An Overview of the Polymerization of Cyclosiloxanes - ACS Publications

13

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An Overview of the Polymerization of Cyclosiloxanes J. E. McGRATH, J. S. RIFFLE, Α. Κ. BANTHIA, I. YILGOR, and G. L. WILKES Virginia Polytechnic Institute and State University, Department of Chemistry and Department of Chemical Engineering, Polymer Materials and Interfaces Laboratory, Blacksburg, VA 24061 A review of the polymerization of cycloorgano siloxanes is provided. Cycloorganosiloxanes such as octamethyl cyclotetrasiloxane are the principal intermediate for the formation of high molecular weight polyorganosiloxane polymers. The ring open ing reaction can be initiated through the use of suitable basic or acidic catalysts. The transforma tion of the cyclosiloxanes into linear chains is an equilibrium process characterized by a very low heat of polymerization. At equilibrium conversions one produces 12-15% of cyclic oligomers, which are pre dominantly but not exclusively the cyclic tetramer. Equations from the literature which define these equilibrium are reviewed. The principal mechanisms involved in the anionic ring opening polymerization via initiators such as potassium hydroxide are discussed. A host of other related initiator types have also been used, including the so-called transient catalysts which are based upon quaternary silanolates. The transient catalysts are so described since they rapidly decompose above 130°C to yield inactive byproducts. The anionic polymeri zation of siloxanes is relatively well understood and currently accepted mechanisms are presented. Cationic polymerization of organosiloxanes is also well known but is much less understood. In general, molecular weight vs. conversion curves for the two processes are considerably different and these as pects are reviewed. Molecular weight is often con trolled by disiloxane or low molecular weight siloxane molecules terminated with triorganosiloxy groups. These materials (which are known as end blockers) control the molecular weight due to the

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146

INITIATION OF POLYMERIZATION

fact that their silicon-oxygen bonds can exchange with and incorporate the growing chain. By contrast, the silicon-carbon bonds in such materials are incapable of reacting. Thus at equilibrium one incorporates most of the linear chains between the two triorganosiloxy terminals. Such a process is possible with either anionic or cationic intermediates. This approach has recently permitted the introduction of functional groups such as aminopropyl, carboxy propyl, epoxy propyl, etc. Such oligomers are interesting intermediates for novel segmented copolymers.

Perhaps the f i r s t syntheses of the major types of the p o l y a l k y l s i l o x a n e s were performed by F r i e d e l , Ladenburg, and C r a f t s during the period 1865-71 (_l-6). However, i t was not u n t i l the e a r l y 1900*8 that F. S. Kipping and h i s group demonstrated the siloxane polymeric s t r u c t u r e ( 3 ) . This group prepared a large number of l i n e a r and c y c l i c polymers o f type (R2SiO)fl and HOd^SiOjflH, as w e l l as c r o s s l i n k e d a r c h i t e c t u r e s (RSi03- H3-2N)m* Kipping's studies d i s c l o s e d f o r the f i r s t time that the Si-O-Si group d i f f e r s d r a m a t i c a l l y from the C-O-C group i n i t s r e a c t i v i t y and that the Si-O-Si group could be e a s i l y cleaved by a c i d s , bases, or Lewis a c i d s . Moreover, he was the f i r s t to observe the c a t a l y t i c e f f e c t of acids and bases f o r the now commercially important siloxane r e d i s t r i b u t i o n and r i n g opening polymerization r e a c t i o n s . Following t h i s period, commercial production o f siloxane polymers was r e s t r a i n e d by the absence of convenient methods f o r monomer synthesis. In the 1930's, Rochow at General E l e c t r i c (4^7,8) and R. Muller (nine months l a t e r ) i n Germany apparently independently discovered what i s known as the " d i r e c t process" ( 9 ) . This process was e s s e n t i a l for the industry since i t allowed f o r the economical manufacture of the family of methylchlorosilanes necessary f o r siloxane production d i r e c t l y from s i l i c o n and a l k y l (mostly methyl) c h l o r i d e s . C o n t r o l l e d h y d r o l y s i s of the various organohalos i l a n e s then provided the c y c l i c trimers and tetramers which could be used f o r polymerization. Some of the important terminology and nomenclature f o r siloxanes i s reviewed i n Tables 1 and 2. Some important p h y s i c a l constants (10) f o r c h l o r o s i l a n e s are a l s o given i n Table 3. n

Although there are a v a r i e t y of routes presently a v a i l a b l e for siloxane production, only two are of commercial importance. These include h y d r o l y t i c reactions o f organohalosilanes or organoalkoxysilanes and r e d i s t r i b u t i o n type polymerizations of

In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


13.

MCGRATH ET A L .

