The grafting of polyamide 6 (PA6) onto polystyrene (PS) can rely on the use of a copolymer of styrene (St) and 3-isopropenyl-alpha, alpha-dimethylbenzene isocyanate (TMI), PS-co-TMI, to activate the polymerization of epsilon-caprolactam (CL) in the presence of sodium epsilon-caprolactam (NaCL) as an anionic catalyst. This article is aimed at answering the following key questions. First, do all the isocyanate moieties of the PS-co-TMI participate in the activation of the polymerization of CL? Second, what are the composition of the resulting polymer product and the structure of the resulting graft copolymer? The results show that the isocyanate moieties had all participated in the activation of the polymerization, implying that each isocyanate moiety has led to the formation of a PA6 graft. The as-polymerized product was composed of a pure PS-g-PA6 graft copolymer, homo-PA6, and unreacted CL. Moreover, when the composition of a PS-co-TMI/CL/NaCL system was fixed, the mass ratio between the PA6 grafts and PS backbone of the pure PS-g-PA6 graft copolymer was almost a constant and was almost independent of its molar mass. (C) 2008 Wiley Periodicals, Inc.