Role of surface-adsorbed water in the horizontal polymerization of trichlorosilanes

The high surface area of porous silica gel enables characterization of the amount of adsorbed water needed for horizontal polymerization of trichlorosilanes for the first time. Silica gel samples were individually exposed to relative humidity levels of 30%, 40%, 50% and 60% prior to reaction, and the amount of adsorbed water was determined by weighing. Optical microscopy shows that polymer microspheres, unattached to the silica, form in the case of 60% humidity, indicating that this level is in excess of the optimum coverage of water. Microanalysis reveals a monotonic increase in carbon with increased humidity, and ²⁹Si NMR spectra reveal increased cross-linking of the siloxanes as the humidity increased to 50%. HPLC is shown to be a very sensitive technique for assessing the coverage of the silica substrate: zone tailing is sharply minimized at 50% humidity. These results point to 50% as the optimal humidity for dense horizontal polymerization of trichlorosilanes. The amount of adsorbed water at 50% humidity and room temperature is 22±3 μmol/m², which approaches the stoichiometric amount required to hydrolyze the trichlorosilanes.