Cyclic polymers

CYCLIC POLYMERS

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CYCLIC POLYMERS (Second edition) Edited by

J. ANTHONY SEMLYEN University of York, U.K.

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

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>..F(MM$456!+#-4,-!N8#; >..F(MM! 90 %) when the polystyrene MW is less than 8000 but that the yield decreases for higher MW polymers. For example, at the cyclization yield is 57%, which is still very good when compared to the yields of other methods. The structure of the cyclic polystyrene has been confirmed by their MALDITOF spectra [35]. Macrocyclic polystyrene with and 35 have been prepared from bifunctional linear polystyrene [36]. The linear polymer is prepared from 3-lithiopropionaldehyde diethylacetal as the protected functional initiator and 2,2,5,5-tetramethyl-1-(3-bromopropyl)-1-aza-2,5-disilacyclopentane as the protected bifunctional terminating agent. Accordingly,

is obtained. The preparation of the linear polymer for the cyclization involves five steps and four purifications on silica gel to give

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The cyclization step proper involves the formation of an amide bond at high dilution. After silica gel chromatography the cyclic fraction is isolated with an overall yield between 30 and 35 %. 10.2.3 Cyclic diblock copolymers The obvious interest in cyclic block copolymers is in the comparison of their microphase morphology with the morphology of linear di- and triblock copolymers homologues. Cyclic polymers can only form double loops in strongly segregated microphases while linear triblock copolymers form loops and bridges. The earliest synthesis of a synthetic cyclic block copolymer predates the systematic studies of the 1980s [37]. An original combination of living anionic polymerization and equilibration cyclization was used. The isolated polystyrene containing fraction is a mixture of general formula

where n varies between 1 and 4. A more rigorous synthesis of cyclic PS-PDMS has been given recently [38,39]. The two-ended poly(dimethylsiloxane-bstyrene-b-dimethylsiloxane) (PDMS-PS-PDMS) is prepared with the Lithium counterion in THF. Dichlorodimethylsilane is the cyclizing agent. It is worth mentioning that alternative approaches were unsuccessful [38,39]. Nevertheless, with the adopted method it is still difficult to produce high MW monodisperse PDMS blocks. At high conversion and long polymerization times redistribution reactions begin to dominate the hexamethylcyclotrisiloxane polymerization [40]. The PDMS content is varied between 5 and 73 %. The cyclic nature of the diblock copolymer is deduced from the delayed elution in SEC. The and NMR spectra do not show resonances for end groups. It is likely that the higher MW samples are contaminated by linear homologues due to inadvertent termination reactions. In the same studies the synthesis of a

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linear CBABC and the corresponding cyclic ABCB polymer is described. In these polymers A is PS, B is 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane and C is PDMS. Ma and Quirk have synthesized cyclic poly(styrene-b-butadiene)s [41,42]. They used 1,3-bis(1-phenylethylenyl)benzene (DPPE) to prepare a dilithium initiator for the consecutive polymerization of butadiene and styrene in benzene in the presence of excess lithium alkoxide. The triblock copolymer was diluted 10-fold in cyclohexane and cyclized with dichlorodimethylsilane or with DPPE. The cyclic polymer was isolated and identified by its reduced intrinsic viscosity.

Deffieux et al. made poly(styrene-b-2-chloroethyl vinyl ether) copolymers from an anionically prepared polystyrene [43]. They converted the acetal to an ether function with trimethylsilyliodide and then polymerized the 2-chloroethyl vinyl ether monomer cationically. Ring closure was catalyzed by Cyclic block copolymers of MWs between 4000 and 7500 were obtained [43]. Ishizu and Ichimura prepared poly(isoprene-b-styrene-b-isoprene) and capped the copolymer with 3-bromopropyl groups [18]. An interfacial

cyclization technique was used to prepare the diblock ring. No rigorous proof of the final structure was given. The morphology of a number of cyclic diblock copolymers and their linear triblock parent polymers has been compared in the medium and strong segregation regime [44]. The lamellar spacing of the cyclic copolymers is 0.91 to 0.95 time smaller than the linear copolymer spacing. Such small difference is in agreement with theory but requires that utmost experimental care be used. 10.2.4 Tadpole or semicyclic polymers

Tadpole polymers are hybrid polymers consisting of a cyclic and one or more linear fragments. The homopolymer tadpole with one linear branch is completely defined by the mass fraction in the ring. In the case of block copolymer tadpoles several topological isomers are possible depending on the location of the different blocks in the polymer. Not all possibilities have been experimentally explored to date. Quirk and Ma prepared a cyclic polybutadiene with two linear polystyrene chains [41]. They coupled living polystyryllithium with DPPE. The still living polymer is used to initiate two chains of polybutadiene which are cyclized with dichlorodimethylsilane.

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It is acknowledged that the SEC properties of such polymers will depend considerably on the mass fraction in the cyclic segment. The separation from the linear or branched homologue is smaller the smaller the ring fraction in the polymer. Deffieux et al. worked out a strategy to prepare cyclic poly(2chloroethyl vinyl ether) with a polystyrene tail [45]. See Scheme 2.

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SCHEME 2.

The cyclization was achieved with The total molecular weight is in the range 6000 to 9000. More recently Kubo et al. were able to attach a linear polystyrene onto a preformed cyclic polystyrene [46]. The amide bond formed in the ring closure was used to react with an -carboxyl polystyrene. Schematically,

10.2.5 Bicyclic and tricyclic polymers

It is obvious that the synthetic control required for producing a sizable quantity of bicyclic polymer is even greater than for the cyclic polymer. Nevertheless, several groups took on this challenging synthesis. Antonietti and Fölsch prepared difunctional living polystyrene from sodium naphthalene in THP at -40°C [47]. The double cyclization is performed with 1,2bis(dichloromethylsilyl)ethane. The cyclic yield is about 30% when the cyclization is performed in a 0.5 % solution on a polymer with SEC analysis is inconclusive. However, the doubling of the molecular weight was proved by light scattering and osmotic pressure measurements. The small size of the bicyclic polymer prevented accurate size determination. Some geometric restriction is also seen in the broadened spectrum of the bicyclic relative to the linear polymer. Madam et al. reacted two-ended living

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polyisoprene with

From the SEC analysis a polymer contraction was

observed but the product seems not to have been isolated and further characterized. The association of the lithium carbanion in the cyclization medium is held responsible for the exceptionally good yield of the bicyclic polyisoprene. Kudo at al. followed a more classic organic chemistry approach [46]. Their anionically prepared polystyrene ring is closed by an amide bond and this was reduced to a secondary amine. Two rings are then linked through a glutaric acid bridge. Deffieux and his collaborators approached the synthesis of bicyclic polymer differently. They started from a tetrafunctional initiator with two acetal and two styrenic functional groups. Two acetal functions are used to produce two-ended poly(2-chloroethyl vinyl ether) by cationic polymerization [48]. In the cyclization step, the acetal ends of the precursor are activated to a carbocationic group that rapidly reacts with the two remaining styrenic groups

of the initiator. The linear and cyclic polymer can be isolated and their properties can be compared. Relatively small bicyclic polymers of 2500 < MW