A study on copolymers of styrene with a methacrylamide containing benzofuran side group Their monome

Abstract

Keywords: Benzofuran; Copolymerization; Free Radical Polymerization; Methacrylamide; Monomer Reactivity Ratio; Thermal Properties

Introduction

Polymers containing functional groups with different properties have been used in various applications as functional materials. In this regard, polymethacrylates with benzofuran ring in the side group have gained more and more interest lately. One of the most frequently used methods to produce materials with desired properties using monomers containing these functional groups is copolymerization. Functional methacrylate copolymers have many biomedical applications in industry. Drug delivery systems, dental and knee prosthesis, biosensors are some of these areas of applications. Studies have aimed to improving properties such as biocompatibility, bioactivity and long-lasting durability in their areas of use [1-4]. Benzofuran and its derivatives have been investigated for their biological activities [5-8], optical properties [9, 10], thermal properties [11] and these studies are still ongoing. For example, one study reported that 2-acetyl benzofurans can be used for anti-cancer treatment [12]. In another study, anticancer, antibacterial and antioxidant properties of some benzofuran modified compounds were investigated. As a result of the study, it was reported that these compounds showing antiproliferative effect are promising [13]. Some researchers who have studied the effect of benzofurane structures containing bromine for a long time against cancer have found that these structures exhibit quite important effects against leukemia [14]. For these reasons, the synthesis of methacrylamides containing benzofuran groups and further investigation of such polymers will lead to promising results. But when synthesizing a copolymer, it is important to have information about the reactivity ratios of the monomer of the copolymer to be synthesized [15-17]. These reactivity ratio values provide an insight into the design and composition of the product.. As a result, copolymer composition is critical for some applications [18, 19]. In this context, the knowledge of monomer reactivity ratios in methacrylate copolymers containing benzofuran structure synthesized in our study and having many properties enables the use of these copolymers in various application areas. For example, this use could be the production of disposable materials such as gloves, vials and pipettes made from styrene-based polymers containing benzofuran in their structure. In order to determine monomer reactivity ratios, which is such important information, linear and nonlinear mathematical methods as well as some experimental kinetic data are frequently preferred [20-23]. One of the important properties of polymers is their thermal behavior. The fundamental thermal data are significant in terms of the use of polymer materials. The change in mass of a polymer as a function of time and temperature is determined by thermogravimetric analysis [24]. From this analysis it is possible to obtain information about the thermal stability of a polymer, the degree of thermal degradation reaction and the activation energy of thermal degradation

[11]. These properties change drastically according to the groups found in the structure of the material [25]. The change in the thermal stability of methacrylate polymers due to the addition of side groups, especially heterocyclic groups, has attracted the interest of researchers [26]. The thermal behavior of some methacrylate copolymers containing benzofuran has also been studied and reported to exhibit high thermal stability. For example, Demirelli et al. examined methacrylate polymers with different benzofuran concentration and reported that thermal stability increased with increasing benzofuran units in the structure. In addition, they reported that the composites of these structures made with graphite oxide have semiconducting properties at all temperatures studied [27]. Styrene is one of the most important monomers for copolymers and composites, which are used today in an increasingly wide range of applications. Therefore, polystyrene is found in many commonly used products. Polystyrene is used in the medical field, especially in the production of implants and devices [28, 29]. It is also widely used in disposable materials such as gloves and vials. Polystyrene is also preferred in the production of laboratory equipment such as petri dishes, pipettes and sterilization trays [30, 31]. In addition to its low cost and thermal stability, its durability, lightness, and ease of processing make polystyrene a suitable candidate for these uses. Due to the recent increasing demand for single-use medical materials, the need for such medical polymers continues to increase significantly. In many studies in the literature, poly(styrene-block-isobutylene-block-styrene) block copolymers have been frequently used for biomedical applications due to their processability, biocompatibility and stability. [32,33]. In addition, the thermal stability, biological, optical and electrical properties of new polymers containing styrene units in their structure have been frequently studied in the literature [34-38]. The benzofuran ring is a fundamental structure found in many medically important compounds. But a vast majority of studies on benzofurans are aimed at synthesising new benzofuran derivatives with small molecular weight by various organic synthesis reactions and investigating their biological properties. When the literatures are examined, studies on the synthesis and properties of benzofuran containing polymers are quite limited [27, 39-41]. Due to the properties mentioned above, P(NBMM-co-St) copolymers can be used in various manufacturing and medical fields, especially in biomedical applications. Therefore, in our study, the monomer reactivity ratios of P(NBMM-co-St) copolymers with functional and characteristic properties to be used for the desired purpose were determined. At the same time, the thermal behaviors of these polymers, which reveal their properties such as processability and resistance to high temperatures, were investigated and their thermal degradation kinetics were studied.

