Phosphazene Elastomers - American Chemical Society


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9 Phosphazene Elastomers

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Synthesis, Properties, and Applications

ROBERT E. SINGLER and GARY L. HAGNAUER Polymer Research Division, Army Materials and Mechanics Research Center, Watertown, MA 02172 RICHARD W. SICKA Firestone Tire and Rubber Company, Central Research Laboratories, Akron, OH 44317 The synthesis of a unique class of polymers with a phos­ phorus-nitrogen backbone is described, focusing on poly(dichlorophosphazene) and poly(organophosphazene) elastomers. Melt and solution polymerization techniques will be illustrated while briefly indicating the role of catalysts which give significantly improved rates of conversion and reproducibility in polymer properties. The elucidation of chain structure, molecular weight and polymer yield by various dilute solution techniques will be considered. Poly(dichlorophosphazene) is the common, though hydrolytically sensitive, precursor of a large number of poly(organophosphazene) polymers. Examples of tech­ nologically significant elastomers are shown which are obtained through the use of selected side group substituents attached to the phosphorus-nitrogen backbone. This article concludes with a brief mention of alternate synthetic methods which may lead to useful phosphazene polymers. The study of open-chain polyphosphazenes has drawn increasing attention during the past several years (1-4) • The polyphosphazenes are highly f l e x i b l e chains of alternating phosphorusnitrogen (P-N) atoms with two substituents attached to phosphorus. Many d i f f e r e n t polyphosphazenes with a wide range of bulk properties and s o l u b i l i t i e s have been prepared by varying the type of substituent attached to the P-N backbone. Interest stems from the greater control achieved i n the polymerization processes and appreciation of the technological potential of these polymers. This paper updates developments of the past several years (4) with an emphasis on the polymerization process and technological developments of the elastomers. F i n a l l y , a b r i e f mention w i l l be made of related polyphosphazenes which currently are a t t r a c t i n g interest• NOTE: This chapter is Part II in a series. 0097-6156/84/0260-0143$06.00/0 © 1984 A m e r i c a n C h e m i c a l Society

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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POLYMERS FOR FIBERS A N D

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The Polymerization

ELASTOMERS

Process

The most f a c i l e route (Figure 1) for the preparation of the phosphazene polymers has been the ring opening polymerization of hexachlorocyclotriphosphazene (I) to give open-chain poly(dichlorophosphazene) ( I I ) , followed by direct replacement of the chlorine atoms with organonucleophiles to give h y d r o l y t i c a l l y stable polymers (IV, V, VI). It i s now well recognized (3) that care must be taken i n the p u r i f i c a t i o n of I and i n monitoring of the polymerization process i n order to avoid the formation of extensive branching and crosslinking ( I I I ) i n favor of soluble polymer ( I I ) . The polymer must be soluble i n order to replace the chlorine atoms with organo-substituents and thereby obtain a h y d r o l y t i c a l l y stable polymer. The conventional polymerization route used i n the laboratory has been the high-temperature (250 C) melt polymerization of I. Unfortunately, for this type of polymerization, the time required for a suitable conversion to soluble polymer i s markedly dependent on trimer purity, and minor impurities often lead to insoluble polymer. To avoid these problems a suitable catalyst system was needed. Various investigators (3,4) have shown water, Lewis acids and organometallic compounds, used both i n the bulk and i n solution, can serve as catalysts for the preparation of I I . Some examples are i l l u s t r a t e d i n Figure 2. Among the most promising catalyst systems are the boron halides and the boron halide-triphenylphosphate complexes studied by Fieldhouse, Graves and Fenske (5-7)• Yields of II up to 83% could be obtained using the boron trichloride-triphenylphosphate complex with I i n the melt at 220°C f o r 40 hr (6). Results from these studies (5-7) and independent studies by Horn and Kolkmann (8-10) suggest that the function of the Lewis acid catalysts i s rather complex. In addition to a simple ring-opening i n i t i a t i o n step, ligand exchange processes to form mixed phosphorus and boron compounds may be involved. Evidence includes phosphorus-31 NMR data and the observation that c y c l i c phosphazene esters w i l l react with boron t r i c h l o r i d e to give mixed substituted polyphosphazenes of low molecular weight (9). In contrast to the use of c y c l i c phosphazene trimers, l i n e a r phosphonitrilic chloride polymers were reported from the reaction of open-chain p h o s p h o n i t r i l i c chloride oligomers with ammonia or ammonium chloride (11). The polymers (II) obtained appear to have a lower MW than those described e a r l i e r , and they may be more useful for applications such as f i r e resistant coatings and foams which are discussed i n a subsequent section of this paper. U n t i l recently, molecular weight and other d i l u t e solution studies focused on the h y d r o l y t i c a l l y stable organo-substituted polymers. However, direct characterization of II can now be conducted on a routine basis using high performance l i q u i d chromatography (12-15). The catalysts described above generally give lower molecular weights than those obtained i n the uncatalyzed bulk process (Figure 2). Comparative measurements on both the chloropolymer (II) and derived products (IV, V, VI) have shown that the high molecular weight of the chloropolymer i s retained during the substitution process (12), although d i f f e r e n t

