SUMMARY: The free radical copolymerization at 50 8C of styrene with butyl acrylate was carried out in bulk. Although the copolymer composition is well described by the Mayo-Lewis terminal model, the copolymerization rate is not. As many other systems recently examined, the data are well represented by the "implicit penultimate effect" model. Values of monomer and radical reactivity ratios are qualitatively rationalized in terms of the enthalpic and entropic models.