Electroactive Aniline Oligomers of Well-Defined Structures and Their

Chapter 24

Electroactive Aniline Oligomers of Well-Defined Structures and Their Polymeric Derivatives 1,6

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Yen Wei , Shuxi L i , Xinru Jia , Ming-Hsiung Cheng , Mat W. Mathai , Jui-Min Yeh , Wei L i , Susan A. Jansen , Zhi Yuan Wang , Chuncai Yang , Jian Ping Gao , Moshe Narkis , Arnon Siegmann , and Bing R. Hsieh 1

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Department of Chemistry, Drexel University, Philadelphia, PA 19104 Department of Chemistry, Temple University, Philadelphia, PA 19121 Department of Chemistry, Carleton University, Ottawa K1S 5B6, Canada Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Xerox Corporation, The Wilson Center for Research and Technology, 800 Phillips Road, MS 114-39D, Webster, NY 14580

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A general strategy for the synthesis o f aniline oligomers w i t h controlled molecular weight, narrow or unit molecular weight distribution and designable end-groups has been developed based on the theory o f non-classical or reactivation chain polymerization. A series o f oligomers have been prepared by oxidative polymerization of aniline i n the presence o f N-phenyl-1,4-phenylenediamine or 1,4phenylenediamine as initiator. The molecular weight o f the oligomers is controlled by varying the amount o f initiator. Generally, lower oligomers can serve as the initiators to build higher oligomers. The oligomers with m i n i m u m 4 nitrogen atoms and 3 phenylene rings exhibit similar characteristic redox behavior and electroactivity as polyaniline. Electronic conductivity of the oligomers o f 7 or 8 aniline units approaches that o f polyaniline. Solubility o f the oligomers is much improved over that o f conventional p o l y a n i l i n e . V a r i o u s functional groups can be introduced to the oligomers either by proper selection o f starting materials or by post-synthesis modifications v i a common organic reactions. The functionalized oligomers undergo further polymerizations to afford a variety o f new electroactive materials, including polyamides, polyimides, polyureas, polyurethanes, polyacrylamides and epoxy polymers. There are numerous potential applications for the oligomers and their polymeric derivatives.

Conductive polymers, such as polythiophene, polyaniline, polypyrrole, poly(pphenylene vinylene), etc., have been studied extensively and explored for numerous potential applications because o f their high conductivity and, probably more importantly, their electroactivity and other unique properties. " Recently, there have 1

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Corresponding author.

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©1999 American Chemical Society

In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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been many encouraging developments towards commercialization of the electroactive polymers for applications such as electroluminescent devices, corrosion resistant 5

coatings and electrostatic dissipation coatings and fabrics. However, one problem associated with large-scale commercial applications is the limited processibility of electroactive polymers. F o r instance, p o l y a n i l i n e solution i n N - m e t h y l - 2 pyrrolidinone ( N M P ) often forms gels and unsubstituted polythiophene and polypyrrole are not soluble in common organic solvents. Furthermore, among the inherent drawbacks of the electroactive polymers, like most synthetic polymers, are the non-unity polydispersity i n molecular weight and the existence of structure


defects, which become obstacles for many electronic and optical applications that demand the materials of well-defined structures and high chemical purity. In an effort to solve these problems, our research i n the last several years has focused on the synthesis of electroactive oligomers of well-defined structures. The basic idea of our approach has been that the electroactive oligomers with the same repeating unit structures as their corresponding polymers can be prepared to have well-defined structures, controllable molecular weights, very narrow or unit polydispersity and designable functional end-groups. Because o f the l o w molecular weights, these oligomers should have much enhanced solubility and, therefore, could be purified, thoroughly characterized and studied for their physical and chemical properties. O n the other hand, since the end-groups can be readily varied to polymerizable functionalities, the oligomers function as monomers for further polymerization to yield a variety of new polymers, which possess both excellent mechanical properties of typical polymers and the electroactivity o f the conducting polymers to a certain extent. It is particularly noteworthy that electroactive oligomers often exhibit similar or even improved physicochemical properties. A s examples, the 6

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aniline oligomers appear to have good or even better anticorrosion properties. ' The electronic properties of the aniline octamer were reported to be comparable to polyaniline.

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Defect-free thiophene oligomers such as unsubstituted or (Χ,ω-

substituted sexithiophenes inherently possess the basic or much improved electronic 9

and optical properties of polythiophene. The p-phenylenevinylene oligomers have also been synthesized and investigated for electroluminescence applications. In this article, we demonstrate our oligomer approach to the electroactive polymeric materials with oligoanilines as specific examples. The synthesis, properties and further polymerization of amino-terminated trianilines and other higher oligomers are reviewed and discussed. The synthesis o f several new polymers i n c l u d i n g polyureas and polyurethanes derived from the α,ω-end-functionalized aniline oligomers is presented. Some of the potential applications of the oligomers and their polymeric derivatives are described. A General Strategy for the Synthesis of Aniline Oligomers There have been a number of methods reported for the preparations of aniline oligomers in the l i t e r a t u r e . ' Most of them involved multi-step synthetic reactions or unstable reagents. Since later 1980s, W e i and coworkers have been interested in the development of fundamental understanding of the polymerization mechanism and, therefore, better methodologies for the synthesis of electroactive polymers including 8

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p o l y a n i l i n e " , polythiophene *

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and polypyrrole . A new general theory has been

proposed as "non-classical chain polymerization" or "reactivated chain polymerization".

