Abstract Free radicals produced by mechanical destruction of polymethylmethacrylate, polyethyl-methacrylate, polybutylmethacrylate and polyallylmethacrylate has been studied by the ESR method. The radicals decay in two temperature ranges around −40° and 50° (or + 5°). It is supposed that the decay of free radicals trapped in the polymer matrix in the first temperature region is caused by transfer reaction of the growing-chain radical and sidegroups or of the transfer reaction between these groups, which depends both on their dimensions and chemical constitution. In the second thermal region, decay of radicals is determined by the character of T g transition and the chemical nature of the radical itself. Activation energies were found to be near zero at the lower temperatures and 24 kcal/mole for polymethylmethacrylate, 21 kcal/mole for polyethylmethacrylate, 10 kcal/mole for polybutylmethacrylate and 3 kcal/mole for polyallylmethacrylate at the higher temperatures.