De Novo 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 5 -C 5 Me 5 )Hf(Me)[N(Et)C(Me)N(Et)]}[B(C 6 F 5 ) 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, f PMCH, 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 f PMCH = 0.09 ( Ia ) and 0.23 ( Ic ), respectively, and a third more complex morphology being observed for Ib ( f PMCH = 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.