Thermal reactions, without chain scission

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. 

A method of incorporating between 5% to 45% maleic anhydride into polypropylene without chain scission or viscosity increase is described. The method entails an initial thermally induced ene reaction followed by the free radical addition of the anhydride to the polymer backbone. 

The structure —CHC1—CH2—CO—CH2 — was found by Kwei [99] in polyvinylchloride after photo-oxidation. Such j3 chloroketones decompose by the Norrish type I mechanism without loss of chlorine atoms. Hydrogen chloride is obtained only when polyvinylchloride is photo-oxidized above 30°C [98]. It seems that zipper dehydrochlorination plays little role in the reaction occurring on exposure to ultraviolet light at temperatures below 150°C in the presence of air [97], and that hydrogen chloride is mainly a product of thermal decomposition rather than photolysis [98], The following mechanism can be proposed which takes into account the experimental results namely, that chain scission and crosslinking occur simultaneously on irradiation at 253.7 nm [100] and that carbon dioxide is evolved, while an absorption band at 1775 cm-1 (ascribed to peracids) is detected in the infrared spectrum [98]. 

Where a polymer has functional groups in the main chain, typical of polymerization by step-reaction chemistry or chain poljunerization of heterocyclic monomers, the fimctional groups in the chain are often the weakest and may nndergo reactions which do not produce radicals. Thermal degradation by random scission, without significant yield of volatiles is typical of polymers such as polyesters, polyamides, and polyurethanes, all of which undergo thermolysis by reaction at the functional groups (5). 

The decomposition temperature of PLA is normally 230—260°C. Therefore, it is considered to be safe for room temperature applications. PLA is seldom used at elevated temperatures, such as the boiling point of water, because PLA tends to lose its structural properties at temperatures >60°C. Although PLA is unlikely to release toxic substances extensively, residues of plasticizer or oligomers still need further attention. PLA undergoes initial thermal decomposition at temperatures above 200°C by hydrolysis reaction followed by lactide reformation, oxidative main-chain scission, and inter-or intramolecular transesterification reaction (Jamshidi et al., 1988). Thermal decomposition can occur at 200°C without catalysts, but it requires higher temperatures to induce a faster and more prevalent reaction (Achmad et al., 2009). 

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