An Introduction to Inorganic and Organometallic Polymers - ACS

Jan 7, 1988 - The subtitle delineates elements of primary focus: "Macromolecules Containing Silicon, Phosphorus and Other Inorganic Elements." The ter...
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Chapter 1

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An Introduction to Inorganic and Organometallic Polymers Kenneth J. Wynne Chemistry Division, Office of Naval Research, Arlington, V A 22217-5000

"Inorganic and organometallic" in the context of t h i s Symposium is meant to describe macromolecules which usually contain inorganic elements in the chain and organic moieties as pendant groups. The s u b t i t l e delineates elements of primary focus: "Macromolecules Containing S i l i c o n , Phosphorus and Other Inorganic Elements." The term "macromolecules" implies that the subject matter includes chain molecules that may be b u i l t up of repeat u n i t s , as well as more complex r i n g , branched, or crosslinked species (for example, see Organo-Oxo-Element Macromolecules Related to Sol-Gel Processes, and contributions by Murray (p. 408) or Seyferth (p. 143)). P r i o r reviews concerning inorganic and organometallic macromolecules are contained in texts by Stone and Graham (1), Andrianov (2), Borisov (3), Allcock (4,5), and Voronkov (6), and volumes based on previous ACS symposia edited by Rheingold (7), and Carraher, Sheats and Pittman (8, 9). The present e f f o r t is t o p i c a l in nature, and the order of presentation approximates that of the Symposium presentations. The impetus for t h i s symposium volume is the considerable progress which has been made in the l a s t few years in inorganic and organometallic macromolecules. T o t a l l y new macromolecules have been brought into existence by the development of new synthetic methods or improvement of known synthetic routes. Thus, Neilson (p. 283) describes a new polymerization reaction that gives high molecular weight poly(diorganophosphazenes), -(R PN)n -, in which organic pendant groups are bonded through P-C bonds. These polyphosphazene analogs of polysiloxanes were not previously accessible, and the development of structure-property relationships in this new subclass of macromolecules w i l l no doubt y i e l d important information and perhaps s i g n i f i c a n t applications. 2

