Kaila M. Mattson

External regulation of controlled polymerizations.

Polymer chemists, through advances in controlled polymerization techniques and reliable post-functionalization methods, now have the tools to create materials of almost infinite variety and architecture. Many relevant challenges in materials science, however, require not only functional polymers but also on-demand access to the properties and performance they provide. The power of such temporal and spatial control of polymerization can be found in nature, where the production of proteins, nucleic acids, and polysaccharides helps regulate multicomponent systems and maintain homeostasis. Here we review existing strategies for temporal control of polymerizations through external stimuli including chemical reagents, applied voltage, light, and mechanical force. Recent work illustrates the considerable potential for this emerging field and provides a coherent vision and set of criteria for pursuing future strategies for regulating controlled polymerizations.

The Fluid-Mosaic Model, Homeoviscous Adaptation, and Ionic Liquids: Dramatic Lowering of the Melting Point by Side-Chain Unsaturation†

Proposed by Singer and Nicolson in 1972, the fluid-mosaic model holds that the phospholipid bilayer is a dynamic twodimensional solvent milieu. Its proper function is closely tied to its “fluidity”, and that is often quantified by reference to the melting point, Tm (increased fluidity corresponds to a lower Tm value). The fluid-mosaic model is highly evocative of the emerging picture of nanoscale structuring in ionic liquids (ILs), and just as the function of phospholipid bilayers is tied to the Tm value, so too is the utility of ILs. Whereas the former often have low Tm values despite being composed of charged species with long aliphatic appendages, the fluidity of ILs generally decreases when progressively longer aliphatic appendages are used. It is a challenge to design imidazolium ILs (the most common IL class) that incorporate progressively more lipophilic structural elements while keeping their melting points below room temperature (Figure 1). Indeed, the Tm values of these ILs begin to rise dramatically once an appended Nalkyl group exceeds seven carbon atoms in length. Herein we report that by using an approach modeled on homeoviscous adaptation (HVA), ILs with very long alkyl appendages and very low Tm values can be prepared. This discovery may have significant implications for IL use in enzymatic catalysis, lubricants, heat-transfer fluids, and gas storage and separation, among other applications. Widely accepted as a mechanism by which the melting temperature of cell membranes is modulated, HVA is the incorporation into cell membranes of phospholipids with “kinked” tail structures. It is argued that the packing efficiency of the collective membrane hydrophobic components is diminished by the presence of these phospholipids and that increased fluidity results. A comparison of the Tm value of distearoylphosphatidylcholine with that of dioleylphosphatidylcholine provides a dramatic example of how much impact this seemingly trivial difference can have. The former, with its linear, saturated C18 tails has a Tm value of 58 8C; the latter, with its “kinked” C18 tails (each of which incorporates a cis-alkenyl group), has a Tm value of 22 8C. This effect is also at the heart of the Tm difference between the solid triacyl glycerols called fats, and those that are liquid at room temperature known as oils. In both instances, the effect is probably entropic in nature, as in the case of anthracene (“linear”, Tm = 217 8C) and phenanthrene (“kinked”, Tm = 99 8C). Accordingly, we hypothesized that ILs with long, unsaturated, aliphatic tail structures would, like the corresponding phospholipids, have significantly lower Tm values than their counterparts with saturated appendages. To test the validity of our hypothesis by measuring their Tm values, we prepared a series of lipid-inspired ILs in a threestep process from high-purity (99 + %) fatty-alcohol mesylates, 1-methylimidazole, NaI, and NaTf2N. [12] Each of the ILs (Scheme 1) had a long alkyl appendage identical to that in a natural fatty acid. Compounds 1, 3, and 8 feature fully saturated C16, C18, and C20 side chains, respectively, and their Figure 1. Influence of alkyl-chain length on Tm in N-alkyl N-methylimidazolium salts and the corresponding n-alkanes. The graph includes data from the literature and newly synthesized ionic liquids.

Chemoselective Radical Dehalogenation and C–C Bond Formation on Aryl Halide Substrates Using Organic Photoredox Catalysts

Despite the number of methods available for dehalogenation and carbon-carbon bond formation using aryl halides, strategies that provide chemoselectivity for systems bearing multiple carbon-halogen bonds are still needed. Herein, we report the ability to tune the reduction potential of metal-free phenothiazine-based photoredox catalysts and demonstrate the application of these catalysts for chemoselective carbon-halogen bond activation to achieve C-C cross-coupling reactions as well as reductive dehalogenations. This procedure works both for conjugated polyhalides as well as unconjugated substrates. We further illustrate the usefulness of this protocol by intramolecular cyclization of a pyrrole substrate, an advanced building block for a family of natural products known to exhibit biological activity.

Engineering Surfaces through Sequential Stop-Flow Photopatterning.

Solution-exchange lithography is a new modular approach to engineer surfaces via sequential photopatterning. An array of lenses reduces features on an inkjet-printed photomask and reproduces arbitrarily complex patterns onto surfaces. In situ exchange of solutions allows successive photochemical reactions without moving the substrate and affords access to hierarchically patterned substrates.

A simple and rapid route to novel tetra(4-thiaalkyl)ammonium bromides

A simple approach for the preparation of symmetrical quaternary ammonium bromides employing thiol–ene click chemistry is used to synthesize tetra(4-thiaalkyl)ammonium bromides. This approach allows the incorporation of a variety of alkyl moieties onto the nitrogen center with a one-step synthesis involving easy work-up, no side reactions and environmentally friendly reagents. To elucidate information regarding the behaviour of this novel class of compounds, comparisons to tetraalkylammonium analogues have been made. These include melting points, activity as phase-transfer catalysts, and conformational predictions from computational modelling. All results are consistent in indicating stronger bonding between the quaternary cation and the anion for the salts with 4-thiaalkyl chains as compared to those with n-alkyl chains.

Triazine-mediated controlled radical polymerization: New unimolecular initiators

Triazine-based unimolecular initiators are shown to mediate the controlled radical polymerization of several monomer classes, yielding polymers with low dispersities, targeted molecular weights, and active chain ends. We report the modular synthesis of structurally and electronically diverse triazine-based unimolecular initiators and demonstrate their ability to efficiently control the radical polymerization of modified styrene monomers. Copolymerizations of styrene with butyl acrylate or methyl methacrylate were conducted to highlight the monomer family tolerance of this system. Notably, in the case of methyl methacrylate and styrene, up to 90 mol% methyl methacrylate comonomer loadings could be achieved while maintaining a controlled polymerization, allowing the synthesis of a range of block copolymers. This class of triazine-based mediators has the potential to complement current methods of controlled radical polymerization and marks an important milestone in ongoing efforts to develop initiators and mediators with high monomer tolerance that are both metal and sulfur-free.