Facile fabrication of polymer network using click chemistry and their computational study

Click reaction is a very fast, high yield with no by-product, biocompatible, tolerant to surrounded medium, and very specific cycloaddition reaction between azides and alkynes to form triazole. They are widely being employed in the synthesis of various polymeric materials. Here, the design, fabrication and characterization of hydrogel prepared using click reaction have been reported. At first, telechelic acetylene precursor for click reaction is prepared from diisocyanatohexane and propargyl alcohol in the presence of triethylamine. The azide derivatives of poly(hydroxyethylmethacrylate), i.e. poly(HEMA), are successfully prepared following two different routes. In route 1, esterification of bromopropionic acid is performed with HEMA monomer using N,N′-dicyclohexylcarbodiimide/4-dimethylaminopyridine (DCC/DMAP) as a catalyst followed by replacing bromide by azide moiety. Free radical polymerization of the fabricated monomer is then performed under N2 atmosphere using azobisisobutyronitrile (AIBN) as an initiator. In route 2, polymerization of HEMA has been carried out first, then modification of the polymer with azide group via successive steps to obtain azide derivative polymer for click reaction. The hydrogel is prepared by a very fast, highly specific, and simple click reaction between azide derivative polymer and telechelic acetylene precursor using copper as a catalyst. The structures of derivatives of azide-functionalized HEMA, acetylene precursors and hydrogels are confirmed by FTIR and 1H-NMR spectroscopy. The optimized structure of each precursor is determined, and their chemical and thermodynamic parameters are computationally studied in detail.

alcohol (1.54 g, 27.39 mmol) and 1,6-diisocyanatohexane (2.1 g, 12.45 mmol) were added to the reaction vessel. The reaction vessel was immersed in an ice bath and stirred for 24 h. It was then quenched with 2 M HCl and cleaned by ethyl acetate. Anhydrous MgSO 4 powder was used to dry the organic part by magnetic stirring for 30 min. Then it was filtered, and the solvent was removed at reduced pressure to obtain oily residue of compound (1) with a 79% yield.
Synthesis of 2-((2-bromopropanoyl)oxy)ethyl methacrylate (2): A 50 ml round bottom flask was filled with 2-bromopropinoic acid (200 mg, 1.31 mmol), DCC (276 mg, 1.34 mmol) and dichloromethane (4 ml). The reaction mixture was cooled in an ice bath. HEMA (170 mg, 1.31 mmol), DMAP (0.02 g) and dichloromethane (4 ml) solution were added dropwise for 30 min under vigorous stirring. The reaction mixture was then permitted to swirl for 1 h at 0°C and then at room temperature for 18 h. The insoluble compound was separated by filtration, and the filtrate was extracted with dichloromethane followed by washing multiple times with saturated NaHCO 3 aqueous solution and pH 1 aqueous solution in a separatory funnel. The organic part was dried by adding anhydrous MgSO 4 powder. After filtration, the solvent was removed at reduced pressure to obtain oily residue of compound (2)   Synthesis of 2-((2-azidopropanoyl)oxy)ethyl methacrylate (3): The compound (2) (340 mg, 1.28 mmol), NaN 3 (160 mg, 2.46 mmol) and 8 ml DMF were added into a 50 ml round bottom flask. The reaction mixture was then allowed to simmer at room temperature for 12 h. After completion of the reaction, dichloromethane was added to the mixture and extracted by deionized water. The sample was dried and collected following the same procedure mentioned earlier for compound (2), and the total yield is 58%.
Synthesis of poly[2-((2-azidopropanoyl)oxy)ethyl 2-methylbutanoate] (4): DMF (7 ml) was taken into a 50 ml round bottom flask and degassed for 20 min. The synthesized compound (3) (150 mg, 0.66 mmol) and initiator AIBN (1%) were carefully added to the reaction flask. The reaction mixture was permitted to swirl under the nitrogen atmosphere at 60°C for 6 h. The crude product was removed using dichloromethane and water. The solvent was removed by rotary evaporator; a pure and sticky type compound (4) was obtained with a 53% yield.