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

INORGANIC AND ORGANOMETALLIC POLYMERS

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2

A n o t h e r example o f i n t e r e s t i n g new i n o r g a n i c polymers is f o u n d in the work o f L a s o c k i (p. 166), who r e p o r t s the s y n t h e s i s of p o l y s i l a z o x a n e s , - [ ( R SiOj^CR^SiNR) ] -, and f i n d s s u r p r i s i n g l y b e t t e r t h e r m a l s t a b i l i t y compared wïtfi t h e i r p o l y s i l o x a n e a n a l o g s . The d e s i g n o f f u n c t i o n a l i z e d polymers w i t h a s p e c i f i c u t i l i z a t i o n is seen in new p o l y s i l o x a n e s used by Z e l d i n (p. 199) as phase t r a n s f e r c a t a l y s t s . N o v e l f u n c t i o n a l p o l y p h o s p h a z e n e s have been r e p o r t e d as w e l l by A l l c o c k (p. 250). The i n t r o d u c t i o n o f t r a n s i t i o n m e t a l c y c l o p e n t a d i e n y l , m e t a l c a r b o n y l and c a r b o r a n e m o i e t i e s i n t o p o l y p h o s p h a z e n e macromolecules is r e p r e s e n t a t i v e o f t r u l y n o v e l c h e m i s t r y a c h i e v e d a f t e r c a r e f u l model s t u d i e s w i t h corresponding molecular systems. West (p. 6 ) , M i l l e r (p. 4 3 ) , Z e i g l e r ( 1 0 ) , and Sawan (p. 112) o u t l i n e the s y n t h e s i s of a wide v a r i e t y o f s o l u b l e , p r o c e s s a b l e p o l y d i o r g a n o s i l a n e s , a c l a s s o f polymers which not l o n g ago was t h o u g h t t o be i n t r a c t a b l e . M a t y j a s z e w s k i (p. 78) has found s i g n i f i c a n t improvements in the s y n t h e t i c method f o r p o l y d i o r g a n o s i l a n e s y n t h e s i s as w e l l as new s y n t h e t i c r o u t e s t o unusual s u b s t i t u t e d polydiorganosilanes. S e y f e r t h (p. 21, 143) r e p o r t s s y n t h e t i c r o u t e s t o a number o f new p o l y c a r b o s i l a n e s and p o l y s i l a z a n e s w h i c h may be used as p r e c u r s o r s t o c e r a m i c m a t e r i a l s . New c a t a l y t i c p o l y m e r i z a t i o n r o u t e s t o p o l y s i l a n e s ( H a r r o d , (p. 89)) and p o l y s i l a z a n e s ( L a i n e , (p. 124)) have been d i s c o v e r e d . S i n g l e r ( p . 268) d e s c r i b e s t h e use o f B C l ^ in t h e more e f f i c i e n t s y n t h e s i s of - ( P N C 1 ) -, the s t a r t i n g h i g h polymer f o r most polyphosphazene p o l y m e r s c u r r e n t l y under i n v e s t i g a t i o n . These r e s u l t s in the a r e a o f p o l y m e r i z a t i o n c a t a l y s t s a r e i m p o r t a n t , as the s y s t e m a t i c development o f e f f i c i e n t c a t a l y t i c r o u t e s f o r i n o r g a n i c and o r g a n o m e t a l l i c macromolecules w i l l make such m a t e r i a l s more g e n e r a l l y a c c e s s i b l e and u t i l i z a b l e . T h i s r e s e a r c h is a l s o c l o s e l y r e l a t e d t o u n d e r s t a n d i n g o f mechanisms o f c h a i n growth. E f f o r t s aimed a t the e l u c i d a t i o n o f p o l y m e r i z a t i o n mechanisms i n c l u d e t h o s e o f S i n g l e r (p. 268) in p o l y p h o s p h a z e n e s , L i p o w i t z (p. 156) in p o l y s i l a z a n e s , and Z e i g l e r and W o r s f o l d in p o l y s i l a n e s . In c o n t r a s t w i t h c a r b o n c h e m i s t r y , the mechanisms o f p o l y m e r i z a t i o n r e a c t i o n s l e a d i n g t o i n o r g a n i c and o r g a n o m e t a l l i c m a c r o m o l e c u l e s a r e o f t e n not w e l l u n d e r s t o o d . Such s t u d i e s are c r i t i c a l in e l u c i d a t i n g pathways o f c h a i n growth, t e r m i n a t i o n , and b r a n c h i n g so t h a t t h e s e f e a t u r e s may be c o n t r o l l e d . O f t e n t i m e s m e c h a n i s t i c s t u d i e s l e a d t o more e f f i c i e n t s y n t h e t i c methods, f o r example b y i m p r o v i n g y i e l d s o r s h o r t e n i n g r e a c t i o n times. In the s e c t i o n on b o r o n - c o n t a i n i n g p o l y m e r s , P a i n e (p. 378), N e i l s o n ( p . 385), and P a c i o r e k (p. 392) p r e s e n t p i o n e e r i n g work aimed a t the p r e p a r a t i o n o f l i n e a r c h a i n m a c r o m o l e c u l e s w i t h B-N backbones. T h i s work is s i g n i f i c a n t because c o n d e n s a t i o n r e a c t i o n s o f B-N compounds tend to p r o d u c e compounds w i t h r i n g s t r u c t u r e s r a t h e r t h a n c h a i n s . One o b v i o u s p o t e n t i a l u t i l i z a t i o n o f such n o v e l macromolecules is as p r e c e r a m i c m a t e r i a l s , much in the same way as p o l y a c r y l o n i t r i l e is used as a p r e c u r s o r f o r c a r b o n . O r i e n t a t i o n o f t h e B-N polymer may be t r a n s f e r r e d in p a r t t o the c e r a m i c s o l i d s t a t e a g a i n in manner s i m i l a r to c a r b o n c h e m i s t r y . However, the p r o p e r t i e s o f BN p o l y m e r s and c e r a m i c m a t e r i a l s d i f f e r g r e a t l y from t h e i r c a r b o n a n a l o g s due t o l o c a l i z e d e l e c t r o n i c s t a t e s in t h e BN bond. The consequences o f t h i s c o n t r a s t i n g e l e c t r o n i c s t r u c t u r e on m a t e r i a l s p r o p e r t i e s w i l l be i n t e r e s t i n g t o see as t h i s new r e s e a r c h a r e a develops. 9

