Polystyrene-block-poly(n-butyl acrylate) block copolymers were prepared from 4-oxo-2,2,6,6-tetramethylpiperidinooxy (4-oxo-TEMPO) capped polystyrene macroinitiators at a high temperature, 165 degrees C. It was found that the number-average molecular weight of PBA chains in block copolymers could reach above 10,000 rapidly at early stage of polymerization with a narrow polydispersity index of 1.2-1.4, but after that, the polymerization seemed to be retarded. Furthermore, according to the kinetic analysis, the concentration of of 4-oxo-TEMPO was increased mainly by the hydrogen transfer reaction of hydroxylamine (4-oxo-TEMPOH) to growing radicals during polymerization. This increase in 4-oxo-TEMPO concentration could retard the growth of polymer chains. The rate constant of the hydrogen transfer reaction of 4-oxo-TEMPOH to growing radical, k(H), estimated by the kinetic model is about 9.33 x 10(4) M-1 s(-1) at 165 degrees C. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2833-2842, 2009