Synthesis and Study of Phenyl-Capped Tetraaniline as an

Chapter 9

Synthesis and Study of Phenyl-Capped Tetraaniline as an Anticorrosion Additive Downloaded by COLUMBIA UNIV on September 2, 2012 | http://pubs.acs.org Publication Date: February 28, 2003 | doi: 10.1021/bk-2003-0843.ch009

Wanjin Z h a n g 1,*, Youhai Yu1, Liang C h e n 1, Huaping M a o 1, Ce W a n g 1, and Yen W e i 2

1Department of Chemistry, Jilin University, Changchun 130023, Peoples Republic of China Department of Chemistry, Drexel University, Philadelphia, PA 19104

2

Abstract: Phenyl-capped tetraaniline was synthesized via a pseudo-high-dilution technique. The mechanism most likely involves the formation of an intermediate trianiline salt and the subsequent coupling reaction of the trianiline salt with diphenylamine. Preliminary tests on the performance of phenyl-capped tetraaniline as an anticorrosion additive to a bicomponent polyurethane lacquer on cold rolled mild steel were performed. The results show that phenyl-capped tetraaniline is not only able to retard the formation of rust, but also exhibits throwing power and the ability of removing rust.

Introduction

PolyanUine is one of the most studied conjugated polymers because of its convenience of synthesis and air stability. In order to elucidate its conducting mechanism, oligomers were synthesized by researchers in the past. The first oligoanOine, tetramer called zuirin, was reported without a comprehensive analysis in 1907 . In 1968, Honzl reported a synthetic method which involved the condensation of small oligoanilines (i.e., dimer and trimer) with diethyl 1}

156

2)

© 2003 American Chemical Society

In Electroactive Polymers for Corrosion Control; Zarras, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

157 succinoylsuccinate, followed by hydrolysis, decarboxylation, and aromatization. Phenyl-capped tetraaniline (PCAT) and hexaaniline can be synthesized by this method. In 1986, based on a modified Honzl's approach, Wudl and his coworkers synthesized a phenyl-capped octamer, which had essentially the same electrical and spectroscopic properties as polyaniline. More recently, other methods for the synthesis of oligoanilines have been emerged. Wei et al. has shown a one-step method for synthesizing amine-capped trianiline . An Ullmann coupling reaction between acetanilide and 4iodonitrobenzene was used in an iterative coupling/reduction sequence followed by deacetylation to afford parent trianiline and tetraaniline . In 1998, Buchwald and co-workers reported a palladium-catalyzed synthetic approach for the synthesis of oligoanilines and their derivatives Wudl's phenyl-capped octamer in the doped form was re-synthesized with a little difference in UV/Vis spectrum by this method. A literature report of a blue solid deposited onto platinum metal in the electrochemical oxidation of aniline indicated that the aniline polymer could be prepared as a coating on metal, though unrecognized as such, in a laboratory 134 years ago . In the past 15 years, specific observations on the anticorrosion effect of polyaniline have come from Mengoli et al., DeBerry, Ahmad, and MacDiarmid. The use of conducting polymers as coatings for protecting mild steel from corrosion was discussed by Troch-Nagels et a l . Recent reports from Wessling and from Epstein and Jasty give a support to the claim that polyaniline electrically effects on the passivation of steel or iron. In our previous work, we reported a general method to synthesize parent oligoanilines, such as tetramer, octamer and hexadecamer . In this work, we present a pseudo-high-dilution technique to synthesize PCAT and the results of its anticorrosion performance. 3 )

4)

Downloaded by COLUMBIA UNIV on September 2, 2012 | http://pubs.acs.org Publication Date: February 28, 2003 | doi: 10.1021/bk-2003-0843.ch009

5)

8)

9)

10)

n)

1}

13)

14)

15)

Experimental

Materials. The following chemicals were used as received: diphenylamine (98%, Aldrich), p-phenylenediamine (98%, Aldrich), ammonium persulfate (98%, Shengyang Chemical Factory, China), ammonium hydroxide (30%, Changchun Chemical Factory, China), hydrochloric acid (37%, Changchun Chemical Factory, China) dimethylformamide (99%, DMF, Beijing Chemical Factory, China), acetaldehyde (40%, Tianjin Nankai Chemical Factory, China),


158 bicomponent polyurethane lacquer (BP-lacquer, Xingtan Wancai Coating Ltd., Sunde, China).

