Polyphosphazene Elastomers: Synthesis ... - American Chemical Society


of the Navy-National Bureau of Standards Test Program (54). Alternate Synthesis .... David W. Taylor, Naval Ship R&D Center, Annapolis, MD. 1978, NASA...
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ROBERT E . SINGLER and G A R Y L . H A G N A U E R Army Materials and Mechanics Research Center, Polymer Research Division, Watertown, M A 02172 RICHARD W. SICKA Firestone Tire & Rubber Co., Central Research Laboratories, Akron, O H 44317

The synthesis of a unique class of polymers with a phosphorus nitrogen backbone is described, with an emphasis on poly(dichlorophosphazene) and poly(organophosphazene) elastomers. Poly (dichlorophosphazene) can be prepared by high temperature melt or solution polymerization techniques, with or without the use of catalysts. High performance GPC and other dilute solution techniques have been used to monitor yield and to analyze molecular weight, molecular weight distribution, and chain structure. Although poly(dichlorophosphazene) is an elastomer, i t must be modified in order to obtain long term hydrolytic stability and other useful properties. From a common poly(dichlorophosphazene) intermediate, one can introduce a variety of substituents giving polyorganophosphazenes with a wide range of physical properties. Some of the useful properties of phosphazene elastomers and their technological significance will be shown. This article concludes with a brief mention of alternate synthetic methods which may lead to useful polyphosphazene elastomers. The study of open-chain polyphosphazenes has attracted increasing attention i n recent years, both from the standpoint of fundamental research and technological development. These polymers have been the subject of several recent reviews (1-6). Interest has stemmed from the continuing search for polymers with improved properties for existing applications as well as for new polymers with novel properties. The polyphosphazenes are highly f l e x i b l e chains of alternating phosphorusnitrogen atoms with two substituents attached to the phosphorus atom. Although the properties of the polyphosphazenes are influenced to a degree by the molecular weight and chain structure, the properties are determined largely by the size and the nature of the substituent attached to the phosphorus0097-6156/82/0193-0229$06.00/0 © 1982 American Chemical Society In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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AND

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nitrogen (P-N) backbone. For the purpose of t h i s paper, we shall c l a s s i f y polyphosphazenes into two groups, the poly (halophosphazenes) and the poly(organophosphazenes). Both groups contain elastomers, and several poly(organophosphazenes) are currently of interest for commerical development.

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Poly(halophosphazenes) Poly(dichlorophosphazene). Most of the interest i n the poly(halophosphazenes) has centered around poly(dichlorophosphazene) . The thermal conversion of hexachlorocyclotriphosphazene (I) to the rubbery poly(dichlorophosphazene) has been known since the turn of the century (7). This transparent inorganic polymer exhibits many of the properties of a good elastomer except that prolonged exposure to a moist atmosphere results in hydrolysis and degradation with an attendant loss of useful physical properties. Early attempts to s t a b i l i z e this "inorganic rubber" by replacing the chlorine groups with organic substituents were unsuccessful and interest was rather limited. However, i n 1965, Allcock demonstrated that stable poly(organophosphazenes) could be prepared from soluble, open-chain poly(dichlorophosphazene) (8,9) , and work with these inorganic backbone polymers has increased s i g n i f i c a n t l y . The polymerization of hexachlorocyclotriphosphazene (I) has been the subject of numerous investigations (9,10). The polymerization reaction (Figure I) i s markedly influenced by the presence of trace impurities which was one of the d i f f i c u l t i e s encountered i n e a r l i e r investigations. The conventional route i s a melt polymerization of highly purified trimer (NPCl^K or a mixture of trimer and a small amount of tetramer fNrCl,-), , sealed under vacuum i n glass ampoules, at approximately 250°C. Proper selection of time and temperature i s necessary to obtain I I and avoid the formation of cross-linked matrix ( I I I ) . More recent studies (4,10) have shown that various acids and organometallic compounds can serve as catalysts for the preparation of I I . The advantages include lower polymerization temperatures, higher y i e l d s , lower molecular weights, and the use of conventional large scale equipment. Examples include bulk polymerizations using H^O (11), Et«Al„Cl« (12), CrCl .6H 0 (13), (C H 0) P0-BC1 (147, polyphosphoric acid catalyst i n trichlorobenzene (15) and sulfur catalyst i n decalin (16). Recently, gel permeation chromatography (GPC) and other d i l u t e solution techniques have been d i r e c t l y applied to the characterization of I I (17-20). In our laboratory we have examined I I prepared by the uncatalyzed bulk and solution polymerization processes. Polymers obtained from the former process have high molecular weights (MWs) and broad molecular weight distributions (MWDs, Mw/Mn=5). The d i l u t e solution 2

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In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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11.

