Thermal Degradation of Common Chain-Growth Polymers
Thermal degradation of polymers is conveniently studied by pyrolytic methods. The polymer litera ture contains many reports on such studies conducted at various temperatures in inert atmospheres, in air, or in vacuum. The volatile products are usually monitored with accompanying measurements of the weight loss per unit time. The reaction rates are thus measured by:
1. Loss of molecular weight as a function of temperature and the extent of degradation
2. The quantity and the composition of the volatile and nonvolatile products of degradation
3. The activation energy of the degradation process Ageneral scheme for thermal degradations of chain-growth polymers by free-radical reactions can be written as follows:
Random type chain scission:
Chain-end type scission:
In a thermal chain depolymerization, the degradation can also be random. Rupture occurs at various points along the chain. The products are various size fragments of the polymer, usually larger than individual monomers. Both reactions shown above can take place simultaneously in the same polymer chain or only one of them might take place exclusively. Also, chain depropagation may not necessarily be initiated from the terminal ends of the macromolecules, though such depolymer izations are more common. They may instead start from some points of imperfection in the chain structures. These imperfections might consist of incorporated initiator fragments, or peroxides, or ether links. They might have formed as a result of oxygen molecules being present during chain growths. Weak points in polymer backbones can also be at locations of some tertiary hydrogens. Each individual polymer will depolymerize at its own specific rate and the degradation products will be peculiar to the particular chemical structure of the polymer [446–452]. For instance, poly(methyl methacrylate) can be converted almost quantitatively back to the monomers. The depolymerization occurs from the terminal end in an unzipping fashion with the overall molecular weight decreasing slowly in proportion to the amount volatilized. Polyethylene, on the other hand, undergoes scission into longer olefin fragments. Very little monomer is released. At the same time, the molecular weight tends to decrease rapidly and only a small amount of volatilization takes place. The two polymers are good examples of extreme behavior of chain-growth polymers. Most of the chain-growth polymers, however, fall between the two. The depolymerization reaction of chain-growth polymers generally occurs by a free-radical mechanism and the reactions are similar. If depropagation is the major portion of the degradation process, then the molecular weight reduction is proportional to the quantity of the monomer that forms. If, on the other hand, chain transferring is the major portion of the degradation process, then there is rapid loss of molecular weight with little accompanying monomer accumulation and the reaction products are large segments of the chains. The rate of depolymerization exhibits a maximum. When depropagation take splace at an elevated temperature, at a rate that is equal to the propagation in a free-radical polymerization, then the temperature of the reaction is a ceiling temperature. Termination can take place by disproportionation. Secondary reactions, however, may occur in the degradation process depending upon the chemical structure of the polymer. Such side reactions can, for instance, be successive eliminations of hydrochloric acid, as in poly (vinyl chrolide), or acetic acid as in poly (vinyl acetate).
