Mechanistic Studies of Redox-Switchable Copolymerization of Lactide and Cyclohexene Oxide by a Zirconium Complex

Several aspects of the copolymerization of l-lactide (LA) and cyclohexene oxide (CHO) by a redox-switchable zirconium catalyst, (salfan)­Zr­(OtBu)2 (salfan = 1,1′-bis­(2-tert-butyl-6-N-methylmethylenephenoxy)­ferrocene), were examined, such as the mechanism of cyclohexene oxide polymerization, the reactivity of [(salfan)­Zr­(OtBu)2]­[BArF] (BArF = tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate) toward lactide, and comonomer effects on polymerization rates. Experimental methods and DFT calculations indicate that the likely mechanism of CHO polymerization by [(salfan)­Zr­(OtBu)2]­[BArF] is coordination insertion and not a cationic pathway, as employed by the majority of cationic catalysts. Furthermore, DFT calculations showed that the polymerization of LA by [(salfan)­Zr­(OtBu)2]­[BArF] is not thermodynamically favored, in agreement with experimental results. Finally, we found that the conversion times of CHO or LA from block to block correlate with the amount of monomer left from the previous block rather than other factors.