A new type of acid-labile cationic copolymer consisting of a hydrophilic poly(ethylene glycol) (PEG) block and a polymethacrylamide block bearing tertiary amines linked by acid-labile ortho ester rings in side chains (PAOE), with defined chain length, had been synthesized via RAFT polymerization. The copolymers could efficiently bind and condense plasmid DNA at neutral pH into narrowly dispersed nano-scale polyplexes. The hydrolysis of ortho ester group in the side-chains of PAOE followed a distinct exocyclic mechanism and the rate of hydrolysis was much accelerated at mildly acidic pH, resulting in the accelerated disruption of polyplexes and the release of intact plasmid DNA. The three polymers were not toxic to cultured COS-7 cells as measured by MIT assay. As expected, PEG segments of the PEG-b-PAOE copolymers prevented nonspecific transfection of COS-7 cells. Once conjugated to a targeting ligand to enhance cell-specific entry, PEG-b-PAOE with its pH-triggered DNA release properties may achieve efficient intracellular delivery of DNA or other nucleic acid therapeutics. (C) 2014 Elsevier Ltd. All rights reserved.