SINGLER E T A L .

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Polyphosphazene Elastomers

parameters indicate the polymers obtained at low conversions are randomly coiled i n solution. Representative GPCs are given i n Figure 2. Note the bimodal d i s t r i b u t i o n at 100 hr; t h i s may indicate a change i n the polymerization mechanism at longer times and higher conversions (20). In contrast to the uncatalyzed bulk polymerization described in the preceeding paragraph, a catalyzed solution polymerization using polyphosphoric acid i n trichlorobenzene (TCB) gave a s l i g h t l y lower MW for [email protected]) but with a s i g n i f i c a n t l y narrower MWD, Mw/Mn^l.5 (Figure 3). No gel formation was observed i n t h i s experiment. The solution polymerization proceeds more rapidly than the uncatalyzed bulk polymerization, but gel formation (4%) was observed during the solution polymerization after 16 hrs. at 22% overall conversion (20). The elastomeric properties of poly(dichlorophosphazene) have been the subject of various investigations over the years. Probably most of these investigators were studying poly(dichloro­ phosphazene) i n the p a r t i a l l y crosslinked state. Most of this was summarized by Allcock (9). More recently, highly p u r i f i e d , uncrosslinked I I has been examined i n the s o l i d state (21). The unstressed polymer i s amorphous at room temperature, but c r y s t a l l i z a t i o n can be induced by cooling or stretching techniques. The glass t r a n s i t i o n temperature, measured by Torsional Braid Analysis, i s -66°C (22). Poly(difluorophosphazene). A b r i e f mention w i l l be made of poly(difluorophosphazene) (NPF^)^, which was f i r s t reported by Seel and Langer (23) and has been investigated i n greater d e t a i l by Allcock (24). I t i s prepared by the bulk polymeriza­ tion of hexafluorocyclotriphosphazene (NPF^)^ 350°C. Poly(difluorophosphazene) i s an elastomer with a glass t r a n s i t i o n temperature of -96°C (for (NPC1 ) , Τ = -66°C) and a c r y s t a l l i n e melting temperature of -68°C. f f ca?e i s taken during the trimer p u r i f i c a t i o n and polymerization, (N^^^n ^ ^ ^ as an uncrosslinked white elastomer which can be reacted with organometallic agents to prepare poly(organophosphazenes) with phosphorus-carbon bonds (25). a t

2

n

C a n

INPFζ.1η

1)

C-H-Li

2)

CF CH 0Na

e

0

t a i n e c

9

3

2

Poly(organophosphazenes) Synthesis-Structure-Properties. Poly(dichlorophosphazene) i s important as an intermediate for the synthesis of a wide range of poly(organophosphazenes) (Figure I ) . The nature and size of the substituent attached to phosphorus plays a dominant role i n determining the properties of the polyphospha-

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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CI

A N DRUBBER

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CI

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CROSSLINKED MATRIX

OR / {N=P}

Figure 1.

X

lOAr {N=P}

X

\

NRR' {N=P}

OR

OAr

NRR'

IV

V

VI

X

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

Melt Polymerization 250 C

Figure 2. GPC studies of melt polymerized poly(dichlorophosphazene). Cumulative C(M) and differential F(log M) MWD of II obtained at 60 and 100 h. Ref. 20. In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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11.

siNGLER E T A L .

RLog ΛΛ)

Polyphosphazene Elastomers

233

1

55

60 Logio M

Figure 3. Cumulative C(M) and differential F(log M) MWD of solution polymer­ ized II. Solvent for the polymerization was trichlorobenzene (TCB). Ref. 20.

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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zenes. I f complete replacement of chlorine i s not achieved upon substitution, the resulting polymer may be an elastomer with properties often quite different from the f u l l y substituted polymer. The glass transition temperatures (T s) vary from -84°C for [NP(0CH CH ) ] to around 100°C for §he poly(anilinophosphazenes) [NPtNHAr7 ï . The poly(organophosphazenes) vary from elastomers to f l e x i b l e f i l m forming thermoplastics and glasses at room temperature. Some are highly solvent r e s i s t a n t , whereas others l i k e certain poly(aminophosphazenes) are water soluble. This wide range of properties, based on the same polymer chain, i s unique i n polymer chemistry. f

