Binuclear aluminum Lewis acid and its behavior in the polymerization of methyl methacrylate and n-butyl acrylate

Abstract

Bimetallic aluminum complex [Al(mbmp)Me]₂ chelating 2,2′-methylenebis(6-tert-butyl-4-methylphenol) (mbmpH₂) was synthesized and utilized as a powerful Lewis acid for Lewis pair polymerization (LPP). The hydrolysis of [Al(mbmp)Me]₂ with water resulted in the formation of dimeric hydroxide [(O∼∼CH₂∼∼O)AlOH]₂, with phenol oxygen atoms adopting μ–η¹-η² mode. Single-crystal X-ray analysis showed a C₂ axis of symmetry, which relates the coordination spheres of each Al center. Combining [Al(mbmp)Me]₂ with commercially available Lewis bases, including phosphines and carbenes, afforded frustrated Lewis pairs, which were employed for methyl methacrylate (MMA) and n-butyl acrylate (nBA) polymerization. Methyl methacrylate (MMA) was polymerized at 25 °C in toluene in a high-speed manner (TOF = 2400 h⁻¹) by [Al(mbmp)Me]₂/PCy₃ to give poly(methyl methacrylate) with high molecular weights and narrow molecular weight distributions (M_n = 14 500 g mol⁻¹, M_w/M_n = 1.1). MALDI-TOF Ms indicated the formation of both a cyclic chain end and linear chain end. The different requirements of Lewis acid (LA)/Lewis base (LB) matching for MMA and nBA polymerization were revealed by polymerization experiments. In contrast to MMA LPP, nBA LPP needs a weaker Lewis acid and stronger Lewis base to ensure the polymerization proceeds. In comparison with the monomeric Al Lewis and its counterpart phosphine LB in MMA polymerization, the bimetallic organoaluminum exhibited better control of molecular weight and molecular distribution of the resulting polymer and required only equal molar ratios of LA/LB to complete the polymerization. The reaction mechanism was tentatively proposed through a bimetallic Al Lewis acid synergetic effect.