Weixian Xi

Spatial and Temporal Control of Thiol-Michael Addition via Photocaged Superbase in Photopatterning and Two-Stage Polymer Networks Formation.

Photochemical processes enable spatial and temporal control of reactions, which can be implemented as an accurate external control approach in both polymer synthesis and materials applications. “Click” reactions have also been employed as efficient tools in the same field. Herein, we combined photochemical processes and thiol-Michael “click” reactions to achieve a “photo-click” reaction that can be used in surface patterning and controlled polymer network formation, owing to the ease of spatial and temporal control through use of photolabile amines as appropriate catalysts.

Clickable Nucleic Acids: Sequence‐Controlled Periodic Copolymer/Oligomer Synthesis by Orthogonal Thiol‐X Reactions

Synthetic polymer approaches generally lack the ability to control the primary sequence, with sequence control referred to as the holy grail. Two click chemistry reactions were now combined to form nucleobase-containing sequence-controlled polymers in simple polymerization reactions. Two distinct approaches are used to form these click nucleic acid (CNA) polymers. These approaches employ thiol-ene and thiol-Michael reactions to form homopolymers of a single nucleobase (e.g., poly(A)n ) or homopolymers of specific repeating nucleobase sequences (e.g., poly(ATC)n). Furthermore, the incorporation of monofunctional thiol-terminated polymers into the polymerization system enables the preparation of multiblock copolymers in a single reaction vessel; the length of the diblock copolymer can be tuned by the stoichiometric ratio and/or the monomer functionality. These polymers are also used for organogel formation where complementary CNA-based polymers form reversible crosslinks.

Visible-Light-Initiated Thiol-Michael Addition Polymerizations with Coumarin-Based Photobase Generators: Another Photoclick Reaction Strategy

An efficient visible-light-sensitive photobase generator for thiol-Michael addition reactions was synthesized and evaluated. This highly reactive catalyst was designed by protecting a strong base (tetramethyl guanidine, TMG) with a visible-light-responsive group which was a coumarin derivative. The coumarin-coupled TMG was shown to exhibit extraordinary catalytic activity toward initiation of the thiol-Michael reaction, including thiol-Michael addition-based polymerization, upon visible-light irradiation, leading to a stoichiometric reaction of both thiol and vinyl functional groups. Owing to its features, this visible-light photobase generator enables homogeneous network formation in thiol-Michael polymerizations and also has the potential to be exploited in other visible-light-induced, base-catalyzed thiol-click processes such as thiol-isocynate and thiol-epoxy network-forming reactions.

Wavelength-Selective Sequential Polymer Network Formation Controlled with a Two-Color Responsive Initiation System

We report a wavelength-selective polymerization process controlled by visible/UV light, whereby a base is generated for anion-mediated thiol-Michael polymerization reaction upon exposure at one wavelength (400-500 nm), while radicals are subsequently generated for a second stage radical polymerization at a second, independent wavelength (365 nm). Dual wavelength, light controlled sequential polymerization not only provides a relatively soft intermediate polymer that facilitates optimum processing and modification under visible light exposure but also enables a highly cross-linked, rigid final material after the UV-induced second stage radical polymerization. A photobase generator, NPPOC-TMG, and a photo-radical initiator, Irgacure 2959, were selected as the appropriate initiator pair for sequential thiol-Michael polymerization and acrylate homopolymerization. FT-IR and rheological tests were utilized to monitor the dual cure photo-polymerization process, and mechanical performance of the polymer was characterized at each distinct stage by dynamic mechanical analysis (DMA). By demonstrating complete light control in another sequential polymerization system (thiol-Michael and thiol-ene hybrid polymerization), this initiator pair exhibits great potential to regulate many other coupled anion and radical hybrid polymerizations in both a sequential and controllable manner.