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

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1. WYNNE

Introduction

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A p l u r a l i t y o f p a p e r s in t h i s volume c o n c e r n l i n e a r c h a i n m a c r o m o l e c u l e s . Fundamental t o u n d e r s t a n d i n g t h e p h y s i c a l and m e c h a n i c a l b e h a v i o r and c h e m i c a l and p h y s i c a l s t a b i l i t y o f t h e s e m a c r o m o l e c u l e s is a f a m i l i a r i t y w i t h phase t r a n s i t i o n b e h a v i o r , an a r e a w e l l known in o r g a n i c polymer c h e m i s t r y ( 1 1 ) . As with organic p o l y m e r s , amorphous and s e m i c r y s t a l l i n e i n o r g a n i c and o r g a n o m e t a l l i c macromolecules a r e known. C r y s t a l l i n i t y a r i s e s f r o m m a i n c h a i n o r d e r o r s i d e c h a i n c r y s t a l l i z a t i o n as d i s c u s s e d by S i n g l e r (p. 268) f o r polyphosphazenes and West (p. 6) and M i l l e r ( p . 43) f o r p o l y s i l a n e s . The l a t t e r work d e m o n s t r a t e s that c r y s t a l l i z a t i o n b e h a v i o r p l a y s a c r i t i c a l r o l e in c o n t r o l l i n g main c h a i n c o n f o r m a t i o n and o p t i c a l t r a n s i t i o n s in p o l y ( d i o r g a n o s i l a n e s ) . Thus, i m p o r t a n t s t r u c t u r e / p r o p e r t y r e l a t i o n s h i p s a r e emerging t h a t a r e r e l e v a n t t o e l e c t r o n i c and o p t i c a l m a t e r i a l s a p p l i c a t i o n s f o r these m a t e r i a l s . In a d i f f e r e n t v e i n , s i d e chain c r y s t a l l i z a t i o n has r e s u l t e d in t h e f i r s t l i q u i d c r y s t a l l i n e i n o r g a n i c and o r g a n o m e t a l l i c macromolecules, v i z . , unusual p o l y ( d i a l k o x y phosphazenes) d e s c r i b e d by A l l c o c k (p. 250) and S i n g l e r (p. 2 6 8 ) . In t h i s c a s e , t h e f l e x i b l e n a t u r e o f t h e P-N c h a i n p l a c e s s t r i n g e n t s t r u c t u r a l r e q u i r e m e n t s on t h e n a t u r e o f t h e pendant group. Unique c o m b i n a t i o n s o f p r o p e r t i e s c o n t i n u e t o be d i s c o v e r e d in i n o r g a n i c and o r g a n o m e t a l l i c macromolecules and s e r v e t o c o n t i n u e a h i g h l e v e l o f i n t e r e s t w i t h r e g a r d t o p o t e n t i a l a p p l i c a t i o n s . Thus, A l l c o c k d e s c r i b e s h i s c o l l a b o r a t i v e work w i t h S h r i v e r (p. 250) t h a t l e d t o i o n i c a l l y c o n d u c t i n g p o l y p h o s p h a z e n e / s a l t complexes w i t h t h e h i g h e s t ambient temperature i o n i c c o n d u c t i v i t i e s known f o r p o l y m e r / s a l t e l e c t r o l y t e s . E l e c t r o n i c c o n d u c t i v i t y is found v i a t h e p a r t i a l o x i d a t i o n o f u n u s u a l p h t h a l o c y a n i n e s i l o x a n e s (Marks, p. 224) which c o n t a i n s i x - c o o r d i n a t e r a t h e r t h a n t h e u s u a l four-coordinate Si. P a r t o f t h i s symposium was d i r e c t e d t o t h e s y n t h e s i s , p r o p e r t i e