Materials and characterizations
All chemicals and reagents in this work were analytical grade and used without further purifications. In order to confirm the structure, the Fourier transform infrared (FT-IR) spectrophotometer (FTIR-8400, Shimadzu, Japan) was used in the 4000-500 cm −1 region. At 60°C, the samples were oven-dried and mixed grinding samples with pure KBr (Sigma-Aldrich, Germany) crystals to perform FT-IR spectra of the solid and powdered samples. The powder mixture was then compressed manually to form a pellet and placed in the sample chamber for measurement. By analysing the nuclear magnetic resonance ( 1 H-NMR) spectra, the structures of the compounds synthesized in various steps were confirmed. The 400 MHz Bruker BPX-400, 1 H-NMR spectrometer was used to analyse chemical shifts (δ) in ppm and the coupling constant ( j) in Hz using CDCl 3 as a solvent and tetramethylsilane (TMS) as an internal standard. The chemical shifts of samples were recorded compared to the protons peak of TMS. Simultaneous thermogravimetry-differential scanning calorimetry (STA/TG-DSC) of STA 449 F3 Jupiter ® , NETZSCH-Gerätebau GmbH, Germany were used to perform thermal studies. Approximately 5 mg of dried and powdered sample was taken in a platinum sample pan and analysed from room temperature to 900°C under a nitrogen atmosphere at a heating rate of 10°C min −1 . The nitrogen gas was purged at a flow rate of 20 ml min −1 . For optimization of the structure, calculation of vibrational frequency and molecular orbital for molecules, the most common functional DFT was employed for quantum calculations [27]. The VAMP code of Materials Studio version 8.0 was employed for optimization and calculation based on DFT [28,29]. After optimization, the HOMO, LUMO frontier molecular orbitals and their magnitudes were analysed to obtain electrostatic potential map, electron density and thermodynamics, and so on. After analysis, the compounds were converted into a SMILES file for toxicity evaluation which was performed by amdetSAR online database system.

Results and discussion
The telechelic acetylene derivative (1) was prepared with a 79% yield by the reaction of 1,6diisocyanatohexane with propargyl alcohol in the presence of triethylamine at room temperature for 24 h in dry THF (figure 1). The characteristic absorption bands of compound (1) are observed at 2945, 1687, 1146, 2131 and 3310 cm −1 for the ν(C-H), ν(C=O), ν(C-O), ν(C≡C) and ν(N-H), respectively. The 1 H-NMR spectrum of compound (1) displays a peak at δ 4.69 ppm due to the successful incorporation of C≡CH proton. The other characteristic peaks at δ 3.18-3.23 ppm for (-CH 2 −N-) protons, peak at δ 4.83 ppm for alkoxy protons, peaks at δ 1.27-1.62 ppm for alkyl protons and peak at δ 7.28 ppm for (N-H) proton also appear.
The azide-functionalized polyHEMA (4/8) was synthesized via two different routes that differ only in the polymerization initiation time of HEMA (scheme 1). In the first route, polymerization of HEMA was performed after esterification and insertion of azide functionalization, whereas in the second route, polymerization of HEMA was carried out first prior to the azide functionalization. The 2-((2bromopropanoyl)oxy)ethyl methacrylate (2) (2)  royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 8: 202056 performed first in DMF solvent using AIBN as an initiator at 75°C for 24 h (scheme 1). The use of V-50 as an initiator at 60°C to 75°C for 6-18 h was unable to produce polyHEMA (6) (electronic supplementary material, table S2). There is no absorption band of C=C bond observed for compound (6) in FT-IR spectrum (figure 2b) that indicates the complete polymerization of the monomer. Other characteristics bands of compound (6) are also noted at 2933, 1745 and 1154 cm −1 for the ν(C-H), ν(C=O) and ν(C-O), respectively. The rest of the steps, i.e. esterification of poly-HEMA (7), azide insertion (8) and hydrogel (9) synthesis, are following the same synthetic procedure as mentioned in route 1. The successful formation of compound (7), (8) and hydrogel (9) are likewise confirmed from the FT-IR and 1 H-NMR spectra. The quantum calculation using VAMP code of Materials Studio was successfully exploited to predict the geometry, electronic structure and reactivity of the synthesized compounds [30][31][32][33][34][35]. From the frontier molecular orbitals, the HOMO, LUMO and electrostatic potential mapping are obtained to understand the possible electronic transition of the compounds and to find the electrophilic and nucleophilic interaction area in the molecule ( figure 4). The value of HOMO for all molecules is found in the range from −10.324 to −6.005 eV, while for LUMO, it is ranging from −0.053 to −3.972 eV ( which leads to their biomedical applications [36][37][38]. The ΔE values for monomers are in the range of 8.0 to 10.0 eV while it is changed for polymers (4/8). A similar trend is found for compound (6) and compound (7). The ionization potential, electron affinity, electrophilicity, chemical potential and electronegativity values also validate the specific chemical reactivity of the compounds (table 1). Figure 4 illustrates the HOMO and LUMO frontier orbitals, HOMO lies around the electronegative atom, oxygen and in alkyl chains, and LUMO belongs to the electropositive atom, particularly nitrogen atom, and also in alkyl chains, although the polymer skeleton or body is completely free from both of HOMO and LUMO parts. The hardness of these compounds is recorded in the range of compound (2) compound (4) hydrogel (5) compound (6) compound (7) compound (8) hydrogel ( (6) and ( f ) compound (7). For HOMO, the red colour denotes the positive node, and the deep blue colour denotes the negative node of an orbital. For LUMO, the green colour denotes the positive node, and the violet colour denotes the negative node of an orbital. royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 8: 202056 and compound (7) have lower binding affinity due to their high binding energy. The free energy, entropy, dipole moment, binding energy, nuclear energy, electronic energy and heat of formation are calculated after the completion of optimization (electronic supplementary material, table S3). The compound (7) has a higher electrostatic potential energy difference than the others. The three-dimensional electrostatic potential mapping of all compounds shows the high surface area and charge region (positive and negative charge) (figure 5). The hydration energy of compound (2) is the lowest Table 1. HOMO, LUMO and chemical reactivities of compound (1), (2), (3), (4), (6), (7) and (8)  royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 8: 202056 (−1.71 kcal mol −1 ), i.e. its solubility in water is high. But the compound (4/8) and compound (7) are poorly soluble in water and possess high hydration energy. The compound (6) has a negative partition function (logP) value of −1.14; therefore, it is hydrophilic, biocompatible and a suitable candidate for drug design. The rest of the compounds show a positive value of logP, which demonstrates their hydrophobic nature to be used unfavourably as a drug. The online database admetSAR was used for the estimation of toxicological data, particularly aquatic and non-aquatic toxicity for all precursors [39]. The data (electronic supplementary material, table S6) show the biodegradable and non-biodegradable natures of the compounds so that they are less likely to harm the environment if they are mixed with any element of the environment even after their use. The compounds used in the study respond negatively to these human ether-a-go-go inhibition and carcinogenicity parameters.
The swelling ratios of hydrogels (5/9) at room temperature increase significantly with increasing time, and their equilibrium swelling ratio are found as 4.0 and 5.0, respectively (figure 6a). The swelling ratio of hydrogel (9) is slightly higher than hydrogel (5), as the polymerization of HEMA in the first step makes a more organized hydrophilic polymer that accommodates a large amount of water in the network. After water interaction with hydrophilic and hydrophobic sites, the osmotic driving force allows more water to be absorbed by the network. Finally, a level of swelling equilibrium is established by the balance between the retraction force and the infinite dilution force. Both hydrogels (5/9) reach their equilibrium swollen states within 15 h.
The initial weight loss of either hydrogels (5/9) is originated from the loss of bound water from the polymer networks. The decomposition temperature is in the range of 190-480°C for hydrogel (5) and 200-480°C for hydrogel (9), with negligible ash residue (figure 6b). The decomposition of polymer network for hydrogel (5) is at 480°C, which is slightly higher than hydrogel (9). The ash residue for hydrogel (5) is found 6%, which is marginally higher than hydrogel (9) with 1.5%.

