The initial stage of the di-TEMPO (N,N'-bis(4-(2,2,6,6-tetramethylpiperidin-1-yloxyl))diaminobutane) controlled radical polymerization of styrene was investigated by electron spin resonance (ESR) method in a temperature range of 300-410 K. Processes corresponding to three subsequent temperature subranges 300-360, 360-375, and 375-410 K have been discussed. The decrease of concentration of the di-TEMPO biradicals is faster than for the TEMPO monoradicals in the range 370-390 K. The reduced mobility of capping groups is postulated to be responsible for the differences between TEMPO monoradical and di-TEMPO biradical mediators. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2036-2041,2010