Cationic Polymerization of Cyclic Ethers Initiated by Superacid Esters

10 Cationic Polymerization of Cyclic Ethers Initiated by Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 13, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0006.ch010

Superacid Esters T A K E O S A E G U S A and S H I R O

KOBAYASHI

Department of Synthetic Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan

Introduction Superacids are more a c i d i c than 100% s u l f u r i c a c i d , the most f r e q u e n t l y used strong a c i d solvent (1,2). With t h i s respect f l u o r o s u l f u r i c , c h l o r o s u f u r i c , and t r i f l u o r o m e t h a n e s u l f o n i c a c i d s are f a m i l i a r among superacids (3). 2,4,6-Trinitrobenzenesulfonic a c i d i s a l s o a superacid s i n c e it forms a s t a b l e trialkyl oxonium salt (4). Superacid e s t e r s have long been known, e.g., e t h y l c h l o r o s u l f a t e was prepared more than a century ago (5). However, little a t t e n t i o n has been paid on the r e a c t i o n of superacid e s t e r s (6) until r e c e n t l y A l d e r et al., have reported that methyl and e t h y l f l u o r o s u l f a t e s are very e f f e c t i v e a l k y l a t i n g agents f o r n i t r o g e n and/or oxygen c o n t a i n i n g compounds ( 7 ) . We have found that methyl and e t h y l e s t e r s of f l u o r o s u l f u r i c and c h l o r o s u l f u r i c a c i d s are good initiators f o r the c a t i o n i c ring-opening p o l y m e r i z a t i o n of t e t r a h y d r o f u r a n (THF), oxetane, 2-oxazoline, and propylene oxide (8). At almost the same time Smith and Hubin have reported s e m i - q u a n t i t a t i v e s t u d i e s of the THF p o l y m e r i z a t i o n initiated by superacid d e r i v a t i v e s i n c l u d i n g methyl trifluoro methanesulfonate (9,10). Very r e c e n t l y we have performed k i n e t i c s t u d i e s of the ring-opening p o l y m e r i z a t i o n of cyclic ethers initiated with superacid e s t e r s (3,11-14). The e s t e r s employed were e t h y l f l u o r o s u l f a t e (EtOSO F), chlorosulfate (EtOSO Cl), tri fluoromethanesulfonate (EtOSO CF , "triflate" EtOTf), and 2,4,6trinitrobenzenesulfonate ( " t r i n i t a t e " EtOTn) ; cyclic ether monomers being THF and 3,3-bis(chloromethyl) oxetane (BCMO). K i n e t i c analyses were c a r r i e d out by means of our "phenoxyl endcapping" method (15,16) as w e l l as of Η and F nmr spectroscopy. These methods allowed the d i r e c t determination of the concentra t i o n of the propagating species [P*]. Thus, we have d i s c l o s e d new f i n d i n g s which are c h a r a c t e r i s t i c to the superacid e s t e r initiated systems of the THF and BCMO polymerizations (11,12). The present article d e s c r i b e s these r e s u l t s (3,11,12)· C a t i o n i c ring-opening polymerizations of c y c l i c ethers initiated with t y p i c a l Lewis a c i d c a t a l y s t s (conventional initiators) have 2

2

2

3

1

19

150

In Polyethers; Vandenberg, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10.

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r e c e n t l y been reviewed by us Polymerization of

151

Cyclic Ethers

(15,16).

THF

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K i n e t i c s of the THF Polymerization by Phenoxyl End-Capping Method (3). In determining [P*] during the THF p o l y m e r i z a t i o n i n i t i a t e d by superacid e s t e r s the phenoxyl end-capping procedure i s shown i n Scheme 1. The r e a c t i o n of sodium phenoxide with the

