Which average of copolymer composition does NMR provide?

Abstract

AbstractWhile in molar mass determinations different averages are clearly distinguished, such differentiation is usually not performed when dealing with the composition of copolymers or polymer blends. The present article shows that the mol ratio calculated by nuclear magnetic resonance (NMR) usually provides neither a weight nor a number average. Only if the composition does not vary with molar mass, or if both copolymer units are of equal molar mass, a weight average mol ratio is obtained from NMR measurements. The frequent assumption that NMR yields a number average composition is incorrect, therefore. However, the mass fraction calculated from NMR corresponds to the weight average mass fraction.