Synthesis of Conjugated Polymers via Polymer Elimination Reactions

Chapter 32

Synthesis of Conjugated Polymers via Polymer Elimination Reactions

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch032

Samson A. Jenekhe Physical Sciences Center, Honeywell, Inc., 1071 Lyndale Avenue, South Bloomington, MN 55420 The transformation of nonconjugated polymers into conjugated polymers using elimination reactions is described. Heterocyclic conjugated polymers containing alternating aromatic and quinonoid sections in the main chain are synthesized by chemical or electrochemical redox elimination reaction on soluble precursor polymers containing sp -carbon atom bridges between the aromatic heterocyclic units. Progress of the redox elimination process is followed by infrared and electronic spectra as well as by cyclic voltammetry. A reaction mechanism in which the precursor polymer undergoes a redox reaction followed by loss of the bridge hydrogens is proposed. The resulting conjugated aromatic/quinonoid polymers generally have very small semiconductor band gaps in accord with predictions of recent theoretical calculations. A brief review of related syntheses of conjugated polymers from nonconjugated precursor polymers is also given. 3

Conjugated polymers are currently of wide interest because of their e l e c t r o n i c (1), electrochemical (2), and nonlinear o p t i c a l (3) properties which originate from their delocalized îi-electron systems. Unfortunately, the high density and geometrical d i s p o s i t i o n of π-bonds i n conjugated polymers which confine the desirable electronic and o p t i c a l properties also make them more insoluble and i n f u s i b l e r e l a t i v e to nonconjugated polymers. The many synthetic routes to conjugated polymers may be c l a s s i f i e d into two broad categories: (1) those i n which the target conjugated polymer i s obtained d i r e c t l y from conventional addition and condensation polymerization processes (4), and (2) those involving transformation of an existing nonconjugated precursor polymer into the target conjugated polymer (jj). The l a t t e r approach i s especially a t t r a c t i v e since the nonconjugated precursor polymer can be more readily processed into films and other forms p r i o r to conversion to the 0097-6156/88/0364-0443$06.00/0 © 1988 American Chemical Society

In Chemical Reactions on Polymers; Benham, Judith L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


444

CHEMICAL REACTIONS ON POLYMERS

conjugated d e r i v a t i v e . Also, the nonconjugated precursor polymer route w i l l allow the tuning of physical properties of the conjugated derivative, including density, morphology, c r y s t a l l i n i t y , and electronic and o p t i c a l properties. Among the possible polymer reactions which could be used to convert nonconjugated polymers into conjugated ones are elimination, addition, and isomerization (5), but our primary interest here i s polymer elimination reactions. One of the e a r l i e s t known synthesis of a conjugated polymer by elimination on a nonconjugated precursor was achieved by Marvel et a l (6) who demonstrated that poly acetylene was obtained by elimination of HC1 from polyvinyl chloride as i l l u s t r a t e d i n Figure 1. Feast and h i s co-workers have recently described an elegant synthesis of polyacetylene (PA) films by thermal elimination of aromatic hydrocarbons such as 1,2-bis(trifluoromethyl)benzene, naphthalene and anthracene from films of soluble nonconjugated precursor polymers (7), Figure 2. The r e s u l t i n g polyacetylene films, now known i n the l i t e r a t u r e as Durham polyacetylene, have physical properties that are s i g n i f i c a n t l y d i f f e r e n t from the Shirakawa polyacetylene: Durham PA i s largely amorphous and has a non-fibrous morphology compared to Shirakawa PA which i s highly c r y s t a l l i n e and fibrous (8-9). Also recently, Lenz, Karasz and co-workers (10) have reported the synthesis of poly(p-phenylenevinylene) (PPV) films by thermal elimination of ( G ^ ^ S and HC1 from poly (p-xylene-α-dime thy 1sulfonium c h l o r i d e ) , a soluble p o l y e l e c t r o l y t e (see Figure 3). Highly oriented Durham PA and PPV films with stretch r a t i o s up to 20 have been obtained by stretching the precursor polymer films during the transformation to conjugated derivatives; such oriented films when doped give r i s e to conductors with large anisotropy i n e l e c t r i c a l conductivity. The Durham route to polyacetylene and the above PPV synthesis demonstrate the r i c h potentials of synthesis of conjugated polymers v i a elimination reactions on nonconjugated precursor polymers. Recently, we discovered a novel type of polymer elimination reaction f o r producing conjugated polymers from the c l a s s of nonconjugated polymers containing a l t e r n a t i n g sp3-carbon atom (-C(R)H-) and conjugated sections i n the main chain (11-13). Under chemical or electrochemical oxidative or reductive conditions such polymers undergo i r r e v e r s i b l e reactions leading to loss of the bridge hydrogens, converting the sp3-carbon to sp2-carbon as i l l u s t r a t e d i n Figure 4, where D or A denotes π-electron donor or acceptor conjugated sections. I n i t i a l observations on this redox elimination reaction were made on poly(3,6-N-methylcarbazolediyl methylene) (PMCZM) (11) and poly(3,6-N-methylcarbazolediyl benzylidene) (PMCZB) (12). For example, upon oxidation of PMCZM and PMCZB with bromine or iodine i t was shown that a bridge hydrogen was eliminated as HBr or HI r e s u l t i n g i n f u l l y conjugated doped derivatives which had dc conductivities (0.1-1 S/cm) as high as the parent poly(3,6-Nmethylcarbazolediyl) (11-12). Figure 5 shows the o p t i c a l absorption spectra of a f i l m of PMCZM before and a f t e r a prolonged exposure to iodine vapor. The large red s h i f t , corresponding to a narrowing of the o p t i c a l band gap or increase i n the degree of conjugation i s to be noted. Also, i t i s s i g n i f i c a n t that elimination of the bridge hydrogens (-C(R)H-) i n PMCZM and PMZCB produced conjugated polymers containing quinonoid sections i n the main chain. These i n i t i a l