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9

Poly(alkoxyphosphazenes) (IV) and Poly(aryloxyphosphazenes) (V). These classes (IV, V) of poly(organophosphazenes) are worthy of interest since they contain both semicrystalline thermop l a s t i c s and elastomers (Figure 4). This dramatic change i n properties can arise by employing certain nucleophiles or combinations of nucleophiles i n the substitution process (Figure 5). Homopolymers prepared from I I such as [NPiOCI^CF^K] are f l e x i b l e film-forming thermoplastics (8). With the i n t r o duction of two or more substituents of s u f f i c i e n t l y d i f f e r ent size, elastomeric mixed substituent polymers are obtained (Figure 5). This principle was f i r s t demonstrated by Rose (26) who used a mixture of fluoroalkoxides during the subs t i t u t i o n step to obtain a f luoroelastomer [NP^CH^CF^ (OCH^F^) ] with excellent chemical resistance and low temperature f l e x i b i l i t y (T = -77°C). The substitution reaction can be further modified to allow for the addition of a small amount (^1%) of a reactive pendant group along with the fluoroalkoxides providing "terpolymers" with more reactive curing sites to f a c i l i t a t e crosslinking and rubber processing. n

With the mixed substituent polymers or copolymers, structures such as INP(OR)(0R )] are only an average representation, since a random distribution of pendant groups i s more l i k e l y (Figure 5). This assumes an equal preference for incoming groups during the substitution process. I f the groups are not s u f f i c i e n t l y different i n size or nature, the copolymers such as [NP(0C.H,-4-Cl)(0C.H -4-C H_)] or[NP(OC.H-)(OCLH,4-OCH )] can o V c r y s t a l l i n e ( 2 7 M • * 6 5 6 4 The effect of the substituent on the properties of the polyphosphazenes i s not f u l l y understood. For instance, [NP(OCH ) 1 and [NPiOC^CH^)»]^ homopolymers are elastomers (8,29). Synthesis using lithium, i n contrast to sodium, salts i s claimed to produce rubber-like fluoroalkoxyphosphazene polymers (30). The presence of unreacted chlorine or low molecular weight oligomers can affect the bulk properties (31,32). Studies with phosphazene copolymers both i n solution and in the bulk state (29,33-38) indicate a rather complex structure, which points out the need for additional work on the chain structure and morphology of these polymers. f

/

o

4

3

n

3

n

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

n

11.

SINGLER ET A L .

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Polyphosphazene Elastomers

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SEMI CRYSTALLINE THERMOPLASTICS OCH CF 2

{P=N}

0 - ©

3

{P=N}

n

OCH CF 2

{P=N}

n

CXQ

3

C

3

H

n

O^QCI

ELASTOMERS OCH2CH3 {P=N}

{P=N}

n

ÔCH CH 2

Figure 4.

OCH2C3F6CF2H

3

{P=N}

n

OCH CF 2

o - @ n

O-Q-C2H5

3

Typical poly(organophosphazene) plastics and elastomers.

OR 2 NaOR OR

OR I -N = P I OR

OR'

I -N=PI OR

OR' I -N = P I OR'

Figure 5. Contrasting synthesis of homopolymers and copolymers with possible copolymer structures.

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Poly(aminophosphazenes) (VI). The poly(aminophosphazenes) also cover a range of properties, but only a few are elastomeric in nature. The poly(anilinophosphazenes) are glassy materials (2,3). Many poly(aminophosphazenes) with a l i p h a t i c pendant groups are f l e x i b l e thermoplastics; some are water soluble. When diethylamine i s used i n the substitution process, approximately 50% substitution occurs. This polymer (VII) i s an elastomer [NPCl ] 2

NH(C H ) j N P C ! N ( C H ) ]

n

2

5

2

2

5

2

n

NH R jNP(NHR)N(C H .) ] ?

2

[

2

n

VII VIII which can be used to prepare f l e x i b l e thermoplastic mixed substituent copolymers (VIII) and terpolymers. The copolymer [NP(0CH CF )N(C H ) ] was prepared v i a VII (39). This mixed alkoxyaminophosphazene was similar i n properties to [NP(OCH CF ) ] . 2

