A series of random copolymers of 2-isopropyl-2-oxazoline and ethylenimine (PiPOX-ran-PEI) with PEI contents 5-84% were prepared by partial acidic hydrolysis of poly(2-isopropyl-2-oxazoline) (PiPOX). Dynamic and electrophoretic light scattering were utilized to investigate aqueous solution properties of the copolymers. Above the phase transition temperature of PiPOX, well-defined nanoparticles (mesoglobules) typically ranging in size 200-500 nm, depending on the heating protocol, and surface potential reaching +35 mV, were formed. The mesoglobules consisted of a core dominated by PiPOX and a thin positively charged shell from PEI moieties which mediated formation of polyplexes with DNA. Upon complexation with DNA, performed at 65 degrees C and at varying N/P (amine-to-phosphate groups) ratios, well-defined polyplex particles with dimensions ranging between 110 and 250 nm and typically negative zeta potential were formed. Cooling down to the physiologically relevant temperature of 37 degrees C resulted in a slight-to- moderate increase of the size due to swelling of the polyplex particles and reduction of the negative 1 potential. The copolymers with PEI contents >= 15% can be considered perspective for delivery of polynucleotides. (C) 2017 Elsevier Ltd. All rights reserved.