Synthesis and characterization of poly(L-lactide-co-ε- caprolactone) copolymers: influence of sequential monomer addition on chain microstructure

A series of copolymers with various compositions were synthesized by one-step and two-step bulk ring-opening polymerizations of L -lactide (LA) and ε-caprolactone (CL) using stannous octoate [Sn(Oct)₂] and 1-hexanol as the initiating system. For the sequential two-step polymerization, a poly(ε-caprolactone) (PCL) prepolymer was polymerized first to a percent conversion of approximately 70% and LA then added in order to produce a copolymer with a chain microstructure different from that obtained from the corresponding one-step reaction. The resulting copolymers were characterized using a combination of nuclear magnetic resonance spectroscopy (¹H- and ¹³C-NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and gel permeation chromatography (GPC). The average sequence lengths of the lactidyl ( ) and caproyl ( ) units, the degree of randomness (R) and the transesterification coefficient (T_(II)) were calculated from the ¹³C-NMR spectra. The appearance of a signal due to CapLCap sequences was directly attributable to transesterification of lactidyl (LL) units. It was found that both and values from the two-step syntheses were significantly longer than from the corresponding one-step syntheses, leading to different semi-crystalline morphologies and chain microstructures. The copolymers all showed at least some blocky chain structure as a result of the significant difference in monomer reactivity (LA > CL) between LA and CL. Thermal properties including stability depended on both composition and chain microstructure which could be controlled by the method of synthesis. From their DSC curves, the two-step copolymers were seen to be semi-crystalline whereas the one-step copolymers were mainly amorphous. A more blocky microstructure, as obtained from the two-step method, appeared to result in a lower thermal stability. Copyright © 2007 John Wiley & Sons, Ltd.