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Abstract:
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 tert-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 tert-butyl ester R‘CO2tBu, 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. tert-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 (Mn/Mw = 1.26−1.48), but with molecular weight much higher than the expected Mn values, consistent with poor initiation efficiency and/or instability.
