The molecular motions underlying the dielectric and dynamic-mechanical 8 relaxation in poly-(methyl methacrylate) (PMMA) have been elucidated in detail by means of two-dimensional (2D) and threedimensional (3D) 13C exchange NMR of the carboxyl moiety and 2D 2H exchange NMR of the methoxy group. The identity of the motions observed by NMR and the ^-relaxation dynamics is proved by the agreement of the measured correlation times. The selective-excitation "3D" NMR spectrum proves that, for every mobile side group, a relatively well-defined motion between two potential-energy minima occurs. The 2D spectral pattern shows that the OCO plane of the side group undergoes 180°(±20°) flips. Experiments with multiple exchange and selective saturation for analysis of the growth of exchange signals (MESSAGE) prove that the molecular motions responsible for the 8 relaxation are associated with a distribution of correlation times, which appears to be bimodal with both mobile and trapped side groups. Consistently, analysis of the integral 2D exchange intensity shows that around 330 K only about 50% of the side groups participate in the large-amplitude dynamical process on the time-scale of the ^-relaxation correlation time. The 2D 2H NMR spectra, while exhibiting narrowing due to methyl-group rotation around the 0-CH3 bond, exclude any significant motion of the methoxy group around the C-OCH3 bond. Both the 13C and the 2H 2D NMR spectra provide compelling evidence that the side-group flip is accompanied by a main-chain rearrangement which can be characterized as a random rotation around the local chain axis with a 20°r oot-mean-square amplitude. This is ascribed to the fact that the asymmetric side group, after the flip, does not fit into its original environment. These findings explain both the dielectric and the dynamic-mechanical 8 relaxations of PMMA.

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The energy gaps of poly@-aryleneviny1ene)s with phenylene, naphthylene and anthrylene subunits are determined by extrapolating the UV/VIS spectroscopic data of the oligomers. The band structure of the three polymers was theoretically investigated. The topology-, geometry-and correlation-factors were determined upon which the energy spectra of this class of one-dimensional n-electron systems depend.

Side-group liquid crystalline polymers were investigated by 1 D and 2D solid state 2H NMR methods in the glassy state. The side-group dynamics of liquid crystalline systems were compared to those of amorphous systems of the same molecular architecture. The onset of fast rotational motion of the phenylene rings additionally to phenylene flips was detected at considerably lower absolute and reduced temperature in the liquid crystalline systems indicating the influence of the ordered state on local dynamics. In 2D exchange spectra, the lineshapes are particularly sensitive towards possible slow motion of the flip axis itself which could be envisaged to fluctuate within the orientational distribution of the mesogen. However, the experimental spectra for the investigated sidegroup liquid crystalline polymers show no indication of the characteristic features, thereby proving that the flip axis is rigid within f3' even after a large number of flips.