii, Ozone and its Reactions with Organic Compounds, Institute of Chemical Physics – Russian Academy of Sciences, 1973. [DSc Thesis] 144. C.E. Thorpe and A.J. Gaynor, inventors; Cudahy Packing Company, assignee; US2857410, 1958. 145. C. Noller, J. Carson, H. Martin and K.S. Hawkins, Journal of the American Chemical Society, 1963, 36, 1, 24. 146. J. Treacy, M. Curley, J. Wenger and H. Sidebottom, Journal of the Chemical Society - Faraday Transactions, 1997, 93, 16, 2877. 147. R.D. Cadle and C. Schadt, Journal of the American Chemical Society, 1952, 74, 6002. 148. J.J. Buffalini and A.P. Altschuller, Canadian Journal of Chemistry, 1965, 43, 2243. 149. P.L. Hanst, E.R. Stephens, W.E. Scott and R.C. Doerr in Ozone Chemistry and Technology, Eds., J.S. Murphy and J.R. Orr, The Franklin Institute Press, Philadelphia, PA, USA, 1975, p.958. 150. T. Vrbaski and R.J. Cvetanovic, Canadian Journal of Chemistry, 1960, 38, 1053. 151. J. Heickelen, Journal of Physical Chemistry, 1966, 70, 477. 152. J. Herron and R. Huie, Journal of Physical Chemistry, 1974, 78, 2085. 153. H. O’Neil and C. Blumstein, International Journal of Chemical Kinetics, 1973, 5, 397. 154. S.M. Japan, C. Wu and H. Niki, Journal of Physical Chemistry, 1974, 78, 8.
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Ozonation of Organic and Polymer Compounds 155. S. Toby, F.S. Toby and B.A. Kaduk, International Journal of Chemical Kinetics, 1976, 8, 25. 156. B.J. Finlayson, J.N. Piits Jr. and R. Atkinson, Journal of the American Chemical Society, 1974, 96, 5356. 157. L. Hull, I. Hisatsune and J. Heicklen, Canadian Journal of Chemistry, 1973, 51, 1504. 158. S.M. Japan, C. Wu and H. Niki, Journal of Physical Chemistry, 1976, 80, 2057. 159. F.S. Toby and S.Toby, Journal of Physical Chemistry, 1976, 80, 2313. 160. D.A. Hansen and J.N. Pitts, Chemical Physics Letters, 1975, 35, 569. 161. F.S. Toby and S. Toby, International Journal of Chemical Kinetics, 1974, 6, 417. 162. W. Pritzkow and G. Schappe, Journal für Praktische Chemie, 1969, 311, 689. 163. D.G. Williamson and R.J. Cvetanovic, Journal of the American Chemical Society, 1968, 90, 14, 3668. 164. D.G. Williamson and R.J. Cvetanovic, Journal of the American Chemical Society, 1968, 90, 16, 4248. 165. S.D. Razumovskii, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1970, 335. 166. S.W. Benson, The Foundations of Chemical Kinetics, Mir Publishers, Moscow, 1964. 167. E.F. Caldin, Journal of the Chemical Society, 1959, 3345. 168. S.D. Razumovskii and G.E. Zaikov, Zhurnal Organicheskoi Khimii, 1972, 8, 464. 169. S.D. Razumovskii, L.M. Reutowa, G.A. Niazashvili, I.A. Tutorskii and G.E. Zaikov, Doklady Academii Nauk SSSR, 1970, 194, 1127. 170. L.P. Hammett, Physical Organic Chemistry: Reaction Rates, Equilibria and Mechanisms, McGraw-Hill Book Company, New York, NY, USA, 1940, p.188.
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Ozonolysis of alkenes in liquid phase 171. S.D. Razumovsky and Y.N. Yuriev, Zhurnal Organicheskoi Khimii, 1968, 4, 1716. 172. S.D. Razumovsky and Y.N. Yuriev, Tetrahedron Letters, 1967, 8, 40, 3939. 173. R. Huisgen, Journal of Organic Chemistry, 1976, 41, 403. 174. R. Huisgen, Angewandte Chemie, 1963, 75, 604, 742. 175. E. Bernatek, P. Kolsaker and T. Ledaal, Tetrahedron Letters, 1963, 4, 20, 1317. 176. P.D. Bartlett and M.R. Rice, Journal of Organic Chemistry, 1963, 28, 3351. 177. W. Woods, R. Carboni and J. Roberis, Journal of the American Chemical Society, 1956, 78, 5653. 178. V.T. Aleksanyan, H.E. Sterlin and A.A. Mel’nikov, Izvestitya Akademii Nauk SSSR, Seriya Physica, 1958, 22, 1073. 179. E. Wiberg and B. Nist, Journal of the American Chemical Society, 1961, 83, 1226. 180. N. Allinger, Journal of the American Chemical Society, 1958, 80, 1953. 181. H. Krieger, Suomen, Khem, 1962, 38, 136. 182. L.M. Sverdlov and E.P. Krasnov, Optika i Spektroskopiya, 1959, 6, 334. 183. H. Paub, J. Sange and A. Kausmann, Chemische Bericht, 1965, 98, 1789. 184. A.A. Popov, S.D. Razumovskii, V.M. Parfenov and G.E. Zaikov, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1975, 282. 185. N.D. Gil’chenok, S.D. Razumovskii, V.K. Tziskovskii, Catakytic Reaction in Liquid Phase, Nauka Publishers, Alma-Ata, Khazakhstan, 1967, p.189. 186. E. Amis, Influence of the Solvent on the Mechanisms of Chemical Reactions, Mir Publishers, Moscow, 1968. 187. M.G. Buligin and G.E. Zaikov, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1968, 491. 188. S.D. Razumovskii and Yu.N. Yur’rev, Neftekhimiya, 1966, 6, 737.
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Ozonation of Organic and Polymer Compounds 189. D.R. Kerur and D.L.M. Diaper, Canadian Journal of Chemistry, 1973, 51, 3110. 190. D.L.M. Diaper, Oxidation and Combustion Reviews, 1973, 6, 145. 191. P.S. Bailey, C. Abshire and S. Manthia, Journal of the American Chemical Society, 1960, 82, 6136. 192. P.S. Bailey, J.A. Thompson and B.A. Shoulders, Journal of the American Chemical Society, 1966, 88, 4098. 193. E. Briner and D. Frank, Helvetica Chimica Acta, 1938, 21, 1297. 194. E. Dallwigk and E. Briner, Helvetica Chimica Acta, 1958, 41, 1030. 195. E. Briner, S. Fliszar and G.P. Rossetti, Helvetica Chimica Acta, 1964, 47, 2041. 196. R.H. Bauer and M. Senfarth, Berichte, 1930, 63B, 2691. 197. I.S. Plotnikov, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1916, 1563. 198. R. Wilke and P. Chang, American Institute of Chemical Engineering and Science, 1966, 21, 999. 199. T.V. Alfrai, Chemical Reactions of Polymers, Mir Publishers, Moscow, 1967, p.9. 200. P. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, USA, 1953, p.41. 201. A.M. Kotliar and N.J. Moravetz, Journal of the American Chemical Society, 1955, 77, 3692. 202. A.N. Giazer and E.L. Smith, Journal of Biological Chemistry, 1961, 236, 2948. 203. A.A. Berlin, A.A. Sayadyan and N.S. Enikolopyan, Vysokomolekulyarnye Soedineniya Seriya A, 1969, 11, 1893. 204. S.D. Razumovskii, G.A. Niazashvili, Yu.N. Yur’ev and I.A. Tutorskii, Vysokomolekulyarnye Soedineniya Seriya A, 1971, 13, 195. 205. Yu.S. Zuev and V.F. Molofeevska, Degradation and Stabilisation of Rubbers, Gostkhimizdat, Moscow, 1960, p.27. 248
Ozonolysis of alkenes in liquid phase 206. S.D. Razumovskii, O.N. Karpuhin, A.A. Kefely, T.V. Pokholok and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya A, 1971, 13, 782. 207. A.A. Kefely, S.D. Razumovskii and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya A, 1971, 13, 803. 208. K. Nakanisi, Infrared Spectra and the Structure of Organic Compounds, Mir Publishers, Moscow, 1965. 209. CRC Atlas of Spectral Data and Physical Constants for Organic Compounds, Ed., J.G. Grassell, CRC Press Inc., Boca Ratan, FL, USA, 1973. 210. R. Murray and P. Story, Ozonation in Chemical Reactions of Polymers, Mir Publishers, Moscow, 1967. [In Russian] 211. L.W. Crane, P.J. Dynes and D.H. Kaelble, Journal of Polymer Science: Polymer Letters, 1973, 11, 533. 212. H. Kissinger, Analytical Chemistry, 1957, 21, 1702. 213. G. Gyulai and E.J. Greenhow, Thermochimica Acta, 1973, 6, 254. 214. K.W. Ho, Journal of Polymer Science, Part A, 1986, 24, 2467. 215. Yu.S. Zuev, Elastomer Cracking under Peculiar Operating Conditions, Khimia Publishers, Moscow, 1980. [In Russian] 216. G.G. Egorova, V.S. Shagov in Synthesis and Chemical Transformation of Polymers, Leningrad State University, Leningrad, 1986. [In Russian] 217. K.W. Ho and J.E. Gutman, Journal of Polymer Science: Part A Polymer Chemistry, 1989, 27, 2435. 218. B.I. Tikhomirov, O.P. Baraban and A.Y. Yakubchik, Vysokomolekulyarnye Soedineniya Seriya A, 1969, 11, 306. 219. P. Dole Robbe, Bulletin de la Societe Chimique de France, 1980, 3160. 220. K.J. McCullough and M. Nojima in Organic Peroxides, Ed., W. Ando, John Wiley & Sons Ltd, Chichester, UK, 1992. 221. D. Brazier, Rubber Chemistry and Technology, 1980, 53, 437. 222. L.F.R. Cafferata, G.N. Eyler, E.L. Svartman, A.I. Canizo, and E.J. Borkowski, Journal of Organic Chemistry, 1990, 55, 3, 1058.
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Ozonation of Organic and Polymer Compounds 223. C.W. Jefford, A. Jaber, J. Boukouvalas and P. Tissot, Thermo-chimica Acta, 1991, 188, 337. 224. A.C.Baldin in The Chemistry of Functional Groups: Peroxides, Ed., S. Patai, Interscience Publishers, London, UK, 1983. 225. M.J. Hackathorn and M.J. Brock, Rubber Chemistry and Technology, 1972, 45, 1295. 226. M.J. Hackathorn and M.J. Brock, Journal of Polymer Science, Part A-1, 1975, 13, 4, 945. 227. D.W. Brazier and N.V. Schwartz, Thermochimica Acta, 1980, 39, 7. 228. The Sadtler Handbook of Proton NMR Spectra, Sadtler, Philadelphia, PA, USA, 1978. 229. S.M. Ellerstein in Analytical Calorimetry, Eds., R.S. Porter and J.O. Johnson, Plenum Press, New York, NY, USA, 1968.
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4
Degradation and Stabilisation of Rubber
In the light of the contemporary search for highly efficient technologies, the studies in the field of degradation and stabilisation of elastomers, vulcanisates and rubber products receive considerable attention. The demand for producing new articles with new properties, the growing production of elastomers and the expanding of the fields of their application, together with the higher technical requirements and the worsening of the exploitation conditions [1-3] make the research in this field important and necessary. The study of the kinetics and mechanism of polydiene degradation and stabilisation in real conditions makes possible: 1) Establishment of the definite steps responsible for the degradation processes that result in deterioration of the physicochemical and mechanical properties of elastomers and vulcanisates and the products from them. 2) Development and application of efficient methods for stabilisation that prevent or retard the degradation processes. 3) More complete utilisation of raw materials and prolongation of the ‘life’ of rubber goods. 4) Creation of scientific-quantitative bases for ensuring and prediction of the storage and exploitation terms of rubber products [4-6]. As ozone is one of the basic factors resulting in polydiene degradation we focus our attention on investigating its effect on rubber goods both alone and in combination with oxygen, temperature and mechanical stresses. We have studied the degradation processes in pure systems such as nonfilled, radiation vulcanised elastomers, in sulfur vulcanised model mixtures and real mixtures and products. The term degradation stands for the loss of physicochemical, mechanical and exploitation properties of elastomers and rubber products associated with molecular mass reduction under the action of various degradation agents. The contributions by various researchers and companies on this problem so far have served as a base for our studies. In regard to the processes of stabilisation we have directed our studies to the synthesis of antiozonants, elaboration of their protective mechanisms, development of highly
251
Ozonation of Organic and Polymer Compounds efficient antiozonants and antioxidative compositions and highly stabilised elastomers systems. We have also followed the role of the various components of rubber mixtures such as photostabilisers, antifatigue agents, and so on. [7]. Stabilisation is a general term used to describe the process of preventing the various materials from the harmful effect of degradation agents. Aiming at the development of an appropriate scientific basis for prediction of the behaviour of elastomers and rubber products during storage and exploitation we have successfully applied the Arrhenius equation.
4.1 Ozonolysis of Elastomers in Elastic State As we have already shown, the ozonolysis of olefins and elastomers in the liquid phase is an electrophilic process in regard to ozone. The rate of ozonolysis depends slightly on the temperature and decrease if substituents at the double bonds have electronwithdrawing properties. It is also dependent on the olefin configuration, the transisomers being more reactive by about 10-fold than the cis-isomers. The ozonolysis of elastomers in solution is governed by molecule mobility and macromolecule conformation. In general, the values of the rate constant for the elastomers are lower as compared with those of olefins but the yield of the C=C cleavage products is higher for polymers. The main reasons for these observations are: 1) The low mobility of polymer chains hinders the ozone access to the C=C bonds in the macromolecule. 2) The formation of primary ozonides (PO) is also hindered since the sp2-sp3 rehybridisation in the transition state is accompanied by reorganisation of large polymer units. 3) The latter makes the PO decomposition slower than the olefins PO. 4) After the PO decomposition to zwitterion (carbonyl oxide - CO) and carbonyl compound (CC), the low mobility of the polymer fragments prevents the appropriate orientation of CO and carbonyl compound that leads to ozonide formation which although being unstable does not change the molecular weight of the elastomer, and 5) The low mobility of the polymer residual fragments increases the possibility for the CO interaction outside of the kinetic cage which in its turn enhances the degradation efficiency. The physicochemical and mechanical properties of rubbers in solid-elastic state are determined mainly by their chemical structure, macromolecule configuration and
252
Degradation and Stabilisation of Rubber conformation, the segmental mobility, the type and the properties of the supramolecular structures, defects in the mass and on the surface, and so on [8-17]. The segmental mobility, supramolecular structure and the ratio between the amorphous and crystalline phase play an essential role in the degradation process of elastomers in solid state. It is known that the diffusion of various gases including ozone in the amorphous phase is more facilitated than in the crystalline one. The greater access of the C=C bonds to ozone attack in this phase together with the higher ozone solubility provide suitable concentration conditions for acceleration of the process [18]. The degradation reaction that takes place on the boundary layer is facilitated by the concentration of stresses in the linking phase units. There should be mentioned the reactions of monomolecular decomposition of the various intermediate polymer compounds, including polymer zwitterions. The surface defects as well as those in the solution are actually the reaction centres from which the degradation process starts. They are also the entry for the ozone degradation In the course of their exploitation the rubber materials are subjected to considerable mechanical stresses resulting from elongation, load, tearing, shrinking in static and dynamic conditions. These stresses play an important role in accelerating the degradation process acting as mechanical and thermal promoters of polymer molecules decomposition. Usually the mechanical stresses are converted into heat without deteriorating the rubber articles. They are also accumulated and concentrated, to a smaller extent, as stresses in net defects, interphase surfaces, in surface defects, thus changing the supramolecular structure, configuration and conformation of the macromolecule through retardation of the relaxation processes. We have made an attempt to evaluate the contribution of the main factors responsible for the degradation of elastomers and their vulcanisates. In this connection we have compared the rate of oxidative and ozone degradation. It has been established that the first one is proportional to the rate of the chain oxidation process as described by: WO2 (kp.(Wi/kt)1/2.[RH]
(4.1)
where; kp is chain propagation rate constant = 1 s10-1 M-1.s-1; kt is the rate constant of chain termination = 1 s107 M-1.s-1; Wi - initiation rate; [RH] = 14 M - rubber concentration. Substituting the known values for the constants and concentration in Equation 4.1 the following expression for the rate of oxidative degradation is obtained: Wox (0.014.(Wi)1/2
(4.2)
253
Ozonation of Organic and Polymer Compounds The second rate is proportional to the rate of ozone reaction with the elastomer: Woz = koz.[O3].[RH]
(4.3)
where: koz = is the rate constant of ozone interaction with C=C bonds 105M-1.s-1; [O3] the real ozone concentration in the atmosphere = 1 s10-8 M. Again by using the known values we get: WO3 = 0.014
(4.4)
The comparison of Equations 4.2 and 4.4 reveals that the two rates differ by the multiplier (Wi1/2) which is always smaller than unity. This fact can be regarded as the main reason that makes the study and fight against ozone degradation so important. It is true that ozone degradation takes place in the presence of other degradation agents like oxygen, heat, light, mechanical stresses, etc., which complicates further the research in this field. Our experiments show that this problem can be satisfactorily solved as linear combination of its one-dimensional components:
n
J = c1Jc 2Jc x Jx =
3 c xJx
x=1
where: c - constant; and J- parameter related to the degradation process. What is the main result arising from the ozone effect on rubber samples? Rubber samples pre-stretched (E) by more than 10% and then exposed for a definite time in an ozone atmosphere are covered by a network of cracks that grow with time and finally break down [20-23]. The longitudinal axis of the cracks depending on the deformation force (up to elongations b200%) and the rubber type is rectangular (orthogonal) to the extension direction, while at higher deformations the crack orientation begins to change and may reach a parallel orientation. Another interesting phenomenon which has been observed by many authors [25-27] is the occurrence of critical deformation. At E = 10-50% depending on the type of the elastomer a maximum in the degradation rate is observed with a simultaneous increase of the total area and cracks depth (Figures 4.1 and 4.2) [24].
