Characterization of the Repeat Unit Sequence of Poly(lactide-<i>co</i>-glycolide)

Established methods of characterizing repeat unit sequence by measuring changes in comonomer concentration during copolymerization and inferring the resulting copolymer sequence are inapplicable to poly(lactide-co-glycolide) due to reverse and transesterification reactions. In this case, measurement of repeat unit sequences must be based on direct measurement on the copolymer. Here, the assignment of the 13C NMR spectrum of the glycolyl methylene of poly(lactide-co-glycolide) to specific compositional ester tetrads is reported using insights provided by a stochastic model regression analysis of experimental reversible transesterification copolymerization data. The relative integrals of the deconvoluted compositional tetrad signals can be interpreted within the context of a model for repeat unit clustering that describes the cumulative population of homosequences with at least nG adjacent glycolyl esters, including the resonant glycolyl ester. The decay of the relative population of homosequence length nG is described by an exponential decay f(n) = e(1–n)/σb with the parameter σb describing the degree of clustering of glycolyl ester units. An empirical model describes the time-evolution of σb as a function of reaction time, which evolves due to comonomer reversibility and copolymer transesterification.