Accessing Equilibrious Modulation of Activity, Molecular Weight, and Tacticity for Syndiotactic Propylene Polymerization at Industrial Relevant Temperatures via Rational Design of Single-Site Catalysts

Single-site catalysts (SSC) have been widely applied for olefin coordination polymerization to generate high performance polyolefins. However, regular optimization of the structures of SSC would only enhance certain polymerization behaviors, such as polymerization activity, molecular weight, and syndiotacticity ([rrrr]) of syndiotactic polypropylene (sPP). In this work, we report the synthesis of Zr complexes Zr1, Zr2, and Zr3 bearing 3,5-tBu2, 2-Me, and 2-iPr substituted phenyl moiety and their application to syndiotactic propylene polymerization at industrial relevant temperatures based on the gradient modulation strategy of cyclopentadienyl-fluorenyl (Cp-Flu) ansa-zirconocenes. Equilibrious modulation of activity, molecular weight, and syndiotacticity was successfully achieved at industrial relevant temperatures. Zr2 exhibited equilibrious polymerization performance with high activity of 2.1 x 107 and 4.5 x 107 g/(mol<middle dot>h), high molecular weight of 60.2 x 104 and 36.5 x 104, and high syndiotacticity of 87.1% and 80.6% at 30 degrees C and 50 degrees C, respectively. Tensile tests and hysteresis experiments of sPPs were performed to establish a relationship between mechanical and elastic properties with syndiotacticity and molecular weight of sPPs. Demonstrated by the molecular structures and topographic steric maps of ansa-zirconocenes, the equilibrious polymerization performance of Zr2 and Zr3 is attributed to the remote modulation of 2-substituted aryl groups, rather than the steric hindrance around the Zr center.