2024 Volume 7 Issues 1–3

Open Access

INEOS OPEN, 2024, 7 (1–3), 7–8  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF Electronic supplementary information DOI: 10.32931/io2404a

Preparation of Polydimethylsiloxane Copolymers by the Azide–Alkyne
Huisgen Cycloaddition Method

Abstract

The copolymers containing alternating siloxane and urethane units in the chain as well as ester moieties were synthesized by the azide–alkyne cycloaddition without application of a solvent and copper salts as catalysts. The resulting copolymers were analyzed by NMR spectroscopy and gel permeation chromatography.

Key words: polydimethylsiloxanes, urethanes, copolymers, click chemistry, cycloaddition.

 

Introduction

Polydimethylsiloxanes (PDMSs) are extensively used in research and industry owing to their unique properties, such as chemical inertness, high elasticity, biocompatibility, wide range of operating temperatures, hydrophobicity, resistance to ultraviolet radiation and others. However, their main drawback is the low mechanical strength, which limits their application scope [1–3].

The azide–alkyne cycloaddition reaction catalyzed by monovalent copper salts (CuAAC) is a versatile and simple approach that ensures efficient interaction between molecules, including polymeric systems, under different conditions and allows achieving a stereoregular structure of the product under mild conditions [4, 5]. It allows for a modular assembly of polymers and easy alternation of the units. However, the presence of catalyst impurities in the resulting compound may be undesirable (e. g., for in vivo application).

The use of the 1,3-dipolar azide–alkyne Huisgen cycloaddition implies the reaction proceeding without the participation of copper salts and allows obtaining a product that does not require the purification from the catalyst. However, this approach has disadvantages such as non-stereoselectivity of the resulting compounds (a mixture of 1,4- and 1,5-triazoles) and more severe reaction conditions (the temperature above 100 °C). This method is often used to modify polymers and to cross-link silicones [6]. Therefore, the goal of this study was to obtain PDMS copolymers with urethane/ester units in the main polymer chain by the AAC method without copper catalysts.

Results and discussion

A series of copper-free azide–alkyne cycloaddition reactions were carried out using blocked alkyne difunctional monomers and telechelic PDMS (Scheme 1). The reactions were carried out upon stirring in an oil bath at 140 °C. The reaction course was monitored using ¹H NMR spectroscopy by the disappearance of -CH₂-N₃ signals of the propylene unit at 3.24 ppm.

Scheme 1. General scheme of the azide–alkyne cycloaddition.

The resulting copolymers were analyzed by ¹H NMR spectroscopy (Fig. 1). The spectra revealed the absence of signals at 2.43–2.46 and 3.24, which correspond to the signals of the acetylene and CH₂ units at the azide group, and the appearance of signals at 7.57–7.74, which correspond to the signals of a mixture of 1,4- and 1,5-triazoles.

Figure 1. Comparison of the ¹H NMR spectra of the resulting copolymers.

The ratio of the integral intensities of the proton signals corresponded to the expected one.

The copolymers were also studied by gel permeation chromatography (GPC). The results obtained are presented in Table 1. The number-average molecular weight varied from ~10000 to 14000 amu. The polydispersity index ranged from 1.9 to 2.8.

Table 1. Results of the GPC analysis of the copolymers obtained

Copolymer	Mn	Mw	Mw/Mn
1	10900	20900	1.94
2	14300	30900	2.16
3	12200	34900	2.85

Experimental section

General remarks

Propargyl alcohol (99%), adipic acid (ABCR), benzene (reagent grade, Khimmed), and p-toluenesulfonic acid (Ruskhim) were purchased from commercial sources. Bis(3-azidopropyl)polydimethylsiloxane with a molecular weight of 3000 amu [7], di(prop-2-yn-1-yl) hexane-1,6-diyldicarbamate [8], and di(prop-2-yn-1-yl) [methylenebis(4,1-phenylene)]dicarbamate [9] were synthesized according to the published procedures.

Analysis and general methodology

The GPC analysis was performed on a Shimadzu LC-10A series chromatograph (Japan) equipped with an RID-10A refractometer and SPD-M10A diode matrix detectors. The analytical separation was performed using a 7.8 mm × 300 mm Phenomenex column (USA) filled with Phenogel sorbent with pore sizes of 15–500 Å. THF was used as an eluent. Polystyrene was used as a standard. The temperature was 40 ± 0.1 °С. The flow rate was 1mL/s.

The ¹H NMR spectra were recorded at room temperature on a Bruker Avance 300 spectrometer (Mannheim, Germany) in CDCl₃. The spectra were processed using the MestReNova software (v. 12.0). The residual solvent signal (7.26 ppm) was used to reference the ¹H NMR spectra.

Syntheses

The detailed procedures for the synthesis of the copolymers are presented in the ESI.

Conclusions

The copper-free AAC reactions afforded the triazole-containing copolymers with alternating fragments based on oligomeric polydimethylsiloxane and urethanes or esters in the main polymer chain. The resulting copolymers were studied by GPC and NMR spectroscopy. The occurrence of the click reaction and the formation of polydimethylsiloxane–triazole copolymers with the average molecular weights ranging from 10000 to 14000 amu were confirmed.

Acknowledgements

The synthesis of the copolymers and PDMS was performed with financial support from the Russian Science Foundation, project no. 23-43-00057. The synthesis of the monomers was carried out with the aid of the Government of the Tula Region (Decree No. 899 of 30 December 2021) under agreement No. 11 of 7 September 2022 of the TSPU, within the program world-class scientific and educational center "Tulatech" of 10 August 2022.

The molecular-weight distribution and NMR spectroscopic studies were performed with financial support from the Ministry of Science and Higher Education of the Russian Federation (FFSM-2024-0001) using the equipment of the Collaborative Access Center "Center for Polymer Research" of ISPM RAS and within the framework of the program of state support for the centers of the National Technology Initiative (NTI) on the basis of educational institutions of higher education and scientific organizations (Сenter NTI "Digital Materials Science: New Materials and Substances" on the basis of the Bauman Moscow State Technical University).

Electronic supplementary information

Electronic supplementary (ESI) information available online: the ¹H NMR spectra, GPC curves, and synthetic procedures for di(prop-2-yn-1-yl) adipate. For ESI, see DOI: 10.32931/io2404a.

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