AbstractThe radical polymerization of styrene was investigated in the presence of six different hydrazine derivatives. These compounds were selected in view of the nitrogen radicals, either from the aminyl or the hydrazyl type, resulting upon carbon radical attack. A distinct dependence of the rate of polymerization on the nature of hydrazine substituents was detected in thermally as well as radically (AIBN) induced polymerization. 1,2‐Diacetylhydrazine (13), 1,2‐bis(p‐nitrophenyl)hydrazine (16), 1,2‐diacetyl‐1,2‐diphenylhydrazine (15), and 1,2‐dimethylhydrazine (12) act as retarding compounds whereas 1,2‐diphenylhydrazine (14) and tetraphenylhydrazine (1) are inhibitors. The transfer constants were calculated from the Mayo equation to be 0,02 for12, 0,06 for13, 0,1 for15, 0,8 for16, 0,9 for14, and 5,6 for1. Any influence of polymerization temperature on the chain transfer constant was not observed. From model reactions only hydrazyl radical