The Hidden Isodimorphic Crystallization of Poly(ε-Caprolactone-<i>Ran</i>-ω-Pentadecalactone) Copolymers

Poly(ε-caprolactone-<i>ran</i>-ω-pentadecalactone) (PCL_<i>x</i>-PPDL_<i>y</i>) copolymers were synthesized by using ring-opening polymerization with <i>Candida antarctica</i> lipase B as a catalyst across various compositions. The aim was to study their crystallization behavior and ascertain whether they are isomorphic or isodimorphic. Differential scanning calorimetry, polarized light optical microscopy, <i>in situ</i> wide- and small-angle X-ray scattering, and Fourier-transform infrared spectroscopy were employed to assess the crystallization mode. Various crystallization conditions were used to investigate their influence on the comonomer inclusion/exclusion balance. The copolymers exhibited pseudoeutectic behavior across all compositions, crystallizing in either PPDL-type or PCL-type unit cells and conformations, independent of crystallization conditions. This indicates that they are isodimorphic, contrary to previous reports. Self-nucleation tests showed that the <i>Domain II</i> width decreases with increasing comonomer content, supporting isodimorphism. The pseudoeutectic point was observed at CL contents above 83%, which explains the previously unrecognized isodimorphic character of these copolyesters.