Polylactones, 52. Tin Carboxylates as Initiators ofɛ-Caprolactone

Eleven different tin carboxylates were used as initiators of the ring-opening polymerization of e-caprolactone. All polymerizations were conducted with 2 M solutions in chlorobenzene at 80°C and time/conversion curves were determined up to 96–99% conversion. The following structure-reactivity relationships were found. Noncyclic initiators, such as Bu2Sn(acetate)2 or Bu2Sn(benzoate)2 are more reactive than cyclic initiators such as Bu2Sn succinate and Bu2Sn phthalate, respectively. Electron withdrawing substituents enhance the reactivity. Bu2Sn(acetate)2 is more reactive than Bu3Sn acetate and Sn(II) bis(2-ethylhexanoate) is more reactive than Bu2Sn(2-ethyl-hexanoate)2. Traces of moisture to play the role of efficient co-initiators. This conclusion is supported by the following observations. The degrees of polymerization (DP) largely exceed the monomer/initiator (M/I) ratios. Furthermore, the DPs increase with the purity of e-CL suggesting that the monomer itself carries traces of moisture. The co-initiation with water and the incorporation of a few acyl endgroups were confirmed by MALDI-TOF mass spectroscopy. CH2OH endgroups were also found in all polyesters by 1H NMR spectroscopy, whereas only traces of acyl endgroups were detectable. Tetrabutyldistannoxane bisacetate is far more reactive than Bu2Sn bisacetate. Addition of an alcohol to a tin carboxylate allows a control of the molecular weight, and, as previously reported by several research groups, accelerates the entire polymerization process.