chemical modifications of polymers find increasing application to produce new fiber-forming polymers and new fibers of improved properties. there are two approaches: the first one is to introduce new side groups without disturbance of the polymer backbone structure and the other one $em dash$ to introduce new monomers or longer blocks to the polymer backbone. in the case of polyethylene terephthalate/ /pet/ modification both the methods are used. in the reported experimental study, the authors have carried out syntheses of pet copolymers containing both the side functional groups /sulfonic/ and polyglycol blocks. the aim was to elucidate the relationships between the copolymer properties and their structure and chemical composition. data are presented which demonstrate that it is possible to obtain various bi- and tricomponent pet-containing block copolymers of wide range of melting point and viscosity depending on the copolymer type and its chemical composition. 6 refs