A highly efficient metal-free protocol for the synthesis of linear polydicyclopentadiene.

We have achieved breakthroughs for the efficient synthesis of linear polydicyclopentadiene (pDCPD) via photoredox mediated metal-free ring-opening metathesis polymerization (MF-ROMP). Molecular weight control from 1 to 16 kDa can be targeted in a straightforward manner. Assisted by this method, the Tg–Mn dependence of linear pDCPD is investigated and reported for the first time.

An Ion‐Pairing Approach to Stereoselective Metal‐Free Ring‐Opening Metathesis Polymerization.

Stereochemistry can have profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP. Ion-pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to >98%. High cis content was obtained from relatively larger counterions, toluene as solvent, low temperatures (-78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.

Advances in Polymerizations Modulated by External Stimuli

Synthetic polymer chemistry endeavors to imitate the spatial and temporal control exhibited within biological systems to obtain well-defined polymeric materials with unique structures, properties, and applications. This is often approached through the development of dynamic catalyst (or initiator) systems that use external stimuli to elicit discrete, site-specific transformations that impact the polymerization. Herein we highlight developments in polymerizations that are modulated by external stimuli, with particular focus on those systems that enable notable changes in kinetics, monomer selectivity, polymer architecture, or tacticity. Examples of external stimuli include chemical oxidants or reductants, light, applied voltage, and mechanical force.

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

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.

Molecular Weight Control via Cross Metathesis in Photo‐Redox Mediated Ring‐Opening Metathesis Polymerization

The use of π‐conjugated polymers (CPs) in conductive hydrogels remains challenging due to the water‐insoluble nature of most CPs. Conjugated polyelectrolytes (CPEs) are promising alternatives because they have tunable electronic properties and high water‐solubility, but they are often difficult to synthesize and thus have not been widely adopted. Herein, we report the synthesis of an anionic poly(cyclopentadienylene vinylene) (aPCPV) from an insulating precursor under mild conditions and in high yield. Functionalized aPCPV is a highly water‐soluble CPE that exhibits low cytotoxicity, and we found that doping hydrogels with aPCPV imparts conductivity. We also anticipate that this precursor synthetic strategy, due to its ease and high efficiency, will be widely used to create families of not‐yet‐explored π‐conjugated vinylene polymers.

Hybrid Photo-induced Copolymerization of Ring-Strained and Vinyl Monomers Utilizing Metal-Free Ring-Opening Metathesis Polymerization Conditions

We introduce the hybrid copolymerization of two disparate monomer classes (vinyl monomers and ring-strained cyclic olefins) via living photopolymerization. The living character of the polymerization technique (metal-free photo-ROMP) is demonstrated by consecutive chain-extensions. Further, we propose a mechanism for the copolymerization and analyze the copolymer structure in detail by high-resolution mass spectrometry.

Integration of Metal-Free Ring-Opening Metathesis Polymerization and Organocatalyzed Ring-Opening Polymerization through a Bifunctional Initiator

We have investigated the use of metal-free ring-opening metathesis polymerization (MF-ROMP) in combination with organocatalyzed ring-opening polymerization (o-ROP) to produce diblock copolymers with highly disparate block compositions via exclusively metal-free methods. Use of a bifunctional initiator bearing a vinyl ether as organic initiator for MF-ROMP and an alcohol for initiation of o-ROP allowed for investigation of three synthetic approaches: 1) sequential polymerization with isolation of the intermediate macroinitiators, 2) simultaneous bidirectional polymerizations, and 3) “one-pot” sequential monomer addition. Macroinitiators formed by first conducting o-ROP were successfully used in subsequent MF-ROMP to prepare diblock copolymers. Simultaneous MF-ROMP and o-ROP was thwarted by incompatible cross-combinations of catalysts and monomers. Finally, a straightforward “one-pot” synthesis of block copolymers, using o-ROP followed by MF-ROMP, was realized by sequential addition of each monomer-catalyst combination.

Bidirectional Metal-Free ROMP from Difunctional Organic Initiators

Ditopic initiators were evaluated for bidirectional organocatalyzed ROMP. Incorporation of monomer was found to be successful for both inward and outward polymer growth, stemming from divinyl ethers with different relative orientation of alkoxy moieties. Macroinitiators were also used to prepare triblock and graft copolymers that were found to be easily cleaved with acid catalyst.

Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization

We have investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization (ROMP). Polynorbornene, poly(exo-dihydrodicyclopentadiene), and poly(endo-dicyclopentadiene) were found to have cis olefin contents of 23%, 24%, and 28%, respectively. Additionally, the cis/trans ratio remained consistent during the course of norbornene polymerization. Polymer tacticity was evaluated by quantitative 13C NMR spectroscopy, which revealed each polymer to be largely atactic. Specifically, the three polymers were estimated to be 33%, 58%, and 55% syndiotactic, respectively. In parallel, we also explored the ability to produce diblock copolymers from norbornene and exo-dihydrodicyclopentadiene. Successful diblock copolymerization was achieved using either monomer order. In each case, however, we observed results consistent with chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) when reaction times were extended.

Expanded Functionality of Polymers Prepared Using Metal-Free Ring-Opening Metathesis Polymerization

Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is an alternative to traditional metal-mediated ROMP that avoids the use of transition metal initiators while also enabling temporal control over the polymerization. Herein, we explore the effect of various additives on the success of the polymerization in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP. The use of protected alcohol monomers allows for homo- and copolymers to be prepared that contain functionality beyond simple alkyl groups. Several other functional groups are also tolerated to varying degrees and offer insight into future directions for expansion of monomer scope.