Polymerization of Cyclosiloxanes

147

TABLE l . COMMON TERMINOLOGY OF SILOXANES Formula

Equivalent formula

( 3>3 0.5

M e

(CH ) SiO

Me SiO

CH

Si0

3

C H

3

S 1 0

2

1,5

3

S i 0

0.5

M D

2

MeSi0

Symbol

1>5

Τ

CH (CgH )SiO

MePhSiO

D'

(CgH ) SiO

Ph SiO

D'

(CH )HSiO

MeHSiO

D'

Si0

Si0

Q

3

5

5

3

2

2

2

2

American Chemical Society Library 1155 16th St. N. W. In Initiation of Polymerization; F., et al.; Washington, D. C. Bailey, 20038 ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

148


EXAMPLES OF NOMENCLATURE

TABLE 2.

Structural formula

"MDT" formula

3

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3

NM

3

Me S iOS iMe OS i M e 3

2

3

2

hexamethyldisiloxane

MDM

3

S i M e

Me SiO (SiMe 0)

2

3

M

D

0

octamethyltrisiloxane decamethyltetrasiloxane

M 2

Me Si—Ο—Si—Me 2J I 2

D.

0

Ο

Systematic name

octamethylcyclotetras i l o x a n e (methy1tetramer)

4

Ο

Me S i — 0 — S i M e 2

2

Ph Si—Ο—Si—Phu \ ι 2 z

octaphenylcyclotetras i l o x a n e (phenyltetramer)

0

Ph Si—0—Si—Ph 2

2

MePhSi—0—Si—MePh

I

I

ο

ο

I

1,3/5,7 t e t r a m e t h y l 1,3,5,7-tetraphenylcyclotetrasiloxane (methylphenyltetramer)

I

MePhSi—0—Si—MePh MT

3

3

Me SiOSiHMeOSiMe 3

3

MD'M ,

I

Me PhSiOSiMePhOSiPhMe M D M 2

2

Abbreviated Examp l e :

Me S i — 0 — S iMe ~ 3

l

1,1,3,5,5-pentamethyl1,3,5-triphenyltrisiloxane

-SiPh—(OSiMe )

2

3

1

5

1,1,1,3,5,5,5-heptamethyltrisiloxane

IUPAC

sometimes i s termed MDT M C H Si0 . 6

1,1,1,3,5,5,5-heptamethyl3-trimethylsiloxytrisiloxane

3

(Me SiO) S i M e

2

1

2

where Τ , i n t h i s c a s e , i s

1 > 5


13.

MCGRATH ET AL.


Où

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CO

CO

u_ ο ζ ο ι—ι I— Ο

cuo

XMÛ

•rΟ CO

cr» CO i s cr» cr» cr» cr» 1 m cr» ο LD σι cr» cr» 00

ZD

C "

s- ω -σ

4-

00 00

CO CvJ

Ο

Ο

cr» CO CM 00 cr» CO CO CO CO

,—

,—

Ι—

ω c

Où

CO

CVJ

Ι

Ο

I—

·Γ-

CO

ο Où ο _ J >-

•r-CM CO T 3 C

ω ο

ο CO

is

1

(x)

1

!

\

/

R'-Si-O-Si-R'

+

(y) R-Si-R

R-Si-R

\

1

(5)

/ *Si-

0

I R

1 R

R

I

I

R'-Si-O- -Si-0 — S i - R *

I

I

I

R

-R

- mR

+

catalyst

c y c l i c structures

3

d i s t r i b u t i o n s i n l i n e a r / c y c l i c e q u i l i b r a t e d polymer systems. Up to that time, t h e o r i e s d e s c r i b i n g molecular weight d i s t r i b u t i o n s had not considered the presence o f c y c l i c s t r u c t u r e s . Jacobson and Stockmayer s theory i s based on the premise that the proportion o f a macrocyclic species o f u n i t s i n e q u i l i b r i u m with l i n e a r components i s r e l a t e d to the p r o b a b i l i t y o f coincidence o f 1


152


the terminal atoms o f the sequence of χ u n i t s . In development of t h i s theory, the f o l l o w i n g processes were considered: Process 1:

-My-

f=* -My_ -

Process 2:

-M -

F=*

x

x

+

-Μ χ

C-M

x

M = monomer unit


C-M = c y c l i c form o f the monomer unit χ and y = degrees o f polymerization Process 3 denotes the sum of processes 1 and Process 3:

-M -

zzr -M _ ~ +

y

x

y

2. C-M

x

An e q u i l i b r i u m constant, K , for r i n g formation ( f o r process 3) was derived from entropy r e l a t i o n s h i p s and i s shown i n equations 6, 7, and 8 for processes 1, 2, and 3, r e s p e c t i v e l y . The parameter "P" i n these equations was described by Jacobson and Stockmayer as the p r o b a b i l i t y that a chain would be i n an appropriate c o n f i g u r a t i o n for r i n g formation (equation ^ ) . Jacobson and Stockmayer a c t u a l l y set equal to (vxb ) which meant i m p l i c i t l y that was to be set equal to < r > , the mean-square end-to-end distance of the unperturbed chain. The term " x ( r = o ) i n equation 9 denotes the Gaussian d i s t r i b u t i o n f u n c t i o n expressing the density of end-to-end v e c t o r s , r , i n the v i c i n i t y of r = 0. The existence o f t h i s f u n c t i o n i n the entropy term x

x

x

0

W

Δ β

(1)

=

(6)

k In A s

AS( ) 2

(7)

k In

VP (8)

AS(3) = k In R

χ

s


13.

