Open-to-Air RAFT Polymerization on a Surface under Ambient Conditions

Oxygen, light, and mechanical force mediated radical polymerization toward precision polymer synthesis

The synthesis of polymers with well-defined composition, architecture, and functionality has long been a focal area of research in the field of polymer chemistry. The advancement of controlled radical polymerization (CRP) has facilitated the synthesis of precise polymers, which are endowed with new properties and functionalities, thereby exhibiting a wide range of applications. However, radical polymerization faces several challenges, such as oxygen intolerance, and common thermal initiation methods may lead to side reactions and depolymerization. Therefore, we have developed some oxygen-tolerant systems that directly utilize oxygen for initiating and regulating polymerization. We utilize oxygen/alkylborane as an effective radical initiator system in the polymerization, and also as a reductant for the removal of polymer chain ends. Moreover, we employ the gentler photoinduced CRP to circumvent side reactions caused by high temperatures and achieve temporal and spatial control over the polymerization. To enhance the penetration of the light source for polymerization, we have developed near-infrared light-induced atom transfer radical polymerization. Additionally, we have extended photochemistry to reversible addition-fragmentation chain transfer polymerization involving ion-pair inner-sphere electron transfer mechanism, metal-free radical hydrosilylation polymerization, as well as carbene-mediated polymer modification through C-H activation and insertion mechanisms. Furthermore, we propose a new method for polymerization initiation synergistically triggered by oxygen and mechanical energy. This review not only showcases the current advancements in CRP but also outlines future directions, such as the potential for 3D printing and surface coatings, and the exploration of new heteroatom radical polymerizations. By expanding the boundaries of polymer synthesis, these innovations could lead to the creation of new materials with enhanced functionality and applications.

Xanthate Disulfide-Mediated RAFT Polymerization Toward Oxygen Tolerance

In recent years, the fully oxygen-tolerant reversible addition-fragmentation chain transfer (RAFT) polymerization has been emerging as a versatile and powerful tool for preparing well-defined polymers. In this contribution, the symmetrical diethyl xanthogen disulfide (XD) is successfully used as an initiator, chain transfer agent, and termination (iniferter) agent to directly regulate the open-to-air RAFT polymerization. The high end-group fidelity of polymer demonstrated by nuclear magnetic resonance (1H-NMR) and matrix-assisted laser desorption/ionization time-of-flight mass spectra indicates that sulfur-centered xanthate radical can initiate and regulate RAFT polymerization of methacrylate (MA) under open conditions. It is believed that sulfur-centered xanthate radical initiation strategy provides a powerful and minimalist tool for fully oxygen-tolerant RDRPs.

l-DOPA-Based Polymer Coatings via Oxidative Radical Polymerization and Their Antifouling Applications

Bioinspired coatings that mimic the adhesive properties of mussels have received considerable attention for surface modification applications. While polydopamine chemistry has been widely used to develop functional coatings, 3,4-dihydroxyphenyl-l-alanine (l-DOPA), a key component of mussel adhesive proteins, has received less attention because, compared to dopamine, it is relatively difficult to form effective coatings on solid substrates in mildly alkaline solutions. Although several methods have been explored to improve the efficiency of l-DOPA coatings, there is still a need to expand the l-DOPA-based surface chemistry. Herein, we report a simple yet efficient approach to forming poly(l-DOPA)/polymer coatings via radical polymerization with various acrylic monomers. In this one-step, dip-coating process, free radicals generated by the oxidation of l-DOPA either coupled to form poly(l-DOPA) or initiated the polymerization of the acrylic monomers, resulting in polymer coatings on a wide range of substrates. The feasibility of this coating process was confirmed by ellipsometry, contact angle goniometry, X-ray photoelectron spectroscopy, and atomic force microscopy. As a potential application, the antifouling properties of the poly(l-DOPA)/poly(oligo(ethylene glycol) methacrylate) coatings were investigated against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus), showing a significant reduction in bacterial adhesion. We think that this study broadens the scope of l-DOPA-based surface chemistry and provides a new avenue for the development of biomimetic functional coatings in materials science and surface engineering.

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.