Et-E-0(CH ) ^ -0^> 2

4

n

Scheme 1 propagating species and with the unreacted i n i t i a t o r should q u a n t i t a t i v e l y produce the polymer phenyl ether and phenetole, r e s p e c t i v e l y . The quant i t a t ivenes s of the r e a c t i o n between sodium phenoxide and the propagating c y c l i c oxonium has been e s t a b l i s h e d i n the previous s t u d i e s by using Lewis a c i d c a t a l y s t s (17,18). Therefore, the r e a c t i o n s of sodium phonoxide with superacid e s t e r s of EtOS0 F and E t O S 0 C l were examined (Table I ) . From the r e s u l t s shown i n Table I Reaction 1 has been regarded 2

2

as q u a n t i t a t i v e (3). This i n d i c a t e s that the phenoxyl endcapping method can be used f o r the k i n e t i c a n a l y s i s of the THF p o l y m e r i z a t i o n by superacid e s t e r i n i t i a t o r s as i n the case of Et 0+BF^~ i n i t i a t o r system (19). The i n i t i a t i o n r e a c t i o n can be formulated as 3

EtOS0 X 2

+

(2)

On the b a s i s of an S 2 i s given by N

mechanism the r a t e equation of

initiation


POLYETHERS

152 Table I . Reactions of EtOSOJC (X=F and CI) with Sodium Phenoxide a t Room Temperature a

EtOS0 X ^ 2

NaOC H

X

6

Reaction Time

Yield

(min)

(%)

(mmol)

(mmol)


b) 5

0.99

F

4.0

10

96

0.59

F

2.9

30

95

1.15

Cl

3.5

5

93

b) S o l u t i o n i n 6 ml of THF.

a) S o l u t i o n i n 3 ml of THF. c) Based on EtOS0 X. 2

c )

Determined by UV a n a l y s i s . B u l l . Chem. Soc. Japan (3).

d[I] dt

Μ [I] [M]

(3)

I n t e g r a t i o n of Equation 3 gave

[ I ]n[I] L 1 J

η

r = ki I [M]dt T

(4)

Λ

t

where

i s the r a t e constant of i n i t i a t i o n . I t has already been e s t a b l i s h e d that the propagation of the c a t i o n i c p o l y m e r i z a t i o n of THF i s expressed by

^y^Tl

> 0 ( C H ) - ( Q · A"

· A"

2

4

(5)

According to a bimolecular mechanism the r a t e equation of propagation i s given by

d[M] dt

= k [P*] f[M] - [H]ej D

(6)

where [M] and [M]e represent the instantaneous and e q u i l i b r i u m monomer c o n c e n t r a t i o n s , r e s p e c t i v e l y , and kp i s the r a t e constant of propagation. I n t e g r a t i o n of Equation 6 leads to

F i g u r e 1 shows the [P*] - time (curve A) and monomer conversion - time (curve B) r e l a t i o n s h i p s i n the THF polymerizat-


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153

Cyclic Ethers

i o n i n i t i a t e d by EtOSO F i n C I ^ C l a t 0°C. A f t e r 20 hr, [P*] reached to 63% of the feed c o n c e n t r a t i o n of the i n i t i a t o r . The presence of the i n d u c t i o n p e r i o d (curve B) i n d i c a t e s a slow i n i t i a t i o n followed by a f a s t propagation. P l o t s of Equations 4 and 7 were made on the b a s i s of the data of curves A and B, respectively. In both cases s t r a i g h t l i n e s passing through the o r i g i n were obtained, the slopes of which gave values of k i = 0.33 Χ ΙΟ"- 1/mol-sec and kp = 0.66x10-3 1/mol-sec, r e s p e c t i v e l y . Table I I l i s t s the k i n e t i c data of three superacid ester i n i t i a t o r s as w e l l as of an oxonium species of EtgO+'BF^*".


5

Table I I .

K i n e t i c Data of the THF P o l y m e r i z a t i o n with Ester I n i t i a t o r s i n CH C1 S o l u t i o n 2

EtOS0 F 2

Superacid

2

EtOS0 Cl 2

EtOS0 CF 2

+ - a) EtÔBF^

3

Initiation kiXlO

5

at 0°C

0.33

0.38

0.80

6.1

b

)

(l/mol» sec) Δ Η * (kcal/mol)

13.5

12.8

10.5

16.4

AS* (e.u.)

