Alkyl(aryl)hydropolysiloxanes—IV. The kinetics of the reaction of unsaturated compounds with dimethylmethylhydropolysiloxanes

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Alkyl(avyl)hydropolysiloxanes--I V 5. 6. 7. 8. 9. 10. 11.

G. V. SCHULZ and U. CANTOW, J . Polymer Sci. 1O: 79, 1953 R. M. FUOSS and V. P. STRAUSS, Ann. New York Acad. Sei. 51: 836, 1949 R. M. FUOSS and U. P. STRAUSS, J. Polymer Sei. 3: 602, 603, 1948 B. H. ZIMM, J. Chem. Phys. 16: 1093, 1099, 1948 F. W. BILLMEYER and C. B. de THAN, J. Amer. Chem. Soc. 77: 4763, 1957 A. SILBERBERG, J. ELIASSAF and A. KATCHALSKY, J. Polymer Sci. 23: 259, 1957 A. OTH and P. DOTY, J. Phys. Chem. 56: 43, 1952

ALKYL(ARYL)HYDROPOLYSILOXANES--IV. THE KINETICS OF THE REACTION OF UNSATURATED COMPOUNDS WITH DIMETHYLMETHYLHYDROPOLYSILOXANES * L. A. G R I G O R ' E V A

and V. O.

REIKHSFEL'D

Lensoviet Technological Institute, Leningrad

(Received 19 April 1963)

IN CONTINUATION of studies [1] of the kinetics and mechanism of the addition of unsaturated hydrocarbons and their derivatives to dimethylmethylhydropolysiloxanes (DMMHPS) a study has been made of the kinetics of the reaction of ~-methylstyrene and methyl methacrylate (MMA) with copolymers I, II and I I I [1], of the general formula [(CH3)2SiO]n [(CHa)HSiO]x [(CHa)2SiO]m, where lOO ~,,~'~

n

r

~ -2

.'so

g'2o I ¸

o

I

80

120

l

/8O

Time,rain

FIG. 1. Effect of atmospheric oxygen on the rate of addition of g-methylstyreno to DMMHPS : 1 - - u n d e r nitrogen ; 2 - - i n air ; continuous curves-- 100 ° ; broken curves -- 50 °.

x = l , 4 and 5 respectively, in the presence of solutions of H2PtCle.6H~O in isopropanol or tetrahydrofuran. In a preliminary stage the effect of oxygen, the ratio of the reagents and the catalyst concentration on reaction rate was studied for the ~-methylstyrene* Vysokomol. soyed. 6: No. 6, 988-993, 1964.

1088

L. A. GRIGOlVV.VAand V. O. REIKHSFEL'D

copolymer I system. I t was found that the reaction rate is higher in an inert gas atmosphere t h a n in air (Fig. 1). Variation of the concentration of the copolymer in ~-methylstyrene from 3% to 20% had practically no effect on the reaction rate. Increase in the catalyst concentration had a favourable effect, but with increasing catalyst concentration this effect became relatively less. There is evidently a limit above which the effect of change in catalyst concentration becomes insignificant (Fig. 2.). The effect of temperature (at constant concentrations of polymer and catalyst) on the rate of addition of ~-methylstyrene and MMA to copolymer I I I is shown in Figures 3a and b respectively. Analysis of the ldnetic data enables some suggestions to be made concerning a possible mechanism of addition in the presence of Speier catalysts in general and in the present particular case. There is no universally accepted view on this problem at the present time. Some authors [2-4] consider that the addition of unsaturated compounds to silicon hydrides follows an ionic mechanism. Ponomarenko et al. [5] assume a radical mechanism. Our results indicate that Io0

. so

/r"

~ 40

•

° °

30

90 Time, rain

150

FIG. 2. Effect of catalyst concentration (mole/100 g of mixture) on the rate of addition of ~-methylstyrene to DMMHPS: 1--0.5× 10-5; 2--1.3× 10-5; 3--2.5× 10-5. this reaction is clearly autocatalytic and comprises at least two stages, namely formation of the catalyst (reduction of chloroplatinic acid to platinum) and addition of the vinyl derivative at the Si--H bond. The autocatalytic nature of the reaction is manifested particularly clearly at low temperatures (Fig. 3a, curve 1). At higher temperatures (above 50 °) reduction of the H~PtC16 is evidently more rapid t h a n the addition reaction and the curve does not show a catalyst-formation period. Presumably the addition occurs on the surface of colloidal platinum formed in the reaction mixture. The following facts support this suggestion. Firstly homopolymers are not formed during the addition process, even with a monomer as reactive as styrene. This is readily explained when the inhibiting effect of colloidal platinum on the polymerization of styrene is borne in mind [6], but difficult to explain if it is assumed t h a t radicals or ions are present in the system. Secondly, as mentioned above, oxygen retards the

1089

Alkyl(aryl)hydropolysiloxanos--IV

addition reaction, and this is probably associated with hindrance to formation of the catalyst (reduction of H~PtCl~). Finally the catalytic adtion of metallic platinum on a process of this type is well kno~m. a

40

/~. i/~'/~"1t

2O "Co 0 r..... ,~ c..)

