Steadily Controllable Anionic Ring-Opening Polymerization of ε-Caprolactam at High Temperature with a Low...

The direct melt spinning of polyamide 6 (PA6) after polymerization remains formidably challenging. The conventional hydrolysis polycondensation (HP) of ε-caprolactam (CL) requires long polymerization and postpurification times due to low conversion, while the typical alkali-metal-initiated anionic ring-opening polymerization (AROP) of CL suffers from uncontrollable molecular weight at high temperatures. Here, we proposed a controllable AROP of CL with a low Mg-initiator dosage, especially suitable for direct melt spinning. With a low initiator dosage (I₀/M₀ = 1/1000-1/3000), it enables controllable regulation of M_n (4.7-38.5 kg mol^-1) and remains stable for a relatively long period above T_m (220 °C), simultaneously maintaining high conversion (ca. 95%) and a quasi-living character. The cyclic dimer residual was restricted to an unprecedented level (<0.01 wt %). The proposed simple system is more efficient and greener than the traditional HP and AROP techniques, offering a promising approach for producing PA6 fibers via direct melt spinning.