Polysilane High Polymers: An Overview - ACS Publications - American

Chapter 2

Polysilane High Polymers: An Overview

Downloaded by GEORGETOWN UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: January 7, 1988 | doi: 10.1021/bk-1988-0360.ch002

Robert West and Jim Maxka Department of Chemistry, University of Wisconsin, Madison, WI 53706 The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of s i l i c o n atoms, which show unique properties resulting from easy delocalization of sigma electrons in the s i l i c o n - s i l i c o n bonds. Polysilanes may be useful as precursors to s i l i c o n carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors.

The p o l y s i l a n e s are compounds c o n t a i n i n g c h a i n s , r i n g s , o r t h r e e d i m e n s i o n a l s t r u c t u r e s of s i l i c o n atoms j o i n e d by c o v a l e n t bonds. R e c e n t l y , p o l y s i l a n e h i g h polymers have become the s u b j e c t of i n t e n s e r e s e a r c h in numerous l a b o r a t o r i e s . These polymers show many unusual p r o p e r t i e s , r e f l e c t i n g the easy d e r e a l i z a t i o n of sigma e l e c t r o n s in the s i l i c o n - s i l i c o n bonds. I n f a c t , the p o l y s i l a n e s e x h i b i t b e h a v i o r u n l i k e t h a t f o r any o t h e r known c l a s s of m a t e r i a l s . In t h i s c h a p t e r , an i n t r o d u c t i o n and overview of p o l y s i l a n e c h e m i s t r y w i l l be p r e s e n t e d , c o n c e n t r a t i n g on the l i n e a r h i g h polymers ( p o l y s i l a n e s ) and t h e i r t e c h n o l o g i c a l a p p l i c a t i o n s . P o l y s i l a n e polymers were reviewed in 1986,(1) and a more g e n e r a l review of p o l y s i l a n e c h e m i s t r y appeared in 1982.(2) Historical P o l y ( d i p h e n y l s i l y l e n e ) may have been prepared as e a r l y as the 1920's by F. S. K i p p i n g , the g r a n d f a t h e r of o r g a n o s i l i c o n c h e m i s t r y ; but the p o l y m e r i c or o l i g o m e r i c p r o d u c t s were not c h a r a c t e r i z e d . The f i r s t c e r t a i n p r e p a r a t i o n of a l i n e a r p o l y s i l a n e came in 1949, when C h a r l e s Burkhard of the G e n e r a l E l e c t r i c Company Research Laborat o r i e s , p u b l i s h e d a c l a s s i c paper d e s c r i b i n g the s y n t h e s i s of p o l y ( d i m e t h y l s i l y l e n e ) , ( M e 2 S i ) . ( 3 ) The polymer was o b t a i n e d by condensing d i m e t h y l d i c h l o r o s i l a n e w i t h sodium m e t a l , in essence the same p r o c e s s used today f o r the s y n t h e s i s of p o l y s i l a n e s . Burkhard d e s c r i b e d ( M e 2 S i ) q u i t e c l e a r l y and a c c u r a t e l y , as an i n s o l u b l e , i n f u s i b l e , and g e n e r a l l y q u i t e i n t r a c t a b l e m a t e r i a l . I tisnow c l e a r t h a t p o l y ( d i m e t h y l s i l y l e n e ) is a t y p i c a l among p o l y s i l a n e s , but t h i s n

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0097-6156/88/0360-0006$06.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


