absorbable heterochain polymers carrying acid or basic groups , capable of ionic conjugation with basic or acidic bioactive agents , respectively , are produced by free - radical addition of unsaturated functional monomers onto absorbable liquid polymers and subsequent generation of reactive functionality or by direct copolymerization of a carboxylic initiator with cyclic monomers to produce liquid carboxyl - bearing copolyesters or polyester carbonates .