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

SINGLER E T A L .

Phosphazene Elastomers

CROSSLINKED

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MATRIX

Figure 1.

Synthesis of poly(dichlorophosphazene) and poly(organophosphazenes)•

Figure 2.

Contrast uncatalyzed and catalyzed [NPd^lg polymerization reactions. Examples are given of catalysts which are e f f e c t i v e i n producing high MW poly(dichlorophosphazene) (3-5, 7).

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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146

POLYMERS FOR FIBERS A N D

ELASTOMERS

nucleophiles can make minor modifications i n the molecular weight (MW) and molecular weight d i s t r i b u t i o n (MWD). The polymerization k i n e t i c s and mechanism of the BCl^ catalyzed polymerization of I are currently under i n v e s t i g a t i o n (16). Both solution and melt polmerization reactions are being studied, using s i z e exclusion chromatography and laser l i g h t scattering techniques to monitor and characterize the products. Polymer y i e l d s close to 100% with l i t t l e or no gel formation are obtained with BCl^ at temperatures as low as 150 C. The MW and MWD of poly(dichlorophosphazene) samples obtained from the catalyzed polymerization reaction are d i f f e r e n t from those obtained f o r the high temperature uncatalyzed process. For example, uncatalyzed polymerization reactions run at 250 C t y p i c a l l y y i e l d very high MW polymer with broad MWD's; eg, M = 2.4(10 ) and M = 5.6(10 ) g/mol(15^. By way of contrast, a B C l catalyzed polymerization run at 170 C yielded a polymer with,a lower MW and narrower MWD; eg, M - 8.3(10 ) and M = 2.2(10 ) g/mole. Preliminary studies also indicate that the BCl^ catalyzed solution process i s quite d i f f e r e n t from the catalyzed melt process (16). W

r

3

Poly(organophosphazenes) The Substitution Process. Poly(dichlorophophazene) serves as an important intermediate for the synthesis of an increasing number of poly(organophosphazenes). Property variations can be e a s i l y achieved by changing the nature and s i z e of the substituent attached to the P-N backbone. The poly(alkoxy- and aryloxyphosphazenes) (IV,V) are elastomers or f i b e r forming semicrystalline thermoplastics, whereas the poly(aminophosphazenes) (VI) generally are amorphous, glassy materials. I n i t i a l work involved the use of simple nucleophiles during synthesis i n order to demonstrate that h y d r o l y t i c a l l y stable polymers could be prepared (17,18) • Subsequent workers have shown that using two or more nucleophiles during the substitution process could have a profound e f f e c t on the properties. This synthetic modification i s outlined i n Figure 3. If OR i s OCH CF , a semicrystalline thermoplastic [NP(0CH CF )J i s obtained, whereas i f OR - OCH CF and OR* - O C H ^ F ^ Che result i s an amorphous "hybrid copolymer" (19)• If the nucleophiles used i n the s u b s t i t u t i o n process are comparable i n r e a c t i v i t y , then a random d i s t r i b u t i o n of substituents would be expected for the copolymers. When the nucleophiles have s u f f i c i e n t l y d i f f e r e n t r e a c t i v i t y , such as with c e r t a i n meta and para substituted phenoxides, then the preference for unsymmetrical, -NP(0R)(0R )-, rather than symmetrical, -NP(0R)(0R)-, s u b s t i t u t i o n may occur under c e r t a i n reaction conditions. Differences between nominally s i m i l a r copolymers have been observed with d i f f e r e n t i a l scanning calorimetry (20), but, to date, spectroscopic measurements such as phosphorus-31 NMR have been unable to d i s t i n g u i s h between possible copolymer structures. 3