1 5 - 2 4

In this theory, the polymer chain propagation is accomplished by reactions

of a reactive chain end ( M * ) with incoming monomer ( M ) leading to formation of a m

non-reactive or dormant product with higher molecular weight ( M + i ) . This dormant m

chain is then reactivated chemically (e.g., oxidation or reduction, etc.), physically (e.g., heating or radiation, etc.) or biologically (e.g., enzymatic action) to become a reactive chain end ( M

m +

i * ) , which reacts with a monomer to complete another chain


growth step yielding again a dormant chain ( M

M

m

+

i

; ^ M Reactivation

M +

2):

_

m

+

1

w

* — • • M Chain growth

m

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This new class o f polymerization process is neither a classical chain nor a classical step polymerizations. Examples of this polymerization could include many synthetical and natural polymerizations, such as the oxidative polymerization of anilines and other aromatic c o m p o u n d s , the recently emerged l i v i n g radical p o l y m e r i z a t i o n s ' , in which dissociation of the capping groups can be considered as the reactivation step to allow the addition of one or more monomers before the recapping, and biological polymerizations for the biosynthesis of nucleic acids, proteins and polysaccharides. Under proper conditions, the polymerization could be stopped at desired stages by not reactivating the dormant chains, implying that the non-classical chain polymerization could become a living polymerization. Application of this new theory has led to the development of a general, one-step synthesis o f aniline oligomers of well-defined structures with controllable end-groups and molecular weights. In this method, various amounts of aromatic amine additives, such as 1,4phenylenediamine ( P D A ) or N - p h e n y l - l , 4 - p h e n y l e n e d i a m i n e ( P P D A ) , were introduced into the aniline polymerization system i n aqueous HC1 solution with ammonium persulfate as oxidant at about -5 °C. The reaction conditions were very similar to conventional aniline p o l y m e r i z a t i o n , except for the presence o f the additives. A s illustrated i n Scheme 1 with P P D A as example, the aniline dimer P P D A can be considered as a chain initiator or a "dormant" chain ( M ) , w h i c h could be reactivated to reactive species such as nitrenium ions, iminium ions or others ( M * ) by oxidation, while aniline is considered as the monomer ( M ) because it has a higher oxidation potential than the M c o m p o u n d . The highly reactive M * attacks the aniline monomer at the para position followed by deprotonation to afford a "dormant" aniline trimer ( M i ) , i.e., N-phenyl-4,4'-diaminodiphenylamine. Since the trimer has an even lower oxidation potential, it w i l l be oxidized to form the reactive M i * , which again reacts with another aniline monomer resulting i n M 2 . The process repeats to yield higher oligomers and p o l y m e r s . 15-24

25

26

3

m

m

16

m

m

m +

m +

M +

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The aniline dimer P P D A plays a similar role as the chain initiator i n a classical chain polymerization. Therefore, the amount o f P P D A added to the aniline polymerization should significantly affect the molecular weight of polymers. Indeed, we have found that the molecular weight decreases as the amount o f P P D A is increased (Fig. 1). A t 20 mol-% P P D A , the number-average molecular weight (Mn) In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

387

Scheme 1 NH2

-20-j Reactivation
1.0 d L / g ) , w h i c h were e

determined in dilute N M P solutions containing 5 wt% of L i C l at 25.0 C . The readily attainable high molecular weights i n this system could be attributed to the high reactivity of the isocyanate groups. The electronic properties and characteristics of Downloaded by UNIV QUEENSLAND on June 22, 2013 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch024

both chemical and electrochemical redox reactions of the polyureas were found to be quite similar to those of the aniline trimers. Engineering and Potential Technological Applications of the Oligomers There are many possible technological applications for the aniline oligomers and both o f their small molecular and polymeric derivatives. L i k e polyaniline, the oligomers and their derivatives could be used, for instance, as electroactive materials i n fabricating electrochromic, electroluminescent, biosensor, and electroactuator devices, c h e m i c a l l y or electrochemically tunable gas-separation membranes, anticorrosion and antistatic coatings, rechargeable batteries, e t c . Taking advantage of their well-defined structures and designable end-groups, properly functionalized oligomers could form a variety o f molecular or supramolecular assemblies for potential electronic and optical applications. B y introducing silane or a l k o x y s i l y l groups i n the oligomers, one should be able to prepare electroactive organic-inorganic hybrid or nanocomposite materials v i a the sol-gel reactions . The oligomers or their metal complexes could find potential catalysis applications. L i k e many aromatic amines, the aniline oligomers i n their reduced forms may serve as antioxidants and free-radical absorbents. The oligomers may also serve as a component i n conventional or conducting polymer blends to renter the blends electroactive or more processible. It has been demonstrated by electrochemical ' and X-ray photoelectron spectroscopic studies that the anticorrosion performance of the aniline oligomers is as g o o d as, and sometimes better than, that o f p o l y a n i l i n e . Besides the electroactivity, the polymers containing the oligomer building blocks should have many other desired chemical and physical properties, such as good mechanical strength and thermal stability, for applications as structural materials. M a n y of the above-mentioned applications have been explored or are currently under investigation in our laboratories. 1-5

