Following Sharpless' visionary characterization of several idealized reactions as click reactions, the materials science and synthetic chemistry communities have pursued numerous routes toward the identification and implementation of these click reactions. Herein, we review the radical-mediated thiol-ene reaction as one such click reaction. This reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield. Further, the thiol-ene reaction is most frequently photoinitiated, particularly for photopolymerizations resulting in highly uniform polymer networks, promoting unique capabilities related to spatial and temporal control of the click reaction. The reaction mechanism and its implementation in various synthetic methodologies, biofunctionalization, surface and polymer modification, and polymerization are all reviewed.

Thiol–Ene Click Chemistry

Bioactive modification of poly(ethylene glycol) hydrogels for tissue engineering

This article summarizes the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels are attractive scaffolding materials owing to their highly swollen network structure, ability to encapsulate cells and bioactive molecules, and efficient mass transfer. Various polymers, including natural, synthetic and natural/synthetic hybrid polymers, have been used to make hydrogels via chemical or physical crosslinking. Recently, bioactive synthetic hydrogels have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mechanical properties, as well as an extracellular matrix-like microenvironment for cell growth and tissue formation. This article addresses various strategies that have been explored to design synthetic hydrogels with extracellular matrix-mimetic bioactive properties, such as cell adhesion, proteolytic degradation and growth factor-binding.

Design properties of hydrogel tissue-engineering scaffolds

Zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices

Additional functionality can be incorporated into liquid crystalline materials by using phase segregation and self-assembly. Intermolecular interactions such as hydrogen bonding and ionic interactions play key roles in the formation of these complex structures. One-, two-, and three-dimensional phase-segregated structures on various scales of length are formed by self-assembly of a variety of partially incompatible molecules. Such structures can enhance anisotropic properties such as ionic conductivity.

https://science.sciencemag.org/content/sci/295/5564/2414.full.pdf

Self-Assembly of Phase-Segregated Liquid Crystal Structures

Photopolymerizable diacrylate monomers dissolved in fluid-layer smectic A and smectic C liquid crystal (LC) hosts exhibited significant spatial segregation and orientation that depend strongly on monomer structure. Small, flexible monomers such as 1,6-hexanediol diacrylate (HDDA) oriented parallel to the smectic layers and intercalated, whereas rod-shaped mesogen-like monomers such as 1,4-di-(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylbenzene (C6M) oriented normal to the smectic layers and collected within them. Such spatial segregation caused by the smectic layering dramatically enhanced photopolymerization rates; for HDDA, termination rates were reduced, whereas for C6M, both the termination and propagation rates were increased. These polymerization precursor structures suggest novel materials-design paradigms for gel LCs and nanophase-separated polymer systems.

https://science.sciencemag.org/content/sci/275/5296/57.full.pdf

Effects of Monomer Structure on Their Organization and Polymerization in a Smectic Liquid Crystal

Development and characterization of photopolymerizable biodegradable materials from PEG–PLA–PEG block macromonomers

Despite their potential for a variety of applications, copper nanoparticles induce very strong inflammatory responses and cellular toxicity following aerosolized delivery. Coating metallic nanoparticles with polysaccharides, such as biocompatible and antimicrobial chitosan, has the potential to reduce this toxicity. In this study, copper nanoparticles were coated with chitosan using a newly developed and facile method. The presence of coating was confirmed using x-ray photoelectron spectroscopy, rhodamine tagging of chitosan followed by confocal fluorescence imaging of coated particles and observed increases in particle size and zeta potential. Further physical and chemical characteristics were evaluated using dissolution and x-ray diffraction studies. The chitosan coating was shown to significantly reduce the toxicity of copper nanoparticles after 24 and 52 h and the generation of reactive oxygen species as assayed by DHE oxidation after 24 h in vitro. Conversely, inflammatory response, measured using the number of white blood cells, total protein, and cytokines/chemokines in the bronchoalveolar fluid of mice exposed to chitosan coated versus uncoated copper nanoparticles, was shown to increase, as was the concentration of copper ions. These results suggest that coating metal nanoparticles with mucoadhesive polysaccharides (e.g. chitosan) could increase their potential for use in controlled release of copper ions to cells, but will result in a higher inflammatory response if administered via the lung.

/pdf/chitosan-coating-of-copper-nanoparticles-reduces-in-vitro-3i8o0e82qq.pdf

Chitosan coating of copper nanoparticles reduces in vitro toxicity and increases inflammation in the lung

Polymer dispersed liquid crystals (PDLCs) are a class of electrooptic materials most often formed by polymer-induced phase separation, a one-step fabrication technique often based on photopolymerization of the commercial thiol−ene mixture NOA65. To allow further understanding regarding PDLC formation, this work systematically examines processing variables that influence thiol−ene-based PDLC morphology and subsequent performance, namely polymerization kinetics, polymer gel point, and liquid crystal (LC) phase separation. PDLC formulations containing a wide range of thiol and ene monomers were examined as a function of monomer (thiol and ene) functionality, thiol−ene stoichiometry, and ene monomer composition. Simultaneous examination of polymer evolution and LC phase separation by real-time infrared (RTIR) spectroscopy shows that both polymerization kinetics and the gel point of thiol−ene PDLC formulations are influential on the extent of LC phase separation. Increasing monomer functionality (both thiol an...

Polymerization Kinetics and Monomer Functionality Effects in Thiol−Ene Polymer Dispersed Liquid Crystals

Primary objectives in templating of polymer systems include control over polymer structure and reaction behavior in pursuit of functionally advanced materials. Through such control, properties and features not accessible through traditional bulk polymerization are possible. In this review, recent developments in polymer templating are discussed that utilize benefits afforded through photopolymerization in the templating process. Photopolymerization not only is widely used and well-suited for templating applications but also is a highly effective tool with which to probe the kinetic behavior and structural evolution of templated polymers. Polymer templating encompasses a number of technologies and methods. Photopolymerization is prevalent in a number of these areas including linear chain templating or catalytic polymerization, organized particle templating, molecular imprinting, templating of assembled vesicles, and polymer templating in liquid crystals. In these systems the unique aspects of photopolymeri...

C. Allan Guymon

Papers

Effects of Monomer Structure on Their Organization and Polymerization in a Smectic Liquid Crystal

Development and characterization of photopolymerizable biodegradable materials from PEG–PLA–PEG block macromonomers

Chitosan coating of copper nanoparticles reduces in vitro toxicity and increases inflammation in the lung

Polymerization Kinetics and Monomer Functionality Effects in Thiol−Ene Polymer Dispersed Liquid Crystals

Photopolymerization in Polymer Templating