Scalable Air-Tolerant μL-Volume Synthesis of Thick Poly(SPMA) Brushes Using SI-ARGET-ATRP

We present a facile procedure for preparing thick (up to 300 nm) poly(3-sulfopropyl methacrylate) brushes using SI-ARGET-ATRP by conducting the reaction in a fluid film between the substrate and a coverslip. This method is advantageous in a number of ways: it does not require deoxygenation of the reaction solution, and the monomer conversion is much higher than usual since only a minimal amount of solution (microliters) is used, resulting in a tremendous reduction (∼50×) of wasted reagents. Moreover, this method is particularly suitable for grafting brushes to large substrates.

Chemical antifouling strategies in sensors for food analysis: A review

Surface biofouling induced by the undesired nonspecific adsorption of foulants (e.g., coexisting proteins and cells) in food matrices is a major issue of sensors for food analysis, hindering their reliability and accuracy of sensing. This issue can be addressed by developing antifouling strategies to prevent or alleviate nonspecific binding. Chemical antifouling strategies involve the use of chemical modifiers (i.e., antifouling materials) to strongly hydrate the surface and reduce surface biofouling. Through appropriate immobilization approaches, antifouling materials can be tethered onto sensors to form antifouling surfaces with well-ordered structures, balanced surface charges, and appropriate surface density and thickness. A rational antifouling surface can reduce the matrix effect, simplify sample pretreatment, and improve analytical performance. This review summarizes recent developments in chemical antifouling strategies in sensing. Surface antifouling mechanisms and common antifouling materials are described, and factors that may influence the antifouling effects of antifouling surfaces and approaches incorporating antifouling materials onto sensing surfaces are highlighted. Moreover, the specific applications of antifouling sensors in food analysis are introduced. Finally, we provide an outlook on future developments in antifouling sensors for food analysis.

Curcumin-Based Universal Grafting of Poly(OEGMA) Brushes and Their Antibacterial Applications

Catechol and/or pyrogallol groups are recognized as crucial for the formation of polyphenol coatings on various substrates. Meanwhile, studies on polyphenolic molecules that do not contain such groups are relatively rare. The key molecule in turmeric-based universal (i.e., substrate-independent) coatings is curcumin, which contains no catechol or pyrogallol groups. As chemically reactive hydroxyl groups would remain after curcumin coating, it is hypothesized that curcumin coating can serve as a reactive layer for controlling interfacial properties. In this study, a curcumin-based surface modification method is developed to graft polymer brushes from various substrates, including titanium dioxide, gold, glass, stainless steel, and nylon. α-Bromoisobutyryl bromide, a polymerization initiator, is introduced to the curcumin-coated substrates via esterification; subsequently, poly(oligo(ethylene glycol) methacrylate) (poly(OEGMA)) is grafted from the surfaces. Compared to the control surfaces, poly(OEGMA)-grafted surfaces significantly suppress bacterial adhesion by up to 99.4%, demonstrating their antibacterial properties. Considering its facile and versatile surface modification, curcumin-based polymer grafting can be an efficient method for controlling the chemical/physical properties of surfaces in a substrate-independent manner.

Advances in design of polymer brush functionalized inorganic nanomaterials and their applications in biomedical arena

Grafting of polymer brush (assembly of polymer chains tethered to the substrate by one end) is emerging as one of the most viable approach to alter the surface of inorganic nanomaterials. Inorganic nanomaterials despite their intrinsic functional superiority, their applications remain restricted due to their incompatibility with organic or biological moieties vis-à-vis agglomeration issues. To overcome such a shortcoming, polymer brush modified surfaces of inorganic nanomaterials have lately proved to be of immense potential. For example, polymer brush-modified inorganic nanomaterials can act as efficient substrates/platforms in biomedical applications, ranging from drug-delivery to protein-array due to their integrated advantages such as amphiphilicity, stimuli responsiveness, enhanced biocompatibility, and so on. In this review, the current state of the art related to polymer brush-modified inorganic nanomaterials focusing, not only, on their synthetic strategies and applications in biomedical field but also the architectural influence of polymer brushes on the responsiveness properties of modified nanomaterials have comprehensively been discussed and its associated future perspective is also presented. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Cell-membrane-inspired polymers for constructing biointerfaces with efficient molecular recognition

