Isotactic Polymers with Alternating Lactic Acid and Oxetane Subunits from the Endoentropic Polymerization of a 14-membered Ring

Zhang, D.; Xu, J.; Alcazar-Roman, L.; Greenman, L.; Cramer, C. J.; Hillmyer, M. A.; Tolman, W. B.
Macromolecules 2004, 37, 5274.

A new isotactic, perfectly alternating polymer of (S)-lactic acid and oxetane was synthesized by the entropically driven ring-opening polymerization of a 14-membered cyclic diester (S,S-3) mediated by a zinc-alkoxide catalyst. The polymer (S,S-PMOD) was characterized by NMR spectroscopy, size exclusion chromatography (SEC) and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Under polymerization conditions the equilibrium concentration (0.17 +/- 0.05 M) of the monomer in toluene was essentially independent of temperature and the average thermodynamic parameters for the ring-opening polymerization were found to be DH_p^o = 0.2 +/- 0.7 kJ mol^-1 and DS_p^o = +16 +/- 2 J mol^-1 K^-1 (standard state [S,S-3] = 1.0 M). Theoretical calculations of the standard state enthalpy (DH_p^o) for ring-opening polymerization of various oxocyclics, including S,S-3, were well correlated with experimental values. Kinetic studies showed that the polymerization of S,S-3 was slower [k_p = 0.82 +/- 0.04 M^-1 s^-1 and k_dp = 0.12 +/- 0.04 s^-1 at 25 ^oC] than that of lactide [k_p = 2.2 M^-1 s^-1] using the same zinc-alkoxide catalyst. Differential scanning calorimetry (DSC) of S,S-PMOD showed a glass transition temperature of -30 ^oC for samples with molecular weights between 5-72 kg mol^-1. In support of the potential utility of S,S-PMOD as a PLA plasticizer, complete miscibility of the polymeric components was demonstrated by the observation of single T_g values for a series of blends of S,S-PMOD and atactic PLA.

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