Instrumentation.

BR Spectra of KBr powder-pressed pellets were recorded on a Nicolet Impact 410 Spectrometer. H-NMR was run on a Varian Unity-400 spectrometer, referenced to internal tetramethylsilane, d -DMSO as solvent Mass spectra were performed on an LDI-1700 laser desorption ionizationflyingtimespectrometer (Biomolecular Co., USA). Elemental analysis results were obtained on a Perkin Elmer 2400 CHN elemental analysis instrument. l


6

Synthesis. PCAT was synthesized by a two-step method:

a.

Preparation o f Schiff-base a.

9.7g of p-phenylenediamine was dissolved in a mixture solvent of 250mL ethanol and 20mL acetaldehyde in a 500ml beaker. 10ml of concentrated H2SO4 was added into the solution slowly under a magnetic stirring. After 0.5 h, the reaction mixture was filtered through a Buchner-funnel. The precipitate was washed with ethanol three times. The obtained Schiff-base a without further purification was used for the following reaction. The experiment was carried out at room temperature in air.

b.

Preparation of PCAT.

3.2g of Schiff-base a and 10.4g of diphenylamine were dissolved in 200ml DMF containing 20ml distilled water and 15ml concentrated HC1. The reaction system was cooled to 0 · . Then, a solution of (NFU^Og (10.2g in 1M HC1 of 40ml) was added into the reaction system at about 60 drops/min under a strong stirring. The resulting solution was stirred for another 1 hour at the same temperature. It was then transferred into a 2000mL-beaker containing 1000ml distilled water to get a precipitate. The precipitate was collected by vacuum filtration through a Buchner-funnel and washed by lOOOmL HC1 (0.1M) three times and dedoped by 0.1 M ammonium hydroxide. Upon drying at 4 0 · under vacuum, the crude product was obtained as a blue-black powder. This was reduced by phenyl-hydrazine in DMF and recrystallized from 1,4-dioxane. A


159 typical elemental analysis for C30H26N4: Cal.: C, 81.42; H, 5.92; N, 12.66; Found: C, 80.58; H, 5.75; N, 12.95.

Test of anticorrosion performance. lg of PCAT was suspended in 9.5g of component b of the BP-lacquer under a magnetic stirring. After standing for ten minutes, the suspension was ground with a colloidal mill for 30 minutes to get a 5%-PCAT-lacquer.. Several pieces of cold rolled steel plates with a size of 5x10cm were cleaned to get rid of grease with a sandblast and then washed with acetone, and dried. For comparison, one of the steel plates was coated with the 5%-PCATlacquer, the other was coated with the pure BP-lacquer. The coatings on the plates were dried under ambient condition for several days. For the examination of anticorrosion ability, the experiments were divided into three groups: 1). A nick of 0.5x2cm was scraped in the coating sheets of the samples with and without PCAT before immersion in 0.1 M HQ, respectively. A fresh HC1 solution was used instead each week. Four weeks later, the coating was removed from the sheets with dimethylformamide. 2). The 5%-PCAT-lacquer was coated on a plate in a form of circle with a diameter of > 0.1cm. 3). The 5%-PCAT-lacquer was coated on the upper and lower sides of the plates. After drying, it was immersed in a 0.1 M HC1 solution for one week and then put in air for one month.


