Kinetics of bulk photo-initiated copper(i)-catalyzed azide–alkyne cycloaddition (CuAAC) polymerizations

Abstract

Photoinitiation of polymerizations based on the copper(i)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction enables spatio-temporal control and the formation of mechanically robust, highly glassy photopolymers. Here, we investigated several critical factors influencing photo-CuAAC polymerization kinetics via systematic variation of reaction conditions such as the physicochemical nature of the monomers; the copper salt and photoinitiator types and concentrations; light intensity; exposure time and solvent content. Real time Fourier transform infrared spectroscopy (FTIR) was used to monitor the polymerization kinetics in situ. Six different di-functional azide monomers and four different tri-functional alkyne monomers containing either aliphatic, aromatic, ether and/or carbamate substituents were synthesized and polymerized. Replacing carbamate structures with ether moieties in the monomers enabled an increase in conversion from 65% to 90% under similar irradiation conditions. The carbamate results in stiffer monomers and higher viscosity mixtures indicating that chain mobility and diffusion are key factors that determine the CuAAC network formation kinetics. Photoinitiation rates were manipulated by altering various aspects of the photo-reduction step; ultimately, a loading above 3 mol% per functional group for both the copper catalyst and the photoinitiator showed little or no rate dependence on concentration while a loading below 3 mol% exhibited 1st order rate dependence. Furthermore, a photoinitiating system consisting of camphorquinone resulted in 60% conversion in the dark after only 1 minute of 75 mW cm−2 light exposure at 400–500 nm, highlighting a unique characteristic of the CuAAC photopolymerization enabled by the combination of the copper(i)’s catalytic lifetime and the nature of the step-growth polymerization.

Reduced shrinkage stress via photo-initiated copper(I)-catalyzed cycloaddition polymerizations of azide-alkyne resins

“…BisGMA/TEGDMA (70/30) comonomers solution was used as received from ESSTECH. Fusion silane coupling agent was used as received from George Taub Products & Fusion Co., Inc. Bis(6-azidohexyl) (1,3-phenylenebis(propane-2,2-diyl))dicarbamate (BZ-AZ) and 1-(prop-2-yn-1-yloxy)-2,2-bis((prop-2-yn-1-yloxy)methyl)butane (AK) were synthesized according to a previously reported procedure [60]. All azides were synthesized according to the azide safety rules and handled with appropriate precaution when working with monomers, resins, and polymers in small quantities [62].…”

Blue-light activated rapid polymerization for defect-free bulk Cu(i)-catalyzed azide–alkyne cycloaddition (CuAAC) crosslinked networks

“…The developed blue-light sensitive Type II photosensitizing system includes optimized concentrations of camphorquinone and a hydrogen-donating tertiary amine as an accelerator to afford polymer networks that are not only completely defect-free under all experimental conditions but also show rapid polymerization. This methodology to obtain bulk photo-CuAAC network additionally takes advantage of ‘dark polymerization’, wherein the network continues to crosslink after irradiation is ceased, owing to the persistence of catalytic Cu(I) species 32–33 . The photo-CuAAC-based polymerization maintains a step-growth molecular weight evolution producing characteristic high functional group conversions with a narrow glass transition region.…”

“…38 and in bulk polymerization by Song et al . 33 at moderately elevated temperatures (35 °C). Under blue-light during photopolymerization of vinyl monomers, the use of a co-initiator amine species in conjunction with CQ has been utilized as a kinetic accelerator 39 .…”

“…The amine was added to monomers in 1:1 CQ:amine ratio (w/w) (i.e., 0.35 wt% of 5 and 0.35 wt% of 6 of the total monomer weight in the CuAAC formulation) and polymerized under 470 nm filtered light of intensity 40 mW cm −2 to yield a bubble free network. The formulations not containing the accelerating amine exhibit only 5% conversion at equal irradiation intensity and time; it is possible that the excited CQ could abstract an H-atom from the PMDETA (Cu-ligand) 18, 43 or from the urethane group on monomer 1 33, 44 . The control sample with no CQ and no amine shows negligible conversion throughout.…”

“…One of the main attributes of photo-CuAAC reactions is their ability to continue reaction after the initiating light has ceased (i.e., dark-polymerization) 19, 32–33 . In Fig.…”