| Abstract
| - A new isotactic, perfectly alternating polymer of (S)-lactic acid and oxetane was synthesizedby the entropically driven ring-opening polymerization of a 14-membered cyclic diester (S,S-3) mediatedby a zinc alkoxide catalyst. The polymer (S,S-PMOD) was characterized by NMR spectroscopy, sizeexclusion chromatography, and matrix-assisted laser desorption ionization mass spectrometry. Underpolymerization conditions the equilibrium concentration (0.17 ± 0.05 M) of the monomer in toluene wasessentially independent of temperature, and the average thermodynamic parameters for the ring-openingpolymerization were found to be ΔHp° = 0.2 ± 0.7 kJ mol-1 and ΔSp° = +16 ± 2 J mol-1 K-1 (standardstate [S,S-3] = 1.0 M). Theoretical calculations of the standard state enthalpy (ΔHp°) for ring-openingpolymerization 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 forsamples 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 bythe observation of single Tg values for a series of blends of S,S-PMOD and atactic PLA.
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