[acs symposium series] inorganic and organometallic polymers volume 360 (macromolecules containing...

Chapter 20

Phosphazene Polymers: Synthesis, Structure, and Properties

Robert E. Singler, Michael S. Sennett, and Reginald A. Willingham

Army Materials Technology Laboratory, Watertown, MA 02172-0001

An overview of the synthesis and characterization of a unique class of polymers with a phosphorus-nitrogen backbone i s presented, with a focus on poly(dichloro-phosphazene) as a common intermediate for a wide variety of poly(organophosphazenes). Melt and solution polymerization techniques are illustrated, including the role of catalysts. The elucidation of chain structure and molecular weight by various dilute solution techniques is considered. Factors which determine the properties of polymers derived from poly(dichlorophos-phazene) are discussed, with an emphasis on the role that the organic substituent can play in determining the fi n a l properties.

The study of open-chain polyphosphazenes has a t t r a c t e d i n c r e a s i n g a t t e n t i o n i n recent years, both from the standpoint of fundamental research and t e c h n o l o g i c a l development. The polyphosphazenes are long chains of a l t e r n a t i n g phosphorus-nitrogen atoms w i t h two s u b s t i t u e n t s attached to phosphorus. These polymers have been the subject of s e v e r a l recent reviews (1-3). I n t e r e s t has stemmed from the c o n t i n u i n g search f o r polymers w i t h improved p r o p e r t i e s f o r e x i s t i n g a p p l i c a t i o n s as w e l l as f o r new polymers w i t h novel p r o p e r t i e s .

Figure 1 provides an overview of the two step synthesis process, pioneered by A l l c o c k (4) and i n use today by a number of workers and l a b o r a t o r i e s : formation of a s o l u b l e r e a c t i v e polymer intermediate ( I I ) from which i s derived a l a r g e number of polymers v i a s u b s t i t u t i o n r e a c t i o n s .

Since the i n i t i a l d i s c l o s u r e by A l l c o c k , workers have sought to answer various questions: 1) What i s the nature of the pol y m e r i z a t i o n process (mechanism)? 2) What i s the s t r u c t u r e of poly(dichlorophosphazene) that d i s t i n g u i s h e s i t from the i n s o l u b l e " i n o r g a n i c rubber" ( I I I ) ? 3) The s u b s t i t u t i o n process gives a seemingly endless v a r i e t y of products. What are the l i m i t a t i o n s or

This chapter is not subject to U.S. copyright. Published 1988, American Chemical Society

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

20. SINGLERETAL. Phosphazent Polymers 269

CROSSLINKED MATRIX

III

HNRR'-Et3N

OR / loAr \ NRR' r I ι / Γ ' I ι ? I η

{ N = P } X { N = P } X ± N = P } X

OR OAr NRR'

IV V VI

Figure 1. Synthesis of poly(dichlorophosphazene) and poly(organophosphazenes).

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270 INORGANIC AND ORGANOMETALLIC POLYMERS

c o n t r o l l i n g f a c t o r s i n the s u b s t i t u t i o n process? 4) How do the above f a c t o r s c o n t r o l the p r o p e r t i e s of the poly(organophosphazenes) (eg. IV, V, VI)? 5) Are any of these polymers t e c h n o l o g i c a l l y u s e f u l or of commercial i n t e r e s t ?

This paper w i l l provide an overview of the p o l y m e r i z a t i o n processes and the p r o p e r t i e s of poly(dichlorophosphazene). This paper w i l l a l s o discuss the various f a c t o r s which i n f l u e n c e the p r o p e r t i e s of the poly(organophosphazenes) and show how these f a c t o r s have r e s u l t e d i n a c l a s s of polymers w i t h a wide range of p r o p e r t i e s , i n c l u d i n g s e v e r a l examples of current commercial importance.

