Functionalization of poly(p-phenylenevinylene) polymers with pendant nitroxide groups

Synthetic

ELSEVIER

Metals

84 (1997)

Functionalization of poly@-phenylenevinylene)

333-334

polymers with pendant nitroxide groups

Ahmed Iraqi,* Andrew Mlynski, Ashley Heard, John C. Walton and Joe A. Crayston School of Chemistry, Purdie Building, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9ST, U.K.

Abstract In the search for novel organic ferromagnetic polymers we devised synthetic routes to poly-p-phenylenevinylenes with nitroxide functional groups. Reaction of the Grignard reagent derived from the ROC6H4(CH&Br precursor (R=CH3, n=4; R=nC6Hl3, n=6) with CO2 gave an acid which was chloromethylated and subsequently coupled to the paramagnetic TEMPOL group. The paramagnetic monomers could be readily converted to the diamagnetic h*ydroxylamine salts to permit full NMR characterization. The paramagnetic monomers were polymerized via the KOtBu route to the corresponding PPV. Keywords: conjugated 1.

Poly (phenylene polymers.

vinylene)

and derivatives,

Coupling

reactions,

Magnetic

films,

Special-purpose

functionalized

Introduction Poly (p-phenylene vinylenes), PPVs, have had considerable impact in device applications such as light emitting diodes[7]. however, there are no reports on PPV functionalization with redox active groups. This prompted us to investigate the molecules whose synthesis is outlined in the scheme below. Functionalization of PPVs with paramagnetic and redox active centres could lead to polymers with promise as magnetic thin films in data storage devices and electroactive coatings in chemical sensors.

The synthesis of new polythiophene derivatives with improved conductivity and processing properties is an important part of conducting polymer research[l,2]. Notable recent advances include the regioregular synthesis of polyalkylthiophenes[3]. We have recently used this strategy ourselves in the synthesis of a regio-regular aryloxyalkylthiophene[4]. Another goal is the addition of paramagnetic or redox-active groups to the polymer backbone that could give properties such as ferromagnetism[5,6].

OH

OR RBr

Br(CH&Br

~ 6

W$$

CH201HCl

~

KJxo3

OH

OH

WH 2

1

Jp WH

>

2)nm

2H

WH

5

R = -CH,, n = 4. R = -C6H13, n =6. Present

address:

School of Physics 4YB, England, U.K.

and

Chemistry,

University

of Lancaster,

0379-6779/97/$17.00 Q 1997 Elsevier Science S.A. All rights reserved PII SO379-6779(96)04024-6

OW

2)&O

2H

3

J$ c,Jyc~KJiiis~

4

Lancaster,LA1

2hBr

dnF=o

*

or KOtBu

O(CH2>nF=O

A.

334

2.

Iraqi et al. /SyntheticMetals 84 (1997) 333-334

Experimental

Details of syntheseswill be given elsewhere[8]. Only coupling of the nitroxide group to form the hexyloxy monomer5 and its polymerizationare given here. Preparation

of nitroxide

monomer 5 (R=n-he&,

n=6)

The acid derivative 4 (3.0 g, 7 mmol) was refluxed at 65°C for 1.5 h in excessoxalyl chloride(40 cm3). The excess oxalyl chloride was then removedto leave a brown oil. This was dissolvedin CHxC12(20 cm3) and addeddropwiseto an ice-cold solution of excess triethylamine (4 cm3) and 4hydroxy-2,2,6,6-tetramethylpiperidinyl-oxy (TEMPOL, Aldrich) (1.23 g, 7 mmol) in CH&lz (25 cm3)over a periodof 1 h. The mixture was allowedto stir at room temperaturefor 3h, washedwith very dilute HCl(aq) andwater (2 x 40 cmm3), then dried over MgSO+ The CH2C12 wasremovedto give an orange-brown oil which was washed with pet. ether and diethylether to remove excessTEMPOL. It was then dried undervacuumto a tacky, orange-brownsolid(3.41 g, 83%). Preparation

of tosylate derivative of 5 (R=n-hexyl,

n=6)

