Functionalization of Polystyrene with Cyclic

REGULAR CONTRIBUTED ARTICLES

I. L. Onder, A. Okudan* Department of Chemistry, Selcuk University, Konya, Turkey

Functionalization of Polystyrene with Cyclic Anhydrides and Their Spectroscopic, Adhesive and Corrosive Characterizations Polystyrene (PS) has been chemically modified with cyclic anhydrides such as glutaric anhydride (GA), citraconic anhydride (CA) and phthalic anhydride (PA) in the presence of BF3.O(C2H5)2 in chloroform and succinic anhydride (SA) in 1,2-dichloroethane. Some important reaction parameters were determined in order to optimize the acylation process. ATR FTIR and 1H NMR studies indicate that the acylation reaction can introduce both carboxyl group and double bond (for only citraconic anhydride) onto pendant aromatic groups of the polymer. The adhesion properties and corrosion resistance of the acylated PS on metal surface under various conditions have been investigated.

1 Introduction Polymer chemistry has been the subject of studies concerning synthesis and applications (Gaylord et al., 1992; Wu et al., 1993; Lee and Ahn, 1999). The synthesis of new polymeric materials can be carried out by either the polymerization of new monomers or the chemical modification of polymers with an appropriate technique (Gaylord et al., 1989; Harrison et al., 1975). During recent years, polymer chemists have increasingly focused on the need to maximize efficacy because of the widespread use of polymers in high physicomechanical materials and the interest in improving their performance, adhesion, and photosensitivity. Chemical modification has great importance because of the impossibility of polymerization of the modified polymers with proper monomers (Wafaa, 2008). Modification reactions of polymers can be accomplished by use of a number of different reagents, like epichlorohydrin, organic anhydrides in the various reaction conditions or by the characteristic reactions such as alkylation, halogenation, nitration and sulfonation (Gibson, 1980; Kurbanova et al., 1979; 1976; Seymour and Carraher, 1988). The use of polymer materials, especially polymers with polyfunctional groups, has increased because of the demand of modern techniques with chemical modification (Wafaa, 2008). PS without additional * Mail address: Ahmet Okudan, Department of Chemistry, Selcuk University, 42075 Konya, Turkey E-mail: [email protected]

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functional groups has limited some properties such as thermal stability, mechanical strength, adhesion capability, corrosion resistance and photosensitivity. For this reason, the desired physical and mechanical properties of PS can be obtained by anchoring some functional groups (Kenyon and Wough, 1958; Swiger, 1975; Blanchette, 1958; Frechet et al., 1979). Thus, the undesired properties of PS can be improved using different reagents by modification reactions. By use of these modified polystyrenes, it was shown that new coating systems with improved heat resistance, anticorrosive and impact properties can be obtained (Kurbanova et al., 1985; 1990; Krohmalmy et al., 1990). PS, which is widely used in industrial applications currently, is also used for the production of plastic materials used instead of metals in technological applications. Modifications on the side chain of the macromolecule are possible and frequently employed. For example, introduction of carboxyl groups in phenyl rings of polystyrene (Letsinger et al., 1964; Ayres and Mann, 1965) can be easily carried out by cleavage of benzoylated or chlorobenzoylated polystyrene and by oxidation of commercial formyl resin and chloromethylated polystyrene (Harrison and Philip, 1975). Acylation of benzene with a cyclic anhydride, maleic anhydride (as a model system), was demonstrated by Pummerer and Buchta (1936). Pummerer used Lewis Acid in these studies. The acylation reactions of PS with organic anhydrides are very important for the synthesis of polyfunctional PS (Kurbanova et al., 2000; Wang et al., 2002). The reactions of anhydrides with commercial polymers were investigated extensively such as polyethylene (Gaylord and Mishra, 1983), polypropylene (Mitsuaki and Masuasu, 1967), polystyrene (PS) (Okudan, 1998; Li et al., 2002; Zheng et al., 2007; Chen et al., 2008) in the presence of catalysts. In our previous works, we investigated the chemical modification of polystyrene with maleic anhydride by Friedel-Crafts reactions (Kurbanova et al., 1996, 1997, 1998). The products were resistant to heat, impact, and various substrates and could be used as polymeric plate materials (Kurbanova et al., 1997, 1998). The outstanding virtue of this method is well suited for the preparation of high molecular mass of styrenic polymer based ionomers with substituted groups situated randomly along the polymer chain (Kurbanova et al., 1996). The aim of this work is to modify PS with different cyclic anhydrides by Friedel-Crafts acylation reaction (Fig. 1), to ex-

