Abstract Investigation of the effect of the α-CH 3-group on low-temperature postpolymerization in devitrifying matrices leads to the following conclusions. 1. The low-temperature postpolymerization in supercooled alcohol solutions ( T g ∼ 102 K) is quite efficient with acrylic monomers. Inactivity of their methacrylic analogues under these conditions is attributed to steric screening of the unpaired electron in the growing radicals. 2. As the temperature is raised and the CH 3-group vibration intensity increases, the screening effect fades. Thus in devitrifying water-alcohol solutions of NaAA and NaMAA, at higher temperatures the postpolymerization is efficient in both cases. Data on copolymerization of NaAA and NaMAA indicate that at ∼ 170 K the steric limitations due to the CH 3-group are eliminated. 3. In a glycerine matrix ( T g ⋍ 195 K ), all the acrylic and methacrylic monomers studied show efficient polymerization over virtually the same temperature range. It is concluded that the a-CH 3-group in methacrylic monomers appreciably affects their polymerization activity only at low temperatures, where steric screening of the growing polymer radical becomes important.