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 C2v-, C2-, and Cs-ligated neutral zirconocene bis(ester enolate) complexes effectively polymerize ϵ-CL at 80 °C with high (>90%) initiator efficiencies. The Cs-ligated complex Ph2C(Cp)(Flu)Zr[OC(OiPr)CMe2]2 (1) also promotes highly efficient polymerization of l-LA and is at least 100 times more reactive than the C2v- and C2-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-iPr2C6H4)N(CH2)3N(2,6-iPr2C6H4)]Zr(OiPr)2[OC(NMe2)CHMe2] (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 iPrOH 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.