Alternating copolymers of butadiene and propylene: Properties and characterization

Alternating copolymers of butadiene and propylene (BPR) made by a solution polymerization technique with a modified catalyst of the Furukawa type have highly regular structures. The constitutional repeating unit consists of one propylene and one 1,4‐ trans ‐butadiene unit. No chain imperfections were detected, except for ≦2% 1,4‐ cis ‐butadiene units. The configurations of the chiral tertiary carbon atoms in the propylene units could not be directly determined by spectroscopic methods. The intrinsic viscosities of the samples studied lie between 1.1 and 1.8 dL/g. The high molecular weight fractions ( M > 2 × 10 5 ) show long‐chain branching. Crystallization effects, namely, a decrease in specific volume with time, and melting are observed after annealing unoriented samples at temperatures between −30 and −50°C. X‐ray diffraction shows that the structure of the ordered phase depends on polymerization conditions, varying between a liquid crystal‐like close packing of parallel chain segments and imperfect crystalline, three‐dimensionally periodic arrangements of chains. Both in the noncrystalline and crystalline ordered phases, the molecules are arranged in layers with a spacing of 0.47 nm. The crystalline chains have nonplanar and not fully extended conformations with a repeat distance of 0.72 nm. The rate of crystallization and the relative decrease in volume (≦1.3%) are small compared to stereoregular polymers like natural rubber and high‐cis polybutadiene. In crosslinked specimens, stretched 1:6 to 1:10, the crystalline or liquid crystal‐like structure persists up to +60°C. The imperfect nature of the crystallization of BPR is caused by stereoisomerism of the chiral carbon atoms. The chains are probably composed of short tactic sequences separated by tactic arrangements.