Features of network formation in the chain crosslinking (co ...

Download PDF
4 downloads 0 Views 358KB Size Report
Comment
1Polymer Institute, University of Essex, Colchester, U. K., on leave from Institute of. Organic and Polymer Technology, 50-370 Wroclav, Poland. Summary.
Polymer Bulletin 3, 19-25 (1980)

Polymer Bulletin 9 by Springer-Verlag 1980

Features of Network Formation in the Chain Crosslinking (Co) Polymerization Karel Du~ek, Henryk Galina I and Josef Mike~

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia 1Polymer Institute, University of Essex, Colchester, U. K., on leave from Institute of Organic and Polymer Technology, 50-370 Wroclav, Poland Summary A n a l y s i s of e x p e r i m e n t a l r e s u l t s on the chain (co) p o l y m e r i z a t i o n of b i s u n s a t u r a t e d m o n o m e r s s u g g e s t s that c y c l i z a t i o n plays a d e c i s i v e role a l r e a d y at the onset of the reaction. At m e d i u m and e l e v a t e d c o n c e n t r a t i o n s of the b i s u n s a t u r a t e d compound, c o m p a c t m i c r o g e l - l i k e p a r t i c l e s are formed, w h i c h give rise to a gel m a i n l y by p a r t i c i p a t i n g in the p o l y m e r i z a t i o n r e a c t i o n only w i t h p e n d a n t d o u b l e bonds in t h e i r surface layer, and not w i t h d o u b l e bonds in the interior. This m e c h a n ism is r e f l e c t e d in an a p p a r e n t d e c r e a s e in r e a c t i v i t y of the p e n d a n t d o u b l e bonds. Introduction E x p e r i m e n t a l e x p e r i e n c e g a i n e d in the r e c e n t d e c a d e s shows that the p r o c e s s of n e t w o r k f o r m a t i o n by the chain c r o s s l i n k i n g (co) p o l y m e r i z a t i o n , p a r t i c u l a r l y at its b e g i n n i n g , e x h i b i t s the g r e a t e s t d e v i a t i o n from the course c o r r e s p o n d i n g to the t r e e - l i k e m o d e l of n e t w o r k f o r m a t i o n (cf.e.g. S T O C K M A Y E R 1943, F L O R Y 1953, G O R D O N and T E M P L E 1976). A n a l y s i s of e x p e r i m e n t a l results has led to the p r e d o m i n a n t c o n c l u s i o n (cf.e.g. KAST and F U N K E 1979, DU~EK 1979) that the cause of such special features lies in a strong c y c l i z a t i o n w i t h r e s p e c t to the f o r m a t i o n of i n t e r m o l e c u l a r c r o s s l i n k s . This general v i e w has been lately q u e s t i o n e d (OKASHA et al. 1979), and a v i e w has been f o r w a r d e d that at the b e g i n n i n g of the process i n t r a m o l e c u l a r cycles do not play any m a j o r role and e x p l a n a t i o n of d e v i a t i o n s is to be r a t h e r sought in a d e c r e a s e d r e a c t i v i t y of p e n d a n t d o u b l e bonds. It has also b e e n c l a i m e d that the cop o l y m e r s t r u c t u r e has b e e n n e v e r a c c u r a t e l y r e l a t e d w i t h the p a r a m e t e r s of copolymerization and that the r e a c t i v i t y of p e n d a n t d o u b l e bonds is g e n e r a l l y ignored. In this study we try to r e e x a m i n e both t h e o r e t i c a l a r g u m e n t s and p u b l i s h e d e x p e r i m e n t a l results, i n c l u d i n g some novel e x p e r i m e n t a l data on the c h a i n c r o s s l i n k i n g

0170-0839/80/0003/0019/$01.40

20

(co)polymerization. L o g i c a l r e a s o n i n g s show that the c h a i n c h a r a c t e r of the p o l y m e r i z a t i o n p r o c e s s r e s u l t s in that at the very o n s e t of ( c o ) p o l y m e r i z a t i o n the monomer(s) c o n t a i n so few g r o w i n g m a c r o r a d i c a l s and t e r m i n a t e d c h a i n s w i t h p e n d a n t d o u b l e bonds that the p r o b a b i l i t y of an interm o l e c u l a r r e a c t i o n of the r a d i c a l w i t h the p e n d a n t d o u b l e b o n d of a n o t h e r chain, ~, is very small (zero in the limit of zero c o n v e r s i o n ) , w h i l e the p r o b a b i l i t y of an i n t r a m o l e c u l a r r e a c t i o n of the r a d i c a l w i t h p e n d a n t d o u b l e b o n d s on the same chain, o, is finite in this limit (Fig.l). T h e s e facts lead to h i g h v a l u e s of the

t2 i o o

o 9

o

o

~ ~ "

