Catalytic properties of cross-linked polystyrene grafted diorganotins in a model transesterification and the ring-opening polymerization of ε-caprolactone

The catalytic properties of cross-linked polystyrene grafted diorganotins of the types [P-H]_(1-t)[P-(CH₂)₁₁-Sn-w- BuCl₂]_t and [P-H]_(1-t){[P-(CH₂) ₁₁-Sn-n-BuCl]₂O)_t/2, in which [P-H] represents the monomeric unit of the cross-linked polystyrene matrix and t is the degree of functionalization, were compared. The grafted chlorodistannoxane catalyst was completely characterized by elemental analysis, IR and Raman, ¹¹⁹Sn high-resolution magic angle spinning (hr-MAS) NMR and ¹¹⁷Sn CP-MAS NMR spectroscopy. In spite of the spatial constraints and the potential steric demand resulting from cross-linking, the grafted C11 chlorodistannoxane exhibited the same dimeric ladder complex arrangement as its nongrafted n-butyl analog in the solid state and in solution. Moreover, ¹¹⁹Sn hr-MAS NMR and ¹¹⁷Sn CP-MAS NMR spectroscopy appeared to be complementary techniques for the assessment of the tin(IV) functionality of the grafted tetraalkyldichlorodistannoxane. The catalytic activity of the compounds was assessed in a model reaction, the transesterification reaction of ethyl acetate and n-octanol, as well as in the ring-opening polymerization (ROP) of ε-caprolactone. Residual tin trace analysis was performed on the acquired reaction products. Whereas tin contents of the order of 20 ppm were found in the reaction products of the transesterification, tin leaching values were overall higher for the ROP, being less unfavorable, however, for the distannoxane (±70 ppm) than for the dialkyltin dichloride (±290 ppm). Quantitative liquid ¹H NMR was applied to assess the conversions in both reactions, but also to determine various molecular parameters of the synthesized poly-caprolactones. Furthermore, size exclusion chromatography was used to determine their polydispersity index. It was also demonstrated that ¹H and ¹¹⁹Sn hr-MAS NMR enable the chemical integrity and recycling ability of the catalyst to be assessed.