Enzyme-catalyzed ring-opening polymerization of ε-caprolactone in supercritical carbon dioxide

We report the ring-opening polymerization reaction of c-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(c-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(epsilon-caprolactone) (PCL) in the complete absence of potentially toxic organic solvents or metal catalysts.