R ~"~

Fig.l c o n c e n t r a t i o n ratio of intra- and i n t e r m o l e c u l a r c r o s s links, o/s, and to its d i v e r g e n c e in the limit of zero c o n v e r s i o n . A n o n z e r o v a l u e of o d e p e n d s on the poss i b i l i t y of c o n t a c t s b e t w e e n the r a d i c a l and the d o u b l e b o n d and the c h e m i c a l r e a c t i v i t y of the p e n d a n t d o u b l e bond. For d i v i n y l m o n o m e r s , the c h e m i c a l l y - d e p e n d e n t r e a c t i v i t y of the p e n d a n t d o u b l e b o n d may be w e a k e r or s t r o n g e r than, or close (e.g., for e t h y l e n e d i m e t h acrylate) to the r e a c t i v i t y of d o u b l e bonds in the m o n o mer. The p r o b a b i l i t y of the c o n t a c t is given by c h a i n f l e x i b i l i t y and by steric h i n d r a n c e s (if any) in the n e i g h b o u r h o o d of the r e a c t i n g sites. The c h a i n flexib i l i t y is u s u a l l y s u f f i c i e n t for the f o r m a t i o n of i n t r a m o l e c u l a r links; for linear and w e a k l y c r o s s l i n k e d chains, steric factors s h o u l d play no i m p o r t a n t role. Hence, no g e n e r a l causes e x i s t w h i c h w o u l d impede c y c l i z a t i o n from p r e d o m i n a t i n g o v e r the i n t e r m o l e c u l a r r e a c t i o n at the b e g i n n i n g of p o l y m e r i z a t i o n . C o m p u t e r s i m u l a t i o n of the p o l y m e r i z a t i o n of a d i v i n y l m o n o m e r on a cubic l a t t i c e p e r f o r m e d by us d e m o n s t r a t e d strong c y c l i z a t i o n , w h i l e i n d i c a t i n g at the same time that a c o n s i d e r a b l e part of p e n d a n t v i n y l s w e r e not able to react b e c a u s e of steric reasons. Experimental

Evidence

The c h a i n c r o s s l i n k i n g (co) p o l y m e r i z a t i o n is c h a r a c t e r i z e d by the f o l l o w i n g m a i n e x p e r i m e n t a l features:

