Synthesis of hybrid block copolymers via integrated ring-opening metathesis polymerization and polymerization of NCAw

Hybrid copolymers, which contain segments with distinct structures and properties, are useful materials in various applications such as novel plastics, polymeric catalysis, smart materials, self-assembly, and bionanotechnology. They can be prepared through conjugation of hybrid polymeric segments or polymerization with corresponding initiators. The latter approach is particularly attractive as the hybrid polymers with tuneable molecular weights (MWs) and narrow polydispersities (PDIs) of each block may be readily achieved via controlled polymerizations. In parallel to the studies of utilizing AB-type dual initiator to facilitate two distinct, controlled polymerizations from the initiators A and B of the same centre, there is also significant interest in preparing linear hybrid copolymers via controlled, sequential polymerizations, using a mono-initiator for the synthesis of the first polymeric block followed by transformation of the active chain end of the resulting first block using chain transfer agents (CTAs) or terminating agents (TAs) to a macromolecular initiator with a desired terminal group for the synthesis of a second polymer block. Here, we report the utilization of the CTA approach to synthesize polypeptide-b-poly(oxa)norbornene hybrid copolymers, via controlled ring-opening metathesis polymerization (ROMP) followed by ring-opening polymerization of amino-acid N-carboxyanhydrides (NCAs). We recently reported the integration of ring-opening polymerization (ROP) of NCAs and ROMP to prepare polypeptide-grafted brush-like polymers in a one-pot fashion (Scheme 1a). The strategy involves the ROMP of a norbornene monomer containing N-trimethylsilyl (N-TMS) amine followed by the controlled ROP of NCAs mediated by the pendent N-TMS amine group, a method we developed recently. We reasoned that the excellent compatibility between N-TMS amines and ROMP catalysts could also be utilized to prepare linear hybrid block polymers by using an N-TMS amine functionalized cis-alkene as CTA. Consequently, the N-TMS terminal groups, which are amenable for NCA polymerization yet compatible with olefin metathesis catalyst, can be transferred to the polyolefin chain ends, making the synthesis straightforward with no need to protect the amine group (Scheme 1b). CTA (2) was designed as a symmetrical, N,N0-bis(TMS)diamine functionalized cis-olefin (Scheme 2) instead of an N-TMS amine functionalized terminal alkene because the former usually leads to a higher degree of chain-end functionalization than the latter. 2 was synthesized in a few steps from inexpensive, commercially available starting materials, and fully characterized by H-NMR and C-NMR (see ESIw). To test whether 2 can act as a CTA and give complete end-capping of ROMP polymers, Grubbs catalyst 1 was mixed with 2 (5–6 equiv.) and stirred for 1 h to ensure complete carbene exchange, yielding a N-TMS bearing ruthenium catalyst (Step 1, Scheme 2). The colour gradually turned brown from green, indicating the olefin exchange and the formation of S3. The ROMP monomer M1 (30 equiv.) was then added directly to the S3 solution and the polymerization proceeded rapidly (Step 2, Scheme 2). We expected that an N-TMS amine terminated ROMP polymer (S4, Scheme 2) with an active chain-end would be generated from S3. Since the ROMP of M1 was several orders of magnitude faster than the cross-metathesis of 2, the chain termination of the ROMP polymer by excess 2 (Scheme 2, Step 3) would happen