254
Degradation and Stabilisation of Rubber
12
10
1
8
6
3 4
2
2
0 0
20
40
60
80
100
120
, %
Figure 4.1 Dependence of the rate of crack growth (v) on deformation during ozonation: 1 - Natural rubber (NR), [O3] = 1.5 x 10-3 mol/m3; 2 - Butadiene styrene rubber - SKS-30 with dominant trans-configuration of the butadiene units, [O3] = 0.9 x 10-3 mol/m3; 3 - Polychloroprene rubber (PCR) - with predominant trans-configuration, [O3] = 4.5 x 10-3 mol/m3
35
1
30 25 20 15
2
10
3
5 0 0
200
400
600
800
, %
Figure 4.2 Dependence of the time breakdown on the deformation: 1 - NR, 3 SKS-30, [O3] = 4.5 x 10-4 mol/m3; 2 - PCR, [O3] = 2.2 x 10-3 mol/m3
255
Ozonation of Organic and Polymer Compounds From the Figures 4.1 and 4.2 can be seen that the maximum rate of ozone degradation is registered in the range of critical deformations. This effect being more pronounced in the case of NR. Different points of view are put forward by different authors to explain the nature of this phenomenon. For that reason we have made an attempt to summarise and analyse the basic and widespread opinions related to the mechanism of this observation. Newton [28] tried to explain this anomalous phenomenon by the mutual effect of cracks and relates the extreme character to the cracks number and sizes. He assumed that at small deformations a few cracks are formed in energetically rich surface defects, and an increase in the deformation energy begins to concentrate on energetically poorer defects thus transforming them in to reaction cores for crack formation. At an E equal to the critical deformation, the number of the reaction centres reaches its maximum and, the degradation rate reaches its maximum values too. At higher E values, the cracks begin to influence each other, thereby slowing their mutual growth and retarding the degradation. The further increase in E results in accomulation of the deformation energy in the lips of the cracks - they increase their own depth and size leading to a rise of the degradation rate. Bradden and Gent [29, 30] considered that the crack formation and growth is closely related to the appearance of critical deformation whereby a sufficient amount of energy is accumulated for the formation of new surfaces. Solomon and Tellamon [31, 32] put forward the opinion that the ozone cracking is an unique phenomenon that is characteristic only for elastomers with double bonds. Based on the similarity between static wear and corrosion decay, Zuev [33, 34] assumed that elastomer’s degradation is related to the static wear of the vulcanisates, accelerated by the strong corrosive action of ozone. The basic argument in favour of this explanation is the observed similarity in the change of the rubber durability on exposure to ozone with the change of hardness and resistance to sheare at deformation. He considered, however, that the nature of the corrosive agent is of no importance since the specificity of the chemical reaction is of minor importance. He attributed the observed changes only to the extent of change of the polymer orientation and its acceleration or relaxation resulting from the applied deformation. The analysis of the mechanisms discussed previously reveals principal differences between them with respect to the nature of the critical deformation. Some of the mechanisms explain the crack formation only by physical or only by chemical reactions. The simultaneous action of these two factors has not been considered in explaining the existence of critical deformations. In our opinion the mechanism of ozone effect on rubber specimen could be much better understood if the study is concentrated on the initial steps of the reaction prior to the appearance of the first cracks [35-37]. Figures 4.3 and 4.4 show the changes in the the quantity of the C=C bonds, and the accumulation of C=O-groups during
256
Degradation and Stabilisation of Rubber ozonation of the SKI-3 sample, vulcanised by gamma-radiation, depends on the elongation. 1 - D1720/D1640 -C=O-groups; 2 - D1640/D1375 - C=C-groups; ATR - KRS-5 - 45o; [O3] = 7 x 10-5 mol/m3, at room temperature. [ATR - attenuated total reflection; KRS-5 - (TlBr-TlI) is a gorgeous red crystal commonly used for attenuated total reflection prisms for IR spectroscopy.
1.8
>C=O 1.6 1.4 1.2
1 1.0 0.8 0.6
2 0.4
>C=C
4
A4T1
Rectangular, with furrow tight side
>2
A1T3
Oval, with some cross fibers
>3
A3T2
Cyrillic letter (ZH)
>1.7
A2T3
Poor definite rectangular
3
A2T2
SKD, (1,4-cis-polybutadiene Russia) SKI-3/SKD Bulex 1500, (Polybutadiene styrene 70:30 - product of Bulgaria) Bulex M27, (oil-27% filling polybutadiene-styrene rubber - Bulgaria)
Note: * - also for filled sample; ** at oval - the ratio of ellipse axes, at rectangular - the ratio of faces; A - number of cracks: A1 = 1-5, A2 = 5-40, A3 = 40-120, and A4>120; T - depth of cracks; T1 - surface cracks; T2 - cracks b1 mm, T3 - cracks >1 mm, and T4 - cracks through the total depth of the specimen studied.
The process of crack formation was determined by means of reflected-light optical spectroscopy methods using metalographic-MMR-4 LOMO microscope (Russia) with a magnification of x600 [39]. The shape of the cracks depends on the rubbers type. They are shown in Figure 4.6. In this case cracks are formed when the molecular interactions result in visible supramolecular changes. The parameters of cracking are time of first appearance (to), shape and sizes of cracks and time of break down (tc) of the samples depend on: 1) the elongation extent; 2) the extent of static and/or dynamic deformations and their frequencies; 3) rubber type; 4) ozone concentration; 5) temperature; 6) for vulcanisates - ingredients and the preparation conditions [40]. Scheme 4.1 shows the influence of external deformation on ozone interaction with rubbers.
259
Ozonation of Organic and Polymer Compounds
1
2
3
4
Figure 4.6 Shape of the cracks depends on the type of rubber at E = 20%, 20 oC, 10 ppm ozone. 1 - NR; 2 - SKI-3; 3 - SKD; 4 - Bulex 1500
In this case, F the force acts along the longitudinal axis of the macromolecules. The first step which is not shown in the Scheme includes the PO formation which decomposes further to zwitterion and aldehyde. The applied force facilitates the O-O and C-C bond cleavage as part of the stretching mechanical energy is converted into bond energy thus forming zwitterion and aldehyde. The next step, as suggested by the Criegee mechanism, includes the reaction of the zwitterion in or outside the cage. The force application impedes the ozonides formation since it hinders the orientation of the broken ends of the macromolecules. Thus the macromolecule is cleaved at the place of the double bonds and a crack is formed. This Scheme exemplifies the relation between the molecular changes and their influence at the macro level [41-45]. The picture of crack formation depends dramatically on the extent of elongation. When natural rubber samples elongated to different extents, were ozonated, it was found out that the number of cracks linearly increases with increasing elongation up to 82% and the crack depth goes through the maximum at 32%. According to our analysis, the degradation during the time of processing is dominated by one or another reason, but is totally the result of the joint action of different factors and the main one among them is the chemical reaction of ozone with the double bonds of the polymeric chains straining under the stress.
260
Degradation and Stabilisation of Rubber
Scheme 4.1
4.2 Antiozonants The diene elastomers: natural and synthetic butadiene, isoprene, butadiene-styrene rubbers which compose the major part (over 80%) of all rubber products are generally exposed to ozone degradation. Synthetic rubbers such as polychloroprene, butadienenitrile, polyisobutylene, ethylene-propylene, fluoro, silicon, polyurethane, and so on are relatively more stable when exposed to ozone. In our studies we have focussed our attention on the ozone degradation of conventional rubbers. As we have already pointed out, the ozone concentration in the atmosphere varies in the range of 0.2-0.6 ppm. However, it can reach values up to 60-100 ppm in some areas, arising from the photochemical oxidation of organic compounds in the presence of nitrogen oxides in the polluted city atmosphere (smog), photochemical decay of algae and sea organisms on the seashore, the ultraviolet (UV) and microwave radiation, electric charges and other radiation in atmospheric oxygen. As the degradation rate is proportional to the ozone concentration (Equation 4.3) it is obvious that in these areas the degradation will increase almost two-fold.
261
Ozonation of Organic and Polymer Compounds The basic protection against ozone involves the application of small amounts up to 10 phr (parts per 100 rubber) of organic, inorganic, polymeric and other additives, called antiozonants as components of the rubber compositions. The creation and selection of antiozonants is a complex problem and irrespective of the numerous empirical approaches there is no, so far generally accepted, quantitative theory on this problem. There are, however, several basic requirements and rules which direct the synthesis and selection of the compounds suitable for application as antiozonants: They should: UÊ ÃÕÀiÊ «À}i`Ê âiÊ ÀiÃÃÌ>ViÊ vÊ Ì
iÊ ÀÕLLiÀÊ >ÌiÀ>ÃÊ Õ`iÀÊ ÃÌ>ÌVÊ >`Ê dynamic conditions. UÊ iÊÌ
iÀ>ÞÊÃÌ>Li]Ê
>ÛiÊ>Ê>ÌÃVÀV
}ÊivviVÌÊ>`ÊLiÊwÀiÊÃ>vi° UÊ iÊV«>ÌLiÊÜÌ
ÊÌ
iÊÀÕLLiÀÊÝÌÕÀiÃ]ÊÌÊ`ÃVÕÀ]Ê`vvÕÃiÊi>ÃÞÊÊÌ
iÊ«ÞiÀÊ matrix and not be easily washed away. UÊ iÊVÕÀiÃÃÊ>`ÊÃÌ>LiÊ`ÕÀ}ÊÃÌÀ>}i° UÊ iÊÃ>vi° UÊ /
iÊ À>ÜÊ >ÌiÀ>ÃÊ >`Ê ÌiV
}iÃÊ vÀÊ Ì
iÀÊ «Ài«>À>ÌÊ Ã
Õ`Ê LiÊ i>ÃÞÊ available. In addition, according to the assumed mechanism of ozone interaction with rubber solutions, discussed in the previous chapter, the following specific requirements can be formulated: 1. Antiozonants (AO) can preserve the elastomers if: Ê
UÊ
Ê "Ê>ÀiÊÀi>VÌi`ÊÜÌ
ÊâiÊv>ÃÌiÀÊÌ
iÊâiÊÜÌ
Êi>ÃÌiÀÊ>`ÊvÊ>ÌÊÌ
iÊÃ>iÊ time the AO diffusion rate from the bulk to the surface of the elastomers is correspondingly.
Ê
UÊ
Ê"Ê>ÀiÊvÀi`ÊV«iÝiÃÊÜÌ
ÊâiÊÀÊ`iV«Ãi`ÊÌÊV>Ì>ÞÌV>Þ°
Ê
UÊ
Ê"Ê>ÀiÊ`i>VÌÛ>Ìi`Ê r ÊL`ÃÊLÞÊvÀ}ÊV«iÝiÃÊÜÌ
ÊÌ
i°
Ê
UÊ
Ê
iÊ"Ê>ÀiÊ«ÞvÕVÌ>ÊV«Õ`ÃÊÌ
iÞÊV>Êii«ÊÌ
iÊiVÕ>ÀÊÜ}
ÌÊvÊ / the elastomers by networking of the broken parts of the macromolecules.
2. Antiozonants and their ozonation products should physically prevent the ozone having access to the C = C bonds by: Ê
UÊ
262
i}ÊiÀÌÊV«Õ`ÃÊÌÜ>À`ÃÊÌ
iÊâiÊ>ÌÌ>V°
Degradation and Stabilisation of Rubber Ê
UÊ
Ê >VÌ>Ì}ÊÌ
iÊÀi>Ý>ÌÊvÊÌ
iÊÃÌÀ>ÊvÀi`]Ê°i°]ÊÌ
iÞÊ>VÌÊ>ÃÊ>Ìv>Ì}ÕiÊ additives.
It is obvious that practically all these requirements cannot be satisfied by a single compound. That is why the stabilisation of elastomers is carried out by using stabilising systems - mixtures of various antiozonants which could ensure a complex protection, both chemical and physical one. The rate constants of ozone reaction with amines, hydroquinolines, thiosemicarbazides, some aminophosphorus compounds, thiophenols, aminophenols, and so on are up to 3-4 orders higher than with elastomers [48]. Among them the derivatives of N,Nasubstituted-p-phenylenediamines (PPHDA), the salts of dialkyldithiocarbamates, dihydroquinolines, and so on are the most powerful antiozonants. Some inorganic compounds such as dithiocarbamate and dithiophospate complexes of some alkaliearth and transition metals also exhibit good antiozonant properties. It has been also found that the oxides of the latter are good catalysts of ozone decomposition. However, the rate constants of ozone reaction with saturated organic compounds such as paraffins and their oxygen-, halogeno-, nitrile, nitro-, thio-, sulfo-, phosphocontaining derivatives, waxes, i.e., paraffin mixtures with definite molecular mass distribution, polyolefins, saturated fluoro-, chloro- and silicon containing elastomers [48] appear to be much lower (up to 4-6 orders) than with elastomers. These compounds prevent the degradation process by reducing the ozone access to the elastomer, form surface layers, and at the simultaneous application with low molecular antiozonants guarantee favourable conditions for their diffusion to the reaction zone. Saturated polymer additives also operate through a kinetic factor associated with the decrease of the surface C=C bonds. Many organic and inorganic metal-containing compounds and complexes, the active and nonactive fillers in the rubbers compositions act as deactivators of the double bond. Such compounds are those formed during the vulcanisation of rubbers with ions of copper, magnesium, nickel, zinc and other metals present. The formation of complex and surface compounds with participation of the C=C bonds substantially decrease their reactivity towards the ozone action. The derivatives of PPHDA and dihydroquinolines also exert crosslinking functions and the latter are the best known as antifatigue additives.
4.2.1 Mechanism of Action The use of stabilising additives as antiozonants depends primarily on the type of rubbers and the field of their application for producing rubber goods and car and trunk pneumatic tyres [50].
263
Ozonation of Organic and Polymer Compounds The selection of a particular antiozonants is based on considering the probability of its interaction with the various ingredients of the rubber composition and its compatibility with the vulcanising agents [51]. PPHDA being one of the most efficient and widely applied antiozonants in the rubber industry has been intensively studied with a view of elaborating its mechanism of action. The most widespread mechanism for the antiozonants action is the kinetic approach based on the concurrent interaction of ozone with antiozonants and polydiene [52-56]. A characteristic peculiarity of these antiozonants is that they alone or their ozonation products can form a protective surface film [57] thus preventing the ozone having access to rubber [58]. Lorenz and Parks [35] considered that the antiozonants react mainly via three reaction routes: 1) Directly with ozone; 2) With the peroxide compounds reducing them; 3) With two polymer molecules containing aldehyde groups. Rakovsky and co-workers [59, 60] related the antiozonants efficiency to the inhibition of free radical reactions initiated by ozone. Braden and Gent [62] distinguished two basic classes of antiozonants: 1) antiozonants that prevent the crack growth, and, 2) antiozonants that increase the critical energy of crack growth. Storey and co-workers, [62] attributed the high antiozonant activity of PPHDA to the presence of two conjugated nitrogen atoms, one of which can act as a nucleophile due to the presence of the second one. The different efficiencies of antiozonants is due to the various nucleophility of the nitrogen atoms, which in its turn affects the decomposition rate of the peroxide compounds on the rubber surface. The stronger the nucleophilic effect is, the higher the rate of nitroxyl radical formation. The latter initiate reactions with short kinetic chains and form cross bonds thus reducing the number of adjacent double bonds. Alkyl substituents that increase the nucleophility of the nitrogen atoms increase the antiozonants activity in comparison with those without aryl substituents. This action of PPHDA is confirmed by the fact that ozone penetration in rubber is at depth of about 1.3 s 10-5 m. Other authors stated that the bifunctional antiozonants prevent the growth and formation of cracks in elastomers through combining the two zwitterions, obtained
264
Degradation and Stabilisation of Rubber from the decomposition of two PO, thus forming a new bond between the broken ends of the macromolecules [63, 64]. The protective action of antiozonants is also associated with the relative stability of the free radicals generated from them during the ozone degradation of elastomers [65]. Michaelis [66] has shown that the more effective PPHDA lead to the formation of stable semiquinone structures while the less active ones form unstable ones. On the basis of these results Barnhard and Newby [67] assumed that the stable semiquinone structure prevents the macromolecule degradation through destroying the ozonides and peroxides. In our studies on SKI-3 degradation in the presence of various PPHDA it has been demonstrated that some of them exhibit initiating properties in dependence on the applied deformations [68, 69]. The foregoing survey shows that the antiozonant action of PPHDA is a rather complex and ambiguous process. We believe that the study on the kinetics and mechanism of antiozonants action and the establishment of relationships for prediction of their efficiency or for the synthesis of new antiozonants with definite properties is of great practical importance [70-73]. Bailey suggested the following pathways for ozone interaction with di-tert-butylamine as a low molecular model of PPHDA- antiozonants: H (C 4H 9) 2NH + O
(C 4H 9) 2NOOO
3
H (C 4H 9) 2NOOO
(C 4H 9) 2NO + O
(C 4H 9) 2NO + HO
2
(C 4H 9) 2NOOO
3
O C 4H 9NO 2 + C 4H 9O + O
(C 4H 9) 2NOOO
2
O
(C 4H 9) 2NOOO
+
(C 4H 9) 2NOC 4H 9 +
(C 4H 9) 2NO
C 4H 9NO 2 + O
2
O OOO (C 4H 9) 2NOC 4H 9 + O
C 4H 9O
C 4H 9O + RH
(C 4H 9) 2NOC 4H 9
3
CH 3COCH
3
+ CH
(C 4H 9) 2NO + C
4H 9O
+ O
3
C 4H 9OH + R
265
Ozonation of Organic and Polymer Compounds The electron spin resonance (ESR)-spectra characteristic for the nitroxyl radical, as well as the 2-methyl-dinitromethane, tert-butyl alcohol, acetone and oxygen which were obtained confirm this mechanism. One molecule of amine decomposes up to four ozone molecules. If one takes into account the possibility of ozone interaction with radicals, without nitrogen, then the number of decomposed ozone molecules should be greater which additionally enhances the amine efficiency. The influence of ozone on pure rubber and antiozonants solutions in inert solvents has been the subject of numerous investigations. These studies have contributed to the discovery of some of the basic steps of antiozonants transformations and rubber decomposition products, which can undergo mutual interactions and affect the elastomers degradation process [74-76]. Upon the ozone reaction with elastomers with low C=C bonds content or with electronaccepting substituents at the C=C bonds such as butylrubber, polychloroprene, and so on, the basic reactions of degradation are related either to the chain-radical route or molecular reactions of ozone addition to the C-H bonds and isomerisation of RO2 and RO radicals. We have evaluated the role of ozone as an initiator of chain-radical oxidation processes in Chapter 1. The protection of such rubbers is accomplished via the antiozonants reaction with ozone thus converting the active oxy- and peroxyradicals into non active molecular products. Thiosemicarbamates and thiosemicarbazides [77, 78] have been used as good, noncolouring antiozonants instead of PPHDA. These compounds easily donate electrons and the values of k for their reaction with ozone amount to 106-107 M-1.s-1 (by about 2-3 orders higher than k found for the ozone reaction with C=C bonds in elastomers) and Ea = 1.3 kcal/mol [79, 80]. It is known that the value of the Ea of diffusion is of the same order while that for solid state may be higher. This suggests that the degradation reaction will be limited by the mass transfer rather than by the change of temperature. For this reason the antiozonants should be compatible with the rubber mixture and should easily diffuse in the polymer matrix. As we have mentioned previously the stabilising compositions are composed of various antiozonants or of antiozonant blends with polyolefins and saturated elastomers such as butyl rubber or triple copolymers of ethylene with propylene and diene, with ozone resistant elastomers like chorine, fluorine or nitrile rubber, and so on. The antiozonants action can be enhanced by the use of synergetic mixtures and the ozone resistance of the rubber products can be increased by using antiozonants mixtures spread on the surface. The latter are antiozonants solutions in gasoline, chloroform, tetrachloromethane, turpentine, and so on.
