It is difficult for the naked external gene to become internalized due to its similar charge to the extracellular membrane surface. Various cationic copolymers and cationic lipids have been reported to show different effects, but the exact mechanistic processes remain unclear. Polymers may interact with DNA to form DNA polyplexes through hydrophobic, entropic and electrostatic interactions. The net charge, size and structure of the complexes formed may influence cell transfection efficiency, but it remains largely unknown about how to control the gene-polymer complexation to optimize cell transfection and cytotoxicity. This work aims to rationalize the molecular processes by investigating complexation between a model diblock cationic copolymer and DNA.