The copolymerization of methyl methacrylate and styrene mechanochemically initiated by the grinding of quartz in the monomer mixture was investigated by using a vibrating ball mill. The effect of the grinding of quartz on the copolymerization was investigated by characterizing both the polymer formed and the ground quartz. The copolymerization proceeds apparently from the quartz grindings and was closely related to the total surface area of the ground quartz. The molecular weight distributions of the copolymers formed were unimodal as found in a homopolymer of only methyl methacrylate. It was clear from the composition analysis of the copolymer that the mechanochemical copolymerization of methyl methacrylate and styrene proceeded with a radical polymerization mechanism because of radicals on the quartz surface produced by the grinding. The monomer reactivity ratios obtained by the FinemanRoss method suggested that the copolymers formed were alternating copolymers.