Fabrication of devices that accurately recognize, detect, and separate target molecules from mixtures is a crucial aspect of biotechnology for applications in medical, pharmaceutical, and food sciences. This technology has also been recently applied in solving environmental and energy-related problems. In molecular recognition, biomolecules are typically complexed with a substrate, and specific molecules from a mixture are recognized, captured, and reacted. To increase sensitivity and efficiency, the activity of the biomolecules used for capture should be maintained, and non-specific reactions on the surface should be prevented. This review summarizes polymeric materials that are used for constructing biointerfaces. Precise molecular recognition occurring at the surface of cell membranes is fundamental to sustaining life; therefore, materials that mimic the structure and properties of this particular surface are emphasized in this article. The requirements for biointerfaces to eliminate nonspecific interactions of biomolecules are described. In particular, the major issue of protein adsorption on biointerfaces is discussed by focusing on the structure of water near the interface from a thermodynamic viewpoint; moreover, the structure of polymer molecules that control the water structure is considered. Methodologies enabling stable formation of these interfaces on material surfaces are also presented.

Oxygen Tolerance in Surface-Initiated Reversible Deactivation Radical Polymerizations: Are Polymer Brushes Turning into Technology?

Over the past three decades, the development of reversible deactivation radical polymerizations (RDRP), and advancements toward more user-friendly and accessible experimental setups have opened the door for nonexperts to design complex macromolecules with well-defined properties. External mediation, improved tolerance to oxygen, and increased reaction volumes for higher synthetic output are some of the many noteworthy technical improvements. The development of RDRPs in solution was paralleled by their application on solid substrates to synthesize surface-grafted "polymer brushes" via surface-initiated RDRP (SI-RDRP). This Viewpoint paper provides a current perspective on recent developments in SI-RDRP methods that are tolerant to oxygen, especially highlighting those that could potentially enable scaling up of the synthesis of brushes for the functionalization of technologically relevant materials.

Oxygen-demanding Photocontrolled RAFT Polymerization under Ambient Conditions

A photocontrolled reversible addition-fragmentation chain transfer (RAFT) process is developed by initiating polymerization through a 1,3-diaminopropane-triethylborane (DAPTB)-diphenyl iodonium salt (Ph₂ I⁺ ) complex (DAPTB/Ph₂ I⁺ ) under ambient temperature and atmosphere. Upon demand, this air-stable DAPTB/Ph₂ I⁺ complex is photolyzed to liberate a reactive triethylborane that consumes atmospheric oxygen and generates ethyl radicals, which initiate and mediate RAFT polymerization. Controlled RAFT polymerization is thus achieved without any prior deoxygenation using a novel RAFT chain transfer agent, BP-FSBC, which contains both benzophenone and sulfonyl fluoride moieties. Furthermore, the kinetics of polymerization reveal that the reaction process is rapid, and well-defined polymers are produced by a 61% conversion of 2-hydroxyethyl acrylate (HEA) within 7 minutes and 77% conversion of N,N-dimethylacrylamide (DMA) within 10.5 minutes. The temporal and spatial control of this photopolymerization is also demonstrated by an "on/off" switch of UV irradiation and a painting-on-a-surface approach, respectively. In addition, active chain ends are demonstrated by preparing block copolymers by chain extension and click sulfur(VI)-fluoride exchange (SuFEx) postreaction using RAFT-derived macrochain transfer agents. This article is protected by copyright. All rights reserved.

End-functionalized polymers by controlled/living radical polymerizations: synthesis and applications

This review focuses on end-functionalized polymers synthesized by controlled/living radical polymerizations and the applications in fields including bioconjugate formation, surface modification, topology construction, and self-assembly.

Plant-inspired quercetin thin films: universal coatings and their postfunctionalization for non-biofouling applications

We developed quercetin surface chemistry that can be applied for various substrates and is able to postfunctionalize for hemocompatible coatings.

Polymer brush growth by oxygen-initiated RAFT polymerization on various substrates

We developed a user-friendly method for growing polymer brushes on various substrates in the presence of air by O2-SI-RAFT polymerization and mussel-inspired surface chemistry. In particular, PHEMA brushes grew to 856 nm on the titanium substrate.