High-resolution 500 MHz solution-state 1H and 13C NMR spectra of various poly(lactides) indicate at least hexad stereosequence sensitivity. The poly(lactides) were prepared in vials by melt polymerization of various combinations of l-lactide, d-lactide, and meso-lactide at 180 °C for 3 h using tin(II) bis(2-ethylhexanoate) (tin(II) octoate) as the catalyst in a 1:10 000 ratio. The intensity distribution of the various stereosequence resonances in the NMR spectra indicates a preference for syndiotactic addition during the polymerization process. Minimal evidence of transesterification was observed for these polymerization conditions.

Small-angle X-ray scattering (SAXS), optical microscopy and differential scanning calorimetry were used to establish relationships between crystallization, microstructure, and equilibrium melting temperature (T m 0) for random copolymers of poly(l-lactide-co-meso-lactide). T m 0s derived using the Gibbs−Thomson and data-fitting23 approaches were found to be in excellent agreement and decreased significantly with increasing meso-lactide concentration. Analysis of these equilibrium melting temperatures using models for copolymer crystallization leads to the conclusion that the meso defects are rejected from the crystalline regions. From analysis of the SAXS data, it was concluded that the copolymers contained significant interfibrillar regions whose concentration increased with higher comonomer content. Spherulitic growth rates were strongly dependent on meso content and were analyzed using the Lauritzen−Hoffman kinetic theory of crystal growth.

This paper presents a continuation of our earlier research on the crystallization and solidstate structure of polylactide copolymers. The focus here is on random copolymers containing predominately L-lactide and small amounts (1.5, 3, and 6%) of D-lactide. As expected, degrees of crystallinity and spherulite growth rates decrease substantially with increasing D-lactide content in the copolymers. The importance of defect arrangement (isolated vs paired stereochemical defects) was demonstrated by comparison to our earlier research on L-lactide/meso-lactide copolymers. At a given degree of supercooling, measured lamellar thicknesses decrease significantly with increasing R stereoisomer concentration: e.g., by more than a factor of 2 (compared to poly(L-lactide)) for the 6% D-lactide copolymer. The results of small-angle X-ray scattering experiments indicate that a significant amount of noncrystalline material resides between lamellar stacks. Equilibrium melting points were estimated for the copolymers using the Gibbs-Thomson approach, and the values conform with predictions of the model of Wendling and Suter in the exclusion limit. Taken together with the significant reduction in lamellar thickness and crystallinity, these results point to substantial rejection of D-lactide (and meso-lactide) defects from S stereoisomer crystals. However, experiments by others on similar copolymers suggest that a significant amount of R (or R-R) isomers can be included in S crystals under certain crystallization conditions. Some speculation about the origin of these differences is presented.