In the present work, anomalous shapes of free radical (FR) accumulation curves Found under PMMA copolymers irradiation in the T ≥ 300°K temperature range have been studied. With the irradiation dose increase the FR concentration in PMMA increases up to a definite maximal value and then starts falling. It is stated that appearance of slopes on the FR accumulation curves is not associated with possible changes of the FR relaxation characteristics. The second order of the FR decay reactions is estimated with the activation energy E = 8.3 kcal/mol. It is suggested that the FR decay processes in the irradiation temperature range T ≥ 300°K are due to the fast decay of macrochains according to the “unzipping” mechanism.