Mechanistic study of Bu₂SnCl₂-mediated ring-opening polymerization of ε-caprolactone by multinuclear NMR spectroscopy

The ring-opening polymerization (ROP) of ε-caprolactone (CL) was carried out in toluene at 100°C with n-propanol (nPrOH) in the presence of Bu₂SnCl₂. It comes out that the molar mass of the polyester chains can be predicted from the initial monomer-to-alcohol molar ratio in accordance with a controlled ROP mechanism involving an O-acyl cleavage of the monomer to selectively form (α-propyloxy)(ω -hydroxy)poly(ε-caprolactone) chains. In order to gain fundamental understanding of the mechanistic factors governing the polyester chain growth, advanced ¹H, ¹³C, and ¹¹⁹Sn NMR investigations were performed in situ in [D₈]toluene, as well as with model solutions that contained Bu₂SnCl₂ and binary mixtures of the components at various concentrations and temperatures. This has enabled us to propose a mechanism in which Bu₂SnCl₂ behaves as a catalyst, while nPrOH is the actual initiator. It involves non-aggregated, six-coordinate Bu₂SnCl₂ complexes in which ligands exchange fast on the ¹¹⁹Sn NMR observational timescale, and the simultaneous interactions of CL and alcohol function in such a way that it favors insertion/propagation reactions over transesterification ones, up to high monomer conversion.