MCGRATH ET AL.

Polymerization

of

153

Cyclosiloxanes

V = Volume of the system. σ

=

Symmetry number for the chain species ( 2 f o r organosiloxanes).

Α

σ

=

Α

σ

^ = Symmetry number for the r i n g species of "x" repeating u n i t s = 2x f o r organosiloxanes). x

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" s Ρ =

W»(r)

3

"

3 ν

dr = 3/2

s

—

(9) 2πνχ

π

Vxb

Z

= X

0

= χ

ν = Number of l i n k s per repeat u n i t , b = Average " e f f e c t i v e l i n k length" 2 = The mean-square end-to-end length averaged over a l l c o n f i g u r a t i o n s of chain s i z e x. X

obviously implies a Gaussian d i s t r i b u t i o n of end-to-end v e c t o r s . This point may, i n f a c t , be a poor assumption for the cases of very short chains (which, i n turn, form small r i n g s ) . The term " v " was defined as the volume element w i t h i n which two termini must meet i n order to form a bond. It appears i n the denominator o f Equation 6 due to the fact that the two atoms which formed the bond broken i n process 1 were constrained to the volume element v p r i o r to the occurrence of process 1. Since o / ( r ) i s the p r o b a b i l i t y of the termini of a^ molecule meeting i n the volume range, v , t h i s f a c t o r a l s o appears i n the entropy term d e s c r i b i n g process 2. The enthalpy terms for the f i r s t two processes presumably cancel each other with the assumption that the r i n g formed i s not small enough to be s t e r i c a l l y s t r a i n e d . T h i s was r a t i o n a l i z e d by noting that the intramolecular bond formed i n process 2 was s i m i l a r i n nature to the one severed i n process 1. The e q u i l i b r i u m constant for process 3 (expressed i n m o l e s / l i t e r ) was derived from equation 8 (equations 10 and 11). The d e f i n i t i o n of "p" i n t h i s context (equation 11) d i f f e r s s

v s

s

w

x

s

3/2

3/2 _3 2πν

Is

_1 N

A

_5/2

3 2πν

_1 (10)

χ 2 2 x

3/2 0

N

A


154


< Γ > ο = Vxb χ

[C-M ] [-M - -] [-M -] x

Κ

χ

=

y


y

[C-M ]

X

x

=

(11) ρ*

s l i g h t l y from i t s "normal" d e f i n i t i o n ( i . e . f r a c t i o n a l extent o f r e a c t i o n ) . Here, "p" i s defined as the extent o f r e a c t i o n o f the f u n c t i o n a l endgroups i n the chain p o r t i o n o f the system. Moreover, "p" could also be i d e n t i f i e d as the r a t i o o f the concentrations o f a c y c l i c species o f s i z e s χ to x-1 (12). The combination o f equations 10 and 11 p r e d i c t e d s e v e r a l interesting points: 1. The c o n c e n t r a t i o n o f x-mer rings was shown not to be a function of d i l u t i o n . The number o f x-mer rings was p r e d i c t e d to increase l i n e a r l y with d i l u t i o n , thereby making the p r o p o r t i o n o f r i n g s i n the system greater as i t i s diluted. Upon incrementally adding solvent, t h i s process e v e n t u a l l y r e s u l t s i n a " c r i t i c a l d i l u t i o n p o i n t " above which only r i n g s are present. 2. The r i n g s formed are small s p e c i e s . As ρ " ^ l ^ K * [C-M ] and [C-M ] approaches p r o p o r t i o n a l i t y to x"~ / . Jacobson and Stockmayer a l s o reasoned that each subset o f species (e.g. r i n g s vs. chains) w i t h i n the o v e r a l l d i s t r i b u t i o n s must be i n e q u i l i b r i u m with i t s e l f at thermodynamic e q u i l i b r i u m . Therefore, they p r e d i c t e d the normal d i s t r i b u t i o n f o r the chain species whether o r not r i n g species were formed. Figure 1 (reproduced from t h e i r 1950 paper) i l l u s t r a t e s the p r e d i c t e d d i s t r i b u t i o n for r i n g and chain s p e c i e s . One problem o f Jacobson and Stockmayer's i n t e r p r e t a t i o n i s observed i n the cases o f very small but unstrained r i n g s , where much higher concentrations o f r i n g s were formed than were p r e d i c t e d . F l o r y and Semlyen (15) explained t h i s d e v i a t i o n by suggesting that not only d i d two termini have to meet w i t h i n a volume " v " i n order to e s t a b l i s h a bond, they also had to approach each other from a s p e c i f i e d d i r e c t i o n . This d i r e c t i o n was s p e c i f i e d by a s o l i d angle f r a c t i o n 6ω/4π. They explained that t h i s term should appear i n the entropy expression for process 1 i n the inverse form, i . e . 4ττ/δω. In process 2, i f the chains were s u f f i c i e n t l y long, there would be no c o r r e l a t i o n between the p r o b a b i l i t y f o r two termini o f a_ molecule to meet w i t h i n v and to approach w i t h i n the s o l i d angle δω. In t h i s case, the term δω/4ττ would be v a l i d f o r i n c l u s i o n i n t o the entropy term f o r process 2 and, hence, when the entropies for processes 1 and 2 were summed, these terms would cancel (and the equation f o r AS(3) from Jacobson and Stockmayer s theory should be v a l i d ) . However, for short chains, the p r o b a b i l i t y o f approach x

x

s

s

1


x

13.

