Quaternary copolymer containing styrene, hydroxystyrene and chromophoric monomers: synthesis, functionalization and nanoagregation

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, Nikolay@microtracers.com

³Leibniz Institute of Polymer Research, Dresden, Germany

Quaternary copolymer containing styrene, hydroxystyrene and chromophoric monomers: synthesis, functionalization and nanoagregation

Introduction

Nowadays there are many works dedicated to the formation of polymer nanoparticles ^1-3. Our interest in this sphere is caused by the possibility of creating scintillator polymer materials able to form nanoparticles and, consequently, to increase their quantum output. There are hardly any works devoted to the nanosized organic scintillators, whereas these materials are of great interest for science and technology. Our attention was attracted by the works by E. W. Meijer et al., where for the formation of nanoaggregates were used functional groups of the 2-ureido-pyrimidinone (UPy), capable of forming between each other a quadruple hydrogen bond and lead to the collapsing of single chains in a solution ^4-6. In the suggested work we describe the production of a copolymer containing the specified amounts of naphthalene and anthracene chromophore links, its functionalizing by means of a UPy-fragment, studies of the nanoaggregation ability and the influence of this aggregation on the fluorescence spectrum.

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; 2-vinylnaphthalene(2VN), 9-vinylanthracene (9VA) then were dried out in vacuum in presence of phosphorous pentoxide to remove of traces of water and solvents and characterized by ¹H NMR. 4-Acetoxystyrene (4AS) (97%, Aldrich) was distilled under reduced pressure prior to use; 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidine was prepared from (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (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 .¹H NMR spectra were collected on the device «Bruker Bio Spin» (¹H 500 MHz, 20 ⁰С, solvent -CDCl₃). Scanning electron microscopy was carried out using Ultra 55 (Carl Zeiss SMT, Jena, Germany).UV/vis absorption spectra were measured on Perkin Elmer Lambda 800 spectrophotometer.

Copolymerization of styrene 2-vinylnaphtalene, 9-vinylanthracene and 4-acetoxystyrene. Mixture of 4-Acetoxystyrene (13.35 mmol, 2.16594g), TEMPO (0.04 mmol, 0.0062g) and 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidine (0.399 mmol, 0.1044g) was dissolved in 8 ml (49.08 mmol, 5.11422g) of styrene. Mixture was divided into two equal parts. One of them was placed in a flask containing 9-vinylanthracene (0.5 mmol, 0.10235g) and 2-vinylnaphthalene (1.67 mmol, 0.25756g). Second part was operated without addition of 9-vinylanthracene and 2-vinylnaphthalene. Traces of oxygen were removed from both parts by three freeze–pump–thaw cycles. Flasks were flushed with dry argon. Both mixtures were stirred overnight at the temperature 130 ⁰C. Prepared copolymers were dissolved in the system CH₂Cl₂:CH₃OH 9:1 and precipitated in methanol three times. Polymers were dried to constant weight in vacuum at temperature 70 ⁰C and characterized by ¹H NMR and UV/vis absorption spectroscopy.

Synthesis of OH-group containing copolymers CPolOH and MPolOH). A. Removal of Acetyl Groups by Hydrazinolysis (Deacetylation) from Precursor Copolymers CPOL and MPOL) (General Procedure). In a 1 L round flask 2 g of (3.35 mmol of acetyl groups) copolymer CPOL were dissolved in 250 mL of dioxane. The solution was stirred for 15 min, and 1 mL (20.6 mmol) of hydrazine monohydrate was added slowly with a syringe. After additional stirring for 24 h the solution was concentrated. The polymer was precipitated in 400 mL of water, filtered, and dried at 40 °C under vacuum. This procedure was repeated twice in order to remove all low molecular byproducts and impurities. After drying in vacuum in presence of phosphorous pentoxide to remove of traces of water 1,53 g of CPolOH was obtained as a white-green powder. According to ¹H NMR spectroscopy all acetyl groups have been quantitatively removed. Similar procedure was used for removal of acetyl groups from copolymer MPOL.

Synthesis of Upy-group containing copolymer CPolOH.

1.5 g of copolymer CPolOH was dissolved in dry toluene in argon atmosphere. The solution was turbid. In this solution was added dry triethylamine to make solution clear.

2.1 g of 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone⁷ was dissolved in hot toluene in argon atmosphere, and then cooled to room temperature. Prepared solution was introduced slowly in solution of CPolOH throw filter with pore size 0.2 μm by stirring. When solution comes to turbid several drops of triethylamine were added to make solution clear. After additional stirring for 24 h the solvent was evaporated, polymer dissolved in acetone, centrifuged and precipitated in hexane throw filter with pore size 0.2 μm. This procedure is repeated three times and resulting powder (CUPOL) dried at 80 °C under vacuum and characterized by ¹H NMR.

Results and Discussion

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

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

According to GPC data, the Mw of copolymers CPOL and MPOL are 15200 and 17300, respectively. The determined molecular weight distributions have been equal to 1.27 and 1.38 for CPOL and MPOL, accordingly. The amount of 4-AS in copolymers has been determined by ¹H NMR spectroscopy.