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I. L. Onder, A. Okudan: Functionalization of PS with Cyclic Anhydrides Succinic anhydride (SA) and phthalic anhydride (PA) were purified by recrystallization from 1,2-dichloroethane and benzene, respectively, followed by sublimation in vacuum. Glutaric anhydride (GA) and citraconic anhydride (CA) were distilled under vacuum before used. Chloroform and 1,2-dichloroethane were dried overnight with calcium chloride, filtered and distilled before used. Other chemicals were commercially available and used without further purification. 2.2 Chemical Modifications of Polystyrene with Cyclic Anhydrides (General Procedure) For modification, PS (5.20 g, 0.05 mol) was dissolved in chloroform (40 mL) in a three-necked flask at 20 8C for 2 h. Then, cyclic anhydride (0.01 mol) was added to the above solution. For modification of PS with SA, 1,2-dichloroethane was used as solvent. After the mixture was stirred for 1 h, BF3.O(C2H5)2 (0.01 mol) was added dropwise at the same temperature and the mixture were stirred for 2 h more. The concentrated organic phase was precipitated with methanol, filtered, washed with hot acetone and subsequently dried overnight under vacuum at 50 8C. The degree of acylation corresponding to carboxylic acid value of the modified polymer was determined by chemical titration, and the data were presented in Table 1.

Fig. 1. Friedel Crafts acylation reaction of PS with cyclic anhydrides

tend the application field of polystyrene and to characterize them by FT-IR and 1H NMR spectroscopies.

2 Experimental

2.3 Measuremets and Analyses

2.1 Materials

ATR FT-IR spectral analysis of the polymer films (0.5 mm) prepared by melt pressing at 140 8C were measured with a Perkin Elmer spectrum 100. 1H NMR spectra were recorded at 25 8C on a Varian 400 NMR spectrometer. Samples for 1H NMR spectroscopy were prepared by dissolving about 20 mg of products in 1 ml deuterated chloroform. Tetramethylsilane was used as an internal reference. Intrinsic viscosities of the

Polystyrene (PS) was purchased from Aldrich and other chemicals were purchased from Merck. Polystyrene (PS) (Mw 230.000) was dissolved in benzene followed by precipitating in methanol and used after drying under vacuum at 50 8C. BF3.O(C2H5)2 was purified by distillation at 123 8C.

Run

Acylation agents

BF3/Anhydride mol

PS/Anhydride mol

Time h

Temperature 8C

AN mg KOH/g

(m : n)a

[g]b

1 2 3* 4 5 6 7 8 9 10 11 12 13 14

GA GA GA GA GA GA GA GA GA GA GA SA PA CA

1:1 1:1 1:1 1:1 1:1 1:1 1:2 0.5 : 1 2:1 1:1 1:2 1:1 1:1 1:1

10: 1 5:1 5:1 5:1 10: 1 3:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1

1 2 2 2 2 2 2 2 3 3 3 2 2 2

10 10 20 30 20 20 20 20 20 30 10 20 20 20

27 31 52 43 29 47 51 18 50 41 31 47 51 50

7:1 6:1 4:1 5:1 7 :1 4 :1 4:1 11 : 1 4:1 5:1 6:1 4:1 4:1 4:1

0.83 0.81 0.80 0.77 0.78 0.80 0.69 0.88 0.67 0.82 0.76 0.83 0.84 0.82

a

Ratio of contents of polystyrene (m) to acylated polystyrene (n) units in macromolecules. [g] in toluene 25 8C (dL/g) *optimum values of the reaction is presented in run 3. b