2. WEST AND MAXKA

7

Polysilant High Polymers

was net r e a l i z e d at the t i m e . The d i s c o u r a g i n g p r o p e r t i e s of p o l y t d i m e t h y l s i l y l e n e ) perhaps c o n t r i b u t e d to the n e g l e c t of t h i s f i e l d over the f o l l o w i n g 25 y e a r s . ( 4 ) In any event, between 1951 and 1975, no papers appeared on p o l y s i l a n e h i g h polymers. However, l i n e a r p e r m e t h y l p o l y s i l a n e s o f the type Me(SiMe2)nMe were prepared and s t u d i e d , e s p e c i a l l y by Kumada and h i s s t u d e n t s , ( 5 ) and c y c l i c p o l y s i l a n e s were b e i n g i n v e s t i g a t e d in s e v e r a l laboratories.(£ι_7) S t u d i e s of the p e r m e t h y l c y c l o s i l a n e s , (Me2Si)« where η = 4 to 7, showed t h a t these compounds e x h i b i t remarkable d e r e a l i z a t i o n of the r i n g sigma e l e c t r o n s , and so have e l e c t r o n i c p r o p e r t i e s somewhat l i k e those of a r o m a t i c hydrocarbons.(6) I n t e r e s t in p o l y s i l a n e polymers was f i n a l l y reawakened by the work of Yajima and H a y a s h i , who found t h a t p o l y ( d i m e t h y l s i l y l e n e ) c o u l d be used as a p r e c u r s o r to s i l i c o n c a r b i d e . ( 9 ) The discovery, or r e d i s c o v e r y , o f s o l u b l e p o l y s i l a n e s at W i s c o n s i n was q u i t e accidental.(10) In one attempt to p r e p a r e c y c l o s i l a n e s c o n t a i n i n g both phenyl and methyl groups, PhMeSiCl2 and M e z S i C U were co-condensed w i t h a l k a l i m e t a l . A polymer was o b t a i n e d i n s t e a d o f the d e s i r e d r i n g compound, and to our s u r p r i s e i t proved to be somewhat s o l u b l e and m e l t a b l e . The i n t r o d u c t i o n of phenyl groups a l o n g the c h a i n breaks up the c r y s t a l l i n i t y of (Me2$i)n polymer. T h i s a d v e n t i t i o u s f i n d i n g l e d to s y n t h e s i s of the " p o l y s i l a s t y r e n e " f a m i l y of Me2Si-PhMeSi copolymers.(11) At almost the same t i m e , s o l u b l e p o l y s i l a n e s were r e p o r t e d by T r u j i l l o { 1 2 ) at Sandia L a b o r a t o r i e s and by Wesson and W i l l i a m s ( 1 3 ) at Union C a r b i d e Co. Research in p o l y s i l a n e polymers grew s l o w l y at f i r s t a f t e r t h i s reawakening. But w i t h i n the past few y e a r s , both the unusual s c i e n t i f i c i n t e r e s t and the t e c h n o l o g i c a l p o s s i b i l i t i e s " of the p o l y s i l a n e s have been r e c o g n i z e d , and a c t i v i t y in t h i s f i e l d has increased sharply. Commercial manufacture of both p o l y ( d i m e t h y l s i l y l e n e ) and " p o l y s i l a s t y r e n e " is now b e i n g c a r r i e d out in Japan, so t h a t these two polymers are r e a d i l y a v a i l a b l e in q u a n t i t y . S y n t h e s i s of P o l y s i l a n e s P o l y ( s i l y l e n e ) polymers are u s u a l l y made by the r e a c t i o n of d i o r g a n o d i c h l c r o s i l a n e s w i t h sodium m e t a l , in an i n e r t d i l u e n t a t temperatures above 100°C. (11) Rapid s t i r r i n g is o r d i n a r i l y used so t h a t the sodium is f i n e l y d i s p e r s e d , speeding the r a t e of r e a c t i o n . E i t h e r homopolymers o r copolymers can be s y n t h e s i z e d : R RR

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P o l y s i l a n e s c o n t a i n i n g Si-Η groups a r e r e a c t i v e in v a r i o u s ways, f o r i n s t a n c e in a d d i t i o n t o o l e f i n s c a t a l y z e d by p l a t i n u m . R e a c t i v i t y of o r g a n i c s u b s t i t u e n t groups has a l s o been o b s e r v e d ; an example is hydrogen h a l i d e a d d i t i o n t o the C=C double bond in p o l y s i l a n e s c o n t a i n i n g c y c l o h e x e n y l e t h y l groups:(18)

P h o t o c h e m i s t r y and C r o s s l i n k i n g When i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t , a l k y l p o l y s i l a n e s undergo s c i s s i o n almost e x c l u s i v e l y , but c r o s s l i n k i n g as w e l l as s c i s s i o n is observed f o r a r y l - s u b s t i t u t e d polysilanes.(19,20) A l t h o u g h the mechanism of p h o t o l y s i s has not been e l u c i d a t e d in d e t a i l , e x h a u s t i v e p h o t o l y s i s of p o l y s i l a n e s (RR'Si)n in the presence of a s i l y l e n e t r a p p i n g agent, t r i e t h y l s i l a n e , l e d t o the s i l y l e n e product E t a S i - S i R R ' - H as w e l l as d i s i l a n e , HSiRR'-SiRR'H.(21) These f i n d i n g s suggest t h a t t h e r e a r e a t l e a s t two primary s t e p s , s i m p l e s c i s s i o n t o s i l y l r a d i c a l s (A) and e l i m i n a t i o n of s i l y l e n e s , R2S1 (B). These two p r o c e s s e s c o u l d a l s o occur simultaneously(C).