32.

JENEKHE

Figure 1.

Figure 2.

Synthesis of Conjugated Polymers

Synthesis of polyacetylene by elimination of HC1.

from p o l y v i n y l chloride

The soluble nonconjugated precursor polymer route to Durham polyacetylene v i a thermal elimination.

H C

PH

3

H +S _ , r

c—c I H

Figure 3.

445

I

H

Synthesis of poly(p-phenylene vinylene) films by thermal elimination on a soluble p o l y e l e c t r o l y t e .



446

SCHEME 2

SCHEME 1

—\

H I

DORA

DORA

\-C-

I

R

R


la Redox

Redox

DORA

•C=]

DORA

f=C-

R

F i g u r e 4.

R

G e n e r a l scheme o f i r r e v e r s i b l e redox e l i m i n a t i o n r e a c t i o n on t h e c l a s s o f nonconjugated polymers c o n t a i n i n g a l t e r n a t i n g -C(R)H- and c o n j u g a t e d s e c t i o n s i n t h e main c h a i n y i e l d i n g c o n j u g a t e d polymers.

CH

ÇHj

3

|

Ç"3

θ

, θ 3

LUndopedPMCZM 2.l dopedPMCZM

Φ 1.2

υ c

2

\2

1

200

600

1100

1600

*

2100

2500

wavelength (nm)

F i g u r e 5.

O p t i c a l absorption spectra of a t h i n f i l m of p r e c u r s o r PMCZM ( 1 ) and a f t e r exposure t o i o d i n e v a p o r ( 2 ) . A l s o shown i s t h e proposed scheme o f redox e l i m i n a t i o n y i e l d i n g a doped c o n j u g a t e d c o n d u c t i n g polymer.

J



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447

observations lead us to pursue the synthesis of conjugated polymers containing alternating aromatic and quinonoid sequences i n the main chain v i a redox elimination on precursors, such as the heterocyclic conjugated polymers of Figure 6. Extensive recent theoretical calculations predict that very narrow band gaps would be obtained i n conjugated polymers containing quinonoid sections (14-18). We recently reported the smallest bandgap i n organic polymers f o r such conjugated polymers (19). In this paper we present r e s u l t s on the polymer redox elimination reaction used i n the synthesis of the polymers i n Figure 6. Preliminary results on electrochemical redox elimination on precursor polymers are also presented. A mechanism of the polymer elimination reaction i s proposed. Related recent experimental observations at other laboratories that can be described within the framework of the scheme of Figure 4 are discussed. Experimental Precursor Polymers. The synthesis and characterization of the nonconjugated precursor polymers containing alternating heteroaromatic units and -C(R)H- i n the main chain are described elsewhere (20-21). These nonconjugated precursor polymers include polythiophenes, polypyrroles and polyfurans; however, only the studies done with some of the polythiophenes and their related conjugated derivatives are described i n d e t a i l here: poly(2,5thiophenediyl benzylidene) (PTB), poly[ a-(5,5'-bithiophenediyl) benzylidene] (PBTB), poly[ a-(5-5 -bithiophenediyl)pacetoxybenzylidene] (PBTAB), and poly[a-(5-5"-terthiophenediyl) benzylidene] (PTTB). These precursor polymers are soluble i n organic solvents including tetrahydrofuran (THF), methylene chloride and Ν,Ν-dimethylformamide (DMF). Thin films used i n spectroscopy and for c y c l i c voltammetry were cast from THF, methylene chloride or DMF. f