MCGRATH ET AL.

155



100

801-

Ιϋ

0

10

20

30

40

50

60

70

D. P. Figure 1. Typical molecular distribution for a ring-chain equilibrium polymer (U).



156

of two t e r m i n i o f one chain w i t h i n v would depend on bond angles, s t e r i c f a c t o r s , e t c . , p a r t i c u l a r to the s p e c i f i c system, and would deviate from δω/4π. In that case the p r o b a b i l i t y "P" i n Jacobson and Stockmayer's theory i n the entropy term would be incorrect. There have been s e v e r a l studies conducted i n v e s t i g a t i n g e q u i l i b r i u m d i s t r i b u t i o n s o f polyorganosiloxanes experimentally. T h e o r e t i c a l and e m p i r i c a l weight f r a c t i o n s o f c y c l i c s and t h e i r d i s t r i b u t i o n s with variances i n χ are o f p a r t i c u l a r i n t e r e s t to the s y n t h e t i c polymer chemist. Equation 10 can be rearranged to give the weight f r a c t i o n , w , o f c y c l i c s (equation 12) f o r high extents o f r e a c t i o n (the e m p i r i c a l comparisons were a l l made s


r

3

3 / w

r

Η,

2

.

I

—

= π

2 I N c A

-3/2

(x C )

(12)

x

x=4

c = T o t a l s i l o x a n e concentration i n g/i MQ = Molecular weight o f a repeat u n i t I = The length o f a siloxane bond. J. = C 2x£ x

0

x

at ρ 1 ) . C o n t r i b u t i o n s from the weight f r a c t i o n o f the s t r a i n e d c y c l i c trimer which i s not a p p l i c a b l e to the theory are neglected. Wright and Semlyen compared t h e i r own (ljO values together with Brown and Slusarczuk's (18) c a l c u l a t e d and experimental values o f K f o r polydimethylsiloxane (PDMS). C a l c u l a t e d values were based on c a l c u l a t i o n s o f < r > derived from F l o r y ' s r o t a t i o n a l isomeric state model f o r PDMS (23). E m p i r i c a l values came from measurements o f the concentrations o f x-meric r i n g s according to Jacobson s and Stockmayer s r e l a t i o n s h i p , K = [C-M ]/p . Brown and Slusarczuk (18) e q u i l i b r a t e d PDMS i n toluene at 110 degrees centigrade and a concentration o f 0.22 g/mi o f s i l o x a n e . Values from that e q u i l i b r a t i o n and Wright's and Semlyen's bulk e q u i l i b r a t i o n (17) as a f u n c t i o n o f χ are compared with t h e o r e t i c a l values i n Figure 2. As p r e d i c t e d , the Κ values i n the range χ = 11-40 were experimentally independent o f d i l u t i o n . In d i r e c t c o n t r a s t , the c y c l i z a t i o n constants for χ = 4-10 increase with d i l u t i o n (the increase becoming more pronounced with decreases i n x ) . Siloxane chains f o r χ greater than approximately 15 agreed w e l l with t h e o r e t i c a l values. x

x

1

0

1

x

x

x

χ

These same authors (16) a l s o compared experimental K values as a f u n c t i o n o f χ f o r a s e r i e s o f bulk e q u i l i b r a t e s o f the s t r u c t u r e —fR(CH3)Si-0-}- wherein R e q u a l l e d H, CH3, CH3CH2, CH3CH2CH2, and C F 3 C H 2 C H 2 i n order to assess the e f f e c t o f the s u b s t i t u e n t s i z e on the e q u i l i b r i u m d i s t r i b u t i o n . The K values x

x

x


MCGRATH ET AL.