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

INORGANIC AND ORGANOMETALLIC POLYMERS

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The s i l y l r a d i c a l s formed in the i n i t i a l s c i s s i o n appear to undergo f u r t h e r r e a c t i o n s , which may be complex. A possible s e c o n d a r y r e a c t i o n is hydrogen t r a n s f e r from an a l p h a carbon atom g i v e Si-Η and a s i l i c o n carbon double bond:(21)

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P h o t o c h e m i c a l c r o s s l i n k i n g of a l k e n y l p o l y s i l a n e s takes p l a c e r e a d i l y , presumably v i a r a d i c a l a d d i t i o n t o the u n s a t u r a t e d C=C l i n k a g e s on a d j a c e n t c h a i n s . C r o s s l i n k i n g of any p o l y s i l a n e can be c a r r i e d out p h o t o c h e m i c a l l y i f the polymer is mixed w i t h a multiply-unsaturated compound such as p h e n y l t r i v i n y l s i l a n e ; a d d i t i o n of s i l y l groups to the c a r b o n - c a r b o n d o u b l e bonds of the a d d i t i v e then p r o v i d e s the c r o s s l i n k i n g . ( 2 0 ) Thermal c r o s s l i n k i n g of p o l y s i l a n e s c o n t a i n i n g p o l y u n s a t u r a t e d c r o s s l i n k i n g a d d i t i v e s can a l s o be c a r r i e d ^ u t , w i t h a f r e e - r a d i c a l i n i t i a t o r such as ALBN. Among the o t h e r c r o s s l i n k i n g systems which have been d e v i s e d , an i n t e r e s t i n g example is the c r o s s l i n k i n g of a l i q u i d m i x t u r e of an o l i g o m e r i c p o l y s i l a n e c o n t a i n i n g Si-Η bonds t o g e t h e r w i t h a p o l y u n s a t u r a t e d compound, brought about by the a d d i t i o n of foPtCU as a h y d r o s i l y l a t i o n c a t a l y s t ; t h i s p r o c e d u r e p r o v i d e s "room temperature v u l c a n i z a t i o n " of polysilanes.(22)


2.

