STRAINED SILOXANE RINGS ON THE SURFACE ON SILICA - THEIR REACTION WITH ORGANOSILOXANES, ORGANOSILANES, AND WATER

The siloxane network on the surface of highly dehydroxylated silica (Cab-O-Sil) contains strained siloxane rings. The =(SiO)(2)= dimer rings are edge-shared tetrahedra, some of which have a lessened reactivity due to an attached silanol. We use infrared spectrometry to show that the dimer rings react with organosiloxane molecules or with water at comparable rates, depending on the structure of the organosiloxane. A polar bond component is a necessary condition for a rapid reaction with dimer rings, since nonpolar organosilanes react orders of magnitude slower than does water or organosiloxanes. Our data show that the important features of the reacting bond that control the rapidity of the reaction are its polarity, steric accessibility, and bond strain. The reactions require approximately 6 orders of magnitude of gas exposure to go from 1% to 99% completion, which shows that the surface is highly heterogeneous. We have applied a simple model of the surface's heterogeneity to our kinetic data, incorporating a linearly distributed activation energy in the ring population. Using this model, a fit to the data shows the spread in activation energy is approximately 40 kJ/mol in all cases. Our results show that the rapid organosiloxane reaction with dimer rings produces hydrolytically stable coupling points that cannot be readily synthesized with conventional silane chemistry.