2

2

3

3

f

Phosphazene Fluoroelastomers. The work of Rose (19) along with subsequent e f f o r t s at several laboratories has led to the commer-

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

SINGLER ET AL.

Phosphazene Elastomers

[NPCI ]

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2

[NPCI ]

3

2

n

BULK POLYMERIZATION 250°C, in vacuo, 40-100 hr Soluble polymer ^ 50% yield

1^7 [7] ^ M

n

~ 10 , M 5

w

3 dl/g

~ 10 , M / M 6

w

n

- 10

BULK POLYMERIZATION - CATALYSTS H 0, BCI3, AICI3, Et AI CI 2

3

2

3

CrCl3-6H 0 2

(C6H50)3PO-BCl3 Lower Polymerization Temperatures Higher Rates of Conversion Improved Molecular Weight Control US Patents:

3937790, 4123503, 4137330, 4226840

CATALYZED SOLUTION POLYMERIZATION H3P0 -P 0 , (CF3S03Hg) 0 in CI3C6H4 4

2

5

2

Sulfur in Decalin/Tetralin Atmospheric Pressure, Lower Temperatures,

Inert Atmosphere

BCI3, (C H 0) P0-BCl3 in Cyclohexane 6

5

3

Inert Solvents, Improved Process Control US Patents:

Figure 3.

4005171, 4139598, 4225567, 4327064

Contrasting synthesis of homopolymers and copolyme: with possible copolymer structures.

American Chemical Society Library 1155 16th St., N.W.

In Polymers for Fibers and Elastomers; Arthur, J., et al.; Washington, D . C . Society: 20036Washington, DC, 1984. ACS Symposium Series; American Chemical

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ELASTOMERS

c i a l i z a t i o n of the phosphazene fluoroelastomers by Firestone Tire and Rubber Company under the trademark PNF (1,4) • The elastomer i s a "terpolymer" (Figure 4) with a mixture of alkoxide substituents attached to the P-N backbone. Approximately 65% of the substituents are t r i f l u o r o a l k o x i d e groups, and the remaining 35% are derived from a mixture of teleomer fluoroalkoxides. A small amount (ca. 1%) of a reactive pendant group i s also added during the substitution step to provide more reactive curing s i t e s for crosslinking and rubber processing. Extremely high molecular weights (ca. 10 ) and broad molecular weight d i s t r i b u t i o n s are d i s t i n c t i v e features of these elastomers. The glass t r a n s i t i o n temperature i s -68 C (I)• The phosphazene fluoroelastomers can be compounded and processed to give an excellent balance of properties (Table I) which make them suitable for a variety of applications which require the combination of fuel and o i l resistance with a wide service temperature range ( 4 ) . I n i t i a l development work focused on demanding applications i n aerospace, m i l i t a r y , petrochemical and gas pipeline areas. The phosphazene fluoroelastomers are currently being used or evaluated i n both m i l i t a r y and commercial applications, and their continuing demand i n these areas i s now established. Some examples are shown i n Figure 5 (21). More recently there has been a surge of interest i n PNF f o r biomedical a p p l i c a t i o n s . The work at the Gulf South Research Institute (GSRI) i s quite promising for a p r a c t i c a l dental application of the PNF materials (22,23). Examples of PNF soft denture l i n e r s are shown i n Figure 6. PNF was solution blended with methyl methacrylate (MMA) monomer along with BaSO, and CdSSe pigment. Ethylene g l y c o l dimethacrylate (EGDMA) crosslinker completed the MMA interpenetrating polymer network. The GSRI-PNF soft putty was cured at 100 C using lauroyl peroxide as a crosslinking agent with MgO acid scavanger. This mixture was cured d i r e c t l y onto a hard poly(methylmethacrylate) PMMA baseplate. The mechanical properties of this composite, including peel bond strength, were reported to be quite suitable for the a p p l i c a t i o n . An SEM photomicrograph (Figure 7) of the peel test specimen indicates there i s an excellent bond of the elastomer to the PMMA baseplate beads. Since the PNF and MMA are reacting simultaneously, an attempt was made to i d e n t i f y domains of either polymer at high magnification using SEM with an EDAX attachment. No concentrations of phosphorus or fluorine were found at magnifications up to 100.000X. Thus the extent of homogeniety i n this polymer blend i s unknown, but i t may be high. Toxicity screening indicated that these PNF soft denture compounds have very low acute t o x i c i t y compared to some currently used denture l i n e r materials. C l i n i c a l t r i a l s are planned f o r the near future (23)• Another challenge i n the biomedical materials area i s the search for synthetic materials with improved blood compatibility for a r t i f i c i a l heart devices and other organs. An early study by Wade (24) using a series of poly(organophosphazenes) showed these polymers i n the u n f i l l e d state are as biocompatible as s i l i c o n materials. More recent blood compatibility studies using radiation crosslinked PNF showed excellent hemo compatibility