266
Degradation and Stabilisation of Rubber
4.2.2 Synthesis The organic compounds used as antiozonants and antioxidants usually contain various numbers of heteroatoms: N1, N2, N3, N4, N1O2, O2S1, O1, P2S4, and so on. The synthetic methods for their preparation depend on the type of the antiozonants and the initial products which are mostly obtained from petroleum.
4.2.2.1 Paraphenylenediamine In our [81-83] and other studies [84-84] it has been convincingly established that among the great variety of chemical compounds used as antiozonants the substituted N,N-dialkyl, diaryl- or alkylaryl-PPHDA are the most powerful antiozonants exhibiting a broad spectrum of useful properties. The modern tendency in PPHDA antiozonant preparation is closely associated with the creation of new high-end technologies. The latter require that the whole process of additives preparation including the separate reactions of amination, alkylation (arylation), reduction and oligomerisation up to the commercial state of the antiozonants should be carried out in one reactor by using a highly selective and efficient catalytic system. The semi-products for PPHDA preparation: p-chloronitrobenzene, p-dinitrobenzene, p-chlorophenol, p-dichlorobenzene, phenylp-phenylenediamine, and naphthyl-p-phenyldiamine, in fact become semi-products for producing the commercial antiozonants. The search is basically directed to expanding the range of raw materials by including low-cost and available products from petroleum processing, coke preparation, and so on.
4.2.2.1.1 Alkylation in the Presence of Hydrogen Catalytic alkylation in the presence of hydrogen is widely applied to synthesis and industrial preparation of PPHDA-antiozonants. Catalysts based on cerium, colbalt, nickel, palladium, platinum and zirconium are most suitable for reducing the alkylation process. This is a process of introducing an alkyl group into the molecule by the interaction of amino, nitro or nitroso compounds with ketones, aldehydes or alcohols in the presence of hydrogen. The most efficient antiozonants are obtained upon alkylation with ketones, mainly, unsymmetrical one like methylethylketones with alkyl substituent varying from C3 to C5-C7. The most used ketones are methyl ethylketone (MEK), methyl isobutylketone, methyl isoamylketone, methyl hexylketone, and so on. Alcohols should be preliminarily dehydrogenated, which makes the process more expensive.
267
Ozonation of Organic and Polymer Compounds N,N´-dialkyl-PPHDA is obtained from a mixture of p-nitroaniline or p-phenylenediamine with at least 2 moles of aldehydes or ketones, hydrogenated at a temperature of 100200 oC and a hydrogen pressure of 1-20 MPa in the presence of CuO, Cr2O3 and BaO or their mixtures [87]. Other catalysts used for the same purpose are: CaO, Ce2O3 and BaO, Ba, CuCrO2 or Cu/ZnO [88], with a ratio between the ketone and the p-nitroaniline in the range from 2:1 to 10:1 under hydrogen pressure of 140 at a temperature over 100 oC. The interaction of paranitroaniline with MEK in the presence of copper chromite at PH2 – 5 MPa and temperature of 110-160 oC leads to N,Na-di-sec-butyl-PPHDA. The reaction proceeds according to the scheme: H 2N
NO2 + 3H2
H 2N
NH2 + 2H2O
H 2N
NH2 + 2 CH3COC2H5 + 2H2
C2H5CH(CH3)HN
NH(CH3)HCH5C2H5
Patents [91, 92] concerning the conversion of paranitroaniline into N,Na-di-sec-butyl PPHDA, the most suitable conditions for this reaction are a temperature of 180-200 o C, PH2 = 3-6 MPa and a molar MEK: PPHDA ratio = 10. Other conditions for the alkylation of p-nitroaniline with MEK are presented in [91-99]. Copper chromite has been used as a catalyst not only in the above reaction but also in the preparation of N-(1-Methyl-hexyl)-N´-phenyl-benzene-1,4-diamine(4C6H5NCH6H4NHCH(CH)3C5H11) from 4-aminodiphenyl amine and methyl hexyl ketone in a ratio of 1:2 [100]. Substitution of 4-CH3O-4-CH3-pentene-2-one for methyl hexyl ketone leads to the formation of 4(4-CH3O-4-CH3-pentyl)-amino diphenyl amine [101, 102]. Alkylation with a sec-butyl alcohol proceeds in the presence of 15% Ni, 5% Cu and 1% Mn on SiO2 and PH2 = 1.4 MPa [103]. The yield of the final product increases after re-distillation of aniline or p-nitroaniline [104, 105]. When PPHDA or N-aryl-N-(4-aminophenyl) amine is alkylated with furfurol or tetrahydrofurfuryl alcohol, N, N´-difurfuryl-PPHDA is obtained on the Pd/C LiAlH4 or Raney-nickel catalyst [106, 107]:
H2N
NH2
+
2
CH2OH O
CH2 HN O
268
NH CH2 O
5% Pd/C
Degradation and Stabilisation of Rubber If a mixture of diphenylamine and furfuryl alcohol is allowed to stand for 80 days at room temperature, 4,5-di(diphenylamino) cyclopentene-2-one with antiozonant properties is obtained [107]. Diatomite-supported nickel (63%) is used for the preparation of N(1,3-dimethylbutyl)Na-phenyl-PPHDA from 4-amino-diphenylamine and methyl isobutyl ketone in a ratio of 1:2 [108, 109]:
C6H5-C6H4-NH2 + CH3COCH2CH(CH3)2 o
atm, 165 C }50-60 }}}} }m
(CH3)CH2CH(CH3)CH2NH-C6H4-NH-C6H5
Modification of nickel catalysts with cobalt leads to an increase of its selectivity [110]. This catalyst is very active and can cause hydrogenation of the phenol ring. That is why addition of negligible amounts of organic sulfur compounds such as sulfates, mercaptans, thio acids and sulfides to the reaction mixture is recommended [111, 112]. Selenium and tellerium as catalysts preserve their activity for a long time. They are not poisoned by sulfur compounds (no purification of hydrogen is needed) and lead practically to no side reactions [113, 114]. Nickel catalysts are also used during the conversion of N-alkyl-4-nitrosoaniline into N-alkyl-N-phenylenediamine, while iron-copper-chromium catalysts are utilised in the reaction of the N-alkyl-N-phenylenediamine with ketones. The yield (74%) can be enhanced up to 96.3% by the addition of 0.34% triphenyl methane [115, 116]. One of the most effective antiozonants, N-(1,3-di-methyl-butyl)- N´-phenyl-PPHDA is obtained from nitro-, nitroso- or p-aminophenylenediamine and iso-butylmethyl ketone in the presence of Pd/C catalyst under hydrogen atmosphere [117-121]. The reaction proceeds according to the following general scheme:
269
Ozonation of Organic and Polymer Compounds
NH
NO2
H2 NH
NO
+
C H 3C O C H 2C H ( C H 3) 2 -H 2O
NH 2
NH
NH
N H C H ( C H 3) C H 2C H ( C H 3) 2
If alkylation is conducted with acetone at a hydrogen pressure ranging from 0.5 to 30 MPa and a temperature of 100-250 oC, the final products will be N-isopropylNa-phenyl-PPHDA [122, 123]: C6H5NHC6H4NO + CH3COCH3 = C6H5NHC6H4NHCH(CH3)2 The yield can be enhanced by modification of the catalyst with thiocarbamide [125]. Another catalyst that is used is platinum (0.2-2%) on an appropriate support. This catalyst is reduced at a temperature of 95 oC and its activity is enhanced by addition of (CH3)2S or hexyl mercaptan [126, 127]. When platinium catalysts are used, the initial p-nitrosodiphenylamine should contain neither sulfur nor chlorine since these elements deactivate the catalyst [128]. N,N´-di-(1-ethyl-3-methylphenyl)-PPHDA, N,N´-di-(1-methyl-heptyl)-PPHDA and N,N´-(1,3-dimethyl-pentyl)-PPHDA obtained on 1% Pt/C belong to the widely applied antiozonants [129, 130]. N,N´-di-(1,3,3-trimethyl)-PPHDA with a mp of 98 oC is more appropriate from a technological point of view than are liquid PPHDA. It is obtained from p-nitroaniline and methyl-neopentylketone according to the scheme:
270
Degradation and Stabilisation of Rubber
O 2N C 6H 4N H 2
+
2C H 3C O C H 2C ( C H 3) 3
C H C O O H , Pt/ C 3
P
H
2
( C H 3) 3C C H 2C H ( C H 3) N H C 6H 4N H C H ( C H 3) C H 2C ( C H 3) 3
The preparation of lower-toxicity antiozonants such as N-(1,3-dimethylbutyl)- N´(1,4-dimethylpentyl)-PPHDA and N-isopropyl- N´-methylpentyl-PPHDA is described in [131-133]. The presence of halogens in the aliphatic part of the PPHDA increase their activity as is shown for the example of N-(2-chloroallyl)-N-phenyl-PPHDA. It has a 75% higher activity than N-alkyl-N´-phenyl-PPHDA [134]. A mixture of differently substituted PPHDA is prepared by reducing alkylation of 4-nitrodiphenyl or (N-isopropyl-p-nitroaniline, p-nitroaniline or PPHDA) with two or more ketones at a temperature of 25-120 oC, a hydrogen pressure of 2.8 MPa and 1% Pd/C catalyst [135, 136]. Hydrogenation of a mixture of O2NC 6H 4NH 2 and the ketones CH3COCH 2 CH(CH 3) 2 and CH 3COCH 2CH 2CH(CH 3) 2 in a ratio of 1:2 leads to 4.2% p-(CH 3) 2CHCH 2CHCH 3NH 2C 6H 4 and 41.8% (CH 3) 2CHNC 6H 4NHCH(CH 3) CH2CH(CH3)2 [137]. When the ratio between the ketones is 1:1, about 20% of symmetric di-sec-alkyl-PPHDA is also obtained [137]. The products of the reaction between p-nitroaniline and a mixture of methyl-iso-butylketone and methyl-isoamylketone (1:1) are 18.6% N,N´-di(1,3-dimethylbutyl)-PPHDA, 30.6% N,N´-di(1,4dimethylpentyl)-PPHDA and 50.2% N-1,3-dimethylbutyl- N´-(1,4-dimethylpentyl)PPHDA. We have synthesised a number of complex catalysts based on Pt(II) and Pd(II) with phenyl-azo-2-naphthol: (1), 1,2,4-trianthraquinone (purpurin) (2), 8-oxyquinoline (3), anthranilic acid (4), chloroanilic acid (5) and dimethylglyoxime (6) on G-Al2O3 [138]. The complexes are obtained by the treatment of G-Al2O3 supported ligands (mentioned previously) with aqueous solutions of K2PtCl4, K2PdCl4, Pd(OAc)2 at a metal ion:ligand ratio of 1:2.5:
271
Ozonation of Organic and Polymer Compounds
N 2
N
A l2O 3
+
N
K 2M eC l 4
-K C l -H C l
O
N
Me
OH
N
O
N
A l2O 3
1 A l2O 3 O O
O
HO
O
O
O
A l2O 3
Me
;
O
O
N
OH
O Me
O
N 3
2
A l2O 3
O C
H 2N
O 4
CH 3 O O
Me C
A l2O 3
OH
Cl
O
Cl
O
O
O
5
CH 3
N
N Me
H
Cl
O
Cl
HO
Me A l2O 3
O
O
NH2
O
N
O O
H
N
O A l2O 3
H 3C
CH 3
O
O Me
HO
O
N
O 7
6 A l2O 3
The supported metal content is 0.5-3 wt%. In a separate test with the homogeneous and immobilised complex (1) we have established that the immobilisation does not affect the catalyst activity. This parameter has been evaluated in a model reaction of nitrobenzene hydrogenation in various solvents (Table 4.2).
272
Degradation and Stabilisation of Rubber
Table 4.2 Catalytic activity of Pd(II)-complexes in nitrobenzene hydrogenation to aniline. Catalyst weight: 0.1 g, Pd = 0.62%, nitrobenzene = 1 mmol, 67 ºC, PH2 = 10,325 Pa. Catalysts
Solvent
Rate of Hydrogenation [mol H2/(min.g.atom Pd)]
Pd(purpurin)2
Octane
62.3
Pd(purpurin)2
Heptane
47.2
Pd(purpurin)2
Decane
32.0
Pd(purpurin)2
Cycloheptane
22.0
Pd(purpurin)2
DMFA
21.2
Pd(purpurin)2
Ethanole
22.4
Pd(purpurin)2 + 8-oxyquinoline
Octane/DMFA
59.5/27.1
Pd(purpurin)2 + phenylazonaphthol
Octane/DMFA
59.5/27.1
Pd(purpurin)2 + dimethylglyoxime
Octane/DMFA
47.7/30.9
Pd(purpurin)2 + chloroanile
Octane/DMFA
18.5/16.9
DMFA = N,N-dimethylformamide
The catalysts preserve their activity for a long time, even after 14-fold application. These catalysts hydrogenate nitrobenzene to phenylhydroxylamine in DMFA and to aniline in alcohols. The reaction kinetics was followed by determination of the ozone uptake, chromatographically (high-performance liquid chromatography, gas and thinlayer chromatography), the nitrobenzene consumption and phenylhydroxylamine and aniline formation. The strong complexing functions of DMFA stabilise phenylhydroxylamine and the reaction stops kinetically at the absorption of 2 moles hydrogen, while in alcohols (ethanol, iso-propanol) the reaction goes on up to 3 mol ozone uptake forming aniline. The hydrogenation of p-nitroaniline in DMFA in the presence of Pd(purpurin) gives a quanitative yield of p-phenyldiamine.
4.2.2.1.2 Alkylation in the Absence of Hydrogen Substituted p-phenyldiamines can be obtained without using hydrogen and expensive noble metals as catalysts [139, 140]. When N-alkyl-4-nitroaniline is mixed with
273
Ozonation of Organic and Polymer Compounds n-hexanol and 59% NaOH, and heated for 1 hour at a temperature of 100 oC, it yields monosubsituted PPHDA: RHNC6H3(R1)NO2 +2R2CH2OH m RHNC6H3(R1)NH2 where: R is a C1-C12 alkyl or naphthyl radical, and R1 is a C1-C12 alkyl radical. Monosubstituted PPHDA can also be obtained by interaction of N-substituted aminophenols with ammonium in the presence of an acid catalyst (Lewis acid): RHNC6H4OH + HNH2 m RHNC6H4NH2 + H2O Other catalysts used in this reaction are FeCl3 or a mixture of free iodine and powdery iron in a ratio of 6:1 [141, 142]: RNH2 + HOC6H4NH2 (RNHC6H4NH2 + H2O where: R = C6H5-, C6H4CH3 or C6H11-. Heating at 215-220 oC in an nitrogen atmosphere of a mixture of aniline, cyclohexylamine and N-(1,3-dimethylbutyl)-p-aminophenol in the presence of iodine, powdery iron and NH 4Cl leads to the formation of 10% N,N´-(1,3dimethylbutyl)-PPHDA, 5.5% N-cyclohexyl-N´-(1,3-dimethylbutyl)PPHDA, 30% N-(1,3-dimethylbutyl)-N´-phenyl-PPHDA, 8% N-cyclohexyl-Na-phenyl-PPHDA and 9% N,N´-diphenyl-PPHDA as final products [143]. Substituted N,N´-diphenyl-PPHDA with low melting points appear after a reaction of hydroquinone with a mixture of amines (35% aniline, 25% toluidine and 40% xylidines) during heating in the presence of a catalyst (FeCl3) of condensation [144]. Reaction of substituted PPHDA with thiols or formaldehyde in an acid medium results in their conversion into active stabilisers having a complex behaviour against the action of oxygen, ozone, light and heat [145-147]. Their general formula is: R1-S-CH2-C6H2-(R2)(R3)-NH-C6H4-NH-R4 where: R1 is alkyl C1 - C20, aryl C6 - C14, alkylaryl; R2 and R3 are H, alkyl C1-C7, aryl C6-C14 and R4 is [-C6H4-CH2-S-R1].
274
Degradation and Stabilisation of Rubber Alkylated 2,4-toluenediamines are good antiozonants and antioxidants, which interrupt the chain-radical process taking place during the oxidation of polymeric materials [148]. A mixture of 2,4-toluenediamine, dicyclopentadiene and pentene is stirred in the presence of a catalyst (12% Li and 87% Si) under elevated pressure in a nitrogen atmosphere. A mixture of 3-(cyclo-pent-2-enyl)-2,4-toluenediamine and 5-(cyclopent-2-enyl)-2,4-toluene is obtained. The conversion of 2,4-toluenediamine amounts to 64%.
