Depropagation depolymerization

The addition of radicals and, in particular, propagating radicals, to unsaturated systems is potentially a reversible process (Scheme 4.46). Depropagation is cntropically favored and the extent therefore increases with increasing temperature (Figure 4.4). The temperature at which the rate of propagation and depropagalion become equal is known as the ceiling temperature (rc). Above Tc there will be net depolymerization. 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

Even non-construction applications of polymers may gain importance in the future. If a cheap polymer could be synthesized which liberated gaseous products by depolymerization at the required rate at the proper time, it might serve as an energy accumulator. A sufficient number of other possible non--construction applications of polymers, made possible by their controlled depropagation, exist that it cannot be excluded that some of them will be soon realized. 

On the initiating particles IM, and IM2, copolymer chains grow from the monomers M, and M2. IM, and IM2 cannot undergo depolymerization and termination and transfer do not occur in the system. The propagation constant, fcp, depends only on the kind of monomer and of the growing end of the macromolecule (irrespective of its length). The depropagation constant is determined by the type of the last two monomers... 

Depolymerization of poly(a-methylstyrene) has been studied also in solution in various solvents ". The results have been interpreted in terms of a dhain reaction initiated by random scission, followed by a rapid depropagation, no transfer and a kinetic chain... 

On exposure to 253.7 nm radiation polymethylmethacrylate degrades rapidly to monomer at temperatures above 130° C in vacuo [9]. The depolymerization process involves photochemical initiation, depropagation and mutual termination of reaction chains. The over-all quantum yield is greater than 102. 

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