Characterization of the Repeat Unit Sequence of Poly(lactide-co-glycolide)

Abstract

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 n_G adjacent glycolyl esters, including the resonant glycolyl ester. The decay of the relative population of homosequence length n_G is described by an exponential decay f(n) = e^(1–n)/σ_b with the parameter σ_b describing the degree of clustering of glycolyl ester units. An empirical model describes the time-evolution of σ_b as a function of reaction time, which evolves due to comonomer reversibility and copolymer transesterification.