4.2.2.2 Hydroquinolines Hydroquinolines are prepared by the classical method of Scraup using a reaction of aromatic amines with aldehydes or ketones in the presence of oxidants and acid catalysts according to the following scheme:
OHC OHC
NH2 +
CH
CH 2
CH 2
CH 2 N H
HOHC CH CH 2 N H
-H O 2
N H
A mixture of aniline and C8-C15-alkylated amines (45% C12, 15% C11 and 12% C10) in an acid medium is heated up to a temperature of 140 oC for 12 hours, after which acetone is added. The final product is an oil containing 58% 6-dodecyl-1,2dixydro-2,2,4-trimethylquinoline, 6% dodecylaniline and oligomer of 1,2-dihydro2,2,4-trimethylquinoline [149, 150]. Addition of acetone to 4,4a-(ethylenedioxyl)-dianiline for 12 hours in the presence of benzenesulfonic acid at 150-160 oC yields 6,6a(ethylenedioxyl)-di-1,2-dihydro-2,2,4trimethylquinoline [151]. When 2,2,4-trimethyl-1,2-dihydroquinoline is allowed to polymerise in the presence of hydrochloric acid, an active polymeric antiozonant is obtained. The monomer is formed during reaction of aniline with acetone at a temperature of 120 oC in the presence of p-toluenesulfonic acid as a catalyst [152]. The polymer is isolated by dissolution of the reaction mixture in benzene, followed by neutralisation and evaporation of the solvent. The reaction mixture can be dispersed in water and then granulated. The antiozonants obtained are usually applied in a combination with N-phenyl- N´-isopropyl-PPHDA [153, 154].
275
Ozonation of Organic and Polymer Compounds
4.2.2.3 N,N´-Disubstituted Hexahydropyrimidines These are nonstaining antioxidants [155], which are prepared by condensation of cyclohexanone or its derivatives with ammonia. Tetrahydropyramidine is formed first and then reduced to hexahydropyrimidine whose pyramidine ring is open to form diamine. The two amino groups form bonds with aldehydes and yield N,N´substituted hexahydropyrimidine. The reaction proceeds as follows:
3
+ 2N H 3 O
N
-H 2O
NH
H2
HN
NH
H2
( C H 3) 2C H C H O , H 2
H 2N
NH
C H 2O
( C H 3) 2C H C H 2H N ( C H 3) 2C H C H 2H N
NH
NH
4.2.2.4 N-Substituted Dimethylpyrols These compounds are also nonstaining antiozonants [156]. They are obtained from 1,4-dicarbonyl compounds passing through an enol intermediate which reacts with amines giving:
CH 3 Z -alkylene-N CH 3 where Z is substituted NH2; RNH-; R2N-; R-NH-C(S)-N=N- or -(R)N-C(S)-NH2
4.2.2.5 Enamines The enamines are nonstaining antiozonants with the general formula: (R1)(R)N-HC=C(R2)(R3)
276
Degradation and Stabilisation of Rubber where: R1, R2 and R3 are alkyl groups with C1 - C6. These compounds are prepared by condensation of amines with ketones. The amines used are pyrolidine, pyperidine, substituted morpholines, and so on: RR1NH + (R2)(R3)CHC(R4)O m (R)(R1)N-(R4)C=C(R2)(R3) + H2O If the ketones are unsymmetric, a mixture of products is obtained. Condensation of 2,6-dimethylmorpholine with 2-ethylhexanal yields 1-(2,6-dimethylmorpholino)2-ethyl-hexene-1 which is usually applied for natural rubber and butadienestyrene rubber [157]: RCH2CHO +HN(R')2 m RCH2CH(OH)N(R')2 k (RCH=CHN(R')2 + H2O
CH 3 O
CH 3
N H + O H C -C H ( C 2 H 5 ) ( C H 3 ) C H 3
N -C H =C ( C 2H 5) ( C H 3) C H 3
O
CH 3
CH 3
4.2.2.6 Nitrone Compounds The C-substituted or unsubstituted aryl-N-substituted nitrones are appropriate not only as antiozonants but also as antifatigue inhibitors [158]. Particularly effective in this respect are aldonitrones of the type:
R2
R4
R1
C H =N -R 6 R3
R5
O
where: R1 is a mono- or disubstituted aminogroup; R2 and R3 - alkyl or alkoxy groups; R4, R5 = R2 and R6 - iso-propyl, sec-butyl or tert-butyl group.
277
Ozonation of Organic and Polymer Compounds The nitrone compounds may be prepared by a variety of ways, for example: (a) by oxidation of the corresponding aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) hydroxylamines; (b) by reaction of aryl ketones with primary hydroxylamine; (c) by N-alkylation of the corresponding oxime; (d) by reaction of the corresponding ketimine with primary hydroxylamine; (e) by oxidation of the corresponding N-substituted imine. The most used nitrone is 4-hydroxy-3,5-dimethyl-N-tert-butylnitrone obtained when N-tert-butyl-hydroxylamine and 3,5-dimethyl-4-hydroxybenzaldehyde in a ratio of 1:1 are dissolved in absolute alcohol and the mixture is allowed to stand for five days at room temperature: CH 3
CH 3 CH O
HO
+ H O N H -C ( C H 3 ) 3
CH 3
C H =N -C ( C H 3) 3
HO CH 3
O
4.2.2.7 Derivatives of 3(5)-Methylpyrazone The general formula of these compounds is:
where: R is -N
;
N
The ozone resistance of rubber mixtures based on natural rubber with ozonine-16 (pyperidinomethyl-3,5-methylpyrazole) and ozonine-51 (hexamethyleneimino-3,5methylpyrazole) exceeds 1.5 to 2 times that of mixtures containing Plastanox 2246 (2,2a-methylene-bis-4-methyl-6-tert-butylphenol) and is close to that reached by tributylcarbamide. A mixture of ozonine-16 or ozonine-51 with 2246 in a ratio of 1:1 displays the highest activity [159].
278
Degradation and Stabilisation of Rubber
4.2.2.8 Enolethers These compounds are used as antiozonants for natural and synthetic rubber. They are slightly staining. The following enolethers are synthesised: 2H-pyran-2-methyl(3,4-dihydro-2H-pyran-2-carboxylate); 3,4-dihydro-2,5(dimethyl or diisobutyl, didecyl)-2H-pyran-2-methyl-(3,4-dihydro-2,5-dimethyl-2H-pyran-2-carboxylate) with the general formula:
R1
R1
O C H 2 -O -C O
O
where: R is alkyl, aryl or alkylaryl radical. By condensation of 3,4-dihydro-2H-pyran-2-carboxyaldehyde and aluminium isopropylate with stirring for 4 hours at 40 oC, 75% of 3,4-dihydro-2H-pyran-2methyl-(3,4-dihydro-2H-pyran-2-carboxylate) is obtained [160]. The mixture of different enoethers results in better antiozonant activity. For example, a mixture of hexanediol-1, 6-di-(2-methyl-prop-1-ethyl)ether and 1,1a-bis-(2-methyl-prop-1epoxy)-dietheylether is prepared by aldol condensation of iso-butyl aldehyde and 1,2-hexanediol in the presence of toluene sulfonic acid, quinoline and cyclohexane as a solvent [161].
4.2.2.9 Ethers Ethers with the general formula:
are odourless antiozonants, where R1 is a hydrocarbon group which may contain a heteroatom; X is S or O; R2 and R3 are H, -CH3. These ethers are used in combination with R-CO-NH-NH2 (where: R is an alkyl group with C3-C10) and exert high antiozonant ability [162].
279
Ozonation of Organic and Polymer Compounds
4.2.2.10 Cyclic and Acyclic Acetals and Ketals Cyclic and acyclic acetals and ketals are nonstaining antiozonants with general formula [163-166]:
where: n and m range from 0 to 12; R1, R2=H, alkyl group with C1-C3, alkylene group containing halogens, cyclo or bicycloalkenes with C5-C7, aryl groups with C6 -C10 or alkylaryl groups. They are prepared by the reaction of alcohols with aldehydes or ketones in the presence of catalysts: RCHO + 2RCH2OH m RCH(OCH2R)2 (R)2CO + 2RCH2OH m (R)2C(OCH2R)2 The catalysts used are: H2SO4, H3PO4, p-toluene sulfonic acid, boron trifluoride, and so on. These cyclic acetals are usually applied with microcrystalline waxes [167]. Acetals and ketals with unsaturated cyclic groups are less volatile, have no odour and are compatible with rubbers [168].
4.2.2.11 Other Classes of Compounds
4.2.2.11.1 Bis-Alkylaminophenoxy Alkanes Bis-alkylaminophenoxy alkanes with the general formula:
(where: X=halogen, R= sec-alkyl radical with C3-C8, R' is -CH2-, -CH2-CH2-) are obtained from nitrophenol by reducing alkylation in the presence of Pt/C as a catalyst [169].
280
Degradation and Stabilisation of Rubber
4.2.2.11.2 Derivatives of 2,2,7,7,-Tetramethyl-1,4-Diazocyclopentane The derivatives of 2,2,7,7-tetramethyl-1,4-diazocyclopentane with the formula:
where: n is 1, Y is H, CH3, O and -NH, R denotes alkyl groups with C1 = C20; where n is 2, R is alkylene groups with C1-C20, cycloalkene groups with C7 -C18 or alkylaryl groups with C5 - C12, are also applied as antiozonants for polymers. Their preparation is accomplished in a reaction of 1,4-diazocyclopentane-5-one with isocyanate [170].
4.2.2.11.3 Bis-Cyclopentadienyl Compounds Bis-cyclopentadienyl compounds with the general formula:
where: X is A,A´-p-xylilene or 4,4´-oxydiphenyl-dimethylene, which are good antiozonants [171].
4.2.2.11.4 Alkylnaphthenes Alkylnaphthenes are applied as stabilisers of cables and insulating materials. They are obtained by alkylation of naphthaline with dodecane chloride in the presence of AlCl3 at 80 oC or 1-hexadecanol in the presence of BF3 at 130 oC: C10H8 + C12H25Cl j B-C12H25-C10H7 + HCl C10H8 + C16H33OH m B-C15H30-C10H7 + H2O The yield is 60% [172].
281
Ozonation of Organic and Polymer Compounds
4.2.2.11.5 Aminomethylene Derivatives of Furane Aminomethylene derivatives of furane are cheaper and nonstaining antiozonants [173].
4.2.2.11.6 Lactams Strong antiozonants are the lactams with the general formula:
where n = 3-20; m = 1-2; R - alkyl or phenyl radical. Comparison based on antiozonants activities indicates E-caprolactam and caprylactam to rank first [174].
4.2.2.12 Sulfur-Containing Compounds The derivatives of isothiocarbamate used as antiozonants for natural rubber, butadiene rubber, isoprene rubber and butadienestyrene rubber are noncolouring and nonstaining, and have the general formula: R4-N=(SR1)-N-R2R3 where: R1, R2, R3 and R4 are alkyl C1-C30, phenylalkyl C1-C6 or cycloalkyl radicals [175]. A high antiozonant activity is exhibited by sulfides R-S-R [176], where R is [C6H5NHC6H4NHCO]nX, n is 1 or 2, X denotes methylene, ethylene or 1,2-propylene radicals. When R = H, the sulfides are obtained by a reaction of 4-aminodiphenylamine with acids having the general formula: H-S-X-(-SO2H)n
282
Degradation and Stabilisation of Rubber If R' = -S-R, the acids should have the formula (HOOC)n-X-S-S-X-(COOH)n, (dithioacetic, B-dithiopropionic). The sulfides obtained are bis-(4-anilinophenyl aminocarbonyl methyl)disulfide and bis-(4-aminophenyl aminocarbonyl ethyl) disulfide. These antiozonants are easily soluble in rubber compositions. Sulfonate antiozonants with the general formula R1R2SO3N(R3OH) (R is alkyl radical with C1-C13, R2 - aryl radical with C1-C6, R3 -alkylene with C4-C6) are nonstaining, nontoxic and, above all, cheap to prepare [177]. The initial substances used are alkylated products, resulting from the preparation of dodecyl benzene (hydrocarbon radicals with C24-C30) which is sulfonated with oleum and then reacted with alkanolamines at a pH of 7.5. The sulfides of 3,4-dehydropyperidine are efficient antiozonants. They have the general formula:
where: X = S, SO, SO2; Y, Y* = methyl, isopropyl, sec-butyl radicals. Di-4-(3,4-dihydro-2,2,6,6-tetramethyl-piperidine)-sulfide, sulfoxide or sulfone are prepared by interaction of:
with a gaseous mixture of hydrogen chloride and hydrogen sulfide [178, 179]. Ozone resistance is observed with the products of mercaptoaldehyde condensation with phenols [180-183]:
283
Ozonation of Organic and Polymer Compounds
where: R, R1 are H, CH3, tert-butyl radicals. Triazinedisulfide derivatives to be used as antiozonants are relatively easily prepared by a reaction of benzthiazoldisulfide with 2(4-morpholinyldithio)benzthiazole and 1,3,5-triazine dithiol in the presence of a neutralising agent (NaOH) [184, 185]. In comparison with zinc-dialkyldithiophosphates, dithiol derivatives exhibit a higher antiozonant activity. They have the general formula:
where: R1 and R2 are alkyl radicals with C1-C12; T is H or SR2; R2 denote: s alkyl radicals with C1-C20. The reaction of alkylated phenols with sulfur in the presence of a basic catalyst yields 4-(3,5-diisopropyl-4-hydroxyphenyl)-1,2-dithiole-3-thione [186, 187]:
HO
S S
2-Vinylphenothiazine enhances the stability of rubber products towards ozone, temperature and repeated deformation [188]. Butoxyethyl N,N´-dimethyldithiocarbamate is used as antiozonant of light rubbers [189]. 284
Degradation and Stabilisation of Rubber Nitrile antiozonants with the general formula:
R1
S
(CH)n
C
N
R2 where: R1 - alkyl radicals with C1-C18 or (CH2)n-CN and R2 - H, CH3 or C2H5 are nonstaining antiozonants and they are characterised with low toxicity (LD50 = 5300 mg/kg) and low production expenses [190].
4.2.2.13 Si-Containing Compounds Aminoorganosilanes with the general formula: RnSi(CH2NXY)4-n where: R - alkyl radical; X - aryl, alkoxyaryl, aminoaryl, phenyl, and naphthyl radical; Y - alkylaryl, aminoaryl radicals show a good antiozonant activity. Dimethyl-bis-tert butylhydroxysilane used for the stabilising of butyl rubber is prepared by interaction of 3,5-di-tert-butyl-4-hydroxy-benzyl alcohol with dimethylchlorosilane [191, 192]. The oligodiphenylsiloxyquinone, which is a good thermostabiliser, is isolated by heterophase condensation of diphenylchlorosilane with aromatic diols in the presence of suitable catalysts [193].
4.2.2.14 P-Containing Compounds Phosphites and phosphates of 5-norbornene-2-methanol are used as antiozonants of polychloroprene rubber and have the following general formula: HP(O)(OR)2, (RO)3P, (RO3)PO where: R is 5-norbornene-2-yl methyl. The interaction of PCl3 with 5-norbornene-2-methanol in a pyridine medium results in a mixture of bis-(5-norbornene-2-methyl) phosphite (17%), tris-(5-norbornene-2methyl) phosphate (22%) and tris-(5-norbornene-2-methyl) phosphate (15%). The
285
Ozonation of Organic and Polymer Compounds reaction with POCl3 leads to the formation of phosphates only [194]. Aryl phosphites are applied as stabilisers of synthetic rubber [195, 196]. Organic polymers are stabilised by amide esters of phosphonic acid having the general formula: P(ZR)(Z'R')(NR2R3), where: Z, Z' - O, NH; R2, R3, R and R' - linear or cyclic, substituted or unsubstituted aliphatic or aromatic radicals [197].
4.2.3 Application The stabilising systems on the basis of well-known inhibitors and their mixtures developed by various authors are presented in Table 4.3.
Table 4.3 Antiozonant (AO) systems for rubbers and specific protective properties No.
Stabilising system
Elastomer
Protecting ability
Reference
EPDM
Increasing the O3 resistance at elongation
[198]
1.
a) Para-phenylene-diamine (PPHDA) type AO; b) Imidazole
2.
a) N-Phenyl N´-isopropyl-PPHDA – (4010 NA) b) N-Pheny-B-naphthylamine (Neozone D)
Diene rubbers
O3
[199]
3.
a) Reaction products of acetone with diphenylamine b) Zn-2-mercapto-toluyl imidazole
EPDM
O2, O3, t0
[200]
4.
a) 2,6-di-tert-butyl-cresol b) 4,4a-thio-bis (6-tert-butyl)-mcresol (SC)
Styrenebutadiene rubbers (SBR)
O3, t0
[201]
286
Degradation and Stabilisation of Rubber
Nitrile rubbers
O3
[202]
a) N,N´-bis(2-hydroxy-4,6dimethylbenzyl)-piperazine b) 2,2a-methylene-bis (4-methyl, 6-tert-butylphenol) - (2246)
NR
t0
[203]
7.
a) Na-aluminium-methylsilicate
Carboxylate synthetic latexes
O3, nonstaining, nontoxic
[204]
8.
a) PPHDA type antiozonants – 1-4 php b) Imidazole – 0.2-4 php
Chlorosulfonated polyethylene
t0, O2
[205]
9.
a) PPHDA type AO – 0.1-5 php b) p-Di-nitrosoarene – 0.1 php
NR
O3, O2
[206]
10.
a) PPHDA type AO b) Bis-maleimide a:b=80-20:20-80
Vulcanisates
O3
[207]
11.
a) N,N´-Diphenyl-PPHDA, 0.1-5 php b) Petroleum type waxes, 0.1-10 php
Isoprene rubber
O3, nonstaining
[208]
12.
a) Polymerised 2,2,4-trimethyl1,2-dihydro-quinoline (Flectol H) b) 4010 NA c) N-Phenyl N´- (1,3dimethylbutyl) - PPHDA (S13) a:b:c = 50:25:25
NR
O3, inhibitor of flex cracking, antifatiguing
[209]
13.
a) N,Na-Diphenyl-PPHDA b) N,Na-Ditoluyl - PPHDA a:b = 10-50:90-50
NR
O3, non-staining
[210]
14.
a) 4010 NA b) Montmorillonite
NR
O3
[211]
5.
Cd salts of I. ArNRArNR7-R1-COOH II. ArNHAr-R4-COOH III. OH(R5)(R6)ArR2 -COOH R - H, halogen atom, C1-C8 alkyl; R1 - C1-C20 alkylene, -CO-R2; R2 - C1-C20 alkylene; R3 - C1-C8 alkylene; R4 - -OCO-R2, R5 and R6 - H or C1-C8 alkyl, R7 - H or C1-C4 alkyl
6.
