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Dipole Moments of Isotactic and Syndiotactic Poly(methyl methacrylate) and Their Temperature Dependence

Dipole moments of isotactic (iso) and syndiotactic (syn) poly(methyl methacrylate) (PMMA) samples were determined from measurements of dielectric constants and densities in dilute benzene solution over the temperature range from 25 to 55°C. The dipole moments and their temperature coefficients showed marked tacticity dependence. It was found that the iso-PMMA gives greater values of the dipole moment than those for the syn-PMMA; e.g., 1.457 D (=μ, dipole moment per constitutional repeating unit) was obtained for the iso-PMMA, and 1.323 D for the syn-PMMA at 25°C. With increasing temperature, the dipole moment of the iso-PMMA showed little change and remained nearly constant, while that of the syn-PMMA increased continuously, giving 1.399 D at 55°C. Experimental dipole moment ratios, Dr=〈m2〉/nμ02=〈μ2〉/μ02, (m, dipole moment of polymer; μ0, dipole moment of the constitutional unit; n, the number of units;) were compared with the calculated ones, which leads to the conclusion that rotational conditions regarding the orientation of polar side groups in the PMMA chains are highly restricted.