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Analysis of Molecular Mass Distributions of Styrene Methylmethacrylate Copolymers Using Size Exclusion Chromatography Data

Size exclusion chromatography (SEC) is the common method for the determination of molecular mass distributions (MMD) of polymers. MMD data in the form of relative frequency vs. molecular mass relationships are readily converted into average molecular mass values using simple calculations. However, more advanced analysis procedures may extract much more information from SEC data even for rather simple polymer molecules, such as linear statistical copolymers of vinyl monomers. Thus, using proper normalization procedures, MMD data may be put in a form suitable for the description with different theoretical distribution functions. The deduction of the best matching function, together with corresponding optimum parameter values may reveal (co)polymerization reaction mechanism and valuable kinetic data. In this paper such a procedure is described and applied on a model system. Statistical copolymers of styrene and methylmethacrylate of different compositions were synthesized by the radical copolymerization in the batch isothermal reactor with toluene as a solvent and AIBN as an initiator. The whole range of compositions was covered. Copolymer MMDs were obtained by the size exclusion chromatography (SEC) and average copolymer compositions by the elemental analysis. Molecular mass distributions of synthesized statistical copolymers of styrene and methylmethacrylate were described with lognor, Flory, Schulz and Tung theoretical function, respectively. The best match between experimental and calculated data was obtained by the Schulz distribution. The alpha-parameter of Schulz's function was related to the average molecular mass and showed the maximum approximately at the midpoint of the copolymer composition range, in connection to the observed minimal copolymerization rate. The b-parameter was found to be constant for all the investigated copolymers and related to the constant value of polydispersity index. This feature indicated that there was no significant change in the copolymerization reaction mechanism throughout the entire copolymer composition range.

Size exclusion chromatography; Copolymers of styrene and methyl methacrylate; Molecular mass distribution; Schulz distribution function

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