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

9.

SINGLER E T AL.

Phosphazene Elastomers

PNF

GUM RUBBER

^OCH CF 2

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149

3

--N = P \)CH (CF ) - CF H J 2

2

X

2

n

x • 1, 3, 5, 7, . . . SPECIFIC GRAVITY = 1.75 SOLUBILITY - KETONES, ESTERS, THF, ALCOHOLS T = -68°C g

Figure 4.

Chemical structure and properties of phosphazene fluoroelastomer, PNF gum rubber.

Table I PHOSPHAZENE FLUOROELASTOMER COMPOUNDING FORMULATIONS

VULCAN IZATE PROPERTIES

POLYMER

100

SILICA 1 CARBON BLACK? CLAY J

30-60

BRITTLE POINT*

-65°C

TR - 10*

-550C

PETROLEUM RESISTANT

SILICONE GUM 1 FLUOROSILICONEJ

, '"

i n

SILANE COUPLING AGENT MgO

i U

2

PEROXIDE OR SULFUR PRESS CURE 20-30 min AT

2

175°C

COMPRESSION SET (%)*

35-90 10-50

70 hr AT 149°C

2 1-3 170°C

POSTCURE (OPTIONAL) 4 hr AT 175°C

ASTM D-1329

GOOD THERMAL STABILITY (LONG TERM) SHORE A HARDNESS

2-10

(8-HQ) Zn (STABILIZER)**

*ASTM D-746

LOW TEMPERATURE FLEXIBILITY

TENSILE STRENGTH, MPa psi

6.9-17 1000-2500

ELONGATION, % 100% MODULUS, MPa psi

*ASTM D-395

75-250 2-10 290-1450

*ZINC 8-QUINOLINOLATE

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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ELASTOMERS

Figure 5.

Phosphazene fluoroelastomer (PNF) compounds were used to prepare molded rubber parts for diverse a p p l i c a t i o n s . Shown here include 0-rings f u e l hoses, shock absorption and v i b r a t i o n damping mounts. Photograph courtesy of the Firestone Tire and Rubber Co.

Figure 6.

Phosphazene fluoroelastomer (PNF) gum stock and soft denture l i n e r s which were prepared at Gulf South Research I n s t i t u t e . Photograph courtesy of Firestone Tire and Rubber Co.

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

9.

Phosphazene Elastomers

SINGLER E T AL.