WEST AND MAXKA


Electronic

Polysilane High Polymers

11

Spectra

The p o l y s i l a n e polymers a l l show s t r o n g e l e c t r o n i c a b s o r p t i o n bands in the u l t r a v i o l e t r e g i o n , f a l l i n g between 300 and 400 nm.(19) A b s o r p t i o n s p e c t r a f o r s o l u t i o n s of a few polymers are shown in F i g u r e 2. The e l e c t r o n i c t r a n s i t i o n s are of o-c* type, r e f l e c t i n g e x t e n s i v e d e r e a l i z a t i o n of σ-electrons in the c a t e n a t e d s i l i c o n atoms. The a b s o r p t i o n maxima depend on the nature of the o r g a n i c substituents. Simple, unhindered d i a l k y l p o l y s i l a n e s absorb near 300 nm, but the i n t r o d u c t i o n of s t e r i c a l l y - h i n d e r i n g groups s h i f t s the maximum t o l o n g e r wavelength. A r y l groups d i r e c t l y a t t a c h e d t o s i l i c o n a l s o produce bathochromic s h i f t s ; as an example, (PhMeSi)n has i t s a b s o r p t i o n maximum at 340 nm. The l o n g e s t wavelength maxima are found f o r the s o l u b l e d i a r y l p o l y s i l a n e s r e c e n t l y r e p o r t e d by M i l l e r et a l . , ( 2 4 ) which have A x 395 nm. Thus both e l e c t r o n i c and s t e r i c e f f e c t s i n f l u e n c e the a b s o r p t i o n s p e c t r a of p o l y s i l a n e s . (25) A r y l s u b s t i t u e n t s can i n f l u e n c e the e l e c t r o n i c spectrum by a l l o w i n g m i x i n g of sigma w i t h p i s t a t e s , as d i s c u s s e d in a r e c e n t paper from the NTT r e s e a r c h group.(26) Many p o l y s i l a n e s a l s o show s t r i k i n g changes in t h e i r uv s p e c t r a w i t h temperature.(27) An example is shown in F i g u r e 3 f o r ( n - p e n t y l 2 S i ) in hexane s o l u t i o n . As the temperature is lowered the o r i g i n a l a b s o r p t i o n band at 313 nm d e c r e a s e s and a new band at 356 nm grows in. These changes are r e v e r s i b l e , a l t h o u g h m i c r o c r y s t a l s a p p a r e n t l y form i f the s o l u t i o n is kept f o r a time at low temperatures, making the r e v e r s a l q u i t e slow. The changes w i t h s t e r i c h i n d r a n c e and temperature must be due to c o n f o r m a t i o n a l e f f e c t s a l o n g the polymer c h a i n s . I t is now g e n e r a l l y agreed t h a t the thermochromism r e s u l t s from an i n c r e a s e in the p r o p o r t i o n of t r a n s - t o gauche c o n f o r m a t i o n s in the polymer c h a i n as the temperature is d e c r e a s e d . S i m i l a r l y , the i n t r o d u c t i o n of s t e r i c a l l y h i n d e r i n g s u b s t i t u e n t s c o u l d i n c r e a s e the amount of t r a n s junctions. E v i d e n c e in f a v o r of t h i s model is p r e s e n t e d in s e v e r a l r e c e n t papers,(28) as w e l l as the chapter by M i c h l in t h i s volume. Pure o l i g o m e r i c p o l y s i l a n e s of moderate c h a i n l e n g t h would be v e r y u s e f u l f o r d e t e r m i n i n g c o n f o r m a t i o n a l p r e f e r e n c e s and s t u d y i n g c o n f o r m a t i o n a l changes, but none are y e t a v a i l a b l e . The c l o s e s t a p p r o x i m a t i o n is the c y c l i c oligomer, ( M e 2 S i ) i e . The c r y s t a l s t r u c t u r e f o r t h i s compound, i l l u s t r a t e d in F i g u r e 4, shows t h a t i t has a c o n f o r m a t i o n q u i t e u n l i k e t h a t f o r o r g a n i c 16-membered r i n g s . T y p i c a l 1 6 - r i n g c a r b o c y c l e s e x h i b i t a "square" or d i a m o n d - l a t t i c e type s t r u c t u r e w i t h e i g h t t r a n s and e i g h t gauche t o r s i o n a l a n g l e s . An example is 1 , 1 , 8 , 8 - t e t r a m e t h y l c y c l o h e x a d e c a n e , which has e i g h t t r a n s j u n c t i o n s between 175.6 and 180°, and e i g h t gauche t o r s i o n s between 50.3 and 59.3°, a l l normal v a l u e s . (30) In c o n t r a s t , the c y c l o s i l a n e (Me2Si)i& shows no t o r s i o n a l angles in e i t h e r the normal t r a n s or normal gauche range.(30) I n s t e a d i t has e i g h t "gauche- e c l i p s e d " j u n c t i o n s between 83.9 and 93.4°, and e i g h t " t r a n s - e c l i p s e d " angles between 158.0 and 169.5°. These r e s u l t s suggest t h a t the view of p o l y s i l a n e polymers as c o n s i s t i n g of t r a n s and gauche l i n k a g e s may be o v e r s i m p l i f i e d . In the p o l y s i l a n e s , i n t e r m e d i a t e t o r s i o n a l a n g l e s may be much more important than they are in carbon polymers. m a

n




F i g u r e 1. G e l permeation chromatograph f o r ( P h M e S i ) n ( M e 2 S i ) copolymer, m=n, showing bimodal m o l e c u l a r weight d i s t r i b u t i o n . m

250

300 Wavelength, nm

350

400

F i g u r e 2. UV s p e c t r a of s o l u t i o n s of h i g h m o l e c u l a r weight p o l y s i l a n e s in THF a t 23°C; ( — ) , (n-DodecylMeSi)n ; (···) (n-Hexyl Si) ; ( ), ( C y c l o h e x y l M e S i ) n [ i n c y c l o h e x a n e ] ; ί ), (PhMeSi)n. 2

Q




WEST AND MAXKA

F i g u r e 4. ORTEP diagram from x - r a y c r y s t a l s t r u c t u r e of (Me2Si)ie. Hydrogens have been o m i t t e d f o r c l a r i t y .


14


Even more c o m p l i c a t e d changes take p l a c e in s o l i d f i l m s of p o l y s i l a n e polymers as the temperature is changed. These are d i s c u s s e d in the c h a p t e r by M i l l e r et a l . in t h i s volume.(31) Nmr

