Cocrystallization in copolymers of α-olefins II—Butene-1 copolymers and polybutene type II/I crystal phase transition

Abstract

The cocrystallizing behaviour shown by copolymers of butene with a number of linear and branched α-olefins made with titanium trichloride-diethyl aluminium chloride catalysts has been investigated by x-rays over a range of copolymer compositions and the effect of copolymerization on the spontaneous polybutene Type II (PB II) to Type I (PB I) crystal phase transition has been studied in unoriented specimens and fibres. Degrees of crystallinity, crystal phase changes from those of the relevant homopolymers including lattice expansion and contraction and helix modification, and melting points have been observed. Pentene forms a highly cocrystallizing system based on PB I; propylene shows limited, and ethylene practically no ‘isomerism of monomer units’. These three comonomers markedly accelerate the PB II/I transition, and in sufficient amount cause direct crystallization from the melt to PB I. Hexene and octene units can enter the PB II lattice but α-olefins with C > 8 do not. Butene-3-methyl butene is a highly cocrystallizing system; 4-methylpentene and 4,4-dimethylpentene copolymers show limited and no cocrystallization respectively. Linear α-olefins with C > 5 and the branched comonomers retard the PB II/I transition and in sufficient amount stabilize the PB II form completely. For comonomers which enter the PB II lattice, stabilization is attributed to sdteric inhibition of the change from a PB II 115 (or more likely 4011) helix to a more tightly coiled 31 (PB I) helix. For those that do not enter the lattice, it is believed that the large units act as ‘stops’ at the fold boundaries, which prevent the helical chains being drawn through the crystallites and thus inhibit the transition. The X-ray patterns of some of the copolymers show loss of short range order while preserving long range order. Phase separation and lack of randomness in the copolymers are discussed; the degree of copolymerization attained with this highly stereospecific catalyst system is shown to be related to the degree of cocrystallization possible in each copolymer.