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Experimental Procedures

Materials and Characterization The monomer namely, N-[2-(4-brombenzoyl)benzofuran-3-yl]-2-methacrylamide (NBBM) was synthesized through a method from the literature [42]. Methanol, ethanol, triethylamine, acetone and methacryloyl chloride (Sigma-Aldrich) were commercial products with analytical grade and were used as received. The comonomer styrene (Sigma-Aldrich) was cleaned up from inhibitor first by being washed with aq. NaOH (5%). Later, it was dried over MgSO4. Azobisisobutyronitrile (AIBN) (Merck) was purified by recrystallisation using the chloroform-methanol mixture. A Perkin Elmer Spectrum FT-IR spectrometer was used for the FT-IR measurements. A 600 MHz Avance III HD 600 NMR spectrometer with CDCl3 as the solvent was used for the NMR measurements. The molecular weights (Mn and Mw) of the polymers were determined using an Agilent 1100 series gel permeation chromatograph equipped with an RI detector and calibrated to poly(methyl methacrylate)

standards. Thermal data were obtained using a Perkin Elmer DSC-8000 instrument at a heating rate of 20 oC min-1 in an N2 atmosphere and a Perkin Elmer SII 7300 model TGA/DTA device at a heating rate of 10 oC min-1 in an N2 atmosphere. Synthesis of NBBM-ST Copolymers Conventional free radical polymerization method was applied for the synthesis of copolymers. The typical example for synthesizing copolymers here is as the following. The N-[2-(4-brombenzoyl)-benzofuran-3-yl]-2methacrylamide was placed in a polymerization tube with a commercial monomer styrene (St) at certain ratios by mole. The monomers were dissolved in the 1,4-Dioxane: tetrahydrofuran (3:2) solvent which is 3 times the total weight taken for polymerization. As initiator, AIBN in the ratio of 1% by weight of the total amount of monomers was added. The tube was flushed with nitrogen gas for 10 minutes in order to remove the air from the polymerization tube and was then allowed to polymerize in an oil bath at a temperature of 70 oC. The polymerization reaction was stopped

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after 12 hours. The mixture was precipitated dropwise in ethyl alcohol, filtered and dried under vacuum at 40 oC for 24 hours. The solid product (polymer) was then dissolved in dichloromethane, precipitated again in ethyl alcohol and dried. This purification process was repeated 3 times. Using NBBM and St monomers, a series of copolymers composed of five different compositions by mole was also synthesized by this method. Figure 1 shows the scheme for copolymer synthesis.

Results And Discussion

Characterization of Copolymers The copolymerization of the NBBM monomer with styrene has been carried out using the free radical polymerization method. The percentage compositions of NBBM and St copolymers were experimentally calculated from 1H-NMR spectra and NBBM: St mole ratios are found as; Co-1: 18/82, Co-2: 30/70, Co-3: 43/47, Co-4: 59/41, Co-5: 73/27, respectively. FT-IR and 1H-NMR measurements were used to identify the structures of the copolymers. Figure 2 shows the FT-IR spectra of the copolymers. The peaks related to amide carbonyl (NHC═O) at 1700 cm-1, ketone carbonyl (C═O) [43] at 1612 cm-1 and mono-substituted benzene derived from St monomer at 700 cm-1 have been clearly seen from the FT-IR spectra of copolymers. Additionally, the direct proportional variation of the relative intensity of the peak at 700 cm-1, depending on the molar ratio of the St monomer used in the copolymers, demonstrates clearly the effectuation of the synthesis of the copolymers at different monomer ratios. In the 1H-NMR spectra of NBBM-St copolymers (Fig. 3), the disappearance of the signals of monomeric vinylic protons at 5.68-6.17 ppm demonstrates clearly the completion of copolymerization reaction. In addition, the presence

