CONVEI~SION

OF

AGGRESSIVE

MEDIA

COMMUNICATION

POLYORGANOSILOXANES

I.

MECHANISM

OCTAMETHYLCYCLOTE

OF

TRASILOXANE

IN

THE

HYDROLYSIS

IN AQUEOUS

OF SOLUTIONS

OF INORGANIC ACIDS L. P. Razumovskii, N. A. Markova, A. and G. E. Zaikov

UDC 541.124:542.938:547 .I'128

Yu. V. Moiseev, G. Kuznetsova,

A s t a d y of t h e m e c h a n i s m of t h e h y d r o l y s i s of o r g a n o s f l o x a n e s is of g r e a t s i g n i f i c a n c e f o r t h e i n t e r p r e t a t i o n of d a t a on t h e c h e m i c a l s t a b i l i t y of p o l y o r g a n o s i l o x a n e s , a s w e l l a s on t h e p o l y c o n d e n s a t i o n and c a t a l y t i c r e a r r a n g e m e n t s of t h e s e c o m p o u n d s . T h i s w o r k w a s d e v o t e d t o a s t u d y of the k i n e t i c s and m e c h a n i s m of t h e h y d r o l y s i s of o c t a m e t h y l c y c l o t e t r a s i l o x a n e (D4) in a q u e o u s s o l u t i o n s of i n o r g a n i c a c i d s . T h e r e is no i n f o r m a t i o n in t h e l i t e r a t u r e on t h e m e c h a n i s m of the h y d r o l y s i s of t h i s c o m p o u n d in a q u e o u s s o l u t i o n s of acids.

EXPERIMENTAL

METHOD

D4[(CH3)2SiO]4, bp 176 ~ linear siloxane HO[Si(CH3)20]4H , and polydimethylsiloxane (PDMS) with average degree of polymerization n = 25 were used for the investigation. The compounds were produced according to the methods of [I]. Grade cp HCI, H2SO O and H3PO 4 were used to prepare the solutions of acids: TABLE

I. keff at Various

Acid Concentrations

and Temperatures

Amount t oc keff. 10-7 ]Amount] keff. 10-7 Amount I oc keff. 10-7 of acid, T, ]of acid,] T, ~ ]g/min. cm 2 of acid, T, ~+ g/rain, cm e II~. I+ ~o " g/min, crn 2 19,5

90 80 7O

10,4 5,3 2,8

4t ,1

32,5

90 80 70 60

29,2 t6,2 10,2 5,8

50,0

15,6

90 75

47,2 22,4 9,1 2,8

23,0

H2~0 8O 7O

55,3

60 50 40

34,0 22,i 6,3 3,8 46,0 23,6 16,8

50 40 33 25,5

86,3 33,5 30,4* 16,5

HC 70 55 40 25,5

49,0 17,3 7,3 2,5

27,6

60 50 40 33 25 25

57,0. 24,i 17,5 8,4, 6,1 14,7

60 50 40 25,5

54,3 36,0. 18,3: 8,4

30,9 H3PO~.

631

1,717,55,8 27,91172,8

6O

70

39,6

40

14,5 8,6

6o65

18,3

*Obtained by the PMR method. I n s t i t u t e of C h e m i c a l P h y s i c s , A c a d e m y of S c i e n c e s of t h e USSR. N a u k SSSR, S e r i y a K h i m i c h e s k a y a , No. 11, p p . 2 4 4 8 - 2 4 5 4 , N o v e m b e r , N o v e m b e r 29, 1972.

Translated from Izvestiya Akademii 1973. O r i g i n a l a r t i c l e s u b m i t t e d

9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 Test 17th Street~ New York, N. Y. lO011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

2393

k./O" rain- i

k. f g

30

15

rO J I

0

50

t

100

I

I

IdO

ZOO

,

~,m m z

I

0

I

500

I

I

/JO0

Fig. I

;ig.

