A novel polymeric coupling agent was synthesized through the reaction of polycarbonate (PC) and 3-glycidoxypropyltrimethoxysilane (GPS) in the presence of a quaternary ammonium salt. The reaction in the solid state gave a GPS-modified PC, which had a bimodal molecular weight distribution. The solutions of the GPS-modified PC were effective primers for promoting the adhesion of a siliceous abrasion-resistant coating to the PC substrate.