Elimination Procedures. Chemical redox-induced elimination was performed on precursor thin films by exposure to bromine or iodine vapor or by immersion of films i n hexane solutions of these halides. In order to follow progress of elimination, reactions were also performed on thin films i n a special sealed glass c e l l which permitted i n s i t u monitoring of the electronic or infrared spectra at room temperature (23°C). T y p i c a l l y , the infrared or electronic spectrum of the p r i s t i n e precursor polymer f i l m was obtained and then bromide vapor was introduced into the reaction v e s s e l . In s i t u FTIR spectra i n the 250-4000 cm"* region were recorded every 90 sec with a D i g i l a b Model FTS-14 spectrometer and o p t i c a l absorption spectra i n the 185-3200 nm (0.39-6.70 eV) range were recorded every 15 min with a Perkin-Elmer Model Lambda 9 UV-vis-NIR spectrophotometer. The reactions were continued u n t i l no v i s i b l e changes were detected i n the spectra. C y c l i c voltammetry was performed on precursor polymer thin films cast on platinum electrodes i n order to assess the p o s s i b i l i t y of electrochemical redox elimination and consequently as an alternative means of monitoring the process. A l l electrochemical experiments were performed i n a three-electrode, single-compartment c e l l using a double junction Ag/Ag (AgN03) reference electrode i n 0.1M +

American Chemical Society, Library 1155 16th St., N.W. In Chemical Reactions on Polymers; Benham, Judith L., et al.; Washington, OX. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

448


tetraethylammonium perchlorate (TEAP)/acetonitrile. The voltage regulator was a Princeton Applied Research Model 175 Universal programmer. Current measurement was done using a Princeton Applied Research Model 173 Potentiostat/Galvanostat i n conjunction with Princeton Applied Research Model 376 logarithmic current converter. The c y c l i c voltammograms were recorded on a HP Model 7040A X-Y recorder.


Results Figure 7 shows the FTIR spectra of i n i t i a l precursor PBTB f i l m (A) and at two subsequent times (B and C) during i n s i t u elimination reaction with bromine vapor at 23°C. Several main changes i n the infrared spectrum of the precursor f i l m are observed. The a l i p h a t i c C-H stretching v i b r a t i o n bands i n the 2800-3000 cm~l region attributed to the bridge sp3 C-H are s i g n i f i c a n t l y modified, generally decreasing with elimination reaction time and eventually disappearing. In contrast, the absorption bands i n the 3060-3250 cm-1 region attributed to the side chain phenyl and thiophene C-Hs remain r e l a t i v e l y the same. A new gas phase absorption band at 2400-2800 cm"l assigned to evolved HBr gas appears as seen i n Figure 7 (B and C). The gas phase absorption band attributed to HBr gas appeared i n the f i r s t spectrum a f t e r bromine was introduced into the reaction c e l l and i n t e n s i f i e d with reaction time. New absorption bands appeared and intensity of e x i s t i n g ones increased i n the carbon-carbon double bond absorption region, 1000-1600 c n r l . There was no evidence of a new band i n the 500-650 cm~l region that would be a t t r i b u t a b l e to a C-Br bond due to substitution or bromination reactions on the polymer chain. These IR spectra r e s u l t s c l e a r l y show that the bridge hydrogens (-C(R)H-) of PBTB are eliminated as HBr by reaction with bromine, y i e l d i n g the anticipated aromatic/ quinonoid conjugated polymers of Figure 6. The e l e c t r o n i c absorption spectra at d i f f e r e n t times of elimination reaction on PBTB are shown i n Figure 8. As-synthesized PBTB i s a green polymer (Curve 1, Figure 8) due to p a r t i a l conversion of the -C(R)H- to -C(R)= bridges i n the main chain (19-21). A f t e r 15 min. reaction time, the e l e c t r o n i c spectrum i s greatly red-shifted with the absorption edge now located at about 1200 nm (1.03 eV). Progress i n elimination further moves the absorption edge to longer wavelength as curves 2 to 13 i n Figure 8 show. No observable changes i n the e l e c t r o n i c absorption spectra were seen a f t e r 24 hours (Curve 13). V i s u a l l y , the sample changes from green (Curve 1) to m e t a l l i c gray i n c o l o r . The absorption edge determined from Curve 13 i s about 1500 nm (0.83 eV). This represents an extremely large increase i n the degree of ττ-electron d e l o c a l i z a t i o n . The r e l a t i v e l y sharp absorption edges are also noteworthy. Figure 9 shows the e l e c t r o n i c absorption spectrum of a PTTB f i l m which has undergone extensive but incomplete reaction with bromine i n a non-in-situ experiment. The absorption spectrum i s that expected f o r a one-dimensional conjugated polymer. The sharpest absorption edge i s a t about 1490 nm (o.83 eV) and the absorption maximum i s located at 1240 nm (1.0 eV). Thus, this material has a bandgap of about 0.83 eV. Note that two small