40

80 7[me, rain

fO0

I~C

b

• 3o

80

'~0

• •

2 • o

• e o•

~

• •

I

0

120

I

GO

I

120 180 Time, min

I

I

240

300

FIG. 3. Effect of temperature on the rate of addition to DMMHPS: a--~-methylstyrene: 1--30°; 2--50°; 3--100°; b--methyl methacrylate : 1--70°; 2--80°; 3--90 °. The mechanism of addition can be represented as follows: CHa

I

-~Si--O~ H

,

R

/.\\!

I

CHa--CH2R,

'- CH~--CI-IRj which involves the formation of a n-complex in which the n-electrons of the double b o n d and the d-orbital of the silicon atom take part. The kinetic results fit the first order reaction equation satisfactorily if the reacting unit is taken to be the polymer segment {[(CH3)2SiO]¢9[(CHa)HSiO]} (A), of molecular weight 575.3, and if it is assumed that there is a sufficiently large excess of the second reagent. This is illustrated b y Figures 4a and b, which show the relationship log Co/C=f(~ ) at various temperatures for the reaction of copolymer I I I w i t h a-methylstyrene and MMA respectively. I t should be

1090

L. A. GRIOOR'EVAand V. O, REIKHSFEL'D

n o t e d t h a t w h e n t h e degree o f c o n v e r s i o n is fairly h i g h t h e r e is a p r o g r e s s i v e d i v e r g e n c e fron~ linearity. This is a s s o c i a t e d ~rith t h e f a c t t h a t as t h e silicon h y d r i d e g r o u p s in t h e p o l y m e r are c o n s u m e d t h e a s s u m p t i o n t h a t t h e u n i t A is t h e kinetic u n i t b e c o m e s less valid.

logco/c

o

0

20

~ Co/C

I 0

40

~0

Eme , mt'n

./o" 8O

240

160 Erne , rain

FIG. 4. Dependence of log c0/c on: a--~-methylstyrene: •--30°; 2--50°; 3--100°; b--methyl methacrylate: 1--70°; 2--80°; 3--90°.

The rate constants of the reactions were found from Figures 4 a and b, and f r o m t h e t e m p e r a t u r e d e p e n d e n c e o f these c o n s t a n t s t h e energies o f a c t i v a t i o n were o b t a i n e d (Fig. 5). T h e s e figures are g i v e n below.

Added monomer k × l O ~, rain -~: at 30 ° ,, 50 ° ,, 70 ° ,, 80 ° ,, 90 ° ,, 100° E, kcal/mole

~-Methylstyrene

Methyl methaerylate

1"76 6"96

0.24 0.61 1.26

32"71 8"8

20.1

1091

Alkyl(aryl)hydropolysiloxanes--IV

I t is evident t h a t a-methylstyrene is considerably more reactive t h a n MMA. This is in accord with the proposed reaction mechanism, which implies t h a t t he monomer with the higher electron density at the double bond will be the more reactive. I n our case this monomer is a-methylstyrene [7]. This is confirmed

_1.4 J

-2"2

26

2'8

'

30

I

I

32

34

~ x 104

FIG. 5. Dependence of log k on I/T: 1--g-methylstyrene; 2--methyl methacrylate. by application of the Alfrey-Price scheme to the present c a s e - - t h e value of e for ~-methylstyrene is --1.2 and for methyl methacrylate ~-0.4 [8]. I n order to examine the effect of the distribution of the hydrogen-containing groups in th e polymer chain on the rate of addition the reaction of MMA and a-methylstyrene with eopolymers I, I I and I I I was studied. 700

Q

8O o

o

~

~

*2 ~o i

2O I

30

I

f

90

150

I

0 20 1-[me , r a i n

I

f

f

GO

lO0

140

o

FIG. 6. Effect of distribution of hydrogen-containlng units in DMMHPS on addition rate (catalyst concentration 1.3x 10-s mole/100 g): 1--copolymer I; 2--copolymer I I ; 3-- copolymer I I I ; a-- methyl methacrylate, 90°; b -- ~-methylstyrene, 50°. Methyl m e t h a c r y l a t e reacts at different rates with the three copolymers b u t a-methylstyrene reacts at the same rate. This is seen from the rate curves in Figures 6a and b, and from the values of the rate constants calculated from Figures 7a and b and given below (k x 10-2 rain-Z).