of aromatic protons of the styrene monomer with the other aromatic protons in the structure at 6.23-8.40 ppm, aliphatic protons at 0.43-2.61 ppm and NH proton at 10.58 ppm in the spectrum supported the expected copolymer structures. Both FT-IR and 1H-NMR results were found to agree with the predicted chemical structure of the copolymers [41, 43]. Determination of the Monomer Reactivity Ratios The 1H-NMR spectra were used to calculate the percentage compositions of the NBBM-St copolymers. The NH protons at 10.58 ppm in the PNBBM units and the aromatic protons at 6.23-8.40 ppm in the NBBM-St units were taken as a basis for the calculation of composition percentages (Table 1). Copolymer compositions were calculated from the following Equations. (1) If it is to simplify; (2) Where m1 and m2 are the mole fractions of NBBM and St, respectively in the copolymer. Since the total mole fraction is always equal to 1, m2=1-m1 if m1+m2=1. The calculation method of the copolymer compositions for the Co-1 polymer sample is as follows. (3) It is calculated as % m1 = 0.18 and % m2 = 1 - m1 = 0.82 For the other samples, compositions were determined by similar calculations using the peak heights obtained

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Table 1. Monomer compositions in feed and in the copolymer Sample

Integral values of aromatic CH protons b Integral values of NH protons a

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through the 1H-NMR spectra. The copolymer conversions were calculated gravimetrically [44]. The parameters r1 and r2, known as monomer reactivity ratios, represent the reactivity ratio of NBBM and St, respectively. Both the feed composition and the reactivity ratio of the monomers affect the composition of the copolymers. Therefore, knowing these ratios is important for synthesizing copolymers with desired properties. To estimate the monomer reactivity ratios of the synthesized copolymers were used the Kelen Tüdös (K-T) [45] and Fineman-Ross (F-R) [46] Equations 4-5. The Notations in the equations were described in Table 2. (4) (5) Graphs of η versus ξ and G versus H were plotted for Kelen-Tüdös and Finemann-Ross methods, respectively (Fig. 4,5). From the straight line slope and intercept, monomer reactivity ratios of NBBM and St were determined and presented in Table 2.

When the calculated r1 and r2 values ​​are examined; the r1 value of NBBM is smaller than the r2 value of St monomer, indicating that the NBMM monomer has a lower reactivity than the St monomer and the polymer chain has a higher tendency to add St monomer. That is, there are more St monomers and fewer NBBM monomers in the copolymer chain. The reason of this can probably be interpreted as that the NBBM monomer with large-volume groups have sterically difficulty in adding its own monomer and so tends to add the other monomer, styrene. Gel Permeation Chromatography (GPC) The GPC chromatograms of NBBM-St copolymers are monodispers (Fig. 6). The average molecular weights (Mn and Mw) and polydispersities (PI) of the copolymers, given in Table 3, were determined from the GPC measurements. It is known that PI value for natural polymers is 1, and this value is close to 1 for living and controlled polymers. The copolymers we synthesized were synthetic and their PI values ​​ranged from 1.51 to 1.26 in GPC measurements. Thermal Properties The glass transition temperatures of the NBBM-St copolymers have been determined from the DSC curves

α=√Hmax.Hmin=0.817 M1; Mole fraction of NBBM in the feed M2; Mole fraction of St in the feed m1; Mole fraction of NBBM in the copolymer m2; Mole fraction of St in the copolymer

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Figure 6. GPC curves of synthesized copolymers. Table 3. Molecular weights and polydispersity index values of copolymers Sample

