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 epsilon -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 degreesC. Statistically (ANOVA), two significant regions were observed. Solvents having log P values from -1.1 to 0.49 showed low propagation rates (less than or equal to 30% epsilon -CL conversion in 4 h) and gave products of short chain length (M-n less than or equal to 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 epsilon -CL to toluene (wt/vol) ratios from 1:1 to 10:1. The ratio 1.2 was selected since, for polymerizations at 70 degreesC, 0.3 mL of epsilon -CL and 4 h, gave high monomer conversions and M-n values (similar to 85% and similar to 17 000 g/mol, respectively). Increasing the scale of the reaction from 0.3 to 10 mt 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 degreesC. For example, within only 2 h at 90 degreesC (toluene-d(8) to epsilon -CL, 5:1, similar to1% protein), the % monomer conversion reached similar to 90%. Also, the controlled character of these polymerizations as a function of reaction temperature was evaluated.