Copolyamides based on polyamide-6,6 (PA-6,6) were prepared by solid-state modification (SSM). Para- and meta-xylylenediamine were successfully incorporated into the aliphatic PA-6,6 backbone at 200 and 230 degrees C under an inert gas flow. In the initial stage of the SSM below the melting temperature of PA-6,6, a decrease of the molecular weight was observed due to chain scission, followed by a built up of the molecular weight and incorporation of the comonomer by postcondensation during the next stage. When the solid-state copolymerization was continued for a sufficiently long time, the starting PA-6,6 molecular weight was regained. The incorporation of the comonomer into the PA-6,6 main chain was confirmed by size exclusion chromatography (SEC) with ultraviolet detection, which showed the presence of aromatic moieties in the final high-molecular weight SSM product. The occurrence of the transamidation reaction was also proven by H-1 nuclear magnetic resonance (NMR) spectroscopy. As the transamidation was limited to the amorphous phase, this SSM resulted in a nonrandom overall structure of the PA copolymer as shown by the degree of randomness determined using C-13 NMR spectroscopy. The thermal properties of the SSM products were compared with melt-synthesized copolyamides of similar chemical composition. The higher melting and higher crystallization temperatures of the solid state-modified copolyamides confirmed their nonrandom, block-like chemical microstructure, whereas the melt-synthesized copolyamides were random. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 5118-5129