Phosphazene Polymers: Synthesis, Structure, and Properties - ACS

Chapter 20

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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(dichlorophosphazene) as a common intermediate for a wide variety of poly(organophosphazenes). Melt and solution polymerization techniques are i l l u s t r a t e d , including the role of c a t a l y s t s . The elucidation of chain structure and molecular weight by various d i l u t e solution techniques i s considered. Factors which determine the properties of polymers derived from poly(dichlorophosphazene) are discussed, with an emphasis on the role that the organic substituent can play i n determining the f i n a l properties.

The s t u d y 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 r e c e n t y e a r s , b o t h from the s t a n d p o i n t o f fundamental r e s e a r c h and t e c h n o l o g i c a l development. The polyphosphazenes a r e l o n g c h a i n s o f a l t e r n a t i n g p h o s p h o r u s - n i t r o g e n atoms w i t h two s u b s t i t u e n t s a t t a c h e d t o phosphorus. These polymers have been the s u b j e c t o f s e v e r a l r e c e n t r e v i e w s ( 1 - 3 ) . I n t e r e s t has stemmed from the c o n t i n u i n g s e a r c h 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 n o v e l properties. F i g u r e 1 p r o v i d e s an o v e r v i e w of the two s t e p s y n t h e s i s p r o c e s s , p i o n e e r e d by A l l c o c k (4) and i n use today by a number o f workers and l a b o r a t o r i e s : f o r m a t i o n of a s o l u b l e r e a c t i v e polymer i n t e r m e d i a t e ( I I ) from which i s d e r i v e d a l a r g e number o f polymers v i a substitution reactions. S i n c e 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 t o answer v a r i o u s q u e s t i o n s : 1) What i s the n a t u r e of the p o l y m e r i z a t i o n p r o c e s s (mechanism)? 2) What i s the s t r u c t u r e of p o l y ( d i c h l o r o p h o s p h a z e n e ) t h a t 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 r u b b e r " ( I I I ) ? 3) The s u b s t i t u t i o n p r o c e s s g i v e s a s e e m i n g l y e n d l e s s v a r i e t y o f p r o d u c t s . What a r e the l i m i t a t i o n s o r

This chapter is not subject to U.S. copyright. Published 1988, American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


20.

Phosphazent Polymers

SINGLERETAL.

269

CROSSLINKED MATRIX III

HNRR'-Et3N OR / r

/

I ι

{N=P}

loAr Γ

X

'I ι

{N=P}

\ ?

X

NRR' I

η

±N=P}

X

OR

OAr

NRR'

IV

V

VI

F i g u r e 1. S y n t h e s i s o f p o l y ( d i c h l o r o p h o s p h a z e n e ) and poly(organophosphazenes).