Polymerization

of

MONOMER

UNITS 10

5

1

1

Cyclosiloxanes

1

1

I

I

I

(x) 15

20

I I I I I I I I IIII II


• -

o

%

-1

•

•

Bulk

ο

Solution

3

Calculated

*q "·

-

ο-

-

_

·. *·

\

-3

1

Ι

ι

ι

0.6

0.8

1.0

1.2

1.4

log χ

Figure 2. Molar cyclization equilibrium constants of dimethylsiloxane at 110

an


158


for the smallest unstrained rings (x=4 or 5) were found to increase along the s e r i e s R = H Rb > Κ > L i (33). At equal molar concentrations, the rates using the hydroxide or s i l o x a n o l a t e of the same metal have been found to be s i m i l a r (30). Tetramethylammonium s i l o x a n o l a t e s e x h i b i t a c t i v i t i e s close to the cesium s i l o x a n o l a t e s (33). Tetramethylammonium hydroxide, s i l a n o l a t e , and s i l o x a n o l a t e r a p i d l y decompose above 130°C y i e l d i n g methanol, methoxy t r i m e t h y l s i l a n e , or methoxysiloxane r e s p e c t i v e l y and trimethylamine (34-35)· i n the cases o f the f i r s t two compounds l i s t e d above, the c a t a l y s t breakdown products are f u g i t i v e at the decomposition temperature. Thus, the usual need for c a t a l y s t n e u t r a l i z a t i o n and removal following polymerization i s eliminated. They are o f t e n termed " t r a n s i e n t " c a t a l y s t s (34). At low c a t a l y s t concentrations and i n the absence of "end-blockers", the degree of polymerization i s approximately



-76

-80

-59

-78

-63

MDM

MDgM

MD M

MDM

MDgM

Source:

MD,M

5

4

307.5

290

270

245

229

194

153

99.5

Reproduced with permission from Ref. 10.

-80

MDM

2

-67

MM

Boiling point, °C

0.9180

0.9099

0.9012

0.8910

0.8755

0.8536

0.8200

0.7636

2 0

Density, d

SOME PHYSICAL CHARACTERISTICS OF LINEAR OLIGOMERS

Melting point, °C

5.

Symbol

TABLE


1.3980

1.3970

1.3965

1.3948

1.3925

1.3895

1.3840

1.3774

1

Refractiyi index, ηέ

13.

MCGRATH ET AL.


163

i n v e r s e l y r e l a t e d to the c a t a l y s t concentration (36). Russian authors (4) have noted a d e v i a t i o n i n the l i n e a r i t y o f the 1/X vs. c a t a l y s t concentration f u n c t i o n at high c a t a l y s t loadings. It was suggested that t h i s d e v i a t i o n could be a t t r i b u t e d to the presence o f s t a b l e associated s t r u c t u r e s s i m i l a r such as ^4. n

- + 0

Κ


ι/

\i

Si

Si

κ

0 4

P r a c t i c a l l y , polymerization temperatures are s e l e c t e d on the b a s i s o f the a c t i v i t i e s o f the c a t a l y s t , c y c l o s i l o x a n e , and endblocker used with the aim o f a r r i v i n g at thermodynamic e q u i l i b r i u m w i t h i n an acceptable time period. In the bulk polymerization o f D4, modest temperature changes reportedly do not a f f e c t the f i n a l e q u i l i b r i u m number average molecular weight o f the polymers (37). Rates o f anionic polymerization are influenced by the number o f siloxane u n i t s present i n the monomer r i n g s . Some c h a r a c t e r i s t i c s are given i n T a b l e s 6 and 7 (10). Due to r i n g s t r a i n i n the three unit r i n g s , a l l o f the c y c l o t r i - siloxanes polymerize f a s t e r than the c y c l o t e t r a s i l o x a n e s . In the dimethylsiloxanes, D3 r e p o r t e d l y polymerizes approximately 50 times f a s t e r than D4 ( 4 ) . Several i n v e s t i g a t i o n s have been performed concerning the r e l a t i v e r e a c t i v i t y o f c y c l i c tetramers s u b s t i t u t e d with v a r y i n g amounts o f phenyl and methyl groups (38-42). Andrianov et a l . (42) studied a n i o n i c copolymerizations o f D4 with varying amounts o f o c t a p h e n y l c y c l o t e t r a s i l o x a n e (10-70 mole percent). They found that the rate o f copolymer formation, the v i s c o s i t y o f the r e s u l t i n g copolymers, and the e q u i l i b r i u m y i e l d o f l i n e a r species a l l decreased r e g u l a r l y as the mole percent o f phenyl tetramer was increased i n the r e a c t i o n mixture. They also noted that i n the e a r l y stages o f conversion, although both monomers had become incorporated i n the l i n e a r p o r t i o n to some extent, the polymers formed were enriched with diphenyl u n i t s . On the basis o f e l e c t r o n i c f a c t o r s , i t was reasoned that i n the diphenyl s u b s t i t u t e d tetramer, the s i l i c o n atoms would be more s u s c e p t i b l e to n u c l e o p h i l i c attack. Conversely, the phenyl s u b s t i t u t e d s i l o x a n o l a t e anion, once formed, would be less r e a c t i v e than the methyl s u b s t i t u t e d analog. These same authors (42) a l s o studied the s t r u c t u r e o f the c y c l i c s as e q u i l i b r a t i o n proceeded. R e d i s t r i b u t i o n type steps had e v i d e n t l y occurred even i n the



?

a

Source:

Dg

6

D

5

D

D

4

3

D

D

Symbol

b

Crystals

290

154

245

210 a

175.8

134

Boiling point, °C

Reproduced with permission from Ref. 10.

At 20mm Hg.