I

Z500 n

2

Fig. 1. Dependence of the rate constant of h y d r o l y s i s of D~ in 53% H2SO4 at 79 ~ on the s u r f a c e : a, b) in glass ampoules; c) in a teflon ampoule. Fig. 2. Dependence of the rate constant of the hydrolysis of D4 on the n u m b e r of oscillations p e r minute (n): 1) in 55.9% H2SO4 at 50~ 2) in 41.1~0 H2SO4 at 80 ~ The acid h y d r o l y s i s of the siloxane D4 was investigated at the t e m p e r a t u r e s 25-90 ~ [concentration int e r v a l (% by m a s s ) : 19.5-55.3 for H2SO4; 15.6-30.9 for HC1; and 63.0-79.8 for H3PO4] (Table 1). The kinet i c s of h y d r o l y s i s was studied according to the d e c r e a s e in D 4 in the r e a c t i o n mixture. The amount of D4 was determined on an LKhM-7A c h r o m a t o g r a p h (detector: k a t h a r o m e t e r ; column with c a r r i e r brand INZ600, t r e a t e d with polysiloxane liquid P1V[S-100; c a r r i e r gas He, 40-60 m l / m i n , 155~ The experiment was conducted in sealed ampoules, which were t h e r m o s t a t i c a l l y controlled with an a c c u r a c y of ~0.2 ~ The volume of the aqueous solution of acid was 4 m l , De 0.06 ml. After definite time intervals, ampoules were removed f r o m the t h e r m o s t a t , the r e a c t i o n mixture cooled, cyclic siloxane ext r a c t e d with c h l o r o f o r m , and identical sized samples analyzed. The D4 concentration was determined a c cording to the a r e a of the peaks (average value of three to four m e a s u r e m e n t s ) . P r e l i m i n a r y experiments showed a linear relationship between the a r e a of the peaks and the D 4 concentration. The PMR s p e c t r a of cyclic and linear siloxane, polydimethylsiloxane, and the r e a c t i o n products were obtained at 25 ~ on a Varian HA-100 s p e c t r o m e t e r in benzene, which s e r v e d as an internal standard. To determine the size of the drops, we photographed the reaction mixture in glass ampoules through a 1ViIN-8 m i c r o s c o p e with microphotopacking and instantaneous stoppage of the ampoule (1-2 sec) after mixing at 25 ~ A study of the kinetics of the cohesion of drops after mixing ceased showed that t h e i r radius does not change in 5-7 sec and does not depend on the time of mixing of the iiquids. RESULTS

AND DISCUSSION

Macrokinetics of the Reaction. D 4 is insoluble in aqueous solutions, and its h y d r o l y s i s o c c u r s at the interface. F i g u r e 1 p r e s e n t s the dependence of the r e a c t i o n rate constant of the h y d r o l y s i s of D 4 on the CD4/Co

Cn/Co

m,g

1,o

,405

4o~

45

45 o,o2 I

JO

I

1

7fO

Fig. 3

I

I

250

min

0

dO

700

n]in

Fig. 4

Fig. 3. Dependence of the change in the amount of D4 on the time of h y d r o l y s i s in 15.6% HC1 at 75 ~ Fig. 4. Dependence of the relative concentrations of D 4 and the p o l y m e r on the time in the reaction of h y d r o l y s i s in 55.3% H2SO4 at 33~ 1) consumption of D4; 2) accumulation of polyorganosiloxane.

2394

lg(Keff/aH20)

/

5,g[

f

~

Z

Fig. 5. Dependence of -lgkeff/aI-I2 O on Ho: 1) H2804; 2) HC1; 3) H3PO4:

7,g[- / g / i f / . "

~',

}

!