287
Ozonation of Organic and Polymer Compounds
15.
a) Phenothiazine b) N-Alkyl-N´-phenyl - PPHDA a:b = 20-60:80-40
NR
16.
a) Diisopropyl-di-thiophosphate b) Di-hydro-di-phenylpropane c) 4010 NA a:b:c = 1:0.5:0.5
SBR
O3, light protection
[213]
17.
a) S13 b) Flectol H c) 6-Anilino-2,2,4trimethylquinoline
Diene rubbers
O3, non-staining
[214]
18.
a) 4010 NA b) S13
Diene rubbers
O2, O3, t0
[215]
19.
a) Neozone D – 0.5-2 php b) 9-Phenyl-10-p-vinyl-phenylanthracene – 2-7 php
Vulcanisates
O3, t0
[216]
20.
a) Deca-cis-(nonylphenyl)hepta-cis-(di-propylenglycoloctaphosphite) b) N,N´-di-(2-octyl) – p-phenylenediamine a: 0.75-1.75 php b: 1.25-0.25 php
NR
O2
[217]
21.
a) N-iso-propyl-amino-di-phenylamine b) 2,6-Di-tert-butyl-phenol (Ionol)
Diene rubbers
t0
[218]
22.
a) Phenothiazine – 20-80% b) Flectol H - 80-20%
SBR
thermal, anti-fatiguing
[219]
23.
a) 2246 – 0.3-0.75% b) 2-Mercapto-benzimidazole (MBI) - 0.15-0.5%
NR
O2, O3
[220]
24.
a) 1,1,3-Tris-(2-methyl-4-hydroxy5-tert-butyl-phenyl) butane (Topanol CA) b) 4010NA a: 5-50; b: 95-50 wt%
Synthetic rubbers
O2
[221]
25.
a) 1,3-Bis-pyperidinomethyleneimidazoline-2-thion b) 2246
Diene rubbers
O3
[222]
288
O3
[212]
non-staining
Degradation and Stabilisation of Rubber
26.
a) S13 b) Flectol H
NR
O3
[223]
27.
a) N-(2-aminophenyl)phosphoramidate b) Zn-dihydrocarbyldihydrophosphate
NR and Synthetic rubbers
O3, non-staining
[224]
28.
a) S-Containing sulfur ester b) Amine or phenolic antiozonants
NR and Synthetic rubbers
t0, O2
[225]
29.
a) Topanol CA – 5-50 wt% b) Tributoxyethyl-phosphate - 5550 wt%
Synthetic rubber
O2, increase dispersity and penetration ability
[226]
30.
a) Diethyldithio-carbamate b) Co-diethyldithio-carbamate c) 4010NA
Diene rubbers
O3
227
31.
a) Phenylhydrazone b) Amine or S-containing AO
NR
O2
[228]
32.
a) MBI b) Flectol H
EPDM and diene rubbers
O3, O2
[229]
33.
a) 4010 NA b) Flectol H
Diene rubbers
O3, O2, anti-fatiguing
[230]
34.
a) 4010 NA - 1.7 php b) Flectol H - 1.3 php or c) 4010NA - 1.9 php d) SC - 1.1 php
Isoprene rubber, SBR
t0, O3
[231]
35.
a) 4010 NA b) Flectol H a:b = 1-2.5:0.5-2 php
NR, Isoprene rubber, and SBR
t 0, O 3
[232]
36.
a) 4010 NA b) SC a:b = 1-2.5:0.5-2 php
NR, IR, BSR
O3
[233]
37.
a) N,N´-bis(1,5-dimethylpentan)PPHDA (S77) b) SC a:b = 1-2.5:0.5-1 php
NR, Isoprene, SBR
O3
[234]
38.
a) S77 b) Flectol H a:b = 1:0.5 php
NR, Isoprene rubber, SBR
O3
[235]
289
Ozonation of Organic and Polymer Compounds
39.
a) S77 b) MBI a:b = 1-2.5:0.5:2 php
Isoprene rubber and SBR
O3
[236]
40.
a) 4010 NA b) Flectol H a:b = 1.7:1.3 php
SBR
O3
[237]
41.
a) MBI b) N-Phenyl Na-octyl-PPHDA
NR
anti-fatiguing, O3, t0
[238]
42.
a) 4010 NA b) Neozone D
Vulcanisates
t0
[239]
43.
a) Unsaturated cyclo-aliphatic carboxylated ester, 1 php b) Paraffin - 0.25 php
All rubbers
O3, non-odouring
[240]
44.
a) MBI b) Castor oil
Chloroprene rubbers
O3
[241]
45.
a) 2246 - 0.5 php b) Tetrakis (2,4-di-tertbutylphenyl-4,4a-diphenyl) phosphite – 0.25 php
NR, SBR
O 3, t 0
[242]
46.
a) PPHDA type AO– 1-3 php b) Paraffin – 1-5 php
Vulcanisates
O3, t0
[243]
47.
a) S13 - 1 php b) Naphthenes - 0.5-10 php
Diene rubbers
O3, t0
[244]
48.
a) Di-tert-butyl phenols - 1-5 php b) Zinc salt of mercaptoimidazole - 2-8 php
EPDM
t0, O3
[245]
Note: EPDM: Ethylene propylene diene terpolymer; t0: temperature resistance
We have developed methods for evaluating the antizonant’s efficiency, a series of synergetic stabilising systems for rubber mixtures, highly stabilising elastomers compositions for sidewalls and protectors of light and heavy pneumatic tyres (PT) and highly effective stabilising compositions for hose faces and chambers [247-257]. By applying Equation 1.4 the rate constants of six industrial antiozonants of PPHDA type, were determined and their values are listed in Table 4.4 [258].
290
Degradation and Stabilisation of Rubber
Table 4.4 Rate constants of PPHDA-type antiozonants at 20 ºC in CCl4 No.
Antiozonant
k s 10-7 (M-1.s1)
1.
N,N´-Dinitroso-di-(C7-C9)-alkyl-PPHDA
0.5
2.
N,N´-Di-(2-ethylhexyl)-PPHDA
0.7
3.
N-Phenyl-Na-iso-propyl-PPHDA
1.2
4.
N,Na-Di-(1,4-dimethylpentyl)-PPHDA
1.3
5.
N-iso-propyl-PHDA
1.4
5.
Polymerised 2,2,4-trimethyl-1,2-dihydroquinoline*
2.2
6.
N-Phenyl-N´-cyclohexyl-PPHDA
2.4
* quinoline type antiozonant
An express method for determination of the antiozonant’s efficiency depending on the rubber type is developed. The antiozonants activity is evaluated by comparison the values of the rate constant. This method includes the separate determination of the molecular masses of rubbers in solution with and without antiozonants and of the corresponding amounts of absorbed ozone. The antiozonant efficiency is defined as a ratio between the amount of consumed ozone in an elastomer solution containing antiozonant and the amount of reacted ozone in the antiozonant-free solution (Figure 4.7) through which an equal change of the molecular mass of both the solutions is attained. The registration of the changes begins at initially equal molecular masses, further goes on with maintaining the ratio between the amount of absorbed ozone in solution containing antiozonant and the antiozonant amount in the range from 0.1 to 1 and ends at values of the antiozonants activity vary from 1 to 3. The quantitative determination of the antiozonants efficiency was estimated from the relationship [259]: Ef = $G2/$G1
(4.5)
where: $G1 - the amount of consumed ozone changing the molecular mass of sample 1 from M1 to M2; $G2 - the amount of absorbed ozone changing the molecular mass of sample 2 from M1 to M2, $G2 values were selected in the region whereby the ozone uptake varying from 0.1 to 1 mol corresponds to 1 mol antiozonants.
291
Ozonation of Organic and Polymer Compounds
4.0
3.5
3.0
2.5
2.0
2
1 1.5
G1
G2
1.0 0
5
10
15
20
25
-5
G.10 , mol
Figure 4.7 Variation of the relative viscosity of SKD (0.6 g/100 ml solvent) – (1) and the same solution containing 3.7 mM 4010 NA - (2) at 20 oC
On the basis of the synergistic stabilising system including N,N´-bis-1,4-dimethylpentylp-phenylenediamine (S77) and polymerised 2,2,4-trimethyl-1,2-dihydroquinoline (Flectol H) in a ratio from 0.5 to 2 and in amounts ranging from 1.5 to 4.5 ppm we have prepared a highly stabilising rubber mixture (Table 4.5). The behaviour of model samples was evaluated under conditions of ozone ageing (30 oC, ozone - 6 ppm and elongation deformation of 20%) and atmospheric ageing (20% elongation deformation on a roof in Sofia during the summer in 1982) and the results obtained are shown in Table 4.6. Among the investigated combinations, only system No.3 exhibits synergistic effects. Applying the same proportions and conditions we have also evaluated the synergistic properties of the following systems: No.4 - S77/Sc [4,4-thio-bis-(2-methyl-6-tertbutylphenol)] - 1.8/1.1 php; No.5 - 4010NA/SC = 1.9/1.1 php and No.6 - 4010NA/ Flectol H = 1.7/1.3 php (Table 4.7).
292
Degradation and Stabilisation of Rubber
Table 4.5 Rubber compositions under vulcanisation conditions: 153 oC, 25 min No.
Ingredients
Base sample No.1 (php)
Base sample No.2 (php)
New system No.3 (php)
1.
Natural Rubber (NR)
10.0
10.0
10.0
2.
Bulex M-27 (SBR)
20.0
20.0
20.0
3.
Bulex 1500 (SBR)
30.0
30.0
30.0
4.
Isoprene Rubber (SKI-3)
40.0
40.0
40.0
5.
Pyrolen - highly aromatic resin
7.0
7.0
7.0
6.
Carbon black - PM-50
25.0
25.0
25.0
7.
Carbon black - PM-75
25.0
25.0
25.0
8.
ZnO
3.0
3.0
3.0
9.
Stearic acid
2.0
2.0
2.0
10.
PN-6SH - highly aromatic oil
10.0
10.0
10.0
11.
S77
3.0
-
1.7
12.
Flectol H
-
3.0
1.3
13.
2–Benzthiazolylsulfenmorpholid - Santocure MOR
1.2
1.2
1.2
14.
Sulfur
1.9
1.9
1.9
Table 4.6 Results from ozone and atmospheric ageing tests Sample
Ozone degradation, first cracks (min)
Atmospheric degradation, first cracks (months)
Atmospheric degradation, breaking down (months)
No.1
80
9
11
No.2
20
5
6
No.3
>120
>12
>12
293
Ozonation of Organic and Polymer Compounds
Table 4.7 Results from ozone and atmospheric ageing tests Sample
Ozone degradation, first cracks (min)
Atmospheric degradation, first cracks (months)
Atmospheric degradation, breaking down (months)
No.4
280
-
>12
No.5
120
9
>12
No.6
230
9
11
The stabilising system S77/Vulcanox MB-2A (2-methyl-mercaptobenzimidazole) = 2.9/1.0 php was studied in two compositions: 1) SKI-3 - 50.9; Bulex M-27 - 50.0; carbon black - PM-50 - 45.0; Highly aromatic oil - PN-6SH 5.0; ZnO - 5.0; stearic acid - 2.0; vulkazit CZ (2-cyclohexylmercaptobenzothiazole)-1,2 and sulfur -2.2; 2) SKI-3 -100.0; PM-50 - 45.0; PN-6SH - 5.0; ZnO - 5.0; stearic acid - 2.0; Vulcazit CZ - 1.2 and sulfur - 2.3 (php), vulcanised at 150 oC for 15 and 10 minutes, respectively. These samples with new rubber compositions have shown higher stability than standard ones. That statement is confirmed by the fact that the values of physicomechanical parameters, such as relative elongation and tensile strength are kept unchanged longer during artificial ageing. The samples elongated to 20% at two different temperatures (70 oC and 95 oC) for 21 days. The proposed synergetic stabilising systems were applied for the development of universal reception for PT sidewalls and protectors with high antiozonant and antioxidant stability, containing complex elastomer compositions with reduced content of NR and high PMP: 1) Sidewalls: NR - 10.0; SKI-3 - 40.0; Bulex M-27 - 20; PM-50 - 25.0; Carbon black PM-75 - 25.0; Highly aromatic resin - Pyrolen - 7.0; PN-6SH - 10.0; ZnO - 3.0; stearic acid - 2.0; microcrystalline wax (Ozonshuzvax 111 - BASF) - 2.0, Santocure MOR - 0.9 and sulfur - 1.9 (php) vulcanised at 153 oC for 25 minutes. The rubber specimen resistance towards ozone and atmospheric ageing is found to be superior by 29-40% and 50% during exploitation and storage conditions, respectively, as compared with the usual rubber compositions. 2) Protectors: SKI-3 - 25.0; Bulex 1500 - 50.0; SKD-25 - 25.0; carbon black PM100 - 65.0.; Pyrolen - 2.0; ZnO - 3.0; stearic acid - 2.0; petroleum bitumen
294
Degradation and Stabilisation of Rubber (Rubrax) - 5.0; Ozonshuzvax 111 - 2.0, Santocure MOR - 1.2 and sulfur - 1.9 (php) vulcanised at 153 oC for 25 minutes. The resistance of new rubber samples against artificial ozone and atmospheric impact, is dependent on the stabilising system. Applying a similar approach we have proposed some highly stabilised elastomer compositions for hose sidewalls and chambers possessing high resistance to water and air action. On the basis of the synergetic stabilising systems developed we put forward the following rubber compositions: 1) Hose sidewalls: Bulex 1500 - 100.0; PM-6SH - 9.0; ZnO - 3.0; stearic acid - 1.5; microcrystalline wax - 2.0; Vulcazit CZ- 1.0 and Sulfur - 1.9, vulcanised at 160 o C for 25 minutes. 2) Hose chamber: Oil filled butadiene-styrene rubber (SKMK-30ARKM-15-made in Russia) - 100.0; PM-50 - 90.0; PM-75 - 5.0; PN-6SH - 8.0; ZnO - 3.0; Asphalt 7.0; Stearic acid - 2.0; Vulkazit CZ - 1.3 and Sulfur - 2.0, vulcanised at 160 oC for 15 minutes. 3) Hose chamber: SKMK-30ARKM-15 - 92.0; high molecular styrene resin (KER 1904) - 8.0; PM-94 - 94.0; PN-6SH - 1.4; asphalt - 11.0; ZnO - 2.5; stearic acid - 2.8; Vulkazit CZ - 2.1; N-cyclohexyl-thio-phthalimide - 0.45 and sulfur - 1.8, vulcanisation conditions - 160 oC and 15 minutes. The resistance of the proposed mixtures for sidewalls and chambers during accelerated thermal (70 oC) and ozone ageing is improved if compared to the commonly used stabilising compositions. Thus, depending on the stabilising system they are superior by 1.7 times for the faces mixtures and from 2.0 - to 4.0 times for the chamber ones. The degradation process in real conditions is closely related to two main reactions: oxidation and ozonolysis. The first one is initiated by temperature, static and dynamic deformations, light, radiation, electrons action, defects in the solid body, initiators, catalysts, oxy-reduction agents, bases, acids, and so on. The monomolecular conversions of Rv, ROv and RO2v - radicals or in some cases of the ion intermediate are among the degradation factors which should be taken into account. The ozonolysis degradation is influenced, mainly, by the ozone concentration, deformations and defects and to a smaller extent, by temperature due to the relatively low values of Ea of the ozone reaction of the C=C bonds and the zwitterion reactions. In this case the mono- and bimolecular reactions with zwitterions participation and the peroxide bonds decomposition which replace the strong C=C bonds formed in ozonolysis are responsible for degradation occurrence.