31.5

-32

-3

-44

17.5

64.5

Melting point, °C

1.1770

0.9730

0.9672

0.9593

0.9561

b

Density, d 2 0

INFLUENCE OF RING SIZE ON THE

CHARACTERISTICS OF CYCLIC SILOXANES

TABLE 6.


1.4060

1.4040

1.4015

1.3982

1.3968

2

0

Refractive index, n ,

13.

MCGRATH ET AL.

Polymerization

165

of Cyclosiloxanes

Ο

cno

ο ο Σ

ο

CM

C7>

η

σι

Ν

σ>

^·

VO

ι—

CM ^JCO ^

Ο


CL

>CMQ •ιC

< χ ο CO

-l-> Ο

CM

*

(Ο

Χ

en

τ-

CO

i- ω Η- -ο

CO

ο ο >ο >> •+-> CO LU

Ο

ο

CM

r—

(Tl

Lf)

•ΓCO CD C

in

LO Ν

ί-

Ν

σι

σι

• Ο CM

ι-^

ι—

Ο

Ο

\

ε

η

00

LO

CO

ι— vo σι

Ο

-σ

CL.

Ο

< Ο

ο π: ο.

LO

ι—

>•ιΟ CO

ε ο

en

•ι-

Ο CÛ

ι—

LO

Γ>Η

CO CO

CO CM

CO CO ι—

Ο ι—

rr— ι—

00

stfCO ι—

LO

I— ι—

ο

ε U

α,

ω

i—ι ι—I ο ο

PQ

ο

'ΐCO

Ό Ο U

CO ζπ

ο

^ΐ-

α

I—I

^ -Γ-

^ ^ Ο ^- Ο Ο ΖΠ ^ ·ρCM Ο Ο CO Ο CO ZLZ II ·Γ— CM •ι_C Ο CO ^ CO ΟCO ' CM ΖΠ CO CM CO LL. ΠΙ COI -c π: ο ο ζπ ο Q_

-—

Ο




166

i n i t i a l stages s i n c e mixed c y c l i c s (diphenyl and dimethyl u n i t s i n the same c y c l i c molecules) were observed soon a f t e r the beginning of the r e a c t i o n . In e q u i l i b r a t i o n r e a c t i o n s , hexaorganyl d i s i l o x a n e or low molecular weight siloxanes terminated with t r i o r g a n y l s i l o x y groups i n the r e a c t i o n mixture can regulate molecular weight by a c t i n g as chain t r a n s f e r agents or "end-blockers". The e f f i c i e n c y of these agents depends on the r e l a t i v e amount of p o s i t i v e charge on t h e i r s i l i c o n atoms. Results of v i s c o s i t y measurements as a f u n c t i o n of time using hexamethyldisiloxane endblocker, D4, and tetramethylammonium hydroxide c a t a l y s t (1J3) are shown i n Figure 4. The i n i t i a l large v i s c o s i t y increase was found to be a r e s u l t of the f a s t e r r e a c t i o n of D4 as compared to hexamethyldisiloxane under these c o n d i t i o n s . Contrasted with the above r e s u l t s was the analogous r e a c t i o n with the exception that a s u l f u r i c a c i d c a t a l y s t instead o f tetramethylammonium hydroxide (see Figure 5) was used (13). The shape of the curve i n Figure 5 represents approximately equal r e a c t i v i t y o f D4 and hexamethyldisiloxane towards an a c i d c a t a l y s t . Many compounds with an e l e c t r o n donor character are reported to have an a c c e l e r a t i n g e f f e c t on the a n i o n i c polymerizations. No doubt, these a l t e r the nature of the ion p a i r . Representative "promoters" reported include tetrahydrofuran (43-45), dimethylformamide (46), s u l f o x i d e s (47,48), and cryptâtes (60). C a t i o n i c polymerization of c y c l o s i l o x a n e s i s w e l l known but used much less f r e q u e n t l y than a n i o n i c r e a c t i o n s . The most frequently used c a t a l y s t s include s u l f u r i c acid and i t s d e r i v a t i v e s (4^49-52)· T r i f l u o r o a c e t i c a c i d has also been used to polymerize D4 i n bulk (53,61). The mechanism of a c i d c a t a l y z e d polymerization i s postulated to be that schematically i l l u s t r a t e d i n equations 16 and 17. However, according to the recent text by Odian (54), there i s no evidence for the rearrangement of the s i l i c o n i u m ion shown i n these equations. A l t e r n a t i v e l y , one could propose that the propagating species i s the t e r t i a r y oxonium i o n . R I

Initiation: /

0

Ί

1 Si R

\

+ HOx

\

/ R-Si-R

R-Si-R

\

SiR2~(0SiR2)3

H A

jr

/

R

0

+

R v

. N

/

Si

I R

I H-(OSiR )3-0-Si 2

+

A"

I R


(16)



ON

•—*

Co

ι

ο

fi

1'

•Η.

§'

>β

Ci

>

Η

w

Η

Ο

Ο


168

INITIATION OF

POLYMERIZATION

Figure 5. Viscosity vs. time of reaction for equilibration of 1 mol of hexamethyldisiloxane and 1 mol of £> with 4% H SOj, at room temperature (13). 4

2


13.

