Copper Ligand and Anion Effects: Controlling the Kinetics of the Photoinitiated Copper(I) Catalyzed Azide-Alkyne Cycloaddition Polymerization

Abstract

The kinetics of photoinduced copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) polymerizations were assessed as a function of copper(II) amine-based ligands. Copper(II) bromide ligated with 1,1,4,7,10,10-hexamethylenetetramine (HMTETA) exhibited the fastest kinetics in both Norrish type(I) and type(II) photoinitiating systems. A characteristic induction period is observed with these polymerizations and is manipulated by adding an external tertiary amine in Norrish Type(II) photoinitating systems or by changing the anion of the copper(II) salt. Halides, specifically bromide and chloride, exhibit the fastest kinetics with the smallest induction period in comparison with organic anions, such as bistriflimide and triflate. The temporal control of the photo-CuAAC polymerization is affected by pre-ligation of the copper catalyst, by the presence of certain anions such as acetate, and by specific ligands such as tetramethylethylenediamine (TMEDA).

Figure 1.

Conversion profile of a 1:1 stoichiometric mixture of AZD and YNE monomers in the presence of 1.5 wt% copper(II) bromide pre-ligated to a polydentate amine, and under 470 nm light at 30 mW/cm², containing (A) 1 wt% camphorquinone, (B) 1 wt% Irgacure 819, and (C) 1 wt% camphorquinone and 1 wt% trimethylaniline. For all these samples, light was not irradiated until after 5 minutes after measurement started (indicated in the grey box) to show temporal control over the reaction. All sample thicknesses were kept constant at 0.11 mm. (D) Schematic of the Norrish Type (II) photoinitiation of CuAAC polymerizations with camphorquinone and a tertiary amine.

Figure 2.

Conversion profile between AZD and YNE using different equimolar amounts of copper(II) anion sources ligated to PMDETA in the presence of 1 wt% camphorquinone under 470 nm light at 30 mW/cm². The halides (bromide and chloride) exhibited the fastest reaction kinetics compared to triflate and bistriflimide ions. All sample thicknesses were kept constant at 0.11 mm.

Figure 3.

Conversion profile for the CuAAC polymerization using equimolar amount of AZD and YNE comparing a pre-ligated copper(II) chloride PMDETA ligand vs. a non-ligated copper(II) chloride catalyst where the PMDETA ligand was added separately. No reaction was observed in the pre-ligated copper(II) complex over 10 hours, whereas near full conversion was observed with the non-ligated copper(II) source within 2 hours. All sample thicknesses were kept constant at 0.11 mm.

Figure 4.

UV-Vis spectra over 1000 minutes at a constant pathlength of 1 cm of 100 mM 1-dodecyne and 10 mM of (A) CuBr₂/TMEDA, (B) CuBr₂/PMDETA, and (C) CuBr₂/Me₆TREN in methanol. A decrease in the peak between 600 and 800 nm was observed only for the copper(II) TMEDA ligand indicating copper(II) reduction in the presence of the alkyne. (D) Change in ¹HNMR spectra at time=0 (red) to time=18 hours (blue) for s solution of 100 mM 1-dodecyne and 10 mM copper(II) bromide TMEDA in deuterated methanol. The NMR shows a decrease in the peak at 2.15 ppm and an appearance of a peak at 2.63 ppm.

Scheme 1.

(A) Schematic of the copper(I) catalyzed azide—alkyne cycloaddition reaction. An alkyne moiety (blue) reacts with an azide functionality (red) in the presence of a copper(I) catalyst to yield a 1,4 disubstituted 1,2,3-triazole product. (B) Photo-CuAAC mechanistic cycle: Copper(II) reduces to copper(I) (1) in the presence of a photoinitiator (P.I.) and light, where the photoinitiator breaks into radicals (P.I.*) capable of reducing copper(II) to copper(I). The copper(I) species ligates to the alkyne, forming an intermediate (2). This ligation allows for the formation of a binuclear copper(I) acetylide (3). A nucleophilic attack of the azide on the acetylide carbon creates the intermediate triazolide adduct with two ligated copper species (4) and the subsequent 1,2,3-triazole with a copper moiety on the C-5 position of the triazole intermediate (5). Finally, a 1,4-substituted 1,2,3-triazole is formed (6) and copper(I) is regenerated, completing the cycle.

Scheme 2.

Polydentate amine ligands and monomers used in this study.

Scheme 3.

(A) Schematic of the photo-CuAAC polymerization. An alkyne functional group (blue) reacts with an azide functionality (red) in the presence of copper(I), forming a step-growth network with triazole linkages (purple). (B) The photoinitiation of the reaction occurs by one of two routes: Norrish Type(I) and Norrish Type(II) photoinitiation. In Type(I) initiation, the photoinitiator (shown as RCOR’) undergoes intersystem crossing to form an excited state which then dissociates into two radicals. One or more of these radicals can then reduce copper(II) to copper(I). In Type(II) initiation, the photoinitiator also undergoes intersystem crossing, but then typically abstracts a hydrogen from a co-initiator (H-R”). The resulting co-initiator radical can then reduce copper(II) to copper(I).