Kinetic Model under Light-Limited Condition for Photoinitiated Thiol-Ene Coupling Reactions

Thiol-ene click chemistry has become a powerful paradigm in synthesis, materials science, and surface modification in the past decade. In the photoinitiated thiol-ene reaction, an induction period is often observed before the major change in its kinetic curve, for which a possible mechanism is proposed in this report. Briefly, light soaking generates radicals following the zeroth-order reaction kinetics. The radical is the reactant that initializes the chain reaction of thiol-ene coupling, which is a first-order reaction. Combining both and under the light-limited conditions, a surprising kinetics represented by a Gaussian-like model evolves that is different from the exponential model used to describe the first-order reaction of the final product. The experimental data are fitted well with the new model, and the reaction kinetic constants can be pulled out from the fitting.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications

The use of light to mediate controlled radical polymerization has emerged as a powerful strategy for rational polymer synthesis and advanced materials fabrication. This review provides a comprehensive survey of photocontrolled, living radical polymerizations (photo-CRPs). From the perspective of mechanism, all known photo-CRPs are divided into either (1) intramolecular photochemical processes or (2) photoredox processes. Within these mechanistic regimes, a large number of methods are summarized and further classified into subcategories based on the specific reagents, catalysts, etc., involved. To provide a clear understanding of each subcategory, reaction mechanisms are discussed. In addition, applications of photo-CRP reported so far, which include surface fabrication, particle preparation, photoresponsive gel design, and continuous flow technology, are summarized. We hope this review will not only provide informative knowledge to researchers in this field but also stimulate new ideas and applications to further advance photocontrolled reactions.

UV-Induced Micropatterning of Complex Functional Surfaces by Photopolymerization Controlled by Alkoxyamines

We report on the use of an alkoxyamine (AA) for fabrication of functional micropatterns with complex structures by UV mask lithography. The living character of the polymer surface and the vertical spatial control of the repolymerization reaction from few tens of nanometers to few micrometers were demonstrated. The impact of the main parameters governing the controlled polymerization and the reinitiation process activated by light or heat was investigated. Micropatterning is shown to be a powerful method to investigate the physicochemical molecular phenomena. It is possible to control the polymer microstructure thickness from few tens of nanometers to few micrometers. In the last section, some applications are provided showing the potential of the AA for generating covalently bonded hydrophilic/hydrophobic micropatterns or luminescent surfaces. This demonstrates the high versatility and interest of this route.

Polymerization Behavior and Polymer Properties of Eosin-Mediated Surface Modification Reactions

Surface modification by surface-mediated polymerization necessitates control of the grafted polymer film thicknesses to achieve the desired property changes. Here, a microarray format is used to assess a range of reaction conditions and formulations rapidly in regards to the film thicknesses achieved and the polymerization behavior. Monomer formulations initiated by eosin conjugates with varying concentrations of poly(ethylene glycol) diacrylate (PEGDA), N-methyldiethanolamine (MDEA), and 1-vinyl-2-pyrrolidone (VP) were evaluated. Acrylamide with MDEA or ascorbic acid as a coinitiator was also investigated. The best formulation was found to be 40 wt% acrylamide with MDEA which yielded four to eight fold thicker films (maximum polymer thickness increased from 180 nm to 1420 nm) and generated visible films from 5-fold lower eosin surface densities (2.8 vs. 14 eosins/µm(2)) compared to a corresponding PEGDA formulation. Using a microarray format to assess multiple initiator surface densities enabled facile identification of a monomer formulation that yields the desired polymer properties and polymerization behavior across the requisite range of initiator surface densities.

Copolymerization and Dark Polymerization Studies for Photopolymerization of Novel Acrylic Monomers

The copolymerization behavior and the dark polymerization kinetics of highly reactive novel acrylic monomers were compared to traditional acrylate monomers. Copolymerization of thiol functionalities with novel acrylic monomers was characterized, and it was observed that the inclusion of secondary functionalities such as carbamates, carbonates, and cyclic carbonates, in acrylic monomers significantly alters the relative reactivity of the novel acrylates with thiols. While traditional aliphatic acrylates exhibited propagation to chain transfer ratios ranging between 0.8 (± 0.1)-1.5(± 0.2), the novel acrylates characterized by secondary functionalities exhibited much higher propagation to chain transfer ratios ranging from 2.8(± 0.2)-4(± 0.2). In the dark polymerization studies, the kinetics of the novel acrylates were evaluated following cessation of the UV light. The novel acrylates exhibited extensive polymerization in the dark compared to most traditional acrylates and diacrylates. For instance, cyclic carbonate acrylate was observed to attain 35 % additional conversion in the dark when the UV light was extinguished at 35 % conversion, whereas traditional acrylates such as hexyl acrylate attained only 3 % additional conversion when the UV light was extinguished at 35 %, and a diacrylate such as HDDA attained 15 % additional conversion when the UV light was extinguished at 40 % conversion. Also, through choice of appropriate monomers, the dark polymerization studies were performed such that the polymerization rate was approximately the same at the point the light was extinguished for all these monomers. The copolymerization and dark polymerization studies support the hypothesis that the nature of the propagating species in the novel acrylates is altered as compared to traditional acrylic monomers and polymerizations.

