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.

that stereochemical retention or inversion is possible upon addition of monomer units to the active chain end, providing opportunities for control over polymer tacticity.We recently discovered a method of ROMP that diverges from metal-mediated approaches by employing a vinyl ether initiator, which is activated toward monomer incorporation via one-electron oxidation. [5] Although the polymers obtained from the metal-free approach are superficially the same as those produced via metal-mediated ROMP, the mechanistic distinctions between the two pose unanswered questions regarding synthetic capabilities and structural control from the former ( Figure 2). As a first step toward understanding the stereochemical outcomes of metal-free ROMP, we have investigated the olefin geometry and tacticity of three common polymers: polynorbornene (pNB), polydicyclopentadiene (pDCPD), and polydihydrodicyclopentadiene (pDCPD-H 2 ).Another aspect of structural control from metal-mediated ROMP that has garnered many accolades is the broad ability to produce block copolymer architectures. [6] Although it is thus far evident that metal-free ROMP does This study investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization. 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 13 C 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, this study 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, the results suggested to us that chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) occurred when reaction times were extended.

Materials and Methods. Dichloromethane (CH 2 Cl 2) and acetonitrile (CH 3 CN) were obtained from a solvent purification system. 1 H NMR spectra were recorded on Bruker AVance 300 MHz or 500 MHz spectrometers. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane using the residual protio-solvent as an internal standard (CDCl 3 , 1 H: 7.26 ppm; DMSO-d 6 , 1 H: 2.50 ppm and 13 C: 39.5 ppm). Gel permeation chromatography (GPC) was performed using a GPC setup consisting of: a Shimadzu pump, 3 in-line columns, and Wyatt light scattering and refractive index detectors with tetrahydrofuran (THF) as the mobile phase. Numberaverage molecular weights (M n) and weight-average molecular weights (M w) were calculated from light scattering and refractive index data. All polymerizations were carried out under an inert atmosphere of nitrogen in standard borosilicate 2-dram glass vials purchased from Fisher Scientific with magnetic stirring unless otherwise noted. Irradiation of photochemical reactions was done using a 2 W Miracle blue LED indoor gardening bulb. The output of the blue LED bulb was measured using an Ocean Optics USB2000 spectrometer. The (thio)pyrylium tetrafluoroborate salts were prepared according to literature procedure. 1,2 Norbornene was sublimed prior to use. All other reagents and solvents were obtained from commercial sources and used as received unless otherwise noted. Cyclic Voltammetry. Inside a glovebox under N 2 atmosphere, 1.0 mmol (67 mM) of photo-oxidant and 320 mg (0.200 M) LiClO 4 were dissolved in 15 mL of CH 3 CN from the solvent still in a 3-neck round bottom flask equipped with a stir bar. The flask was sealed with septa pierced with a glassy carbon working electrode (3 mm), a Pt basket counter electrode (Premier Lab Supply), and a 0.1 M Ag/AgNO 3 reference electrode (0.1 M AgNO 3 in dry CH 3 CN). The cell was removed from the glovebox and equipped with a N 2 balloon. The mixture was stirred prior to cyclic voltammetry measurements. Cyclic voltammograms (CVs) were obtained using a BASi Epsilon potentiostat. Ferrocene (0.15 mmol) was added as an internal standard after each voltammogram and a subsequent CV was taken.

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.

Ring-opening metathesis polymerization (ROMP) is widely used for the synthesis of polymers, typically using a transition-metal-based initiator. Much effort based on improvement of catalyst design and polymerization techniques has led to breakthrough applications in diverse areas, including high-performance plastics, drug delivery, bioengineering, and organic photovoltaics. We reported an electrochemical polymerization technique using a vinyl ether as an initiator, which is directly oxidized to a radical cation at a carbon anode during bulk electrolysis. However, polymerization was only observed on the electrode surface, resulting in low yields. We hypothesized the low efficiency was due to the heterogeneous nature of the direct oxidation. Herein we report on the development of a method for the indirect electrochemical oxidation of the initiator via an electrochemical mediator to achieve metal-free-ROMP. Preliminary cyclic voltammetry analyses predicted the feasibility of electron transfer between the mediator and the initiator. In addition to these recent results, ongoing work will developing electrochemical ROMP methods via indirect oxidation of the initiator will be discussed.