Zwitterionic ring opening polymerization of lactide by metal free catalysts: Production of cyclic polymers

Tailor made N ‐heterocyclic carbene (NHC) catalyst precursors namely (+) and (−) 1‐methyl‐3‐menthoxymethyl imidazolium chloride have been synthesized in high yield with a literature modified procedure. A reaction of catalyst precursor with potassium tert butoxide in situ generates the NHC catalyst. The zwitterionic ring opening polymerization of lactide (LA) mediated by a catalytic system composed of NHC catalyst at 25°C under argon atmosphere led to a cyclic poly(lactide) of a high molecular weight with a narrow molecular weight distribution. The cyclic poly(lactide) was characterized by NMR spectroscopy, Gel Permeation Chromatography (GPC) and Matrix‐assisted laser desorption ionization‐time of flight mass spectrometry (MALDI‐TOF MS). The NHC catalysts are active for lactide polymerization in the presence of air and elevated temperatures at 55°C.