Copolymers of styrene and methacrylonitrile have been prepared by free radical polymerisation in the presence of the chain transfer agent ethyl alpha-(t-butanethiomethyl)acrylate (1). Chain transfer constants vary with co-monomer composition, ranging from 0.42 for methacrylonitrile, through a minimum of 0.23 for co-monomers to 0.90 for styrene. Bulk copolymer composition is independent of the amount of chain transfer. The efficiency of the addition-fragmentation mechanism in producing specifically end-functionalised co-oligomers was investigated by H-1 NMR spectroscopy. Spectral peaks are mostly consistent with the expected end groups for all co-monomer feeds. Quantitative measurement of end-group concentrations indicates a consistent deficiency of olefinic end groups of 10-20%. (C) 2000 Elsevier Science Ltd. All rights reserved.