Photo-induced copper(I) catalyzed azide-alkyne cycloaddition polymerization: fundamentals and applications
Date
2018
Authors
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Publisher
University of Delaware
Abstract
The copper(I) catalyzed azide—alkyne cycloaddition (CuAAC) reaction is one of the most utilized click chemistries and has applications that include organic synthesis, bioconjugation, surface modifications, medicinal chemistry and polymer materials. Typical of all 'click' reactions, the CuAAC chemistry is characterized by high yields and selectivity, limited by-products, and proceeds under mild ambient conditions. Recently, significant advances have been made to afford spatiotemporal control to the CuAAC reaction by introducing an in situ photoinitiator with a light source, creating new avenues in materials research including CuAAC shape memory polymers and wrinkles. Moreover, the triazoles formed from CuAAC reactions are rigid and able to undergo hydrogen bonding as well as π-π interactions, allowing for enhanced material performance and desirable mechanical properties.
One potential avenue of photo-CuAAC chemistry is dental materials as a replacement for the currently used methacrylate free radical polymerization system. Despite being widely used in dental composites, current free radical polymerization chemistries that are utilized in these composites suffer from low final methacrylate conversions (only 55-70%), leading to undesired side reactions, mechanical wear and degradation, and promotion of bacterial growth, all of which shorten the overall lifetime of dental materials. The suitability of the photo-induced CuAAC reaction in dental composites is demonstrated where the composites show similar mechanical properties to that of the methacrylate system. Moreover, the new polymerization schemes developed are completely compatible with dental lamps where the reaction can be initiated under blue (470 nm) light. These polymerization are furthermore rapid and can proceed to near full conversion in less than 10 minutes.
Despite the broad impact of the photo-CuAAC reaction, very little work had been done to understand the underlying kinetic limitations of this reaction. Through Fourier Transform Infrared Spectroscopy (FTIR) investigations of different parameters, namely copper(II) concentrations, photoinitiator concentrations, and incident intensity, an optimized kinetic profile of the reaction in both a solvent-based and network forming polymer system has been determined. The results in the solvent-based system show near first-order kinetics on copper and photoinitiator concentrations up to a threshold value in which the kinetics switch to zeroth-order. This kinetic shift shows that the photo-CuAAC reaction is not susceptible from side reactions such as copper disproportionation, copper(I) reduction, and radical termination at the early stages of the reaction. The overall reaction rate and conversion is highly dependent on the initial concentrations of photoinitiator and copper(II) as well as their relative ratios. Furthermore, the performance of the copper(II) catalytic source through synthesizing various copper(II) amine-based ligands (PMDETA, TMEDA, HMTETA, Me6TREN, BPY, DPA) with different counter anions (Cl, Br, TFSI, OTf, OAc) have been assessed. Their impact on the kinetics of a photo-CuAAC network-forming system has also been evaluated. The polymerization rate can be manipulated by changing the counter anion of the copper(II) source. Namely, samples using a copper(II) bromide ligand shows the greatest rate of polymerization. Through these results, a better fundamental understanding of the photo-CuAAC kinetic behavior is provided and, importantly, the applicability of this reaction in polymer systems is expanded.
Using the blue light photoinitiating system, the photo-CuAAC polymerization can be used as a polymer ionic liquid (PIL). Through effective ionization of a difunctional alkyne, the incorporation of charged moieties in a photo-CuAAC network was achieved. Full conversion of these monomers occurred within 30 minutes under mild irradiation conditions. Through a facile exchange of hydrophobic anions for hydroxide, hydroxide conductive polymer networks (>19 mS/cm) are yielded illuminating their potential as membranes in alkaline fuel cell applications. Photo-CuAAC characteristics of this polymerization, such as spatiotemporal control were retained where the network can be easily photopatterned. Furthermore, this reaction has shown potential in other applications such as vitrimers and in copper-metal organic polyhedra.