Mechanistic Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co-polymerization of Epoxides and CO sub(2)

The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co-polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co-polymerization experiments with propylene oxide (PO) and CO sub(2) revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO sub(2) co-polymerization was not stable enough to deactivate the catalyst due to steric repulsion. Put to rest: The most active catalytic system for the co-polymerization of cyclohexene oxide and CO sub(2) was thoroughly investigated toward propylene oxide/CO sub(2) co-polymerization. Experimental results and DFT calculations revealed that the received low conversions are a consequence of the formation of an energetically highly stable resting state as soon as the catalyst is exposed to both propylene oxide and CO sub(2). This enables neither further epoxide ring openings nor CO sub(2) insertion reactions (see figure).