1092

L.A. GRIGOR'EVAand V. O. REIKHSFEL'D Copolymer Methyl methacrylate at 50° ~-Methylstyrene at 90°

I II III 0.794 1"59 1-26 6.96 6.96 6.96

The variation in rate observed in the first case can be explained by the fact t h a t an added MMA molecule can screen (against nucleophilic attack) a neighbouring silicon hydride group by interaction of the carbonyl exygen with the silicon atom, according to the scheme: CH3

CH3

CH3

CH3

CH3

CHs

1 i ~+ I I i I Si--O-- Si----O ~F=Si--O--Si--O--Si--O--Si--O~ I I -)', I i • i

CH3

CH2

~0 H

H

H

CHs

i // CH.---CH -----C---~0--CH.

Zogco/c 08

i/~

Cl

log co/c 0.6'

b

o

0"6 o

o

•

0"4

0-4 • •

0.2 ¢jo

o

o~

,~° •

02

02

"3

(]0

40

0 120 Time, min

80

I

1o

2'o

3O

FTG. 7. Dependence of log co/c on v: 1-- copolymer I ; 2-- copolymer I I ; 3-- copolymer III; a-- methyl methacrylate ; b -- ~-methylstyrene. Obviously this effect is not possible with copolymer I I and in this case the reaction rate is highest. The effect occurs in reaction with copolymers I and I I I and it should be relatively greater when the eopolymer contains blocks of four silicon hydride groups (I) t h a n when it contains blocks of five such groups (III). This is in accord with the kinetic data. The structure of the adduct with ~-methylstyrene is such t h a t interaction with neighbouring silicon hydride groups is not possible: CH.

sl

CH.

I

CH3

I

CH,

i

CH3

1

CH8

i

i--O--Si--O--Si-- O--~Si--O-- Si--O--Si--O ~ , C]H3 CH I s. H] I ~tts tti I-I i CH.--CH--~ and therefore differences in the reaction rates are not observed.

Alkyl(aryl)hydropolysiloxanes--IV

1093

EXPERIMENTAL Starting ~naterials. The synthesis of DMMHPS has been described previously [1]. ~-Methylstyrene and MMA were freshly distilled before use. Kinetic measurements. The experiments were carried out in a three-necked flask provided with a stirrer, thermometer and reflux condenser, in a current of oxygen-free nitrogen. The flask containing the reaction mixture was placed in a water or glycerol thermostat in which the temperature was controlled within ~=0.3°. The process was monitored by determination of the residual active hydrogen content. For this purpose test samples were removed at suitable intervals and decomposed by Mcoholie KOH in a Zerewitinoff apparatus.

CONCLUSIONS (1) A s t u d y has been m a d e o f the kinetics of the a d d i t i o n o f ~ - m e t h y l s t y r e n e a n d m e t h y l m e t h a e r y l a t e to d i m e t h y l m e t h y l h y d r o p o l y s i l o x a n e s of different s t r u c t u r e in the presence of Speier's catalyst. I t is s h o w n t h a t the process is a u t o c a t a l y t i c a n d comprises at least two s t a g e s - - f o r m a t i o n of t h e c a t a l y s t a n d t h e a d d i t i o n reaction. (2) The rate c o n s t a n t s a n d energies o f a c t i v a t i o n for these reactions h a v e been determined. A possible r e a c t i o n m e c h a n i s m is p u t forward. (3) I t is s h o w n t h a t a d d i t i o n to D M M H P S ' s w i t h different distributions of the silicon h y d r i d e g r o u p s can proceed a t different rates if i n t e r a c t i o n occurs b e t w e e n a reactive p a r t o f the a d d e d molecule a n d a n e i g h b o u r i n g silicon a t o m a t t a c h e d to h y d r o g e n . Translated by E. O. PHILLII'S

REFERENCES 1. 2. 3. 4. 5.

6. 7. 8.

V. O. REIKHSFEL'D and L. A. GRIGOR'EVA, Vysokomol. soyed. 6: 969, 1964 J. L. SPEIER, J. A. WEBSTER and G. H. BARNES, J. Amer. Chem. Soc. 79: 974, 1957 L. GOODMAN, It. M. SILVERSTEIN and A. BENITEZ, J. Amer. Chem. Soc. 79: 3073, 1957 R. A. BENKESER and R. A. HICKNES, J. Amer. Chem. Soc. 80: 5298, 1958 V. A. PONOMARENKO, V. G. CHERKAEV and N. A. ZADOROZHNYI, Izv. Akad. Nauk SSSR, Otd. khim. nauk, 1610, 1960 A. D. STEPUKHOVICH, E. A. RAFIKOV and A. L. BORTNICHUK, Vysokomol. soyed. 4 : 85, 1962 G. M. BUItNETT, Mechanism of Polymer Reactions, Interscience Publishers, New York 1954; F. R. MAYO and C. WALLING, Chem. Reviews 46: 191, 1950 T. ALFREY and C. C. PRICE, J. Polymer Sci. 2: 101, 1947