Cationic polycarbonates are promising gene delivery vectors owing to their biodegradability and low toxicity. While previous research has mainly focused on polycarbonates with pendant cationic side chains, main-chain polycarbonates incorporating both cationic and stimuli-responsive groups, along with well-defined degradation profiles, remain underexplored for mRNA delivery. Herein, we synthesized main-chain cationic polycarbonates, Bn-pNC and Bn-p(NC-co-SSC), via ring-opening polymerization of a nitrogen-containing cyclic carbonate (NC), with or without a disulfide-containing cyclic carbonate (SSC), using benzyl alcohol (Bn) as the initiator. The resulting copolymers featured tertiary amine groups alone (Bn-pNC) or a combination of disulfide linkages (Bn-p(NC-co-SSC)) within their backbones. We demonstrated that Bn-pNC exhibited pH-responsive degradation, while p(NC-co-SSC) displayed dual pH- and reduction-responsive degradability. These polymers efficiently complexed with mRNA to form nanosized, positively charged polyplexes. However, only Bn-p(NC-co-SSC) based polyplexes demonstrated both excellent transfection efficiency and biocompatibility in HEK 293T cells. In vivo studies further revealed that Bn-p(NC-co-SSC)-mRNA polyplexes enabled effective transfection at the administration sites via both intramuscular and intranasal routes. Notably, Bn-p(NC-co-SSC)/mRNA polyplexes achieved approximately 100-fold higher transfection efficiency than commercial poly(ethylenimine) following intramuscular administration, with no observable systemic toxicity. Moreover, the Bn-p(NC-co-SSC)-mRNA polyplexes acted as localized drug depots at the muscle site, enabling controlled mRNA release and sustained therapeutic protein expression. Overall, this study highlights the potential of main-chain cationic polycarbonate Bn-p(NC-co-SSC), featuring dual pH- and reduction-responsive degradability, as a safe and efficient mRNA delivery system for future therapeutic applications, particularly in vaccine development and long-acting protein therapies.
