Candida antartica Lipase B Catalyzed Polycaprolactone Synthesis: Effects of Organic Media and Temperature

Abstract

Engineering of the reaction medium and study of an expanded range of reaction temperatures were carried out in an effort to positively influence the outcome of Novozyme-435 (immobilized Lipase B from Candida antarctica) catalyzed ε-CL polymerizations. A series of solvents including acetonitrile, dioxane, tetrahydrofuran, chloroform, butyl ether, isopropyl ether, isooctane, and toluene (log P from −1.1 to 4.5) were evaluated at 70 ^οC. Statistically (ANOVA), two significant regions were observed. Solvents having log P values from −1.1 to 0.49 showed low propagation rates (≤30% ε-CL conversion in 4 h) and gave products of short chain length (M_n ≤ 5200 g/mol). In contrast, solvents with log P values from 1.9 to 4.5 showed enhanced propagation rates and afforded polymers of higher molecular weight (M_n = 11500−17000 g/mol). Toluene, a preferred solvent for this work, was studied at ε-CL to toluene (wt/vol) ratios from 1:1 to 10:1. The ratio 1:2 was selected since, for polymerizations at 70 °C, 0.3 mL of ε-CL and 4 h, gave high monomer conversions and M_n values (∼85% and ∼17 000 g/mol, respectively). Increasing the scale of the reaction from 0.3 to 10 mL of CL resulted in a similar isolated product yield, but the M_n increased from 17 200 to 44 800 g/mol. Toluene appeared to help stabilize Novozyme-435 so that lipase-catalyzed polymerizations could be conducted effectively at 90 °C. For example, within only 2 h at 90 ^οC (toluene-d₈ to ε-CL, 5:1, ∼1% protein), the % monomer conversion reached ∼90%. Also, the controlled character of these polymerizations as a function of reaction temperature was evaluated.