Analysis of molecular weight distributions of styrene-methyl methacrylate copolymers using size exclusion chromatography data

Size exclusion chromatography (SEC) is a common method for the determination of molecular weight distributions (MWD) of polymers. Using appropriate normalization procedures, MWD data may be put in a form suitable for the description by different theoretical distribution functions. In this paper such a procedure is described and applied to a model system. Statistical copolymers of styrene and methyl methacrylate (SMMA) of different compositions were synthesized by radical copolymerization in a batch isothermal reactor with toluene as solvent and azobisisobutyronitrile as initiator. Copolymer MWDs were obtained by SEC and average copolymer composition by elemental analysis. MWDs of SMMA copolymers were correlated with the lognormal, Flory, Schulz, and Tung theoretical function. The best match between experimental and calculated data was obtained by the Schulz distribution. The a-parameter of Schulz's function was related to the average molecular weight and showed its maximum at the midpoint of the copolymer composition range, in connection with the observed minimal copolymerization rate. The variation of the b-parameter was found to be less pronounced; the parameter was related to the polydispersity index. This feature indicated that there was no significant change in the copolymerization reaction mechanism throughout the entire copolymer composition range.