Abstract
The ring‐opening polymerization (ROP) of ε‐caprolactone (CL) was carried out in toluene at 100 °C with n‐propanol (__n__PrOH) in the presence of Bu~2~SnCl~2~. 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 ^1^H, ^13^C, and ^119^Sn NMR investigations were performed in situ in [D~8~]toluene, as well as with model solutions that contained Bu~2~SnCl~2~ and binary mixtures of the components at various concentrations and temperatures. This has enabled us to propose a mechanism in which Bu~2~SnCl~2~ behaves as a catalyst, while __n__PrOH is the actual initiator. It involves non‐aggregated, six‐coordinate Bu~2~SnCl~2~ complexes in which ligands exchange fast on the ^119^Sn 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.