-34

-37

-44

-16

Propagation kpxlO

3

a t 0°C

0.66

1.4

1.7

3.7

(l/mol- sec) Δ Hp^(kcal/mol)

13.0

11.8

11.6

12

A S * (e.u.)

-26

-28

-29

-26

p

a) Taken from (15, 16, 19, 20). b) Data at 2.5°C (20).

B u l l . Chem. Soc. Japan (3).

The i n i t i a t i o n i s a d i p o l e - d i p o l e r e a c t i o n to produce an oxonium i o n (Equation 2 ) . The k^ values of superacid e s t e r s a r e 1/20 - 1/10 of that of Et30+BF~ and a t l e a s t 2 Χ 1 0 times smaller than the kp values i n the superacid e s t e r systems. The a c t i v a t i o n parameters e x h i b i t e d r e l a t i v e l y low Δ Η * ( f a v o r a b l e ) and Δ s£ (unfavorable) v a l u e s . This tendency has o f t e n been observed i n d i p o l e - d i p o l e S^2 r e a c t i o n s producing i o n i c s p e c i e s , e.g., the Menschutkin r e a c t i o n (21) and the oxonium formation r e a c t i o n from superacid e s t e r s and tetrahydropyran (Reaction 8) ( 2 2 ) . 2


POLYETHERS

154

R0S0 X 2

+

"0(3

=

R

'

0

S

° 2

X

"

(

8

)

The kp v a l u e of EtOSC^F i s extremely s m a l l , e.g., about 1/6 of that of Et30BF^T i n i t i a t o r . The kp values of EtOSC^Cl and EtOS0 CF3 are between those of EtOSC^F and Et30 BF4~. Since the a c t i v a t i o n parameters of the propagation are very c l o s e to those of Et30 BF4~", i t i s reasonable to conclude that the propagation i n i t i a t e d by EtOSC^X proceeds mainly v i a an oxonium mechanism (Equation 5 ) . +

2


+

K i n e t i c s of the EtOS02CF3-Initiated P o l y m e r i z a t i o n of THF by and i ^ F Nmr Spectroscopy. I t has been pointed out i n s e v e r a l s t u d i e s (3> Jb 12* 23) that i n the THF polymerization.by a supera c i d e s t e r i n i t i a t o r , e.g., EtOSÛ2CF3, the propagating chain end may be i n the e q u i l i b r i u m of the oxonium (I) - e s t e r (II) species (Equation 9). Our previous study showed that k i n e t i c s of the THF Κ 0S0 CF " 2

I

3

=

0(CH ) 0S0 CF 2

4

2

(9)

3

II

p o l y m e r i z a t i o n i n i t i a t e d with superacid e s t e r s could be c a r r i e d out by % nmr spectroscopy alone ( 3 ) . However, i t was very d i f f i c u l t to v e r i f y d i r e c t l y the e q u i l i b r i u m of Equation 9 due to the l i m i t a t i o n of the s e n s i t i b i t y and r e s o l u t i o n of *H nmr spectroscopy. We d i s c l o s e d that l ^ F spectroscopy i s very powerful to abserve d i r e c t r y both the oxonium counteranion OSO2CF3- (I) and e s t e r species CH2OSO2CF3 ( I I ) . 19

