Artur Mantel¹, Nikolay Barashkov², Irina Irgibayeva¹, Anton Kiriy³ and Volodymyr Senkovskyy³

¹L.N. Gumilyov Eurasian National University, Astana, Kazakhstan

²Micro Tracers, Inc., San Francisco, California, USA

³Leibniz Institute of Polymer Research, Dresden, Germany

Free-radical quaternary copolymerisation of styrene, 4-vinylbenzoic acid, 9-vinylanthracene and 2-vinylnaphthalene: composition of prepared copolymers

Introduction

This study is devoted to the synthesis and investigation of styrene and its carboxy-derivative copolymers containing naphthalene and anthracene chromophore fragments into polymer chain. Copolymers of styrene, including chromophore-containing copolymers has been previously used as materials for efficient plastic scintillators^1-4. Analysis of previously reported data on copolymerization of styrene (Sty) with comonomers, such as 9-vinylanthracene (9VA) and 2-vinylnaphthalene (2VN) shows that 2VN and Sty possess close constants of copolymerization^5,6 and therefore all 2VN participates in reaction. In contrary, 9-VA has been found to be less reactive than Sty and its significant part (at least 50% according to data⁷) does not participate in reaction of copolymerization. There are some experimental evidences that besides reaction of copolymerization, 9VA is capable of interacting with styrene in the following ways ^3,8: a) addition of styryl or polystyryl radicals to the meso-position of the anthracene ring; b) addition of one of several molecules of styrene to the vinyl group of 9VA to form low molecular weight products; substitution of hydrogen atoms in the meso-position of the anthracene ring by styryl or polystyryl radicals.

There are also numerous papers describing the synthesis and application of carboxyl-containing styrene-based copolymers. Carboxyl group is often used for the subsequent modification⁹, which explains our intention to introduce the carboxy group into the chromophore-containing polymer. Carboxylic group is introduced into copolymer by the means of copolymerization of styrene with 4-vinylbenzoic acid¹⁰ (VBA). Taking into account the available literature data, we made an attempt to synthesize the quaternary copolymer, containing the predicted ratio of the four monomers in the chain.

Experimental

Materials. All reagents were purchased from the Aldrich Chemical Co. 2-vinylnaphthalene(2VN), 9-vinylnaphthalene (9VA) and styrene (Sty) were purified from stabilizator, products of oligomerization by silica column with hexane as eluent; 4-vinylbenzoic acid (VBA) was recristallized from water: ethanol (3:7 v/v); 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidine was prepared from TEMPO and (1-bromoethyl)benzene by the method presented below.

Instrumentation. Gel permeation chromatography (GPC) was used to determine the molecular weights and molecular weight distributions, Mw/Mn, of polymer samples with respect to polystyrene standards. The system configuration: THF with flow rate 1.0 ml/min. HPLC-Pump, Ser. 1200, Agilent Technology; ETA-2020 – RI – and viscosity detector from Fa.Bures; MALLS detector from Wyatt.

1H NMR spectra were collected on the device «Bruker Bio Spin» (1H 500 MHz, 20 ⁰Ñ, solvent -CDCl₃, TMS - an internal reference).

UV/vis absorption spectra were measured on Perkin Elmer Lambda 800 spectrophotometer.

Synthesis of 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperi-dine. To the round-bottomed flask containing 0.5008 g of copper bromide, which is closed with a septum and filled with argon , the mixture of 0.73 ml N,N,N',N'',N''-pentamethyldiethylenetriamine and 10 ml dry toluene was added. To another flask equipped with a magnetic stirrer a mixture of 0.538 g (1-bromoethyl)benzene and 0.5 g ÒÅÌÐÎ was added. The air was evacuated from the reaction mixture, 10 ml of dry toluene was added and a reaction flask was flushed with dry argon. The content of first flask was added slowly to the content of second flask and mixture was stirred for 1 hr at temperature 50 ⁰Ñ.

The obtained solution was purified by chromatography using toluene as eluent. After evaporation of toluene, the oily liquid was slowly crystallized in the cold. The resulting white crystals were dried in vacuum and characterized by ¹H NMR.

Copolymerization of styrene 2-vinylnaphtalene, 9-vinyl-anthracene and 4-vinylbenzoic acid. Mixture of VBA (5.5 mmol, 0.815g), TEMPO (0.031 mmol, 0.00478g) and 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidine (0.304 mmol, 0.07940g) was dissolved in 8 ml (69.59 mmol, 7.248g) of Sty. Mixture was divided into two equal parts. One of them was placed in a flask containing 9VA (0.038 mmol, 0.00784g) and 2VN (0.142 mmol, 0.02189g). On next step both parts were subjected to three freeze thaw cycles to remove oxygen, and flasks were flushed with dry nitrogen. Both mixtures were stirred overnight at the temperature 130 ⁰C. Prepared copolymers were dissolved in the system CH2Cl2:CH3OH 9:1 and precipitated in hexane three times. Polymers are drying to constant weight in vacuum at temperature 70 ⁰C and characterized by ¹H NMR.

