Magnetic and asymmetric induced circular dichroism studies of polymers

Abstract

The magnetic circular dichroism (MCD) spectra of syndiotactic and isotactic polymers containing aromatic chromophores have been found sensitive to subtle configurational and conformational differences. For isotactic polymers it was found that as the aromatic ring became further from the main chain, the ratio of B terms for the polymers to that of their model compounds went through a minimum and increased significantly when the aromatic ring was separated from the main chain by four atoms. This enhancement of MCD is thought to be due to bending around of the side chains such that overlap of the aromatic π -- π* transition. This effect was found to be much less pronounced for the syndiotactic polymers because sufficient nearest neighbor overlap was sterically not as good. The ratio B terms for the polymers and model compounds were found to wither increase, decrease or pass through minimum as the polymer coils expanded in various solvents. No correlation or trends were apparent from these results, although it can be concluded that considerable changes in the interaction distances and orientation of the chromosomes are occurring during coil expansion. The MCD results also indicate that the microscopic conformation of isotactic and syndiotactic polystyrene are considerably different in goof solvents. This is the first experimental evidence for this fact by an optical method. The ratio of the dipole strengths for the polymers and model compounds were found to parallel the MCD results, but the ultraviolet techniques was not found to be as sensitive to subtle conformational differences as MCD. Polybenzyl methacrylate and benzyl pivalate were found not to be suitable systems for studying the MCD effect because the B terms of these materials approached zero. ...