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.

Successive Mechanochemical Activation and Small Molecule Release in an Elastomeric Material

We have developed a mechanochemically responsive material capable of successively releasing small organic molecules from a cross-linked network upon repeated compressions. The use of a flex activated mechanophore that does not lead to main chain scission and an elastomeric polyurethane enabled consecutive compressions with incremental increases in the % mechanophore activation. Additionally, we examined the effect of multiple applications of compressive stress on both mechanophore activity and the mechanical behavior of the elastomeric matrix in which the mechanophore is embedded.

“Flex-Activated” Mechanophores: Using Polymer Mechanochemistry To Direct Bond Bending Activation

We describe studies in mechanochemical transduction that probe the activation of bonds orthogonal to an elongated polymer main chain. Compression of mechanophore-cross-linked materials resulted in the release of small molecules via cleavage of covalent bonds that were not integral components of the elongated polymer segments. The reactivity is proposed to arise from the distribution of force through the cross-linking units of the polymer network and subsequent bond bending motions that are consistent with the geometric changes in the overall reaction. This departure from contemporary polymer mechanochemistry, in which activation is achieved primarily by force-induced bond elongation, is a first step toward mechanophores capable of releasing side-chain functionalities without inherently compromising the overall macromolecular architecture.