Design of a New Class of "Hard-Soft" Amorphous, Microphase-Separated, Polyolefin Block Copolymer Thermoplastic Elastomers.

Sequential cyclic/linear/cyclic living coordination polymerization of 1,6-heptadiene (HPD), propene, and HPD, respectively, employing the well-defined and soluble group 4 transition-metal initiator, {(η-CMe)Hf(Me)[N(Et)C(Me)N(Et)]}[B(CF)], provides the stereoirregular, amorphous poly(1,3-methylenecyclohexane)--atactic polypropene--poly(1,3-methylenecyclohexane) (PMCH-aPP--PMCH) polyolefin triblock copolymer () in excellent yield. By varying the weight fraction of the end group, minor component "hard" PMCH block domains, , relative to that of the midblock "soft" aPP domain, three different compositional grades of these polyolefin block copolymers, , were prepared and shown by AFM and TEM to adopt microphase-separated morphologies in the solid state, with spherical and cylindrical morphologies being observed for = 0.09 () and 0.23 (), respectively, and a third more complex morphology being observed for ( = 0.17). Tensile testing of served to establish these materials as a new structural class of polyolefin thermoplastic elastomers, with being associated with superior elastic recovery (94 ± 1%) after each of several stress-strain cycles.