Chapter 24
Polybis(pyrrolyl)phosphazene R. C. Haddon , S. V. Chichester , and T. N. Bowmer 1
1
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AT&T Bell Laboratories, Murray Hill, NJ 07974-2070 Bell Communications Research, Inc., Red Bank, NJ 07701-7020 1
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Current progress in the synthesis and properties of pyrrolylphosphazenes is summarized. The differences in reactivity of the cyclic trimer (NPCl ) , and high polymer (NPCl ) , toward the pyrrolide nucleophile are discussed. Efforts to induce electronic conductivity in the polyphosphazenes are reviewed with particular emphasis on polybis(pyrrolyl)phosphazene. 2 3
2 x
It is clear from the contents of this volume that polyphosphazene chemistry is currently receiving a great deal of attention. Commercialization of these materials is still in its infancy but appears to hold significant potential. Much of the current level of interest in this field stems from the synthetic versatility of the phosphazene polymers, as developed by Allcock and coworkers. " As a result of this, the preparative chemistry of the polyphosphazenes is the most highly developed of all the inorganic polymer systems. This has lead to the synthesis of a wide variety of substituted polyphosphazenes, " many of which were specifically designed for a given chemical, mechanical or biological property. Noticeably absent from the properties exhibited by the polyphosphazenes has been electronic conductivity and in general the materials are insulators. It should be noted, however, that suitably functionalized polyphosphazenes have recently been shown to serve as electrolytes for ionic conductivity. 1
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The question of electronic conductivity in the polyphosphazenes inevitably raises questions regarding the electronic structure of the phosphazene linkage. " This matter has been the subject of controversy in the literature, but experimentally the situation is now well known. In spite of the fact that the phosphazene backbone is fully conjugated, bond equalized and possesses bond lengths which are indicative of partial double bond character, the evidence suggests that these are localized systems. 7
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0097-6156/88/0360-0296S06.00/0 © 1988 American Chemical Society
24. HADDON ET AL.
Poly bis (pyrrolyl)phosphazene
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The possibility of inducing electronic conductivity within the P-N linkage is being actively pursued within a number of research groups, ' but the phosphazenes themselves present a daunting challenge (in spite of their proximity to polysulfurnitride in the Periodic Table). 4,11 14,15
An alternative approach to electronic conductivity in the polyphosphazenes, utilizes the 'outrigger approach' in which the appropriate functionality for electronic mobility is provided by the substituents. A successful implementation of this methology which made use of polyphosphazene bound copper phthalocyanine units was recently reported by Allcock and Neenan. Iodine doping of these materials led to conductivities in the semiconductor range. It is also possible to prepare semiconducting tetracyanoquinodimethane (TCNQ) salts, in which quaternized polyphosphazenes serve as counterions. In a variant of the outrigger approach we have synthesized polybis (pyrrolyl) phosphazene (PBPP); this new polymer undergoes electrochemical oxidation to produce conductingfilmsat the anode. 4
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Apart from the ability of PBPP to undergo electroxidation, this polymer is significantly different from previously reported polyphosphazenes, both in preparation and in properties. Although a complete picture of the chemistry of this polymer has yet to emerge, our current progress will be reviewed in the present article. Preparation of Pyrrolylphosphazenes The first pyrrolylphosphazenes were apparently prepared by McBee and coworkers in 1960, and although the work was never documented in the chemical literature, hexakis- (pyrrolyl) cyclotriphosphazene (1) and octakis(pyrrolyl)cyclotetraphosphazene were described in a Technical Report of the Defense Technical Information Center. Compound 1 was reported to be produced in 26% yield from the interaction of hexachlorocyclotriphosphazene [(NPC^ ) ] with excess potassium pyrrolide in refluxing benzene over a 24 hour period. Lithium pyrrolide and pyrrolyl magnesium bromide were found to be unsatisfactory reagents for the preparation of 1. 19
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The yield of the potassium pyrrolide reaction is considerably improved by the use of a phase transfer catalyst. However, in exploratory work on this substitution reaction we have found that sodium pyrrolide reacts with (NPC^ ) in tetrahydrofuran at room temperature to produce 1 in quantitative yield in less than 3 hours. The structure of the compound is reproduced in Figure 1. 20
