Quarternization of 3-azido-1-propyne oligomers obtained by copper(I)-catalyzed azide–alkyne cycloaddition polymerization

Summary 3-Azido-1-propyne oligomer (oligoAP) samples, prepared by copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) polymerization, were quarternized quantitatively with methyl iodide in sulfolane at 60 °C to obtain soluble oligomers. The conformation of the quarternized oligoAP in dilute DMSO-d 6 solution was examined by pulse-field-gradient spin-echo NMR based on the touched bead model.

Recently, we have investigated the CuAAC polymerization of 3-azido-1-propyne (AP) using 3-bromo-1-propyne as a monomer precursor, and obtained its oligomer, whose backbone is composed of 1,2,3-triazole and methylene moieties (Scheme 1a) [37]. The oligomer obtained was crystalline and adsorbed strongly copper ions. Since the oligomer was soluble only in strong acids and insoluble in water and organic solvents, it was not possible to characterize the oligomer in solution. The observation that the AP oligomer is well soluble in strong acids suggests that protonation of the 1,2,3-triazole moieties improves its solubility. Thus we were motivated to study quarternization of the AP oligomer because it is known that 1,2,3-triazole is quarternized with alkyl halides or others [38,39]. Recently, it has been reported that quarternized 1,2,3-triazole derivatives act Scheme 1: CuAAC polymerization of 3-azido-1-propyne (AP) (a) and quarternization of 3-azido-1-propyne oligomer (oligoAP) with methyl iodide (b). as ionic liquids [40,41], and polymeric ionic liquids have been also prepared from polymers possessing 1,2,3-triazole moieties [42][43][44]. Therefore the quarternized AP oligomer may be applied as a polymeric ionic liquid or a precursor of polymeric ionic liquids. In this study, we investigate quarternization of an AP oligomer with methyl iodide to obtain a soluble oligomer (Scheme 1b) and characterize the quarternized oligomer in dilute solutions. Table 1 lists the conditions and the results of quarternization of the oligomer of 3-azido-1-propyne (oligoAP) with a degree of polymerization n = 14. As can be seen in Table 1, oligoAP was quarternized almost quantitatively with a large excess of methyl iodide in sulfolane at 60 °C for 24 h ( Table 1, run 1). The degree of quarternization (x q ) was determined to be ca. 0.94 by 1 H NMR as described in the later subsection. The degree of quarternization x q can be controlled by altering the amount of methyl iodide used (see runs 2-4 in Table 1). Figure 1 displays a typical example of 1 H and 13 C NMR spectra measured in dimethyl sulfoxide-d 6 (DMSO-d 6 ) for the oligomer quarternized (oligoAPMe, run 1 in Table 1). As can be seen in Figure 1a, the 1 H NMR spectrum exhibits signals ascribable to the methine proton in 1,2,3-triazole and methylene protons in the polymer backbone in the region of 9.4-7.9 and 6.6-5.6 ppm, respectively. The spectrum also contains signals assignable to the methyl groups introduced in the range of 4.6-4.0 ppm. In the 13 C spectrum (Figure 1b), there are signals due to the methylene and methyl carbons at ca. 44 and 30.6 ppm, respectively.

