Ethylene-Triggered Regioselectivity Switch of Dimethylbutadiene in Their Copolymerization: Formation of “Plastic” Rubber and Mechanism

2,3-Dimethylbutadiene (DMB), a very old monomer used for making rubber albeit with poor elastomeric property a century ago, is rather difficult to polymerize due to its steric bulkiness. Herein, we report the successful coordination–insertion homopolymerization of DMB in trans-1,4 selectivity using the designed quinolinyl-functionalized cyclopentadienyl and thiophene-fused cyclopentadienyl scandium complexes to give a plastic product. Based on this, the copolymerization of DMB and ethylene (E) was realized with high activity through the comonomer "positive effect". More interestingly, high 1,2-regioselectivity of DMB was achieved (96.6 mol %) in copolymerization. Density functional theory (DFT) calculation revealed that the regioselectivity switch is triggered by the comonomer E, which coordinates to the active metal center to cause the isomerization of the active species from the Sc-η3-allyl mode to the Sc-σ-alkyl mode. The resultant E/DMB copolymers bear the pseudo-random sequence distribution to exhibit high elongation and strength (595%, 12.2 MPa) contrary to the crystalline properties of PDMB and PE precursors, indicating a rubbery material. This work paves an avenue to make high-performance materials from the commonly used or even abandoned monomers.