A detailed study of the microstructure of methyl methacrylate (MMA) and butyl acrylate (BA) copolymer prepared by group transfer polymerization (GTP) was carried out by using 13 C NMR spectroscopy. Methyl methacrylate and butyl acrylate centred triad molar fraction were determined from the quaternary carbon and carbonyl group resonance signale. The experimental sequence concentrations were in agreement with the values calculated from the terminal copolymerization model and bernoullian statistics. A greater block tendency of BA monomers may be noticed in both experimental and calculated distribution

which is due to the large difference between the reactivity ratios of MMA and BA in group transfer copolymerization.