(Fig. 7). The curves were recorded by heating the samples to 300 oC in a nitrogen atmosphere at a heating rate of 20 o C/min. Table 4 shows the glass transition temperatures (Tg) for the NBBM-St copolymers. The Tg value of styrene homopolymer is 105 oC [47] and that of P(NBBM) is 211 o C. The Tg values of the copolymers have been found in the range of the Tg values of the homopolymers. According to these results, an increase in Tg values from 144 oC to 184 oC was observed as the molar fraction of NBBM monomer in the copolymer increased. The cyclic benzofuran structure

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resulting from the NBBM units in the polymer chain gives rigidity to the polymer chain. This restricts the mobility and flexibility of the chain, leading to an increase in the Tg value [47]. Table 4. Tg values of P(NBBM) and its copolymers Sample

The TGA curves of NBBM-St copolymers in a nitrogen atmosphere were recorded from room temperature up to 500 oC at a heating rate of 10 oC/min (Fig. 8). The mass losses of these curves at different temperatures are given in Table 5. When the TGA curves of the copolymers are examined, it was seen that the most heat-resistant polymer is polystyrene P(St), and the least heat-resistant polymer is the P(NBBM). The temperature at which 50% weight loss occurs in polymeric materials can be taken as a measure to determine the thermal stability of materials [48]. While these temperatures were determined as 423 oC for P(St) and 377 oC for P(NBBM), these values were found between their homopolymers for the copolymers studied. The thermal stability of copolymers with St units in their structure was seen to be higher than that of P(NBBM). This is due to the fact that P(St) is more

Table 5. Decomposition temperatures of the copolymers at various compositions Sample

Pnbbm

m1 Weight fraction of NBBM in the copolymer b : Initial decomposition temperature c Temperature corresponding to maximal mass loss d Temperatures at which the weight loss was respectively 10%, 30%, 50%, 70% and 90% a

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thermally stable. The initial degradation temperatures (Tint.) of P(St), P(NBBM) homopolymers were 323 oC and 279 oC, respectively, and Tint. values of P(NBBM-co-St) copolymer systems were observed in the range of these two values. The maximum degradation temperatures (Tmax ) of the polymer were obtained from derivative of mass loss (DTG) curves were given in Figure 8. Thermal decomposition of both homopolymers and copolymers were occurred in one step. Considering the data obtained from the TGA curves of the copolymers, St units in the copolymer systems studied compared to P(NBBM) caused an increase in Tmax. values. Thermal Decomposition Kinetics Thermogravimetric analysis (TGA) is based on mass loss measurement by heating the sample in an environment with inert gas flow at a constant heating rate. This method has an important role in elucidating the mechanism of physical and chemical events that occur during the degradation of polymers under the influence of heat. The rate of an isothermal decomposition reaction in the solid state can be described by the following equation: (6) In equation 6; α is the degree of conversion, T is the absolute temperature in Kelvin (K), A is the preliminary exponential factor (min-1), E is the activation energy, (kj mol-1), R is the gas constant, (8.314 Jmol-1K-1 ) and f(α) is a function that depends on the reaction mechanism. The solid state reaction mechanism in non-isothermal TG experiments was determined from equation 7, which was obtained by rearranging by integrating both sides. In

this equation, g(α) is the integral function of the transformation, αp is the degree of transformation at the peak temperature, and Tp depends on the peak temperature. The differential expression of g(a) for different solid state mechanisms is shown in Table 6 [49-51]. (7) In this study, thermal stability and activation energy measurements of decomposition have been performed at different heating rates (10, 15, 20 and 25 oC min-1) between room temperature and 500 oC under a nitrogen atmosphere. Kissinger [52], Flynn-Wall-Ozawa (FWO) [53, 54], Tang [55] methods were chosen, which are integral and isoconversional techniques that do not require any mechanism for activation energy calculation. In addition, calculations were made with the non-isoconversial method, the Van Krevelen method [56], and the type of degradation mechanism was determined. In the calculations carried out by these methods, the decomposition rate (α), temperature (T), and heating rate (β) do not change depending on the reaction model. The proposed reaction model remains the same throughout the reaction. The following are the final equations of the methods for calculating the activation energy:

