Neutral metallocene ester enolate and non-metallocene alkoxy complexes of zirconium for catalytic ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1021/om800602s

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 e-caprolactone (ε-CL). All C_2v-, C₂-, 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 ₂C(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 C_2v.,- 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 firstorder kinetics with respect to both [L-LA] and [1], consistent with a monometallic, coordinationanionic propagation mechanism. The structurally characterized non-zirconocene bis(alkoxy) complex [(2,6-ⁱPr₂C₆H ₄)N(CH₂)₃N(2,6-ⁱPr₂C ₆H₄)]Zr(OⁱPr)₂[O=C(NMe ₂)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.