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

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) ( e g . IV, V, V I ) ? 5) Are any o f these polymers t e c h n o l o g i c a l l y u s e f u l o r of commercial i n t e r e s t ? T h i s paper w i l l p r o v i d e an o v e r v i e w of the p o l y m e r i z a t i o n p r o c e s s e s and the p r o p e r t i e s of p o l y ( d i c h l o r o p h o s p h a z e n e ) . This paper w i l l a l s o d i s c u s s the v a r i o u s 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 o f 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 o f polymers w i t h a wide range o f 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 o f c u r r e n t commercial i m p o r t a n c e . Poly(dichlorophosphazene) The p o l y m e r i z a t i o n of h e x a c h l o r o c y c l o t r i p h o s p h a z e n e ( I ) has been the s u b j e c t o f 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 , III) i s markedly i n f l u e n c e d by the presence of t r a c e i m p u r i t i e s . The c o n v e n t i o n a l r o u t e t o I I i s a m e l t p o l y m e r i z a t i o n a t 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 ^ ) ^ , s e a l e d 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 n e c e s s a r y t o o b t a i n I I and a v o i d the f o r m a t i o n 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 p r o c e s s e s , v a r i o u s a c i d s and o r g a n o m e t a l l i c compounds can be u t i l i z e d as c a t a l y s t s t o prepare s o l u b l e polymer, b o t h i n b u l k and i n s o l u t i o n ( 2 ) . The advantages of c a t a l y z e d p o l y m e r i z a t i o n s i n c l u d e lower r e a c t i o n t e m p e r a t u r e s , h i g h e r y i e l d s , and the use of c o n v e n t i o n a l l a r g e s c a l e equipment. S i z e e x c l u s i o n chromatography (GPC) and o t h e r d i l u t e s o l u t i o n t e c h n i q u e s have been a p p l i e d t o the c h a r a c t e r i z a t i o n of I I ( 6 , 7 ) . Polymers o b t a i n e d from the b u l k p o l y m e r i z a t i o n t y p i c a l l y have h i g h m o l e c u l a r w e i g h t s and broad m o l e c u l a r weight d i s t r i b u t i o n s (MWD's). C a t a l y z e d p r o c e s s e s g e n e r a l l y g i v e narrower MWD's but lower m o l e c u l a r weight polymer. A l t h o u g h q u e s t i o n s s t i l l remain as t o the n a t u r e 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 t o be a c a t i o n i c , c h a i n growth, r i n g opening p o l y m e r i z a t i o n p r o c e s s ( F i g u r e 2 ) . E v i d e n c e 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 ^ , f o r m a t i o n of h i g h m o l e c u l a r 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 o b t a i n e d on I I which p o i n t t o randomly c o i l e d polymer c h a i n s r e l a t i v e l y f r e e of l o n g - c h a i n b r a n c h i n g f o r low t o moderate c o n v e r s i o n s t o h i g h polymer. One way t o overcome the m o l e c u l a r 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 o f the " l i v i n g " n a t u r e 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 ) t o the e x i s t i n g polymer t o i n c r e a s e the m o l e c u l a r weight i n a s t e p w i s e f a s h i o n ( F i g u r e 3 ) . T r i m e r i s p o l y m e r i z e d i n the presence of BC1~ i n a t r i c h l o r o b e n z e n e s o l u t i o n i n a s e a l e d ampoule a t 210 °C f o r 48 h o u r s . For the second and t h i r d s t a g e s , t r i m e r i s added i n s o l u t i o n e q u a l t o the amount i n stage 1. The B C l ^ c o n c e n t r a t i o n i s h e l d c o n s t a n t . Each stage i s c a r r i e d t o g r e a t e r than 95 % c o n v e r s i o n . L i g h t s c a t t e r i n g measurements on the polymer o b t a i n e d from s t a g e 3 show MW > 10 , thus c o n f i r m i n g t h a t h i g h m o l e c u l a r weight I I can be o b t a i n e d i n h i g h conversion i n a c a t a l y z e d s o l u t i o n process ( 8 ) .


Phosphazene Polymers

20. SINGLERETAL.

[NPCI l3 — 2

[NPCI J 2

n

BULK - UNCATALYZED HIGH PURITY TRIMER NECESSARY - OTCERWISE GEL FORMATION HIGH POLYMER (MW - 10 ) 6

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

F i g u r e 2. G e n e r a l comments on t h e p o l y m e r i z a t i o n

BCI [NPCI l3

process.

3

2

" [NPCI ] 2

TCB, 210°C SEALED TUBE

n

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 «

1 2 3

13,000 100,000 322,000

n

M » w

37,000 118,000 536,000 ( M ~ 6 χ 10 )

*GPC MW DE7IRMI NATION. *LIGHT SCATTERING.

w

6

f

POLYSTYRENE STANDARDS.

SENNE1T (1986)

F i g u r e 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.