Poly(dichlorophosphazene)

The p o l y m e r i z a t i o n of hexachlorocyclotriphosphazene ( I ) has been the subject of numerous i n v e s t i g a t i o n s ( 5 ) . The r e a c t i o n ( I > I I , I I I ) i s markedly i n f l u e n c e d by the presence of trace i m p u r i t i e s . The conventional route to I I i s a melt p o l y m e r i z a t i o n at 250 °C of h i g h l y p u r i f i e d t r i m e r ( N P C ^ ) ^ , sealed under vacuum i n g l a s s ampoules. Proper s e l e c t i o n of r e a c t i o n time and temperature i s necessary to o b t a i n I I and avoid the formation of I I I . For l a r g e s c a l e i n d u s t r i a l processes, v a r i o u s a c i d s and organometallic compounds can be u t i l i z e d as c a t a l y s t s to prepare s o l u b l e polymer, both i n bulk and i n s o l u t i o n ( 2 ) . The advantages of c a t a l y z e d polymerizations i n c l u d e lower r e a c t i o n temperatures, higher y i e l d s , and the use of conventional l a r g e s c a l e equipment.

Si z e e x c l u s i o n chromatography (GPC) and other d i l u t e s o l u t i o n techniques have been a p p l i e d to the c h a r a c t e r i z a t i o n of I I (6,7). Polymers obtained from the bulk p o l y m e r i z a t i o n t y p i c a l l y have high molecular weights and broad molecular weight d i s t r i b u t i o n s (MWD's). Catalyzed processes g e n e r a l l y give narrower MWD's but lower molecular weight polymer. Although questions s t i l l remain as to the nature of the p o l y m e r i z a t i o n mechanism ( 7 ) , i t i s g e n e r a l l y thought to be a c a t i o n i c , chain growth, r i n g opening p o l y m e r i z a t i o n process (Figure 2 ) . Evidence f o r t h i s i n c l u d e s the e f f e c t i v e n e s s of Lewis a c i d c a t a l y s t s , e s p e c i a l l y B C l ^ , formation of high molecular weight polymer e a r l y i n the p o l y m e r i z a t i o n , and d i l u t e s o l u t i o n parameters obtained on I I which point to randomly c o i l e d polymer chains r e l a t i v e l y f r e e of long-chain branching f o r low to moderate conversions to high polymer.

One way to overcome the molecular weight l i m i t a t i o n s i n a s o l u t i o n c a t a l y z e d process i s by t a k i n g advantage of the " l i v i n g " nature of the p o l y m e r i z a t i o n ( 7 ) . For the B C l ^ c a t a l y z e d p o l y m e r i z a t i o n , one can add monomer ( t r i m e r ) to the e x i s t i n g polymer to increase the molecular weight i n a stepwise f a s h i o n (Figure 3 ) . Trimer i s polymerized i n the presence of BC1~ i n a trichlorobenzene s o l u t i o n i n a sealed ampoule at 210 °C f o r 48 hours. For the second and t h i r d stages, t r i m e r i s added i n s o l u t i o n equal to the amount i n stage 1. The B C l ^ concentration i s held constant. Each stage i s c a r r i e d to greater than 95 % conversion. L i g h t s c a t t e r i n g measurements on the polymer obtained from stage 3 show MW > 10 , thus confirming that high molecular weight I I can be obtained i n high conversion i n a c a t a l y z e d s o l u t i o n process ( 8 ) .

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20. SINGLERETAL. Phosphazene Polymers

[NPCI2l3 — [NPCI2Jn

BULK - UNCATALYZED

HIGH PURITY TRIMER NECESSARY - OTCERWISE GEL FORMATION

HIGH POLYMER (MW - 106)

AT LOW CONVERSION «30%), GEL FREE, 250°C, 40-100 hr

BULK - CATALYZED

TRIMER PURITY LESS CRITICAL

LOWER TEMPERATURES (170°C - 220°C)

WITH HIGHER CONVERSIONS 050%) OF GEL-FREE POLYMER AT SHORTCR TIMES

LOWER MW POLYMER MO*)

SOLUTION - CATALYZED

SAME COMMENTS AS IN BULK - CATALYZED

INERT SOLVENT

GENERAL MECHANISM

CATIONIC - CHAIN GROWTH - RING OPENING

Figure 2. General comments on the pol y m e r i z a t i o n process.

BCI3

[NPCI2l3 " [NPCI2]n

TCB, 210°C SEALED TUBE

STEPWISE PROCESS

FIRST STAGE: 15 wt% TRIMER IN C6H3CI3 (3g/16g). BCI3-0.66g. 48 hr. 210°C. 95% CONVERSION. SOLUBLE POLYMER.