Table 1. Acylation of PS with cyclic anhydrides by Friedel Crafts

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I. L. Onder, A. Okudan: Functionalization of PS with Cyclic Anhydrides acylated polystyrenes were determined by an ubbelohde viscometer in toluene at 25 8C. 2.4 Chemical Analysis 2.4.1 Determination of the Acid Number (AN) The amount of functional groups bonded to the structure of the modified PS was determined by functional group analysis. To determine the number of carboxyl (-COOH) groups in the PS, the polymer sample was refluxed with excessive 0.1 N KOH solution for 1 h. After the mixture was cooled to room temperature, the remaining KOH was titrated with 0.1 N HCI solution. ðV1 & N1 % V2 & N2 Þ ; m where AN is the acid number, V1 is the volume of the added 0.1 N KOH solution (mL), N1 is the normality of KOH solution, V2 is the volume of the consumed 0.1 N HCl solution (mL), N2 is the normality of HCl solution, and m is the amount of the sample (g). ANðmgKOH=g polymerÞ ¼ 56:1 3

2.5 Determination of Coating Properties The 10 % (w/w) of polymer solution in toluene was prepared for the investigation of coating properties of modified PS with different molecular weight. The film with a thickness of 100 to 120 lm was applied to the metal (steel of moderate carbon content with dimensions 50 mm · 100 mm · 1 mm surface). The film was dried first at room temperature, and then in an oven at 70 8C for 1 to 2 h. 2.6 Determination of Adhesion Properties The adhesion property of the modified polymers was determined by the \Lattice notch method". For the determination of the adhesion capability to metal (cold rolled carbon steel with content C, 0.2 %; Mn, 1.5 %; Si, 0.1 %; P, 0.01 %; S, 0.008 % and with dimensions 50 mm · 100 mm · 1 mm), the tested side was blasted and cleaned to apply the coated materials. The each polymer solution was deposited over the metal as a layer (120 to 140 lm). This material was first dried in open air and then in an oven at 50 8C. According to the \Lattice notch method" a thin polymer layer (120 to 140 lm) is formed on the metal surface and the polymer layer is divided into small squares (1 mm · 1 mm) by a razor blade. Insulating tape (10 to 100 mm) is over these squares and the tape is suddenly pulled. In this process, a portion of small squares is separated from the surface of the metal and another portion remains on the surface. Therefore, the percent adhesion is calculated from the number of small squares still remaining on the metal surface. The percent adhesion was calculated as: ða % bÞ 3 100; a where a is the total number of squares, b is the number of squares removed from the substrate (GOST, 1978). Adhesionð%Þ ¼

2.7 Determination of the Corrosion Resistance Polymer coatings (100 to 120 lm thick) on the metal surface were exposed to 3 % NaCl, 10 % NaOH, 10 % HCl solution, pure water, and air for 8 days. After removal of the coated metals from the treating solutions, the amount of decomposition observed by the naked eye was taken as a measure of the corrosion resistance. 3 Results and Discussion 3.1 Synthesis Friedel Crafts acylation reactions are aromatic substitution reactions in which benzene (or a substituted benzene) undergoes acylation when treated with carboxylic acid derivatives (usually acyl halide or anhydride) and a Lewis acid catalyst such as BF3.O(C2H5)2 (Kurbanova et al., 1997) and AlCl3 (Andrew, 1971). These reactions are widely used to modify polystyrene through the side groups (phenyl rings) of macromolecules (Sun et al., 1996). In this study, Friedel Crafts acylation reaction was used to prepare acylated polystyrenes. The precipitated polymer washed with hot acetone to remove unreacted cyclic anhydrides and cyclic anhydride oligomers, if present. It is of great importance to insure total elimination of the unreacted cyclic anhydrides and homopolymers of cyclic anhydrides. Since unreacted cyclic anhydrides and cyclic anhydride oligomers may prevent accurate characterization of the acylated polystyrene. The following parameters such as the amount of BF3.O(C2H5)2, reaction temperature and reaction time were examined in order to optimize the process. The degree of acylation corresponding to carboxylic acid value of the modified polymers was determined by chemical titration and the data are presented in Table 1. The bonding mechanism of cyclic anhydrides to the aromatic ring of the polymer with BF3.O(C2H5)2 is shown in Fig. 1. As shown in Table 1, a significant change in the acid number has been observed initially with increasing catalyst concentration. The results indicate that a relatively higher catalyst concentration (up to the optimum catalyst ratio) which depends on the [BF3.O(C2H5)2/GA] molar ratio is desired to promote acylation efficiency. AN decreases with decreasing the amount of catalyst used according to the optimum reaction condition, not a noticeable change is increasing. However, increasing the amount of catalyst decreases the intrinsic viscosity. Under optimum reaction conditions, one acyl group is connected to the one of each four aromatic ring of PS and acid number is increased up to 52 mg KOH /g (Run 3, Table 1). As shown in Table 1, the optimum reaction conditions were determined as PS/anhydride of 5 : 1 (mol/ mol), anhydride/catalyst of 1 : 1 (mol/mol), time of 2 h and temperature of 20 8C. Accordingly, modification reactions with SA, PA and CA were conducted under optimum conditions. 3.2 FT-IR Analysis To aid the structural elucidation of the cyclic anhydrides-functionalization, PS with carboxyl moieties along the backbone


