Polymerizations of cyclotrimethylenecarbonate (TMC) with SnCl4, SnBr4 and SnI4 were conducted in bulk. The reaction time, the temperature and the monomer/initiator (M/I) ratio were varied, and the influence of these parameters on yields and molecular weights was investigated. Yields above 90% were obtained with all these initiators, but the highest molecular weights (M(w) approximate to 1.5 x 10(5)) resulted from SnI4-initiated polymerizations. SnI4 has the additional advantage that the polycarbonates were free of ether groups when prepared at moderate temperatures (e.g. 60 degrees C). In contrast, SnCl4 and SnBr4 cause partial decarboxylation even at 60 degrees C, and at higher temperatures up to 20 mol% of the carbonate groups may be replaced by ether linkages. Furthermore, mechanistic studies were conducted. SnCl4 and SnBr4 form solid 1:2 complexes with TMC at 20-25 degrees C. Infra-red, H-1 and C-13 nuclear magnetic resonance (n.m.r.) spectroscopies indicate complexation at the carbonyl oxygen. Kinetic measurements of polymerizations in solvents of different polarity suggest cationic polymerization mechanisms. In addition to CH2-OH, CH2Cl and CH2Br endgroups were detected by H-1 n.m.r. spectroscopy. The endgroup analyses of reaction mixtures and isolated poly(TMC) support the assumption of a cationic polymerization mechanism.