1

T

-/

-f

-j

-~

No

s u r f a c e ar,~a of the i n t e r f a c e in 53% H2SOa at 79 ~ The r e a c t i o n r a t e is p r o p o r t i o n a l to the a r e a of the i n t e r face. In [2] it was shown that siloxanes a r e s t r o n g l y a b s o r b e d on a g l a s s s u r f a c e , which m i g h t influence the kinetics of h y d r o l y s i s ; t h e r e f o r e the e x p e r i m e n t s w e r e conducted in teflon a m p o u ! e s . As can be s e e n f r o m Fig. 1, the nature of the s u r f a c e of the a m p o u l e s has p r a c t i c a l l y no effect on the r a t e of h y d r o l y s i s . In the e a s e of mixing (shaking of the ampoules), the r e a c t i o n r a t e constailt i n c r e a s e s sharply; howe v e r , begianing with s o m e f r e q u e n c y of mixing (n = 1500 o s c i l l a t i o n s / m i n ) , the r e a c t i o n r a t e b e c o m e s constant (Fig. 2). During mixing, the cyclic sfloxane is b r o k e n down into d r o p s , and the r e a c t i o n r a t e is p r o p o r t i o n a l t~ the total s u r f a c e of the d r o p s dm/dt

-~

--kef ~ S,

(1)

w h e r e m sad S a r e the m a s s and s u r f a c e of D 4 at the m o m e n t of t i m e t; keff is the z e r o o r d e r r a t e constant of the r e a c t i o n of h y d r o l y s i s . F i g u r e 3 p r e s e n t s a typical kinetic c u r v e of the change in the m a s s of D 4 during the r e a c t i o n of h y d r o I y s i s in 15% HC1 at 75 ~ Analogous c u r v e s w e r e Mso obtained under other conditions. F r o m Fig. 3 it is evident that the r e a c t i o n is p r a c t i c a l l y i r r e v e r s i b l e and is f i r s t o r d e r with r e s p e c t to the m a s s of D 4. To e:~plain the data obtained, we conducted special e x p e r i m e n t s on the d e t e r m i n a t i o n of the size and n u m b e r of d r o p s during h y d r o l y s i s f r o m p h o t o g r a p h s . Both t h e i r a v e r a g e radius (r) and the n u m b e r of drops are v i r t u a l l y unchanged up to the end of the reaction. The value of r was calculated f r o m the curve of the d i s t r i b u t i o a of the n u m b e r of d r o p s as a function of t h e i r r a d i u s . After mixing is stopped, the d r o p s a r e s p h e r i c a l . During mixing, the d r o p s , without changing t h e i r m a s s , acquire the shape of elongated ellipsoids, the s u r f a c e of which differs f r o m the s u r f a c e of a s p h e r e by no m o r e than 20%. On the b a s i s of the data cited, we can suggest the following m o d e l s . a) During the r e a c t i o n the n u m b e r of d r o p s containing D 4 d e c r e a s e s ; r is unchanged, since with this mode of mixing it depends only on the p h y s i c a l p a r a m e t e r s of the s y s t e m (the s u r f a c e tension at the i n t e r face, v i s c o s i t y and r a t e of mixing), [3]. The r e a c t i o n product, a p o l y m e r (the identification of the r e a c t i o n p r o d u c t is d e s c r i b e d below), f o r m s new drops, r of which does not differ significantly f r o m r of the d r o p s of D4; the r e d i s t r i b u t i o n of the r e a g e n t and p o l y m e r is v e r y rapid. The solution of Eq. (1) takes the f o r m

--ln

m =keff m 0

Sot,

(2)

/7"t o

w h e r e m 0 is the initial m a s s of D4; SO is the total s u r f a c e of the drops. b) The p o l y m e r does not leave the drop; the r a t i o of the amount of D 4 and the p o l y m e r on the s u r f a c e of the droic is p r o p o r t i o n a l to t h e i r m a s s e s . The p o l y m e r is a I m o s t e n t i r e l y soluble in D4; t h e r e f o r e , in all p r o b a b i l i t y model b) is m o r e r e a l i s t i c . A solution of Eq. (1) in this c a s e takes the s a m e f o r m . As has a l r e a d y been indicated, the r e a c t i o n r a t e does not depend on the r a t e of mixing, beginning with s o m e f r e q u e n c y of mixing; the d r o p s f o r m e d have a limiting value of r, which depends only on the p h y s i c a l p a r a m e t e r s of the s y s t e m . However, in the case of mixing with the aid of ultrasound (10 W / c m 2, 850 kHz), the r e a c t i c n r a t e constant i n c r e a s e s 10-fold in c o m p a r i s i o n with e x p e r i m e n t s conducted under identical conditions with a f r e q u e n c y of mixing 1500 o s c f l l a t i o n s / m i n . This is evidence that the s u r f a c e of contact of the p h a s e s i n c r e a s e s , i . e . , the s i z e of the d r o p s also depends on the method of mixing of the liquids. Special experimenLs conducted under the s a m e conditions, but in water, showed that t h e r e is no clevage of bonds in D 4.