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Summary and Remarks W e have described a general methodology, which has been developed based on the theory of non-classical or reactivation chain polymerization, for the synthesis of aniline oligomers with controlled molecular weight, narrow or unit molecular weight distribution and designable end-groups. A series of oligomers were prepared by In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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oxidative polymerization o f aniline i n the presence o f N-phenyl-l,4-phenylenediamine or 1,4-phenylenediamine as initiator. The molecular weight of the oligomers is controlled by varying the amount o f the initiator. A s the initiator concentration is increased, the molecular weight decreases and the molecular weight distribution becomes narrower and approaches unity. Generally, lower oligomers can serve as the initiators to build higher oligomers. The oligomers with minimum 4 nitrogen atoms and 3 phenylene rings exhibit similar characteristic redox behavior and electroactivity as polyaniline. Electronic conductivity increases with the number of aniline units i n the oligomers and approaches the value for polyaniline as the number is about 7 or 8.


Solubility of the oligomers is much improved over that of conventional polyaniline. Various functional groups can be introduced to the oligomers either by proper selection o f starting materials i n the oxidative reactions or by

post-synthesis

modifications v i a conventional organic transformations. The functionalized oligomers can also undergo further polymerization to afford a variety o f new electroactive materials, including polyamides, polyimides, polyureas, polyurethanes, polyacrylamides, and epoxy polymers. The electroactive conjugated oligomers o f well-defined structures have many significant advantages over their corresponding polymers. There are also numerous potential applications for the oligomers and their polymeric derivatives. E x p e r i m e n t a l Section The synthesis of various substituted or unsubstituted aniline oligomers was 27

31

29

reported i n previous p u b l i c a t i o n s " . Preparation o f new p o l y aery l a m i d e s , 29

2 9

3 0

4 0

40

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p o l y a m i d e s , p o l y i m i d e s ' ' , polyureas , polyazomethines , 6

and epoxy p o l y m e r s '

27

polyurethanes

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with electroactive oligomer b u i l d i n g blocks was also

reported. The synthesis o f electroactive polyureas containing the aniline trimer (1) was achieved by following a modified procedure for the preparation o f high 43

molecular weight polyureas . aniline trimer ( l )

2 8

A s a typical procedure, 0.581 g (2.0 mmol) o f the

and 2 m g of the catalyst 1,4-diazabicyclo[2,2,2]-octane (Dabac,

98%, Aldrich) were dissolved i n 10 m L anhydrous N-methyl-2-pyrrolidinone ( N M P ) . The blue solution was heated to 100 °C under stirring and nitrogen, to w h i c h a solution o f 0.553g (2.2 mmol) o f freshly distilled

,

4,4 -methylenebis(phenyl

isocyanate) ( M D I , Aldrich) in 11 m L N M P was added dropwise i n a period of 30 m i n . The reaction was allowed to proceed at 100 °C for additional 3 hours. Occasionally, small amounts of N M P solvent were added to the system i f the viscosity became too high. After the reaction was completed, the solution was poured into distilled water under stirring. A dark-violet precipitate was collected by filtration. After washing with water and ethanol several times, the filtration cake was cooled in liquid nitrogen and was crushed and ground into fine powders, which were then extracted with acetone in a Soxhlet apparatus for 48 hours. U p o n drying overnight at 80 °C under vacuum, the polyurea (12, R ' = - P h - C H - P h - ) was obtained in 94 % 2

yield.

Polymerizations of the aniline trimer with toluene-2,4-diisocyanate ( T D I ,

Aldrich) and 1,4-phenylene diisocyanate (PDI, Aldrich) were carried out following


396

the similar procedures to afford corresponding polyureas i n 90% and 96% yields, respectively. Acknowledgments. W e wish to dedicate this article to Prof. A l a n G . M a c D i a r m i d for his pioneering contributions to the field o f conducting polymers. This work was supported in part by A k z o - N o b e l Corporation ( Y W ) , by Harry Stern Foundation v i a a grant to the Drexel-Technion collaborative research projects ( Y W , M N and A S ) , and


by the Natural Sciences and Engineering Research Council of Canada ( Z Y W ) .

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Electroactive Aniline Oligomers of Well-Defined Structures and Their

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