Polymer-Coated Alumina Particles: Correlation of Structure and Chromatographic Performance

A study combining structural characterization by solid-state ¹³C NMR and measurement of chromatographic performance is reported for a series of polymer-coated Unisphere alumina particles. The alumina particles, coated with cross-linked polybutadiene (PBD) or polybutadiene oxide (PBO) or derivatives thereof, can function as the stationary phase in reversed-phase liquid chromatography (RPLC). Cross-polarization magic-angle spinning ¹³C NMR spectroscopy allowed detailed characterization of the polymer-coated samples, including comparison of polymer loading levels, measurement of olefinic vs aliphatic carbon, and determination of 1,2- and 1,4-olefinic units in the PBD portions of the polymers. In situ chemistry, including hydrogenation and reactions of the cross-linked polymers with C₁₀ and C₁₈ aliphatic chains, was also monitored by solid-state NMR. The ability of each of the polymer-coated alumina samples to separate a standard mixture of five organic compounds was evaluated. Correlations between the NMR results and RPLC performance provide insights into those structural features of polymer coatings which are most important in determining their suitability for use in chromatographic separations. In particular, hydrophobicity is found to be a key parameter in determining a polymer coating's chromatographic performance.