Metal-Free Ring-Opening Metathesis Polymerization: From Concept to Creation

Abstract

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.

Metal‐Free Ring‐Opening Metathesis Polymerization with Hydrazonium Initiators**

“…Thus, there has been an interest in the discovery of nonmetal alternatives for ROMP. [30] Most notable in this regard, Boydston developed an elegant photoredox-mediated ROMP that utilized vinyl ethers as the propagating functionality. [31,32] Despite the highly creative and useful nature of this chemistry, issues of generality and scalability present challenges for its wider adoption.…”

“…In addition, certain applications such as those involving biomedical devices or electronics are incompatible with polymeric materials that contain metal impurities. Thus, there has been an interest in the discovery of non‐metal alternatives for ROMP [30] . Most notable in this regard, Boydston developed an elegant photoredox‐mediated ROMP that utilized vinyl ethers as the propagating functionality [31, 32] .…”

The Limited Palette for Photonic Block‐Copolymer Materials: A Historical Problem or a Practical Limitation?

“…Therefore, while ROMP remains the most viable method to unlock the potential of BBCPs as large-area colorants (e.g., automotive paint), alternative backbones and polymerization methods are urgently required that can maintain efficient polymerization at high DPs while achieving a lower production cost. Potential alternative polymerization methods that could be applied towards the effective and inexpensive polymerization of BBCPs include metal-free ROMP, [40] grafting-onto click chemistry, [41] and orthogonal cationic and radical RAFT polymerizations. [42] Finally, recent examples employing supramolecular comb-like BCPs suggest an alternate strategy that bypasses these synthetic challenges, while also offering a degree of tunability via the formulation.…”

Donor modification of thermally activated delayed fluorescence photosensitizers for organocatalyzed atom transfer radical polymerization

“…[16][17][18][19][20][21] As a result of these innovations, photoinduced electron transfer (PET) has become a key feature of metal-free reversible deactivation radical polymerizations. [22][23][24][25][26] In 2017, Huang et al demonstrated O-ATRP using thermally activated delayed fluorescence (TADF) emitters as photocatalysts. 27 TADF is widely deployed in fields ranging from organic electronics, 28 photocatalysis, 29 biological imaging, 30 and chemical sensing 31,32 due to a unique ability to interconvert singlet and triplet excited states.…”