A series of 4-, 5- and 6-carbon natural polyols were studied as monomers for immobilized Candida antarctica Lipase B (CALB) catalyzed polymerizations. Reactions were performed with a molar ratio of adipic acid to 1,8-octanediol to polyol of 1.0:0.8:0.2. Molecular weight increase as a function of reaction time was measured. The following is the relative order of M_w as a function of polyol structure for 46 h polymerizations: D-mannitol (73.0 ± 0.4 K) > erythritol (38.1 ± 4.4 K), xylitol (42.3 ± 2.2 K), ribitol (38.4 ± 2.9 K) > D-glucitol (27.7 ± 2.0 K) > galactitol (11.0 ± 0.9 K). Replicate experiments using a needle valve and a digital vacuum regulator showed that, reproducible results were obtained only with the digital vacuum regulator that more accurately controlled the reduced pressure in reaction vessels. From this limited set of polyols, no apparent correlation was found between polyol chain length and its polymerization by CALB catalysis. Plots of log[η] vs. logM_w were prepared from SEC-MALS-Viscosity analyses of poly(octamethylene adipate) and corresponding terpolymers with galactitol, D-glucitol and D-mannitol. Comparison of exponent a values from slopes of these plots showed copolymers from D-mannitol had the largest degree of branching and, therefore, greatest propensity for combined reactivity at both primary and secondary hydroxyl groups. Differences in alditol reactivity were analyzed with respect to their stereochemical configurations at polyol β-carbons, closest to terminal primary hydroxyl groups, and secondary hydroxyl groups, that upon reaction lead to branching.
展开▼
