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

ABSTRACT

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, {(η5-C5Me5)Hf(Me)[N(Et)C(Me)N(Et)]}[B(C6F5)4], provides the stereoirregular, amorphous poly(1,3-methylenecyclohexane)-b-atactic polypropene-b-poly(1,3-methylenecyclohexane) (PMCH-b-aPP-b-PMCH) polyolefin triblock copolymer (I) in excellent yield. By varying the weight fraction of the end group, minor component "hard" PMCH block domains, fPMCH, relative to that of the midblock "soft" aPP domain, three different compositional grades of these polyolefin block copolymers, Ia-c, 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 fPMCH = 0.09 (Ia) and 0.23 (Ic), respectively, and a third more complex morphology being observed for Ib (fPMCH = 0.17). Tensile testing of Ia-c served to establish these materials as a new structural class of polyolefin thermoplastic elastomers, with Ia being associated with superior elastic recovery (94 ± 1%) after each of several stress-strain cycles.