32.

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449

R


Ill—

Figure 6.

3600

Novel conjugated heteroaromatic by redox elimination.

3200

2800

2400

2000

1600

polymers synthesized

1200

800

400

Wavenumbers (cm ~ ) 1

Figure 7.

FTIR absorption spectra of precursor PBTB thin f i l m (A) and during i n - s i t u elimination (B and C) i n bromine vapor at 23°C. Arrow indicates a superposed gas phase absorption band due to evolved HBr gas.



450


ι

ι

600

1000

01 200

ι 1400

ι

ι

ι

1

1800

2200

2600

2800

Wavelength (nm)

F i g u r e 8.

E l e c t r o n i c a b s o r p t i o n s p e c t r a o f p r e c u r s o r PBTB t h i n f i l m ( 1 ) and c o n j u g a t e d d e r i v a t i v e s (2-13) a t d i f f e r e n t times d u r i n g i n - s i t u e l i m i n a t i o n r e a c t i o n a t 23°C.

1.50

Ο 0.0

'

200

'

'

1

1

600

'

1

'

1

'

1000

'

'

1

'

1400

'

L—J

1

ι i—• 1 ι ι ' 1

1800

2200

'I

2600

Wavelength (nm)

F i g u r e 9.

E l e c t r o n i c absorption spectra of a conjugated derivative of ΡΠΒ.


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451

absorptions can be seen below the gap at 1560 nm (0.79 eV) and 1950 nm (0.64 eV). The two absorptions below the bandgap may be interpreted as evidence of charged bipolarons (dications) (22). However, no such below bandgap absorptions are observed i n the completely reacted material. Some of the results of c y c l i c voltammetrie studies of a f i l m of PBTAB at various stages of redox elimination are shown i n Figures 10A and 10B. The c y c l i c voltammogram of Figure 10A shows the behavior of a precursor PBTAB f i l m on platinum electrode under repeated c y c l i n g at 200mV/s. I n i t i a l l y , an oxidation peak at about 0 V (vs. Ag/Ag ), a main reduction peak at -0.56 V and a small broad reduction peak centered at -0.15 V are observed. This i n i t i a l votammogram i s very unstable under c y c l i n g : ( i ) the redox peaks at -0.56 V and 0.0 V are decreasing; and ( i i ) evolving very broad peaks that form a redox couple are observed i n the range +0.14-0.21 and -0.09 to -0.23 V. After numerous c y c l i n g (-100) over a period of hours the r e s u l t i n g polymer f i l m was washed and i t s c y c l i c voltammogram was again obtained i n a fresh e l e c t r o l y t e . A reversible redox couple with oxidation peak at +0.21 V and reduction peak at -0.24 V was obtained as shown i n Figure 10B. This l a t e r c y c l i c voltammogram i s quite stable under repeated c y c l i n g and i s what one might expect for an electroactive conjugated polymer. These results suggest that the electrochemical redox reaction induces elimination of the -C(R)H- bridge hydrogens, y i e l d i n g -C(R)= bridges. Preliminary measurements of e l e c t r i c a l conductivity of the conjugated derivatives of PBTAB, PBTB and PTTB obtained by the above treatment with bromine vapor are poor semiconductors with a conductivity of the order 10"^S/cm which apparently i s not due to doping. Subsequent electrochemical or chemical doping of these polymers lead to 4-6 orders of magnitude increase i n conductivity. Ongoing studies of the e l e c t r i c a l properties of these conjugated polymers with alternating aromatic/quinonoid units w i l l be reported elsewhere. Similar redox elimination reactions have been performed on several heteroaromatic precursor polymers within the class shown i n Figure 4 and whose conjugated derivatives are shown i n Figure 6. In general, the r e s u l t s and observations are similar to those described here. The major differences observed are due to differences i n the heteroatom (X) while minor differences are observed with d i f f e r e n t R groups. For example, the smallest bandgaps were obtained when X=S i n the polymers of Figure 6. On the other hand, faster elimination k i n e t i c s were observed for aromatic R compared to a l i p h a t i c R side groups.


