Soluble polystyrene(PS)-polyparaphenylene(PPP) block copolymers

Synthetic Metals, 29 (1989) E35- E40

E35

SOLUBLE POLYSTYRENE(PS)-POLYPARAPHENYLENE(PPP) BLOCKCOPOLYMERS

Xing Fu ZHONG ,Bernard FRANCOIS* I n s t i t u t Charles Sadron (CRM - EAHP) (CNRS) , 6 rue Boussingault, 67083 Strasbourg C6dex (France)

ABSTRACT Soluble

block

copolymers polystyrene-polyparaphenylene (PS-PPP) have been

synthesized by aromatisation of PS

poly 1,3 cyclohexadiene block

(PS - PCHD). The PPP blocks are made of short sequences of

copolymers

about 10-11 units

separated by defects. Light

scattering

experiments

show that

these

copolymers are

generally

aggregated in solution. Continuous films of some microns thickness can be cast from these solutions. thermal

treatment

in

inert

atmosphere or

A

under vacuum eliminates the PS

sequence by pyrolysis ,leaving a pure PPP film. Preliminary doping experiments of these materials are reported.

INTRODUCTION Polyparaphenylene is a thermally

stable polymer which was found to

be an

interesting conducting polymer. Several synthesis have been described ,

by

chemical (i-4) or by electrochemical (s'6"7) polymerization processes. However these d i f f e r e n t methods lead to handling as well

as

their

insoluble and infusible polymers sothat their characterization

at

molecular

level is rather

difficult. Other methods proceed via

the chemical

precursor polymers such as diesters derivatives. (I°) A solution

for increasing

copolymers with 0379-6779/89/$3.50

or thermal aromatization of

polycyclohexa

I-3

the PPP s o l u b i l i t y is

f l e x i b l e soluble

soluble

diene(B'9)(PCHD)) or i t s 5-6 to prepare block or

sequences weakening the strong

graft

interaction

© Elsevier Sequoia/Printed in The Netherlands

g36 beetween r i g i d

PPP chains.

polystyrene(PS)-PPP block

We describe

h e r e the

copolymer prepared

polystyrene-polycyclohexa I-3

synthesis of a soluble

by

aromatization

diene copolymer (PS-PCHD)

of

a

. A further thermal

treatment decomposes the PS sequence leaving pure PPP films or powders.

EXPERIMENTAL PART: A - Synthesis of PS-PCHD block copolymer The PS-PCHD block copolymer was prepared by anionic polymerization under vacuum, using the classical the PCHD sequence to through

a

I-4

break-seal technique.The further aromatization of

PPP needs than the

addition.

It

has been shown( I I )

quantitatively observed when the organocompound in non polar the

styrene

introduced The

When this

CHD takes

place

that such an addition is

polymerisation

is

medium. Sec Buli was

polymerisation.

i n i t i a t e d by a lithium

used for the

polymerization

is

i n i t i a t i o n of over,

CHD is

and the copolymerization takes place.

length of

monomer as

polymerization of

PCHDsequences is limited

previously reported

(9)

by tranfer

of active sites to the

But we observed moreover that the new

organometallic compound formed in that reaction

is rapidly decomposed ,for a

part, and r e i n i t i a t e s new chains , for an other part. These reactions lead to a mixture

of

copolymer and homopolymer. The yield of homopolymer depends on

monomer and polystyryllithium concentration . I t can be decreased by increasing the aromaticity of the solvent and the butyllithium purity or by decreasing the temperature

of

polymerisation.

These results

will

be described in details

elsewhere. The exclusive I-4 addition of CHD in the PCHDsequences was verified by NMR and Infrared spectroscopy (12) B- Aromatization of PS-PCHD copolymer to PS-PPP The

choosen

described by about 4

method

for

aromatization of

PCHD sequences

Marvel(8)for homopol~nners PCHD. In

u n i t s were

obtained. I t

is based

t h i s case

on the

was previously PPP sequences of

reaction with c h l o r a n i l at

temperatures ranging from 120 to 150 C

. A complete study of the influence

solvent

concentration

,temperature

,and

reactants

on

the

lenght

of

of PPP

sequences is c u r r e n t l y under i n v e s t i g a t i o n . We v e r i f i e d that the polystyrene is not modified under these experimental conditions. The obtained

yellow

chloroform ) give

powders

,when dissolved

in

a

solvent (THF, toluene,

red brown solutions from which f i l m s were prepared.

