Evaluation of 2-Bromoisobutyryl Catechol Derivatives for Atom Transfer Radical Polymerization-Functionalized Polydopamine Coatings.

The use of α-bromoisobutyryl-functionalized polydopamine (PDA), derived from an in situ mixture with dopamine (DA) and α-bromoisobutyryl bromide, enables surface-initiated atom transfer radical polymerization (SI-ATRP) of a broad range of methacrylate monomers for surface functionalization. Although the putative intermediate 2-bromo-N-(3,4-dihydroxyphenethyl)-2-methylpropanamide 1 has been proposed to account for the SI-ATRP activity of α-bromoisobutyryl-functionalized PDA, there has not been a systematic investigation on the efficacy of other catechol-derived 2-bromoisobutyryl derivatives for SI-ATRP. In this work, a number of catechol-derived ATRP initiators containing the 2-bromoisobutyryl moiety were designed and synthesized, in an effort to investigate the effect of changes in structure on initiator immobilization, and subsequent ATRP performance. The change in the length of the linker unit bearing the 2-bromoisobutyryl moiety, the introduction of a free amine group, or the replacement of the amide with an ester were found to have profound effects on the ability of the molecule to deposit ATRP-initiator-modified PDA coatings, as well as the subsequent SI-ATRP performance. Among the ATRP initiators synthesized, 5-(2-aminoethyl)-2,3-dihydroxyphenethyl 2-bromo-2-methylpropanoate hydrobromide 4·HBr was most efficiently incorporated into ATRP-initiator-modified PDA coatings and also the best at effecting SI-ATRP with 2-hydroxyethyl methacrylate; the high performance of this initiator is likely due to the presence of a free amine and an appropriately long methylene linker unit to the 2-bromoisobutyryl moiety. This methodology was found to be suitable for the functionalization of a range of organic and inorganic surfaces, for the fabrication of high-value surface-grafted polymer brush coatings for various applications.

Direct Evidence for the Critical Role of 5,6-Dihydroxyindole in Polydopamine Deposition and Aggregation.

The definitive role of the intermediate 5,6-dihydroxyindole (DHI) in the formation of polydopamine (PDA) coatings from aqueous dopamine (DA) has not been clearly elucidated and remains highly controversial. Our foray into this debate as reported in this study agrees with some reported assertions that DHI-based coatings are not synonymous with PDA coatings. Our conclusion arises from a systematic comparison of the components and properties of DHI-based coatings and PDA coatings. In addition, through careful copolymerization studies of DA and DHI, our studies reported herein unequivocally suggest that both DA and DHI are partial building blocks for PDA formation. Our results also provide additional evidence of the critical role of DHI in controlling the thickness of PDA coatings, through competitive events between PDA aggregation in solutions and deposition onto substrates. These findings highlight the complex interplay between both DHI and uncyclized DA moieties in the formation of adhesive catechol/amine materials.

The Chemistry of Bioinspired Catechol(amine)-Based Coatings.

Surface coatings are widely used for the protection of underlying materials from erosion or contamination by the external environment, with biomimetic organic coatings based on catecholamine chemistry gaining prominence in recent years. Such coatings have found use in the biomedical field, e.g., in diagnostics, implant manufacturing, and biosensing, with coatings based on polydopamine (PDA) being the most popular. This Review aims to summarize the chemistry of catechol(amine) coatings, in particular the adhesion and cohesion properties of catecholamine-based coatings. This will allow for the design and synthesis of new polymers and coating materials in a more rational manner, enabling the selection of parameters and conditions to precisely control the structure of the materials formed. Particular attention is paid to the formation mechanism, structure, and variables affecting the properties of PDA, which is the most widely reported catechol(amine) coating. The use of other catechol(amine) precursors to synthesize biomimetic coatings is also discussed. A summary of the different methods reported in the literature to effect specific chemical properties on catechol(amine) coatings will allow the reader to best choose the technique to tailor coating properties for specific applications.

Functionalization of Alkenes through Telescoped Continuous Flow Aziridination Processes.

Alkenes can be efficiently aziridinated using highly soluble iminoiodane derivatives under continuous flow conditions. By combining the aziridine generation with nucleophilic ring opening reactions, a variety of products can be made without the need to handle or isolate these potentially hazardous intermediates. Additionally, this chemistry can be used to make and use aziridines that are difficult to isolate and purify because of their high reactivity.

Generation and Ring Opening of Aziridines in Telescoped Continuous Flow Processes.

A simple method for the preparation of a variety of N-sulfonyl aziridines (10 examples) from 1,2-amino alcohols under continuous flow conditions is described. Using flow based methods, the aziridines can be further ring opened with oxygen, carbon, and halide nucleophiles or ring expanded to imidazolines by Lewis acid promoted reaction with nitriles. Telescoping the aziridine generation and ring opening steps together in a microfluidic reactor allows the chemistry to be undertaken with limited exposure to the potentially hazardous aziridine intermediates.