The graft copolymerization of undecylenic acid onto acrylonitrile-butadienestyrene terpolymer (ABS) was initiated with benzoyl peroxide (BPO) in a 1,2-dichloroethane solution. IR spectra confirmed that undecylenic acid was successfully grafted onto the ABS backbone. The influence of the concentrations of undecylenic acid, BPO, and ABS on the graft copolymerization was studied. A reaction mechanism was proposed: the grafting most likely took place through the addition of poly(undecylenic acid) radicals to the double bond of the butadiene region of ABS. A monomer cage effect on the graft reaction was observed to depend on the 1.5 power of the monomer concentration from the experimental results of the initial rate of graft copolymerization. The initial rate of graft copolymerization was written as R-p = 1.77 X 10(-3) [P] [I-2] [M](2.5)/([P] + 2.75 [M](2.5))(2). (C) 2001 John Wiley & Sons, Inc.