Depolymerization of Polycaprolactone to 6-hydroxyhexanoic Acid by Candida antarctica Lipase B in Aqueous Media and in Toluene/Water Emulsions

In today’s world, the widespread utilization of plastics results in millions of tons of waste plastics being generated every year, which ultimately has become a great problem because of its environmental impact. Recycling of plastics can relieve this problem, as it prevents the loss of the material and energy used in their production and contributes to the reduction of CO₂ emissions. Depolymerization of plastic waste back to the original monomers seems to be the soundest alternative for plastics recycling, as it allows the production of the same original polymer or, eventually, the upcycling to other valuable chemicals. Polycaprolactone (PCL), one of the most widely used biodegradable polyesters, was subjected to enzymatic depolymerization to its monomer, 6-hydroxyhexanoic acid (6-HHA), catalyzed by a commercial Candida antarctica lipase B (CalB) following two alternative processes. The first one was carried out in aqueous medium and allowed the complete depolymerization of 100 g/L PCL in less than 24 h in an optimized phosphate-buffered medium. The second one is a novel two-phase system process in toluene/water emulsion, where PCL was initially dissolved in the organic phase and the hydrolysis product, 6-HHA, was accumulated in the buffered aqueous phase through the action of CalB. The highest depolymerizing activity among all the conditions tested, either in aqueous medium or in emulsion, was found in emulsion when the pH buffer was Tris-HCl, producing up to 71.2 g/L 6-HHA in only 5 h of enzyme treatment and achieving complete depolymerization of PCL in less than 24 h.