2

2

Results and discussion 1. Formation of PCAT At the beginning of the experiments, we attempted to use pphenylenediamine and diphenylamine to directly synthesize PCAT. However, we found that a series of higher oligomers as byproducts emerged according to mass spectrometry (Figure 1), although the reaction temperature was decreased to -15 °C. Purifying the compounds appears to be very difficult So, we designed a pseudo-high-dilution technique to synthesize the desired PCAT Similarly, an oxidation coupling reaction was used in the technique and went in two stages. First! p-phenylenediamine and aldehyde were condensed under an acidic conditions to produce an intermediate - Schiff-base a. Then, the Schiffbase a was reacted with an excess amount of phenylenediamine to get end product (Schiff-base arphenylenediamine = 1:2, mol), in view of phenylenediamine being able to self-condense to dimer, which leads to the reaction stopping. The reaction equations are shown in Scheme 1.


160

NH2 +


H 2 N

-V_/~

if

CHjCOH

-HfeN-

Ο

\J~H~\J ~

+

3

a Reduced

'

2 N = C H C h 3

N=CHCH

ΟΊΓΟΊΓΟΤΟΙΤΟ Scheme

1. Synthesis of PCAT

4000

—

442.6

3500 3000 2500 2000 1500

1 1 1 JiJllJ

1000 500

—•

0

1

200

1

1

400

1

1

600

II.

1

•

1

800

1

1

1000

i

1

1200

"

1—

1400

m/z Figure 1. Mass spectrum of PCAT synthesized directly from pphenylenediamine and diphenylamine


161 4


5

6

7

Scheme 2. A Possible Formation Mechanism of PCAT

Based on the results, a reaction mechanism was suggested as shown in Scheme 2. It can be known that a rapid coupling reaction leads to the formation of Sehiff-base c (eq. 4) upon the addition of the oxidizing agent (NFL^SiOg to the mixture solution of Schifif-base a and diphenylamine. The presence of a small amount of water brought by HCI solution was likely to cause the hydrolysis of Schiff-base c to a trianiline salt (eq. 5), which reacted with diphenylamine at once. It means that the trianiline salt could be rapidly consumed by reacting with diphenylamine. The trianiline salt concentration was much lower than that of diphenylamine in the reaction system. The preparation technique is therefore called pseudo-high-dilute technique. The self-coupling reaction of the trianiline salt (eq. 7) can not take place by using this technique and PCAT is the main product. This has been proved by mass spectrum shown in Figure 2.


442.2 a


Mw = 442.5

.

,

.

Ιι

1

„

I

400

200

800

600

1000

m/z Figure 2. Mass spectrum of PCAT synthesized by a psudo-high-dilute technique.

2. The anticorrosive performance of the coating containing PCAT During the examination of anticorrosion ability of PCAT, we surprisingly found that it not only has some features that the other anticorrosion additives do have, but also shows some peculiar deportment that the others do not show. First, it shows a good retardation effect on the formation of rust, as shown in Plate 1. Therightpicture shows the appearance of the steel surface with the pure BP-lacquer and the left side with the 5%-PCAT-lacquer. It is not difficult to observe that many rusty spots appear on the right plate and at the nick, whereas on theleft photo a smooth surface is given in contrast. The anticorrosion mechanism is related to the intrinsic property of PCAT having three types of oxidation state (oxidation state, median state, and reduction state). In the presence of water, iron is oxidized to a layer of Fe(OH) converting to Fe 0 afterwards and PCAT was reduced (eq. A). After that, PCAT is oxidized to the primary state in air (eq. B). The Fe 0 layer formed ensures that the steel plates are not corroded. 3

3

4

3

4

0

Fe + 3PCAT+ + 3H 0 == Fe(OH) + 3PCAT + 3H 0 + 2H 0 + PCAT* = PCAT + 40H~ 2

3

5

2

2

+

a b



163

Plate 1. The retardation phenomenon of PCAT. The surface of steel plates were coated with the pure BP lacquer (right) and with the 5%-PCAT-lacquer (left). T h i s figure c a n also b e f o u n d i n t h e c o l o r i n s e r t .