Results and Discussion
In the aromatic region of the 13 C NMR spectrum, the signals at 141.4, 135.2, 133.5, and 124.3 ppm are ascribable to the carbons in the 1,2,3-triazole ring. This observation indicates that the methylation occurred on both the 2-and 3-positions in 1,2,3-triazole under the present conditions, although the quarternization of 1,2,3-triazole usually occurs on the 3-position [40,43,44]. From the ratio of the integrals of signals due to the methylene and methyl protons, x q was calculated as listed in Table 1. The 1 H NMR spectrum confirms that oligoAP was successfully quarternized and the oligoAP quarternized with methyl iodide was soluble in DMSO-d 6 .
The C, H, and N contents of the oligoAPMe of x q = 0.94 (C, 22 19.94). The elemental analysis data indicated that the oligoAPMe did not contain inorganic compounds although it was not possible to remove completely inorganic compounds from oligoAP even after washing repeatedly with a saturated aqueous solution of ethylenediaminetetraacetic acid (EDTA) and 5% nitric acid [37]. These observations indicate that quarternization weakens the interaction with metal ions.
While oligoAP was insoluble in all the solvents examined, including, DMSO, water, methanol, acetone, THF, and toluene [37], the oligoAPMe of x q = 0.94 was soluble in DMSO and slightly soluble in water and methanol. The solubility of oligoAPMe in DMSO increased with x q . The oligoAPMe of x q = 0.94 was insoluble in less polar solvents, acetone, THF, and toluene.
The conformation of quarternized oligoAP in DMSO-d 6 was examined by pulse-field-gradient spin-echo (PGSE) NMR (see Supporting Information File 1). An oligoAPMe sample of n = 11 and x q = 0.96 was used for the experiment. The intensities of echo signals at ca. 8.9 ppm were evaluated as a function of the intensity of the pulse-filed-gradient for dilute DMSO-d 6 solutions of the oligoAPMe of three different concentrations, as can be seen in Figure 2a. Using the Sterjkal   1.03 nm [45]. This value is larger than the experimental R H (= 0.95 nm) for oligoAPMe. If d of the rodlike touched bead model is chosen to be 0.55 nm, the theoretical R H agrees with the experimental one. However, 0.55 nm seems to be too thin as seen from Figure 3a. If the oligomer is viewed as a touched bead wormlike chain model with d = 0.7 nm and the persistence length (q) = 2 nm, the theoretical R H [45,46] is in agreement with the experimental one (cf. Figure 3c). A more precise conformational analysis of oligoAP quarternized should be studied in future.

Conclusion
Quarternization of oligoAP was investigated using methyl iodide to obtain soluble oligomers; oligoAP was quarternized quantitatively with a large excess of methyl iodide in sulfolane at 60 °C. The oligoAPMe of x q = 0.94 was soluble in DMSO, indicating that quarternization makes the oligoAP soluble. The 1 H and 13 C NMR spectra for the oligoAPMe indicated that the methylation occurred on both the 2-and 3-positions in 1,2,3-triazole under the present conditions. The hydrodynamic radius R H of oligoAPMe of n = 11 and x q = 0.96 in DMSO-d 6 dilute solutions was determined to be 0.95 nm by PGSE NMR to study the conformation of the quarternized oligomer. The conformation of oligoAPMe was further discussed based on the touched bead model. Since the oligoAPMe obtained in this study was solid and the iodide counterions were not significantly dissociated in DMSO, it is difficult to apply the oligoAPMe as a polymeric ionic liquid. However, if appropriate quarternization reagents and counterions are employed, a new type of polymeric ionic liquids may be provided.

Experimental
IR spectra were recorded on a JASCO FT/IR-8300 spectrometer equipped with a JASCO ATR PRO410-S cell. 1 [47][48][49]. The strength of pulsed gradients (g) was increased from 6.36 × 10 −1 to 43.1 gauss cm -1 . The time separation of pulsed field gradients (Δ) and their duration (δ) were 0.10 and 2.0 × 10 −3 s, respectively. The sample was not spun and the airflow was disconnected. The shape of the gradient pulse was rectangular, and its strength varied automatically during the course of the experiments.
Samples of oligoAP were prepared by CuAAC polymerization using BrP and CuSO 4 ·5H 2 O/NaAsc as a monomer precursor and a catalyst, respectively, in 70-90 % yield, as described previously [37]. The oligoAP samples were purified by washing repeatedly with water (2 × 60 mL), a saturated aqueous solution of EDTA (3 × 60 mL), and then water (2 × 60 mL). The degrees of polymerization (n) were estimated to be ca. 11 and 14 by NMR and IR [50].

Supporting Information
Supporting Information File 1 Estimation of the apparent self-diffusion coefficient for the oligomer quarternized.