Initiation of Polymerization - ACS Publications - American Chemical

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Catalysis by Water-Soluble Imidazole-Containing Polymers Downloaded by UCSF LIB CKM RSCS MGMT on November 29, 2014 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch002

C. G. OVERBERGER and RICHARD TOMKO The University of Michigan, Department of Chemistry and the Macromolecular Research Center, Ann Arbor, MI 48109 A summary of our recent publications describing water-soluble imidazole containing polymers is presented. Copoly[1-alkyl-4- or 5-vinylimidazole/4(5)vinylimidazole] (I), copoly[vinylamine/4(5)-vinylimidazole] (II), and dodecane-block-poly[ethylenimine-graft-4(5)-methylimidazole] (III) have been used to investigate the hydrophobic interaction in esterolytic reactions. A brief survey of related work is presented along with our current work. Several reviews have been published describing our work on imidazole catalysis.(1-4) Other investigators in this field have published comprehensive review articles comparing synthetic catalysts to enzymes.(5-9) The esterolytic activity of the three catalyst systems reviewed in this paper have been compared to poly[4(5)-vinylimidazole] (PVIm) in 28.5% ethanol-water solutions since PVIm is not soluble in water. However, within each system, minor changes in catalyst apolarity illustrate the effect of hydrophobic interactions on the rate of esterolysis. The substrates consisted of a series of neutral p-nitrophenyl esters (Sn) and negatively charged 4-acyloxy-3-nitrobenzoic acids (Sn ), where n denotes the number of carbons in the acyl chain. -

I

II

0097-6156/83/0212-0013$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UCSF LIB CKM RSCS MGMT on November 29, 2014 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch002

14

INITIATION

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Copolymers of l - a l k y l - 4 - or 5 - v i n y l i m i d a z o l e and 4 ( 5 ) - v i n y l imidazole (I) were synthesized with v a r i a t i o n s i n the a l k y l chain length and the 4 ( 5 ) - v i n y l i m i d a z o l e content.(10) The i n c o r p o r a t i o n of a l k y l a t e d imidazole residues i n t o the backbone of p o l y [ 4 ( 5 ) v i n y l i m i d a z o l e ] not only provided e f f i c i e n t regeneration of the c a t a l y s t s , but a l s o imparted water s o l u b i l i t y to the copolymers. (11) These copolymers were compared with mixtures of t h e i r component homopolymers having the same v i n y l i m i d a z o l e content. The pKa values of the imidazole residues i n the copolymers decreased with decreasing v i n y l i m i d a z o l e content. The increased a c i d i t y was a t t r i b u t e d to a d e s t a b i l i z a t i o n of the protonated imidazoles by i n t r a m o l e c u l a r i n t e r a c t i o n with the a l k y l a t e d i m i d a zole residues. There was a l s o a s l i g h t decrease i n pKa as the length of the a l k y l imidazole s i d e chain was i n c r e a s e d . The imidazole pKa values were higher i n water (4.99-5.92) than i n 28.5% ethanol-water (4.94-5.80)(12) due to enhanced s t a b i l i t y of the charged residues i n the more polar medium. The pKa values for the copolymers were lower than the pKa values f o r PVIm i n 28.5% ethanol-water ( 5 . 7 8 - 6 . 2 0 ) . The pH-rate p r o f i l e s f o r the copolymers i n the h y d r o l y s i s of S i n 28.5% ethanol-water were s i m i l a r to PVIm.(13) The copolymers were more e f f i c i e n t c a t a l y s t s than a mixture of the component homopolymers due to the increased b a s i c i t y of the copolymers. Lengthening the a l k y l s i d e chain from one to three carbons had l i t t l e e f f e c t on the r a t e of hydrolysis. Bell-shaped pH-rate p r o f i l e s s i m i l a r to PVIm were observed i n the h y d r o l y s i s of the Sn s e r i e s . ( 1 4 ) There was a maximum 2

