polymers Article Effect of Different Flame-Retardant Bridged DOPO Derivatives on Properties of in Situ Produced Fiber-Forming Polyamide 6 Jelena Vasiljevi´ c 1, * , Marija ˇ Colovi´ c 2, * , Nataša ˇ Celan Korošin 3 , Matic Šobak 2 , Žiga Štirn 2 and Ivan Jerman 2 1 Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerˇ ceva 12, 1000 Ljubljana, Slovenia 2 National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia; Matic.Sobak@ki.si (M.Š.); Ziga.Stirn@ki.si (Ž.Š.); ivan.jerman@ki.si (I.J.) 3 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Veˇ cna pot 113, 1000 Ljubljana, Slovenia; natasa.celan@fkkt.uni-lj.si * Correspondence: jelena.vasiljevic@ntf.uni-lj.si (J.V.); marija.colovic@ki.si (M. ˇ C.); Tel.: +386-120-03200 (J.V.); +386-147-60323 (M. ˇ C.); Fax: +386-120-03270 (J.V.) Received: 18 February 2020; Accepted: 10 March 2020; Published: 13 March 2020   Abstract: The production of sustainable and effective flame retardant (FR) polyamide 6 (PA6) fibrous materials requires the establishment of a novel approach for the production of polyamide 6/FR nanodispersed systems. This research work explores the influence of three different flame-retardant bridged 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivatives on the comprehensive properties of in situ produced PA6/FR systems. To this end, in situ water-catalyzed ring-opening polymerization of ε-caprolactam was conducted in the presence of three different bridged DOPO derivatives, e.g., one P−N bond phosphonamidate derivative and two P−C bond phosphinate derivatives. The selected bridged DOPO derivatives mainly act in the gas phase at the temperatures that relatively match the PA6 pyrolysis specifics. The effects of the FRs on the dispersion state, morphological, molecular, structural, melt-rheological, and thermal properties of the in situ synthesized PA6 were evaluated. The specific advantage of this approach is one-step production of PA6 with uniformly distributed nanodispersed FR, which was obtained in the case of all three applied FRs. However, the applied FRs differently interacted with monomer and polymer during the polymerization, which was reflected in the length of PA6 chains, crystalline structure, and melt-rheological properties. The applied FRs provided a comparable effect on the thermal stability of PA6 and stabilization of the PA6/FR systems above 450 ◦ C in the oxygen-assisted pyrolysis. However, only with the specifically designed FR molecule were the comprehensive properties of the fiber-forming PA6 satisfied for the continuous conduction of the melt-spinning process. Keywords: PA6; bridged DOPO derivatives; in situ polymerization; thermal stability; flame retardancy 1. Introduction The semicrystalline linear polyamide 6 (PA6) polymer is one of the most important synthetic fiber-forming polymers because of its excellent processing properties and good mechanical and tribological properties. An additional advantage of the PA6 is its chemical recyclability, which enables the re-usability and the possibility for closing the loop on the circular economy [1]. The applications of PA6 textile filament yarns range from carpets, apparel, and upholstery to tire cords, seat belts, automotive and other transportation interior and seats covers, parachutes, tents and sleeping bags, etc. For such applications, increased thermal stability and flame retardancy are required in order to comply with fire safety regulations. Polymers 2020, 12, 657; doi:10.3390/polym12030657 www.mdpi.com/journal/polymers