SI-29 AND C-13 NMR CHARACTERIZATION OF MIXED HORIZONTALLY POLYMERIZED MONOLAYERS ON SILICA-GEL

A horizontally polymerized monolayer of mixed C18 and C3 trifunctional silanes on silica, and conventional chromatographic phases, are studied by NMR spectroscopy. For horizontal polymerization, quantitative C-13 and Si-29 NMR measurements reveal that a dense and highly polymerized monolayer of bonded groups is formed on the surface. The conventional polymeric surface, by contrast, is oligomeric in nature: the ratio of bonded silanes to reacted substrate silanols is only slightly more than 1:1, and the average oligomer size is 5 units. The data reveal insignificant vertical polymerization. C-13 NMR measurements show that the C18 chain dynamics for the mixed horizontally polymerized monolayer are similar to those for the monomeric C18 monolayer, whereas the C18 chain dynamics for a conventional polymeric phase are significantly slower. These results indicate that the C18 chains are randomly interspersed with the short C3 chains in the horizontally polymerized mixed phase.