Copolymerizations Involving Dienes

The ability to determine which copolymerization model best describes the behavior of a particular comonomer pair depends on the quality of the experimental data. There are many reports in the literature where different workers conclude that a different model describes the same comonomer pair. This occurs when the accuracy and precision of the composition data are insufficient to easily discriminate between the different models or composition data are not obtained over a wide range of experimental conditions (feed composition, monomer concentration, temperature). There are comonomer pairs where the behavior is not sufficiently extreme in terms of depropagation or complex participation or penultimate effect such that even with the best composition data it may not be possible to conclude that only one model fits the composition data [Hill et al., 1985 Moad et al., 1989]. 

The sequence distributions expected for the different models have been described [Hill et al., 1982, 1983 Howell et al., 1970 Tirrell, 1986] (Sec. 6-5a). Sequence distributions obtained by 13C NMR are sometimes more useful than composition data for discriminating between different copolymerization models. For example, while composition data for the radical copolymerization of styrene-acrylonitrile are consistent with either the penultimate or complex participation model, sequence distributions show the penultimate model to give the best fit. 

The termination rate constants and molecular weights for the different copolymerization models have also been studied for purposes of discriminating between different copolymerization models [Buback and Kowollik, 1999 Landry et al., 1999]. 

The first case is the copolymerization of monomer A with diene BB where all the double bonds (i.e., the A double bond and both B double bonds) have the same reactivity. Methyl methacrylate-ethylene glycol dimethacrylate (EGDM), vinyl acetate-divinyl adipate (DVA), and styrene-p- or m-divinylbenzene (DVB) are examples of this type of copolymerization system [Landin and Macosko, 1988 Li et al., 1989 Storey, 1965 Ulbrich et al., 1977]. Since r = Yi, Fi = f and the extent of reaction p of A double bonds equals that of B double bonds. There are p[A] reacted A double bonds, p[B] reacted B double bonds, and p2[BB] reacted BB monomer units. [A] and [B] are the concentrations of A and B double bonds, 

A second case is the copolymerization of A and BB in which the reactivities of the two groups are not equal but are, instead, r and rx, respectively. In this case the critical extent of 

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Copolymerizations involving dienes such as the copolymerization of isobutylene with isoprene are important from the industrial point of view (3,4). Isoprene acts as a strong chain-transfer and terminating agent in the carbocationic polymerization of isobutylene (161), and at high concentration it leads to a cross-linked, insoluble product (180). Because of the limited composition range available for analysis, determination of the reactivity ratios in this system is rather difficult. Reactivity ratios published for the isobutylene/isoprene system are listed in Table 7. 

An alternative explanation by Korotkov 254) is available and involves complexa-tion of the active centers by the diene involved. This involves diene solvation of the active center (or centers for the associated species) where it was assumed that styrene is less effective in solvation. Since the advance of this concept by Korotkov 254) in 1958, subsequent evidence, which has been noted previously in this review, has accrued which indicates that aggregated carbon-lithium species can indeed form such complexes via interaction with it-electrons. Obviously, though, the role, if any, of monomer solvation in these copolymerizations remains to be elucidated in detail. 

It is known that crown ether possesses the ability to form stable complexes with alkali cation and also the ability to solubilize these salts in hydrocarbon solvents Furthermore, crown ether/metal cation complexes can serve as catalysts in reactions involving ionic intermediates It is felt that such a system would provide us valuable information regarding the stability of allylic sodium Our early publication has achieved this goal This paper is a review of crown ether and its application in the areas of anionic polymerization and copolymerization of diene ... 

Ebdon and coworkers22 "232 have reported telechelic synthesis by a process that involves copolymerizing butadiene or acetylene derivatives to form polymers with internal unsaturation. Ozonolysis of these polymers yields di-end functional polymers. The a,o>dicarboxy1ic acid telechelic was prepared from poly(S-s tot-B) (Scheme 7.19). Precautions were necessary to stop degradation of the PS chains during ozonolysis. 28 The presence of pendant carboxylic acid groups, formed by ozonolysis of 1,2-diene units, was not reported. 

Structure and Composition of Diene Copolymers. One finds that most of the reported copolymerization studies on butadiene or isoprene involve styrene as comonomer. In part this is due to the early interest in styrene-butadiene synthetic rubbers. The free radical produced copolymers (GRS, usually about 20—25% styrene units) contain about 20% of its butadiene fraction in the 1,2 form. The ratio of 1,2 to 1,4 units is little affected by polymerization variables such as temperature, conversion and styrene content (39). Butadiene and styrene copolymers contain 50 to 60% 1,2-diene units when prepared by sodium catalysts at 50° (39). This behaviour is once more significantly different when lithium is used in place of sodium as can be seen in Table 3. 

Although the Pd-catalyzed alkene-CO copolymerization reaction must involve a series of acylpalladation reactions, it is outside the scope of this chapter. And, the readers are referred to recent reviews and pertinent references cited therein [27-29]. As such, the cyclic carbonylation reactions of dienes were of limited synthetic utility because of difficulties in controlling regiochemistry and other aspects of importance in fine chemicals synthesis. Whatever the reasons might have been, little had been reported further until the 1980s. 

The copolymerization of furan and 2-methylfuran with dienophiles such as maleic anhydride leads to polymer structures with furan pendent functionality. Furan, 2-methylfuran, and 2,5-dimethylfuran have been copolymerized with acrylic monomers (51,52) and acrylonitrile (52,53). The furan ring of furan, 2-methylfuran, and 2,5-dimethylfuran participates as a diene in a free radical copolymerization with acrylonitrile. The initial step for furan and for 2,5-dimethylfuran is the attachment of an acrylonitrile radical at the 2-position, but for 2-methylfuran, the attack is at the-5-position. Propagation proceeds by the attack of the furan radical on an acrylonitrile molecule, to leave one olefinic bond in the structure derived from the furan ring. If this bond is in the 4,5- or 2,3-position, it may be involved in a second additional reaction by the return of the propagating chain. 

Note that styrene and conjugated dienes can be copolymerized to yield statistical or block copolymers. The latter process, which involves additions of one monomer to a living polymer of the other monomer, is described in the following section. 

Pd complex-catalyzed copolymerization of alkene and CO affords the polyketones via alternating insertion of the two monomers [166, 167]. The polymer growth involves migratory insertion of CO into the metal-carbon bond as a crucial step, which is unique to the late transition metal complexes such as Ni, Pd, Rh, and Co. The copolymerization of allenes and methylenecydopropanes with CO has attracted much less attention than the alkene-CO copolymerization, although it would provide further functionalized polyketones due to the dual functionality of the dienes and the derivatives. 

The kinetics of copolymerization provides a partial explanation for the copolymerization behavior of styrenes with dienes. One useful aspect of living anionic copolymerizations is that stable carbanionic chain ends can be generated and the rates of their crossover reactions with other monomers measured independently of the copolymerization reaction. Two of the four rate constants involved in copolymerization correspond at least superficially to the two homopolymerization reactions of butadiene and styrene, for example, and k, respectively. The other... 

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