Strained surface siloxanes as a source of synthetically important radicals

Abstract

The calcination of pure amorphous silica at temperatures of up to 850 °C results in the formation of strained siloxane rings, which at room temperature, are capable of undergoing homolytic cleavage at room temperature to generate radicals when in the presence of an appropriate substrate. These surface radicals are shown to act as radical initiators for the polymerisation of methyl acrylate (MA). Silica calcined at 600 °C (SiO₂₍₆₀₀₎) was found to be the most active radical initiator. For example, in hexane at 70 °C, a yield of poly(methyl acrylate) (PMA) of 77% was obtained with a polydispersity index (PDI) of 1.4 after 24 h. In supercritical CO₂ (sc-CO₂) at 210 °C for 1 h, a yield of PMA of 85% with a PDI of 1.3 was obtained. The use of polyhedral oligo-silsesquioxanes (a6b0 and a7b3) as silica surface model compounds confirms the need for strained siloxane rings to induce radical polymerisation. Mass spectroscopic studies of silsesquioxane a6b0 in the presence of MA reveal the presence of numerous addition products resulting from the homolytic breaking of Si–O–Si bonds to form potential reaction intermediates.