21

(i) G e l a t i o n in a r i n g - f r e e s y s t e m s h o u l d o c c u r if one c r o s s l i n k issues f r o m one w e i g h t a v e r a g e p r i m a r y c h a i n (one r e p e a t u n i t in the chain is c r o s s l i n k e d ) . E x p e r i m e n t a l c r i t i c a l c o n v e r s i o n s of m o n o m e r s are h o w e v e r s h i f t e d t o w a r d s h i g h e r v a l u e s by a f a c t o r of the o r d e r of m a g n i t u d e 100-102 . This f a c t o r i n c r e a s e s w i t h i n c r e a s i n g c o n t e n t of the b i s u n s a t u r a t e d m o n o m e r and i n c r e a s i n g d e g r e e of p o l y m e r i z a t i o n of the p r i m a r y chains (cf.e.g. HOLT and S I M P S O N 1965, S P U R R et al. 1959, S T O R E Y 1965, W E S S L A U 1967, M A L I N S K ~ et al. 1971, ITO et al. 1975, O K A S H A et al. 1980, G A L I N A et al. 1980). The s m a l l e s t shift is o b s e r v e d for very low conc e n t r a t i o n s of the b i s u n s a t u r a t e d m o n o m e r s and very short p r i m a r y chains (e.g., d i a l l y l p h t h a l a t e ) . As expected, c r i t i c a l c o n v e r s i o n i n c r e a s e s w i t h dilution. (2) The c o n t e n t of d o u b l e r e a c t e d d i v i n y l u n i t s in l o w - c o n v e r s i o n (co)polymers is r e l a t i v e l y high, u s u a l l y c o n s i d e r a b l y e x c e e d i n g the value n e e d e d for att a i n i n g the gel point in a r i n g - f r e e system. E s p e c i a l l y in c o p o l y m e r s w i t h a h i g h e r c o n t e n t of b i s u n s a t u r a t e d c o m p o n e n t , u n s a t u r a t i o n v a r i e s w i t h c o n v e r s i o n only very little or not at all. In the same systems, uns a t u r a t i o n (or the f r a c t i o n of d o u b l e r e a c t e d d i v i n y l units) u s u a l l y v a r i e s only little w i t h d i l u t i o n (cf. e.g. H A W A R D 1954, H O L T and S I M P S O N 1956, M I N N E M A and S T A V E R M A N 1968, A S O 1959, W E S S L A U 1967, M A L I N S K Y et al. 1971, S O P E R et al. 1972, B R A U N and B R E N D L E I N 1973b, F U N K E et al. 1975, ITO et al. 1975, SHAH et al. 1978, D U ~ E K and S P ~ V A C E K 1980, G A L I N A et al. 1980). On the o t h e r hand, O K A S H A et al. (1979) found the c o n t e n t of units w i t h p e n d a n t v i n y l only s o m e w h a t lower than that of all the d i v i n y l u n i t s in the c o p o l y m e r . (3) C o p o l y m e r i z a t i o n of m e t h y l m e t h a c r y l a t e w i t h p e n d a n t d o u b l e bonds of p o l y ( e t h y l e n e d i m e t h a c r y l a t e ) is s l o w e r than w i t h d o u b l e bonds of e t h y l e n e d i m e t h a c r y l a t e (ASO 1959). (4) W i t h i n c r e a s i n g l e n g t h (flexibility) of the b r i d g e in the d i v i n y l c o m p o n e n t , there is a d e c r e a s e in u n s a t u r a t i o n and an i n c r e a s e in the f r a c t i o n of d o u b l e r e a c t e d d i v i n y l units, b o t h intra- and interm o l e c u l a r l y (SHAH et al. 1978). (5) M o l e c u l a r m a s s e s of l o w - c o n v e r s i o n (co)polym e r s are m u c h lower than v a l u e s c o r r e s p o n d i n g to a r i n g - f r e e reaction, b e i n g in the e x t r e m e case close to m o l e c u l a r m a s s e s of p r i m a r y chains (GALINA et al. 1980). The study of c o p o l y m e r s w i t h d e g r a d a b l e c r o s s links has r e v e a l e d that the f r a c t i o n of i n t e r m o l e c u l a r c r o s s l i n k s is low (MINNEMA and S T A V E R M A N 1958, B R A U N and B R E N D L E I N 1973, M R K V I ~ K O V A and K R A T O C H V ~ L 1980), m u c h m o r e c r o s s l i n k s are of the r i n g - f o r m i n g type. (6) The i n t r i n s i c v i s c o s i t y of l o w - c o n v e r s i o n (co)polymers is very low w i t h the e x c e p t i o n of the range close to the gel point and r a t h e r d e c r e a s e s w i t h the c o n t e n t of the d i v i n y l c o m p o n e n t of the c o p o l y m e r