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


5

643 1,183 686

58 71 79

28.5% EtOH-water 28.5% EtOH-water 28.5% EtOH-water

7.1

7.1 28.5% EtOH-water 28.5% EtOH-water

6.85

6.85

n

P(l,4Me)

P(l,5Me)

m

n

50% Homo Mix

P(VIm-l,5Me)

50% Homo Mix

25.4

5.45 1,264

58

28.5% EtOH-water

7.1

1 2

P(VIm-l,4Me)

2 5

P(EIIm -C )

Footnotes

on n e x t

page

4.1 (7) 4.7

(7)

(7)

0

—

10

10

g

h

(8) (7.2)

5.2

5.37

5.02 1,325

52

Water

7.0

l 2

5

5.13

3,566

107

Water

7.0

8 5

P(EIIm -C )

k

7.1

5.35 1,734

88

Water

7.0

P(EIIm)

m

77 51

5.11

11,409

984

Water

8.0

J

Ρ (Vim. ..-VAm) 0/

(8) (7)

1.89(7)

84 47

5.40

10,706

1,101

Water

8.0

J

P(VIm -VAm)

75

PVIm

(7)

(7)

(pH)

7 (7) 18.2 (8)

1,409

110

28.5% EtOH-water

7.1

1

g

6.00

10,316

338

20% EtOH-water

8.0

f

PVIm

11.5

7.00

26

2

58

S

20% EtOH-water

pKa °

L

-1 -1 b (M min )

8.0

S12

k

Imidazole

N

16

}

δ

n

.-l t»

8

i

6.95

2

m

40

S

(

70

k

ι /w-1 cat

on C o p o l y m e r s o f V i n y l i m i d a z o l e s

Water

a

Length

Solvent

Chain

8.0

pH

inAlkyl

Imidazole ^

1

of Variations

Catalyst

Effect



table

4

In 28.5% EtOH-water; data taken from Reference 35.

Data taken from Reference 10.

h

1

13.

S t r u c t u r e I I I w i t h PEI blocks of DP 25 or 85; data taken from Reference 18.

S t r u c t u r e I 50:50 Copolymer of 4 ( 5 ) - v i n y l i m i d a z o l e and l-methyl-4- or 5 - v i n y l i m i d a z o l e compared to a 50% mixture of homopolymers. Reference 10.

Homopolymers of l-methyl-4- or 5 - v i n y l i m i d a z o l e ; data taken from Reference 11.

1

m

n

PEI-graft-4(5)-methylimidazole; data taken from Reference 18.

S t r u c t u r e I I c o n t a i n s 67 or 75% v i n y l i m i d a z o l e r e s i d u e s ; data taken from Reference 15.

In 25% EtOH-water; data taken from References 11 and

8

^ Data taken from Reference 14.

Data taken from Reference 34.

^ Data taken from Reference 17.

0.02.

5

ο water s o l v e n t contained 3.3% a c e t o n i t r i l e , 26 C.

[ C a t a l y s t ] = 5 χ 10~ M, [ s u b s t r a t e ] = 5 χ 1θ" Μ.

T r i s b u f f e r , μ = 0.02,

to

° Determined at 25 or 26°C, μ =

b

a

Footnotes



2.

