The influence of reversal of propagation reactions on copolymer composition has been studied for three systems: (1) anionic copolymerization of vinyl mesitylene (M1) and α-methylstyrene (M2) in tetrahydrofuran (THF), (2) radical copolymerization of styrene (M1) and methyl methacrylate (M2) ino-dichlorobenzene (ODCB), and (3) cationic copolymerization of styrene (M1) and α-methylstyrene (M2) in methylene chloride and in bulk.14C-labeled M2was used in systems (1) and (3).