Gradient Modulation of Ansa-Metallocenes for Syndiotactic Propylene Polymerization: Multi-Stage Remote Substituent Effects

The production of economical and sustainable thermoplastic all-polyolefin composites has been a grand challenge in polymer chemistry. Polypropylene ranks at the top in the life cycle assessment but generally needs alien materials or modifications to attain certain desired material properties. Syndiotactic polypropylene exhibites the benefits of high impact resistance, processability, transparency, and elasticity and might help to meet the sustainable purpose. In this contribution, the gradient modulation strategy of cyclopentadienyl-fluorenyl ansa-zirconocenes with 4-^tBu, 3,5-Me₂, and 2,6-Me₂ substituted phenyl moiety (Zr1, Zr2, and Zr3) has been developed and evaluated in propylene polymerization with controlled polymerization activity, molecular weight, syndiotacticity, unsaturated chain ends, and mechanical properties of obtained syndiotactic polypropylenes (sPPs). It is elucidated for the first time that 2,6-, 3,5- and 4- substituents are responsible for high molecular weight (up to 80.3 × 10⁴ g moL⁻¹), high activity (up to 5.0 × 10⁷ g/(mol·h)), and high syndiotacticity (rrrr up to 81.1%), respectively. It is found by the unsaturated chain end analysis that 1,2-insertion followed by fast β-H elimination was the dominate pathway for syndiotactic polypropylene formation. Additionally, characterized by tensile tests and hysteresis experiments of selected sPP samples, high tensile strength of sPP-Zr2-30 (26.14 MPa) and high elastic property of sPP-Zr3-50 (SR = 49.2%) is demonstrated. Installation of phenyl rings, which feature large steric hindrance, flexible rotation, and multiple substitutive sites, to the fluorenyl presents an emerging pathway for potentially more sustainable polypropylene production.