295
Ozonation of Organic and Polymer Compounds
4.2.4 High Molecular Antiozonants A number of highly efficient low molecular antiozonants because of their low solubility in polymers, volatility, blossom ability, and ability to be washed and hydrolysed cannot be used in many cases. In view of this, creation, proposing and application of highly effective high molecular antioxidants (HMAO) are quite modern investigations and provoke an intensive research. This is because the HMAO are able to overcome the forgoing disadvantages. The elastomer’s stabilisation by HMAO can be accomplished via two basic routes: by addition of preliminary prepared polymer stabiliser to the parent polymer or by chemical bonding of the active additive in the polymer matrix. The first approach for stability improvement is more and more often recommended. The ozone resistant polymer component may be polyvinylchloride polyvinyl chloriode (PVC) [260], ethylene-vinylacetate-copolymer [261], polychloroprene rubber copolymer of butadiene and butene n-butene, without C=C bonds [263], chlorosulfopolyethylene [264], and so on. However, recently the EPDM are attracting much attention, particularly, their application as protective polymers. This is associated with two main reasons: 1) highly protective action; 2) negligible effect on important vulcanisate parameters [265, 266]. Among them EPDM with ethylidenenorbornene or cyclopentadiene [267] as a third copolymer are the most affective antiozonants additives. The ratio between the EPDM concentration and the basic elastomer group varies from 1:9 to 30:70, mostly from 10:90 to 25:75 and the addition of up to 5 php of PPHDA to the rubber composition is highly recommended [268-271]. Andrews proposed that the HMAO particles mechanically prevent the growth of microcracks which are formed under ozone action [272]. This suggests that HMAO should form a continuous disperse medium which can block the microcracks formation and development in the phase of the second polymer. However, this factor is necessarily but not sufficient to ensure high ozone protective properties of HMAO. Another mechanism for protection includes the blocking of the C=C bonds by the HMAO molecular segments thus reducing the ozone assess to them. This can be realised only in case of high degree of mutual polymers dispersion, i.e., at the formation of monophase systems. A mixture of butadienenitrile rubber (SKN 40) and PVC [273] is such an example. In the general case of a two phase system the ozone-protective action of HMAO is determined by the dispersion level and their ability to cover the sample surface [273, 274]. Polymers containing antioxidant groups can be prepared by the following reactions:
296
Degradation and Stabilisation of Rubber 1. Interaction of polydienes with nitroso compounds, i.e., p-nitrosoamines [275], giving PPHDA bound to the rubber [276, 277]:
H2
H
+
H O -N
O =N
NH Ph
NH
NH Ph
NH Ph
The nitroso compound can be easily added to rubber simultaneously with the other ingredients. 2. Interaction of unsaturated polymers containing epoxy-groups with primary aromatic amines [278]: -CH2-CH-CH-CH2- + H2N-Ar
-CH2-CH-CH-CH2 HO NHAr
O
or with 2,6-di-tert-butyl-4-alkylphenols derivatives [279]: t.-But -CH 2 C H CH CH 2 - + HX ( CH2 ) n O X = -NH-, -OO-, -O-; n = 2-3
OH t.-But
H2 C HC X( CH2 ) n HCOH
t.-But OH . t.-But
H2 C
Mostly, the HMAO obtained by modification via reaction (1) are equal or superior to N-phenyl-B-naphthylamine (Neozone D) and practically do not differ and even in some cases are superior to N,Na-diphenyl-p-phenylenediamine, 4010NA, etc., while those prepared by reaction (2) are similar in their antiozonants action to 2,6-di-tert-butyl-4-methylphenol (Ionol). There are also other polymer similar conversions for introduction of stabilising groups. During the functionalisation of butadiene-metacrolein copolymer with aromatic amines and phenols [280] or treating of hydrochlorinationed cis-1,4-polyisoprene with 2-tert-butyl-resorsinol, the efficiency of the HMAO obtained is higher than that of Neozone D, 2,2-methylene-bis-(4-methyl-6-tertbutylphenol)-2246 and that of the HMAO based on the diene elastomer treated with the antioxidant with aliphatic unsaturation such as N-(4-anilinophenyl)-
297
Ozonation of Organic and Polymer Compounds metacrylamide (CH2=CH-CO-NH-Ph-Ph). The reaction can be conducted in solution, emulsion or in solid polymer. The stabiliser is added through stirring in a mill or closed mixer for 5-10 minutes at a temperature ranging from 20 to 160 oC in the presence of peroxides initiator [281]; phosphorisation of diene rubber with P2S5 and further treatment with aromatic amines [282] resulting in functionalising of the polymer with functional groups of the type: CH3 -CH2-C=C-CH2-CH2S=P-NH-
containing P-S and N-P bonds which are energetically weaker than the C-C and C-H bonds. 3. Copolymerisation of diene monomers (butadiene with styrene, acrylonitrile, isoprene, 2-chlorobutadiene) in the presence of a third monomer with groups capable of inhibiting the oxidation processes [283-285] such as compounds with secondary aminogroups, i.e., Ph-NH-Ph-CO-C(CH3)=CH2 or phenol type antioxidants [286, 287], i.e., HO-Ph-CH2-CH2-O-CO-C(CH3)=CH2. The mechanism of the protective action of HMAO is determined by: 1) The more efficient termination of the chain in oxidative degradation and ozone scavenging as a result of the more uniform distribution of HMAO. 2) Structurising of the polymer during recombination of HMAO molecules and elastomer radicals or its zwitterions [289]. 3) The itramolecular synergetic action in polyfunctional HMAO. Some other factors, like P-conjugation, configuration, conformation and supramolecular structures in elastomers [288] also affects the HMAO efficiency. The kinetics of HMAO stabilisation is controlled by the simultaneous presence of the stabiliser and elastomer radical/zwitterion in the kinetic cage [289] rather than by the HMAO diffusion in the polymer matrix which is the case with lowmolecular antiozonants. The protective action of HMAO based on nitroso compounds is due to the deactivation of the impurities catalysing the oxidation, for example iron, as a result of the formation of complex compounds [275]. HMAO are most widely applied for stabilising
298
Degradation and Stabilisation of Rubber vulcanisates which are continuously exposed to the avtion of higher temperatures, various solvents, oils, water and static load under operating conditions.
4.2.4.1 Ethylene-Propylene Rubber With Diene Monomer (EPDM) Recently the EPDM are considered as the most promising HMAO for vulcanisates [290-293]. However, EPDM addition can result in deterioration of PMP of the vulcanisates which is mainly due to: different compatibility of the vulcanising agents in EPDM and the diene elastomer and the insufficient compatibility of the elastomers used. This problem can be successfully solved by the proper choice of the vulcanising groups which can ensure most similar kinetics of elastomers vulcanisation. In our studies we have used Keltan 312 (dicyclopentadiene as a third monomer) as a HMAO since it has a higher rate of vulcanisation at temperatures over 150 oC. Based on literature data three vulcanisation groups have been selected: sulfur/2morpholinothiobenzothiazole (MBS) (I); sulfur/MBS/dibenzothiazoldisulfide (Altax) (II) and sulfur/diphenylguanidine (DFG) (III). Their efficiency was evaluated on rubber mixtures based on: Bulex, SKD and SKI-3 and their combinations [290, 294-298]. The kinetic curves obtained by the standard method with Moncanto TM-100 rheometer at 155 oC show that similarly to other elastomers Keltan 312 undergoes an effective structurisation under the influence of the vulcanising systems used (Table 4.8). The addition of Altax to (I) forming a system (II) increases the rate constants of vulcanisation by more that 7% and the structurising effect for all rubbers including Keltan 312 is more than 25% (Table 4.9, deformation at 300% elongation). It has been found that systems I and II are very suitable for covulcanisation of Keltan 312 with Bulex 1500 and SKD and to a smaller extent with SKI-3, namely, due to the different vulcanisation rates. These are large differences in the vulcanisation rates using system III. The small differences in the vulcanisation rate of Keltan 312 with diene elastomers favours the formation of homogenous vulcanisation structure which preserve or improve their PMP. The optimum ratio of Keltan 312/diene elastomers, according to our studies, should not exceed 20/80 as the best ratios are found in the range of 5/95-15:85. On the basis of the results obtained we have proposed rubber compositions for PT protectors in diene rubber/Keltan 312 ratio of 95:5 thus reducing the SKI-3 content. PMP of the vulcanisates with I and II are listed in Table 4.10.
299
Ozonation of Organic and Polymer Compounds
Table 4.8 Rate constants of rubber mixture vulcanisation (k) with various vulcanising groups (I-III) at 155 oC No.
Elastomer
I, k (min-1)
Ratio
II, k (min-1)
Ratio
III, k (min-1)
Ratio
1.
Bulex - M27
0.19
1.46
0.22
1.50
2.30
8.80
2.
Bulex - 1500
0.14
1.10
0.14
1.00
1.97
7.60
3.
SKD
0.31
2.40
0.38
2.70
4.57
13.30
4.
SKI-3
0.75
5.80
0.81
5.80
3.00
11.50
5.
Keltan 312
0.13
1
0.14
1
0.26
1
Table 4.9 PMP of vulcanisates based on Bulex 1500 (1), SKD (2), SKI-3 (3) and Keltan 312 (4) in dependence on the vulcanising system (I and II). Property
Unit
1I
2I
3I
4I
1II
2II
3II
4II
Force of strain
kg/cm2
186
130
222
167
202
140
237
170
Tension at 300% elongation
kg/cm2
75
52
73
91
90
58
87
117
Relative elongation
%
555
460
565
455
540
455
545
445
The ozone resistance of the studied protector compositions increases in the presence of Keltan 312 in the elastomer composition (Table 4.11). It is seen that the type of the vulcanisation group does not affect the ozone resistance of the vulcanisate. The analysis of the results reveals that the amount of Keltan 312 in the PT protector compositions should be up to 5 php. This content ensures high PMP and enhances substantially their ozone resistance. Among the vulcanising groups used, sulfur/MBS/ Altax = 1.9/0.7/0.5 turns to be the most appropriate one.
300
Degradation and Stabilisation of Rubber
Table 4.10 PMP of protective type vulcanisates of elastomer compositions Bulex 1500:SKD:SKI-3 = 50:27:23 (1) and (1):Keltan 312 = 90:10 (2) with vulcanising systems: I - sulfur/MBS = 1.9/1 php and II - sulfur/MBS/Altax = 1.9/0.7/0.5 php 1I
2I
2II
Time of prevulcanisation at 130 oC (min)
48
51
40
Force of strain (kg/cm2)
175
150
162
Tension at 300% elongation (kg/cm2)
102
70
85
Relative elongation (%)
515
490
500
8
14
10
62-63
60
60
Elasticity (%)
25
23
23
Strength of tear (kg/cm)
5.2
4.9
4.9
Gurvich residual deformation (%)
14.6
8.9
10.2
o
40
40
41
Property
Residual prolongation (%) Shore A hardness
Heat formation ( C - according to Gurvich)
Table 4.11 Ozone resistance of elastomer compositions from Table 4.10 stabilised by 4010NA/Flectol H = 1.7 + 1.3, at 6 ppm of ozone, 30 oC Time (min)
1I
2I
2II
20
A1T1
45
A1T2
A1T1
A1T1
130
A1T3
A1T2
A1T2
365
T4 - break down
A1T3
A1T3
T4 - breakdown
T4 - breakdown
660
4.3 Apparatus for Ozone Resistance Determination The ozone resistance of vulcanisates is tested by various accelerated methods both under laboratory and real conditions [299, 300]. The laboratory methods are divided
301
Ozonation of Organic and Polymer Compounds into two groups. The first one, using up to 6 ppm ozone, is intended for testing of natural rubber, polybutadiene, polyisoprene, butadiene-styrene rubbers which are usually more susceptible to ozone action and the second one, with 0.01-0.15% ozone concentration, is applied for testing of ozone resistant rubbers such as polychloroprene rubber, butylrubber, and so on. The time of cracks appearance (t0) and breakdown (tC) depends on E and [O3] and in the different standards they are evaluated at various deformations and ozone concentrations. This is the reason why the results from the different tests can not be properly compared [301-303]. The apparatus for testing are with fixed parameters and do not allow the tests to be conducted according to the different standards. This has stimulated us to design new more suitable equipment which could make possible the application of different standards along with selection of appropriate test conditions [304]. The scheme of the apparatus developed by us for evaluation of the vulcanisate’s ozone resistance is depicted in Figure 4.8.
220 V
8
7
9 6-10 kV atm
O2
1
O2
2
3
O2
4
O 3 /O 2
5
6
Figure 4.8 Block scheme for ozone resistance determination of vulcanisates. 1 - oxygen source; 2 - reducing valve; 3 - gas dryer; 4 - ozonator; 5 - test chamber; 6 - UV analyser of ozone; 7 - stabiliser; 8 - autotransfomator; 9 - high voltage transformer
The ozone chamber with capacity of 20 litres is equipped with an ozone inlet and outlet system and ventilator for equalisation of ozone concentration.
302
Degradation and Stabilisation of Rubber The frames for deformation of the samples were designed according to our original schemes. In one frame can be placed up to eight samples. Three frames can be placed in the test chamber, or a total of 24 samples can be tested simultaneously. Ozone was prepared by passing oxygen or air (preliminary dried) through a pipe generator at an electrode voltage of 5-8 kV. The ozone analysis was carried out spectrophotometrically at 254 nm by means of an ADS-3 (double bond analyser, produced at the Institute of Chemical Physics, Russian Academy of Sciences). The device sensitivity is about 10% volume. The samples are rubber strips with size 25 x 5 x 1 mm. The ozone concentration may vary from 4 to 10-5 vol%, and can reach 200%. We have investigated the ozone resistance of Bulex-1500-based vulcanisates by means of the designed test unit. The sample compositions were as follows: Bulex1500 - 100, carbon black (PM-75) - 50, ZnO - 3, PN-6SH - 3, N-cyclohexyl-2benzothiazolsulfenamide (CZ) - 1.2, sulfur - 1.8 and 4010NA - 0, 1.5, 2, 2.5, 3 or 3.5 php, vulcanisation at 160 oC for 10 and 15 minutes. The test results are demonstrated in Figure 4.9.
14
2.0
12
2.5
10
1.5
8
1.0 6
0
4 2 0
0
10
20
30
40
50
, %
Figure 4.9 Dependence of the time of crack breakdown on elongation at 42 ppm ozone, 20 oC and different concentrations of 4010NA
303
Ozonation of Organic and Polymer Compounds As Figure 4.9 shows, the fastest breakdown of the samples takes place at E = 1015% - the range of critical deformation. Before and after that point, the curves are increase relativly fast. That experimental fact can be explained with appearance of critical phenomena [299]. The stabilisation of vulcanisates at higher E is also related to the decrease of the degradation rate as a result of the increase of the system potential energy due to the hindered sp2-sp3 transition [305]. At E 35 h
-
-
7.
S77/Flectol H (P)
155
A1T1
32
8.
4010NA/Flectol H (P)
140
A1T1
32
9.
S77/SC (P)
23
A1T2
32
10.
4010NA/SC (P)
>32 h
-
-
311
Ozonation of Organic and Polymer Compounds
4.5 Prediction The ageing and deterioration of PMP of manufactured rubber articles during their storage and exploitation is related to the change of the molecular mass of the polymer molecules. Among the numerous chemical and physical factors resulting in polymer degradation, as it has been already mentioned, two basic factors should be outlined.
4.5.1 Oxidation Oxidation proceeds via two reaction routes: a mainly radical pathway, which includes the formation of alkyl (R), alkoxy (RO) and peroxy (RO2) radicals and in some particular cases, ionic which gives cations, anions, metals and metalloid species at different oxidation states. The reactions of monomolecular decomposition, cyclisation and disproportionation are the basic reactions which lead to molecular mass reducing. It is known that any oxidation process begins with the reaction of chain initiation, i.e., the reaction of free valence generation. Some of the initiators of oxidation degradation are: UÊ ÝÞ}i]Êâi UÊ V
iV>ÊÌ>ÌÀÃ UÊ Ìi«iÀ>ÌÕÀi UÊ }
Ì UÊ À>`>Ì UÊ iV
>V>Ê>VÌ UÊ V>Ì>ÞÃÌÃ UÊ `iviVÌÃ UÊ ÃÕÌ>iÕÃÊ>VÌÊvÊÌ
iÊÌ>ÌÀÃÊ`iÃVÀLi`Ê«ÀiÛÕÃÞ Temperature, defects and mechanical effects are the main initiators for the oxidation of rubber articles - PT and rubber goods during their usage.
4.5.2 Ozonolysis Ozonolysis takes place when ozone attacks the elastomer double bonds. The reaction is accelerated by the presence of defects and application of mechanical deformations 312
Degradation and Stabilisation of Rubber while temperature has an insignificant effect. The main reactions responsible for occurrence of degradation process are the reactions with participation of polymer zwitterions. They replace the strong C=C bonds with the relatively weak peroxide bonds. The latter are rapidly decomposed thus decreasing the molecular mass. Polymeric zwitterions can also interact with low molecular compounds or undergo monomolecular decomposition which also results in reduction of the molecular mass. On predicting the variations in the service properties of manufactured articles one should bear in mind the proceeding of these two reactions. Considering the prolonged duration (years) of storage and exploitation of rubber goods, it is very important to look for and propose accelerated tests for estimation of their behaviour thus providing reliable information for prediction of the storage and usage terms of rubber products. Some accelerating tests are applied: UÊ iÌiÀ>ÌÊvÊÌ
iʫ
ÞÃViV
>V>Ê«>À>iÌiÀÃÊ**®Æ UÊ Ìë
iÀVÊ>}i}Ê>ÌÊÌi«iÀ>ÌÕÀiÃÊvÊÇäÊ°C, 85 °C and 100 °C for 56 days. The estimation of PMP involves the determination of the change of two parameters: tensile strength and relative elongation in % at a definite temperature: aT= (1 - AT/A0) x 100%
(4.7)
where: Ao and ATare the values of the parameter before and after ageing time T. The experimental results from the accelerated test according to this standard are shown in Figures 4.13a and 4.13b. It is obvious that the stabilising system that preserves the PMP of the samples for a more prolonged time at the three temperatures studied is more efficient in its protective action. We have analysed the results obtained and established that the change of A can be quite reliably described by the exponential law: AT= A0.exp(-kT)
(4.8)
where: k is the rate constant of A change and then: a = 1 - exp(-kT)
(4.9)
313
Ozonation of Organic and Polymer Compounds 40 0
90
0
100 C
35
100 C 80 0
30
85 C
70 0
85 C
25
60 50
20 0
15
0
70 C
40
70 C 30
10 20
5
10
0
0
-5
-10
0
10
20
30
40
50
60
0
10
20
Time, days
30
40
50
60
Time, days
(a)
(b)
Figure 4.13 Variation of tensile strength (a) and relative elongation (b) in time depending on the temperature at E = 20% for Bulex-1500 based vulcanisate with 3 php Santoflex 77
The experimental points of Figure 4.13 are linearised when plotting in coordinates ln(1-a)/T) ((Figures 4.14a and 4.14b).
0.0
0.0
-0.1
-0.4 0
70 C
0
70 C
-0.8
-0.2 0
-1.2
85 C
0
-0.3
85 C -1.6
-0.4
0
100 C
0
100 C 0
10
20
30
40
50
-2.0
60
Time, days
(a)
0
10
20
30
40
50
60
Time, days
(b)
Figure 4.14 Semilogarithmic anamorphosis of experimental points of Figure 4.13, where ln(S) is strength and ln(E) is relative elongation
The value of k estimated from the curve tangent is (3, 5 and 7) x 10-3 days-1 and (11, 20 and 20) x 10-3 days-1 for the three temperatures and the two parameters, respectively. The dependence of k on temperature is described by the Arrhenius equation: k = k0.exp(-Ea/RT)
314
(4.10)
Degradation and Stabilisation of Rubber The activation energy of the parameter - tensile strength and that of the relative elongation amounts to 7.25 and 8.3 kcal/mol, respectively, (Figure 4.15). The calculated values of k at 20 oC for the parameters are 5 x 10-4 and 1.4 x 10-3 days-1, respectively. According to the standard requirements, any product whose PMP properties have not been changed by more than 20% during storage, i.e., up to values of ln (Ao/ AT) = 2.23, can be used in practice. Then the time of its suitability upon storage, the temperature of which does not usually exceed 20 oC, can be determined by the following expression: T20 = 2.23/k
(4.11)
where: k is a constant at 20 oC.
