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This review provides a comprehensive analysis of modern scientific and practical approaches to reactive extrusion (REX) of polyethylene terephthalate (PET) - a method that allows integrating the stages of synthesis, modification and processing of the material in a single high-performance technological process. A wide range of chemical agents is considered, including di- and polyepoxides, isocyanates, dianhydrides, bisoxazolines, biscaprolactams and organic phosphites, which act as chain extenders and promoters of chemical modification reactions of PET. The mechanisms of interaction of these reagents with carboxyl and hydroxyl end groups of the polymer, which are formed as a result of its thermal and hydrolytic degradation, are considered in detail. The influence of each type of modifier on the increase in molecular weight, change in melt flow index, rheological characteristics and complex of physicomechanical properties of the material is analyzed. Particular emphasis is placed on the use of structure-forming agents (nucleators) and reactive impact modifiers to optimize the supramolecular structure and overcome the natural brittleness of PET. The work systematizes the literature data and experimental experience of the article authors on the restoration of the properties of secondary PET and formulates recommendations for the selection of optimal systems for obtaining high-quality polymer matrices for engineering plastics and polymer composite materials based on them.