Neutral Metallocene Ester Enolate and Non-Metallocene Alkoxy Complexes of Zirconium for Catalytic Ring-Opening Polymerization of Cyclic Esters

Four neutral zirconocene bis(ester enolate) and non-zirconocene bis(alkoxy) complexes have been employed for ring-opening polymerizations and chain transfer polymerizations of l -lactide ( l -LA) and ϵ-caprolactone (ϵ-CL). All C 2 v -, C 2 -, and C s -ligated neutral zirconocene bis(ester enolate) complexes effectively polymerize ϵ-CL at 80 °C with high (>90%) initiator efficiencies. The C s -ligated complex Ph 2 C(Cp)(Flu)Zr[OC(O i Pr)═CMe 2 ] 2 ( 1 ) also promotes highly efficient polymerization of l -LA and is at least 100 times more reactive than the C 2 v - and C 2 -ligated analogues. The l -LA polymerization by 1 exhibits living characteristics, producing PLA with quantitative isotacticity (no sign of monomer epimerization) and controlled molecular weight. This polymerization follows first-order kinetics with respect to both [ l -LA] and [ 1 ], consistent with a monometallic, coordination–anionic propagation mechanism. The structurally characterized non-zirconocene bis(alkoxy) complex [(2,6- i Pr 2 C 6 H 4 )N(CH 2 ) 3 N(2,6- i Pr 2 C 6 H 4 )]Zr(O i Pr) 2 [O═C(NMe 2 )CHMe 2 ] ( 2 ) has been synthesized in quantitative yield from the reaction of the zirconium bis(amido) precursor with isopropyl isobutyrate. Complex 2 behaves as a single-site catalyst for polymerizations of both ϵ-CL and l -LA, unlike the comparative homoleptic zirconium tetraisopropoxy complex, which shows multisite behavior for the same processes. Both the zirconocene 1 and the non-zirconocene 2 catalyze efficient chain transfer polymerization in the presence of i PrOH as a chain transfer reagent (CTR) for the catalytic production of PLA and PCL, respectively; however, the metallocene system is more robust (in terms of ligand stability and maintaining the polymerization rate) toward an excess of a protic CTR than the present non-metallocene system.