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.