To overcome the unconventional polymerization behavior of single-site symmetrical metallocene catalyst in the presence of borate

Abstract

The β-H transfer of a propagating chain shows an unconventional polymerization behavior with single-site catalysts. In this work, we seek to address these challenges in the presence of different alkylaluminum cocatalysts. Adjusting the amount and type of cocatalyst in the polymerization process can affect molecular weight distribution (MWD) and chain ends formed via β-H transfer. The cocatalyst's elevated triethylelaluminum (TEA) by more than 50 % resulted in a drop in isoprene (IP) concentration, Mw, and activity, while pure TEA exhibited a smaller [C*]/[Zr] % than both pure Triisobutylaluminum (TIBA) and TIBA/TEA mixture. These results indicate that the type of alkylaluminum and its varying ratio affect the activation of metallocene catalysts. Secondly, this study also includes chain termination and transfer reactions and how comonomers participate in overcoming these issues. For this complicated process, we additionally performed the quench-labeling metal-polymer bonds technique (quantitative experiments (catalyst's active site)) leading to building a kinetic model that plays a significant role in mechanistic studies. The quantitative of active sites results indicate that the lower propagation rate constant (kp) was increased by nearly 20 % with TEA (25 %), and continuously increased with the TEA amount, meaning that TEA is a noticeable essential component in the cocatalyst for higher kp. The higher kpE and kpIP with TIBA might be associated with faster propagation and lower chain transfer reactions to Al−iBu and active centers and faster re-initiation of Al−iBu. The active center and 3,4 connections of IP are increased with increased TEA, meaning that the active site based on Al−Et bonds is facilitating more coordinated-to-IP through 3,4 connections. Finally, to clarify the impact of cocatalysts on MWD, a detailed theoretical splitting of the corresponding GPC was conducted. However, compared to PE MWD, TEA decreases the MWD of the polymers and becomes narrow. It is evident that introducing IP as a comonomer and TEA as a combined cocatalyst significantly reduces the chain transfer reaction that occurs during PE.