(Invited) Electron-Transfer Approaches to Metal-Free Ring-Opening Metathesis Polymerization

Abstract

We have recently discovered a method of performing ring-opening metathesis polymerization (ROMP) via an electrochemically-mediated process involving electron rich olefin initiators and norbornene-based monomers. The method expands upon the powerful, metal-mediated approaches toward ROMP that have been successful for the production of polymers with precise structure and function. A key distinction is our ability to integrate organocatalysis and electro-organic synthesis to circumvent the need for metal-based initiators. In this way, electromediated ROMP has opened the doorway to metal-free protocols that have the potential to provide materials with unique optical, electronic, and materials properties. Inspired by the pioneering work of Chiba and coworkers, we explored the hypothesis that anodic oxidation of vinyl ethers could be used to initiate and mediate polymerization of strained cycloalkenes via a ROMP mechanism. Toward this end, we evaluated vinyl ethers in combination with norbornene as a common ROMP monomer to determine the viability of electromediated ROMP. Although initial conversions of norbornene to polynorbornene were low, we were able to confirm initiation by the vinyl ether and successful polymer formation. Consideration of the key limiting factors in our initial electromediated ROMP conditions, which appeared to be poor solubility of norbornene and polynorbornene, led us to consider photoredox and indirect electrolysis approach. We will discuss recent successes with organic photo-oxidants capable of facilitating metal-free ROMP in high conversions with excellent temporal control. In addition, we will present our latest results on the use of organic mediators to facilitate metal-free ROMP via indirect electrolysis.