151

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compared to currently used materials (25,26) . Elastomer weight gains due to i n v i t r o l i p i d sorption were measured f o r PNF candidates and several commercially available materials. The PNF materials were shown to be quite resistant to l i p i d absorption and can p o t e n t i a l l y serve as an e f f e c t i v e barrier coating f o r soft implants (27). F i r e Resistant Materials. Much of the current impetus f o r polyphosphazene development stems from the need f o r new polymers with improved thermal s t a b i l i t y and flame resistance combined with low smoke evolution and t o x i c i t y . Additives containing elements such as phosphorus, nitrogen and halogens are known to enhance f i r e resistance of organic polymers, but this often i s accomplished at the expense of increased smoke evolution and formation of toxic combustion products i n a f i r e s i t u a t i o n . Compounded polyphosphazenes o f f e r excellent potential f o r f i r e resistance i n part because phosphorus, nitrogen and halogen are part of the polymer, which tends to reduce the evolution of smoke and toxic gases upon combustion. For example, the poly(fluoroalkoxyphosphazenes) can break down to give r e l a t i v e l y stable products (28), and do not necessarily evolve large amounts of hydrogen f l u o r i d e during the degradation process (29). It has also been demonstrated that polyphosphazenes can be prepared which o f f e r f i r e resistance without incorporating halogen i n the side chain (30-33). The halogen free poly(aryloxyphosphazenes) elastomers (APN) show excellent potential f o r applications such as f l e x i b l e foams, coatings, and wire coverings (Figure 8 ) . The f e a s i b i l i t y of using the APN closed c e l l foams as f i r e r e t a r dant thermal i n s u l a t i o n has been demonstrated by a Department of the Navy National Bureau of Standards Test Program (34). Further development and evaluation of the aryloxyphosphazene elastomers for applications such as shown i n Figure 8 i s l i k e l y to continue. Alternate Synthesis Approaches

Phosphazene

Thus f a r , the survey of phosphazene elastomers has been based on the formation and modification of poly(dichlorophosphazene). Although a large variety of polymers can be prepared by this approach, there are l i m i t a t i o n s i n the preparation of polyphosphazenes with phosphorus-carbon bonds • The reaction of poly(dichlorophosphazene) with organometallic agents, such as RMgX or RLi, results mainly i n decomposition and not the desired polymers [NPR J . There are three possible approaches to the preparation or polyphosphazenes with phosphorus-carbon bonds: polymerization of substituted trimers, poly(difluorophosphazene), and thermolysis of small l i n e a r molecules. These three approaches w i l l be discussed i n turn. 2

Polymerization of Organosubstituted Cyclophosphazenes. Whereas hexachlorocyclotriphosphazene (I) reacts to give open chain high polymer i n good y i e l d , the f u l l y organosubstituted derivatives generally do not. There are intermediate cases such as mono- and diorganosubstituted cyclotriphosphazenes (trimers) which can form high MW polymers (38)• One noteable example (39) i s the

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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POLYMERS F O R FIBERS A N D E L A S T O M E R S

Figure 7.

Scanning electron microscopy photo micrograph showing the boundary layer between the GSRI-PNF compound and a hard PMMA baseplate. Note the absence of a polymer domain structure i n the PNF compound Photograph courtesy of Gulf South Research Institute (23).

Figure 8.

Aryloxyphosphazene elastomers (APN) o f f e r excellent potential for applications such as : A, closed c e l l foam thermal i n s u l a t i o n with high f i r e retadancy and low smoke generation (32,34); B, pigmented APN coatings i n aluminum substrates with low flammability, low flame spread, and low smoke (35); C, APN i n s u l a t i o n and cable jacketing (36); D, open c e l l APN comfort cushioning (37). Photograph courtesy of Firestone Tire and Rubber Company ( 4 ) .

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

9.

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Phosphazene Elastomers

SINGLER ET AL.

polymerization of monomethylpentachlorotrimer (VII), which a f t e r chlorine replacement gives a h y d r o l y t i c a l l y stable elastomer (VIII) with a T = -50°C. The estimated molecular weight was 1.5(10 ). The presence of the methyl group i s s u f f i c i e n t to disrupt c r y s t a l l i n i t y which i s found i n the related homopolymer [NP(0CH CF.)] . z J n 6

0

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CH

CI

3

//

r

, CI

v

C

K

CH

\

N

N II / CI n

i

P /

i

i

CI

CI

P'