S p e c t r a and C o n f i g u r a t i o n 1 3

2 9

P r o t o n , C and S i NMR s p e c t r a f o r p o l y s i l a n e s have been recorded.(32) The p r o t o n NMR p r o v i d e l i t t l e s t r u c t u r a l i n f o r m a t i o n , but i n t e g r a t i o n of areas under the p r o t o n resonances is q u i t e u s e f u l f o r d e t e r m i n i n g the c o m p o s i t i o n of copolymers. The S i NMR s p e c t r a are of p a r t i c u l a r i n t e r e s t because they r e f l e c t the c o n f i g u r a t i o n of the polymer c h a i n . ( 3 2 ) Some Si s p e c t r a of a l k y l p o l y s i l a n e s a r e shown in F i g u r e 5. Symmetricallys u b s t i t u t e d polymers such as (n-hexyl2Si)η have no c h i r a l i t y s i n c e t h e r e can be a p l a n e of symmetry through each s i l i c o n atom. The S i resonance is t h e r e f o r e a s i n g l e narrow l i n e . However f o r d i a l k y l p o l y s i l a n e s w i t h two d i f f e r e n t a l k y l groups on each s i l i c o n , (RR'Si)n, each s i l i c o n atom is a c h i r a l c e n t e r and the resonance f o r a p a r t i c u l a r s i l i c o n w i l l depend upon the r e l a t i v e s t e r e o c h e m i s t r y of o t h e r nearby s i l i c o n atoms. For such polymers, a r a t h e r symmetrical c l u s t e r of peaks is observed ( F i g u r e 5 ) . These r e s u l t s are c o n s i s t e n t w i t h a t a c t i c s t r u c t u r e s , h a v i n g a s t a t i s t i c a l ( B e r n o u l l i a n ) d i s t r i b u t i o n of r e l a t i v e c o n f i g u r a t i o n s . ( 3 2 , 3 3 ) For a r y l p o l y s i l a n e s the r e s u l t s are q u i t e d i f f e r e n t . ( 3 4 ) The S i NMR f o r (PhSiMe)n is shown in F i g u r e 6; i t c o n s i s t s of t h r e e broad l i n e s w i t h r e l a t i v e i n t e n s i t y 3:3:4, each l i n e e v i d e n t l y c o n t a i n i n g a c l u s t e r of r e s o n a n c e s . The p a t t e r n s f o r o t h e r a r y l a l k y l p o l y s i l a n e s d i f f e r , but in g e n e r a l two or t h r e e broad resonances are found; none of the a r y l compounds s t u d i e d so f a r has g i v e n a symmetrical p a t t e r n l i k e those observed f o r the a l k y l p o l y s i l a n e s of F i g u r e 5. The r e s u l t s f o r a r y l s i l a n e s are not f u l l y understood, but the s p e c t r a may r e f l e c t p a r t i a l t a c t i c i t y in these polymers. Further work is needed; s t u d i e s of model compounds w i t h known r e l a t i v e c o n f i g u r a t i o n would be p a r t i c u l a r l y h e l p f u l . S i l i c o n - 2 9 NMR of p o l y s i l a n e copolymers a l s o shows g r e a t promise, e s p e c i a l l y f o r d i s t i n g u i s h i n g b l o c k - l i k e from f u l l y random copolymers. 2 9


2 9

2 9

2 9

T e c h n o l o g i c a l A p p l i c a t i o n s of

Polysilanes

P o s s i b l e ways in which p o l y s i l a n e s may be u s e f u l i n c l u d e , 1. As p r e c u r s o r s to s i l i c o n c a r b i d e c e r a m i c s ; 2. As p h o t o i n i t i a t o r s in r a d i c a l r e a c t i o n s ; 3. As p h o t o c o n d u c t i v e m a t e r i a l s , and 4. As p h o t o r e s i s t s in m i c r o e l e c t r o n i c s . The l a s t of these uses w i l l be t r e a t e d in the c h a p t e r by M i l l e r , ( 3 1 ) and so w i l l not be covered here. P o l y s i l a n e s as P r e c u r s o r s to S i l i c o n C a r b i d e . The o r i g i n a l p r o c e s s f o r thermal g e n e r a t i o n of s i l i c o n c a r b i d e ceramic from p o l y m e r i c p r e c u r s o r s was p i o n e e r e d by Yajima and Hayashi.(9) The s t a r t i n g m a t e r i a l s are e i t h e r p o l y ( d i m e t h y l s i l y l e n e ) or the c y c l o s i l a n e (Me2Si)&. T h e r m o l y s i s of these m a t e r i a l s at 400-450°C l e a d s to a complex s e r i e s of changes in which i n s e r t i o n of CH2 groups takes p l a c e i n t o many of the S i - S i bonds, l e a v i n g hydrogen bound to silicon.


2.

WEST AND MAXKA

(nHex S0 ζ η o


(nHexSiMe)

(nBuSIMe)

π

π

15

(nPrSIMe) ·»


1 ppnn

F i g u r e 5. solution.

2

9

S i NMR s p e c t r a f o r d i a l k y l p o l y s i l a n e s in benzene

8

F i g u r e 6.

2 9

29

Si

Sl.ppm

NMR f o r (PhSiMe)n



16

(Me Si) 2

450°C Ar

n

\

H

Polysilane High Polymers: An Overview - ACS Publications - American

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