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

Abstract

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, and matrix-assisted laser desorption ionization mass spectrometry. 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 ΔHp° = 0.2 ± 0.7 kJ mol-1 and ΔSp° = +16 ± 2 J mol-1 K-1 (standard state [S,S-3] = 1.0 M). Theoretical calculations of the standard state enthalpy (ΔHp°) 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 (kp = 0.82 ± 0.04 M-1 s-1 and kdp = 0.12 ± 0.04 s-1 at 25 °C) than that of lactide (kp = 2.2 M-1 s-1) using the same zinc alkoxide catalyst. Differential scanning calorimetry of S,S-PMOD showed a glass transition temperature of −30 °C for samples with molecular weights between 5 and 72 kg mol-1. In support of the potential utility of S,S-PMOD as a PLA plasticizer, the complete miscibility of the polymeric components was demonstrated by the observation of single Tg values for a series of blends of S,S-PMOD and atactic PLA.