Model studies of methacrylate chain transfer polymerization mediated by cationic zirconocene tert-butyl enolate

Enolizable ketones and thiols have been investigated as potential transfer agents for methyl methacrylate (MMA) polymerization mediated by the Cp2ZrMe2/B(C6F5)(3) system. Addition of 1-10 equiv of acetophenone, acetone, or thiophenols inhibits polymerization, while the system tolerates the presence of tent-butylthiol (tBuSH). In this case, a moderate decrease in the molecular weight of the PMMAs is observed. The stoichiometric reactivity of these organic acids toward the cationic ester enolate complex [Cp2Zr(THF)(O(tBuO)C=CMe2)](+)[MeB(C6F5)(3)](-) (1), which models the active species for MMA polymerization, has been investigated. Ketones undergo aldolization reactions with 1 to generate species that are inactive in MMA polymerization. Thiols readily cleave the Zr-O bond of 1 to give [Cp2Zr(SR)(THF)](+) cations (R = tBu, 4; SC6H4-p-Cl, 6; SC6H4-o-OMe, 7). The crystal structure of 7 has been determined. In the presence of a tent-butyl ester R'CO(2)tBu, thiolato complexes 4, 6, and 7 smoothly decompose into the corresponding cationic carboxylato complex [Cp2Zr(THF)(O2CR')](+) (R' = iPr, 5; CH3, 9) and thiol RSH, with release of isobutene. tent-Butylthiolato complex 4 and the in situ combination Cp2Zr(StBu)Me/B(C6F5)(3) polymerized quantitatively MMA in toluene to yield PMMAs with narrow dispersity (M-n/M-w = 1.26-1.48), but with molecular weight much higher than the expected M-n values, consistent with poor initiation efficiency and/or instability.