Enzymatic polymerization can offer metal-free routes to polymer materials that could be used in biomedical applications. To take advantage of the unique properties of ionic liquids (ILs) for enzyme stability, monocationic ionic liquid (MIL) and dicationic ionic liquid (DIL) were used to promote the ring-opening polymerization of epsilon-caprolactone (epsilon-CL) using Candida antarctica lipase B as catalyst. Considering the molecular weight (M (n) ) and reaction yield of the resulting polymer (PCL), high density and viscosity of ILs would be good, especially in the case of DIL. With the same total alkyl chain length, the density and viscosity of [C-4(C(6)Im)(2)][PF6](2) were higher than that of [C(12)MIm][PF6]. Using a lipase/CL/ILs ratio of 1:20:20 (by wt) for 48 h at 90 A degrees C, the highest M (n) and reaction yield of PCL were 26,200 g/mol and 62 % with [C-4(C(6)Im)(2)][PF6](2), while the M (n) and reaction yield of PCL obtained in [C(12)MIm][PF6] were 11,700 g/mol and 37 %.