Reactions of Gas Phase Monomers with Radicals on the Surface of Silica: Reversibility of the Radical Addition Reaction in the SO2−Alkene System

Abstract

Chemically and mechanically activated silica provides a convenient system to prepare surface stabilized radicals of the chosen type. A combination of the volumetric techniques and electron spin resonance spectroscopy was used to characterize the reactions of para- and diamagnetic defects on the surface of the activated silica with simple molecules (H2, O2, CO, CO2, SO2) and monomers (ethylene, tetrafluoroethylene, cyclopentene, methyl acrylate, and allyl alcohol) from the gas phase. Bimolecular rate constants as high as 106 L·mol-1·s-1 could be determined. A significantly strong adsorption of the molecules containing polar groups was shown to be a limitation for the kinetics experiments with surface-grafted radicals. The study of the kinetics of the cross-reactions in the alternating radical copolymerization of SO2 with alkenes was carried out in the regime of the “stopped chain growth”. The reversibility of the SO2 addition to alkyl radicals was observed in real time with the stability (at 298 K) of the sulfonyl radicals decreasing in the order ⋮SiC2F4SO2• (Keq > 1016 L·mol-1) > ⋮SiC2H4SO2• (Keq = 4.9 × 108 L·mol-1) > ⋮SiC5H10SO2• (Keq = 6.8 × 107 L·mol-1). The rate constants at 298 K for the SO2 addition were found to be >107, 59 (ΔEa ∼ 9.3 kJ·mol-1), and 1.7 × 104 L·mol-1·s-1 in this sequence. The corresponding rate constants for the SO2 dissociation were 1.2 × 10-7 and 2.5 × 10-4 s-1 for the ethylsulfonyl and cyclopentylsulfonyl radicals; tetrafluoroethylsulfonyl radicals were stable in the temperature range studied (up to 373 K). An activation energy of ∼9 kJ·mol-1 for the SO2 addition to ethyl radicals was determined, while a reverse reaction was characterized by 87.6 kJ·mol-1 activation energy, in agreement with the previous work. Alkenes were found totally unreactive toward sulfonyl radicals at temperatures up to 373 K and pressures up to 1 atm (Keq < 10-6 L·mol-1). The implications of these results for the mechanism of the alternating copolymerization are discussed.