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 isotacticcopolymers (iP4MPHD) incorporating methylene-1,3-cyclopen-tane (MCP) cyclic co-units with concentrations in the range 4.4−17.6 mol % have been synthesized by using the dimethylpyr-idylamidohafnium/organoboron catalyst. The influence of theMCP cyclic co-unit on the crystallization behavior and themechanical properties of the isotactic poly(4-methyl-1-pentene)(iP4MP) homopolymer has been investigated in detail. iP4MPHDcopolymers with comonomer content up to 11 mol % crystallize inform II of iP4MP from the polymerization solution and in thestable form I of iP4MP from the melt, whereas the sample with thehighest concentration (17.6 mol %) of 1,5-hexadiene (1,5-HD) isamorphous and does not crystallize from either solution and melt. All crystalline samples exhibit high melting temperatures, alwaysabove 120 °C, and a controlled glass transition temperature close to the room temperature (28−30 °C). Incorporation of MCP unitsinto iP4MP chains produces an improvement in flexibility and allows tailoring of deformability while retaining high mechanicalresistance and transparency of the homopolymer. Interestingly, the high deformability is maintained at low temperature (50 °Cbelow the glass transition temperature), suggesting a cooperative role of both amorphous and crystalline phases in the deformationmechanism that enhances ductility. All stress−strain curves of the different copolymers present an unusual second maximum atstrains higher than the yielding point. Diffraction patterns recorded during deformation have revealed that this second maximum isassociated with the crystallization under stretching of a highly disordered crystalline mesophase never described in the literature