Shunsuke Chatani

The power of light in polymer science: photochemical processes to manipulate polymer formation, structure, and properties

As the demand for polymeric materials transitions towards the need for customizable, high value, specialty polymeric materials, the ability to use light to initiate various physicochemical changes in polymers represents one of the most powerful and rapidly evolving approaches. Whether for polymer formation, polymer modification, shape change, or inducing smart material responses, light has the unique capacity for enabling 4D manipulation of each of those processes. Given the simple, 3D ability to focus light on a targeted voxel and the even simpler ability to turn a light on and off to facilitate temporal control, light has been used widely in various polymer modifications. Further, in addition to the ability to enhance the control of various reactive processes, due to the much greater energy available in a photon as compared to the thermal energy available, light enables chemical processes to occur at ambient conditions that are otherwise inaccessible without heating. In particular, within the polymer chemistry field, light has been used to cause bond formation, bond degradation, and isomerization, with subsequent reactions including polymerization, polymer degradation, polymer functionalization, and responsive changes in properties of smart materials. Here, this article attempts to provide a fundamental basis for the various photochemical processes implemented in polymer systems, followed by selected examples of that implementation in various polymerization, functionalization, degradation, and other reactions.

Relative reactivity and selectivity of vinyl sulfones and acrylates towards the thiol–Michael addition reaction and polymerization

The reactivity, selectivity and kinetics of vinyl sulfones and acrylates in base and nucleophile-catalyzed thiol–Michael addition reactions were examined in detail in this study. The vinyl sulfones react selectively and more rapidly with thiols in the presence of acrylates, which was clearly indicated from reactions of hexanethiol (HT), ethyl vinyl sulfone (EVS) and hexyl acrylate (HA) at a molar ratio of 2 : 1 : 1. EVS reaches 100% conversion with minimal consumption (<10%) of HA, which demonstrates the high selectivity of vinyl sulfones over acrylates. The reaction rate of EVS with HT was approximately 7 times higher than that of HA. A detailed study of the kinetics of the nucleophile-catalyzed thiol–Michael addition reaction was carried out, and it was shown that the delay observed in the initial stages of the nucleophile-catalyzed thiol–Michael addition reaction is due to the relatively slow attack of the nucleophiles on the vinyl. The presence of protic species other than thiols in the reaction mixture has also been shown to significantly impede the reaction rate, and in extreme cases, has been shown to inhibit the Michael addition reaction. These results provided a better understanding of the conditions under which the thiol–Michael addition reaction can or cannot be considered as a click reaction. Finally, the high reaction selectivity of vinyl sulfones over acrylates via thiol–Michael addition reaction in ternary systems is used to control gelation behavior in crosslinked polymer networks formed by thiol–Michael addition reactions.

Ester-free Thiol-X Resins: New Materials with Enhanced Mechanical Behavior and Solvent Resistance

A series of thiol-Michael and radical thiol-ene network polymers were successfully prepared from ester-free as well as ester-containing monomer formulations. Polymerization reaction rates, dynamic mechanical analysis, and solvent resistance experiments were performed and compared between compositions with varied ester loading. The incorporation of ester-free alkyl thiol, vinyl sulfone and allylic monomers significantly improved the mechanical properties when compared with commercial, mercaptopropionate-based thiol-ene or thiol-Michael networks. For polymers with no hydrolytically degradable esters, glass transition temperatures (Tgs) as high as 100 °C were achieved. Importantly, solvent resistance tests demonstrated enhanced stability of ester-free formulations over PETMP-based polymers, especially in concentrated basic solutions. Kinetic analysis showed that glassy step-growth polymers are readily formed at ambient conditions with conversions reaching 80% and higher.

Thiol-Michael addition miniemulsion polymerizations: functional nanoparticles and reactive latex films

Thiol-Michael addition polymerization is successfully implemented in a miniemulsion polymerization system. By off-stoichiometric polymerization between thiols and acrylates, inherently functionalized particles are facilely prepared in a single step. We demonstrate that the latex films from such particles are readily available for further modification and second-stage photo-curing.

Development of glassy step-growth thiol-vinyl sulfone polymer networks.

Thermomechanical properties of neat phosphine-catalyzed thiol-Michael networks fabricated in a controlled manner are reported, and a comparison between thiol-acrylate and thiol-vinyl sulfone step-growth networks is performed. When highly reactive vinyl sulfone monomers are used as Michael acceptors, glassy polymer networks are obtained with glass transition temperatures ranging from 30 to 80 °C. Also, the effect of side-chain functionality on the mechanical properties of thiol-vinyl sulfone networks is investigated. It is found that the inclusion of thiourethane functionalities, aryl structures, and most importantly the elimination of interchain ester linkages in the networks significantly elevate the networks glass transition temperature as compared with neat ester-based thiol-Michael networks.

Thermoreversible Folding as a Route to the Unique Shape-Memory Character in Ductile Polymer Networks

Ductile, cross-linked films were folded as a means to program temporary shapes without the need for complex heating cycles or specialized equipment. Certain cross-linked polymer networks, formed here with the thiol-isocyanate reaction, possessed the ability to be pseudoplastically deformed below the glass transition, and the original shape was recovered during heating through the glass transition. To circumvent the large forces required to plastically deform a glassy polymer network, we have utilized folding, which localizes the deformation in small creases, and achieved large dimensional changes with simple programming procedures. In addition to dimension changes, three-dimensional objects such as swans and airplanes were developed to demonstrate applying origami principles to shape memory. We explored the fundamental mechanical properties that are required to fold polymer sheets and observed that a yield point that does not correspond to catastrophic failure is required. Unfolding occurred during heating through the glass transition, indicating the vitrification of the network that maintained the temporary, folded shape. Folding was demonstrated as a powerful tool to simply and effectively program ductile shape-memory polymers without the need for thermal cycling.