Synthesis and characterization of poly(butylene succinate-co-butylene malate):: A new biodegradable copolyester bearing hydroxyl pendant groups

A new biodegradable copolyester, poly(butylene succinate-co-butylene malate) P(BS-co-BM), has been preliminarily prepared with optically active centers and lateral hydroxyl functional groups via a four-step synthetic strategy. First, an optically active benzyl-protected dimethyl malate was synthesized from a starting material of (S)-dimethyl malate and purified with good yield. Then, copolyester poly(butylene succinate-co-benzyl-protected butylene malate), P(BS-co-BBM), was prepared through a skilled condensation copolymerization of the benzyl-protected dimethyl malate, dimethyl succinate, and 1,4-butanediol in the presence of titanium tetraisopropoxide as the catalyst. Finally, a Pd/C catalyzed hydrogenation was applied to eliminate the benzyl protection group in a mixed solution of THF and methanol; thus the target copolyester P(BS-co-BM) was attained. On the other hand, physical properties of the synthesized copolyesters were systematically characterized by means of nuclear magnetic resonance spectrometer, Fourier transformed infrared spectrometer, gel permeation chromatography, optical polarimeter, quantitative hydroxyl titration, and thermal analytical instruments. The experimental evidence demonstrated a successful construction of the product P(BS-co-BM) bearing lateral hydroxyl functional groups. It was also revealed that the lower BBM unit content was in the benzyl-protected optically active P(BS-co-BBM) copolyester, the higher melting point T-m, crystallinity, the broader molecular distribution, and the lower glass transition temperature T-g would be detected, and these results can be accounted for the presence of bulky lateral benzyl moieties. In contrast, the deprotected product P(BS-co-55 mol % BM) showed a higher T-m, crystallinity and lower T-g than its counterpart P(BS-co-55 mol % BBM). Interestingly, a thermal stability as high as that of the linear PBS was observed for P(BS-co-55 mol % BM) while a strong BBM unit content dependence of thermal stability was detected for the benzyl-protected copolyester P(BS-co-BBM)s. Therefore, these results may be beneficial for the new optically active P(BS-co-BM) bearing hydrophilic hydroxyl functional groups as a potential biomaterial.