Polyamide 12, T-polyamide-6 (PA-12, T-PA-6) block copolymers were synthesized by anionic polymerization of caprolactam using a PA-12, T macrocoinitiator (McI). PA-12, T McI and its precursors are soluble in molten caprolactam allowing for both the McI step-growth polymerization and anionic polymerization to be performed in one-pot. It was found that the competing reaction rates of caprolactam ring-opening polymerization and McI transamidation are both deterred by a common ion effect using CaCl2 and soluble materials were obtained using > 1 mol % CaCl2. Without CaCl2, the reaction mixture solidifies in less than 30 s and produces crosslinked materials. To understand this effect, PA-12, T McI reactions with caprolactam were performed with 1-10 mol % CaCl2, and polymer structures were characterized using C-13 NMR and dilute solution viscometry. These data were then correlated with unique thermal properties and swelling behavior of the block copolymers. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2271-2280, 2011