A general synthetic route to high molecular weight poly(p-xylylene)-derivatives: a new route to poly(p-phenylene vinylene)

ELSEVIER

Synthetic

Metals

69 (1995) 509-510

A general synthetic route to high molecular weight poly(p-xylylene)-derivatives

a new route to poly(p-phenylene F. Louwet, D. Vandetzande, Laboratorium

for Organic Chemisty, Limburg Division chemistry, Universitaire

:

vinylene)

J. Gelan

University, Institute for Material Research, Campus, B-3590 Diepenbeek.

Abstract A new synthetic route to high molecular weight poly(p-xylylene)-derivatives & has been investigated. If the substituents on the ethylene segment could be eliminated &&) this method provides a new route to PPV from a precursor polymer that is amenable to conventional non-aqueous processing techniques. The effect of basic reagents on the high molecular weight 4 has been explained as a degradation mechanism. Whether this polymerisation occurs via a free radical ionic mechanism is still under discussion.

1. INTRODUCTION Poly(p-phenylene vinylene) (PPV), which use in optoelectronic applications shows an increasing interest, can be obtained as a processible high quality film by the Wessling process [1] : this method involves the heating of a high molecular weight water soluble precursor polyelectrolyte, which is the result of the treatment of a,a’-bis-(tetrahydrothiophenium chloride)-p-xylene with sodiumhydroxide. In this paper we will show a new route to high molecular weight, non-ionic poly(p-xylylene)-derivatives 4 which can be converted to PPV on heating. 2. RESULTS

AND

DISCUSSION

2.1.

General scheme Recently our research group has shown a novel route (scheme 1) to stable non-aqueous processible precursor polymers for PPV [2]. The general idea of this new route is the base-induced polymerisation of p-xylene-derivatives 1 which have to fulfill two requirements : (i) in the a-position of the pxylene there’s need for a polarizer, which can stabilize the anion formed on the acid-base equilibrium, and (ii) a leavinggroup in the a’-position is necessary to obtain the pquinodimethane-intermediate 1, which is the actual monomer.

M+-/

po1

00 :,+=I

_-L;_

‘L

I I

I

0.

-L

r

2

la Pol = SO-Ph If? Pol = SO-nBu L=Cl Scheme

k u

3

Pol = SO*-Ph Pol = SO*-nBu

1. General scheme

0379-6179/95/$09.50 6 1995 Elsevier Science S.A. Ail rights reserved 0379-6779(94)02548-D

SSDI

Polymerisation The monomers were the a-chloro-a’-alkyl(aryl)sulfinyl(&&Q and a-chloro-a’-alkyl(aryl)sulfonyl-p-xylenes (m. Early results showed that treatment of h with NaH in tetrahydrofuran (THF) gave only low molecular weight oligomers [2]. However if the polymerisation solvent is changed to a more polar, aprotic solvent the molecular weight increases tremendously. The poly(p-xylylene)-derivatives 4, described in this study, were obtained by the polymerisation of 1 in N-methyl-pyrrolidinone (NMP) or dimethylformamide (DMF) with NaH at -20°C. All solutions were thoroughly flushed with nitrogen. The base was added at one go under a constant flow of dry nitrogen while a vigorous stirring was maintained. All reactions were terminated after Ih by pourring the reaction mixture into water whereupon an aqueous solution of HCl was added until a pH=7 was reached. The polymer precipitated and was filtered. GPC characterization was performed in NMP against polystyrene standards. Figure 1 shows a typical GPC trace from the polymerisation of u in DMF. A large amount of polymers with lower molecular weight values and oligomers were formed during the reaction. 2.2.

PO1

PO1

or an

PPV

510

F. Louwet

et al. I Synthetic Metals 69 (1995) 509-510

The possible mechanism for such a degradation process is depicted in scheme 2. It contains the same two steps as during : a proton abstraction and a 1,6-elimithe polymerisation nation. Due to the fact that the polarizer remains within the polymer, an anion can be formed within the polymer chain. Subsequently, a 1,6-elimination results in a quinoid structure at the end of the chain. In the next step an anion, probably the anion of the monomer or the Cl--1eavinggroep or the anion of the solvent, attacks this quinoid structure and hence the polymer is cutted into two pieces.

** ii

0

A_.2

4

6

8

10

11

elutievolume(ml)

Figure

1. GPC from the polymerisation of u in DMF at -20°C with NaH after lh (*Mn=480,000; Mw=3,000,000 and **Mn=2,400; Mw=4,200)

This phenomenon has also been observed for the polymerisation of la_c. By following the molecular weight with reaction time and with conversion, calculated from the chloride concentration, we learned that this lower molecular weight fraction is formed after the formation of the high molecular weight polymers. By precipitation of a CHC13solution of this crude polymer mixture into THF/diethylether (50/50), only the high molecular weight polymer can be recovered. TGA-experiments and FT-IR characterization [2] showed that, if 4a_b are heated under vacuum trans-PPV is formed.

+yyq)q__

-

2.3.

Polymer

\/

1.

Nicdegradation

11 Figure 2 shows GPC chromatograms of purified high molecular weight 4 which was treated with NaH in NMP at -20°C. After 1h molecular weights decreased tremendously and after 24h no high molecular weight polymer could be recovered. These results indicate the degradation of 4 in basic conditions.

Scheme 2. Degradation 3.

: a chain scission mechanism

CONCLUSIONS

We described a new route to high molecular weight poly(pxylylene)-derivatives. With the proper substituens on the ethylene segment, i.e. sulfinyl-groups, PPV is formed on heating. The treatment of the high molecular weight polymers with basic reagents results in the degradation (chain scission) of the polymer. 4. REFERENCES [I]

b

0

2

4

6

elutievolume

8

10

12

R.A. Wessling, J. Polym. SC., Polym. Symp., 72 (1985) 55. [2] F. Louwet, D. Vanderzande and J. Gelan, Synt. Met., 52 (1992) 125.

(ml) ACKNOWLEDGEMENTS

Figure 2. GPC from & (*Mp=470,000) reacted with NaH in NMP at -20°C after lh (**Mp=%,OOO) and after 24h (***Mp=2,300).

This work was supported

by a grant from I.W.O.N.L..