“Constrained Geometry” Titanium Complexes:  Exceptionally Robust Systems for Living Polymerization of Methacrylates at High Temperature and Model Studies toward Chain Transfer Polymerization with Thiols

The 1:1 combination of Ti{CGC}Me₂ (1; CGC = Me₂Si(Me₄C₅)(tBuN)) with B(C₆F₅)₃ was found to feature a so far unrevealed thermal robustness in methacrylate polymerization that enables it to operate in a broad temperature range (0−100 °C) with a living behavior. Highly effective (576 kg PMMA·mol Ti^-1·h^-1) and productive (monomer-to-Ti ratio up to 5000) homopolymerization of methyl methacrylate (MMA) and effective diblock and triblock copolymerization of MMA with butyl methacrylate (BMA) were thus achieved at 80 °C. The robust “constrained geometry” titanium system has been used to investigate thiols as possible chain transfer agents in MMA polymerization. Neutral alkylthiolato and thiophenolato complexes [Ti{CGC}(X)(Y)] (2, X = Me, Y = tBuS; 3, X = Me, Y = o-MeOC₆H₄S; 4, X = Y = iPrS; 5, X = Y = PhCH₂S) have been synthesized by protonolysis of 1 with thiols and shown to polymerize MMA once activated by a Lewis acid such as B(C₆F₅)₃. Combinations 1/B(C₆F₅)₃/tBuSH polymerized quantitatively MMA in toluene to yield PMMAs with narrow polydispersity (M_w/M_n ≅ 1.10), but no effective chain transfer was evidenced, whatever the conditions used. The stoichiometric reaction of tBuSH and o-MeOC₆H₄SH with the cationic enolate complex [Ti{CGC}(O(OiPr)CCMe₂)(THF)]⁺[MeB(C₆F₅)₃]^- (8) revealed that thiols do cleave the Ti−O(enolate) bond of 8 to give the alkylthiolato and thiophenolato titanium cationic species; however, this pathway proceeds remarkably slowly in comparison with that with a similar Zr−O(enolate) bond.