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.