-3.6
-4.0
2 -4.4
-4.8
-5.2
1
-5.6
-6.0 2.65
2.70
2.75
2.80 3
2.85
2.90
2.95
-1
1/T.10 , K
Figure 4.15 Arrhenius relationships of the rate constants on temperature
Applying the values of k at T20 for vulcanisates in Figure 4.13 it has been found out that the variation in the tensile strength and relative elongation values by 20% will be reached after 12 and 5 years. These values are in good correlation with those observed by us during the long-term storage of these unforced vulcanisates (stored from 1982 at temperatures of 20-25 oC). We have registered 18% and 34% change of these two parameters after 12 years storage. This observation gives us reason to assume that this accelerated test can be successfully used for predicting the service properties of rubber products in real storage conditions:
315
Ozonation of Organic and Polymer Compounds UÊ }i}ÊÊ>Ìë
iÀVÊi`ÕÊÜÌ
ÊVÀi>Ãi`ÊâiÊVÌiÌÊÕ`iÀÊÌ
iÊvÜ}Ê conditions: temperature of 30 p 2 oC, 3 ppm ozone and tensile deformation E = 20%, registering the time of the first crack appearance, the breakdown time and the change of the crack pattern. UÊ }i}ÊÊ>ÀÊ>ÌÊÀi>ÌÛiÞÊ
}
ÊÃÌÕÀiÊiÛiÃÊ>`ÊiiÛ>Ìi`ÊÌi«iÀ>ÌÕÀi° UÊ }i}ÊÊ>ÀÊ>ÌÊVÞVVÊÌi«iÀ>ÌÕÀi]Ê16Ê>`Ê,ÊÀi}i° UÊ }i}ÊÊÌ
iÊ«iÊÕ`iÀÊ-w>ÊV>ÌVÊV`ÌÃÊ>ÌÊE = 20%. UÊ iÌiÀ>ÌÊ vÊ «iÕ>ÌVÊ ÌÞÀiÊ ÃÌÀ>}iÊ ÌiÀÊ >VVÀ`}Ê ÌÊ Ì
iÊ iÌ
`Ê vÊ accelerated thermal ageing. At present the usage and storage terms are determined on the basis of the results observed during the real terms of PT storage and exploitation. These terms are 3 and 5 years for car and truck tyres, respectively, that corresponds to 60,000 and 60,000 km. According to the Russian standard these values amount to 45,000 and 70,000 km for car and truck tyres, respectively. The corresponding terms are guaranteed by the standard PMP values of vulcanisates and PT element specimen, the simulation tests of randomly selected PT from production and the observations related to the real life of PT. The analysis of PMP of the protected mixtures reveals that the modern compositions and materials’ properties can ensure a working guarantee period of 80,000 and 100,000 km for car and truck tyres, respectively. The existent standard guarantees 45% and 60% of them leaving a considerable reserve. The main factor for such a high reserve is due to the insufficient level of the tyres stabilisation. Thus, during the accelerating run test, the serial tyres of 165SR/13 size are broken down at 4,500 p 400 km and of 9.00R/20 size at 2,500 p 400 km, while prepared with new stabilising system give 15.5% and 24% more distance, respectively. Next are presented the recipes and the acceleration run test parameters of the new tyres with new stabilising system: UÊ /
iÊÌÞÀiÃÊvÊÃâiÊ£Èx-,É£ÎÊ>`Ê°ää,ÉÓä\ The stabilising system of sidewalls and protectors is unified and it is: Antiozonants: 4010NA – 1.9 php and Santowhite Crystals – 1.1 php; Parffin wax - Ozonschutzvaxe 111 - 2.0 php;
316
Degradation and Stabilisation of Rubber The two tyre sizes are broken down at 5200 p 300 km run and 3100 p 200 km run at a speed of 80 km/h during the acceleration test, respectively. These data clearly manifest the great significance of the stabilisation of rubbers and rubber materials and the effect of the new stabilising systems offered by us. In our opinion the conformity with PMP is a necessary but not sufficient condition for evaluation of the future behaviour of rubber-based articles due to the unobligatory application of accelerated ageing tests. The standard values of PMP guarantee the protection of the rubber products against harmful factors arising from the different road conditions. However, the prediction of the useful life of rubber products could not be laid on a scientific basis without knowledge of the tendency and namely of the rate of PMP variations under the effect of various degradation agents. The physicomechnical properties are closely related to the elastomers molecular mass and its preservation with time [325, 326]. Based on general considerations the rate of degradation of unstabilised diene elastomer will result in 100-fold loss of molecular mass for one year, i.e., from 105-6 to 103-4, which will lead to its complete breakup of the article. For these reasons the adoption of accelerated methods for evaluation of degradation and for predicting purposes, as obligatory is of great practical importance. Actually it represents a simplified version of the modification developed by us, of BS 8800-77. For that purpose the vulcanisates are subjected to thermal ageing at three temperatures T1, T2 and T3, 50, 70 and 90 oC (100 oC). The ageing duration is determined by the variation of a defined PMP by 80% at the highest temperature applied. First the curves describing the change of PMP with time at the three temperatures are plotted afterwhich they are replotted in coordinates log(1/T)(1/T) where (is the time of 40% change of PMP for the three temperatures. Thus, across the points corresponding to 10, 30 and 50% change of PMP at 70 oC are drawn lines parallel to the line with coordinate (1/T40)(1/T2). Thus the nomogram obtained allows the extrapolation at various temperatures and PMP values. The application possibility of this nomogram is demonstrated by the data in Tables 4.15 and 4.16.
317
Ozonation of Organic and Polymer Compounds
Table 4.15 Experimental (T8090, T8050) and calculated (T2025) storage terms of vulcanisates for protectors and sidewalls containing different stabilising systems T8090 (days)
T8050 (days)
T2025 (days)
Side wall - car tyre*
162
1300
2250
Protector - car tyre*
53
600
1125
Side wall - truck tyre*
126
1010
1900
Protector - truck tyre*
41
410
770
2-5-02A
220
1800
3100
2-5-02B
260
2100
4000
2-5-02C
196
1570
2910
2-5-02D
197
1580
2960
1-5-02A
156
1600
3000
1-5-02B
135
1400
2625
1-5-02C
120
1200
2250
1-5-02D
127
1270
2380
Vulcanisate
Note: *: recipe of the tyre plant ‘Vida’-Bulgaria originally from the French Company - Rhone-Poulenc. Symbols 1-5-02 (A-D) are protector compositions proposed by us for car and truck tyres after an unified reception for car and truck tyres protectors were prepared containing: Bulex 1500 - 50; SKI-3 - 25; SKD - 25; carbon black PM-100 - 65; ZnO - 3; stearic acid - 2; softener PN-6SH - 13; Rubrax - 5; Pyrolen -2; MBS - 1.2; Sulfur - 1.9; stabiliser - 3 php: A - 4010NA /Flectol H = 1.7:1.3; B - 4010NA/SC = 1.7:1.3; C S77/Flectol H = 1.7:1.3 and D - S77/SC = 1.7:1.3. Symbols 2-5-02 (A-D) denote the vulcanisates composition developed by us for sidewalls of car and truck tyres: NR - plasticated - 10; SKI-3 - 40; Bulex 1500 - 30; Bulex M27 - 20, Carbon black PM-75 - 25; Carbon black PM-50 - 50; ZnO - 3; stearic acid - 2; PN-6SH - 13; Pyrolen - 7; MBS - 0.9; sulfur - 1.9; stabiliser - 3 php: A - 4010AN/Flectol H = 1.7:1.3; B - 4010NA/SC = 1.7:1.3; B - S77/Flectol H = 1.7:1.3 and D - S77/SC = 1.7:1.3).
The simultaneous application of this method and BS 8800-77 for evaluation the behaviour of static dynamic strained specimen coupled with the data from ozone and atmospheric ageing can be regarded as a reliable base for prediction the storage and exploitation terms of tyres.
318
Degradation and Stabilisation of Rubber
Table 4.16 Experimental (T8090, T8050) and calculated (T2025) storage terms of produced tyres (their potectors and sidewalls) containing different stabilising systems T8050 (days)
T2025 (days)
Test run (km)
134 82 127 67
1071 820 960 670
2000 1540 1800 1260
4500 2500
165-SR-13 (SW) - recipe A 165-SR-13 (P) - recipe A 9.00-R-20 (SW) - recipe A 9.00-R-20 (P) - recipe A
188 128 191 132
1500 1150 1530 1320
2810 2400 2870 2480
5100 3100
165-SR-13 (SW) - recipe B 165-SR-13 (P) - B 9.00-R-20 (SW) - B 9.00-R-20 (P) - B
198 154 182 135
1590 1540 1460 1350
2980 2890 2740 2530
5300 3000
165-SR-13 (SW) - C 165-SR-13 (P) - C 9.00-R-20 (SW) - C 9.00-R-20 (P) - C
192 113 300 125
1530 1130 2400 1250
2880 1950 4500 2340
5300 -
165-SR-13 (SW) - D 165-SR-13 (P) - D 9.00-R-20 (SW) - D 9.00-R-20 (P) - D
190 119 300 115
1520 1190 2400 1150
2850 2230 4500 2160
2000
Tyres
T8090 (days)
VIDA - base 165-SR-13 (side wall (SW)) 165-SR-13 (protector (P)) 9.00-R-20 (SW) 9.00-R-20 (P) NEW
4.6 Efficiency of Antiozonants Under the Conditions of Various Deformations Data from the literature show that the efficiency of antiozonants is decreased at application of large deformations [327]. However, the intimated studies have not been carried out and there is no generally accepted view on this problem. We have investigated the behaviour of the following elastomers: SKI-3 with cis-1,4units content of 95%, unsaturation level - 94-98%; SKS-30, trans-1,4-units - 33%,
319
Ozonation of Organic and Polymer Compounds cis-1,4-units - 43.4%, 1,2-units - 23.3%, ash - 0.2% and residual monomers content of 0.15% wt in a broad range of stretch deformations using some of the most widely used antiozonants: PPHDA, hydroquinoline, dithiocarbamates, mono, bis and trisphenol compounds. The rubbers were pre-purified as follows: dissolution in CCl4 (1% solution) and precipitation by adding a three-fold volume of methyl alcohol followed by drying. The stabiliser was added to the concentrated rubber solution before preparing the films for it’s uniformly distribution over the whole sample volume. Then the rubber specimen without sulfur and promoters were subjected to radiation vulcanisation with a 20 MRad dose (Co source) at intensity of 0.9 MRad/h, while the sulfur-containing samples were vulcanised at 150 - 1700 C for 10 - 20 minutes. The degree of crosslinking the samples was determined by the swelling method [328]. The IR spectra (the method of multiple attenuated total reflection (ATR) with KRS-5 crystal number of reflections - 20, 45o, penetration 10-40 Mm) were monitored during the exposure duration of the specimen to ozone atmosphere - 10 ppm and 20 oC. The IR-spectra of butadiene-styrene rubber in a transsmition (1) and reflection (2) mode is given in Figure 4.16.
Figure 4.16 IR spectra of SKS-30, 1 – transmission 2 – reflection.
320
Degradation and Stabilisation of Rubber IR ATR-spectra has a good resolution and can be used for quantitative studies. The correlation time (Tc) that stands for the rotation mobility of the nitroxyl radical was estimated on the basis of the ESR spectra of the paramagnetic marker - 2,2,6,6tetramethyl-pyperidine-1-oxyl - b1017 spin/cm3, introduced through evaporation [329]:
Tc = 6.65.$H.[(I+/I-)1/2-1].10-10
(4.12)
where: $H - the width of the lowfield spectrum components; I+, I- - intensity of the final spectrum components, in high and lowfield, respectively. The stress relaxation in the rubber specimen in ozone atmosphere (50 ppm) was measured at a temperature of 17 oC in the 5-160% deformation range. Based on the experimental curves the time of chemical relaxation (T), which is inversely proportional to the chemical relaxation rate vr , can be estimated using the following expression [330]:
S/S0=exp(-t/T) ((1-t/T),
(4.13)
where: S is the equilibrium deformation obtained by extrapolation to zero time in the linear part on the relaxation curve; t - time. The initial parts of the relaxation curve allow the determination of the physical relaxation. In order to study the effect of antiozonants in a strained specimen it is necessary to know the structural changes occurring in the rubbers under the influence of applied deformations (ratio between the amorphous and the crystalline phase). Though the crystallisation processes in cis-1,4-polyisoprene rubbers are well studied [328], due to the specific degree of crosslinking at the doses applied by us, we have to define the deformation range for this particular case. Visible reflections of the crystalline phase appear at deformations over 300% (Figure 3.5a, b). All tests were conducted in 0-100% deformation range whereby the rubber crystallisation is of minor significance [328]. It is well known, [331] that the rates of many chemical reactions in polymers depend on the segmental mobility of the macromolecules. For studying the influence of the stretching deformations on the segmental mobility some experiments with rubber samples containing a nitroxyl radical were carried out. The dependence of the correlation time of the marker - radical on the elongation degree E is presented in Figure 4.17.
321
Ozonation of Organic and Polymer Compounds It is seen from the figure that the value of Tc for SKI-3 samples does not change with E which testifies the lack of change in the rotational diffusion of the marker - radical in the used deformation range. However, with SKS-30 (Russia), a decrease of the rotational diffusion of the nitroxyl radical is observed as the value of Tc rises with the increasing of the E. These results suggest that tensile deformation in SKI-3 up to 100% does not change substantially the value of Tc, and consequently the rotational mobility of the antiozonants, whose molecules are very similar in structure and composition to that of the probe-radical. Moreover the translational mobility of the latter does not change too since the rotation and translational diffusion are interconected [332].
5.5
2
5.0
1 4.5 4.0 3.5 3.0 2.5 0
20
40
60
80
, %
Figure 4.17 Dependence of the marker - radical correlation time of (Tc) on elongation level (E). 1 - SKI-3; 2 - SKS-30
Data from the literature [333] clearly shows the close relationship between the antiozonant’s efficiency and its rate constant in its reaction with ozone. In Table 4.7 are listed the rate constants (k) which we have measured by using the bubbling method [334]. The rate constants of ozone reaction with amine stabilisers are about 2-3 orders higher than those with olefins and about 3-4 orders as compared with phenol stabilisers (No.13 and 14). Among all the stabilisers studied only N-dithiocarbamate exhibits a similar antiozonant efficiency (No.15). The difference in their reactivity
322
Degradation and Stabilisation of Rubber towards ozone is related to their different structure and various donor properties of the substituents. However, it can be accepted that they exhibit similar protective ability with respect to ozone, particularly if one considers that in real conditions the diffusion of antiozonants plays a very important role. Upon radiation vulcanisation the antiozonants are stable and only in amines with long alkyl groups, the alkyl chain is cleaved (No.4 and 6) with preservation of the PPHDA structure while at 200 oC (the maximum vulcanisation temperature) in an inert atmosphere the amine does not undergo any changes.
Table 4.17 Rate constants of ozone with antiozonants in CCl4 at 25 oC No.
Antiozonant
k x 10-6 (M-1.s-1)
1.
4-Methyl, N,N´-bis (iso-propyl)-m-PHDA
24
2.
N-Phenyl, N´-iso-propyl-PPHDA
12
3.
N-iso-propyl-p-anisidine
14
4.
N,Na-Bis (1,4-dimethylpentyl)-PPHDA
13
5.
N,Na-Bis-(1,3-dimethylpentyl)-PPHDA
100
6.
4-Methyl, N,N´-bis-(2-ethylhexyl)-m-PHDA
7.3
7.
N-Phenyl, N´-1,3-methylbutyl-PPHDA
25
8.
N,N´-(Dinitroso, di-sec-octyl)-PPHDA
5
9.
N-Phenyl-B-naphthylamine
20
10.
N-Phenyl, N´-cyclohexyl-PPHDA
24
11.
N,N´-Bis (phenyl)-PPHDA
27
12.
Diphenylamine
40
13.
2,2a-Methylene-bis-(6-tert-butyl-4 ethylphenol)
5 x 10-3
14.
2,2a-Methylene-bis-(6-tert-butyl-4-methylphenol)
4 x 10-3
15.
Ni-bis-(N-dibutyldithiocarbamate)
16.
2,2,4-Trimethyl-1,2-dihydroquinoline
17.
2,2a-Thio-bis-(6-tert-butyl-4-methylphenol)
1 0.8 4.5 x 10-3
323
Ozonation of Organic and Polymer Compounds The efficiency of antiozonants was evaluated by: 1) the kinetics of C=O bonds formation, and, 2) the deformation reduction. The rate of change of the 1720/1640 cm-1 ratio, depending on the stabilising system used is depicted in Figure 4.18.
2
60 50 40 30
3 20
6 10
0
1
7
0
45 0
10
20
30
40
50
, %
Figure 4.18 Initial rates of C=O accumulation in SKI-3 in dependence on antiozonants (1-7) or without antiozonants (0) at [O3] = 1 x 10-6 M, 20 oC
Upon ozonolysis of the SKI-3 samples without antiozonant (0) the accumulation of the reaction products was linear in the whole range of deformations applied. At the same time the constancy in the value of $D for antiozonants No.1, 4, 5, 6 (the designation from Table 4.17) indicates the absence of any visible chemical changes on the sample surface. Antiozonants No.2, 3 7 are also efficient in the absence of deformation. It should be noted that $D rises at relatively small values of (while further increase in stress deformation results in a decrease of the rubber oxidation level (Figure 4.19). The existence of critical deformation was also observed for the pure rubber samples and the specimen containing antiozonants No.2, 3 and 7.
324
Degradation and Stabilisation of Rubber
1.4 1.2 1.0 0.8
2
0.6 0.4 0.2
1
0.0 0
10
20
30
40
50
, %
Figure 4.19 Dependence of the oxidation level of SKI-3 (D1720/1640) on E: 1 without antiozonants; 2 - in the presence of 4010NA
As has already been mentioned this phenomenon is characterised by a maximum crack propagation rate and a minimum longevity being observed in a certain range of tensile deformations [327, 328]. The critical elongations were investigated by means of relaxometer. The relaxation curves of ozone-induced SKI-3 samples are demonstrated in Figure 4.20. Highest rate of stress decay is observed in unstabilised rubber sample at E = 20% (the curve is not shown in Figure 4.20) while in the presence of antiozonants under the same conditions the rate of stress decay declines slightly with time. It has been established that at higher deformations (50%, 100% and 160%) the presence of antiozonants in the rubber specimen reduces their ozone resistance as manifested by the relatively higher relaxation rate than the expected one. This supposition is exemplified by the dependence of logarithm of the chemical relaxation time on the tensile deformation without antiozonants (1) and with antiozonants (2) shown in Figure 4.21. It is clearly seen that the presence of antiozonants exerts a substantial stabilising effect in the initial range of small deformations.