22

(cf.e.g. ZIMM et al. 1958, SOPER et al. 1972, G A L I N A et al. 1980). It is m u c h lower than the value c a l c u l a t e d for randomly d i s t r i b u t e d branches. It is also close to the intrinsic v i s c o s i t y of m i c r o g e l s w i t h the same m o l e c u l a r mass o b t a i n e d by e m u l s i o n c o p o l y m e r i z a t i o n (HOFFMANN 1974). (7) In l o w - c o n v e r s i o n soluble c o p o l y m e r s of styrene and ethylene d i m e t h a c r y l a t e only one part of the c o p o l y m e r is r e f l e c t e d in h i g h - r e s o l u t i o n IH-NMR spectra; the "invisible" f r a c t i o n i n c r e a s e s w i t h increasing c o n t e n t of the divinyl c o m p o n e n t (SP~VACEK and DU~EK 1980). (8) Sometimes at a h i g h content of the d i v i n y l c o m p o n e n t the sample becomes t e m p o r a r i l y turbid b e y o n d the gel point, w h i c h indicates a c e r t a i n i n h o m o g e n e i t y (HORIE et al. 1975). Discussion The e x p e r i m e n t a l data given above provide sufficient e x p e r i m e n t a l e v i d e n c e on the i m p o r t a n c e of cyclization, but also indicate the p o s s i b i l i t y of a considerably reduced r e a c t i v i t y of the p e n d a n t vinyl. The shift of the gel point may be a l t e r n a t i v e l y e x p l a i n e d by the (extremely) low r e a c t i v i t y of pendant vinyls or by strong cyclization, but the c o m p a r a t i v e l y h i g h content of double reacted b i s u n s a t u r a t e d units already at the very b e g i n n i n g of the reaction, the low f r a c t i o n of i n t e r m o l e c u l a r c r o s s l i n k s w i t h respect to their total number, the e x t r e m e l y low i n t r i n s i c viscosities of l o w - c o n v e r s i o n polymers and their c o m p a c t ness u n a m b i g u o u s l y suggest cyclization. On the other hand, there exists e x p e r i m e n t a l e v i d e n c e that the apparent r e a c t i v i t y of the p e n d a n t vinyl may be very low: the lower c o p o l y m e r i z a t i o n rate of the pendant vinyl, the w e a k d e p e n d e n c e or i n d e p e n d e n c e of the u n s a t u ration of (co)polymers on c o n v e r s i o n up to high conv e r s i o n s or an increase in the f r a c t i o n of units w i t h p e n d a n t double bonds w i t h i n c r e a s i n g c o n c e n t r a t i o n of the b i s u n s a t u r a t e d monomer. A q u e s t i o n thus arises why the extent of c y c l i z a t i o n d e c r e a s e s w i t h the i n c r e a s i n g c o n t e n t of units of the b i s u n s a t u r a t e d m o n o m e r in the polymer, w h i l e a c c o r d i n g to Fig.l it should i n c r e a s e (there are several p o s s i b i l i t i e s h o w to close a ring). This seeming p a r a d o x can be r e s o l v e d taking into a c c o u n t that it is just cyclization, w h i c h forms internally c r o s s l i n k e d and rigid structures w i t h pendant double bonds "buried" inside. The ability of the internal double bonds to react, even w i t h small m o b i l e m o l e c u l e s of monomers, is m u c h reduced or c o m p l e t e l y suppressed. M I N N E M A and S T A V E R M A N (1958) called this p h e n o m e n o n shielding. The rigid s t r u c t u r e may be loosened by e.g. e x t e n d i n g the s e p a r a t i o n of d o u b l e bonds in the c r o s s l i n k i n g agent, w h i c h leads to a rise in the a p p a r e n t reactivity of the p e n d a n t vinyl. The