2395

F r o m a d e t e r m i n a t i o n of the dependence of the r e a c t i o n r a t e on the value of the s u r f a c e (m 0 is 10 t i m e s as g r e a t as in the e x p e r i m e n t s with mixing in ampoules), it is p o s s i b l e to d e t e r m i n e k = kef f (S o /m0). By c o m p a r i n g the values of k obtained f r o m t h e s e e x p e r i m e n t s with k found f r o m e x p e r i m e n t s with mixing in ampoules (under analogous conditions), it is p o s s i b l e to d e t e r m i n e S of the drops. Knowing the total volume (V) and s u r f a c e of the drops, it is p o s s i b l e t o d e t e r m i n e t h e i r radius ~ = 3 V/S. The value of r is p r a c t i c a l l y independent in the nature of the acid, concentration, and t e m p e r a t u r e u n d e r the conditions of o u r e x p e r i m e n t s . It should be n o t e d that r = (6.8 9 1.7). 10 -4 cm, d e t e r m i n e d f r o m p h o t o g raphy, and Y = (8.6 • 1.0) 9 10 -4 c m , calculated f r o m the kinetic data on the dependence of the r e a c t i o n r a t e constant on S, a r e in s a t i s f a c t o r y a g r e e m e n t .

T A B L E 2. L o g a r i t h m i c Values of the Rate Constants of H y d r o l y s i s of D 4 f o r Various Acids at 25 ~

t

I84 I 84

aII,O

H,SO~ t9,5 32,5 4t ,i 50,0 55,3

t,07 2,00 2,62 3,40 3,93

7,71 7,20 6,86 6,33 5,82

t5,6 23,0

1,62 2,53 3,20 3,63

HC1 7,09 6,6i 6,28 5,84

~,6

30,9

t

0,05 0,15 0,26 0,44 0,61 0,15 0,30

]

