Comparison of the molecular dimensions of di‐alkyl and aryl‐alkyl‐substituted polysilanes in solution using SEC/LS

Abstract

Polysilanes provide an opportunity for exceptional control of the chain hindrances to rotation through the choice of substituents on each backbone silicon. Two alkyl substituents on each silicon result in a large characteristic ratio of at least 19 for poly (di‐n‐hexylsilane), determined by extrapolation of intrinsic viscosities. Bulky aromatic substituents provide even more hindrance to backbone rotations, and can be expected to result in a more extended polymer chain. Direct measurement of the dimensions of these polymers by scattering techniques has been limited by the small quantities available, and by the polydispersity of samples. The recent introduction of light‐scattering detectors for size exclusion chromatography enables the simultaneous measurement of light scattered at as many as 15 scattering angles as the fractionated polymer elutes from the column. Determination of both M and the root‐mean‐square radius of gyration Rg of narrow fractions eluting from a column allows determination of the Rg M relation over as much as a decade in M with less than a milligram of sample. Values of Rg and M across the distribution have been determined for alkyl and aryl substituted polysilanes with this technique. Estimation of Rg,0/M unperturbed by long‐range interactions is made by an extrapolation procedure. The dependence of Rg,0 on M across the distribution is compared among the different substituents and with other measurements reported for these polymers. © 1994 John Wiley & Sons, Inc. Copyright © 1994 John Wiley & Sons, Inc.