We report the design and synthesis of pH-responsive vesicular nanoparticles (NPs) based on amphiphilic random copolymers as novel drug carriers for chemotherapy. The random copolymers were photopolymerized from acryloyl chloride, followed by assembly in acetone. The resulting vesicular PAC NPs with an acrylate-functionalized shell were hydrolyzed in aqueous solution to form NPs with a carboxylated shell, which were further modified with branched poly(ethyleneimine) (PEI) to obtain positively charged PEI NPs. The resulting NPs maintained their structures and were pH-sensitive. The effects of molecular weight and concentration of PEI on the grafting efficiency, surface charge, and pH-sensitivity of PEI NPs were investigated by Fourier transform infrared (FT-IR) spectroscopy, electron microscopy, dynamic light scattering and zeta-potential measurements, respectively. We examined two packaging strategies to load the anticancer drug molecule, doxorubicin hydrochloride (Dox), into the NPs by encapsulating Dox inside the cavity of NPs during the assembly of the PAC NPs and by coupling Dox to the shell of PEI modified NPs. The first showed ∼30 wt% Dox loading efficiency, which was 15 times more than the latter, likely due to the larger loading capacity of both within the cavity and on the NP shell. We then tested in vitro delivery of Dox using cavity and surface loaded PEI NPs to HEK 293T cells. At the optimal concentration of NPs (9 μg mL⁻¹), approximately 10 times more Dox were delivered compared to the naked Dox with a low level of cytotoxicity (over 90% relative cell viability).