7,66 7,05 6,60 9 5,89 5,2i 6,94 6,31

0,42 0,55

5,86 5,29

0,32

6,86 6,02 5,4l

H3P04 ~63,0 72,8

70,8

2,0~ 2,74 3,26

1

7,18 6,56 6,20

0,54 0,79

Table 1 p r e s e n t s the values of keff calculated a c c o r d i n g to Eq. (2) at v a r i o u s t e m p e r a t u r e s and acid concentrations. The activation e n e r g y does not depend on the nature and concentration of the acids and is equal to 12.8 • 2.2 k c a l / m o l e . Identification of the Reaction P r o d u c t s . The h y d r o l y s i s of the cyclic siloxane p r o c e e d s in s e v e r a l s t e p s . The f i r s t step is clevage of the sitoxane bonds, p r o b a b l y with the f o r m a t i o n of a l i n e a r siloxane; the second is a f u r t h e r c o n v e r s i o n of the p r o d u c t s . Evidently the m o s t p r o b a b l e p r o c e s s e s will be ionic p o l y m e r i z a t i o n and polycondensation with the f o r m a t i o n of h i g h - m o l e c u l a r p r o d u c t s . The PMR method was used to identify the r e a c t i o n p r o d u c t s . The c h e m i c a l shifts of the protons in benzene (5, ppm) w e r e as follows: cyclic siloxane - 0.26, r e a c t i o n product - 0.34, l i n e a r siloxane HO[(CH3)2SiO]4H - 0.26 and - 0.27, PDMS (n = 25) - 0 . 3 2 . The signals of the l i n e a r siloxane HO[(CH3)2SiO]4H w e r e not detected in the r e a c t i o n m i x t u r e . -It was shown that this compound is converted to the r e a c t i o n product f a r m o r e r a p i d l y than D 4. On the b a s i s of the m o l e c u l a r data, as well as the data of [4], the signals of the r e a c t i o n p r o d u c t s can be a s signed to the signal of p o l y o r g a n o s i l o x a n e s . F r o m the kinetic c u r v e s of the v a r i a t i o n of the intensity of the s i g n a l s of D 4 and the r e a c t i o n product - polyorganosiloxane - (Fig. 4), it is evident that the s u m m a r y c o n c e n t r a t i o n of the protons of D 4 and the p o l y m e r r e m a i n s constant up to 50% conversion. In the case of f u r t h e r decomposition of D4, the s u m m a r y concentration of protons drops. This m a y be due to p a r t i a l l o s s of p o l y m e r during the e x t r a c t i o n with b e n zene and to adsorption on the g l a s s . It was shown in [2] that the adsorption of p o l y o r g a n o s i l o x a n e s inc r e a s e s with 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 . The r a t e constants of h y d r o l y s i s , obtained by the PMR method a r e in s a t i s f a c t o r y a g r e e m e n t with the r a t e constant obtained b y the c h r o m a t o g r a p h i c method (see Table 1). Thus, the limiting step of the h y d r o l y s i s of the cyclic siloxane D4 is clevage of one siloxane bond (opening of the ring), and the final product is l i n e a r PDMS w i t h n -> 25. In the c a s e of long t i m e s of expos u r e , the f o r m a t i o n of an e l a s t o m e r is o b s e r v e d . M e c h a n i s m of the Reaction. At the p r e s e n t t i m e t h e r e a r e s e v e r a l viewpoints on the m e c h a n i s m of the acid cleavage of sil0xanes. It is suggested [5-7] that the cleavage of siloxanes o c c u r s a c c o r d i n g to an SN1 m e c h a n i s m , a c c o r d i n g to which t h e r e is a rapid and equilibrium protonation of the oxygen of the siloxane bond, followed by slow decomposition of the protonated f o r m into silanol and silyl cations; the l a t t e r r e a c t r a p i d l y with a nucleophilic agent, f o r e x a m p l e , alcohol H~

H

~SiOSi~-~ ~- _~SiOSi_---@ H

~_SiOSi_----~- ~SiOH -~ | @

~--Si | ~- HOR ~ __--___SiOR-}- H @

(Rapid) (Slow) (Rapid)

A b i m o l e c u l a r m e c h a n i s m is also suggested [8, 9], according to which the limiting step is interaction of a nucleophilic agent with the protonated f o r m of the siloxane

2396

H~ H ~_Si0Si~-~ ~ ~-~SiOSi~ @

(Rapid)

H H ~-SiOSi~ -l- R0it ~- ~_~Si0H + ~=SiOR (Slow) @ 9 tt ~ S i O t / ~ _~SiOtl + H O (Rapid)

According to [10-12], elevage of the sfloxane bond p r o c e e d s through s i x - m e m b e r e d activated c o m p l e x e s , in which e i t h e r only the aeid m o l e e u l e (oxygen-containing acids), or an acid m o l e c u l e and a water m o l e e u l e (hydrohalic acids), or a w a t e r m o l e e u l e and a hydroxonium ion p a r t i c i p a t e : Si Si,---r--O-~-~H T I __, ~SiOS020H + ~__Si0H I

_~_Si0Si-~--+ H2SO, --*

e// \OH /Si ~Si0Si~--~ + HX + H~0 ---, i ~ . ~ s

=---Si0Si_.=_+ H~0 + H30--,

-~ ~ S i X -t- --SiOH + H ~

Si 1 S i ~ O - - -H

, ~

r

H -~

2--sioH + H,O*

In e s t a b l i s h i n g the m e c h a n i s m of the r e a c t i o n of h y d r o l y s i s , it is n e c e s s a r y to have information on the proton, ttion of the sfloxane bond and the nature of the limiting step of the reaction. It is known that the protonation of s f l a n e s [13] and t r i o x a n e [14] is d e s c r i b e d by the acidity function H 0. If we a s s u m e that the protonation of sfloxanes p r o c e e d s a c c o r d i n g to an analogous m e c h a n i s m , then the concentration of the p r o tonated f o r m of [(CH3)2SiO] 4 is found a c c o r d i n g to the equation