Controlled polymerization chemistry to graft architectures that influence cell-material interactions

Acrylate monomers were photografted from polymer substrates to create cell responsive chemically and biologically active surfaces that manipulate cell response. Three monomers, polyethylene glycol monoacrylate (MW 375 g/mol) (PEG375A), a monomeric extra-cellular matrix protein, and a cell-cleavable fluorescent monomer, were spatially photopatterned from a base substrate. The base substrate consisted of a dithiocarbamate (DTC) functionalized urethane diacrylate/tri(ethylene glycol)diacrylate copolymer and was shown to non-specifically support NIH 3T3 fibroblast cell adhesion. The DTC-containing polymer was further modified by grafting PEG375A to demonstrate selective blocking of cell-material interactions. Next, acrylated collagen type I was patterned onto polymer substrates to further promote specific cell interactions (i.e. by presenting cell-adhesive moieties). Hydrophilic PEG375A grafted patterns were shown to prevent 3T3 fibroblast adhesion to polymer in spatially grafted regions, while biologically active acrylated collagen type I promoted cell-surface interactions. Collagen type I was grafted at varying densities (0-7.5 pmol/grafted square), and the extent of cell adhesion and spreading were evaluated for each of these graft densities using fluorescence microscopy. Finally, methacrylated carboxyfluorescein diacetate (CFDA) was synthesized and photografted onto a cell-adhesive substrate as a cell sensing mechanism. The acetate groups found in the structure of CFDA cleave in the presence of cells. This cell-responsive substrate results in fluorescence indicative of acetate-group cleavage associated with cell interactions that occurs in patterned regions on polymer surfaces. Collectively, the results herein show the utility and application of a spatially and temporally controlled photografting process for designing cell responsive polymer surfaces.

A water-activated pump for portable microfluidic applications

An on-chip micropump for portable microfluidic applications was investigated using mathematical modeling and experimental testing. This micropump is activated by the addition of water, via a dropper, to ionic polymer particles that swell due to osmotic effects when wetted. The resulting particle volume increase deflects a membrane, forcing a separate fluid from an adjacent reservoir. The micropump components, along with the microfluidic components, are fabricated using the contact liquid photolithographic polymerization (CLiPP) method. The maximum flow rate achieved with this pump is 17 microL per minute per mg of dry polymer particles of 355-425 microm in diameter. The pump flow rate may be controlled by adjusting the particle size and amount, the membrane properties, and the channel dimensions. The experimental results demonstrate good agreement with an analytical model describing the particle swelling and its coupling with resistive forces from the bending membrane, viscous flow in the microchannel, and interfacial effects. Key features of this micropump are that it can be placed directly on a microdevice, and that it requires only a small amount of water and no external power supply to function. Therefore, this pumping system is useful for applications in which a highly portable device is required.

Living Radical Polymerization by the RAFT Process—A First Update

This paper provides a first update to the review of living radical polymerization achieved with thiocarbonylthio compounds (ZC(=S)SR) by a mechanism of Reversible Addition–Fragmentation chain Transfer (RAFT) published in June 2005. The time since that publication has witnessed an increased rate of publication on the topic with the appearance of well over 200 papers covering various aspects of RAFT polymerization ranging over reagent synthesis and properties, kinetics, and mechanism of polymerization, novel polymer syntheses, and diverse applications.

Mechanistic Modelling and Network Properties of Ternary Thiol - Vinyl Photopolymerizations

Ternary thiol–vinyl polymerizations offer a unique platform for improved control over polymerization kinetics and network properties as compared to both binary thiol–vinyl systems and traditional (meth)acrylic systems. Therefore, this study seeks to improve the fundamental understanding of the complex ternary thiol–vinyl systems to enable enhanced control over polymerization kinetics, network evolution, and, ultimately, network properties. The polymerization kinetics and material properties afforded by thiol–triazine–methacrylate systems are investigated. The ternary kinetics are successfully predicted by understanding the reaction mechanisms of the corresponding binary components. In ternary thiol–ene–(meth)acrylate systems, the variation in stoichiometric ratios of thiol and ene does not significantly impact material properties as in thiol–ene- or thiol–(meth)acrylate systems. Further, the ternary systems also provide unique polymer properties such as high glass transition temperature with narrow transition widths.