325
Ozonation of Organic and Polymer Compounds
1.0
0.8
0.6 160% 0.4
100% 50%
0.2
20% 0
10
20
30
40
Time, min
Figure 4.20 Stress relaxation (S/S0) in SKI-3 at different E on ozonation duration. Antiozonants - No.1 = 2 s 10-6 M
4.0
1
3.5
2
3.0
2.5 0
50
100
150
, %
Figure 4.21 Dependence of the chemical relaxation on samples without antiozonants (1) and with antiozonant addition (2)
326
Degradation and Stabilisation of Rubber
4.7 Effect of Vulcanisate’s Structure The antiozonant efficiency also depends on the chemical structure of rubbers [335, 336]. Different points of view are put forward with respect to this relationship. For example, one of them supposes that the stabiliser activity is related antibatic to its ability to form hydrogen bonds with polymers [337]. For evaluating the role displayed by the chemical nature of elastomers on antiozonants efficiency we have investigated several rubbers specimen - NR, SKI-3, SKD and SKS-30. Mixtures with the following composition: Elastomer - 100 php, ZnO - 3.0, Stearin - 1.0, Vulcazit CZ - 1.0, sulfur - 1.8, antiozonants - 3.0 were vulcanised at 160 oC for 15 minutes. We have studied the following antiozonant systems: No.2, 4, 15, 16 and 17 (Table 4.17). We have measured the relaxation time (T) which is inversely proportional to the crack growth rate vp [337]: vp = -u(S/S0)/uT = -(S0/Sd).u(F/F0)/uT,
(4.14)
where: F - the stress applied to the sample cross section S; S0 - initial stress; Sdequilibrium stress resulting from the fast physical relaxation. Figure 4.22 shows typical curves of stress decay in the presence of ozone. Curve 1 is assigned to the occurrence of fast physical relaxation taking place within several seconds, up to one minute and does not change with time. It is characterised by a slight decline at the beginning which is related to the physical re-orientation of the macromolecules and the constancy of the parameter F/F0 with the time after that. This region designated in the figure as (1) is observed in all curves. In the presence of ozone (curves 2 and 3) the pattern changes radically. Moreover, curve 2 clearly shows three regions: I - fast physical relaxation up to 1 minute, II - chemical relaxation up to 5 minutes, a linear stress decay is observed accompanied by visible changes on the sample surface, and consequently with cracks originating on them, III - after 5 minutes - chemical relaxation which accelerates the decay of stress and is associated with cracks appearance and propagation. If one assumes the rate of region II as unity, then the acceleration for region III will be 3.2 times. Curve 3 is characterised by an absence of region II since immediately after region I (physical relaxation) portions of cracks grow at a constant rate. The slow down of the stress decay is about 1.7 times as compared with the rate for curve 2, region II and remains constant up to 18 minutes. The greater elongations change the surface rheology making it energetically more isotropic and thus the risk of appearance and growth of separate cracks to sizes resulting in sharp decrease of the sample resistance is decreased (region II in curve
327
Ozonation of Organic and Polymer Compounds
2). The addition of antiozonants (curve 4) leads to marked expansion of region II with almost zero value of change rate, up to 15 minutes, and in region III (after 15 minutes) the rate of relaxation decay is by a factor of 2 smaller than that of region II in curve 2.
1.0 1
II I
0.8
III
II
III 4
0.6
3
III
0.4
0.2
2 0
5
10
15
20
25
Time, min
Figure 4.22 Relaxation curves of SKI-3 samples without stabiliser and with stabiliser addition in air and ozone atmosphere - 9 x 10-4 mol/m3 at 20 oC. 1 - without antiozonants, in air, E = 10%; 2 - without antiozonants, in ozone atmosphere, E = 10%; 3 - without antiozonants, in ozone atmosphere, E = 100%; 4 - in the presence of 5 php of 4010NA, in an ozone atmosphere, E = 10%
Another parameter characterising the process of ozone interaction with rubber samples is the change in crack growth rate (vp) with E (Figure 4.23). The rate of cracks growth depends on the ozone concentrations as it is shown in Figure 4.24.
328
Degradation and Stabilisation of Rubber 30 25 20
5
15 10
3 5 0
4
6
2 1 0
20
40
60
80
100
120
140
160
180
, %
Figure 4.23 Dependence of vr on E for SKI-3 without antiozonants (curves 1, 3, 5) and in the presence of antiozonants - 4010NA (curves 2 and 4), S77 (curve 2) and N-iso-propyl-p-anisidine (curves 2, 4, 6) in amounts of 1, 3 and 5 php, respectively, at [O3] = 5 x 10-4 mol/cm3 - (curves 1 and 2), 6.4 s 10-2 (curves 3 and 4) and 9 x 10-4 mol/cm3 (curves 5 and 6). Note: the experimental points for antiozonants - 4010AN and S77 on curves 2 and 4 could not be seen for their values are very close to those of the third antiozonants.
1 -1.0
2
-1.5
-2.0
-2.5
-3.0
-3.5
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
3
lg [O3], mol/m
Figure 4.24 Dependence of log vp for SKI-3 samples, E = 20%, at 20 oC on log [O3]; 1 - without antiozonants; 2 - in the presence of 3 php of 4010AN
329
Ozonation of Organic and Polymer Compounds It is seen that the value of T' is constant in the stationary zone of stress decay and has the same meaning as the relaxation time T' in Tobolski’s formula [338]. Figure 4.25 shows the dependence of the value of log n on the degree of tensile deformation in the vulcanisates SKI-3 (a), NR (b), SKD (c) and SKS-30 (d). A similar pattern is observed only in two cases: SKI-3 and NR. This similarity is not only qualitative but also quantitative both for the control (curve 1) and the protected samples. The shape of all the curves, irrespective of the presence of a stabiliser, is also quite similar; the lowest stability (the highest rate of cracks growth rate of cracks) is observed in the range of critical deformations E = 10-20%. However, with an increase in the degree of strething the stability increases (the parameter Tc rises).
8.5
b a
7.0
8.0
2 0
7.5
6.5 4, 14 17
6.0
7.0
4
16
6.5
5.5
2
6.0
0
5.0
15
16
5.5 4.5
5.0 0
4.0 0
20
40
60
80
100
120
140
20
40
60
160
80
100
120
140
160
, %
, %
8.5
7.5
d
c
8.0
2
7.0 0
15 16
6.5
17 6.0
7.5
4
7.0
5.5
6.5
5.0
6.0
4.5
5.5
4.0
5.0
17 0 15 14 16
4.5
3.5 0
20
40
60
80
, %
100
120
140
160
0
20
40
60
80
100
120
140
160
, %
Figure 4.25 Relationship between log nT and the degree of deformation E in specimen from SKI-3 (a), NR (b), SKD (c) and SKS-30 (d) at ozone concentration of 6.4.10-8 M, (the number of the curves correspond to the number of the stabiliser in Table 4.17)
330
Degradation and Stabilisation of Rubber The stability increase can be explained by the decrease in the reactivity during the extension of cis-fragment -C-C=C-C- in all the reactions accompanied by the rearrangement of two carbon atoms at the double bond from sp2 into sp3 state [339] and it is well known that ozone attacks the double bond with the formation of a primary ozonide. There also exists another interpretation of the ‘critical deformation’ phenomenon, based on the mechanism of orientation strengthening [335]. However, as shown by ESR spectral studies performed with the use of a marker radical [340, 342], at least up to 200%, the tensile deformation of such elastomers of SKI-3, NR and SKD does not change the polymer matrix rigidity (segmental mobility of macrochains remains constant). That is why in these cases orientation strengthening cannot, in our opinion, play any significant role. A different situation is registered in the case of SKS-30. The stretching of its vulcanisate raises substantially the rigidity of its polymer matrix [340]. It is with the structuralphysical changes that the rise in (value with the growing E) observed in Figure 4.25c, seems to be associated. An efficient stabiliser for SKI-3 and NR under the chosen testing conditions is the widely applied antiozonant - 4010NA. In the whole range of deformations this stabiliser slows down the growth of cracks, which follows from the comparison of curves 2 and 1 in Figure 4.26 (curve 2 lie above curve 1). This slowing down, however, is not very great: in SKI-3 the vr values decreases by a factor of approximately 1.6, in NR - 1.3. Compositions containing other stabilisers are less resistant to ozone effect under these conditions, as compared with the control samples. The similarity in the results obtained for SKI-3 and NR is caused by the identity of their monomer unit. The significant role of the macrochain chemical nature is confirmed by the difference in the results obtained for elastomers with other monomer units. A relatively small difference in the structure of cis-1,4-polyisoprene (the presence of CH3-substituent) leads to a quite significant difference in the stabiliser efficiency. Thus, it is seen from Figure 4.18c that SKD vulcanisate is stabilised much better than SKI-3 and NR. Apart from 4010AN, other stabilisers prove also to be efficient in the whole deformation range: No.4 (Figure 4.26b, curve 3) and No.15 (the curve is not given because it is identical to curve 1 and lies above it), in the region of small deformations the stabilising effect is manifested by No.16 (up to 60%, curve 4) and No.17 (up to 20%, curve 5). It should be emphasised that for SKD samples especially a decrease in the efficiency of stabilisers with the growing of E is a characteristic. The only exception is antiozonants No.15. Table 4.18 illustrates the stabilising effect, expressed as a degree of a decrease
331
Ozonation of Organic and Polymer Compounds in the crack growth rate at various deformations of samples, as compared with samples that contain no stabilisers.
8
8
SKI-3
NR
2
7
2
7
1
5 1
3, 5, 6
6
3, 4, 6, 7 6
7 4 5
5
4
4
3
3
0
50
100
150
0
50
Elpngation, %
100
150
Elongation, %
9
9
SKS-30
SKD 8
2
3
3
8
5
7
7
6
5 5
4 1
4
1
2, 4, 7
6 5
6
4
3
3
0
50
100
150
0
Elongation, %
50
100
150
Elongation, %
Figure 4.26 Dependence of log nT on the degree of deformation E in: a) SKI-3; b) NR; c) SKD; d) SKS-30, [O3] = 6.4 s 10-2 mol/cm3, 20 oC . The numbers of the curves correspond to the numbers of the samples in Table 4.17
Table 4.18 Degree of a decrease in the crack growth rate in SKD samples in the presence of a stabiliser, compared with samples without antiozonants, at different deformations Antiozonant
E= 20%
E= 30%
E= 60%
E= 100%
No.4
-
14.7
5
2.4
No.2
10
3.6
1.4
1.5
No.16
8.1
2.2
1.1
0.7
No.15
1.4
1.6
1.6
1.6
332
Degradation and Stabilisation of Rubber We have observed a decrease in the antiozonant efficiency with an increase in the degree of cis-1,4-polyisoprene deformation. Thus, in Figure 4.25b the highest stability effect of 4010NA (curve 2) is displayed for E = 10% (two-fold decrease in vp; and at larger deformations the effect decreases. But, as it follows from the above data, that this effect is much more pronounced in the case of SKD. Therefore, one can speak of a different nature of log nTc = f(E) dependencies obtained for the protected vulcanisates of butadiene and isoprene rubbers (Figure 4.25 a-c). For SKS-30 vulcanisate one can note the following peculiarities. This elastomer, as well as SKD, is stabilised by No.4 in the whole range of E (curve 3). Inhibiting effect is also exerted by No.17 at all deformations (curve 5), although to a smaller extent than No.4. All other stabilisers slow down ozone degradation only at small E (up to 20-30%), and at large deformations their presence has an opposite effect - accelerate the cracks growth. Another similarity of SKS-30 with SKD manifests itself in a decrease in the efficiency of stabilisers with the growth of E, although it is not so substantial as in SKD. This is demonstrated by the data on the decrease in vp in strained SKS-30 samples in the presence of antiozonants (Table 4.19).
Table 4.19 Degree of a decrease in the crack growth rate in SKS-30 samples in the presence of a stabiliser, compared with samples without antiozonants, at different deformations. Antiozonant
E = 10%
E = 20%
E = 60%
E = 100%
E = 150%
No.4
10
8
5
4
3
No.17
6.7
-
1.4
1.3
1.7
No.14
2.3
0.8
0.8
0.8
0.8
No.15
2.3
1.2
0.8
0.8
0.8
In our opinion, the results presented in Figure 4.20 cannot be explained by the quenching of ozone molecules by the stabiliser, i.e., with the help of a mechanism based on a higher antiozonants reactivity with respect to ozone in comparison with the polymer C=C bonds. This follows from the data obtained related to the influence of the macromolecule chemical structure on the stabilisers efficiency. Most clearly this is seen for No.2 and 4. The rate constants of their reactions with ozone are practically the same and equal to 1.2 x 107 and 1.3 x 107 M-1.s-1, respectively, [341, 342]. Their protective effect, however, differs considerably and depends on the type of the polymer. Indeed, stabiliser No.2 raises and No.4 lowers the stability of SKI-3 and NR samples, whereas for SKD and SKS-30 stabiliser No.4 is a very efficient one.
333
Ozonation of Organic and Polymer Compounds These facts can only be explained by taking into account the active role played by the polymer matrix itself. It should be noted that this role is hardly caused by the intermolecular interaction of the polymer with the stabiliser, since the elastomers in question are by their nature of low polarity. From the standpoint of intermolecular interaction it is difficult to explain the considerable difference between SKI-3 and NR, on the one hand, and SKD and SKS-30, on the other. Most probably, the stabilisers enter into a chemical reaction with the products of the polymer oxidative degradation (crosslinking of the cut-off fragments). In this case the nature of the monomer unit can predetermine the course of the rate of such a reaction, thereby influencing the antiozonants efficiency. From this point of view it proves possible to explain the No.4 efficiency in the two cases of SKD and SKS-30 because they have similar chemical fragments resulting from butadiene polymerisation. And, possibly, it is with the products of ozone oxidation of these very fragments that a given stabiliser interacts effectively. In polyisoprene chains a similar chemical process most probably does not take place at all or occurs with appreciably lower rate. Thus, it seems that, as noted above, two factors (in the case of polyisoprene samples) are being superimposed [343]: the interaction of antiozonants with ozone and the formation of a protective film on the polymer surface, whereas in the presence of butadiene units another means of protection appears - crosslinking of the brokendown antiozonant fragments. The latter, though more effective, are quite sensitive to deformation. Indeed, stretching must facilitate the drawing apart of the molecule cut-off ends and thereby impede their crosslinking. The conclusion put forward for the active role of the chemical nature of the polydiene monomer unit is in agreement with the data from the literature data. It is known that the two end aldehyde groups resulting from the ozonation of the polybutadiene monomer unit are capable of reaction with diamines, thus restoring the broken polymer chain. The ozonation of the polyisoprene monomer unit leads to aldehyde and ketone grups formation. That is the reason why diamines cannot restore the length of the polyisoprene macromolecule [345]. In conclusion it should be noted that the ozone degradation of diene polymers is a result of ozone reaction with the C=C bonds in the polymer macromolecule whereby the C=C bond is broken or is replaced by the weak peroxide bonds in ozonides and in di- and polyperoxides. The most susceptible areas to ozone attack are the defects on surface, amorphous and interphase regions in the elastomer structure. The application of external deformations in all cases accelerates the ozone degradation transforming the strain energy during C=C bonds cleavage. The critical deformations, in our opinion, are determined by the initial rates of ozone interaction with the elastomer and the restoration of the degraded surfaces in addition to the mechanisms cited in literature.
334
Degradation and Stabilisation of Rubber The ozone stabilisation of elastomers is accomplished by using antiozonant systems which can impede the harmful ozone effect by exerting simultaneous protective action, both chemically and physically. They include ozone quenching, C=C bonds and ozone deactivators, stucturising bi- and polyfunctional compounds, which connect the cut-off ends of the macromolecules and resulting in fast stress relaxation, compounds that facilitate the antiozonants transport from the volume to the reaction centre, physical deactivators and concentration diluters.
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5
Quantum Chemical Calculations of Ozonolysis of Organic Compounds
Quantum chemical calculations of organic compound ozonolysis are continuously expanding and cover more and more new compounds and separate stages in already accepted mechanisms of interaction [1-10]. The reaction of ozone with C=C bonds [11-20] is one of the reactions that has attracted much attention. Here should be mentioned the fundamental contributions of Godart [20-22], Cremer [6, 7, 24], Rouz [17, 19], etc., who have applied various ab initio methods. A number of important and interesting ozone reactions, however, have remained out of the scope of research interest [21-25]. In this connection we devoted our studies on quantum chemical calculations of the ozonolysis of organic compounds containing H, O, S, N and P atoms. The transition states (TS) depending on the atom to which the ozone attack is directed, are denoted in the following manner: hydrogen bound to carbon atom - TS1, hydrogen bound to heteroatom - TS2 and heteroatom - TS3. We have carried out the calculations by using a MOPAC6 program, of which the details, parameterisation and practical use are described in references [26-83].
5.1 Alkanes The calculations were performed according to the method of the reaction coordinate, whereby all coordinates of the reagents are optimised with the exception of the fixed reaction coordinate. The latter varied from 3-4 to 5 Å with a gradient from 0.2 to 0.05 Å. The transition states for the reactions investigated are described as follows: CH3-(H2)C-H...O3
(CH3)2(H)C-H...O3
(CH3)3C-H..O3
(1) Linear transition states that correspond to the hydrogen-atom abstraction mechanism; the C-H bond, being the bond of lowest bond energy, is the reaction centre of the molecule: CH3-H2C-H...O3
(CH3)2-HC-H...O3
(CH3)3C-H..O3
359
Ozonation of Organic and Polymer Compounds (2) Cyclic transition states, which accounts for the 1,3-cycloaddition mechanism, including a transitory state with 5-coordinated carbon and resulting in hydrotrioxide formation in one step:
C
H O
O O
A summary of the results from calculating the kinetics of the ozone reaction with methane, ethane, propane and iso-butane with linear transition states of the TS1 type are shown in Figure 5.1. The calculations with cyclic transition states are not given because we failed to observe the formation of any structure resulting from the C-H bond cleavage and formation of two new bonds C-O and O-H. The data in Figure 5.1 support the ozone reaction with C-H bonds. For methane, the C-H bond cleavage occurs at a reaction coordinate within 0.95 Å and 0.9 Å with a simultaneous drop of $H from 96.3 kcal to 75.3 kcal. In the case of ethane this takes place at a reaction coordinate from 1 Å to 0.95 Å and a change of $H from 79.8 kcal to 11.9 kcal. For the ozone reaction with propane the C-H bond break occurs at a reaction coordinate from 1.05 Å to 1 Å and $H changes from 61.7 kcal to 46 kcal. The reaction coordinate for iso-butane C-H bond fission lies between 1.05 Å and 1 Å and $H changes from 54.2 to –25.9 kcal. The analysis of the foregoing results reveals the following sequence of increasing reactivity of the studied alkanes: methane<ethane<propane