Concentration of anthracene and naphthalene fragments in copolymer CPOL has been determined by means of the UV-absorption spectroscopy using the mixture CH₂Cl₂:CH₃OH (9:1 v/v) as a solvent (Table 1).

Table 1. Content of monomers 9VA, 2VN and 4AS in reaction mixture and in copolymer.

Monomer	Mol. % in reaction mixture	Mol. % in copolymer CPOL
Styrene	73.5	79.03
Acetoxystyrene	20	17.58
9-vinylanthracene	1.5	0.31
2-Vinylnaphtalene	5	3.08

Nanoagregation of Upy-group containing copolymers.

Table 2 summarizes data about the ratios between a good solvent (dichloromethane) and bad solvent (methanol) which were combined together for making samples of modified copolymer CUPOL, which has Upy-group, and unmodified copolymer MPOL with a starting concentration C_start. These solvent systems were used for scanning electron microscopy analysis (SEM)

Table 2. Concentrations and solvent systems for solutions of MPOL and CUPOL.

Copo-lymer	C_start (g/l)	System 1		System 2		System 3
	C_start (g/l)	CH₂Cl₂ (ml)	MeOH (ml)	CH₂Cl₂ (ml)	MeOH (ml)	CH₂Cl₂ (ml)	MeOH (ml)
MPOL	4.44	0.1	1.5	0.4	1.5	1.6	0
CUPOL	4.61	0.1	1.5	0.4	1.5	1.6	0

Samples for SEM analysis were prepared by placing 10 μl of each from all 6 combinations of polymer/solvent presented in Table 2 on the surface of a silica plate, following by full evaporation of the solvents (Table 3).

Table 3. SEM pictures of samples prepared from all 6 combinations of polymer/solvent presented in Table 2

	System 1	System 2	System 3
MPOL	Magnification = 4.10 KX	Magnification = 4.00 KX	Magnification = 15.00 KX
CUPOL	Magnification = 5.00 KX	Magnification = 30.00 KX	Magnification = 20.00 KX

Presented data indicate that the average size nanoparticles prepared from copolymer CUPOL/System 2 and CUPOL/System 3 is about 90 nm.

Figure 2 illustrates data related to absorbance and fluorescence spectra of copolymer CUPOL in solutions of THF (A, B, C) and mixture of THF/MeOH (68:32 v/v) (D, E, F). The choice of the solvent systems was related to our intention to observe the intra- and interpolymer interactions as a function of solvent-induced coil changes. It is known⁸ that for naphthalene-containing polymers the ratio of monomer to excimer fluorescence (I_m / I_e) is very dependent on solvent (and/or coil density). The general observation is that a poor solvent (mixture THF/MeOH) enhances the excimer fluorescence (420-475 for naphthalene fragments) at the expense of the monomer fluorescence (310-340 nm). In these solvents, the coil density is increased such that naphthalene-naphtalene separations are decreased, which in turn leads to a higher density of excimer-forming sites. In fact, the comparison of fluorescence spectra of copolymer CUPOL which has no anthracene units ( l=0 in structure presented in Figure 1) dissolved in THF and THF/MeOH shows the ratios I_m / I_e equal to 40.0 and 32.9, respectively (excitation at 285 nm). This observation helps to explain the differences in the intensity of fluorescence presented in Figure 2B and 2E where the same excitation wavelength has been used. Taking the intensity of emission at 420 nm as a parameter for monitoring, it is easy to estimate that due to contribution of excimer emission of naphtalene fragments in this spectral region the increase in the intensity measured for THF/MeOH solution compared to THF solution is changing from 12.3% to 23.0% when the concentration of copolymer CUPOL has been decreased from 7.5 to 1.9 g/L. The nature of this concentration dependence is not completely clear because it is expected that the ability to form excimer sites is usually enhances with increasing concentration of chromophore groups. It is interesting that increasing excitation wavelength to 391 nm (Figures 2C and 2F) leads to the opposite effect in terms of intensity of fluorescence: solution on THF has a higher intensity that solution in mixture THF/MeOH. That observation can be considered as a additional confirmation of provided explanation related to the contribution of excimer emission of naphtalene fragments which is excited by wavelength 285 nm, but not wavelength 391 nm.

Figure 2. Absorbance (A, D) and fluorescence spectra of CUPOL in solutions of THF (A,B,C) and mixture of THF/MeOH (D, E, F) at excitation 285 nm (B,E) and 391 nm (C,F) at three different concentrations: 7.5 g/L (solid line); 3.7 g/L (dotted line) and 1.9 g/L (dash-dotted line).

Conclusions

Synthesis of copolymers containing styrene, 9-vinylanthracene, 2-vinylnaphthalene and 4-hydroxystyrene, and fictionalization of them by 2-ureido-pirimidinone has been reported. Aggregation ability of the functionalized copolymer with formation of nanoparticles has been investigated. Absorbance and fluorescence properties of the functionalized copolymer in two different solvent systems have been studied and interpreted in terms of specific ability of naphthalene-containing polymer dissolved in a poor solvent to enhance the excimer fluorescence at the expense of the monomer component.

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