SECOND STAGE: NEW TRIMER SOLUTION ADDED TO POLYMER. IBCI3J ~ CONSTANT. SAME t, T, % CONVERSION.

THIRD STAGE: REPEAT

STAGE M n « M w »

1 13,000 37,000 2 100,000 118,000 3 322,000 536,000 ( M w ~ 6 χ 106) f

*GPC MW DE7IRMI NATION. POLYSTYRENE STANDARDS. *LIGHT SCATTERING.

SENNE1T (1986)

Figure 3. S o l u t i o n p o l y m e r i z a t i o n w i t h BC1~. Stepwise process.

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Poly(organophosphazenes)

The synthesis of poly(organophosphazenes) represents probably the best example of a c e n t r a l theme of i n o r g a n i c macromolecules: Pre p a r a t i o n of a r e a c t i v e polymeric intermediate, p o l y ( d i c h l o r o p h o s -phazene), and subsequent use i n a wide v a r i e t y of s i d e group replacement r e a c t i o n s (Figure 1 ) . This concept has been demonstrated i n a number of l a b o r a t o r i e s (3) and has provided a wide v a r i e t y of polymers w i t h d i f f e r e n t p r o p e r t i e s .

Table I serves to i l l u s t r a t e how the nature and s i z e of the s u b s t i t u e n t attached to the P-N backbone can i n f l u e n c e the p r o p e r t i e s of the poly(organophosphazenes). The g l a s s t r a n s i t i o n temperatures range from -84 °C f o r (NP(OCH 2CH 3> 2) n to around 100 °C f o r the poly(anilinophosphazenes). Polymers range from elastomers to f l e x i b l e f i l m forming thermoplastics or glasses at room temperature.

In the case of poly(alkoxyphosphazenes) (IV) or poly(aryloxyphos-phazenes) (V) a dramatic change i n p r o p e r t i e s can a r i s e by employing combinations of s u b s t i t u e n t s . Polymers such as (NP(0CH 2CF 3) 2) and ( N P ( O C 6 H 5 ) 2 ) n are s e m i c r y s t a l l i n e thermoplastics (Table I ) . W?th the i n t r o d u c t i o n of two or more s u b s t i t u e n t s of s u f f i c i e n t l y d i f f e r e n t s i z e , elastomers are obtained (Figure 4 ) . Another requirement f o r elastomeric behavior i s that the s u b s t i t u e n t s be randomly d i s t r i b u t e d along the P-N backbone. This p r i n c i p l e was f i r s t demonstrated by Rose ( 9 ) , and subsequent work i n s e v e r a l i n d u s t r i a l l a b o r a t o r i e s has l e d to the development of phosphazene elastomers of commercial i n t e r e s t . A phosphazene fluoroelastomer and a phosphazene elastomer w i t h mixed a r y l o x y side chains are showing promise f o r m i l i t a r y and commercial a p p l i c a t i o n s . These elastomers are the subject of another paper i n t h i s symposium (10).

Studies have shown that not a l l phosphazene copolymers are n e c e s s a r i l y elastomers (11,12). Figure 5 c o n t r a s t s s e m i c r y s t a l l i n e homopolymers w i t h an elastomeric copolymer, and then w i t h some s e m i c r y s t a l l i n e a r y l o x y copolymers. Note i n Figure 5 that there i s a decreasing order of c r y s t a l l i n i t y from top to bottom. The intermediate cases represent two c l a s s e s of c r y s t a l l i n e copolymers which are d i s t i n g u i s h a b l e by t h e i r thermal t r a n s i t i o n behavior and X-ray c r y s t a l s t r u c t u r e p a t t e r n s . Increasing the d i f f e r e n c e s i n the s i z e and nature of the s u b s t i t u e n t s on the phenoxy r i n g w i l l produce amorphous copolymers, but the polyphosphazene u n i t c e l l appears to be unusually t o l e r a n t of p e r t u r b a t i o n s on a more l i m i t e d s c a l e (12). As evidenced by the s t r u c t u r e s i n Figure 5, some care must be taken i n s e l e c t i n g s u b s t i t u e n t s to achieve d e s i r e d p r o p e r t i e s , e s p e c i a l l y i f the g o a l i s to prepare amorphous polymers.