Table 6. Algebraic Expressions for g(α) for the most frequently used mechanisms of solid-state processes Symbol

Phase boundary controlled reaction (One-dimensional movement)

Phase boundary controlled reaction (contraction volume) One-dimensional diffusion

Random nucleation with two nuclei on the individual particle

Random nucleation with three nuclei on the individual particle

Three-dimensional diffusion (Ginstling-Brounshtein equation) Random nucleation with one nucleus on the individual particle

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(11) Where; R gas constant, A pre-exponential factor, g(α) differential conversion function, α degradation ratio (α=(wi-wt)/(wi-wf)), E activation energy. To evaluate the thermal degradation kinetics of the studied copolymer systems, the copolymer containing 30% NBMM and 70% styrene (Co-2) was chosen as a reference copolymer. The activation energy for P(NBBM) and copolymer (Co-2) was calculated from the slope of the ln(β/ T2max) versus 1000/Tmax plot according to the Kissinger method. The activation energy of P(NBBM) and Co-2 for this method were calculated as 146.66 and 158.52 kj mol-1, respectively (Fig. 8a,b). The activation energy was calculated from the 1000/T plot against log β at different % conversions using the Flynn-Wall-Ozawa method (Fig. 10a,b) and the results (Ea values) are given in Table 7. Using this method, the average

Figure 9. Kissinger method applied to the experimental data at different heating rates (a:P(NBBM), b: P(NBBM-coSt)).

activation energies for P(NBBM) and Co-2 were calculated as 150.7 and 160.89 kj mol-1, respectively. With the Tang method, activation energies were calculated by using Equation 10 and drawing ln(β/T 1.894661) and 1/T graphs (Fig. 11a,b) for different conversion percentages, and the results are given in Table 8. With this method, the average activation energy of P(NBBM) and Co-2 were calculated as 142.33 and 151.326 kj mol-1, respectively. The activation energy values obtained by Tang are very close to those calculated by the other two methods. In many studies, the activation energy of polystyrene has been reported as approximately 200 kj mol-1 [57, 58]. Activation energy values are higher for copolymers than for homopolymers. In other words, the added styrene units increased the activation energy. There are studies in the literature confirming this result. These methods, which are in use by some authors for the control of models of the degradation mechanism, have also been used in our study [51, 59]. Using the Van Krevelen Equation (11), the activation energy for each g(α) function listed in Table 8 was obtained from a constant heating rate in the range log[g(α)/T2] - logT.

Figure 10. Flynn–Wall–Ozawa method applied to the experimental data (3–60%),(a:P(NBBM), b: P(NBBM-co-St)).

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Table 7. Activation energy (Ea) values obtained with the Flynn– Wall–Ozawa method for P(NBBM) and P(NBBM-co-St) P(NBBM)

Average

energy values obtained are compatible with those computed by the Kissinger method. From these tables it can be seen that the P(NBBM) at an optimum heating rate of 10 oC/min occurs with the R3 mechanism (Ea: 143.29 kj mol-1 (phase boundary controlled reaction (contraction volume)). These values are close to the values obtained from FWO (150.7 kj mol-1) and Kissinger (146.66 kj mol-1) which are methods that do not depend on the mechanism. In addition to the calculation made for the Co-2 for termodegaraditon mechanism of copolymer, at an optimum heating rate of 10 oC/ minute takes place with the R3 mechanism (Ea: 151.46 kj mol-1). These values are close to the values obtained from FWO (160.89 kj mol-1) and Kissinger (158.52 kj mol-1) which are methods that do not depend on the mechanism. However, the strong correlation (0.976) is the correlation that corresponds to 10 oC/min for Co-2 at the preferred site with the value obtained by the Van Krevelen method.

