Synthesis of Four-Arm Star Poly(L-Lactide) Oligomers Using an In Situ-Generated Calcium-Based Initiator

Using an in situ-generated calcium-based initiating species derived from pentaerythritol, the bulk synthesis of well-defined four-arm star poly(L-lactide) oligomers has been studied in detail. The substitution of the traditional initiator, Stannous octoate with calcium hydride allowed the synthesis of oligomers that had both low PDIs and a comparable number of polymeric arms (3.7-3.9) to oligomers of similar molecular weight. Investigations into the degree of control observed during the course of the polymerization found that the insolubility of pentaerythritol in molten L-lactide resulted in an uncontrolled polymerization only when the feed mole ratio of L-lactide to pentaerythritol was 13. At feed ratios of 40 and greater, a pseudoliving polymerization was observed. As part of this study, in situ FT-Raman spectroscopy was demonstrated to be a suitable method to monitor the kinetics of the ring-opening polymerization of lactide. The advantages of using this technique rather than FTIR-ATR and H-1 NMR for monitoring L-lactide consumption during polymerization are discussed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4736-4748, 2009