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 Cp 2 ZrMe 2 /B(C 6 F 5 ) 3 system. Addition of 1−10 equiv of acetophenone, acetone, or thiophenols inhibits polymerization, while the system tolerates the presence of tert -butylthiol ( t BuSH). 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 [Cp 2 Zr(THF)(O( t BuO)C CMe 2 )] + [MeB(C 6 F 5 ) 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 [Cp 2 Zr(SR)(THF)] + cations (R = t Bu, 4; SC 6 H 4 - p -Cl, 6; SC 6 H 4 - o -OMe, 7 ). The crystal structure of 7 has been determined. In the presence of a tert -butyl ester R‘CO 2 t Bu, thiolato complexes 4, 6, and 7 smoothly decompose into the corresponding cationic carboxylato complex [Cp 2 Zr(THF)(O 2 CR‘)] + (R‘ = i Pr, 5; CH 3, 9 ) and thiol RSH, with release of isobutene. tert -Butylthiolato complex 4 and the in situ combination Cp 2 Zr(S t Bu)Me/B(C 6 F 5 ) 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.