3

2

3

2

2

5

2

n

n

Applications Although questions s t i l l exist concerning the structure of polyphosphazene elastomers, several of these polymers are of technological interest and are undergoing commercial development (3-5). Poly(fluoroalkoxyphosphazene) Elastomers. Much of the current interest i n the phosphazene fluoroelastomers i s based on the outstanding combination of properties inherent i n these polymers including petroleum resistance, low temperature f l e x i b i l i t y , thermal and oxidative s t a b i l i t y and ozone resistance. These properties have stimulated work for both m i l i t a r y and commercial applications such as a r c t i c fuel hoses and gaskets (40,41), seals and 0-rings (42,44), coated fabrics (45,46), channel sealants (47) and biomedical materials (48). The phosphazene fluoroelastomers can be compounded and processed using conventional methods to give an excellent balance of physical properties (Table 1). This polymer was introduced commercially by Firestone Tire and Rubber Company under the trademark PNF (49). Typical end items which are undergoing evaluation or are i n use are shown i n Figure 6. Fire Resistant Elastomers. The poly(aryloxyphosphazene) elastomers offer excellent f i r e resistance without incorporating halogen i n the polymer or as an additive. These polymers are self-extinguishing i n a i r and generate only moderate noncorrosive smoke and a minimum of toxic combustion products upon combustion (50-53). The poly(aryloxyphosphazene) elastomers (APN®) have excellent potential for applications such as

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

*ASTM D-746

ASTM D-1329

VULCAN IZATE PROPERTIES

10-50

COMPRESSION SET (%)*

"ZINC 8-QUINOLINOLATE

100% MODULUS, MPa psi

ELONGATION, %

TENSILE STRENGTH, MPa psi

2-10 290-1450

75-250

6.9-17 1000-2500

35-90

SHORE A HARDNESS

70 h r AT 149°C

175°C

-65°C -55°C

GOOD THERMAL STABILITY (LONG TERM)

PETROLEUM RESISTANT

f

BRITTLE POINT* TR - 10

LOW TEMPERATURE FLEXIBILITY

*ASTM D-395

POSTCURE (OPTIONAL) 4 hr AT 175°C

170°C

1-3

PEROXIDE OR SULFUR

PRESS CURE 20-30 min AT

2

2

2-10

(8-HQ) Zn (STABILIZER)**

MgO

2

2-10

SILICONE GUM 1 FLUOROSILICONEJ

SILANE COUPLING AGENT

30-60

100

FORMULATIONS

SILICA CARBON BLACK; CLAY

POLYMER

COMPOUNDING

PHOSPHAZENE FLUOROELASTONIER

Table I

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ELASTOMERS

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ELASTICITY

open and closed-cell foams (Figure 7) and wire coverings. The f e a s i b i l i t y of using these elastomeric foams as f i r e retardant thermal insulation has been demonstrated by a Department of the Navy-National Bureau of Standards Test Program (54).

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Alternate Synthesis - Future Developments The survey of phosphazene elastomers so far has been based on the poly(halophosphazene) synthesis. The reason for this i s that while ( N P C l ^ and (NPFO react to give open chain high polymer, f u l l y organo substituted c y c l i c derivatives generally do not. There are intermediate examples such as mono- and diorganosubstituted cyclotriphosphazenes which y i e l d open chain polymers under certain conditions (58). One interesting example (59) i s the polymerization of monoalkylpentachlorocyclotriphosphazene monomers, N^P^Cl^R followed by chlorine replacement to give open chain polymers I-(N=P(OR ) ) -N=P(OR )R-] . This polymer (R=CH , R =CH CF ) i s an elastomer with a Τ =-50°C. The presence or the methyl group i s sufficient to d?srupt the c r y s t a l l i n i t y as observed i n the related homopolymer lNP(0CH„CF ) ] . f

f

2

f

2

o

L

o

J ζ η

N-Silylphosphinimines. U n t i l recently, no general method has been available which allows complete incorporation of the desired substituents before polymerization. The synthesis of polyphosphazenes with direct phosphorus-carbon bonds has been possible only i n a few cases (25,60). A new method which holds promise involves the synthesis of suitably constructed N-silylphosphinimines which upon heating, eliminate substituted silanes to give polyphosphazenes (61). This procedure was R R i

!

Me SiN=P-X 3 ι R

• Me SiX + +N=P^3 ι η R

0

0

f

used to prepare poly(dimethylphosphazene) (XOCIUCF^, R=R =CH ) (61). One can easily imagine that with a suitable selection of substituents, R^R , new phosphazene elastomers with novel properties might be accessible. 3

f

Other Applications. Thus far the phosphazene fluoroelastomers (PNF) and aryloxyphosphazene elastomers (APN) have moved to the commercial stage. In addition to elastomers, phosphazenes are being investigated as f l u i d s , resins and p l a s t i c s . Other areas which hold promise include f i r e resistant paints (55), fiber blends and additives, agrichemicals and herbicides, drug release agents and e l e c t r i c a l l y conducting polymers (6). The large number of different pendant groups with widely varied chemical functionality which can be attached to the

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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siNGLER

E T AL.