F,,37

RESULTS A- Characterisation of CODOlvmers - Length of PPP sequences in PS-PPP copolymers The existence of PPP sequence is demonstrated by the uv/vis spectrum of copolymers in solution or film : a typical spectrum is represented on fig 3. It presents a wide band the maximum of which is located at 331 nm. It is well known that the absorption band shifts towards higher wavelength when the length of the PPP sequence increases. By interpolation of published spectroscopic data the mean length of PPP sequences in this copolymer is found to be about 10-11. As the original PCHDblocks are much longer, i t is clear that the PPP sequences are separated by defects. - Molecular weight of copolymers We report here the study of two PS-PPPcopolymers prepared from PS-PCHD copolymers with similar PS sequences (Mw =25000 and 30000) but two different PCHD sequences (3000 and 10000): samples I and 2. The sample I was studied in THF solution by Gel Permeation Chromatography (GPC) with an apparatus equipped with refractive index and light scattering detectors. This PS-PPPcopolymer has a maximum absorption at 320 nm. It presents in GPC two peaks with similar areas: the f i r s t one corresponds practically to the molecular weight (25000) of the precursor copolymer but the second one corresponds to very high molecular weights , confirmed by the light scattering data: a value Mw = 10 IO6 was estimated with a ratio Mw/Mn = 1.3. It is partially aggregated. A partial sample.

reversibility

of

the

aggregation process was observed with this

The sample 2 was analysed by light scattering in THF solution. The maximumof absorption was in that case 331 nm indicating longer PPP sequences that in the former sample. A classical Zimm's plot is reported in fig I. I

"

I

J

-

(Kc/')',oE~7~A~~ , 9 ~

/

Figure1 :

~

Zimm's plot of light scattering by THF solutions of PS-PPP copolymer 2

4.4

/

3.g

2)+K c _ _

A

i

i

J . ,

E38

A mean radius of gyration of Such

a large

value

about 400 Angstroems is deduced from these data.

corresponds to

approximative value MW = 2 106 mean dn/dc = 0.2 A study

of

a strongly

aggregated polymer. An

is calculated for these aggregates (assuming a

).

the

variation

increasing d i l u t i o n p r a c t i c a l l y linear

of

of

the

light

scattering

a copolymer solution

variation

in

at gO°C angle for an

THF was carried

was observed .The size

out. A

of aggregates does not

vary appreciably with d i l u t i o n . No micellar c r i t i c a l concentration is

visible

in this case . A small depolarization factor of the scattered l i g h t was measured indicating a weak optical anisotropy

of particles.

The aggregates are then

basically

spherical. This conclusion was confirmed by scanning electronic microscopy. A dispersion

of

solution

Trichorobenzene is

in

these

aggregates was observed by l i g h t scattering when a heated above

I00°C.

After

cooling

a

reagglomeration takes place which is then irreversible up to I20°C. These two examples show that PPP in solvents

a molecular dispersion of block copolymers PS-

at room temperature can be achieved only for very

short PPP

blocks. Nevertheless stable aggregated solutions of sample 2 up to 20 % were prepared in THF. This aggregation explains the

lack

of PPP signals which has been observed in

the NMR spectrum of PS-PPP copolymers(11). Indeed the mobility of PPP chains in the core of these aggregates is probably restricted to such an extent that they can escape detection

in

high

resolution

NMR. Only the

spectrum of the

polystyrene shell can be obtained. - Thermal s t a b i l i t y of

PS-PPP

The thermogravimetric analysis of

PS-PPPcopolymers shows a rapid

weight loss

at 400°C ,due to polystyrene decomposition. At 500°C under argon atmosphere i t remains,

for

example, about 32% of

containing 38% of PPP . The residue at

the

initial

weight

for

a copolymer

800°C accounts for 19% of the i n i t i a l

weight. This result shows the formation of heat-resisting PPP and is in good agreement with the copolymer composition deduced from others methods. - Preparation of pure PPP films from PS-PPP copolymer Films were cast on quartz plate or K Br crystals from PS-PPP solutions. They were heated at 420"C for some minutes in nitrogen atmosphere. An I.R spectrum of such a f i l m ,before and after heating, is reported in f i g 2 .