OVERBERGER

AND

TOMKO

lmidazole-Containing Polymers

17

around pH 7 where there were s i g n i f i c a n t amounts of both protonated imidazoles f o r e l e c t r o s t a t i c a t t r a c t i o n and c a t a l y t i c a l l y a c t i v e n e u t r a l imidazoles. PVIm was a b e t t e r c a t a l y s t than the copolymers and the mixtures of component homopolymers i n 28.5% ethanol-water through the Sn s e r i e s . There was l i t t l e d i f f e r ence i n r e a c t i v i t y between the copolymers and the mixture of t h e i r component homopolymers i n the h y d r o l y s i s of S2 · However, the copolymers were b e t t e r c a t a l y s t s from Si* to S i β . In general, the r e a c t i v i t y of the copolymers increased with 4 ( 5 ) v i n y l i m i d a z o l e content, the s u b s t r a t e chain l e n g t h , and the 1 - a l k y l imidazole s i d e chain l e n g t h . F i n a l l y , the second order r a t e constant ( k t ) e x h i b i t e d a greater i n c r e a s e with Sn and the c a t a l y s t s i d e c h a i n length i n water than i n 28.5% ethanol-water, where hydrophobic i n t e r a c t i o n s are weaker. The copolymers hydrolyzed Si2 about f i v e times f a s t e r i n water than i n 28.5% ethanol-water. c a

Copolymerization of 4 ( 5 ) - v i n y l i m i d a z o l e with vinylamine increased the water s o l u b i l i t y of the c a t a l y s t and, t h e r e f o r e i t s e s t e r o l y t i c a c t i v i t y . ( 1 5 ) Copoly[4(5)-vinylimidazole-vinylamine] (II) had considerably lower pKa values f o r the imidazole residues i n water (5.11-5.40) than the imidazole residues f o r PVIm i n 28.5% ethanol-water (5.78-6.20).(14) Apart from the d i f f e r e n c e i n s o l v e n t , the p o l y e l e c t r o l y t e e f f e c t from the s t r o n g l y b a s i c amine residues (pKa = 7.35-7.74 i n water) was r e s p o n s i b l e f o r the lower pKa v a l u e s . The pH-rate p r o f i l e s f o r the h y d r o l y s i s of S2 with these copolymers i n water are s i m i l a r to PVIm i n 28.5% ethanol-water. However, the copolymers are very e f f i c i e n t c a t a l y s t s even at the lower pH values compared to PVIm which i s l e s s e f f i c i e n t than imidazole as a catalyst.(10,13) This greater c a t a l y t i c e f f i c i e n c y was due to the a v a i l a b i l i t y of more n e u t r a l imidazole residues (lower pKa) and the more compact c o i l i n the aqueous media. Both of these f a c t o r s i n c r e a s e the imidazole-imidazole cooperative interactions.(5^ There was no evidence f o r e i t h e r cooperative i n t e r a c t i o n s i n v o l v i n g vinylamine u n i t s or vinylamine c a t a l y z e d aminolysis of the substrate.(16) The pH-rate p r o f i l e s f o r the copolymers with S2 d i d not r e s u l t i n the bell-shaped curves found i n previous studies.(10,14) The continuous r a t e i n c r e a s e with pH ( s i m i l a r to the PVIm and copoly[l-alkyl-4(5)-vinylimidazole-4(5)-vinylimidazole] hydrol y s i s of S2) was r a t i o n a l i z e d by an e l e c t r o s t a t i c a t t r a c t i o n of the substrate to the primary ammonium residues i n the backbone while the imidazole residues remained c a t a l y t i c a l l y a c t i v e . The copolymers hydrolyzed S ~ about three times f a s t e r i n water than PVIm i n 20% ethanol-water at pH 8. (14) The r a t e constants increased continuously i n going to longer chain substrates (S2 to Si2 ). Therefore, hydrophobic i n t e r a c t i o n s begin to p l a y a dominant r o l e even with Si* as opposed to PVIm where the r a t e constant decreased from S2~ to Si*"". (11,14,17) The best copolymer 2



