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  • 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 / ' I ? I

    { 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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    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) (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 measure-ments 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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    In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

  • 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, 250C, 40-100 hr

    BULK - CATALYZED

    TRIMER PURITY LESS CRITICAL

    LOWER TEMPERATURES (170C - 220C)

    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, 210C SEALED TUBE

    STEPWISE PROCESS

    FIRST STAGE: 15 wt% TRIMER IN C6H3CI3 (3g/16g). BCI3-0.66g. 48 hr. 210C. 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 ~ 6 106) 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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