Conclusion

Figure 11. Tang method applied to the experimental data (3–60%),( a:P(NBBM), b: P(NBBM-co-St)). The activation energies and the correlation between 3% and 60% conversion at different heating rates (10, 15, 20 and 25 oC/min) are shown in Tables 9 and 10. The activation

Copolymer systems were successfully synthesized by free radical polymerization using N-[2-(4-brombenzoyl)benzofuran-3-yl]-2-methacrylamide and styrene monomers in five different ratios. The structures of the copolymers were characterized by FT-IR and 1H-NMR spectroscopic methods and found to be in accordance with the expected structure. Experimentally, the percentage compositions of P(N-[2-(4-brombenzoyl)-benzofuran-3-yl]-2methacrylamide-co-Styrene) copolymers were calculated through 1H-NMR spectra. The glass transition temperatures of the copolymers were determined from DSC curves. The glass transition temperature of P(NBMM) is 211 oC, which is a high temperature. It was observed that the glass transition

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Table 8. Activation energy (Ea) values obtained with the Tang method for P(NBBM) and P(NBBM-co-St) P(NBBM)

Average

Table 9. Activation energy (Ea) values obtained for P(NBBM) with the Van Krevelen method for several solid-state processes at heating rates of 10, 15, 20 and 25 oC min-1 Symbol A2

temperature increased from 144 oC to 184 oC depending on the increase in the molar ratio of N-[2-(4-brombenzoyl)benzofuran-3-yl]-2-methacrylamide in the copolymer. This increase in the glass transition temperatures was due to the decrease in the free volume caused by the rather large benzofuran ring in the N-[2-(4-brombenzoyl)-benzofuran3-yl]-2-methacrylamide monomer. When the TGA curves were examined, it was determined that the thermal stabilities of the copolymers of NBBM with St were higher than the thermal stability of P(NBBM). It is clear that the

increase in the thermal stability of the copolymers is due to the styrene units in the polymer chain. Because polystyrene was found to have the highest thermal stability among the polymers studied. While the maximum degradation temperature of Poly(N-[2-(4-brombenzoyl)-benzofuran-3-yl]2-methacrylamide) was 347 oC, the maximum degradation temperature of Polystyrene was 423 oC. A similar situation was seen in the maximum decomposition temperatures. It was determined that the maximum decomposition temperatures increased in copolymers with St units compared

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Table 10. Activation energy (Ea) values obtained for P(NBBM-co-St) with the Van Krevelen method for several solid-state processes at heating rates of 10, 15, 20 and 25 oC min-1 Symbol

to P(NBBM). The reactivity values for the P(N-[2-(4brombenzoyl)-benzofuran-3-yl]-2-methacrylamide-coStyrene) system are calculated for Kelen-Tüdõs as; r1:0.62, r2: 1.09 and for Finemann-Ross as; r1:0.61, r2: 1.07. r2 value of styrene monomer being larger than r1 value of N-[2-(4brombenzoyl)-benzofuran-3-yl]-2-methacrylamide monomer means that both monomers prefer styrene radicals. This indicates that there is a tendency towards random copolymerization where the styrene units in the copolymer are more [60, 61]. For the activation energy calculation, the well-known equivalent Kissinger, Flynn-Wall-Ozawa, Tang and Van Krevelen integral techniques were selected. In many studies, the activation energy of polystyrene has been reported to be approximately 200 kj mol-1 [57, 58]. The presence of styrene units in the copolymer structure synthesized in the study caused an increase in activation energy values compared to the homopolymer. This increase is an expected result when similar studies are examined in the literature. Calculation results for the analysis of the thermodegradation mechanism showed that the copolymer follows the slowdown (R3) type solid state thermodegradation mechanism. The R3 mechanism calculated by Van Krevelen is compatible with the results obtained by Kissinger and Flynn-Wall-Ozawa methods at heating rates of 10 and 15 oC min-1.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

Statement On The Use Of Artificial Intelligence

Artificial intelligence was not used in the preparation of the article.

Acknowledgements

We would like to thank Harran University Scientific Research Project Unit (HÜBAK) for the financial support in carrying out this study with the project number HÜBAK-13098.

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