272



Poly(organophosphazenes) The s y n t h e s i s of poly(organophosphazenes) r e p r e s e n t s p r o b a b l y the b e s t example of a c e n t r a l theme of i n o r g a n i c macromolecules: P r e p a r a t i o n of a r e a c t i v e p o l y m e r i c i n t e r m e d i a t e , 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 ( F i g u r e 1 ) . T h i s concept has been demonstrated i n a number of l a b o r a t o r i e s (3) and has p r o v i d e d a wide v a r i e t y o f polymers w i t h d i f f e r e n t p r o p e r t i e s . T a b l e I s e r v e s t o i l l u s t r a t e how the n a t u r e and s i z e of the s u b s t i t u e n t a t t a c h e d t o 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 p o l y ( o r g a n o p h o s p h a z e n e s ) . The g l a s s t r a n s i t i o n temperatures range from -84 °C f o r ( N P ( O C H C H > ) t o around 100 °C f o r the p o l y ( a n i l i n o p h o s p h a z e n e s ) . Polymers range from e l a s t o m e r s t o f l e x i b l e f i l m f o r m i n g t h e r m o p l a s t i c s or g l a s s e s a t room t e m p e r a t u r e . I n the case o f p o l y ( a l k o x y p h o s p h a z e n e s ) ( I V ) o r p o l y ( a r y l o x y p h o s phazenes) (V) a d r a m a t i c change i n p r o p e r t i e s can a r i s e by employing c o m b i n a t i o n s o f s u b s t i t u e n t s . Polymers such as ( N P ( 0 C H C F ) ) and ( N P ( O C H ) ) a r e s e m i c r y s t a l l i n e t h e r m o p l a s t i c s ( T a b l e 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 o f 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 , e l a s t o m e r s are o b t a i n e d ( F i g u r e 4 ) . Another requirement f o r e l a s t o m e r i c b e h a v i o r i s t h a t 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 a l o n g the P-N backbone. T h i s 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 t o the development of phosphazene e l a s t o m e r s of commercial i n t e r e s t . A phosphazene f l u o r o e l a s t o m e r and a phosphazene elastomer w i t h mixed a r y l o x y s i d e c h a i n s a r e 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 e l a s t o m e r s a r e the s u b j e c t of a n o t h e r paper i n t h i s symposium ( 1 0 ) . S t u d i e s have shown t h a t not a l l phosphazene copolymers a r e n e c e s s a r i l y elastomers (11,12). Figure 5 contrasts s e m i c r y s t a l l i n e homopolymers w i t h an e l a s t o m e r i c copolymer, and t h e n 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 c o p o l y m e r s . Note i n F i g u r e 5 t h a t t h e r e i s a d e c r e a s i n g o r d e r of c r y s t a l l i n i t y from top t o bottom. The i n t e r m e d i a t e c a s e s r e p r e s e n t two c l a s s e s o f c r y s t a l l i n e copolymers which a r e d i s t i n g u i s h a b l e by t h e i r t h e r m a l t r a n s i t i o n b e h a v i o r 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 . I n c r e a s i n g the d i f f e r e n c e s i n the s i z e and n a t u r e 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 c o p o l y m e r s , but the polyphosphazene u n i t c e l l appears t o be u n u s u a l l y 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 ( 1 2 ) . As evidenced by the s t r u c t u r e s i n F i g u r e 5, some c a r e must be t a k e n i n s e l e c t i n g substituents to achieve desired 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 t o p r e p a r e amorphous polymers. The s i d e c h a i n 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 y e t a n o t h e r way. Whereas ( N P ( 0 C H C H ^ ) ) i s amorphous, i n c r e a s i n g the s i d e c h a i n l e n g t h by u s i n g l o n g c h a i n a l c o h o l s can r e s u l t i n polymers w h i c h a r e s e m i c r y s t a l l i n e ( 1 3 ) . Presumably these polymers assume more of the c h a r a c t e r of p o l y ( e t h y l e n e o x i d e ) , as the s i d e c h a i n l e n g t h 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. T a b l e I p r o v i d e s examples o f phosphazenes w i t h two f i r s t o r d e r t r a n s i t i o n s denoted by T ( l ) and Tm. The T ( l ) i s an i n t e r m e d i a t e t r a n s i t i o n t o a p a r t i a l l y ordered s t a t e . Between T ( l ) 2

3

2

n

2

6

5

2

3

2

n

2

2


n

SINGLERETAL.


Table I. Summary of T r a n s i t i o n and Decomposition Temperatures (°C) f o r Various Polyphosphazenes

V

POLYMER

33·

-66

[Cl2PNln [(CH3CH20)2PNI

-84

[(CF3CH 0) PNJ

-66

90

240*

360

6

160

390

380

-24

66

370

380

4

167

365

410

n

2


T(U*

2

[(C H 0) PNl 6

5

2

n

n

i(3-CIC H 0) PNI 6

4

2

[(4-CIC H 0) PNl 6

4

2

I([CH ] N) PN] 3

2

2

i(C H NH) PN] 6

5

2

n

n

-4

n

105

n

i(4-CH OC H NH) PN] 3

6

4

2

3

2

7

2

-77

n

[(CF CH 0)(HCF C F CH )PN] 3

2

2

3

6

2

[(C H 0)(4-C H C H 0)PN] 6

5

2

5

6

4

[(C H )(4-CIC H 0)PNJ 6

5

6

4

266

92

n

i(CF CH 0)(C3F CH 0)PN]

n

n

n

-68 -27 5

77,94

•BY DIFFERENTIAL THERMAL ANALYSIS OR DIFFERENTIAL SCANNING CALORIMETRY. BY THERMAL MECHANICAL ANALYSIS EXCEPT WHERE NOTED. ^THERMAL DECOMPOSITION TEMPERATURES BY THERMAL GRAVIMETRIC ANALYSIS. MOLECULAR WEIGHT CHANGES HAVE BEEN REPORTED BELOW 200°C. f

F i g u r e 4. C o n t r a s t i n g s y n t h e s i s o f homopolymers and c o p o l y m e r s showing p o s s i b l e copolymer s t r u c t u r e s w h i c h a r e randomly d i s t r i b u t e d a l o n g t h e polymer backbone.