MCGRATH ET A L .

Polymerization R

Propagation:

0

~0-Si

+

A"

+

169

of Cyclosiloxanes

·

^

^

SiR -(OSiR )3 2

2

R

I +

^OSiR Hj)^A-^f==i

~ ~ ~ ( 0 S i R ) - 0 - S i A"


2

2

SiR -(OSiR )3 2

4

(17)

R

2

Our own research (55-58, 62-67) has r e c e n t l y been concerned with the p o l y m e r i z a t i o n o f D4 and o f combinations o f D4 and D"4 ( o c t a p h e n y l c y c l o t e t r a - siloxane) i n the presence o f organofunctional endblockers used as molecular weight r e g u l a t o r s . We have s u c c e s s f u l l y obtained dimethyl amino (62), aminopropyl, g l y c i d o x y p r o p y l , carboxypropyl, hydroxybutyl, and hydroxyphenylpropyl terminated s i l o x a n e oligomers o f c o n t r o l l e d molecular weights by these methods. Transient s i l o x a n o l a t e a n i o n i c c a t a l y s t s prepared by r e a c t i n g four moles o f D-4 with one o f tetramethyl ammonium hydroxide at 80°C are e f f e c t i v e f o r e q u i l i b r a t i n g " n e u t r a l " systems such as the epoxy (59), " b a s i c " dimethyl-amino (64) o r aminopropyl (59,j67) end-blockers and D-4. With " a c i d i c " f u n c t i o n a l i t y on the end-blocker, we have s u c c e s s f u l l y u t i l i z e d t r i f l u o r o a c e t i c a c i d f o r the e q u i l i b r a t i o n s . Further d e t a i l s o f the oligomer synthesis and t h e i r u t i l i z a t i o n i n segmented copolymers w i l l be described i n future p u b l i c a t i o n s .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

R. West and T. J . Barton, J . Chem. Ed., 1980, 57 (3), 165. C. Friedel and J. M. Crafts, Ann., 1965, 136, 203. F. S. Kipping, Proc. Roy. Soc., A, 1937, 159, 139). M. G. Voronkov, V. P. Mileshkevich, and Y. A. Yuzhelevskii, The Siloxane Bond, Plenum Press, N. Y., 1978, 2. A. Ladenburg, Ann. Chem., 1971, 159, 259. C. Friedel and J . M. Crafts, Ann. Chim. Phys., 1970, 19 (5), 334. W. Noll, Chemistry and Technology of Silicones, Academic Press, N.Y., (1968). B. B. Hardman and R. W. Shade, Mat. Technol, 26, Spring, 1980. R. J . H. Voorhoeve, Organohalosilanes: Precursors to Silicones, Elsevier, N.Y., (1967). R. Meals, Encyclopedia of Chemical Technology, 18, 2nd edition, 1969, 221-260.


170 11. 12. 13. 14.


15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

INITIATION OF POLYMERIZATION C. Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, London, (1960). B. C. Arkles and W. R. Peterson, J r . , ed., Silicon Compounds, Register and Review, Petrarch Systems, Levittown, Pa., (1979). S. W. Kantor, W. T. Grubb, and R. C. Osthoff, J . Amer. Chem. Soc., 1954, 76, 5190. H. Jacobson and W. H. Stockmayer, J . Phys. Chem., 1950, 18, 1600. P. J . Flory and J. A. Semlyen, J . Amer. Chem. Soc., 2966, 88, 3209. P. V. Wright and J. A. Semlyen, Polymer, 11 (9), 1970, 462. P. V. Wright and J. A. Semlyen, Polymer, 10, 1969, 543. J . F. Brown and G. M. Slusarczuk, J . Amer. Chem. Soc., 1965, 87, 931. M. S. Beevers and J . A. Semlyen, Polymer, 1971, 12 (6), 373. T. C. Kendrick, J . Polym. Sci., 1969, A-2, 7, 297. W. C. Davies and D. P. Jones, Polym. Prepr., 1970, 11, 447 J . C. Saam, D. J . Gordon, and S. Lindsey, Macromolecules, 1970, 3, 4. P. J . Flory, V. Crescenzi, and J. E. Mark, J . Amer. Chem. Soc., 1964, 86, 146. J . B. Carmichael and R. Winger, J . Polym. Sci., A, 1965, 971. J . B. Carmichael, Rubber Chem. Technol., 1964, 40, 1084. J . B. Carmichael and D. J. Gordon, J. Phys. Chem., 1967, 71, 2071. Y. A. Yuzhelevskii, E. G. Kagan, and E. B. Dmokhovskaya, Khim. Geterotsikl. Soedin., 1967, 951. J . B. Carmichael and J . Heffel, J . Phys. Chem., 1965, 2218. J . B. Carmichael, J . Macromol. Chem., 1 (2), 1966, 207. W. T. Grubb and R. C. Osthoff, J . Amer. Chem. Soc., 1955, 77, 1405. V. Bazant, V. Chvalovsky', J . Rathousky, Organosilicon Compounds, 1, Academic Press, N.Y., 15 (1965). K. A. Andrianov, Metalorganic Polymers, Interscience, N.Y., (1965). D. T. Hurd and R. C. Osthoff, J . Amer. Chem. Soc., 1954, 76, 249. A. K. Gilbert and S. W. Kantor, J . Polym. Sci., 1959, 40, 35. A. Noshay, M. Matzner, and T. C. Williams, Industrial and Engineering Product Research and Development (I. & E. C. Product Research and Development), 1973, 12, 268. C. L. Lee and O. K. Johanson, J . Polym. Sci., 1966, A-1 (4), 3013. M. Kucera, J . Polym. Sci., 1962, 58, 1263. K. A. Andrianov and S. E. Iakushkina, Polym. Sci, U.S.S.R., 1960, 1, 221-228.


