Formation of and Coalescence from the Inclusion Complex of a Biodegradable Block Copolymer and α-Cyclodextrin:  A Novel Means To Modify the Phase Structure of Biodegradable Block Copolymers

A well-defined biodegradable block copolymer (PCL-b-PLLA, M<INF>n</INF> = 1.72 × 10⁴, M<INF>w</INF>/M<INF>n</INF> = 1.37) of poly(ε-caprolactone) (PCL) and poly(<SCP>l</SCP>-lactide) (PLLA) was synthesized by a two-step ring-opening polymerization of ε-caprolactone and <SCP>l</SCP>-lactide. Furthermore, we found that α-cyclodextrin (α-CD) molecules may simultaneously thread onto both PLLA and PCL blocks of PCL-b-PLLA to form an inclusion complex (IC). Washing the copolymer−α-CD IC with hot water removed the α-CD, and the copolymer chains were coalesced. Very interestingly, the coalesced copolymer sample shows a great suppression in microphase separation, compared with the as-synthesized copolymer. In contrast to the significant decrease in crystallinity of ca. 50% and up to 79% for PCL and PLLA blocks, respectively, the melting points (T<INF>m</INF>'s) and the cold crystallization temperatures (T<INF>cc</INF>'s) of both PCL and PLLA blocks of the coalesced sample increased in DSC measurements. These results may imply that only small amounts of more extended crystals, with less chain folding, were produced during the process of copolymer coalescence. Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and wide-angle X-ray diffraction (WAXD) measurements were employed to demonstrate formation of the block copolymer−α-CD IC as well as to gauge the suppression of the microphase separation in the coalesced sample.