Results and Discussion

Figure 1 shows the polymerization process and chemical structure of prepared quaternary copolymer (CP). In case when l=n=0, the corresponding copolymer without chromophore fragments (MX) has been prepared.

Figure 1. Copolymerisation of styrene 2-vinylnaphtalene, 9-vinylanthracene and 4-vinylbenzoic acid.

According to GPC data, the Mw of copolymers CP and MX are 30,000 and 30,400, respectively. The determined molecular weight distributions have been equal to 1.09 and 1.16 for CP and MX, accordingly. The amount of VBA in copolymers has been determined by the titration of solution CP or MX in THF after neutralization with a small excess of NaOH/ methanol solution by aqueous 0.1 N HCl solution

¹H NMR spectra of both polymers are depicted in Figure 2. It is obvious that the areas of aromatic protons for both polymers are identical. That´s why it was difficult to determine the amount of 9VA, 2VN and VBA in CP by ¹H NMR spectra.

Figure 2. ¹H NMR spectra of CP (above) and MX (below).

Therefore we used the UV-absorption spectra for quantifying the concentration of anthracene and naphthalene fragments into polymer chain of CP using the system CH₂Cl₂:CH₃OH (9:1 v/v) as solvent. Figure 3 shows the absorption spectra of 4 different concentrations of copolymers CP and MX in the spectral region 300-425 nm which has been used for determination of anthracene concentration.

Figure 3. UV/vis absorption spectra of CP (left, concentration for spectra 1 is 144.7 g/l, 2 – 72.35 g/l, 3 – 36.175 g/l, 4 – 18.087 g/l) and MX (right, concentration for spectra 1 is 220.5 g/l, 2 – 142.1 g/l, 3 – 71.05 g/l, 4 – 35.525 g/l).

It was found that the absorption maximum around 350 nm (peak I on Figure 3) can be used for quantitative determination of anthracene content due to the linear relationship between the intensity of this band and concentration of polymer in solution.

Another spectral region (220-290 nm) and other concentrations of copolymer CP in CH₂Cl₂:CH₃OH (9:1 v/v) solution have been chosen for quantifying content of naphthalene groups in this copolymer (Figure 4).

Figure 4. UV/vis absorption spectra of CP (left, concentration for spectra 1 is 0.05 g/l, 2 – 0.043 g/l, 3 – 0.036 g/l, 4 – 0.029 g/l, 5 – 0.018 g/l) and MX (right, concentration for spectra is 0.049 g/l, 2 – 0.042 g/l, 3 – 0.035 g/l, 4 – 0.028 g/l, 5 – 0.0175 g/l).

In order to determine which wavelength should be used for quantifying naphthalene fragment concentration in CP we compared the absorption of copolymers CP and MX in the spectral region between 220 and 290 nm (Figure 5). It seems that the intensity of absorption at 232 nm is the most distinctive feature in spectrum of copolymer CP which allows to provide the quantitative estimation of naphthalene contribution in the absorption. Figure 5 shows as well the absorption spectrum of model compound 2-ethylnaphtalene which has been chosen for making a calibration graph for quantifying the naphthalene content in copolymer CP.

As for determination of antracene content we used another model compound – 9-methylanthracene. Table 1 summarizes data of spectrophotometrical evaluation of naphthalene and anthracene fragments content in copolymer CP in comparison with amount of corresponding monomers 2VN and 9VA which were used as starting materials for copolymerization. It presents as well the data on content of carboxyl groups in copolymer CP in comparison with concentration of VBA introduced in copolymerization process.

Figure 5. Comparison (left) and difference (solid line, right) between the absorptions of CP (dotted line) and MX (solid line, left) in comparison with 2-ethylnaphthalene (stick-dotted line) as a model compound.

Table 1. Content of monomers 9VA, 2VN and VBA in reaction mixture and in copolymer.

Monomers	Mol. % in reaction mixture	Mol. % in copolymer CP
9VA	0.1	0.0034
2VN	0.37	0.037
VBA	7.26	9.08

Conclusions

Free radical copolymerization of styrene, 2-vinylnaphthalene, 9-vinylanthracene, and 4-vinylbenzoic acid has been reported. Concentration of carboxy groups, as well as concentration of naphthalene and anthracene derivatives in reaction mixture and determined concentration of corresponding fragments in copolymer have been compared. It has been found that copolymer contains about 30 times less of anthracene fragments, about 10 times less of naphthalene fragments than amount of corresponding comonomers which have been introduced into reaction.

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