An NMR study of chain transfer to diols containing both primary and secondary hydroxy groups in the polymerization of ε-caprolactone

Abstract

ε-Caprolactone has been polymerized in the presence of various diols, viz. propane-1,2-diol (PD), butane-1,3-diol (BD) and hexane-1,5-diol (HD). ¹ H and ¹³ C NMR spectroscopy were used to evaluate the structures. The final products were found to be a function of the diol used. It was shown that reactions incorporating PD gave low conversions and/or low degrees of polymerizations when compared with those involving BD or HD. In polymerizations involving BD two ¹³ C resonances could be seen in the carbonyl region, assignable to the ester carbonyls adjacent to the oxymethine and oxymethylene groups derived from the residues of the diol units. Thus, both primary and secondary hydroxy groups were shown to be active in the transfer reaction. Selective decoupling was used to assign the down-field resonance to the carbonyl adjacent to oxymethylene and the up-field resonance to the carbonyl adjacent to oxymethine. However, in the case of the polymerization incorporating BD, approximately 10% of end groups were shown to be secondary alcohols, which are derived from the secondary hydroxy group of BD that does not transfer. In polymerizations involving HD only one carbonyl resonance, which could be assigned to an ester adjacent to the diol residue, was observed. From COSY spectra it was possible to assign a peak due to the ester of the secondary hydroxy. The fraction of secondary chain ends was thus observed to be greater, at ca. 13%, than in the BD polymerizations.