The morphological changes of polyethylenes bearing precisely spaced "defects" are reviewed, focusing on the effects of defect frequency, size, and functionality on crystallization and crystalline structure. The precise defect interval is imparted through acyclic diene metathesis polymerization of structurally symmetric diene monomers. Studies have included structural characterization by differential scanning calorimetry, wide-angle X-ray scattering, small-angle X-ray scattering (SAXS), and infrared spectroscopy. The collective results are presented separately for functionalized polyethylenes and for those containing alkyl chain branches, as these two classes of polymers vary greatly in morphology.

The history of and major advances in the acyclic diene metathesis (ADMET) reaction are described. Because precise branch identity and frequency can be achieved by ADMET polymerizations of symmetrical a,xdienes, polyethylenes with precisely spaced alkyl branches of specified length have been prepared. Investigations of their morphologies and thermal properties have provided valuable insight into the behavior of polyethylene. AD-MET preparation of ethylene copolymers and telechelic oligomers, as well as the properties of these materials, is also discussed.

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