The s i d e chain s u b s t i t u e n t s can a f f e c t the p r o p e r t i e s of the polyphosphazenes i n yet another way. Whereas (NP(0CH 2CH^) 2) n i s amorphous, i n c r e a s i n g the s i d e chain length by using long chain a l c o h o l s can r e s u l t i n polymers which are s e m i c r y s t a l l i n e (13). Presumably these polymers assume more of the character of poly(ethylene o x i d e ) , as the s i d e chain length i n c r e a s e s .

The morphology of the s e m i c r y s t a l l i n e polyphosphazenes i s complex. Table I provides examples of phosphazenes w i t h two f i r s t order t r a n s i t i o n s denoted by T ( l ) and Tm. The T ( l ) i s an intermediate t r a n s i t i o n to a p a r t i a l l y ordered s t a t e . Between T ( l )

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SINGLERETAL. Phosphazene Polymers

Table I. Summary of Transition and Decomposition Temperatures (°C) for Various Polyphosphazenes

POLYMER V T(U*

[Cl2PNln -66 33·

[(CH3CH20)2PNIn -84

[(CF3CH20)2PNJn -66 90 240* 360

[(C 6H 50) 2PNl n 6 160 390 380

i(3-CIC6H40)2PNIn -24 66 370 380

[(4-CIC6H40)2PNln 4 167 365 410

I([CH3]2N)2PN]n -4

i(C6H5NH)2PN]n 105

i(4-CH3OC6H4NH)2PN]n 92 266

i(CF3CH20)(C3F7CH20)PN]n -77

[(CF3CH20)(HCF2C3F6CH2)PN]n -68

[(C6H50)(4-C2H5C6H40)PN]n -27

[(C6H5)(4-CIC6H40)PNJn 5 77,94

•BY DIFFERENTIAL THERMAL ANALYSIS OR DIFFERENTIAL SCANNING CALORIMETRY. fBY THERMAL MECHANICAL ANALYSIS EXCEPT WHERE NOTED.

T̂HERMAL DECOMPOSITION TEMPERATURES BY THERMAL GRAVIMETRIC ANALYSIS. MOLECULAR WEIGHT CHANGES HAVE BEEN REPORTED BELOW 200°C.

Figure 4. Contra s t i n g synthesis of homopolymers and copolymers showing p o s s i b l e copolymer s t r u c t u r e s which are randomly d i s t r i b u t e d along the polymer backbone.

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DECREASING CRYSTALLINITY

{fN}n

0<>CI

CH3

0O-CH3 {P-Nln O Q - C H 3

iP-N}n

{P-N}n

OQ XH3

{p-ίθί1

ELASTOMER

Figure 5. E f f e c t of side chain s u b s t i t u e n t s on polymer c r y s t a l -l i n i t y . Polymers with two para s u b s t i t u e n t s (second row) are more c r y s t a l l i n e than polymers w i t h mixed para and meta s u b s t i t u e n t s .

and Tm i s a mesomorphic s t a t e . However, d e t a i l e d a n a l y s i s (14-16) shows that the polymers w i t h a T ( l ) t r a n s i t i o n are not nematic or smectic i n nature, but r a t h e r have a pseudohexagonal phase e x h i b i t i n g dynamic d i s o r d e r when c h a r a c t e r i z e d by X-ray d i f f r a c t i o n experiments. Polyphosphazenes such as (NPCOCH^CF^W^ have been termed "condis" or c o n f o r m a t i o n a l ^ disordered c r y s t a l s by nWunderlich (17).

To show another example of the e f f e c t of side chain s t r u c t u r e on polymer p r o p e r t i e s , i t has been r e c e n t l y demonstrated that l i q u i d c r y s t a l l i n e s i d e chain phosphazenes can be prepared by at t a c h i n g a mesogenic group through a f l e x i b l e spacer to the phosphazene polymer chain (18). Copolymer V I I (Figure 6) e x h i b i t s a strong r e v e r s i b l e l i q u i d c r y s t a l l i n e phase between 123 and 175 °C. Microscopic a n a l y s i s i n the l i q u i d c r y s t a l l i n e region i s shown i n Figure 7. A s i m i l a r polyphosphazene w i t h a d i f f e r e n t s u b s t i t u t e d phenylazo mesogen side chain has a l s o been prepared which shows l i q u i d c r y s t a l l i n e order (19). Further work i s underway to e l u c i d a t e the exact nature of t h i s s t a t e and to prepare a d d i t i o n a l l i q u i d c r y s t a l l i n e s i d e chain polyphosphazenes.