E39

4000

3500

3000

2000

2500

1500

1000

CM"

Figure 2 : Infra red spectra of PS-PCHD (I) , of PS-PPP (2) (prepared aromatisation of I)

by

and of PPP (3) (partial pyrolysis of 2)

The polystyrene sequence is completely pyrolysed. The resulting film exhibit a UV/Visible spectrum

band p r a c t i c a l l y is

similar

to

identical the

to

published

that

of

the copolymer. Its I.R.

spectrum of Kovacic's PPP with an

important peak at 808 cm-I due to para disubstituted phenyls.A small band at 764 cm-z

is due to monosubstituted

phenyls .

Some reticulations may explain

the bands at 1602 and 860 nm. ( f i g 2 )

B- Doping of PS-PPP copolymer in solution and of PPP films A solution

of PS-PPP (sample 2)

in THF was reacted with potassium metal

mirror in sealed glass vessel equipped with an optical color of the solution became dark

cell . The red-brown

blue. The change of the absorption

spectrum

is reported on f i g 3 . The original band at 331nm vanished progressively and a new band grew at

about 1300-1400 nm on the beginning

of the

reaction. The

maximum of this band gradually shifted to lower wavelenght as the reduction yield increases : the longer PPP sequences are f i r s t l y doped . Pressed PS-PPP pellets

were heated at

420°C and doped by naphthalene K

solutions in THF. A conductivity of about expanded K doped PPP pellets.

10-2 S/cm was measured on these

E40

Figure 3: UV/Vis/NIR spectra of PS -PPP copolymer reduced in THF solution by potassium

0.5

500

1000

1500

Conclusion

Soluble polystyrene - polyparaphenylene copolymer were synthesized and doped Pure PPP films can be prepared by partial

pyrolysis of

these copolymers

Improvements of the synthetic process are in progress. ACKNOWLEDGEMENTS We are indebted to

Dr G.Froyer and the C.N.E.T. for

their scientific and

financial supports. REFERENCES 1- M. Ivory, G.G.Miller, J.M.Sowa, L.W.Shacklette, R.R.Chance and R.H.Baughman J. Chem.Phys. 7 (1979) 1506 2- P.Kovacic and J.Oziomek Macromol.Synth. 2(1966) 23 3- T.Yamamoto and A.Yamamoto Chem. Lett. (1977) 353 4- A.A.Berlin ,V.I.Liogon'kii et V.P.Parini J.Polymer Sc.55,675 (1961) 5- J.F.Fauvarque, M.A.Petit, F.Pfuger, A.Jutand, C.Chevrot Makromol.Chem.Rapid Comm. 4 (1983) 455 6- Rubinstein I J.Electrochem.Soc. 130 (1983) 1506 7- T.Ohsawa, T.Inoue, S.Takeda, K.Kaneto, K.Yoshino Pol.Com.27 (1986) 246 8- C.S.Marvel G.E.Hartzell J.A.C.S. 81 (1959) 448 9- G.Lefebvre and F.Dawans J.of Pol.Sc. part A 2 (1964) 3277 G.Lefebvre ,F.Davans J.of Pol.Sc part A 2 3277 (1982) 10-D.G.H.Ballard , A.Courtis ,I.M.Shirley, S.C.Taylor

J.Chem.Soc.Chem.Comm; (1983) 954 ll-Z.Sharaby ,M.Martan, J.Jagur-Grodinski Macromolecules 15 (1982) 12-X.F.Zhong ,B.Francois Makromol.Chem.Rap.Com. (in press)