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 1^st 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.

Fig. 1

Monomer libraries with systematic structural variations for difunctional azides 2a–2f and multifunctional alkynes 3–8, photoinitiators, and copper catalysts used in the bulk photo-CuAAC photopolymerizations studied here.

Fig. 2

Bulk photo-CuAAC polymerization kinetics as measured by FTIR. 1 : 1 azide : alkyne mixture with varying azide structures, including 2a (closed square), 2b (open triangle), 2c (closed triangle), 2d (closed circle), 2e (open square), 2f (open circle). Each azide was polymerized stoichiometrically with alkyne 3 in the presence of 2 mol% CuCl₂[PMDETA], 4 mol% DMPA per azide functional group, and <0.5 wt% methanol (<0.5 wt% acetone for 2e). Each mixture was irradiated for 5 min (gray shaded area) at 50 °C with 10 mW cm⁻² of 365 nm light following 3 minutes in the dark as a baseline measuring period.

Fig. 3

Bulk photo-CuAAC polymerization kinetics using FTIR. 1 : 1 azide : alkyne mixture with varying alkyne structures, including 3 (open triangle), 4 (closed triangle), 5 (closed circle), 6 (open square), 7 (closed square), 8 (open circle). Each alkyne was polymerized stoichiometrically with azide 2c in the presence of 2 mol% CuCl₂[PMDETA], 4 mol% DMPA per azide functional group, and <0.5 wt% methanol. Each mixture was irradiated for 5 min (gray shaded area) at 50 °C with 10 mW cm⁻² of 365 nm light following 3 minutes in the dark as a baseline measuring period.

Fig. 4

Bulk photo-CuAAC polymerization kinetics using FTIR. 1 : 1.2 azide : alkyne mixture with varying methanol concentration ranging from 0.1 to 12.8% by weight. Azide 2c was polymerized with alkyne 3 in the presence of 2 mol% CuCl₂[PMDETA], 4 mol% DMPA per azide functional group, and methanol. Each mixture was irradiated for 5 min (gray shaded area) at (top) ambient temperature and (bottom) 50 °C with 10 mW cm⁻² of 365 nm light following 3 minutes in the dark as a baseline measuring period.

Fig. 5

Bulk photo-CuAAC polymerization kinetics using FTIR. 1 : 1 azide : alkyne mixture with varying photoinitiators, including TPO (open triangle), I819 (closed triangle), PPD (open circle), CQ (closed circle). Azide 2c was polymerized stoichiometrically with alkyne 3 in the presence of 2 mol% CuCl₂[PMDETA], 2 mol% photoinitiators per azide functional group, and <1 wt% DCM. Each mixture was irradiated continuously (gray shaded area) at 35 °C with 10 mW cm⁻² of 400–500 nm light following 3 minutes in the dark as a baseline measuring period.

Fig. 6

The average initial polymerization rate taken between 10% to 30% conversion from FTIR as a function of the copper concentration. A 1 : 1 azide : alkyne mixture with varying copper and photoinitiator concentration. Azides 2c was polymerized stoichiometrically with alkynes 3 in the presence of different molar ratio of CuCl₂[PMDETA] and CQ per azide functional group, and <1 wt% methanol. Each mixture was irradiated for continuous at 35 °C with 75 mW cm⁻² of 400–500 nm light following 3 minutes in the dark as a baseline measuring period. [PI]/[Cu] = 1 (closed square) and [PI]/[Cu] = 2 (closed circle) indicate the molar ratio of the photoinitiator to copper (PI : Cu) is fixed at 1 and 2 while [PI] = 1 (closed triangle) represents when the photoinitiator concentration is fixed at 1 mol% with varying copper concentration.

Fig. 7

Bulk photo-CuAAC polymerization kinetics using FTIR. 1 : 1 azide : alkyne mixture with varying light intensity, including 75 mW cm⁻² (closed triangle), 50 mW cm⁻² (open triangle), 20 mW cm⁻² (closed square), 10 mW cm⁻² (open square). Azide 2c was polymerized stoichiometrically with alkyne 3 in the presence of 2 mol% CuCl₂[PMDETA], 2 mol% CQ per azide functional group, and <1 wt% DCM. Each mixture was irradiated continuously (gray shaded area) at 35 °C with different light intensities of 400–500 nm light following 3 minutes in the dark as a baseline measuring period.

Fig. 8

Bulk photo-CuAAC polymerization kinetics using FTIR. 1 : 1 azide : alkyne mixture with varying exposure times, including 0 minute (closed circle), 0.5 minute (open circle), 1 minute (closed triangle), 2 minutes (open triangle), 3 minutes (closed square), 60 minutes (open square). Azide 2c was polymerized stoichiometrically with alkyne 3 in the presence of 2 mol% CuCl₂[PMDETA], 2 mol% CQ per azide functional group, and <1 wt% methanol. Each mixture was irradiated at 35 °C with 75 mW cm⁻² of 400–500 nm light following 3 minutes in the dark as a baseline measuring period. Only the start of light irradiation is highlighted in gray dotted line.

Scheme 1

Proposed reaction diagram of one approach to photo-initiated CuAAC-based polymerizations: (a) photoinitiation, copper reduction to form Cu(i), and cycloaddition between azides and alkynes. (b) Side reactions that can potentially occur during the course of the reaction: radical coupling, copper disproportionation, copper oxidation, and alkyne coupling reactions.