The nitroxide 5 (lg, 7 mmol) was dissolvedin 10 cm3 CH$Zlz and addeddropwiseto a solutionof p-toluenesulfonic acid (0.33g, 1.7 mmol) and EtOH (0.25 cm3)in CH2C12(15 cm3) over 1 h, andthen allowedto stir for 2.5 h. The reaction mixture wasthen washedwith water (40 cm3) and dried over MgS04, and the CH&lz removedto yield the sticky brown tosylate salt (1.29 g, 98%). ‘H NMR (aromatic region): 7.65 (2H, d); 7.10 (2H, d); 6.95 (lH, s); 6.80 (lH, s). Polymerization

of 5 (R=n-hexyl,

n=6)

To 5.8 cm3of a0.3 mol dms3THF solutionof 5 (1.28 g, 1.7mmol) wasaddedan equalvolumeof 0.3 mol dms3KO’Bu in THF over 10 mins. Then a further 1.5 aliquotsof KO’Bu wereaddedandthe mixture stirredfor 2 h. The precipitatefrom the reactonmixture wasfiltered off and the THF wasremoved to give a sticky orange-brownsolid. This was dissolvedin CHzC12, precipitatedwith diethylether,redissolvedin CH2C12 washedwith water and then dried over MgS04 to give the polymer (0.42 g). For ‘H NMR characterization the diamagnetictosylate salt of the polymer was prepared in a similarway to that given above for the monomer. 3 . Results

and

Discussion

This route relies on the synthesisof a 4-alkoxyphenol 1 followed by its reaction with 1,4-dibromobutaneor 1,6dibromohexaneto give the bromide 2 which upon reaction with magnesium,then CO,, gives 3 with a carboxylic acid functionality. The acid 3 is then chloromethylatedto give 4 which is involved in an esterification reaction to give the desiredmonomer5. Our efforts were focusedon a polymer having reasonable solubility in organic solvents (for applications as film coatings on electrodes). At first a methoxy functionalised polymerwith an n-butyl spacerwasdesigned(R = CH3, n = 4), since the starting material 1 is commercially available.

Intermediates up to monomer 5 were made and fully characterized. Characterization of paramagneticnitroxide 5 was carried out using p-toluenesulfonicacid and ethanol to produce the equivalent hydroxylamine salt, which was diamagneticand characterizedby NMR spectroscopy.The reaction involves the disproportionation of two nitroxide groups in acidic conditions, one giving the hydroxylamine salt andthe other t’r:eoxoammoniumsalt. The latter is reduced to the hydroxylamine salt after oxidation of ethanol to acetaldehyde.Polymerizationof 5 waseffectedby the KOtBu route, reacting the monomerwith excessbase.However, it wasdifficult to characterizethe polymer sinceit wasinsoluble in most commonorganicsolvents. We then changedR to a hexyl group and increasedthe length of the spacerto the nitroxide substituent(n = 6 instead of 4) with the aim of improving solubility of the final polymer. However, the key 4-hexyloxyphenol starting material cannot readily be preparedfrom hydroquinone as invariably two alkoxy substituentsresult even when a large excessof hydroquinoneis used.We have optimized the yield of the desired product as described elsewhere[8].All the intermediateswere successfullypreparedup to monomer5. The ‘H NMR spectrumof the tosylate salt derived from this material showed the presence of the TOS protons and inequivalent aromatic protons. A small peak in the vinylic region (5.1 ppm) suggested that a small degree of polymerization had already occurred. Polymerization was carried out by reaction with excessKOtBu. The ‘H NMR spectrumof the tosylate salt of the polymer indicated little intensity in the vinylic region and a singlet at 4.65 ppm (>CHCl) suggestingthat only incompletepolymerization to the unconjugatedpolymer had occurred. However, full NMR characterizationof the polymerwasdifficult and morework is neededto elucidateits structure. Magnetic propertiesof the polymerswill be presentedin a future publication[8]. 4.

Conclusions

We have preparedparamagneticpolymer 5 (R=CH,, n=4) which has a low solubility and is difficult to characterize. New polymers with longer spacergroups and solubilizing hexyloxy groupshave now been synthesized. 5.

Acknowledgements

We thank EPSRC(U.K.) for the award of a post-doctoral fellowshipto Dr A. Iraqi. 6.

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