Synthesis, Microstructure, and Properties of High-Molar-Mass Polyglycolide Copolymers with Isolated Methyl Defects

An efficient, fast, and reliable method for the synthesis of high-molar-mass polyglycolide (PGA) in bulk using bismuth (III) subsalicylate through ring-opening transesterification polymerization is described. The difference between the crystallization (Tc ≈ 180 °C)/degradation (Td ≈ 245 °C) temperatures and the melting temperature (Tm ≈ 222 °C) significantly affects the ability to melt-process PGA homopolymer. To expand these windows, the effect of copolymer microstructure differences through incorporation of methyl groups in pairs using lactide or isolated using methyl glycolide (≤10% methyl) as comonomers on the thermal, mechanical, and barrier properties were studied. Structures of copolymers were characterized by nuclear magnetic resonance (1H and 13C NMR) spectroscopies. Films of copolymers were obtained, and the microstructural and physical properties were analyzed. PGA homopolymers exhibited an approximately 30 °C difference between Tm and Tc, which increased to 68 °C by incorporating up to 10% methyl groups in the chain while maintaining overall thermal stability. Oxygen and water vapor permeation values of solvent-cast nonoriented films of PGA homopolymers were found to be 4.6 cc·mil·m-2·d-1·atm-1 and 2.6 g·mil·m-2·d-1·atm-1, respectively. Different methyl distributions in the copolymer sequence, provided through either lactide or methyl glycolide, affected the resulting gas barrier properties. At 10% methyl insertion, using lactide as a comonomer significantly increased both O2 (32 cc·mil·m-2·d-1·atm-1) and water vapor (12 g·mil·m-2·d-1·atm-1) permeation. However, when methyl glycolide was utilized for methyl insertion at 10% Me content, excellent barrier properties for both O2 (2.9 cc·mil·m-2·d-1·atm-1) and water vapor (1.0 g·mil·m-2·d-1·atm-1) were achieved.