Intrinsically Flame-Retardant Polyamide 6 by Anionic Ring-Opening Copolymerization with a Phosphorous-Containing Comonomer

Abstract

A three-step synthetic route is established for the synthesis of a phosphorus-containing ε-caprolactam derivative, starting from cyclohex-2-enone and diphenylphosphinous chloride, followed by a Beckmann rearrangement. 2D NMR spectroscopy reveals that the phosphorus moiety is selectively localized at the 4-position of the caprolactam ring. Different amounts of the phosphorus-containing comonomer are systematically incorporated into polyamide 6 by anionic ring-opening copolymerization with ε-caprolactam at elevated temperatures. Polymerization reactions are initiated by a latent, CO₂ protected N-heterocyclic carbene in the presence of N-acetyl caprolactam and MgCl₂. The phosphorus content is determined by inductively-coupled plasma-optical emission spectroscopy (ICP-OES) analysis. Limiting oxygen indices up to 23.5 are found for polymers containing ≈1 wt.-% of phosphorus. The thermal properties of the copolymers are determined and compared to those of polyamide 6.