Tacticity separation of poly(methyl methacrylate) by multidetector thermal field-flow fractionation

For the first time, thermal field-flow fractionation (ThFFF) has been used for the separation of poly(methyl methacrylate) (PMMA) with regard to molecular microstructure. PMMA exists in three different isomeric forms, namely, isotactic, syndiotactic, and atactic. ThFFF analysis of the different PMMA isomers in tetrahydrofuran, acetonitrile (ACN), and dioxane reveals that isomers with similar molecular weights exhibit different Soret coefficients, and thus different retention times, under identical experimental conditions. Of the three solvents, ACN shows the greatest influence on fractionation of the isomers. The separation according to molecular microstructure is found to be based on the cooperative effects of the normal and thermal diffusion coefficients. Furthermore, it is found that blends of different PMMA isomers with similar molecular weights can be fractionated into their respective isomeric components. The distribution of the isomeric content in an atactic PMMA sample is determined quantitatively by fractionating the sample with ThFFF and subsequently analyzing the fractions by (1)H NMR. The isomeric distributions determined from NMR data correlate well with ThFFF retention data of the samples and thus further highlight the unique fractionating capabilities of ThFFF.