Scope and Limitations of the Use of Grafted Undecyltin Trichloride As a Catalyst for Transesterifications: Effect of Tin Loading on Catalytic Activity, Recyclability, and Leaching

摘要

The effect of the tin loading (functionalization degree t) on the catalytic activity and recyclability is investigated for a polystyrene-grafted undecyltin trichloride catalyst, P-C11-SnCl_3, in transesterification reactions involving either a primary or a secondary alcohol. For the latter, the achieved conversion degree in the first run is about 20% lower than with the primary alcohol. In subsequent runs, the reaction rates are strongly influenced by the tin loading of the catalyst. Unlike low-loaded P-C11-SnCl_3 catalysts (t ≈ 0.10), high-loaded catalysts (t ≈ 0.20) display a simultaneous T_g increase(from 53 ℃ to 107 ℃ after 5 runs) and conversion decrease (from 52% to 15%) upon increasing number of runs, ascribed to reduced mobility of the organotin moieties resulting from undesired cross-linking at the reaction interface. Confirmation for the fact that, in this case, the catalytic performance is dominated by conformational mobility issues is found in the comparison between transesterifications involving either primary or secondary alcohols. Whereas a T_g increase is not associated with a reduced conversion degree for primary alcohols, a clear decrease in conversion is observed for secondary alcohols, illustrating that steric issues are especially pronounced in a low-mobility (high-loaded) system and are of no importance in high-mobility (low-loaded) systems. This also affects the leaching resistance of the compounds, the high-loaded catalysts displaying substantially higher tin leaching (311 ± 278 ppm) than the low-loaded ones (10 ± 8 ppm).