A structure of copolymers of propene and hexene isomorphous to isotactic poly(1-butene) form I

Isotactic copolymers of 1-propene and 1-hexene that contain over 10% and up to about 25% of hexane units in the chain have been shown to crystallize in an unusual crystal modification that differs significantly from the well-known structures of isotactic polypropylene (alpha, beta, and gamma phases) or of isotactic poly(1-hexene) (Macromolecules 2005, 38, 1232). The experimental data obtained by Poon et al. are reinterpreted and analyzed. The new crystal form has a trigonal unit-cell with parameters a = b = 1.717 nm, c = 0.65 nm, very similar to form I of isotactic poly(1-butene) (iPBu) (trigonal cell with parameters a = b = 1.770 nm, c = 0.65 nm). The chain conformation is the 3-fold helix of isotactic polypropylene. The side-chain material in the copolymer with the highest hexene content (25%) is about 12% less than in iPBu form I (all ethyl units). These copolymers of propene and hexene adopt a crystal structure isomorphous to that of a third polyolefin ( polybutene) that, in terms of overall composition, is close to the average of the two parent olefins.