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In the past, most researchers believed that cocatalyst is a passive component that only has the task of "removing impurities" and "alkylating" the active centers of catalyst at the beginning of the polymerization and does not play a specific role in the polymerization. Today, it has been proven that this belief is incorrect, because the use of various cocatalysts leads to the induction of various properties in the final polymer, which is inconsistent with this view. Cocatalyst (negative ion) and active catalytic centers (positive ion) form "ion pairs" that govern the microstructure and arrangement of monomers in polymer chains during polymerization reaction. One of the important factors in the polymerization of ethylene α-olefin is the choice of aluminum alkyl to control the activity and properties of the polymer. In this article, the importance of the role of cocatalyst in the polymerization of Ziegler-Natta, metallocene, and hybrid catalysts, its effect on the catalyst behavior, distribution of active catalytic centers, catalyst activity and properties of polymer products such as molecular weight, molecular weight distribution, wax percentage, comonomer incorporation and physical-mechanical properties such as strength and impact resistance were reviewed. The results showed that each cocatalyst, due to its nature and chemical structure, causes alkylation of active catalytic centers and therefore produces polymers with various properties. Also, the use of a combination of different types of cocatalysts can induce different properties than each of the cocatalysts alone in the polymerization process.

Today, almost all polymer researchers use rheological methods to identify the microstructure of various polymer systems. One of the most popular and widely used methods is the small amplitude oscillatory shear (SAOS) test. SAOS provides a set of valuable information about the microstructure of polymeric materials. Various materials in this test are examined only in the linear viscoelastic region (LVR). However, polymer melts (raw, alloy, composite, etc.) are exposed to a much higher shear rate in processing methods. Therefore, the study of the non-linear behavior of polymers is of particular importance. Fourier transform rheology (FT rheology) is one of the most advanced methods for identifying the microstructure of polymers. The use of large amplitude oscillatory shear (LAOS), which leads to departure from the LVR, is the basis of the method. The stress response to this large deformation has the basic harmonies and involves higher odd harmonies that complicate the response form. In this method, the Fourier transform technique is used to convert a complex response in the time domain to a simple response in the frequency domain. The mentioned response contains valuable information about the microstructure of materials, which is impossible to extract (with this high accuracy) from the output responses in the LVR.