The effects of the repeated number (RN) of aminoethylene (AE) units in polyaspartamide side chains were investigated for polyion complex (PIC)-based siRNA delivery. Reduction of the apparent RN from 3 to 2 by thiourea introduction increased a protonatable amine fraction in AE units at endosomal pH, leading to the efficient endosomal escape of siRNA-loaded PICs.