Synthesis and characterization of random and triblock copolymers of ε-caprolactone and (benzylated)hydroxymethyl glycolide

Abstract

The aim of this study was to develop new polyesters with pendant functional hydrophilic groups suitable for biomedical applications. Therefore, ε-caprolactone (CL) was copolymerized with benzyl protected hydroxymethyl glycolide (BHMG) to introduce, after deprotection, hydroxyl groups into the polyester. Random and triblock copolymers were synthesized in the melt using benzyl alcohol and tin(II) 2-ethylhexanoate as initiator and catalyst, respectively, and deprotected by hydrogenation. The synthesized polyesters before and after deprotection were characterized by NMR, GPC, and DSC measurements. Mixtures of CL and BHMG were polymerized at 110, 130, and 150 °C. A polymerization kinetics study revealed that BHMG was far more reactive than CL at the three reaction temperatures investigated. However, due to transesterification reactions, random copolymers were formed at 150 °C, whereas at 110 °C, the copolymers had a more blocky structure. Taking advantage of this knowledge, ABA triblock copolymers of poly(ε- caprolactone) (PCL, block B) and poly(benzyl protected hydroxymethyl glycolide) (PBHMG, block A) were synthesized. To this end, α,ω PCL-diols were first synthesized at 130 °C by melt polymerization of CL initiated with 1,4-butanediol, which were subsequently chain extended with BHMG at 130 °C. Four triblock copolymers with different PCL (from 2.3 to 4.8 kg/mol) and PBHMG (from 2.0 to 4.3 kg/mol) block lengths were synthesized. The random copolymers (before and after deprotection) were fully amorphous and had their Tg ranging from -29 to -16 °C. Before deprotection, the blocks of the triblock copolymers were fully miscible, while triblock copolymers showed phase separation and were semicrystalline materials after deprotection. The PCL segments crystallize with a Tm ranging from 39 to 46 °C. The latter polymers are interesting for tissue engineering applications because they enable the formation of porous dimensionally stable materials at body temperature.