Mechanistic study of BU2SnCl2-mediated ring-opening polymerization of ε-caprolactone by multinuclear NMR spectroscopy

The ring-opening polymerization (ROP) of epsilon-caprolactone (CL) was carried out in toluene at 100degreesC 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 (alpha-propyloxy)(omega-hydroxy)poly(epsilon-caprolactone) chains. In order to gain fundamental understanding of the mechanistic factors governing the polyester chain growth, advanced H-1, C-13, and Sn-119 NMR investigations were performed in situ in [D-8]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 Sn-119 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.