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20. SINGLERETAL. Phosphazene Polymers 275

Polyphosphazene

OCH2CH2O -Çyn - N - Q - C M - n

-TN-PI L- i-ln

OCH2CF3

Figure 6. General s t r u c t u r e f o r phosphazenes wit h mesogenic side groups. Example i s a mixed s u b s t i t u e n t polymer (VII) where R represents the t r i f l u o r o e t h o x y group and the mesogen wi t h f l e x i b l e spacer i s represented by the c u r l i c u e and rectangular box.

Figure 7. O p t i c a l micrograph of VII showing texture of the mesophase at 182 °C. M a g n i f i c a t i o n 320 X.

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Conclusion

The polyphosphazenes are high molecular weight polymers wi t h a wide range of novel and p o t e n t i a l l y u s e f u l p r o p e r t i e s . The la r g e number of d i f f e r e n t pendant groups w i t h widely v a r i e d 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 p o t e n t i a l of t h i s c l a s s of polymers. Undoubtedly, some of these w i l l hold promise f o r f u t u r e research and development.

Literature Cited

1. Tate, D.P. and Antowiak, T.A. Kirk-Othmer Encycl. Chem. Technol. 3rd. Ed. 1980, 10, 939.

2. Singler, R.E.; Hagnauer, G.L.; Sicka, R.W. In Polymers for Fibers and Elastomers; Arthur, J.C., Ed.; ACS Symposium Series, No. 260. American Chemical Society, Washington, D.C., 1984, p 143.

3. Allcock, H.R. Chem. & Eng. News, March 18, 1985, p 22. 4. Allcock, H.R. Phosphorus-Nitrogen Compounds; Academic Press, New

York, 1972. 5. Hagnauer, G.L. J. Macromol. Sci.- Chem. 1981, A16, 385. 6. Hagnauer, G.L.; Koulouris, T.N. In Liquid Chromatography of

Polymers and Related Materials-III; Jack Cazes, Ed.; Marcel Dekker, Inc.; New York, 1981; p 99.

7. Sennett, M.S.; Hagnauer, G.L.; Singler,R.E.; Davies,G. Macromolecules 1986, 19, 959.

8. Sennett, M.S. unpublished results. 9. Rose, S.H. J. Polym. Sci. Β 1968, 6, 837. 10. Penton, H.R. In Inorganic and Organometallic Polymers;

Zeldin, M.; Allcock, H. and Wynne, K. Eds., ACS Symposium Series, No. xx, American Chemical Society, Washington, D.C., 1987.

11. Dieck, R.L. and Goldfarb, L. J. Polym. Sci. Poly Chem. Ed. 1977, 15, 361.

12. Beres, J.J.; Schneider, N.S.; Desper, C.R.; Singler, R.E. Macromolecules 1979, 12, 566.

13. Allcock, H.R.; Austin, P.E.; Neenan, T.X.; Sisko, J.T.; Blonsky, P.M.; Shriver, D.F. Macromolecules 1986, 19, 1508.

14. Schneider, N.S.; Desper, C.R.; Beres, J.J. In Liquid Crystalline Order in Polymers; Blumstein, Α., Ed., Academic Press, N.Y., 1978, p 299.

15. Kojima, M.; Magill, J.H. Makromol. Chem. 1985, 186, 649. 16. Yeung, A.S.; Frank, C.W.; Singler, R.E. Polym. Prepr. ACS Div.

Polym. Chem. 1986, 27(2), 214. 17. Wunderlich, B. and Grebowicz, J. In Polymeric Liquid Crystals;

Blumstein, Α., Ed.; Plenum Press, New York, 1985, 28, 145. 18. Singler, R.E.; Willingham, R.A.; Lenz, R.W.; Furukawa, Α.;

Finkelmann, H. Macromolecules 1987, in press. 19. Allcock, H.R. and Kim, C. Macromolecules 1987, in press.

RECEIVED September 1, 1987

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[acs symposium series] inorganic and organometallic polymers volume 360 (macromolecules containing...

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