Phosphazene Polymers - American Chemical Society

Chapter 20

Downloaded by KUNGLIGA TEKNISKA HOGSKOLAN on October 11, 2014 | http://pubs.acs.org Publication Date: January 7, 1988 | doi: 10.1021/bk-1988-0360.ch020

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.

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