Polydienes copolymerization

The third possibility to prepare graft copolymers is termed grafting onto . This means that a growing chain B attacks the polymer backbone A with formation of a long branch. This attack can be a chain-transfer reaction or a copolymerization with unsaturated groups, for example, in polydienes. These reactions play an important role in the preparation of high impact polystyrene (see... 

The presence of cross-associated species needs to be considered in the interpretation of copolymerization kinetics. It has been found 269) that the reaction of poly(butadie-nyl)lithium with p-divinylbenzene in benzene solution proceeds at a rate which increases markedly with time. Such a result implies that the poly(butadienyl)lithium aggregate is less reactive than the mixed aggregate formed between the butadienyl-and vinylbenzyllithium active centers. Interestingly, no accelerations with increasing reaction time were found with poly(butadienyl)lithium and m-divinylbenzene nor with poly(isoprenyl)lithium and either the m- or p-divinylbenzenes. This general behavior was subsequently verified 270) by a series of size exclusion chromatography measurements on polydiene stars (linked via divinylbenzene) as a function of conversion. 

An elastomer that is entirely or mostly polydiene is, of course, highly unsaturated. All that is required of an elastomer, however, is enough unsaturation to permit cross-linking. In making butyl rubber (Sec. 32.5), for example, only 5% of isoprene is copolymerized with isobutylene. 

In contrast, in anionic systems in which the solvent may not actually interrupt the propagation process, it may play an active role in controlling both the rate and mode of the chain growth step. This control is perhaps most dramatically illustrated in the case of the organolithium polymerizations in connection with two specific aspects chain microstructure of polydienes and copolymerization of dienes and styrene. 

As indicated in Chapter 1, the polymerization of organic compounds was first reported about the mid-19th century. However, it was not until about 1910 that the simultaneous polymerization of two or more monomers (or copolymerization) was investigated when it was discovered that copolymers of olefins and dienes produced better elastomers than either polyolefins or polydienes alone. The pioneering work of Staudinger in the 1930s and the development of synthetic rubber to meet wartime needs opened the field of copolymerization. 

This is still the most popular method for graft copolymerization of elastomers via free radicals. Free radicals (I) are generated from the same types of initiators which are used for free radical polymerization and copolymerization (see Section 2.4). In general, these radicals are formed in the presence of a polydiene elastomer and a monomer therefore, there are several possible reactions of these initiator-derived radicals which can occur as shown in Eqs. (2.93)-(2.96). The competition between initiation of monomer polymerization (Eq. 

Some commodity plastics have low impact strengths. Of course, the impact strength can be strongly increased by blending with other polymers (see also Section 35.3.5.3), which, in the case of the quite brittle poly(styrene), can be achieved by copolymerization, by graft polymerization in the presence of polydienes, or by styrene polymerization in the presence of poly (2,6-dimethyl phenylene oxide.) Poly(vinyl chloride) is also made less brittle by reinforcing with poly(acrylates). 

The synthesis of Pl-g-PS or PBd-g-PS constitutes another typical example. To achieve that aim, active sites were created, by metallation of the pendant aUylic double bonds of the polydiene backbone by n-BuIi in the presence of TMEDA followed by anionic polymerization or copolymerization of the desired monomer." ... 

Cross-linking can be done by various techniques depending on the chemical composition of the polymer. Thus, if the polymer contains C=C unsaturation, e.g. in polydienes, natural rubber, polyisobutylenes (from copolymerized isoprene), then reaction with sulphur forms —S—S— links. With polysiloxanes or EP rubbers (no C=C) then peroxides are used. With fluorocarbon elastomers, e.g. Viton (a copolymer of vinylidene difluoride and hexafluoropropylene), diamines are used which form H-bonded cross-links. With polyurethanes (formed from diols and di-isocyanates) a controlled number (low) of cross-links are formed during the polymerization by the addition of triols. 

Polydienes constitute an extremely important group of polymers. This group comprises natural rubber (and related natural materials) and its derivatives together with the products of polymerization and copolymerization of conjugated dienes. These various materials make up the contents of this chapter. The importance of the polydienes lies in the fact that they encompass the bulk of the commercial elastomers currently in use. 

The reaction with MA is characteristic for the unsaturated compounds. As a result it is applied to liquid polydienes, to cyclopentadiene and dicyclopentadiene oligomers, to copolymeric aliphatic resins, and to cycloaliphatic resins. 

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