Combining Metal-Free and Metal-Mediated Ring-Opening Metathesis Polymerization for Efficient Synthesis of Bottlebrush Polymers.

Metal-mediated ring-opening metathesis polymerization (ROMP) is a common tool used to synthesize the backbone of bottlebrush polymers by a graft-through approach. However, examples of ROMP oligomers or polymers as macromolecular side chains are limited due in part to the challenges associated with preparing a macromonomer via the same polymerization mechanism as the one used to create the main chain. Accordingly, installation of polymerizable units onto macromonomers produced via ROMP has been achieved in a multistep fashion. In such cases, one metal initiator is used in the preparation of each macromonomer, thus constituting an overall usage of the metal complex that is 1:1 with repeat units in the final bottlebrush polymer product. By leveraging the modularity of organic initiators in metal-free ROMP (MF-ROMP), as well as the differences in monomer selectivities between metal-mediated ROMP and MF-ROMP, we demonstrate a uniquely efficient and low-metal-use route to bottlebrush polymers. Specifically, we found that norbornene imide-functionalized vinyl ether initiators were successful in MF-ROMP to produce macromonomers from tetracyclododecene without compromising the norbornene imide unit. Subsequent metal-mediated graft-through ROMP of the norbornene imide chain end using Grubbs' third-generation catalyst then produced high-molecular-weight bottlebrush copolymers. We report the synthetic methods for this sequential ROMP-ROMP approach, comparative analyses for endo- and exo-isomers of the macromonomers, and thermal characterization of the bottlebrush polymers.