Gradient Ethylene/α-Olefin Copolymer Prepared via In Situ Copolymerization

Synthesis of gradient copolymers through coordination insertion copolymerization is an unmet goal since the prerequisite is not only the copolymerization being performed in a living manner but also the composition drift being sensitive to the change of comonomer concentration. Herein, these challenging issues to ethylene/1-hexene polymerization were simultaneously solved by carefully tuning the steric of the ortho-substituent of the amine bis(phenolate) moiety within the corresponding titanium complex. The titanium complex 1 catalyzed ethylene/1-hexene copolymerization to give gradient copolymers P1–P4 within a certain range of 1-hexene concentrations through in situ polymerization and block copolymer P5 through a sequential feeding strategy. The structure–property relationship between the phase structure and the mechanical property and electric breakdown voltage were elucidated. Compared to P4 containing a tiny crystalline phase, the gradient copolymer P3 with a large discrete crystalline phase shows a much higher electric breakdown voltage (209.6 kV/mm vs 146.6 kV/mm) and tensile strength (23.6 MPa vs 3.8 MPa). With the accumulation of crystalline phase in P2, the electric breakdown voltage slightly decreases to 199.7 kV/mm, but the tensile strength improves to 34.0 MPa, which are similar to those of block copolymer P5 (E_b = 183.7 kV/mm, σ_b = 32.8 MPa) and superior to those of P1 (E_b = 160.7 kV/mm, σ_b = 25.9 MPa) with continuous crystalline phase.