23

c o m p a c t c h a r a c t e r of s t r u c t u r e s w i t h a h i g h e r c o n t e n t of the c r o s s l i n k i n g agent, w h i c h have the c h a r a c t e r of m i c r o g e l particles, is d e m o n s t r a t e d e x p e r i m e n t a l l y by i n t r i n s i c v i s c o s i t y m e a s u r e m e n t s or N M R spectra. The m e c h a n i s m of n e t w o r k f o r m a t i o n via f o r m a t i o n of m i c r o g e l - l i k e p a r t i c l e s due to c y c l i z a t i o n and t h e i r i n t e r l i n k i n g has also b e e n put f o r w a r d in a r e c e n t m o n o g r a p h (IRZHAK et al. 1979). Thus, the a p p a r e n t d e c r e a s e in r e a c t i v i t y is g i v e n by p h y s i c a l factors due to c y c l i z a t i o n , a~d c h a n g e s in c h e m i c a l r e a c t i v i t y g i v e n by the (possibly) d i f f e r e n t c h e m i c a l s t r u c t u r e of the b i s u n s a t u r a t e d m o n o m e r and its c o p o l y m e r i z a t i o n b e h a v i o u r are of s e c o n d a r y importance. The e f f e c t of the v a r i o u s and v a r i a b l e (subs t i t u t i o n effect) r e a c t i v i t y of d o u b l e b o n d s in the b i s u n s a t u r a t e d m o n o m e r in the c o p o l y m e r i z a t i o n has b e e n t h e o r e t i c a l l y t r e a t e d for r i n g - f r e e systems (cf. e.g. G I B B S 1964, W E S S L A U 1967, D U ~ E K 1969, B R A U N and B R E D L E I N 1973a), but - as has b e e n shown above - the use of r i n g - f r e e a p p r o x i m a t i o n is r a t h e r q u e s t i o n a b l e . T h e o r e t i c a l a p p r o a c h e s i n v o l v i n g ring f o r m a t i o n also exist, but they do not take into a c c o u n t the e f f e c t of rings a l r e a d y formed on the c y c l i z a t i o n p r o b a b i l i t y , and may t h e r e f o r e be u t i l i z e d only in cases w i t h a low n u m b e r of rings per p r i m a r y chain, and h e n c e for a low c o n t e n t of the b i s u n s a t u r a t e d m o n o m e r (DU~EK and S P ~ V A C E K 1980). Thus, n e t w o r k f o r m a t i o n in the chain (co)polyme r i z a t i o n of the b i s u n s a t u r a t e d m o n o m e r p r o c e e d s app r o x i m a t e l y so as s h o w n in Fig.2: primarily, c o m p a c t internally crosslinked particles possessing microgel c h a r a c t e r are formed, w h i c h are l i n k e d t h r o u g h t h e i r p e r i p h e r i c d o u b l e b o n d s to y i e l d a gel. Such a c o u r s e is t y p i c a l of m e d i u m and h i g h c o n t e n t s of the b i s u n s a t u r a t e d monomer. D i l u t i o n of the s y s t e m s u p p o r t s i n t r a p a r t i c l e contacts, and this is the m a i n r e a s o n w h y the gel point is s h i f t e d t o w a r d s h i g h e r c o n v e r s i o n s . C y c l i z a t i o n m e a s u r e d by the c o n c e n t r a t i o n of intram o l e c u l a r c r o s s l i n k s i n c r e a s e s only little w i t h d i l u t i o n , b e c a u s e d i l u t i o n has only a small e f f e c t on the i n t e r n a l s t r u c t u r e of the particles. At a low content of the b i s u n s a t u r a t e d c o m p o n e n t and low d e g r e e s of p o l y m e r i z a t i o n of p r i m a r y chains, the c o u r s e app r o a c h e s the c l a s s i c a l c r o s s l i n k i n g process, w h e r e l i n k i n g of b r a n c h e d coils is assumed. However, cycl i z a t i o n can n e v e r be e l i m i n a t e d , u n l e s s the f o r m i n g c h a i n s are e x t r e m e l y rigid. At m e d i u m and h i g h c o n t e n t s of the b i s u n s a t u r a t e d monomer, n e t w o r k f o r m a t i o n r a t h e r r e s e m b l e s f l o c c u lation of s u s p e n d e d particles, and a r a t h e r h e t e r o g e n e o u s gel s t r u c t u r e m a y be e x p e c t e d . P a r a d o x i c a l l y , c o p o l y m e r s c r o s s l i n k e d to h i g h c o n v e r s i o n s a p p e a r e.g. m e c h a n i c a l l y quite h o m o g e n e o u s . This p a r a d o x may be s o l v e d by a s s u m i n g that i n t e r p a r t i c l e spaces are

24

i.occessitXe core

hi(jh conversion tow dilution "homogeneous"

NN

branched mocrom~ecuie preget sloge

high conversion high dilution inhomogeneous

Fig.2 g r a d u a l l y filled in w i t h the f o r m i n g (co) p o l y m e r d u r i n g p o l y m e r i z a t i o n b e y o n d the gel point (occuring u s u a l l y at a c o n v e r s i o n b e l o w 10%), and d i f f e r e n c e s in s e g m e n tal d e n s i t i e s b e c o m e m o r e or less s m o o t h e d out. If an inert solvent is added to the system, w h i c h is u s u a l in the p r e p a r a t i o n of m a c r o p o r o u s s t r u c t u r e s , h e t e r o g e n e i t y is preserved. It seems evident, therefore, that n e t w o r k s obt a i n e d by the c h a i n c r o s s l i n k i n g ( c o ) p o l y m e r i z a t i o n may be s u i t a b l y u s e d as m o d e l n e t w o r k s perhaps only in the e x t r e m e case of the lowest c o n c e n t r a t i o n s of the bisu n s a t u r a t e d monomer. I n v e s t i g a t i o n of the inhomogeneity of such n e t w o r k s close b e y o n d the gel point and of the e f f e c t of i n h o m o g e n e i t y on t h e i r p r o p e r t i e s calls for a m o r e d e t a i l e d study. N o t e added: B e f o r e m a i l i n g this p a p e r the a u t h o r s h a v e come across the r e c e n t f u n d a m e n t a l study of the crosslinking c o p o l y m e r i z a t i o n of m e t h y l m e t h a c r y l a t e by R.S. W H I T N E Y and W . B U R C H A R D (Makromol.Chem. 181,869 (1980)). T h e i r results c o n f i r m the d e c i s i v e role of c y c l i z a t i o n and n o n h o m o g e n e o u s c o u r s e of n e t w o r k formation.

