Synthesis, melt molding and hydrolytic degradation of poly(L-lactide-co-l-methylglycolide) and its composites with carbonated apatite

Abstract

Poly(lactic-co-glycolic acid)s (PLGAs) hold considerable significance for their biomedical applications. Biodegradation and mechanical properties of PLGAs and PLGA-based composites are strongly influenced by lactate/glycolate (L/G) ratio in copolymers, molecular weight characteristics and microstructure of PLGAs. The common approach to PLGAs is based on ring-opening copolymerization of lactides and glycolide, the products of which contain long (L)_n and (G)_n segments. An efficient but expensive approach to PLGAs with given l-G sequences is a segmer assembly polymerization that is hardly applicable for the synthesis of high-MW PLGAs. In the present work, for the first time we synthesized lactate-enriched PLGAs using ring-opening copolymerization of l-lactide (l-LA) with l-methylglycolide (l-MeGL) in 85:15 and 70:30 molar ratios, resulting in l-PLMG 85/15 and l-PLMG 70/30 copolymers. l-PLGA 85/15 with the same L/G ratio as in PLMG 70/30 was synthesized by ring-opening copolymerization of l-LA with glycolide as a sample for a comparison. According to ¹H and ¹³C NMR data and [α]_D measurements, l-MeGL-based PLGAs had a unique microstructure, e.g. macromolecules of l-PLMG 85/15 consisted of L_n sequences with single G insertions. Composites of PLLA and three samples of PLGAs with plate-like carbonated apatite (pCAp) containing 25 and 50 wt.% of the filler were prepared. Rectangular specimens from (co)polymers and (co)polymer composites were obtained by injection molding and studied. Due to the absence of highly reactive (G)_n fragments, l-PLMG 85/15 and PLMG 70/30-based materials demonstrated higher thermal and hydrolytic stability, mechanical testing showed that l-MeGL-based copolymers provide better maintaining of the bending strength in comparison with l-PLGA 85/15 matrix.