We have developed a method to achieve ring-opening metathesis polymerization (ROMP) mediated by oxidation of organic initiators in the absence of any transition metals. Radical cations, generated via one-electron oxidation of vinyl ethers, were found to react with norbornene to give polymeric species with microstructures essentially identical to those traditionally obtained via metal-mediated ROMP. We found that vinyl ether oxidation could be accomplished under mild conditions using an organic photoredox mediator. This led to high yields of polymer and generally good correlation between Mn values and initial monomer to catalyst loadings. Moreover, temporal control over reinitiation of polymer growth was achieved during on/off cycles of light exposure. This method demonstrates the first metal-free method for controlled ROMP.
Publications
3D-Printed Mechanochromic Materials
We describe the preparation and characterization of photo- and mechanochromic 3D-printed structures using a commercial fused filament fabrication printer. Three spiropyran-containing poly(ε-caprolactone) (PCL) polymers were each filamentized and used to print single- and multicomponent tensile testing specimens that would be difficult, if not impossible, to prepare using traditional manufacturing techniques. It was determined that the filament production and printing process did not degrade the spiropyran units or polymer chains and that the mechanical properties of the specimens prepared with the custom filament were in good agreement with those from commercial PCL filament. In addition to printing photochromic and dual photo- and mechanochromic PCL materials, we also prepare PCL containing a spiropyran unit that is selectively activated by mechanical impetus. Multicomponent specimens containing two different responsive spiropyrans enabled selective activation of different regions within the specimen depending on the stimulus applied to the material. By taking advantage of the unique capabilities of 3D printing, we also demonstrate rapid modification of a prototype force sensor that enables the assessment of peak load by simple visual assessment of mechanochromism.
Recent Developments in Organocatalyzed Electro-organic Chemistry
This highlight review focuses on the integration of organocatalysis and electrosynthesis. Specifically, we detail recent advances involving the use of organocatalysts to alter substrate redox potentials through the in situ generation of electroactive intermediates. The ability to redirect organocatalyzed pathways toward oxidative fates using electrolysis techniques has enabled highly efficient transformations that would traditionally require stoichiometric sacrificial oxidants.
1,2-Oxazine Linker as a Thermal Trigger for Self-Immolative Polymers
We have demonstrated the site-specific thermal activation of self-immolative polymers (SIPs) using a bicyclic oxazine as a temperature-sensitive triggering moiety. The oxazine-based trigger was installed at the junction of a SIP-poly(N,N-dimethylacrylamide) (PDMA) diblock copolymer via oxidation of hydroxyurea end groups on the SIP and in situ [4 + 2] cycloaddition with cyclopentadiene-functionalized PDMA. The trigger undergoes a thermally-driven cycloreversion which ultimately leads to initiation of the depolymerization process. The temperature dependence of activation and depolymerization were investigated, along with the mechanism of activation. The relative rates of depolymerization at different temperatures suggested to us that the thermal trigger design may be a good candidate for on-demand activation of SIPs with minimal background triggering.
Kinetic Analysis of Mechanochemical Chain Scission of Linear Poly(phthalaldehyde)
The kinetics of mechanochemical chain scission of poly(phthalaldehyde) (PPA) are investigated. Ultrasound-induced cavitation is capable of causing chain scission in the PPA backbone that ultimately leads to rapid depolymerization of each resulting polymer fragment when above the polymer’s ceiling temperature (Tc). An interesting feature of the mechanochemical breakdown of PPA is that “half-chain” daughter fragments are not observed, since the depolymerization is rapid following chain scission. These features facilitate the determination of rate constants of activation for multiple molecular weights from a single sonication experiment. Additionally, the degradation kinetics are modified with chain-end trapping agents through variation of the nature and amount of small molecule nucleophile or electrophile.
Modeling the Mechanochemical Degradation of Star Polymers
A model for predicting the molecular weight distributions of mechanochemically degraded star polymers has been developed. The model was shown to be in good agreement with experimental distributions and average total molecular weights obtained from ultrasonically degraded three-arm star poly(methyl acrylate)s. Generalization of the model to four- and n-arm star polymers was also achieved. The models are straightforward to use, and thus, all calculations were completed in Microsoft Excel.
Comparison of Mechanochemical Chain Scission Rates for Linear versus Three-Arm Star Polymers in Strong Acoustic Fields
The effect of star versus linear polymer architecture on the rates of mechanochemically induced bond scission has been explored. We determined rate constants for chain scission of parent linear and star polymers, from which daughter fragments were cleanly resolved. These studies confirm a mechanistic interpretation of star polymer chain scission that is governed by the spanning rather than total molecular weight. We further demonstrate the preserved rate of site-selective mechanophore activation across two different polymer structures. Specifically, we observed consistent activation rate constants from three-arm star and linear polymer analogues, despite the Mn of the star polymer being 1.5 times greater than that of the linear system.
Mechanically-Triggered Heterolytic Unzipping of a Low Ceiling Temperature Polymer
Biological systems rely on recyclable materials resources such as amino acids, carbohydrates and nucleic acids. When biomaterials are damaged as a result of aging or stress, tissues undergo repair by a depolymerization–repolymerization sequence of remodelling. Integration of this concept into synthetic materials systems may lead to devices with extended lifetimes. Here, we show that a metastable polymer, end-capped poly(o-phthalaldehyde), undergoes mechanically initiated depolymerization to revert the material to monomers. Trapping experiments and steered molecular dynamics simulations are consistent with a heterolytic scission mechanism. The obtained monomer was repolymerized by a chemical initiator, effectively completing a depolymerization–repolymerization cycle. By emulating remodelling of biomaterials, this model system suggests the possibility of smart materials where aging or mechanical damage triggers depolymerization, and orthogonal conditions regenerate the polymer when and where necessary.
Electrochemical Characterization of Azolium Salts
Redox properties of a series of azolium salts, including benzothiazolium, thiazolinium, thiazolium, triazolium, imidazolium, and imidazolinium salts, have been systematically investigated. The series includes a broad range of N-arylthiazolium salts that collectively demonstrate the ability to fine-tune the reduction potential of the thiazolium ring via electronic modification of the N-aryl moiety. Additionally, a novel class of N-arylthiazolinium salts has been synthesized and characterized. In contrast to what has been observed for imidazolium and imidazolinium counterparts, saturation of the thiazolium backbone to give thiazolinium salts results in a more facile electrochemical reduction.
Organocatalyzed Anodic Oxidation of Aldehydes to Thioesters
A method has been developed for the direct conversion of aldehydes to thioesters via integration of organocatalysis and electrosynthesis. The thiazolium precatalyst was found to facilitate oxidation of thiolate anions, leading to deleterious formation of disulfide byproducts. By circumventing this competing reaction, thioesters were obtained in good-to-excellent yields for a broad range of aldehyde and thiol substrates. This approach provides an atom-efficient thioesterification that circumvents the need for stoichiometric exogenous oxidants, high cell potentials, or redox mediators.