Polystyrene-supported organotin dichloride as a recyclable catalyst in lactone ring-opening polymerization: Assessment and catalysis monitoring by high-resolution magic-angle-spinning NMR spectroscopy

Dialkyltin dichloride grafted to a cross-linked polystyrene, with the formula [P-H]_(1-t)[P-(CH₂)_nSnBuCl ₂]_t (P = [CH₂CH(pC₆H₄)], t = the degree of functionalization, and n = 6 or 11), is investigated as a recyclable catalyst in the ring-opening polymerization (ROP) of e-caprolactone (CL). It is demonstrated that high-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy is an invaluable tool to characterize completely the supported catalyst. The 2D ¹H-¹³C HSQC HR-MAS spectrum, in particular, allowed extensive assignment of the ¹H and ¹³C resonances, as well as accurate measurement of the ⁿJ( ¹H-^117/119Sn) and ⁿJ(¹³C- ^117-119Sn) coupling constants. ¹H and ¹¹⁹Sn HR-MAS NMR spectroscopy is presented as a monitoring tool for catalytic processes based on organotin compounds, particularly for the investigation of the extent to which polymerization residues are observable in situ in the material pores and for the assessment of the chemical integrity and recycling conditions of the grafted catalyst. From polymerization experiments with CL, initiated by n-propanol and with [P-H]_(1-t)[P-(CH₂) _nSnBuCl₂], of various compositions as the supported catalyst, it appears that a partial 'burst' of the polystyrene support occurs when the length of the alkyl spacer is limited to n = 6, as a result of polymer chains growing within the pores of the support. However, extension of the length of the aliphatic polymethylene spacer from 6 to 11 carbon atoms preserves the support integrity and allows the production of catalyst-deprived poly(ε-caprolactone) (PCL) oligomers. A preliminary attempt to recycle the heterogeneous catalyst has shown that very good reproducibility can be obtained, in terms of both catalyst activity and molecular-weight parameters of the as-recovered PCL polyester chains.