Organic Lewis Pairs Based on Phosphine and Electrophilic Silane for the Direct and Controlled Polymerization of Methyl Methacrylate: Experimental and Theoretical Investigations

Fully organic Lewis pairs, combining a phosphine such as tri-n-butylphosphine (PnBu3), tritert-butylphosphine (PtBu3) or tris(2,4,6-trimethoxyphenyl) phosphine (TTMPP) as a Lewis base, and N-(trimethylsilyl)bis(trifluoromethane sulfonyl)imide (Me3SiNTf2) as a Lewis acid, are shown to directly initiate the polymerization of methyl methacrylate (MMA) at room temperature in toluene. A dual reaction mechanism involving an optimal TTMPP:Me3SiNTf2 ratio of 1:2 accounts for the control of the polymerization. Molar masses of poly(methyl methacrylate)’s (PMMA’s) can be varied by the initial [MMA]0/[TTMPP]0 molar ratio. Chain extension experiments confirm that a majority of chains of a TTMPP/Me3SiNTf2-derived PMMA can be reactivated. Both density functional theory (DFT) calculations and stoichiometric studies reveal that TTMPP and Me3SiNTf2 form a P-silyl phosphonium intermediate that is in equilibrium with the corresponding frustrated Lewis pair (FLP). This FLP could correspond to the active form of the initiation...