Metrics & MoreArticle Recommendations CONSPECTUS: Ring-opening metathesis polymerization (ROMP), which is derived from transition-metal-based olefin metathesis, has evolved into one of the most prevalent technologies for making functional polymeric materials in academia and in industry. The initial discovery of and advances in ROMP used ill-defined mixtures of metal salts to initiate polymerization.The initiators most commonly used today, developed with tremendous efforts, are well-defined metal−alkylidene complexes that have enabled a good mechanistic understanding of the polymerization as well as improvement of the initiators' activity, stability, and functional group tolerance.The evolution of ROMP has been decidedly metal-centric, with the path to accolades being paved primarily in ruthenium-, molybdenum-, and tungstenbased systems. Our departure from the ROMP trailhead was inspired in part by recent breakthroughs in radical-mediated polymerizations, whereby their mechanisms were leveraged to develop metal-free reaction conditions. Inventing a metal-free complement to traditional ROMP would essentially involve stepping away from decades of inorganic and organometallic developments, but with the promise of crossing new synthetic capabilities and curiosities.Driven by this motivation, as well as a community-inspired desire to develop "greener" controlled polymerizations, our team pioneered the search for, and discovery of, a wholly organic alternative to traditional metal-mediated ROMP. In this Account, we review our recent efforts to develop metal-free ring-opening metathesis polymerization (MF-ROMP), which is inspired by previous reports in electro-and photo-mediated organic transformations. This work began with an exploration of the direct oxidation of enol ethers and the propensity of the ensuing radical cations to initiate ROMP. To overcome limitations of the electrochemical conditions, a photoredox-mediated method was investigated next, using photoexcited pyrylium salts to oxidize the enol ethers. With this system, we demonstrated the ability to produce ROMP products and temporally control the polymerization. Further investigations into different aspects of the reaction included monomer scope, functional group tolerance, the impact of changing photocatalyst properties, and the ability to control molecular weight. The unique mechanism of MF-ROMP, along with the relative ease of synthesizing enol ether initiators, enabled the preparation of numerous polymeric materials that are hard to access through traditional metal-mediated pathways. At the end of this Account, we provide a perspective on future opportunities in this emerging area.

Photo-redoxm ediated ring-opening metathesis polymerization (photo-ROMP) is an emerging ROMP technique that uses an organic redoxm ediator and av inyl ether initiator,incontrast to metal-based initiators traditionally used in ROMP.T he reversibility of the redox-mediated initiation and propagation steps enable spatiotemporal control over the polymerization. Herein, we explore as imple,i nexpensive means of controlling molecular weight, using alpha olefins as chain transfer agents.T his method enables access to low molecular weight oligomers,a nd molecular weights between 1a nd 30 kDa can be targeted simply by altering the stoichiometry of the reaction. This method of molecular weight control was then used to synthesize af unctionalized norbornene copolymer in ar ange of molecular weights for specific materials applications. Scheme 3. Proposed pathways of chain transfer via cross metathesis and reinitiation of photo-ROMP. Major and minor pathways differentiatedb y regioselectivity of cross metathesis event.Scheme 4. Steric comparison for possible cross metathesis intermediates.

Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl‐addition polynorbornene (VA‐PNB), which has strained bicyclic ring repeat units along the polymer backbone. VA‐PNBs with three different side chains were found to undergo ring‐opening olefination upon sonication in dilute solutions. The sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units into the ring‐open isomer typical of polynorbornene made by ring‐opening metathesis polymerization (ROMP‐PNB). Thermal analysis and evaluation of chain‐scission kinetics suggest that sonication of VA‐PNB results in chain segments containing a statistical mixture of vinyl‐added and ROMP‐type repeat units.

A two-step, one-flask reaction of pyrrole and pentafluorobenzaldehyde was investigated as a streamlined synthetic route to an N-confused porphyrin bearing pentafluorophenyl substituents previously prepared by a stepwise route. A survey of acid catalysts, acid catalyst concentration, DDQ quantity, and reaction time was performed with monitoring by HPLC. The targeted N-confused porphyrin was observed from many reaction conditions. The best condition afforded the N-confused porphyrin in an isolated yield of 10-12% (245-281 mg), providing improved access to this interesting porphyrinoid.

Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl-addition polynorbornene (VA-PNB), which has strained bicyclic ring repeat units along the polymer backbone.V A-PNBs with three different side chains were found to undergo ring-opening olefination upon sonication in dilute solutions.T he sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units into the ring-open isomer typical of polynorbornene made by ring-opening metathesis polymerization (ROMP-PNB). Thermal analysis and evaluation of chain-scission kinetics suggest that sonication of VA -PNB results in chain segments containing astatistical mixture of vinyl-added and ROMP-type repeat units.