Syntheses of cyclic poly(lactones) by zwitterionic ring opening polymerization catalyzed by N‐heterocyclic carbene

Synthesis of cyclic biopolymers from renewable monomers remains a big challenge because of lack of efficient catalysts. The organocatalyst of N‐heterocyclic carbene (NHC), (+)‐1‐methyl‐3‐menthoxymethyl imidazol‐2‐ylidene, is used to prepare cyclic polylactones including poly(ε‐caprolactone) (poly(ε‐CL)), poly(δ‐valearolactone) (poly(δ‐VL)), and poly(ε‐caprolactone‐co‐δ‐valearolactone) (poly(ε‐CL‐co‐δ‐VL)) via zwitterionic ring opening polymerization. The NHC catalyst is founded a highly efficient organic catalyst for the polymerization. The resulting cyclic polymers show a melting temperature (T_m) in a range of 20–60°C, which is dramatically lower than the T_m of cyclic poly(lactide) (T_m = 120–150°C). The resulting copolymer, cyclic poly(ε‐CL‐co‐δ‐VL) owns high molecular weight comparing with corresponding linear poly(ε‐CL‐co‐δ‐VL) produced by other catalysts. The synthesized cyclic homo and copolymers were characterized by ¹H‐, ¹³C‐NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry–thermogravimetric analysis and matrix‐assisted laser desorption ionization‐time of flight mass spectrometry. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013