Enzyme-Catalyzed Ring-Opening Polymerization of ε-Caprolactone in Supercritical Carbon Dioxide

Abstract

We report the ring-opening polymerization reaction of ε-caprolactone in supercritical carbon dioxide (scCO 2 ) using an enzyme catalyst, Lipase B from Candida antarctica supported on macroporous beads (Novozym-435). Ring-opening polymerization of lactones is more commonly performed in organic solvents or in bulk using a Lewis acid catalyst. Recently there has been much interest in the replacement of such catalysts by enzymes. We demonstrate that the enzymatic route is viable in scCO 2, yielding poly(ε-caprolactone) ( M n = 12 000−37 000 g mol -1 ) with molecular weights very similar to those obtained from the same enzyme catalysts in organic solvents, but with lower polydispersities (typical PDI = 1.4−1.6) and higher yields of polymer product (typically 95−98%). In the same process the unique “gaslike” mass transfer properties of scCO 2 can also be exploited to remove quantitatively any unconverted monomer and low molecular weight oligomers by scCO 2 extraction. It is also shown that the enzyme catalyst can be cleaned and recycled using scCO 2, while still producing high molecular weight polymer ( M n = 35 000−37 000 g mol -1 ). Thus, a combination of enzyme catalyst and scCO 2 can be used repeatedly to prepare biodegradable poly(ε-caprolactone) (PCL) in the complete absence of potentially toxic organic solvents or metal catalysts.