Unusual copolymers were prepared by Candida antarctica Lipase B (Novozyme-435) catalyzed copolymerization of omega-pentadecalactone (PDL) with trimethylene carbonate (TMC). Atypical solid-state properties were revealed by thermogravimetric analysis, differential scanning calorimetry (DSC), and X-ray diffraction analyses. Thermal degradation of poly(PDL-TMC) occurs in two steps: the first well above the degradation range of poly(TMC) and the other at a temperature (430 degreesC) comparable with that of poly(PDL) decomposition. Thermal stability of PDL-TMC copolymers increases with randomization of comonomer distribution. All poly(PDL-TMC) investigated are highly crystalline, even those with equimolar comonomer content and close-to-random distribution. This result indicates that PDL and TMC units cocrystallize. The copolymers show two melting processes: the higher one at about 90 degreesC, i.e., close to poly(PDL) melting, and the other 30 degreesC lower. The relative intensity of the two phenomena changes with copolymer microstructure. The X-ray diffraction patterns of poly(PDL-TMC) are practically identical to that of poly(PDL) in the range 10degrees < 2theta < 80degrees. Significant differences appear in the low-angle region below 2theta = 10degrees, indicating in some copolymers the presence of crystals with higher periodicity than poly(PDL) along the chain direction. Comparison of DSC and X-ray results with copolymer microstructure shows that two crystal phases can develop in poly(PDL-TMC): poly(PDL) crystals and - whenever a significant amount of heterodiads are present-another lower melting crystal phase, characterized by a larger fiber axis periodicity and possibly associated with crystallization of alternate PDL-TMC sequences.