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I. L. Onder, A. Okudan: Functionalization of PS with Cyclic Anhydrides was analyzed using FT-IR spectroscopy, and assignments for the characteristic groups were determined. FT-IR was the first spectroscopic tool we used to identify successful acylation of the polystyrene. The FT-IR spectra of the polystyrene and the acylated polystyrenes with SA, GA, PA and CA are given in the range of 3 500 to 600 cm– 1 in Fig. 2a to e, respectively, corresponding to the product coming from entries 12, 3, 14 and 13 in Table. By comparing the above five spectra, it is clear that new bands, which are absent in the spectrum of the polystyrene (trace a), can be ascertained in the spectra of the acylated polystyrenes (traces b, c, d and e). The bands appeared in the range of 1 680 to 1 690 and 1 739 to 1 741 cm– 1 are ascribable to different carbonyl groups of individual ketone and carboxylic acid function for modified polystyrenes, respectively. While the bands belong to ketone group of modified PS with SA, GA and CA are seen at 1 690 cm– 1, the band belongs to ketone group of modified PS with PA is seen at 1 680 cm– 1. The reason of shift is due to that ketone function is a diaryl ketone. Additionally, C-H in plane bending bands belong to aromatic ring of polystyrene were observed in the range of 1 200 – 1 150 cm– 1 (trace a). As distinct from FTIR spectra of PS, a new C-H in plane bending band in the spectra of all modified PS was observed at approx. 1 230 cm– 1 (traces b, c, d and e). 3.3 1HNMR analysis Supporting evidence for the structural elucidation was revealed by 1H NMR analysis. Figure 3 shows the 1H NMR spectra of

polystyrene (A) and acylated polystyrenes with CA (B), GA (C), SA (D) and PA (E). Figure 3A displays typical 1H NMR spectrum of the polystyrene with four peaks around 1.6, 1.9, 6.6 and 7.2 ppm. 1H NMR spectra of anhydrides used in the modification reactions are presented in Fig. 4. After acylation with SA, characteristic signals around 2.8 and 3.6 ppm, due to methylene (CH2) protons of -COCH2CH2COOH moiety and also after acylation with GA, characteristic signals around 2.00, 2.40 and 2.6 ppm, due to methylene (CH2) protons of COCH2CH2CH2COOH moiety, are observed. When acylation with PA, four characteristic signals around 7.2, 7.3, 7.6 and 7.8 ppm, due to aromatic protons moiety, are observed. When acylation with CA, three characteristic signals around 2.1 and 2.3 ppm due to methyl [-CO(CH3)C=CHCOOH or -COCH=C(CH3)COOH] moiety, and at about 5.9 ppm due to methine (-C=CH) moiety are observed. In the modified PS with CA, two methyl (-CH3) signals were observed because citraconic anhydride is not symmetric and can be ring-cleaved from both sides. These new peaks sufficiently prove that the pendant benzene rings have been acylated.

3.4 Adhesion and Corrosion Resistance Properties Adhesion properties and corrosion resistance of modified polystyrene containing carboxyl groups were determined and the results are given in Table 2. As seen in Table 2, adhesion and other coating properties of modified PS varied depending on the amount of the functional groups bound to the structure of polystyrene. It was found that

Fig. 2. FT-IR spectra of polystyrene (a) and the acylated polystyrene with SA (b), GA (c), PA (d), CA (e) in the range 3 500 to 600 cm– 1

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I. L. Onder, A. Okudan: Functionalization of PS with Cyclic Anhydrides

Fig. 3. 1H-NMR spectra of polystyrene (A) and acylated polystyrenes SA (B), GA (C), PA (D), CA (E)

