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 (P^tBu₃) 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 step. Both computational and experimental data support the existence of a cooperative mechanism during the TTMPP/Me₃SiNTf₂ Lewis pair-mediated polymerization of MMA.