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ABSTRACT: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., 1.457D ( = 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, D, = (m 2 )/nµJ = (µ 2 )/µJ, (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.KEY WORDS Poly(methyl methacrylate) / Tacticity / Dipole Moment Ratio / Temperature Coefficient / Conformation / Dilute Solution / The mean square dipole moment (m 2 ) of polymers having polar groups in their constitutional units is a conformation-dependent property like as the unperturbed chain dimension (rJ). Experimental values of the dipole moment can be determined from dielectric constant measurements in solution while theoretical values of (m 2 ) may be calculated using the "rotational isomeric state" model from structural information and conformational energies. Thus, study of dipole moments should provide information regarding the spatial configuration of polymers having polar groups.Dipole moments of poly(methyl methacrylate) (PMMA) have long been studied, and the results of experimental studies have been reported numerously. 1 -6 The value of (m 2 ) of isotactic (iso) PMMA may be different from that of syndiotactic (syn) PMMA, because the conformation of chain molecules should depend on stereochemical constitution of the chain. The dependence of (m 2 ) of PMMA on tacticity was studied experimentally, in the 1960s, by Pohl et al.,4 Salovey, 5 and Roig et al. 6 • More recently, theoretical studies on conformational statistics of iso-and syn-PMMA chains were reported by Flory, Sundararajan, and coworker. 8 -10 The dipole moments of iso-and syn-PMMAs were calculated by Birshtein et al. 11 . However, the reliability of experimental values of the dipole moments previously reported is apparently not 579

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