No abstract

The effects of substrates and solvent on polymer formation, number-average molecular weight (M(n)), polydispersity, and end-group structure for lipase-catalyzed polycondensations were investigated. Diphenyl ether was found to be the preferred solvent for the polyesterification of adipic acid and 1,8-octanediol giving a M(n) of 28 500 (48 h, 70 degrees C). The effect of varying the alkylene chain length of diols and diacids on the molecular weight distribution and the polymer end-group structure was assessed. A series of diacids (succinic, glutaric, adipic, and sebacic acid) and diols (1,4-butanediol, 1,6-hexanediol, and 1,8-octanediol) were polymerized in solution and in bulk. It was found that reactions involving monomers having longer alkylene chain lengths of diacids (sebacic and adipic acid) and diols (1,8-octanediol and 1,6-hexanediol) give a higher reactivity than reactions of shorter chain-length diacids (succinic and glutaric acid) and 1,4-butanediol. The bulk lipase-catalyzed condensation reactions were feasible, but the use of diphenyl ether gave higher M(n) values (42,400 g/mol in 3 days at 70 degrees C). The polydispersity varied little over the conditions studied giving values

Studies of the kinetics and mechanism of Candida antarctica Lipase B (CALB) catalyzed -caprolactone ( -CL) polymerizations in toluene were performed. The kinetic plot of ln ([M]0/[M]t) vs time was carried out to 96% -CL conversion and Mn 11 970. The plot is linear (r 2 ) 0.998), indicating that termination did not occur and the propagation rate is first order with respect to monomer concentration. Changes in the water (e.g., initiator) concentration did not change the polymerization rate but did change the number of chains [R-OH]. Thus, the polymerization is zero order with respect to [R-OH] and initiator concentration. A plot of ln k app vs ln [enzyme] gave 0.7 as the reaction order of the enzyme concentration. The apparent activation energy for Novozyme-435 catalyzed -CL polymerization in toluene is 2.88 kcal mol -1 . This is well below 10.3 kcal mol -1 , the activation energy for aluminum alkoxide catalyzed -CL polymerization in toluene. Upward deviation from linearity for Mn vs fractional -CL conversion and decreases in the number of chains was accentuated by low enzyme water contents and high monomer conversion. These results are consistent with a competition between ring-opening chain-end propagation and chain growth by steplike polycondensations. CALB was irreversibly inhibited by modification with paraoxon at the lipase active site (Ser105). The modified enzyme was no longer active for the polymerization. This supports that the polymerizations studied herein occurred by catalysis at the active serine residue (Ser105) and not by other chemical or nonspecific protein-mediated processes.

Immobilized Candida antarctica Lipase B (Novozyme-435) was studied for bulk polyesterifications of linear aliphatic hydroxyacids of variable chain length. The products formed were not fractionated by precipitation. The relative reactivity of the hydroxyacids was l6-hydroxyhexadecanoic acid approximately 12-hydroxydodecanoic acid approximately 10-hydroxydecanoic acid (DPavg congruent with 120, Mw/Mn 6-hydroxyhexanoic acid (DPavg congruent with 80, Mw/Mn < or = 1.5, 48 h, 90 degrees C). Remarkable improvements in molecular-weight buildup resulted from leaving water in the reaction. By 4 h, without application of vacuum, the DPavg for 12- and 16-carbon hydroxyacids was about 90. In contrast, with identical substrates and water removal, the DPavg at 4 h was about 23. Large differences in the molecular-weight build up of 12-hydroxydodecanoic acid were observed for catalyst concentrations (%-by-wt relative to monomer) of 0.1, 0.5, 1, and 10. Nevertheless, by 24 h, with 1% catalyst containing 0.1% lipase, poly(12-hydroxydodecanoic acid) with Mn 17 600 was formed. For 12-hydroxydodecanoic acid polymerization at 90 degrees C, the catalyst activity decreased by 7, 18, and 25% at reaction times of 4, 24, and 48 h, respectively. Furthermore, the retention of catalyst activity was invariable as a function of the substrates used.

Polymerizations of -CL catalyzed by Novozyme-435 (immobilized Lipase B from Candida antarctica) were studied at temperatures between 20 and 108 °C. The monomer conversion to polymer was remarkably rapid at ambient temperature. At 20 °C by 7 h, -CL conversion and product M n were >97% and 17 800, respectively. Contrary to previous reports, the number of chains formed, as well as the product molecular weight, was almost identical for polymerizations at constant enzyme water content between 60 and 108 °C. Thus, differences in reaction temperature over a 48 °C range did not "free" water from "bound" states so that it could function for chain initiation. At 60 °C, variation in the enzyme water content from 0.6 to 1.9% increased the number of chains formed but did not change the polymerization propagation kinetics. Therefore, the enzyme water content and not the reaction temperature regulated the product molecular weight. In contrast, at 108 °C, an increase in the reaction water content from 0.6 to 1.8% increased both the number of chains and the polymerization propagation kinetics. Explanations for these differences in behavior as a function of temperature and water contents are discussed.