Andrew J. D. Magenau

Electrochemically Mediated Atom Transfer Radical Polymerization

The structure of a polymer can be fine-tuned by rapidly starting and stopping its synthesis. Atom transfer radical polymerization is a versatile technique for exerting precise control over polymer molecular weights, molecular weight distributions, and complex architectures. Here, we show that an externally applied electrochemical potential can reversibly activate the copper catalyst for this process by a one-electron reduction of an initially added air-stable cupric species (CuII/Ligand). Modulation of polymerization kinetics is thereby tunable in real time by varying the magnitude of applied potential. Application of multistep intermittent potentials successfully triggers initiation of polymerization and subsequently toggles the polymerization between dormant and active states in a living manner. Catalyst concentrations down to 50 parts per million are demonstrated to maintain polymerization control manifested in linear first-order kinetics, a linear increase in polymer molecular weight with monomer conversion, and narrow polymer molecular weight distributions over a range of applied potentials.

Controlled Aqueous Atom Transfer Radical Polymerization with Electrochemical Generation of the Active Catalyst

Controlled/living radical polymerizations (C/LRPs) in aqueous media are attractive from both economic and environmental points of view. Aqueous media can be used for the synthesis of a vast array of hydrophilic and hydrophobic polymers, through homogenous and heterogeneous (e.g. suspension and (mini)emulsion) polymerization systems, respectively. Moreover, efficient C/LRPs conducted in aqueous saline buffers can be crucial for the preparation of polymer–biomolecule conjugates under biologically relevant conditions. Atom transfer radical polymerization (ATRP) is one of the most commonly employed C/LRP techniques, enabling the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and specific compositions and architectures. ATRP is often catalyzed by a Cu/Cu system in which the success of this process relies on a rapid and reversible activation/deactivation step. In this dynamic equilibrated system, activators (CuL) react with dormant macromolecular species (RX) to produce propagating radicals (RC) and deactivators (X-CuL; Scheme 1, region delimited by the dashed line). In nonaqueous solvents the equilibrium constant, KATRP= kact/kdeact, is usually small (< 10 ) resulting in a dramatic decrease of [RC] which consequently suppresses bimolecular termination reactions. In contrast to the success of ATRP in organic solvents, aqueous ATRP has been found to suffer from some limitations, specifically with regard to achieving polymerization control and the targeted degree of polymerization (DP). These observed limitations in aqueous ATRPmay result from three main phenomena. First, aqueous ATRP has a relatively large KATRP providing a high [RC] and usually fast polymerizations. Second, the halidophilicity (KX) of Cu L, that is, association of X to CuL, is small therefore diminishing the concentration of deactivator X-CuL. Third, CuL may be unstable in water and may undergo disproportionation. Developing a successful aqueous ATRP requires consideration of all the previously mentioned issues. In fact, improved polymerization control was found by using a high [X ], which helps to suppress deactivator dissociation. Further development of the process has been recently achieved through AGET (activators generated by electron transfer) ATRP. Although this process provides good results in terms of both control and DP, the correct [Cu]/[reducing agent] ratio and appropriate reducing agent are critical for success. The ideal process should have a constant and high Cu/Cu ratio, which is difficult to achieve over the whole polymerization by addition of a single reducing agent. Herein we describe an electrochemical ATRP method (eATRP), aimed to fulfill these criteria. The overall mechanism of eATRP is depicted in Scheme 1. Initially, the reaction mixture contains solvent, monomer, initiator, and CuL (or CuL +X-CuL). Under these conditions, CuL activator is absent in solution, and hence, no polymerization occurs. The onset of polymerization begins only when a sufficient potential (Eapp) is applied to the cathode so that reduction of CuL to CuL occurs at the working electrode. The magnitude of Eapp can be appropriately chosen to achieve a continuous (re)generation of a small quantity of CuL and consequently dictate the [RC]. A living polymerization process is ensured by the combination of a low [RC] and high [CuL]/[CuL] ratio. Furthermore, the polymerization rate and degree of control can be tuned by adjusting the Eapp. The first example of ATRP under electrochemical generation of activator has been recently reported for the successful polymerization of methyl acrylate in acetonitrile. Herein, we describe eATRP of oligo(ethylene glycol) methyl ether methacrylate (OEOMA475) in water. As a catalyst system, CuTPMA (TPMA= tris(2-pyridylmethyl)amine) was selected, which is one of the most active Scheme 1. Mechanism of conventional (delimited by dashed line) and aqueous electrochemical ATRP.

Preparation of cationic nanogels for nucleic acid delivery.

Cationic nanogels with site-selected functionality were designed for the delivery of nucleic acid payloads targeting numerous therapeutic applications. Functional cationic nanogels containing quaternized 2-(dimethylamino)ethyl methacrylate and a cross-linker with reducible disulfide moieties (qNG) were prepared by activators generated by electron transfer (AGET) atom transfer radical polymerization (ATRP) in an inverse miniemulsion. Polyplex formation between the qNG and nucleic acid exemplified by plasmid DNA (pDNA) and short interfering RNA (siRNA duplexes) were evaluated. The delivery of polyplexes was optimized for the delivery of pDNA and siRNA to the Drosophila Schneider 2 (S2) cell-line. The qNG/nucleic acid (i.e., siRNA and pDNA) polyplexes were found to be highly effective in their capabilities to deliver their respective payloads.

Covalently incorporated protein –nanogels using AGET ATRP in an inverse miniemulsion

Using a genetically engineered protein, containing a non-natural amino acid bearing an atom transfer radical polymerization (ATRP) initiator, protein–nanogel hybrids (PNHs) were synthesized by activator generated by electron transfer (AGET) ATRP in an inverse miniemulsion. The route presented is an appropriate synthetic strategy to covalently and site specifically incorporate green fluorescent protein (GFP) into well-defined nanogels. These PNHs were analyzed using dynamic light scattering (DLS), UV-visible fluorescence spectroscopy and confocal microscopy to confirm the successful integration of GFP proteins into each nanogel (NG), while preserving its native tertiary structure.

Facile polyisobutylene functionalization via thiol–ene click chemistry

Thiol–ene click chemistry was adapted to easily and rapidly modify exo-olefin polyisobutylene with an array of thiol compounds bearing useful functionalities, including primary halogen, primary amine, primary hydroxyl, and carboxylic acid.

Genetically targeted fluorogenic macromolecules for subcellular imaging and cellular perturbation

The alteration of cellular functions by anchoring macromolecules to specified organelles may reveal a new area of therapeutic potential and clinical treatment. In this work, a unique phenotype was evoked by influencing cellular behavior through the modification of subcellular structures with genetically targetable macromolecules. These fluorogen-functionalized polymers, prepared via controlled radical polymerization, were capable of exclusively decorating actin, cytoplasmic, or nuclear compartments of living cells expressing localized fluorgen-activating proteins. The macromolecular fluorogens were optimized by establishing critical polymer architecture-biophysical property relationships which impacted binding rates, binding affinities, and the level of internalization. Specific labeling of subcellular structures was realized at nanomolar concentrations of polymer, in the absence of membrane permeabilization or transduction domains, and fluorogen-modified polymers were found to bind to protein intact after delivery to the cytosol. Cellular motility was found to be dependent on binding of macromolecular fluorogens to actin structures causing rapid cellular ruffling without migration.