274


DECREASING

CRYSTALLINITY

0O- 3 CH


{fN}n

{P-Nln OQ-CH

{p-ίθί

1

3

iP-N}n

0CI {P-N}

n

CH3

OQ

XH3

ELASTOMER

F i g u r e 5. E f f e c t o f s i d e c h a i n 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 w i t h two p a r a 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 t h a t the polymers w i t h a T ( l ) t r a n s i t i o n a r e not nematic o r s m e c t i c i n n a t u r e , 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 e x p e r i m e n t s . Polyphosphazenes such as (NPCOCH^CF^W^ have been termed " c o n d i s " o r c o n f o r m a t i o n a l ^ disordered c r y s t a l s by Wunderlich (17). To show a n o t h e r example o f the e f f e c t o f s i d e c h a i n 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 t h a t l i q u i d c r y s t a l l i n e s i d e c h a i n phosphazenes can be p r e p a r e d by a t t a c h i n g a mesogenic group through a f l e x i b l e s p a c e r t o the phosphazene polymer c h a i n ( 1 8 ) . Copolymer V I I ( F i g u r e 6) e x h i b i t s a s t r o n g 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. M i c r o s c o p i c 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 r e g i o n i s shown i n F i g u r e 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 s i d e c h a i n 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 o r d e r ( 1 9 ) . F u r t h e r work i s underway t o e l u c i d a t e the exact n a t u r e o f t h i s s t a t e and t o 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 c h a i n polyphosphazenes. n


20.


SINGLERETAL.

275


Polyphosphazene

OCH2CH2O -Çyn - N - Q - C M - n

-TN-PI L- i-ln OCH2CF3

F i g u r e 6. G e n e r a l s t r u c t u r e f o r phosphazenes w i t h mesogenic s i d e groups. Example i s a mixed s u b s t i t u e n t polymer ( V I I ) where R r e p r e s e n t s the t r i f l u o r o e t h o x y group and the mesogen w i t h f l e x i b l e spacer i s r e p r e s e n t e d by the c u r l i c u e and r e c t a n g u l a r box.

F i g u r e 7. O p t i c a l m i c r o g r a p h o f V I I showing t e x t u r e o f t h e mesophase a t 182 °C. M a g n i f i c a t i o n 320 X.


276


Conclusion The polyphosphazenes a r e h i g h m o l e c u l a r weight polymers w i t h a wide range o f n o v e l 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 l a r g e number of d i f f e r e n t pendant groups w i t h w i d e l y v a r i e d f u n c t i o n a l i t y which can be a t t a c h e d t o t h e P-N backbone demonstrate t h e u n u s u a l m o l e c u l a r d e s i g n p o t e n t i a l o f t h i s c l a s s o f polymers. Undoubtedly, some o f these w i l l h o l d promise f o r f u t u r e r e s e a r c h and development.


Literature Cited 1. 2.

3. 4. 5. 6.

7. 8. 9. 10.

11. 12. 13. 14.

15. 16. 17. 18. 19.

Tate, D.P. and Antowiak, T.A. Kirk-Othmer Encycl. Chem. Technol. 3rd. Ed. 1980, 10, 939. 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. Allcock, H.R. Chem. & Eng. News, March 18, 1985, p 22. Allcock, H.R. Phosphorus-Nitrogen Compounds; Academic Press, New York, 1972. Hagnauer, G.L. J. Macromol. Sci.- Chem. 1981, A16, 385. 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. Sennett, M.S.; Hagnauer, G.L.; Singler,R.E.; Davies,G. Macromolecules 1986, 19, 959. Sennett, M.S. unpublished results. Rose, S.H. J. Polym. Sci. Β 1968, 6, 837. 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. Dieck, R.L. and Goldfarb, L. J. Polym. Sci. Poly Chem. Ed. 1977, 15, 361. Beres, J.J.; Schneider, N.S.; Desper, C.R.; Singler, R.E. Macromolecules 1979, 12, 566. Allcock, H.R.; Austin, P.E.; Neenan, T.X.; Sisko, J.T.; Blonsky, P.M.; Shriver, D.F. Macromolecules 1986, 19, 1508. 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. Kojima, M.; Magill, J.H. Makromol. Chem. 1985, 186, 649. Yeung, A.S.; Frank, C.W.; Singler, R.E. Polym. Prepr. ACS Div. Polym. Chem. 1986, 27(2), 214. Wunderlich, B. and Grebowicz, J. In Polymeric Liquid Crystals; Blumstein, Α., Ed.; Plenum Press, New York, 1985, 28, 145. Singler, R.E.; Willingham, R.A.; Lenz, R.W.; Furukawa, Α.; Finkelmann, H. Macromolecules 1987, in press. Allcock, H.R. and Kim, C. Macromolecules 1987, in press.

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