Intermolecular transfer, depolymerization

The degradation reactions involved in PS include random scission (which reduces the molecular weight of the polymer), depolymerization (which yields monomer), intramolecular transfer reaction (which produces dimer, trimer, etc.), and intermolecular transfer reaction which reduces the molecular weight of the polymer. The initial degradation products from PVC are Cl radicals and HCl (Owen 1984 Ahmad and Mahmood 1996). The structure and composition of PVC and PS and the interaction of various products formed may give rise to some cross products formed from the radicals or molecules which migrate across these phase boundaries and can play an important role in the degradation of blends. 

In contrast to the case of ethylenic monomers where transfer results in chain branching, intermolecular transfer reactions in the polymerization of heterocyclics correspond to exchange reactions between chains, with the number of chains remaining constant. Intramolecular transfer reactions—which is generally the case due to a higher probability of collision—yield inactive cycles which can possibly be the monomer (n = 1) and thus correspond to a depolymerization ... 

For poly(methyl methacrylate), the average ratio of the number of depolymerization steps to the number of chain-transfer plus termination reactions, termed the average zip length, has been calculated from kinetic studies to be approximately 60 (4). The zip length is the average number of monomer units removed from each radical formed via initiation or intermolecular transfer. 

Many workers have argued that the unsaturated chain ends (D) provide the points at which depolymerization is initiated via reaction (b) of Scheme 14. This view has recently been challenged by others,who believe that the formation of radical (A) by random scission and especially following intermolecular transfer provides the principal route to the formation of volatile products. The relative yields of toluene and a-methylstyrene, and in particular the very small amounts of the latter found in the products, tend to support this view and also to suggest that benzylic ends (C) might in fact be less stable than unsaturated ends (D). In PS with only benzylic ends, an increased proportion of toluene in the early stages of degradation has been attributed to additional initiation at these... 

At elevated temperatures, statistical chain cieavage is the dominant reaction its radicais initiate secondary reactions. Intramoiecuiar radical-hydrogen transfer that leads to depolymerization ( zipper mechanism ) releases monomers, dimers, and trimers. Figure 5.172. Intermolecular hydrogen transfer causes degradation in another polymer chain. Figure 5.173. 

It is generally accepted that the other polyacrylates have mechanisms of breakdown in which random scission is important (Scheme 8). In contrast to the situation in the cases of PMMA, PS and PAMS, where one of the two macroradicals depolymerizes to monomer, the radicals produced by homolysis in PMA do not behave in this way. It may be assumed that the more reactive primary radical (B) stabilizes itself by intermolecular and intramolecular transfer of a tertiary H atom and radical (A) may also be involved in this way. The product in either case is a tertiary radical (C) which provides a site for various possible reactions, including scission and crosslinking. Scission leads to an unsaturated chain end and radical (A). 

In addition to depolymerization through p-scission, intra-and intermolecular hydrogen transfer occur. So, primary carbon-centered radicals are expected to isomerize by intramolecular hydrogen abstraction (backbiting) and to form secondary radicals, which are more stable. On the basis of activation and bond dissociation energies, Kuroki et al claimed that backbiting reactions and intermolecular radical transfer reactions are much more likely to occur than depolymerization reactions. ... 

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