Numerous BuSnCl(3)(-), Bu(2)SnCl(2)(-), and Bu(3)SnCl-initiated polymerizations of cyclo(trimethylene carbonate) (TMC) were conducted in bulk. In addition to the initiator, reaction time, temperature, and monomer/initiator (M/I) ratio were varied. Yields above 90% were obtained with all three initiators, but their reactivities decrease in the order BuSnCl(3) > Bu(2)SnCl(2) > Bu(3)SnCl. The maximum molecular weights decrease in the same order. With BuSnCl(3) M(w)s up to 250,000 were obtained. These molecular weights were determined by GPC on the basis of the universal calibration method. In this connection Mark-Houwink equations for two solvents, tetrahydrofuran (THF) and CH2Cl2 were determined and compared with literature data. Furthermore, mechanistic aspects were studied. H-1- and C-13-NMR spectra revealed that BuSnCl(3) forms complexes with the CO-group of TMC, whereas Bu(2)SnCl(3) and Bu(3)SnCl do not cause NMR spectroscopic effects. Kinetic studies in chloroform and nitrobenzene and a comparison with Bu(3)SnOMe suggest that at least BuSnCl(3) initiates a cationic mechanism. However, in contrast to SnCl4 (or SnBr4), BuSnCl(3) does not cause decarboxylation. Regardless of the initiator H-1-NMR spectroscopy revealed CH2OH and CH2Cl endgroups in all cases.