Neat and hybrid poly(2-pyrrolidone), i.e. polyamide 4 (PA4) microparticles containing magnetic or conductive metal, metal oxide, or carbon nanotube fillers were prepared via environmentally-friendly solventless activated ring-opening polymerization of 2-pyrrolidone at 40 degrees C. The method produces high porosity microparticles with diameters of 5-12 mu m and conversion to PA4 of 56-65%. Their structure and properties were characterized by light- and electron microscopy, thermal, spectral and X-ray diffraction techniques. Two crystalline polymorphs, namely alpha- and beta-PA4, were found to coexist at room temperature by X-ray diffraction. The assessment of the adsorption capacity of the PA4 hybrid microparticles toward model protein showed up to 60% efficiency only after 1 h of incubation without any preliminary activation or functionalization. (C) 2018 Elsevier Ltd. All rights reserved.