18

INITIATION

OF

POLYMERIZATION

c a t a l y s t f o r S7 was the more t i g h t l y c o i l e d copolymer with the l e s s e r amine content. The hydrophobic c o n t r i b u t i o n of the s u t s t r a t e _ e v e n t u a l l y o v e r - r i d e s a l l other f a c t o r s i n the h y d r o l y s i s of Si2 where the r a t e s of h y d r o l y s i s f o r the copolymers are n e a r l y equivalent* In the h y d r o l y s i s of Si2 at pH 8, the copolymers were n e a r l y 300 times as e f f i c i e n t as imidazole i n water which makes them j u s t s l i g h t l y b e t t e r c a t a l y s t s than PVIm i n 20% ethanol-water.(14) The more apolar substrates e x h i b i t e d d e c e l e r a t i v e behavior as seen i n other systems where the hydrophobicity of the c a t a l y s t increased due to a c y l a t i o n of the imidazole residues.(14) Solv o l y s i s i n excess substrate revealed that the steady s t a t e conc e n t r a t i o n of a c y l a t e d imidazoles i s much l e s s f o r the copolymers than f o r PVIm. The copolymers i n 20% ethanol-water were more e f f e c t i v e c a t a l y s t s than PVIm with S2 where e l e c t r o s t a t i c i n t e r a c t i o n s are more important than hydrophobic i n t e r a c t i o n s . However, with S12 where apolar i n t e r a c t i o n s are predominant, the copolymers were l e s s e f f i c i e n t c a t a l y s t s than PVIm.(14) Dodecane-block-poly[ethylenimine-graft-4(5)-methylimidazole] copolymers ( I I I ) were e f f i c i e n t c a t a l y s t s f o r the h y d r o l y s i s of p-nitrophenyl e s t e r s i n water.(18) These copolymers d i f f e r from previous poly[ethylenimine] (PEI) catalysts(8.21-24) because they are derived from l i n e a r PEI, i n c o r p o r a t e a w e l l - d e f i n e d apolar binding s i t e (dodecane block) as a p o r t i o n of the polymer backbone, and they c o n t a i n a high weight percent of pendant imidazole groups. Since a l l of copolymers contained the same dodecane block, a greater apolar weight was a s s o c i a t e d with the lowermolecular-weight polymers (fewer PEI u n i t s ) . The pKa values of the pendant imidazoles (5.02-5.13 i n water) and the ethylenimine groups (7.00-7.08 i n water) decreased with decreasing molecular weight of the copolymers. This decrease i n pKa with i n c r e a s i n g a p o l a r i t y i s f u r t h e r evidenced by the l a r g e s t imidazole pKa value (5.35 i n water) belonging to the homopolymer g r a f t , p o l y [ e t h y l e n i m i n e - g r a f t - 4 ( 5 ) - m e t h y l i m i d a z o l e ] , which contains no apolar block. Another f a c t o r lowering the pKa values may be the c l o s e proximity of the protonated ethylenimine units.(15) The pH-rate p r o f i l e f o r the h y d r o l y s i s of S2 showed that imidazole i n water was a 3 - f o l d more e f f e c t i v e c a t a l y s t than the PEI g r a f t s at every pH. At pH l e s s than 7.25, the copolymers were l e s s e f f i c i e n t than the c a t a l y s t c o n t a i n i n g no apolar block. At pH greater than 7.25, the r e a c t i v i t i e s of the copolymers increased which i n d i c a t e d a more e f f i c i e n t use of the apolar binding s i t e . This was due to the greater c o n c e n t r a t i o n of pendant imidazoles a s s o c i a t e d with the apolar domain. Since the lower-molecular-weight copolymers (greater apolar weight) exhibi t e d the greater r a t e enhancements, cooperative i n t e r a c t i o n s were r u l e d out i n favor of hydrophobic e f f e c t s .


2.