s and a p p l i c a t i o n s o f i n o r g a n i c and o r g a n o m e t a l l i c m a c r o m o l e c u l e s w i t h network s t r u c t u r e s . The s e c t i o n on organo-oxo macromolecules r e l e v a n t t o s o l - g e l p r o c e s s i n g a d d r e s s e s t h e i n t e r e s t i n g s y n t h e s i s and c h a l l e n g i n g c h a r a c t e r i z a t i o n e f f o r t s in t h i s a r e a . B r i n k e r (p. 314) o u t l i n e s t h e complex c h e m i c a l and p h y s i c a l f a c t o r s w h i c h a f f e c t network f o r m a t i o n and s t r u c t u r e r e s u l t i n g from t h e h y d r o l y s i s o f a t e t r a a l k o x y s i l a n e . The i n t e r e s t i n g p r o p e r t i e s o f h y b r i d o r g a n i c / i n o r g a n i c network s t r u c t u r e s a r e d e s c r i b e d in the work o f Schmidt ( p . 333) and W i l k e s ( p . 354). In c o n c l u s i o n , some t r e n d s c a n be g l e a n e d from an e x a m i n a t i o n o f t h e c o n t e n t o f t h e symposium as a whole. The growth in r e s e a r c h e f f o r t s a d d r e s s i n g t h e s y n t h e s i s and p r o p e r t i e s o f p o l y ( d i o r g a n o s i l a n e s ) w i l l l i k e l y c o n t i n u e . The u n i q u e p h o t o p h y s i c a l p r o p e r t i e s o f t h i s newly d e v e l o p e d c l a s s o f i n o r g a n i c m a c r o m o l e c u l e s (12) t o g e t h e r w i t h ready s y n t h e t i c r o u t e s w i l l be c o n t r i b u t i n g f o r c e s h e r e , and no doubt new v e c t o r s w i l l a r i s e . A n o t h e r a r e a o f i n c r e a s e d a t t e n t i o n w i l l be organo-oxo macromolecules d e r i v e d from s o l - g e l p r o c e s s i n g methods, e i t h e r a s c o p o l y m e r s o r b l e n d s . Complex d e p e n d e n c i e s o f organo-oxo m a c r o m o l e c u l a r c o m p o s i t i o n and s t r u c t u r e on s t a r t i n g m a t e r i a l s a n d p r o c e s s i n g c o n d i t i o n s ( i n c l u d i n g k i n e t i c e f f e c t s ) w i l l l e a d to c h a l l e n g i n g and i n t e r e s t i n g s c i e n c e . Important m e c h a n i c a l , o p t i c a l and s t r u c t u r a l a p p l i c a t i o n s c o u p l e d a g a i n w i t h emerging s y n t h e t i c approaches w i l l be among t h e d r i v e r s f o r c o n t i n u e d h i g h a c t i v i t y in t h i s area.

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

4

INORGANIC AND ORGANOMETALLIC POLYMERS

F i n a l l y , one a d d i t i o n a l comment c o n c e r n i n g t h e n a t u r e o f p r o g r e s s from i n t e r d i s c i p l i n a r y r e s e a r c h is e v i d e n t from t h e r e s u l t s r e p o r t e d in t h i s Symposium Volume. Once a g a i n i t is s e e n t h a t most r a p i d p r o g r e s s is made when s y n t h e t i c chemists c o l l a b o r a t e w i t h t h e i r c o l l e a g u e s in m a t e r i a l s s c i e n c e o r p h y s i c s t o d e t e r m i n e p r o p e r t i e s o f new i n o r g a n i c and o r g a n o m e t a l l i c po l y m e r s . Acknowledgment The a u t h o r thanks t h e O f f i c e o f N a v a l R e s e a r c h contribution.

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