Synthesis, Modification, and Characterization of l-Lactide/2,2-[2-Pentene-1,5-diyl]trimethylene Carbonate Copolymers

Abstract

This paper explores the copolymerization of l-lactide (l-LA) with 2,2-[2-pentene-1,5-diyl]trimethylene carbonate (cHTC). Since cHTC has a cyclohexene group, this provided a route for preparing poly(lactic acid), (PLA), based chains decorated with controlled quantities of CC substituents. Ring-opening copolymerizations of l-LA with cHTC were successfully conducted in bulk by using AlR3−H2O (R = ethyl, isobutyl), Al(OiPr)3, ZnEt2−H2O and Sn(Oct)2 as catalysts. Comparison of these copolymerizations showed that the Sn(Oct)2 catalyst system gave copolymers of relatively higher molecular weight. Increasing the reaction time of Sn(Oct)2 catalyzed copolymerizations from 6 to 24 h resulted in higher copolymer cHTC content and yield but lower copolymer molecular weight. Variation of the comonomer feed ratio was useful in regulating the content of cyclohexene pendant groups in the copolymer. However, regardless of the catalyst used, the mole percent of cHTC incorporated into the copolymer was lower than that used in the monomer feed. Determination of the comonomer reactivity ratios for Sn(Oct)2 catalyzed copolymerizations gave values of 8.8 and 0.52 for l-LA and cHTC, respectively. All gel permeation chromatography (GPC) traces showed unimodal molecular weight distributions. Determination by 13C-NMR of the copolymer sequence fractions HLL, LLL, LLH, HLH, HL, and LH (H = cHTC units, L = l-lactyl units) showed that they were close to those calculated by assuming a Bernoulli statistical propagation. On the basis of these results and the effects of reaction conditions on the copolymer sequence distribution, a mechanism which involves insertion of cHTC into the polymer chain was proposed. Studies by differential scanning calorimetry (DSC) showed that cHTC units in the copolymers disrupted ordering of the l-PLA crystalline phase. Furthermore, the glass transition temperatures (Tg) ranged from 60 (l-PLA) to 33 °C (P(cHTC)). Conversion of CC to epoxy side groups was successfully carried out by using 3-(chloroperoxy)benzoic acid at room temperature with only small decreases in copolymer molecular weight.