Polymorphism and Phase Transitions of Precisely Halogen-Substituted Polyethylene. (1) Crystal Structures of Various Crystalline Modifications of Bromine-Substituted Polyethylene on Every 21st Backbone Carbon

Detailed crystal structure analysis has been carried out for four crystalline 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 diffraction patterns and polarized FTIR spectral data. This polymer has Br atoms placed regularly on each and every 21st backbone carbon. The precise Br placement along the polyethylene backbone allows drastically different chain conformation and chain packing modes between the group of forms I and I′ and the group of form II and HT phase. In forms I and I′, the molecule is fully extended adopting a planar all-trans zigzag conformation with layers of Br atoms normal to the chain axis. Conformational disorder and mismatch in relative height of Br atom between the neighboring chains distinguish form I from form I′. In forms II and HT phase, the molecular chains bend at the Br substitutional point and take a large zigzag form consisting of long methylene segmental arms. The molecular bends are caused by the generation of nonplanar gauche conformers at the C–C bonds adjacent to the CHBr groups, while the CH2 segments maintain the all-trans conformation. The major difference between form II and HT is conformational disorder within the methylene runs. Heating at T < 65 °C under unrestrained condition causes an irreversible transition from form I′ to form I, and form I transforms irreversibly to form II in a narrow temperature range of 65–66 °C. The higher temperature heating induces the reversible and apparently continuous transition of form II to the HT phase. On the other hand, the tensile stretching at room temperature causes the irreversible transition of forms I and II to the form I′.