25

References ASO C.: J.Polym. Sci. 39, 475 (1959) BRAUN D. and B R E N D L E I N W.: M a k r o m o l . C h e m . 167, 209 (1973) (a) BRAUN D. and B R E N D L E I N W.: M a k r o m o l . C h e m . 167, 217 (1973) (b) DU~EK K.: C o l l e c t . C z e c h . Chem.Commun. 34, 1891 (1969) DUSEK K.: M a k r o m o l . C h e m . , S u p p . 2, 35 (1979) DUSEK K. and S P ~ V A C E K J.: Polymer, in press (1980) FLORY P.J.: P r i n c i p l e s of P o l y m e r Chemistry, C o r n e l l Univ. Press, Ithaca (1953) FUNKE W., B E E R W., and SEITZ V.: P r o g r . C o l l o i d Polym. Sci. 58, 47 (1975). G A L I N A H., DU~EK K., T U Z A R Z., B O H D A N E C K ~ M., and ~TOKR J.: Eur. Polym. J., in press (1980) GIBBS W . E . : J.Polym. Sci. A2, 4809 (1964) G O R D O N M. and TEMPLE W.B.: in C h e m i c a l A p p l i c a t i o n of Graph Theory, A . T . B a l a b a n , Ed., A c a d e m i c Press, N e w Y o r k (1976) H A W A R D R . N . : J.Polym. Sci. 14, 535 (1954) H O F F M A N N M.: M a k r o m o l . C h e m . 175, 613 (1974) HOLT T. and SIMPSON W.: Proc.Roy. Soc. A 238, 154 (1956) IRZHAK V.I., R O Z E N B E R G B.A., and E N I K O L O P Y A N N.S.: N e t w o r k Polymers, Nauka, M o s c o w 1979 (in Russian) ITO K., M U R A S E Y., and Y A M A S H I T A Y.: J.Polym. Sci., Polym.Chem.Ed. 13, 87 (1975) KAST H. and FUNKE W.: M a k r o m o l . C h e m . 180, 1335 (1979) M A L I N S K Y J., K L A B A N J., and DU~EK K.: J.Macromol. Sci.Chem. A5, 1071 (1971) M I N N E M A L. and S T A V E R M A N A.J.: J.Polym. Sci. 29, 281 (1958) M R K V I C K O V A L. and K R A T O C H V I L P.: J.Polym. Sci.,Polym. Chem.Ed., s u b m i t t e d (1980) O K A S H A R., HILD G., and REMPP P.: Eur.Polym. J. 15, 975 (1979) SHAH A.C., H O L D A W A Y I.W., P A R S O N S I.W., and H A W A R D R.N.: P o l y m e r 19, 1067 (1978) SOPER B., H A W A R D R.N., and WHITE E.F.T.: J.Polym. Sci. A-l, iO, 2545 (1972). S P ~ V A C E K J. and DU~EK K.: J.Polym. S c i . , P o l y m . P h y s . E d . , s u b m i t t e d (1980) SPURR R.A., H A N K I N G B.M., and ROWEN J.W.: J.Polym. Sci. 37, 431 (1959) S T O C K M A Y E R W.H.: J.Chem. Phys. ii, 45 (1943) STOREY B.T.: J.Polym. Sci., A3, 265 (1965) W E S S L A U H.: A n g e w . M a k r o m o l . Chem. i, 56 (1967) ZIMM B.H., PRICE F.P., and BIANCHI J.P.: J.Phys.Chem. 62, 979 (1958)

Received July 12, 188o Accepted July 14, 198o