Synthesis and MMA polymerization of chiral ansa-zirconocene ester enolate complexes with C2- and Cs-ligation

A series of chiral ansa-zirconocene ester enolate complexes incorporating C₂- or C_s-symmetric ligands, including neutral rac-(EBI)ZrClOC(OⁱPr)CMe₂] (1), rac-(EBI)Zr(OTf)OC(OⁱPr)CMe₂] (2), rac-(EBI)Zr(OTf)OC(OMe)C(Me)CH₂C(Me₂)C(OⁱPr)O] (3), Me₂C(Cp)(Flu)]ZrMeOC(OⁱPr)CMe₂] (4), and cationic Me₂C(Cp)(Flu)]Zr⁺(THF)OC(OⁱPr)CMe₂]MeB(C₆F₅)₃]⁻ (5), have been synthesized. Within the neutral C₂-ligated zirconocene ester enolate series, the chloride derivative 1 is inactive toward any methyl methacrylate (MMA) additions, the methyl derivative rac-(EBI)ZrMeOC(OⁱPr)CMe₂] adds cleanly only 1 equiv. of MMA, and the triflate derivative 2 can add either 1 equiv. of MMA to form the single-MMA-addition product 3 or multiple equivalents of MMA to form P(MMA). Unlike the C_s-ligated methyl cation Me₂C(Cp)(Flu)ZrMe]⁺, which is inactive for MMA polymerization under various conditions, the C_s-ligated ester enolate cation 5 is moderately active for polymerization of MMA and N,N-dimethylacrylamide at ambient temperature; the resulting P(MMA) has a high molecular weight of M_n = 388 000 Da but a low syndiotacticity of rr] = 64%, and the polymerization conforms to a chain-end control mechanism.