\

CI

3

N=P-N=P-N=P--

25QQC,

^CI

CI

n

N VII

-NaCI

NaOCH2CF

3

CH

OCH2CF

3

3

N=P

N=P

OCH CF 2

3

OCH CF

ELASTOMER, Tg - 50°C

OCH CF 2

3

N=P 2

3

OCH CF 2

3

VIII

The reason that the organo-substituted trimers do not r e a d i l y undergo a ring opening polymerization has been explained on the basis of thermodynamic considerations (38). However, i n the presence of Lewis acids such as A l C l ^ or BCl^, the formation of low to medium MW polyphosphazenes from organo-substituted trimers has been reported (9,40). Apparently, ligand exchange reactions occur to form the more r e a d i l y polymerizable chlorotrimers• Poly(difluorophosphazene). Hexafluorocyclotriphosphazene w i l l polymerize i n the bulk state at 350 C to give poly(difluoro phosphazene). It i s an elastomer with a glass t r a n s i t i o n temgerature of -96 C and a c r y s t a l l i n e melting temperature of -68 C (41). Poly(difluorophosphazene) can be prepared i n the uncrosslinked state and w i l l undergo substitution reactions with amines, alkoxides, and aryloxides (42) • More important i s that i n contrast to poly(dichlorophosphazene), poly(difluorophosphazene) w i l l react with organometallic agents to prepare poly(organophosphazenes) with phosphorus-carbon bonds such as poly(diphenylphosphazene) (43)• N-Silylphosphinimines. A third approach, which has been investigated by Neilson (44,45), involves the synthesis of suitably constructed N-silyphosphinimines. Upon heating, compound IX y i e l d s poly(dimethylphosphazene) with a moderate molecular weight. Other N-silylphosphinimines have been synthesized, and these may eventually y i e l d new phosphazene elastomers with novel properties (45).

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

POLYMERS FOR FIBERS A N D E L A S T O M E R S

154

OCH CF 2

Me SiN=PMe2 3

IX

Me

3

40 hr

Me SiOCH CF 3

2

3

+ -fP - W Me M

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r

w

• 50,000

T -40°C g

Related

Phosphazenes

Although they are not elastomers, other phosphazenes also o f f e r potential i n areas requiring heat and f i r e resistance. Mixed fluoroalkoxy - aryloxy substituted c y c l i c phosphazenes are currently a t t r a c t i n g interest f o r f i r e resistant f l u i d a p p l i cations (46). In other studies, cyclophosphazene matrix resins have been prepared which have potential f o r high temperature adhesive applications (47) and as composite matrix materials (48)• These studies serve to further demonstrate the enhanced thermal s t a b i l i t y and f i r e resistance which can be achieved with these phosphorus - nitrogen systems. Future Trends This paper has reviewed the most recent developments of phosphazene elastomers and touched b r i e f l y i n some phosphazenes of related i n t e r e s t . At the time of t h i s writing, Firestone was the supplier of the PNF and APN elastomers. Ethyl Corporation recently licensed the phosphazene technology from Firestone (49), and thus the continued a v a i l a b i l i t y of these elastomers i s assured for further commercial development• The examples shown i n this paper represent only a f r a c t i o n of the known polymers based on the phosphazene (P-N) backbone. Sulfamic acid catalysts hold some promise f o r f a c i l i t a t i n g the polymerization process (50) and phase transfer catalysts can a i d i n the substitution step (51). Poly(organophosphazenes) having bioactive s i t e s and organometallic substrates have been prepared and are under i n v e s t i g a t i o n . Amino acid esters (52), steroids (53), and b i o l o g i c a l l y active amines such as benzocaine (54) and dopamine (55) have been attached to the phosphazene chain. Water soluble polymers that bear glucose side groups have also been synthesized (56). Poly(organophosphazenes) containing reactive side groups have been used to bind t r a n s i t i o n metal units (57,58) to form polymers which are p o t e n t i a l l y useful for hydrogenation catalysts and other applications. The large number of d i f f e r e n t pendant groups with widely varied chemical f u n c t i o n a l i t y which can be attached to the P-N backbone demonstrate the unusual molecular design potential of this class of polymers. Undoubtedly, some of these w i l l hold promise f o r future research and development•

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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155

Acknowledgments The authors wish to acknowledge the helpful discussions and assistance provided by Dr. Fred Lohr, Firestone, and Dr. Lawrence Gettleman, Gulf South Research I n s t i t u t e , during the preparation of this manuscript.

Literature Cited

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1. 2. 3. 4.

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