AbstractPhenylphosphonyl‐N,N′‐biscaprolactam (I) and phenylphosphonyl‐N,N′‐bis(3,5‐dimethylpyrazole) (II) were synthesized and found to be very efficient activators for the anionic polymerization of caprolactam when used in combination with strong bases such as sodium caprolactam. Polymers obtained in the presence of I and II had generally higher molecular weights and were less sensitive to thermal degradation upon molding than those whose preparation entailed the use ofN‐acetyl‐caprolactam (III) as an activator. Thermal behavior and tensile properties indicated that the structure of these polyamides differs from that encountered in nylon 6 prepared with conventional anionic