Fig. 4. 1H-NMR spectra of anhydrides SA (A), GA (B), PA (C) and CA (D)

it was better to study the chemical modification reactions of PS with GA under optimum reaction conditions in order to understand the functionality effect of polystyrene on the adhesion and corrosion resistance properties of coatings. Likewise, similar results were observed for other anhydrides under optimum reaction conditions. Coating properties of modified PS with different cyclic anhydrides were investigated. The effect of the amount of functional groups bound to the aromatic ring of the PS on the adhesion and corrosion resistance properties of functionalized polymers coated on the surface of the metals was investigated, and it was found that these properties varied depending on the amount of the functional groups bound to the structure of the polystyrene.

It is clearly seen from Table 2 that the adhesion capability of the polymer increases from 53 to 86 when the quantity of bonded carboxyl groups to the aromatic rings of the polystyrene increase up to 52 mg KOH/g (AN). This can be explained in terms of the hydrogen bonding between the –COOH groups and the metal oxide. The unmodified PS shows less adhesion property. The adhesion properties of modified PS improve with increasing of functional groups bonding to PS. In addition, the best adhesion results were obtained for PS modified with GA under optimum reaction conditions. The corrosion resistance properties of coatings were examined by microscopic analysis method for modified PS in 3 % NaCl, 10 % NaOH, 10 % HCl solutions, deionized water,



I. L. Onder, A. Okudan: Functionalization of PS with Cyclic Anhydrides

Cyclic anhydrides

AN mg KOH/g

Adhesion %

3% NaCl

10 % NaOH

10 % HCl

Air

Water absorption %

GA GA GA GA SA PA CA Unmodified PS

52 31 27 18 47 51 50 –

86 65 62 53 83 85 85 2

++ ++ ++ + ++ ++ ++ +

–+ + + + -+ -+ -+ ++

++ ++ ++ + ++ ++ ++ +

++ ++ ++ + ++ ++ ++ +

0.09 0.12 0.12 0.13 0.10 0.09 0.09 –

–, no resistance; –+, little; +, medium resistance, ++, high resistance Table 2. The Modification effect of PS with cyclic anhydrides on the corrosion resistance and adhesion properties

and air. The effect of functional group bound PS on corrosion resistance was investigated and the functionalized PS showed a higher corrosion resistance than PS. It was seen that there was no corrosion on the metal surface in the solution of 5 % NaCl which is the aggressive sea condition in a period of 11 days. But, it was obtained that the resistance against the solution of 10 % NaOH decreased by increasing the amount of carboxyl groups in the structure of polymer. The main reason for this is considered as the interaction of bound carboxyl group with NaOH. Advantage of unmodified PS is the high resistance to NaOH medium because it has no carboxyl groups. But unmodified PS does not have an adhesion property to metal surface and this prevents using it for coating. The water absorption capability of modified PS in 24 h varied between 0.09 % and 0.13 %. Water absorption ability decreased whereas resistance to atmosphere increased by increasing the amount of functional groups in the structure of the polymer. The higher water absorption capability of functional polystyrenes may be explained in terms of the interaction of functional groups with water molecules.

Conclusions The acylation of polystyrene were accomplished by Friedel– Crafts reaction using chloroform and 1,2-dichloroethane as dispersing agent, cyclic anhydrides as acylating agent and BF3.OEt2 as catalyst. An optimum reaction should be carried out at 20 8C with a molar ratio of BF3.OEt2 to cyclic anhydrides of 1/1 and PS to cyclic anhydrides of 5/1. The structures of acylated polystyrenes were confirmed by FT-IR and 1H NMR spectroscopies. The amount of carboxyl groups was determined by chemical titration. It was determined that one acyl group is connected to one of the four aromatic ring of polystyrene. Adhesion and corrosion resistance properties of modified PS coatings depended on the amount of the functional groups bound to the polystyrene. The adhesion properties and corrosion resistance of modified PS improve with increasing of functional groups bonding to PS. The functionalized polystyrene offers possibility for the development of novel polystyrene-based polymer blends and composites, thus extending the application field of polystyrene.

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Acknowledgements The authors thank Selcuk University Research Project Fund (SUBAP 09201020) for support of this work. Date received: July 7, 2011 Date accepted: November 5, 2011

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