Bivariate distribution in PMMA/PBA copolymers by combined SEC/NMR and SEC/MALDI measurements

High conversion copolymers may experience a strong variation in composition as a function of monomer conversion, and since the compositional heterogen eity of copolymers varies as the size of the chains grows, this poses a pro blem in determining the bivariate distribution of masses and composition in copolymers. Although several methods have been proposed to determine bivar iate distributions, relatively little data are available on this important aspect of copolymer characterization, perhaps because of the complexity of the methods available up to the present time. We propose here a generally a pplicable method to determine bivariate distribution of masses and composit ion in copolymers. SEC fractionation of two random copolymers, poly(methyl methacrylate)/polybutyl acrylate (PMMA/PBA) obtained at high and low conver sions, respectively, was used to collect nearly monodisperse fractions that allowed the computation of the corresponding molar masses (by matrix-assis ted laser desorption ionization mass spectrometry, MALDI, measurements) and of compositions (by NMR measurements) for the two copolymers. Bivariate di stribution maps were derived by combining SEC/NMR and SEC/MALDI data with S tockmayer's theoretical distribution to estimate the composition distributi on of each fraction.