In commercial polyethylene (PE), the microstructure, i.e. the branching, determines the macroscopic properties of the polymers as well as its wide applications. To better understand the structure-property-performance relationship, precision polyolefins prepared via acyclic diene metathesis (ADMET) polycondensation chemistry are used as model structures. Due to the intrinsic nature of ADMET polymerization, precision polyolefins can be prepared having, without equivocation, known branch identity and exact location of the branches along the polyolefin backbone.

Precision polyethylenes with alkyl branches are selected as structural models to investigate the influence of branch identity and frequency on polymer morphology and chain dynamics. Several analytical methods are employed, mainly solid state nuclear magnetic resonance (SS-NMR) spectroscopy and wide angle X-ray scattering (WAXS) techniques. Deuterium SS-NMR is mostly appropriate for investigating chain dynamics because of the quandrupolar interaction.

Deuterated precision polymer samples synthesized by selectively labeling the site of interest are studied using advanced ²H and ¹³C SS-NMR. A unique spectrum appears for ADMET precision polymer with a CD₃ branch on each and every 15^th carbon along the main chain: a regular Pake pattern exists with a half static line width, indicating the presence of axial rotation sufficiently fast due to the motions of methyl groups embedded in the crystalline regions. The axial rotation is also observed by ¹³C NMR. For lower branch content, the twist motions are decoupled (pinned defects), while for higher branch contents collective motion (rotator phase) is found. For the first time, the presence of the rotator phase in polyethylene crystalline region has been evidenced by solid state NMR.

Besides SS-NMR, powder X-ray scattering has also been applied to precision polymers. Wide angle X-ray scattering provides important information about crystallinity and the unit cell, as well as the lamellar structure. Combining the results from solid state NMR and X-ray scattering, a clear understanding of morphology has been achieved. When the side chain switches from the methyl group to a bigger substituent, such as the butyl group, the thermal behavior of polymer changes significantly. The influences of the branches on crystallinity and chain packing are investigated by SS-NMR and XRD. It is found that the scattering patterns of methyl-branched and butyl-branched polymers are different from that of the linear ADMET PE. Small branches, such as the methyl group, are incorporated in the unit cell, while the branches as large as or larger than the propyl group are excluded from the unit cell.