MEAN-SQUARE RADIUS OF GYRATION OF ISOTACTIC OLIGO(METHYL METHACRYLATE)S AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTION

The mean-square radius of gyration [S2] was determined by small-angle X-ray scattering and light scattering for 16 samples of isotactic olig o- and poly(methyl methacrylate)s (i-PMMA) in the range of weight-aver age molecular weight M(w) from 4.58 x 10(2) (tetramer) to 1.71 x 10(6) in acetonitrile at 28.0-degrees-C (theta). The fraction of racemic di ads f(r) determined for them by H-1 NMR was ca. 0.01 independently of M(w) except for the oligomers with very small M(w), for which it was s omewhat larger. In contrast to the previous results for atactic (a-) P MMA with f(r) = 0.79 in the unperturbed THETA state, for which the rat io [S2]/x(w) as a function of the weight-average degree of polymerizat ion x(w) exhibits a maximum, it was found to increase monotonically wi th increasing x(w) and level off to its asymptotic value for i-PMMA as in the case of atactic polystyrene (a-PS) with f(r) = 0.59 previously studied. The data obtained were analyzed on the basis of the helical wormlike (HW) chain model to determine the model parameters. The resul ts are lambda-1kappa0 = 2.5, lambda-1tau0 = 1.3, lambda-1 = 38.0 angst rom, and M(L) = 32.5 angstrom-1, where kappa0 and tau0 are the differe ntial-geometrical curvature and torsion, respectively, of the characte ristic helix taken at the minimum zero of the elastic energy of the mo del, lambda-1 is the stiffness parameter, and M(L) is the shift factor . The difference in local chain conformation between i- and a-PMMA is discussed in detail on the basis of the model parameters thus determin ed. It is then concluded that the above difference in the behavior of [S2]/x(w) between i- and a-PMMA arises from the fact that the former i s of weaker helical nature than the latter. In order to illustrate the situation, a picture is given of representative instantaneous contour s of HW Monte Carlo chains for them and also for a-PS.