Thiol−Ene Free-Radical and Vinyl Ether Cationic Hybrid Photopolymerization

Significance of Polymers with “Allyl” Functionality in Biomedicine: An Emerging Class of Functional Polymers

In recent years, polymer-based advanced drug delivery and tissue engineering have grown and expanded steadily. At present, most of the polymeric research has focused on improving existing polymers or developing new biomaterials with tunable properties. Polymers with free functional groups offer the diverse characteristics needed for optimal tissue regeneration and controlled drug delivery. Allyl-terminated polymers, characterized by the presence of a double bond, are a unique class of polymers. These polymers allow the insertion of a broad diversity of architectures and functionalities via different chemical reactions. In this review article, we shed light on various synthesis methodologies utilized for generating allyl-terminated polymers, macromonomers, and polymer precursors, as well as their post-synthesis modifications. In addition, the biomedical applications of these polymers reported in the literature, such as targeted and controlled drug delivery, improvement i aqueous solubility and stability of drugs, tissue engineering, and antimicrobial coatings, are summarized.

Expanding the Thiol-X Toolbox: Photoinitiation and Materials Application of the Acid-Catalyzed Thiol-ene (ACT) Reaction

The acid-catalyzed thiol-ene reaction (ACT) is a unique thiol-X conjugation strategy that produces S,X-acetal conjugates. Unlike the well-known radical-mediated thiol-ene and anion-mediated thiol-Michael reactions that produce static thioether bonds, acetals provide unique function for various fields such as drug delivery and protecting group chemistries; however, this reaction is relatively underutilized for creating new and unique materials owing to the unexplored reactivity over a broad set of substrates and potential side reactions. Solution-phase studies using a range of thiol and alkene substrates were conducted to evaluate the ACT reaction as a conjugation strategy. Substrates that efficiently undergo cationic polymerizations, such as those containing vinyl functional groups, were found to be highly reactive to thiols in the presence of catalytic amounts of acid. Additionally, sequential initiation of three separate thiol-X reactions (thiol-Michael, ACT, and thiol-ene) was achieved in a one-pot scheme simply by the addition of the appropriate catalyst demonstrating substrate selectivity. Furthermore, photoinitiation of the ACT reaction was achieved for the first time under 470 nm blue light using a novel photochromic photoacid. Finally, using multifunctional monomers, solid-state polymer networks were formed using the ACT reaction producing acetal crosslinks. The presence of S,X-acetal bonds results in an increased glass transition temperature of 20 °C as compared with the same polymeric film polymerized through the radical thiol-ene mechanism. This investigation demonstrates the broad impact of the ACT reaction and expands upon the diverse thiol-X library of conjugation strategies towards the development of novel materials systems.

Thiol-Ene Cationic and Radical Reactions: Cyclization, Step-Growth, and Concurrent Polymerizations for Thioacetal and Thioether Units

Thiol-ene cationic and radical reactions were conducted for 1:1 addition between a thiol and vinyl ether, and also for cyclization and step-growth polymerization between a dithiol and divinyl ether. p-Toluenesulfonic acid (PTSA) induced a cationic thiol-ene reaction to generate a thioacetal in high yield, whereas 2,2'-azobisisobutyronitrile resulted in a radical thiol-ene reaction to give a thioether, also in high yield. The cationic and radical addition reactions between a dithiol and divinyl ether with oxyethylene units yielded amorphous poly(thioacetal)s and crystalline poly(thioether)s, respectively. Under high-dilution conditions, the cationic and radical reactions resulted in 16- and 18-membered cyclic thioacetal and thioether products, respectively. Furthermore, concurrent cationic and radical step-growth polymerizations were realized using PTSA under UV irradiation to produce polymers having both thioacetal and thioether linkages in the main chain.

Precisely Tuning Helical Twisting Power via Photoisomerization Kinetics of Dopants in Chiral Nematic Liquid Crystals

It has been paid much attention to improve the helical twisting power (β) of dopants in chiral nematic liquid crystals (CLCs); however, the correlations between the β value and the molecular structures as well as the interaction with nematic LCs are far from clear. In this work, a series of reversibly photo-switchable axially chiral dopants with different lengths of alkyl or alkoxyl substituent groups have been successfully synthesized through nucleophilic substitution and the thiol-ene click reaction. Then, the effect of miscibility between these dopants and nematic LCs on the β values, as well as the time-dependent decay/growth of the β values upon irradiations, has been investigated. The theoretical Teas solubility parameter shows that the miscibility between dopants and nematic LCs decreases with increasing of the length of substituent groups from dopant 1 to dopant 4. The β value of chiral dopants in nematic LCs decreases from dopant 1 to dopant 4 both at the visible light photostationary state (PSS) and at the UV PSS after UV irradiation. With increasing of the length of substituent groups, the photoisomerization rate constant of dopants increases for trans-cis transformation upon UV irradiation and decreases for the reverse process upon visible light irradiation either in isotropic ethyl acetate or in anisotropic LCs, although the constant in ethyl acetate is several times larger than the corresponding value in LCs. Also, the color of the CLCs could be tuned upon light irradiations. These results enable the precise tuning of the pitch and selective reflection wavelength/color of CLCs, which paves the way to the applications in electro-optic devices, information storage, high-tech anticounterfeit, and so forth.

Toughening of photo-curable polymer networks: a review

This review surveys relevant scientific papers and patents on the development of crosslinked epoxies and also photo-curable polymers based on multifunctional acrylates with improved toughness.

Structural engineering of waterborne polyurethane for high performance waterproof coatings

Novel waterborne polyurethane containing fluorine and siloxane (FSPU) for excellent thermal performance, waterproof and mechanical properties.

Thiol-Based 'Click' Chemistries in Polymer: Synthesis and Modification

A series of thiol-based ‘click’ reactions is discussed with an emphasis on highlighting the individual chemistries and noting recent literature examples. This is not an exhaustive review but rather serves to demonstrate the versatility and clear potential of thiol-based chemistry in polymer/materials design, synthesis, and modification.