Bulk Ring-Opening Polymerization of ε-Caprolactone by Zeolitic Imidazolate Framework

Abstract In this work, for the first time, zeolitic imidazolate frameworks (ZIFs = ZIF-8, ZIF-67, and Zn/Co-ZIF), obtained via spray drying, were used as heterogeneous catalysts for the ring opening polymerization of ε-Caprolactone without any co-catalyst or initiator. The catalytic activity of the spray dried ZIFs series varies in the order of Zn/Co-ZIF < ZIF-67 < ZIF-8. It was found that the catalytic activities of ZIFs series correlate with the defects present in their structure. Among active sites of ZIFs, only the acid sites derived from structural defects are supposed to catalyse the polymerization reaction. Under optimal reaction conditions, ZIF-8 shows a superior catalytic activity compared to ZIF-67 or Zn/Co-ZIF and leads to the formation of higher molecular weight linear polycaprolactone. The recyclability test confirmed that the catalyst could be reused and no significant loss of catalytic activity was found after three uses. Graphical Abstract