The mean-square electric dipole moment 〈μ2〉 was determined for 12 samples of atactic (a-) oligo- and poly(methyl methacrylate)s (PMMA), each with the fraction of racemic diads f r = 0.79, in the range of the weight-average degree of polymerization x w from 3 to 1.19 × 103 and for 11 samples of isotactic (i-) PMMA and its oligomers, each with f r ≃ 0.01, in the range of x w from 3 to 5.88 × 102, both in benzene at 30.0 °C. The determination was also made for a syndiotactic (s-) PMMA sample with f r = 0.92 and x w = 3.76 × 102 and for methyl isobutyrate in the same solvent condition. For both a- and i-PMMAs, the ratio 〈μ2〉/x w as a function of x w decreases monotonically with increasing x w for x w ≲ 20 and then approaches its asymptotic value (〈μ2〉/x w)∞, i.e., 1.927 and 2.126 D2 for a- and i-PMMAs, respectively, indicating that the excluded-volume effect on 〈μ2〉 is negligibly small if any. From a comparison of these values of (〈μ2〉/x w)∞ along with that of 〈μ2〉/x w obtained for the s-PMMA sample, it is found that (〈μ2〉/x w)∞ decreases with increasing f r. An analysis of the present experimental values for a- and i-PMMAs is made by the use of the theory for the (unperturbed) helical wormlike chain modified so as to take into account possible effects of chain ends. Then it is shown that the above dependence of 〈μ2〉/x w on x w may be explained by this modified theory with the use of the values of the model parameters already determined from an analysis of the mean-square radius of gyration, if the local electric dipole moment vectors are properly assigned to the initiating and terminating repeat units. Unfortunately, however, any useful information about the main-chain conformations of a- and i-PMMAs cannot be obtained from an analysis of the dependence of 〈μ2〉 on x w.

A neutrino-factory muon storage ring to provide beams for multiple detectors around the world AIP Conf.