Benzyl chloride was grafted onto polystyrene by Friedel Crafts alkylation reaction with $AlCl_3$ in nitrobenzene. The effects of reaction conditions were investigated at various reaction times, catalyst concentrations, monomer to polystyrene ratios, and temperatures. From the general results and the spectral data, a structure for the graft copolymer was proposed. The copolymerization reaction was almost completed within several minutes due to the absence of the induction period and the early attainment of highly branched structure. This formation of highly branched and insoluble polymers seemed to prevent further grafting beyond the percent graft of 200\%.