Chemoenzymatic Synthesis and Study of Poly(α-methyl-β-propiolactone) Stereocopolymers

A chemoenzymatic route from racemic α-methyl-β-propiolactone (MPL) to optically active (R)-poly(α-methyl-β-propiolactone), (R)-PMPL, was demonstrated which involved lipase catalyzed resolution of MPL enantiomers in organic media and subsequent chemical polymerization. From a comparative study of different enzymes and organic media, catalysis by the lipase PS-30 from Pseudomonas fluorescens suspended in diethyl ether was found to be most suitable for MPL enantioselective alcoholysis reactions. MPL having up to 93% (R) content was thus obtained, mixed in various proportions with racemic MPL, and polymerized using CH3COOK/dibenzo-18-crown-6. From a plot of specific rotation vs % enantiopurity, it was estimated that the [α]25D for 100% (R)-PMPL is −38.4° (c 0.8 g/dL, CHCl3). 13C NMR analysis (62.9 MHz) provided information on triad sequence distributions from observation of methine carbons, while the methyl and carbonyl carbons were sensitive to diad sequences. The polymerization of 50% (R)-MPL followed Bernoulli random chain propagation statistics. In contrast, polymerizations of 74% and 93% (R)-MPL deviated from the Bernoulli model. Further analysis showed that these latter polymerizations fit the enantiomorphic-site model. Thus, isoregulating characteristics of the CH3COOK/dibenzo-18-crown-6 catalyst for enantioenriched MPL polymerizations were demonstrated. By increasing the % (R)-content in the monomer feed to 93%, a product was obtained which, by DSC, had Tm and ΔHf values of 131 °C and 22.0 cal/g, respectively. Furthermore, WAXS analysis carried out on a melt-annealed 90% (R)-PMPL sample showed that the polymer was highly crystalline (approximately 73%). FTIR analysis was used to identify vibrational bands at 997, 1010, 1059, 1185, 1249, and 1334 cm-1 which were crystalline-sensitive, while the band at 1465 cm-1 was crystalline-insensitive.