Glycol-functionalized ionic liquids for high-temperature enzymatic ring-opening polymerization

Abstract

Enzymatic ring-opening polymerization (ROP) is a benign method for preparing polyesters, such as polylactides and other polylactones. These reactions are typically carried out at relatively high temperatures (60–130 °C), however, there is a deficiency of enzyme-compatible solvents for such thermally-demanding biocatalytic processes. In this study, we have prepared a series of short-chained glycol-grafted ionic liquids (ILs) based on a phosphonium, imidazolium, pyridinium, ammonium, or piperidinium cationic headgroup. Most of these glycol-grafted ILs exhibit relatively low dynamic viscosities (33–123 mPa s at 30 °C), coupled with excellent short-term thermal stabilities with decomposition temperatures (T_dcp) in the 318–403 °C range. Significantly, the long-term thermal stability under conditions matching those for enzymatic ROP synthesis (130 °C for 7 days) is excellent for several of these task-specific ILs. Using Novozym 435-catalyzed ROP, these ILs are demonstrated to be viable solvents for the enzymatic production of reasonable yields (30–48%) of high molecular mass (M_w ∼20 kDa) poly(L-lactide) and poly(ε-caprolactone) compared to solventless conditions (12–14 kDa).