The bulk polymerization of styrene at 110 degrees C in the presence of 7-oxo-15-azadispiro[5.2.5.1]pentadecanyl-N-oxyl was investigated. The Mn values of poly(St) formed were increased linearly with conversion, and the M-w/M-n values were 1.48-1.54 at 90% conversion. On the basis of the results it was concluded that 1 controlled radical polymerization of St.