+

Discussion The electronic absorption spectra of Figures 8 and 9, and those not shown, reveal that the conjugated polymers of Figure 6 generally exhibit i n t r i n s i c semiconductor bandgaps that are very small. This i s i n accord with extensive recent theoretical calculations which predict that introduction of quinoidal geometry into the main chain of aromatic conjugated polymers (e.g. polythiophene, polypyrrole, and poly-p-phenylene) reduces the bandgap. E s p e c i a l l y relevant are the



452


F i g u r e 10A.

C y c l i c voltammogram o f a p r e c u r s o r PBTAB t h i n f i l m on P t e l e c t r o d e i n T E A P / a c e t o n i t r i l e a t a scan r a t e o f 200 mV/s.

F i g u r e 10B.

C y c l i c voltammogram o f t h e same f i l m i n F i g u r e 10A o b t a i n e d a f t e r washing and r u n n i n g i n a f r e s h TEAP/ a c e t o n i t r i l e a t a scan r a t e o f 200 mV/s.


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453


valence e f f e c t i v e hamiltonian (VEH) calculations of Bredas and co-workers (14-16) which are known to be very accurate i n reproducing experimental data for conjugated polymers. In p a r t i c u l a r , Bredas has shown that as quinonoid structure i s introduced into poly(2,5thiophene) geometry the bandgap decreases l i n e a r l y with increasing quinonoid character (14). More recently, the electronic band structures of the aromatic/quinonoid polymers of Figure 6 have been obtained using MNDO polymer geometry optimized t o t a l energy calculations (23). The theoretical energy bandgaps (23) were found to be i n agreement with the experimental results (19"T7 Mechanism of Redox Elimination. The proposed elimination reaction mechanism i s i l l u s t r a t e d with a methylene-bridged polybithiophene i n Figure 11. I f one considers attempts to oxidize the nonconjugated precursor polymer by electron withdrawal or reduce i t by electron addition, one finds that the added charge cannot be accommodated by d e r e a l i z a t i o n because of the sp3 hybridized carbon (-CH2-) bridges. Consequently, highly unstable charged polymeric chains are generated. Depending on the sign of charge added to the precursor chain, such a high energy unstable intermediate would be best s t a b i l i z e d by loss of HÎ or H" from adjacent bridges, y i e l d i n g the neutral conjugated polymer chain containing alternating aromatic and quinoidal units. This mechanism can be used to explain the electrochemical r e s u l t s . For example, the observed decreasing redox peaks i n the c y c l i c voltammogram of Figure 10A i s attributed to the consumption of the charged intermediates. The optical, absorption data of Figure 9 also appear to evidence bipolarons (dications) as intermediate and hence, consistent with this mechanism. The general scheme of Figure 11 i s f o r both electrochemical and chemical redox elimination. I f we consider the case of oxidative elimination with bromine, generation of the necessary r a d i c a l cation or d i c a t i o n intermediate i s due to: B ^ + 2e" >2Br". The bromide ions presumably then abstract protons from the ionized precursor. Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In p r i n c i p l e , i t should be possible to accomplish t h i s experimentally under s i m i l a r chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et a l (16), including: bandgap (1.17 eV); bandwidth (0.44 eV); ionization p o t e n t i a l (4.2 eV); electron a f f i n i t y (3.03 eV); oxidation potential (-0.20 vs SCE); reduction p o t e n t i a l (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). The two l i m i t i n g s t r u c t u r a l forms of p o l y a n i l i n e are shown i n Figure 12B: ( i ) the nonconjugated leuco base polymer i n which the imine nitrogens are completely protonated (PAN-1); and ( i i ) the neutral conjugated polymer containing alternating aromatic and quinoidal units (PAN-2). The highly conducting (10 S/cm) p o l y a n i l i n e chemically or electrochemically synthesized from a n i l i n e i s believed to be a p a r t i a l l y oxidized form with a structure intermediated between PAN-1 and PAN-2 (25-28). The complicated electrochemical behavior of polyaniline, especially i n aqueous media, i s a r e s u l t of redox reactions coupled with reversible deprOtonation/




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Figure 1 1 .

I l l u s t r a t i o n of the proposed mechanism of redoxinduced elimination.

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Synthesis of Conjugated Polymers via Polymer Elimination Reactions

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