A sequential native chemical ligation – thiol-Michael addition strategy for polymer–polymer ligation†
We systematically investigate sequential native chemical ligation (NCL)-thiol Michael addition as an efficient strategy for the dual modification of polymers. Polymers containing terminal cysteine functional groups were shown to undergo efficient NCL in the presence of amino acid-based and polymeric thioesters. The retention of the native cysteine side chain present at the NCL-junction distinguishes this approach from related multi-step strategies and was exploited for subsequent thiol-Michael additions providing access to diverse polymer architectures and polymer-peptide (single amino acid in this case) conjugates. Careful evaluation of model reactions involving a terminal cysteine functional poly(ethylene glycol), an amino acid thioester, i.e. phenylalanine thioester, and trifluoroethyl acrylate by NMR, SEC and MALDI-ToF MS revealed highly efficient modifications. Expansion of this concept towards oligomeric acrylates based on, e.g. N-acylated poly(amino ester)s (NPAEs) yielded Y-shape (co)polymers with quantitative conversions. The versatility and potential of the sequential modification was further demonstrated by employing a thioester-functionalised poly(2-ethyl-2-oxazoline) (PEtOx) to prepare a 3-arm mikto-arm star terpolymer.