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 (T-c approximate to 180 degrees C)/degradation (T-d approximate to 245 degrees C) temperatures and the melting temperature (T-m approximate to 222 degrees 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 nudear magnetic resonance (H-1 and C-13 NMR) spectroscopies. Films of copolymers were obtained, and the microstructural and physical properties were analyzed. PGA homopolymers exhibited an approximately 30 degrees C difference between T-m and T-c, which increased to 68 degrees 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 O-2 (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 O-2 (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.

Automated summary

This study demonstrates that incorporating methyl groups into PGA polymers can widen the window between crystallization/degradation temperatures and melting temperatures while maintaining overall thermal stability. The distribution of methyl groups in copolymers affects the resulting gas barrier properties.