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}Me-2 (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 degrees C) with a living behavior. Highly effective (576 kg PMMA center dot mol Ti-1 center dot 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 degrees 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 congruent to 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)CCMe2)(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.