OVERBERGER

AND

TOMKO

Imidazole-Containing Polymers

19

There was a continuous i n c r e a s e i n the second order r a t e constant ( k ) from 20- to 100-fold a t pH 9.15 through the Sn s e r i e s . The n e a r l y equivalent c a t a l y t i c e f f i c i e n c i e s of the copolymer and homopolymer g r a f t s suggest that the major hydro phobic i n t e r a c t i o n i n v o l v e d the N-acylimidazole intermediate. However, throughout t h i s s e r i e s , the copolymers were s t i l l b e t t e r c a t a l y s t s than the homopolymer. Analogous to other polymeric imidazole catalysts.(10.14) the copolymer and homopolymer g r a f t s d i s p l a y e d bell-shaped pH-rate p r o f i l e s f o r the h y d r o l y s i s of S2 . The more e f f i c i e n t copolymer c a t a l y s t s contained ethylenimine blocks of higher molecular weight which possessed a stronger e l e c t r o s t a t i c a t t r a c t i o n to the s u b s t r a t e . This a t t r a c t i o n makes these copolymers i n water almost as e f f i c i e n t c a t a l y s t s as PVIm i n 28.5% ethanol-water with S2~ a t pH 7.11.(10) For the Sn~ s e r i e s , again the l a r g e s t r a t e enhancements were d i s p l a y e d by the higher-molecular weight copolymers with the l e s s e r apolar weight. The highest-molecular weight copolymer at pH 7 with Si2"" i s 2.5 times more r e a c t i v e than PVIm i n 28.5% ethanol-water at pH 7.11.(14) however the homopolymer g r a f t i s only s l i g h t l y more r e a c t i v e than PVIm. Therefore, the highest molecular-weight copolymer c a t a l y s t made the most e f f e c t i v e use of both e l e c t r o s t a t i c and hydrophobic interactions. S a t u r a t i o n k i n e t i c s with the highest molecular-weight copolymer i n water i n d i c a t e d that both s u b s t r a t e b i n d i n g and the r a t e of intracomplex breakdown increased through the Sn" s e r i e s . (5,19) The Michaelis-Menten k i n e t i c s f o r PVIm(11,14) i n e t h a n o l water suggested that increased s u b s t r a t e b i n d i n g has g e n e r a l l y c o r r e l a t e d with decreases i n the r a t e of intracomplex breakdown. Therefore, the r e s u l t s from the block copolymers i n d i c a t e that the PEI backbone provided e x t r a s t a b i l i z a t i o n f o r N-acylimidazole h y d r o l y s i s w i t h i n the apolar environment. (5}


c a t

Our current work focuses on g r a f t i n g imidazole pendants onto water s o l u b l e polymers and the s y n t h e s i s of o p t i c a l l y a c t i v e poly[4(5)-vinylimidazole] catalysts. In our c o n t i n u i n g e f f o r t to i n v e s t i g a t e the hydrophobic e f f e c t , ω-(l-imidazolyl)alkanoic a c i d s have been g r a f t e d onto poly(vinylamine) through an amide bond. These g r a f t i n g r e a c t i o n s allow us to vary the a p o l a r i t y of the c a t a l y s t while, ensuring water s o l u b i l i t y and l o c a t i n g the imidazole groups away from the polymer backbone. 1-(S)-(1-Methylb u t y l ) - 4 - v i n y l i m i d a z o l e and l - ( S ) - ( 2 - m e t h y l b u t y l ) - 5 - v i n y l i m i d a z o l e have been copolymerized with v a r i o u s monomers and c r o s s l i n k e d with divinylbenzene. The h y d r o l y s i s of enantiomeric e s t e r s (17,20) i s being s t u d i e d i n order to determine i f these c a t a l y s t s possess e n a n t i o s e l e c t i v i t y . Professor I r v i n g K l o t z and P r o f e s s o r Toyoki Kunitake are well-known c o n t r i b u t o r s to the f i e l d of polymer c a t a l y s i s . K l o t z and h i s a s s o c i a t e s have continued t h e i r i n v e s t i g a t i o n s of branched


