A method polymerizes a monomer to form a polyamide having a reagent incorporated therein. In the method, a masterbatch is formed that includes the reagent. The masterbatch and the monomer are introduced into a reactor, and the monomer is polymerized in the presence of the reagent to form the polyamide having the reagent incorporated therein. The monomer may be a caprolactam monomer that may be polymerized in a VK tube reactor to form polyamide 6 having at least one free acid site. The reagent may be present in an amount of from 1 to 10 parts by weight per 100 parts by weight of the masterbatch. Additionally, the masterbatch and caprolactam may be introduced into the top of the VK tube reactor.