F

nmr spectroscopy (11). F i g u r e 2 shows the ^F nmr spectrum of the THF p o l y m e r i z a t i o n system i n i t i a t e d by EtOS0 CF i n CCI4 a f t e r 48 min a t 13°C. The molar r a t i o of THF to i n i t i a t o r was 5 : 1 . Peak A at S -2.52 ( i n ppm r e l a t i v e to the e x t e r n a l standard of CF3CO2H c a p i l l a r y ) i s assigned to the i n i t i a t o r EtOS02CF3. Peak Β at 8 +0.46 i s due to the oxonium counteranion of I (OSO2CF3-) of the propagating s p e c i e s . F i n a l l y , peak C a t S -2.80is reasonably a s c r i b e d to the e s t e r species of I I (~~-CH OS02CF3). No other peaks were detected during a k i n e t i c run. Thus, the l ^ F spectroscopy provides w i t h a new method f o r the d i r e c t determination of the instantaneous concentrations of i n i t i a t o r and the oxonium and e s t e r species of propagation. F i g u r e 3 shows the v a r i a t i o n s of ([0+] + [E]) (curve A) and [ 0 ] (curve B) as a f u n c t i o n of time under the p o l y m e r i z a t i o n c o n d i t i o n s of F i g u r e 2, where [0+] and [E] denote the concentra t i o n s of the oxonium (I) and e s t e r (II) s p e c i e s , r e s p e c t i v e l y . At the end of the k i n e t i c run ( a f t e r 76 min) 28% of the charged i n i t i a t o r has been r e a c t e d . The molar r a t i o of [0+] to [E] reached to a constant v a l u e of 46 : 54 a t a l a t t e r stage of 2

2

Μ

+


3

10.

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155

Cyclic Ethers

KOBAYASHi

p o l y m e r i z a t i o n ( a f t e r about 30 min). This i n d i c a t e s that the e q u i l i b r i u m of Equation 9 i s a c t u a l l y present i n the THF p o l y m e r i z a t i o n i n i t i a t e d with Et0S0 CF3. S i m i l a r l y the THF polymerization was monitored by 19 nmr spectroscopy i n other four s o l v e n t s . In CHC1«, C ^ C ^ * and benzene s o l v e n t s the f r a c t i o n s of [ 0 ] were 85, 89, and 75% a t 0°C, r e s p e c t i v e l y , a f t e r the e q u i l i b r i u m was reached. In n i t r o benzene, however, the [E] f r a c t i o n was very small, e.g., below 2% a t 0°C throughout the p o l y m e r i z a t i o n . Kinetics. Based on the above nmr r e s u l t s the f o l l w i n g r e a c t i o n s w i l l e x p l a i n the course of the THF p o l y m e r i z a t i o n i n i t i a t e d with E t O S 0 C F (EtOTf). Initiation 2

F


+

2

EtOTf

oQ

+

3

—J

Et-0^_J- OTf" =

Et0(CH ) 0Tf 2

4

(10)

Propagation

+ ^^0Q-0Tf-

k

+

oQ

P(i)

+ /-^0(CH ) -0Q · 2

4

OtCH^-ÔTf

OTf (11)

and

-0(CH ) 0Tf 2

4

+

W)

oQ

—0(CH ) -0Q · 2

4

0(CH ) ^0Tf 2

IXL

- [Ml

[M]

- [M]

l n

2 t

L

rt

=

k

[ p P

e

U

P

)

J

*

(12)

4

The i n t e g r a t e d form of the r a t e equation of propagation expressed as

OTf"

is

] d t

( 1 3 )

0

where kp^ p) i s the apparent r a t e constant of propagation and [P*] represents the t o t a l c o n c e n t r a t i o n of the propagating s p e c i e s , a

e

8

' "

[P*] = [0 ] +

+

[E]

(14)

The e s t e r species (II) i s not dead, but i s thought to be i n h e r e n t l y capable of propagation (Equation 12), s i n c e the e s t e r species EtOTf i n i t i a t e s the THF p o l y m e r i z a t i o n . [P*] was equal to the amount of the reacted i n i t i a t o r . Therefore, the f o l l o w i n g r e l a t i o n s h i p i s derived



156

POLY ETHERS

Time ( hr· * Bulletin of the Chemical Society of Japan Figure 1. Polymerization of THF by EtOSO F at 0°C. [P*]-time (curve A) and monomer conversion-time (curve B) relationships: [I] ο = 4. 8 X JO" mol/l, [M]o = 9.8 mol/l. in CH Cl soluiion (3). x

2

2

2

A

c -3

II

1 -2

-1

B

I

0(ppmf

1

Macromolecules Figure 2. F nmr spectrum of the THF po lymerization mixture by EtOSO^CFs initiator in CCl after 48 min at 13°C (11) 19

h

Time(min)

Macromolecules Figure 3. Polymerization of THF with EtOSO CF monitored by F nmr spectroscopy in CCl at 13°C. Rehtionships of ([0 ] + [E])-time (curve A) and of [0*)-time (curve B): [ M ] o = 5.30 mol/U U] ο = 1.05 mol/l. (11). r

S

19

k

+


10.

