Lipase-catalyzed transformation of poly(lactic acid) into cyclic oligomers

Enzymatic transformations into cyclic oligomers were carried out with the objective of developing chemical recycling of poly(lactic acid)s, such as poly(D),L-lactic acid) (PDLLA), poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA), which are typical biodegradable polymers. They were degraded by lipase in an organic solvent to produce the corresponding cyclic oligomer with a molecular weight of several hundreds. PDLLA (with a M̄_W of 84 000) was quantitatively transformed into cyclic oligomers by lipase RM (Lipozyme RM IM) in chloroform/hexane at 60 °C. PLLA (with a M̄_R of 120 000) was transformed into cyclic oligomer by lipase CA (Novozym 435) at a higher temperature of 100 °C in o-xylene. The oligomer structure was identified by ¹H and ¹³C NMR spectroscopy and MALDI-TOF (matrix assisted laser desorption/ionization-time-of-flight) mass spectrometry.