It was found that diphenyl carbonate reacts with Bu2SnO above 140 degreesC yielding CO2 and Bu2Sn(OPh)(2). This insertion reaction was also successfully applied to a polycarbonate prepared from polytetrahydrofuran diol-1000 and Bu2SnO containing macrocyclic polyTHF dioxides. Bu2SnO reacted analogously with poly(bisphenol A) carbonates yielding tin-containing rapidly crystallizing cyclic oligocarbonates. Their treatment with 1,2-ethanedithiol gave linear oligocarbonates having two 1,2-ethanedithiol gave linear oligocarbonates having two OH end groups. Bu2Sn-containing macrocyclic oligocarbonates reacted in-situ with several carboxylic acid chlorides, forming telechelic oligocarbonates with functional ester end groups. Bu2Sn-containing macro-cycles also reacted in-situ with terephthaloyl chlorides yielding a poly(ester carbonate).