20

INITIATION

OF

POLYMERIZATION

PEI derivatives.(21-24) A s e r i e s of s u b s t i t u t e d aminopyridines, c o v a l e n t l y attached to l a u r y l a t e d PEI, hydrolyzed p_-nitrophenylcaproate ( S ) 50-2000 times f a s t e r than the i s o l a t e d aminop y r i d i n e s . The aminopyridines a r e b e t t e r n u c l e o p h i l e s than imidazole due to t h e i r higher pKa values.(25) K l o t z intends to produce s t e r e o s e l e c t i v e PEI c a t a l y s t s by coupling o p t i c a l l y a c t i v e amino a c i d s to the aminopyridine nucleophile.(26.27) K l o t z has a l s o i n v e s t i g a t e d s a l i c y l a l d o x i m e as a n u c l e o p h i l e i n quaternized PEI and analogous c a t i o n i c m i c e l l e s . (28) Kunitake and h i s coworkers have i n v e s t i g a t e d b i f u n c t i o n a l polymer c a t a l y s t s ( 2 9 ) i n m i c e l l e s ( 3 0 ) and polysoaps.(31) The b i f u n c t i o n a l i n t e r a c t i o n between a hydroxamate n u c l e o p h i l e and a neighboring imidazole group a t the c a t a l y t i c s i t e i s s i m i l a r to the charge r e l a y system of s e r i n e proteases. This i n t e r a c t i o n leads to remarkably a c c e l e r a t e d a c y l a t i o n and d e a c y l a t i o n pro cesses. In the hydrophobic environment of c a t i o n i c m i c e l l e s , where the r e a c t i v i t y of a n i o n i c n u c l e o p h i l e s a r e remarkably enhanced, the o v e r a l l c a t a l y t i c e f f i c i e n c y exceeded even that of α-chymotrypsin a t pH 8. (30) M i c e l l a r monofunctional c a t a l y s t s , nonmicellar b i f u n c t i o n a l c a t a l y s t s , and polysoap-bound b i f u n c t i o n a l c a t a l y s t s were l e s s e f f e c t i v e . Kunitake has a l s o studied the h y d r o l y s i s of £-nitrophenyl e s t e r s i n the presence of aqueous b i l a y e r v e s i c l e s . The r a t e of i n t e r - v e s i c l e S h y d r o l y s i s by a c h o l e s t e r y l d e r i v a t i v e of 4 ( 5 ) imidazole c a r b o x y l i c a c i d was much slower than the i n t r a - v e s i c l e counterpart.(32) R a t e - l i m i t i n g substrate t r a n s f e r was respons i b l e f o r the lower r a t e s and was i n f l u e n c e d by the c r y s t a l l i n e l i q u i d c r y s t a l l i n e transition.(33)


6

2

Acknowledgments : The authors are g r a t e f u l f o r f i n a n c i a l support from the N a t i o n a l Science Foundation under Grants DMR78-13400 and DMR 81-06891, and the Sherwin-Williams Company.

Literature Cited 1. 2. 3. 4.

5. 6. 7. 8.

Overberger, C. G.; Salamone, J . C. Accts. Chem. Res., 1969, 2, 217. Overberger, C. G.; Smith, T. W.; Dixon, K. W. J . Polym. Sci. Symp., 1975, 50, 1. Overberger, C. G.; Guterl, A. C.; Kawakami, Y.; Mathias, Lon J.; Meenakshi, A.; Tomono, T. Pure Appl. Chem., 1978, 50, 309. Pavlisko, J . A.; Overberger, C. G. "Biomedical and Dental Applications of Polymers," Polymer Science and Technology, Vol. 14, Gebelein, C. G. and Koblitz, F. F . , Eds., Plenum Press, New York, 1981. p. 257. Imanishi, Y. J . Polym. Sci., Macromol. Rev., 1979, 14, 1. Kunitake, T.; Okahata, Y. Adv. Polym. Sci., 1976, 20, 159. Kunitake, T.; Shinkai, S. Adv. Phys. Org. Chem., 1980, 17, 435. Klotz, I. M. Adv. Chem. Phys., 1978, 39, 109.


2.

OVERBERGER AND TOMKO

9. 10. 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.

Imidazole-Containing Polymers

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RECEIVED August 24, 1982


Initiation of Polymerization - ACS Publications - American Chemical

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