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Detailedcrystal structure analysis has been carried out for fourcrystalline forms (I, I′, II, and high-temperature phase, HT)of uniaxially oriented specimens from a novel polyethylene-like polymer,−[(CH2)20–CHBr]n– on the basis of the 2-dimensional X-ray diffractionpatterns and polarized FTIR spectral data. This polymer has Br atomsplaced regularly on each and every 21st backbone carbon. The preciseBr placement along the polyethylene backbone allows drastically differentchain conformation and chain packing modes between the group of formsI and I′ and the group of form II and HT phase. In forms Iand I′, the molecule is fully extended adopting a planar all-transzigzag conformation with layers of Br atoms normalto the chain axis. Conformational disorder and mismatch in relativeheight of Br atom between the neighboring chains distinguish formI from form I′. In forms II and HT phase, the molecular chainsbend at the Br substitutional point and take a large zigzag form consistingof long methylene segmental arms. The molecular bends are caused bythe generation of nonplanar gaucheconformers atthe C–C bonds adjacent to the CHBr groups, while the CH2segments maintain the all-transconformation.The major difference between form II and HT is conformational disorderwithin the methylene runs. Heating at T< 65 °Cunder unrestrained condition causes an irreversible transition fromform I′ to form I, and form I transforms irreversibly to formII in a narrow temperature range of 65–66 °C. The highertemperature heating induces the reversible and apparently continuoustransition of form II to the HT phase. On the other hand, the tensilestretching at room temperature causes the irreversible transitionof forms I and II to the form I′. [ABSTRACT FROM AUTHOR]