CBH+

h0

CB K----~-.+"

(3)

If the r e a c t i o n p r o c e e d s a c c o r d i n g to an SN1 m e c h a n i s m , then ktrue ~ 1~n+

keff = KBH-----~ 0 7 ,

(4)

w h e r e ~ r u e and kBH + a r e the t r u e r a t e constant and the b a s i e i t y eonstant of the sfloxane bond, r e s p e c t i v e l y . The r a t i o of the activity coefficients of the protonated f o r m and activated complex is u s u a l l y a s s u m e d to be constant. Our e x p e r i m e n t a l data a r e not d e s c r i b e d by an SN1 m e c h a n i s m . If a w a t e r molecule e n t e r s into the limiting step of the r e a e t i o n (SN2 m e c h a n i s m ) , then ktrue . /BS+ kef f = KBH-------~ n0aH,0 " - ~ .

(5)

A s s u m i n g that fBH+/f ~ does not depend on the concentration and nature of the acid, the e x p e r i m e n t a l data a r e s a t i s f a c t o r i l y d e s c r i b e d by Eq. (5) (Table 2 and Fig. 5), with a slope of 0.9 • 0.1 ( c o r r e l a t i o n coefficient 0.967). CONCLUSIONS 1. The h y d r o l y s i s of o c t a m e t h y l c y c l o t e t r a s i l o x a n e was investigated by the GLC and PMR methods in aqueous solutions of h y d r o c h l o r i c , sulfuric, and p h o s p h o r i c acids in a b r o a d r a n g e of concentrations and t e m p e r a t u r e s under h e t e r o g e n e o u s conditions. 2. The a v e r a g e size of the d r o p s in the r e a c t i o n m i x t u r e during mixing was d e t e r m i n e d e x p e r i m e n t ally. 3. The r e a c t i o n p r o d u c t s w e r e identified. 4. H y d r o l y s i s of o e t a m e t h y l c y e l o t e t r a s f l o x a n e p r o c e e d s a c c o r d i n g to an SN2 m e c h a n i s m .

2397

LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

2398

CITED

K . A . Andrianov, Methods of Heteroorganic Chemistry [in Russian], "Nauka" (1968), p. 40. B . V . Ashmead and M. J. Owen, J. Polymer Sci., A-2, 9, 331 (1971). J . O . Hinze, Amer. Inst. Chem. Engng., 1, 289 (1955). Kang-Jen Lui, Makromolek. Chem., 126, 1"87 (1969). K. Damm, D. Golitz, and W. Noll, Angew. Chemie, 76, No. 6, 273 (1964). K. Damm, D. Golitz, and W. Noll, Z. Anorgan. und Allgem. Chem., 340, 1 (1965). K . A . Andrianov and S. E. Yakushkina, Vysokomolekul. Soed., 1, 613 (1959). Z. Losoeki, J. Kulpinski, and W. Gador, Polymer. Tworz. Wiel'k, 1__55,442 (1970). A . G . Kuznetsova, V. I. Ivanova, and S. A. Golubtsov, Plastmassy, No. 9, 19 (1969). M . G . Voronkov, Heterocyclic Reactions of Cleavage of Sfloxane Bonds [in Russian], INKhS AN SSSR (1961). M . G . Voronkov and L, A. Zhagata, Zh. Obshch. Khimii, 38, 2327 (1968). h~.G. VoronkovandL. A. Zhagata, Izv. AN LatvSSR, Ser. Khim., 507 (1964). L. Sommer, Stereochemistry and Mechanisms of the Reactions of Organosilicon Compounds [Russian translation], "Mir" (1966), p. 144. F . A . Long and D. McIntyre, J. Amer. Chem. Soc., 76, 3240 (1964).