13.

MCGRATH ET AL.

39.

Z. Laita and M. Jelinek, Polym. Sci., U.S.S.R., 1964, 5, 342-353. K. A. Andrianov, S. Ye. Yakushkina, and L. N. Guniava, Vysokomol. soyed., 1966, 8 (12), 2166-2170. K. A. Andrianov et a l . , Vysokomol. soyed., 1970, A14(6), 1268-1276. K. A. Andrianov et a l . , Vysokomol. soyed., 1972, A14(5), 1294-1302. B. Suryanarayanan, B. W. Peace, and K. G. Mayhan, J . Polym. Sci., Polym. Chem. Ed., 1974, 12, 1089. B. Suryanarayanan, B. W. Peace, and K. G. Mayhan, J . Polym. Sci., Polym. Chem. Ed., 1974, 12, 1109. W. A. Fessier and P. C. Juliano, Polym. Prepr., 1971, 12, 151. J . B. Gangi and J . A. Bettelheim, J . Polym. Sci., 1964, A-2, 4011. J . G. Murray, Polym. Prepr., 1965, 6, 163. G. D. Cooper and J . R. Elliott, J . Polym. Sci., A-1 (4). B. Kanner, B. Prokai (Union Carbide Corp.) Ger. Offen. 2,629,138 (C1.C08G 77/78), 13 Jan., 1977, U.S. Appl. 592,129, 30 Jun. 1975. G. Rossmy, R. D. Langenhagen (Goldschmidt), Ger. Offen. 2,714,807, 20, Oct. 1977, Brit. Appl. 76/14,391, 08 Apr. 1976, C. A. 87: 202569z. P. Rosciszeqski, E. Jagielska, K. Bartosiak, Pol. 92,926, 15 Dec. 1977, C. A. 89:75864f. R. E. Moeller (General Electric Co.), Braz. Pedido PI 7,703,261, 20 Feb. 1979, C. A. 90: 188640u. D. T. Hurd, J . Amer. Chem. Soc., 1955, 77, 2998. G. Odian, Principles of Polymerization, 2nd Edition, Wiley, N.Y., 549 (1981). J . S. Riffle, R. G. Freelin, A. K. Banthia, and J . E. McGrath, J . Macromol. Sci. - Chem., 1981, A15(5) 967-998. J . S. Riffle, Ph.D. Thesis, VPI & SU, Blacksburg, Va., Dec. 1980. J . E. McGrath, et a l . , Proceedings of the International Rubber Conference, Kharagpur, India, 1980. J . E. McGrath, J . S. Riffle, I. Yilgor, A. K. Banthia and P. Sormani, Org. Coatings and Plastics Preprints, 1982, 46, 693. J. S. Riffle, I. Yilgör, A. K. Banthia, G. L. Wilkes, and J . E. McGrath, "Epoxy Resins", R. S. Bauer, Editor, ACS Symp. Vol., in press, 1982. S. Boileau, in "Anionic Polymerization: Kinetics, Mechanism and Synthesis," J . E. McGrath, Editor, ACS Symposium Volume 1982, Series No. 166. L. Wilczek and J . Chojnowski, Macromolecules, 1981, 14 (1), 9. T. C. Ward, D. P. Sheehy, J . S. Riffle, and J . E. McGrath, Macromolecules, 1981, 14 (6), 1791.

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171

172


63.

Anionic Polymerization: Kinetics, Mechanism and Synthesis, J. E. McGrath, Editor, ACS Symposium Volume Series No. 166,

64.

J. Ε. McGrath, J . S. Riffle, D. W. Dwight, D. C. Webster and T. F. Davidson, Macromolecules, in preparation, 1982. T. F. Davidson, M. S. Thesis, VPI & SU, Blacksburg, Va.,

(1981).

65.

1980.

66.


67.

S. Tang, E. Meinecke, J . S. Riffle, and J . E. McGrath, Rubber Chem. and Tech., 1980, 54 (5), 1160. J. E. McGrath, J . S. Riffle, A. K. Banthia, I. Yilgor, and G. L. Wilkes, To be published.

RECEIVED October 15, 1982


An Overview of the Polymerization of Cyclosiloxanes - ACS Publications

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