Anionic Copolymerization of Nylon 6/12: A Comprehensive Review Mohammad Mohammadi , Shervin Ahmadi, Ismail Ghasemi, Mohammadreza Rahnama Iran Polymer and Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran This review aims to provide a concise insight into the rapidly growing anionic ring-opening copolymerization of nylon 6/12 and its related structures. This study analyzes the relationship between the structures of polymerization components and the relevant properties to achieve optimum copolymerization formulation. Specific emphasis is placed on how the cata- lyst type and temperature influence the final copolymer structure. The present work reviews available literature about nylon 6/12 synthesis published between 1960 and 2017 and investigates structures linked to polymerization mechanisms behind them. Moreover, experimental results derived from direct/indirect identification methods of struc- tures are presented. The crystalline morphology of different structures was also evaluated. The thermal behaviors of synthesized copolymers are explored comprehensively. Finally, mechanical properties alteration and enhanced water absorption results are reported. POLYM. ENG. SCI., 00:000–000, 2019. © 2019 Society of Plastics Engineers INTRODUCTION Both ring-opening polymerization and copolymerization of cyclic amides (lactams) are of commercial interest for their use in a wide variety of applications [1–5]. Lactam polymerization can be initiated not only by bases but also by acids and water. Initiation by water, known as hydrolytic polymerization, is carried out in the industrial polymerization of ε-caprolactam (CL) to form nylon 6; however, it is seldom used for other lactams [2, 6–9]. Cationic initiation involv- ing positively charged intermediate with acids has not interested many researchers due to the low conversion and the low-molecular weight of polyamides produced by this procedure [9–14]. Anionic polymerization of lactams is typically initiated by a two- component catalyst/activator system, composed of lactamate anions or their precursors, and N-acyllactam (NAL) or similar activating compounds. Such polymerizations which, for example, occur in a reactive injection molding (RIM) process [15, 16] yield polyamides directly from the corresponding lactams. The anionic polymerization, as a unique process typically performed at temperatures below the polymer melting point, has a high reaction rate. Many different stud- ies have led to such a conclusion [1–3, 6]. The anionic copolymerization of different lactam pairs and the properties of their copolyamide have been examined by many researchers since 1960s [17–24] and anionic copolymerization of CL with ω-laurolactam (LL) which are industrially available lactams, has been a frequently explored field of study during the recent decades. Some preliminary work in this field has been done by Kubota and Nowell [25], Šimůnková et al. [26], Frunze et al. [27, 28], and Godovskii et al. [29], who particularly focused on copolyamide 6/12 (nylon 6/12). The ratio of papers published in the field of copolymerization of CL with LL to all published articles regarding the anionic polymeri- zation of CL is negligible. However, the advantages of CL anionic copolymerization along with LL predominate over its disadvantages. The most important benefits of these copolymers include excellent mechanical properties (especially tensile strength), low moisture absorption, low melting point, and acceptable chemical resistance among others [1, 6, 25, 26, 28, 29]. Anionic copolymerization reactions are usually performed in a temperature range of 130–180  C, and the process is started by an appropriate catalyst and activators. Salts of CL sodium, lithium [30], or magnesium bromide [31] are employed as cata- lysts of the copolymerization. Besides, isocyanates [32] and N- substituted carbamoyl lactams [30, 34] have been used as activa- tors to accelerate the process. The choice of initiation system influ- ences the propagation, transacylation, and side reactions. The affected area includes the type of copolymer structure, which is directly influenced by the type of catalyst and synthetic reaction parameters such as copolymerization rate or molecular weight both of which are affected by activator type. However, such reactions are affected not only by the type of initiation system, and active growth centers but also by the concentration of initiation system and reaction conditions which play an undeniable role in the char- acteristics of the final product. In other words, initiation system comprising a catalyst and an activator, and also reaction conditions have a major influence on the polymerization rate, yield, structure, and properties of the synthesized materials, which, in turn, affect the other characteristics of the final polymer [35–39]. The process type should also be considered carefully for the selection of a suit- able initiation system [31, 35–43]. Furthermore, monomers ratio have a substantial effect on the copolymerization degree and consequently the physio-mechanical properties of synthesized copolyamide [31, 34, 42–44]. Ahmadi et al. [40, 41] conducted a comprehensive study on the effect of various catalyst and activator concentrations on the CL’s poly- merization parameters and measured the physical and mechanical properties of final PCL. They found the optimum contents of both cata- lyst and activator in order to achieve the lowest residual monomer as well as enhance mechanical properties. Barhoumi et al. [16] investi- gated the effect of different catalyst/activator concentrations as well as processing parameters on the kinetics of CL polymerization through dynamic rheology and differential scanning calorimetry measurements. They suggested a similar proportion of 4% for both catalyst and activa- tor and a processing temperature around 150  C which resulted in the lowest induction time. The purpose of the present work is to review the conducted studies on the anionic copolymerization of nylon 6/12 copolymers and to examine the effect of feed composition as well as polymerization con- ditions on the physiochemical properties of these copolymers. In addi- tion, different types of initiation systems have been investigated to explore their effect on the final polymer structure. Correspondence to: S. Ahmadi; e-mail: sh.ahmadi@ippi.ac.ir DOI 10.1002/pen.25171 Published online in Wiley Online Library (wileyonlinelibrary.com). © 2019 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2019