The invention relates to a method for preparation of a polyamide 6 copolymer, characterised in that a copolymerization reaction is carried out between: o at least one caprolactam of formula (I): o at least one caprolactam, named substituted caprolactam, in which at least one of the carbon atoms is covalently linked to at least one A moiety, said A moiety being selected from the group formed of: # moieties comprising at least one group of formula: (A) #moieties comprising at least one group of formula: (B) # moieties comprising at least one group of formula: (C) The invention also relates to a polyamide 6, to polyamide 6 filaments and filaments yarns.