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In general, the small molecule substitution chemistry provides an excellent model for the analogous high-polymer synthesis. However, in attempting to extrapolate from the reaction of pyrrolyl salts with (NPC£ ) to the corresponding reaction with the high polymer (NPC^ ) , we discovered significant discrepancies in the two reaction pathways. High yields of 1 may be isolated from the reaction of (NPC/ ) with all of the alkali metal pyrrolide salts which were tested, except lithium. In the case of the polymer (NPC^ ) , 4,13
2
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3
X
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X
INORGANIC AND ORGANOMETALLIC POLYMERS
298
however, we found that some of the alkali metal pyrrolides brought about substantial chain cleavage and occasionally produced the cyclic trimer (1) in high yield. Furthermore there is apparently a facile cross-linking reaction available to PBPP, as a substantial fraction of the isolated polymer is often insoluble (although chemical analyses indicate complete replacement of chloride). There is little mention in the literature of the use of amide salts in substitution reactions on chlorophosphazene precursors. The anilide anion was shown to be a powerful nucleophile in substitution reactions on various trimer derivatives, but investigations of such reactions with the high polymer have not been reported. Where strong nucleophiles (such as amide salts) with low steric requirements are employed, the usual pentacoordinate transition state (Scheme 1), may be a viable reaction intermediate which can undergo alternative modes of decomposition, perhaps involving chain cleavage and/or cross-linking. 22
I
: pI
+
2X
ο ο
ο
ι
+
2x K C «
M)
sl = P —
ό (PBPP)
Nevertheless, through scrupulous purification of the reaction components and rigorous control of the reaction conditions it is possible to isolate the polymer in a state of good purity, by the reaction of potassium pyrrolide with (NPC/ ) in tetrahydrofuran at room temperature (Equation 1). Addition of water to the reaction mixture precipitates the polymer as a white rubbery solid which hardens on drying. A P N M R spectrum of a typical reaction product is given in Figure 2. 2
X
31
Properties of Poly bis (pyrroly I)phosphazene (PBPP) The solubility properties of the polymer are very sensitive to the mode of isolation. PBPP is partially soluble in tetrahydrofuran immediately after precipitation from the reaction mixture, but when the polymer has been dried the solubility is less predictable. 21
24. HADDON ET AL.
Figure 1.
Poly bis (pyrrolyl)phosphazene
Structure of crystalline [ Ν Ρ Ο Κ ^ Η ^ ^ (1)
Scheme 1.
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INORGANIC AND ORGANOMETALLIC POLYMERS
300
31
NP Ν
P
NP Ν
-30 8 (PPM, EXT H P 0 ) 3
Figure 2.
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4
P NMR spectrum of reaction to produce PBPP.
Brittle colorless films of PBPP may be cast from tetrahydrofuran solution. The insoluble portion of PBPP is swelled by the tetrahydrofuran and gives rise to free-standing films on solvent evaporation. Differential scanning calorimetry experiments on PBPP show a glass transition temperature at 40 °C, and some indication of a melting transition at 170°C. PBPP undergoes electrochemical oxidation to produce black films with conductivities in the semiconductor range. More highly conducting materials are produced by heating PBPP in an inert atmosphere (without doping). As may be seen from the thermogravimetric analysis in Fig. 3, PBPP exhibits some resistance toward the thermal depolymerization process which is characteristic of many polyphosphazenes. The conductivity and thermal involatility probably arise from the tendency of PBPP to undergo cross-linking as exemplified in Scheme 2. The structure found for the trimer (Fig. l ) , suggests that these should be viable processes in the polymer. 17
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Future Prospects Although the picture is far from complete, the available evidence suggests that PBPP is rather different from most polyphosphazenes. The polymer may be induced to be an electronic conductor, but perhaps as a result of this tendency to cross-link, the material is more sensitive and difficult to handle than most polyphosphazenes and the thermal depolymerization reaction is inhibited. Nevertheless the synthetic versatility offered by the polyphosphazenes
24. HADDON ET AL.
Polybis(pyrrolyl)phosphazene
Scheme 2.
301
INORGANIC AND ORGANOMETALLIC POLYMERS
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suggests that PBPP and the phthalocyanine derivatives represent the forerunners of a new class of polymers with interesting electronic solid state properties. 18
LITERATURE 1. 2. 3. 4. 5.
CITED
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