SAEGUSA

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k

157

Cyclic Ethers

KOBAYASHi

p(ap) = p ( i )

* i

k

x

+

k

p(e)

· *

( )

x

15

where k p ( i ) and k p ( ) are the r a t e constants of propagation due r e s p e c t i v e l y to the oxonium i o n (I) a n d e s t e r (II) s p e c i e s , and X i and Xe a r e the molar f r a c t i o n s of [0 ] and [Ε], r e s p e c t i v e l y , i . e . , X i + Xe = 1. I t i s reasonable to assume that the magnitude of k p ( ) i s a t l e a s t smaller than that of k-^ which was about 140-280 times smaller than that of k ( i ) (Tables I I I and IV), i . e . , k p ( i ) » k p ( ) . Furthermore, Xe was not so l a r g e , i . e . , below 0.55 i n a l l cases (Tables I I I and I V ) . Therefore, Equation 15 becomes e

+

e


p

e

kp(ap) Consequently,

~

k p ( i ) · Xi

06)

Equation 13 i s converted to

In

[ML - [M] 2 Ê [M] - [M] t

( r

=

k P V

e

U

t

+

d

t

(

1

7

)

0

+

The i n t e g r a t e d values of [P*] i n Equation 13 and [ 0 ] i n Equation 17 were obtained by g r a p h i c a l i n t e g r a t i o n on curves A and Β i n F i g u r e 3, r e s p e c t i v e l y . The monomer conversion was followed by % nmr spectroscopy (3) under the same r e a c t i o n c o n d i t i o n s as F i g u r e 3. Thus, p l o t s of Equations 13 and 17 could be made as shown i n F i g u r e 4 (A) and (Β), whose slopes gave values of kp(ap) = I . 6 X I O - and k p ^ ) = 3 . 6 *10"* 1/mol-sec at 13°C i n C C l ^ . S i m i l a r l y the k^(ap) *d k p ( i ) values were determined at 0 and 25°C (Table I I I ) . I t i s i n t e r e s t i n g to note that the [0 ] f r a c t i o n was not changed a t the r e a c t i o n temperatures 3

3

ai

+

+

Table I I I .

Rate Constants, A c t i v a t i o n Parameters, and the [ 0 ] F r a c t i o n i n the THF P o l y m e r i z a t i o n by E t 0 S 0 C F I n i t i a t o r i n CCI, ) 4 2

3

a

Temp

k

±

X 10

5

k

p(

a

p

)

Χ 10

3

k

p(

i

)

χ 10

3

[0+] Γ &

(°C)

(1/mol-sec)

(1/mol-sec)

(1/mol-sec)

0

0.80

0.84

2.2

45

13

2.1

1.6

3.6

46

25

3.9

3.0

5.7

47

16

13

-12

-24

ΔΗ* 20 (kcal/mol) ASê.u.) a) b)

_5

(%)°

b

)

η

[M] = 5.30 mol/1, [I] = 1.05 mol/1. ο ο A f t e r the e q u i l i b r i u m was reached. Macromolecules (11).


158

POLYETHERS

+

β

ο

ϋ

Ο

ch co oo σ> A

co

CO

•Η U c0 Ν •Η

«4-1

M

lyme


of 0, 13, and 25°C, a f t e r the e q u i l i b r i u m of Equation 9 has been reached. The r e s u l t s of s i m i l a r k i n e t i c s i n other four solvents a r e shown i n Table IV. The k^ values were increased with an i n c r e a s e

ο

υ

rH

Φ

u 4-1 CJ

χ•Η ?Ο

ο CO

CM

Γ*·*

CM

Ο

Cationic Polymerization of Cyclic Ethers Initiated by Superacid Esters

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