Postulation of the mechanism of the selective synthesis of isotactic poly(methyl methacrylate) catalysed by [Zr{(Cp)(Ind)CMe2}(Me)(thf)](BPh4):: A Hartree-Fock, MP2 and density functional study

The bridged, C-1-symmetric, single-component zirconocene [Zr{(Cp)Ind)CMe2}(Me)(thf)](BPh4) (Ind=indenyl, Cp = cyclopentadienyl) polymerises methyl methacrylate (MMA) selectively to isotactic poly(methyl methacrylate) (PMMA) without further cocatalysts or activators. To elucidate the stereoselective steps of the polymerisation of MMA by using this catalyst we studied the propagation steps occurring with the derivative [Zr{(Cp)(Ind)CH2}-O-C(OMe)-C(Me)(Et)}(MMA)] by ab initio calculations at the Hartree-Fock(HF) level of theory. After the initiation step, which consumes the first two MMA molecules, each new catalytic cycle begins with the stereoselective addition of a new MMA molecule at the indenyl side of the zirconocene fragment. At the same time the enolate ring undergoes a stereoselective inplane ring shift to the side opposite to the indenyl ring. These findings are used to postulate a mechanism for the polymerisation that explains the stereoselective synthesis of isotactic PMMA.