The appropriate choice of catalysts and comonomers improved the selectivity of the gas-solid-phase reaction of carbon dioxide into copolymers from glycidyl methacrylate (GNU), which transforms the pendent oxirane moieties to cyclic carbonate moieties. Benzyltributylammonium chloride (BnBU3-NCI) exhibited good and selective catalytic behavior owing to its appropriate mobility in the copolymer film and moderate catalytic activity. n-Butyl methacrylate proved to be an effective comonomer, resulting in higher incorporation efficiency and better selectivity by preventing the cross-linking reaction, allowing one to obtain soluble polymers bearing cyclic carbonate structure.