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 Cs-symmetric ligands, including neutral rac-(EBI)ZrCl[OC(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)]ZrMe[OC(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)ZrMe[OC(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 Cs-ligated methyl cation [Me2C(Cp)(Flu)ZrMe]+, which is inactive for MMA polymerization under various conditions, the Cs-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 Mn = 388 000 Da but a low syndiotacticity of [rr] = 64%, and the polymerization conforms to a chain-end control mechanism.

Graphical abstractWhile the zirconocene methyl cation with Cs-ligation {[Me2C(Cp)(Flu)ZrMe]+} and the neutral methyl zirconocene ester enolate with C2-ligation {rac-(EBI)ZrMe[OC(OiPr)CMe2]} are inactive for MMA polymerization, their respective cationic ester enolate and triflate derivatives are active for polymerization of MMA affording high molecular weight polymers.Figure optionsDownload full-size imageDownload as PowerPoint slide