Conclusion
The poly[2-((2-azidopropanoyl)oxy)ethyl 2-methylbutanoate] (4) polymer was synthesized by free radical polymerization of 2-((2-azidopropanoyl)oxy)ethyl methacrylate (3) monomer which is obtained by esterification reaction followed by insertion of azide group into HEMA monomer. The diacetylene precursor (1) is synthesized by a reaction of 1,6-diisocyanatohexane with propargyl alcohol in the presence of triethylamine at room temperature in a dry tetrahydrofuran solvent. The click reaction of compound (4) and compound (1) is performed in the presence of copper sulfate as catalyst, sodium ascorbate as reducing agent and ethanol/water as a solvent to prepare hydrogel (5). In the second route, poly HEMA (6) was prepared using AIBN as initiator at 75°C for 24 h under nitrogen atmosphere prior to esterification. After esterification, the bromo groups are replaced by azide to produce poly[2-((2-azidopropanoyl)oxy)ethyl 2-methylbutanoate] (8). The hydrogel (9) is prepared by the click reaction of compound (8) and compound (1) by using identical conditions of hydrogel (5) preparation. The formation of hydrogels and all gel precursors are confirmed by FT-IR and NMR spectroscopy. The theoretical studies of hydrogels and all precursor compounds offer evidence for their successful synthesis, reactivities and potential biological activities. The concept of the present royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 8: 202056 work could potentially be used to fabricate well-defined hydrogel from various polymeric systems to overcome many persisting drawbacks of the conventional hydrogels, including fast stimuli sensitivity, mechanical strength and so forth. The hydrogel fabricated using click reaction can be post-treated to have desired functionalities, which may open up a new horizon in biomedical fields. The concept can also be used to join polymer segments having the same and different functionality to obtain smart polymers.