Synthesis and post-polymerisation ligations of PEG-based hyperbranched polymers for RNA conjugation via reversible disulfide linkage

The synthesis of architectural polymers with multiple reactive functionalities offers significant promise as platform technologies for development of nano-medicines that require hybrid biomolecule-nanomaterial components. However, there can often be a mismatch in compatibility between the conditions required for the coupling chemistry, while maintaining stability of the biomolecule. This leads to decreased yields and poor functional fidelity. In this report, we describe the synthesis of hyperbranched polymers, where reversible addition fragmentation chaintransfer (RAFT) polymerization is used to control chain molecular weight, end group functionality and the final size of the hyperbranched polymer. Through optimization of the reaction conditions, we demonstrate that branched polymers with controlled size can be synthesized. The subsequent modification of the end-groups within the branched polymer through coupling to small oligonucleotides is then systematically investigated as a function of coupling chemistry. We demonstrate that to achieve the highest degree of coupling, chain extension of the end-group away from the sterically-hindered core of the polymer is required, and that the use of strained alkyne-azide coupling reactions appear to show the highest level of efficiency under the conditions studied. Indeed, when mixed attachment of both fluorescent dye molecules and oligos is attempted under these conditions, almost quantitative end-group modification is achieved. Overall, we highlight the importance of choosing compatible chemistries that allow efficient coupling of biomolecules to synthetic substrates under mild conditions to achieve optimal reaction performance.