Figure 3. The phenomenon of throwing power of PCAT



164

Figure 4. The phenomenon of "removing rust" of PCAT

Additionally, it is noticeable that the oxide layer can extendfromthe bottom of the coating to the place near the coating sheet, as shown in Figure 3. Namely, as we coated the 5%-PCAT-lacquer on the plate in a circle form, no rust emerged not only in the center but also around the outside of the coating circle. The phenomenon demonstrates that there is an electron transfer among Fe atoms on the surface, which is called throwing power phenomenon. Furthermore, another interesting phenomenon is ability of PCAT "removing rust" (Figure 4). We found that rust can be rapidly formed on the bare surface, as coating a steel plate on its upper and lower sides with the 5%-PCAT-lacquer, followed by putting it in a dilute HC1 solution for one week. However, one month after it stands in air, a part of the rust disappeared again. For this phenomenon, We assume that acid might be a driving force. As a large amount of acid is present, iron is corroded through the acid and a small amount of the doped PCAT is insufficient to prevent the steelfromcorrosion. In air, the excessive acid is evaporated, the doped PCAT is able to reduce the rust to Fe. However, its reduction mechanism is not clear yet, assuming that humidity was low enough that the plates were "dry".

Conclusion

We have developed a pseudo-high-dilution technique to synthesize phenylcapped tetraaniline. The mechanism probably involves the formation of an intermediate trianiline salt and the subsequent coupling reaction of it with diphenylamine. The technique is characteristic of easy operation and high yield. The obtained phenyl-capped tetraaniline has a very good ability of anticorrosion. It is able to not only retard the formation of rust, but also has the function of throwing power and removing rust


165


References 1) Willstatter, R. and Moore, C.W. Ber., 1907,40, 2665. 2) Honzl, J. and Tlustakova, M. J. Polym. Sci. Pt. C, 1968,22, 451. 3) Liu, F.L.; Wudl, F.; Novak, M. and Heeger, A.J. J. Am. Chem. Soc., 1986, 108, 8311. 4) Wei, Y.; Yang, C. and Ding, T. Tetrahedron Lett. 1996,37, 731. 5) Rebourt, E.; Joule, J.A. and Monkman, A.P. Syn. Met. 1997,84, 65. 6) Singer, Rovert Α.; Sadighi, Joseph P. and Buchwald, Stephen L. J. Am.Chem. Soc., 1998,120, 213. 7) Sadighi, Joseph P.; Singer, Rovert A. and Buchwald, Stephen L. J.Am.Chem. Soc., 1998,120, 4960. 8) Lethedy, H. J. Chem. Soc. 1862, 15, 161. 9) Mengoli, G.; Munari, M. T.; Bianco, P.; Musiani, M. M. J. Appl. Polym. Sci. 1981, 26, 4247. 10) DeBerry, D. W. J. Electrochem. Soc. 1985, 132, 1022. 11) Ahmad, N.; MacDiarmid, A. G. Bull. Am. Phys. Soc. 1987, 32, 548. 12) Troch-Nagels, G.; Winand, R.; Weymeersch, Α.; Renard, L.J. Appl. Electrochem. 1992, 22, 756. 13) Wessling, B. Adv. Mater. 1994, 6, 226; Lu, Wei-Kang; Elsenbaumer, R. L.; Wessling, B. Synth. Met. 1995, 71, 2163. 14) Jasty, S.; Epstein, A.J., Polym. Mater.Sci.Eng., 1995, Vol. 72, pp. 565-566. 15) Zhang, W.J.; Feng, J.; MacDiarmid,A.G.and Epstein, A.J. Syn. Met., 1997,84, 119 ; Feng, J.; Zhang, W.J.; MacDiarmid, A.G. and Epstein, A.J. Pro. Soc. of Plastics Engineers Annual Conference, 1997, 1352.


Synthesis and Study of Phenyl-Capped Tetraaniline as an

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