The influence of the catalyst/initiator system and polymerization conditions on the microstructure and physical properties of copolymers of equimolar amounts of 1,3-trimethylene carbonate (TMC) and epsilon-caprolactone (CL) was studied. Statistical copolymers were prepared in the presence of stannous octoate (SnOct(2)) in the bulk at 80 degreesC (14 and 28 d) and 130 degreesC (1 and 3 d). The copolymerization of TMC and CL initiated by yttrium, isopropoxide (Y-5(mu-O)((OPr)-Pr-i)(13)) was performed in solution at room temperature (5 min) and in the bulk at 80 degreesC (2 min). Block copolymers, used as reference materials, were also prepared by sequential polymerization of the monomers in solution at room temperature with yttrium isopropoxide. Both SnOct(2) and yttrium isopropoxide yielded polymers with shorter average monomer sequence lengths at higher reaction temperatures. For the polymerizations with SnOCt(2) a similar effect was observed when the reactions were allowed to proceed for longer periods of time. Independent of the catalyst/initiator system, the statistical copolymers prepared in the bulk at 80 or 130 degreesC were amorphous, with average monomer sequence lengths shorter than 3. The copolymer prepared in solution at room temperature with yttrium isopropoxide was more blocky, with CL sequences long enough to crystallize ((L) over bar (CL) = 3.93). The former copolymers are highly flexible but show low tensile strengths, in agreement with their amorphous structure and low glass transition temperature. The latter copolymer is also flexible but much tougher and stiffer, as it is semi-crystalline. Its properties are characteristic of phase-separated block copolymers.