Octafluoropentyl acrylate [H(CF2CF2)(2)CH2- C(O)CH=CH2] and polycaproamide (PA-6) thread were copolymerized with a tert-butyl hydroperoxide initiator. The fluorine content of the thread was 0.87-1.33% after copolymerization. The creation of ester polyfluorinated alkyl groups on the end of the polyamide macromolecules by the homolytical substitution of hydrogen atoms in the alpha-CH2-group of the -HN-CH2-(CH2)(4)-C(O)- chain led to an increase in the tensile strength of the PA-6 thread. (c) 2006 Wiley Periodicals, Inc.