Stabilization of high molecular weight nylon 6 synthesized by anionic polymerization of ∈-caprolactam

Abstract

To stablilize high-molecular weight nylon 6 synthesized by the anionic polymerization of ∈-caprolactam, a method to prevent polymer degradation by removing catalyst residue was investigated. The removal of catalyst residue in the polymer was efficiently carried out by treating with acids whose pKa in water were between 3 and 7. The stability of the polymer on heating at 200°C in vacuo was highly improved by removing the catalyst residue with treatment by acetic acid (pKa = 4.8) in dimethyl sulfoxide solution. The reached weight average molecular weights (Mw) on heating at 200°C were 6.0 × 104 (initial Mw = 6.2 × 104), and 3.9 × 105 (initial Mw = 7.0 × 105). The decrease of Mw of the high molecular weight polymer without the catalyst residue on heating is considered due to hydrolysis, because the change of Mw agrees with the simulation results based on the assumption that decrease of the molecular weight is caused by hydrolysis. Hydrolysis was negligibly influenced by the catalytic activity of the terminal carboxyl group (provided the terminal concentration less than 0.07 mol%).