Synthesis and Characterization of 4-Methyl-1-Pentene/1,5-Hexadiene Isotactic Copolymers with Enhanced Low-Temperature Mechanical Performance

Novel 4-methyl-1-pentene/1,5-hexadiene isotactic copolymers (iP4MPHD) incorporating methylene-1,3-cyclopentane (MCP) cyclic co-units with concentrations in the range 4.4–17.6 mol % have been synthesized by using the dimethylpyridylamidohafnium/organoboron catalyst. The influence of the MCP cyclic co-unit on the crystallization behavior and the mechanical properties of the isotactic poly(4-methyl-1-pentene) (iP4MP) homopolymer has been investigated in detail. iP4MPHD copolymers with comonomer content up to 11 mol % crystallize in form II of iP4MP from the polymerization solution and in the stable form I of iP4MP from the melt, whereas the sample with the highest concentration (17.6 mol %) of 1,5-hexadiene (1,5-HD) is amorphous and does not crystallize from either solution and melt. All crystalline samples exhibit high melting temperatures, always above 120 °C, and a controlled glass transition temperature close to the room temperature (28–30 °C). Incorporation of MCP units into iP4MP chains produces an improvement in flexibility and allows tailoring of deformability while retaining high mechanical resistance and transparency of the homopolymer. Interestingly, the high deformability is maintained at low temperature (50 °C below the glass transition temperature), suggesting a cooperative role of both amorphous and crystalline phases in the deformation mechanism that enhances ductility. All stress–strain curves of the different copolymers present an unusual second maximum at strains higher than the yielding point. Diffraction patterns recorded during deformation have revealed that this second maximum is associated with the crystallization under stretching of a highly disordered crystalline mesophase never described in the literature.