Mechanism of propagation and degenerate chain branching in the oxidation of polypropylene and polyethylene

Samples of polypropylene with adjacent and isolated hydroperoxide groups have been prepared. The rate constants of free-radical formation from solid hydroperoxides were measured by the inhibitor method. It was found that the free radicals yielded by adjacent hydroperoxide groups are formed more rapidly. The main reaction of free-radical formation in oxidized polypropylene is of the type: ROOH + ROOH → RO + H₂O + RO₂^˙. The average yield of free radicals from polypropylene hydroperoxide is 2–4%. Oxygen has no effect on the yield of free radicals. However, the pressure of oxygen Po₂ affects the rate of degenerate chain branching in polypropylene. The number of adjacent hydroperoxide groups and the rate of initiation increase with Po₂. Consequently, a reaction of the type, R^˙, + RH → RH + R^˙, plays an important part in transport of free valence through solid polymer. This reaction is very fast in polyethylene, and no adjacent hydroperoxide groups are formed. The free radicals from polyethylene hydroperoxide are found to form by a reaction of the type: ROOH → RO^˙ + HO^˙.