The local, temperature dependent structure of poly-(ethylmethacrylate) was studied with wide-angle x-ray scattering. The results were set into context to recent wide-angle neutron scattering results as well as to the results from a multitude of studies of the dynamics of this polymer. The temperature dependence of the wide angle x-ray results point to the development of local order which is backed by the neutron scattering results and which is connected to characteristic temperatures of the relaxation dynamic T(g) and T(c). The poly-(ethylmethacrylate) was studied in its predominantly syndiotactic as well as predominantly isotactic state displaying vastly different local structures as manifested in the x-ray results.

We present solid-state NMR investigations of a series of shape-persistent polyphenylene dendrimers of generation 1-4 with different surface functionalization. Using a combination of traditional static and more advanced magic-angle spinning (MAS) exchange techniques for the elucidation of slow dynamics as well as fast-MAS recoupling techniques for the quantification of dynamic averaging in the megahertz range, we derive a clear picture of the complex molecular dynamics in these systems. Fast processes in the megahertz regime are shown to be restricted to fast vibrations of terminal phenyl rings with amplitudes of up to 40°at most, with a 5-30% fraction of rings performing larger-amplitude motions. Slow processes on the time scale of milliseconds to seconds are also restricted to terminal and doubly para-substituted phenyl rings. This type of motion is characterized by a two-site jump with a mean reorientation angle of 24°and a mean apparent activation energy of 34 kJ/mol and is presumably a concerted process involving several adjacent phenyl rings. The comparison of dendrimers with different surface functionalization allows us to conclude that the molecular dynamics are dominated by intramolecular steric constraints. As for the dependence on dendrimer generation, both the fast and the slow processes follow a trend that is expected from the evolution of the segment free volume at the periphery of the molecules, where most terminal rings are located. We therefore believe that our results represent the first experimental evidence of a class of dendrimers in which the radial segment density distribution is caused by truly extended arms and for which the dense-shell packing limit is reached for generation 4.

The molecular dynamics of polyphenylene dendrimers(e.g., the second generation dendrimer shown in the Figure) is investigated here using advanced solid‐state NMR techniques. It is found that well‐localized, restricted reorientation of single terminal phenyl substituents occurs around fixed axes only, clearly demonstrating the shape persistence of these dendrimers. The synthetic approach to these polyaromatic dendrimers is also described.

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The X-ray crystal structure of Mes 2 SidSiMes 2 ‚ THF (1‚THF) has been determined. The THF molecule is not coordinated to silicon. The two Mes 2 Si units are twisted relative to one another by 13°, while the arrangement at each silicon is coplanar. This conformation differs from that found for 1‚C 7 H 8 (12°twist, 18°p yramidalization) and unsolvated 1 (3°twist, 12, 14°p yramidalization).