Coordinative polymerization brings opportunities for producing well-defined long-chain branched polyolefins specifically by using homogeneous single-site catalysts. Herein, we report a new dual catalytic system for the controlled formation of long-chain branches in a coordinative chain transfer polymerization. The growing polymers on the main aryl-substituted αdiimine nickel catalyst are frequently transferred via a chain transfer agent to the vinyl-producing catalyst, (Bipy) 2 FeEt 2 , where they are released as macromers through β-H elimination. The released macromers are incorporated into the growing polymer structure thanks to the high comonomer affinity of the main catalyst. By recurrence of this reaction cycle, a branch-on-branch structure is produced, which cannot be obtained using previous single or dual catalytic systems. The efficiency of the reaction is studied by performing ethylene and 1-hexene polymerizations at different reaction conditions. By increasing the amount of vinyl-producing catalyst, dramatic effects on the rheological properties are observed including increased dynamic moduli, prolongation of the G′−G″ crossover, and significant thermorheological complexity, all demonstrating formation of long branches. The microstructural analysis using 13 C NMR indicates that short-chain branches are vastly found in all samples due to the chain walking reaction. The spectroscopy and thermal analyses suggest that these short branches decrease by the addition of the vinyl-producing catalyst. This is presumably attributed to the high steric hindrance of the macromers, which forces the catalyst centers to preferentially attach to the primary carbons.

In this paper, using a comprehensive study, we have investigated the effect of various polymerization parameters during the synthesis of bimodal polyethylene resins on their rheological and mechanical properties. Bimodal polyethylene resins were synthesized in two subsequent stages in a lab-scale reactor by manipulating a set of parameters such as C 2 /H 2 ratios in the first and second stages, the split value, and the comonomer type. The results showed that the comonomer type and C 2 /H 2 ratio of the second stage of the polymerization are the most critical parameters that control the final resins' slow crack growth resistance properties. On the other hand, the shear-thinning behavior of the resins is mainly controlled by the first-stage polymers. Although the C 2 /H 2 ratio of the first stage results in a moderate effect on the rheological properties of the final resins, its split value governs the flow characteristics of the final molten polymers under high shear rates.

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