Polyphosphazene Elastomers

239

Figure 6. Phosphazenefluoroelastomers(PNF) were used to prepare various rubber end items, some of which are commercially available. Examples: A, O-rings for rotary and static applications (43); B, extruded items such as hose, tube, and solid splicable stock; C, fuel hose for low temperature service (—57°C) and coated fabric for collapsible fuel storage tanks (40, 45); D, biomedical applications such as soft denture liners (48) and blood compatible parts; E, lip seals (42); F, shock absorption and vibration damping mounts. (Photograph courtesy of the Firestone Tire & Rubber Company.)

Figure 7. A ryloxyphosphazene elastomers (APN) offer excellent potential. Examples: A, closed cell thermal insulation with high fire retardency and low smoke generation (52, 5 4 ) ; B, pigmented APN coatings in aluminum substrates with low flammability, low flame spread, and low smoke (55); C, APN insulation and cable jacketing (56); D, open cell APN comfort cushioning (57). (Photograph courtesy of the Firestone Tire & Rubber Company.)

In Elastomers and Rubber Elasticity; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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P-N backbone demonstrate the unusual molecular design potential of t h i s class of polymers. Undoubtedly, some of these w i l l hold promise for future research and development. References

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Singler, R. E . ; Schneider, N. S.; Hagnauer, G. L. Polym. Eng. and Sci. 1975, 15, 321. Allcock, H. R. Angew Chem. Int. Ed. Eng. 1. 1977, 16, 147. Singler, R. E . ; Hagnauer, G. L . ; Schneider, N. S. Polym. News. 1978, 5(1), 9. Gerber, A. H. ACS Div. Org. Coat Plast. Chem. Pap. 1979, 41, 81; NASA CR 159563, N79-30377. Tate, D. P. and Antkowiak, F. A. Kirk-Othmer Encycl. Chem. Technol. 3rd. Ed. 1980, 10, 936. Allcock, H. R. Makromol. Chem. Suppl. 1981, 4, 3. Stokes, H. N. Amer. Chem. J. 1897, 19, 1782. Allcock, H. R.; Kugel, R. L . ; Valan, K. J. Inorg. Chem. 1966, 5, 1709. Allcock, H. R. "Phosphorus-Nitrogen Compounds", Academic Press, New York, 1972. Hagnauer, G. L. J. Macromol. Sci.-Chem. 1981, A16, 385. Allcock, H. R.; Gardner, J. E . ; Smeltz, Κ. M. Macromolecules 1975, 8, 36. Snyder, D. L . ; Stayer, Jr. M. L . ; Kang, J. W., U. S. Patent 4,123,503, 1978. Pritchard, M. S.; Hilton, A. S.; Stayer, Jr. M. L.; Antkowiak, Τ. Α., U.S. Patent 4,137,330, 1979. Fieldhouse, J. W.; Graves, D. F. U.S. Patent 4,226, 840, 1981. Reynard, Κ. Α.; Gerber, A. H. U.S. Patent 4,257,917, 1981. Halasa, A. F.; Hall, J. E. U.S. Patent 4,225,567, 1981. Hagnauer, G. L. and Singler, R. E. Coat. Plast. Chem. Pap. 1979, 41, 88. Hagnauer, G. L. ACS Symposium Series 1980, 138,239. Adams, Η. E . ; Valaitis, J. K.; Henderson, C. W.; Strauss, E. J. ACS Symposium Series 1980, 138, 255. Hagnauer, G. L. and Koulouris, T. N. "Liquid Chromatogr­ aphy of Polymers and Related Materials - III" Jack Cazes, Ed. Marcel Dekker Inc., New York, 1981. Allcock, H. R. and Arcus, R. A. Macromolecules 1979, 12, 1130. Connolly, Jr., T. M. and Gillham, J. K. J. Appl. Polym. Sci. 1975, 19, 2641. Seel, F. and Langer, J. Z. Anor. Allg. Chem. 1958, 295 (317). Allcock, H. R.; Kugel, R. L . ; Stroh, E. G. Inorg. Chem. 1972, 11, 1120.

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11. SINGLER ET AL. Polyphosphazene Elastomers

25. 26. 27.

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28. 29. 30.

31. 32. 33. 34. 35· 36. 37. 38. 39. 40.

41. 42. 43.

44. 45.

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