Aminated Polystyrene-Copper Complexes as Oxidation Catalysts: The

2 Aminated Polystyrene-Copper Complexes as Oxidation Catalysts: The Effect of the Degree of Substitution on

Modification of Polymers Downloaded from pubs.acs.org by UNIV LAVAL on 07/11/16. For personal use only.

Catalytic Activity G. CHALLA, A. J. SCHOUTEN, G. T E N BRINKE, and H . C. MEINDERS State University of Groningen, Laboratory of Polymer Chemistry, Nijenborgh 16, 9747 AG Groningen, Netherlands

Modification of polymers is a topic in polymer science, because new highly valued or improved applications often require sophisticated chemical structures along the polymer chains. One of such timely domains of interest comprises the development of modified polymers as catalysts for chemical processes. Of course, we do not have in mind catalysts, wherein polymers function as inert supports for the active centers and nomore. In fact, our aim is to develop polymeric catalysts, which combine advantages of the other type of catalysts, viz. (i) the specificity of homogeneous catalysts. (ii) the separability and high stability of heterogeneous catalysts. (iii) the high activity and selectivity of enzymes. In other words, we try to mimic enzymes by attaching centers for homogeneous catalysis to polymer chains; we want to learn from nature how to conduct chemical processes in a cleaner, more selective and milder way. In this respect it is of great importance that we can adapt, just like in enzymes, the micro-environment of the catalytic centers by modification of neighbouring polymer chain segments. From the above it will be clear that the polymer chain carrying catalytic centers has to play an active role during each catalytic cycle. Therefore, we prefer to speak of macromolecular catalysis rather than of polymer catalysis, the more so, as we omitted crosslinked carriers from our studies in order to prevent that diffusion of reactants and products would become rate-determining. Consequently, the practical combination of simple separability and really macromolecular catalysis should be realized by 0-8412-0540-X/80/47-121-007$05.00/0 © 1980 American Chemical Society


8

MODIFICATION OF

POLYMERS

a t t a c h i n g whole c a t a l y t i c a l l y a c t i v e macromolecules to i n e r t n o n p o r o u s o r m a c r o p o r o u s s u p p o r t s . The basic r e s e a r c h f o r such developments w i l l s t i l l imply the s t u d y o f l o o s e , m o d i f i e d m a c r o m o l e c u l e s as m i c r o p h a s e s c o n t a i n i n g c a t a l y t i c c e n t e r s and s u r r o u n d e d by s o l v e n t w i t h o u t s u c h c e n t e r s . The c o n c e p t o f an isolated r e a c t i v e m a c r o m o l e c u l e was i n t r o d u c e d by M o r a w e t z (1) and f u r t h e r d e v e l o p p e d by h i m and o t h e r s , e.g. O v e r b e r g e r (2) and K u n i t a k e (3) f o r p o l y m e r i c c a t a l y s i s o f e s t e r h y d r o l y s i s , I s e (4) f o r c a t a l y s i s of i o n i c r e a c t i o n s by p o l y e l e c t r o l y t e s and T s u c h i d a (_5) f o r c a t a l y s i s by c o o r d i n a t i o n c o m p l e x e s o f transition m e t a l s w i t h p o l y m e r i c l i g a n d s . I n a d d i t i o n , K a b a n o v (6^) t r i e d to o p t i m i z e polymer c a t a l y s i s i n a p r a c t i c a l way by a p p l y i n g b l o c k o r g r a f t - p o l y m e r s w h i c h p a r t l y associate yielding gel-like structures with catalytic domains which remain q u i t e a c c e s s i b l e . We w e r e i n t e r e s t e d i n t h e b e h a v i o u r o f polymeric c a t a l y s t s i n order to c o n f i r m that t y p i c a l polymer e f f e c t s may occur. O x i d a t i v e c o u p l i n g of 2,6d i s u b s t i t u t e d p h e n o l s , as d e v e l o p p e d by Hay (]_) , was c h o s e n as a m o d e l r e a c t i o n and t h e catalytic a c t i v i t i e s of c o o r d i n a t i o n complexes of copper w i t h s e v e r a l p o l y m e r i c t e r t i a r y amines were compared w i t h the a c t i v i t i e s of t h e i r low m o l e c u l a r w e i g h t analogs. The o v e r a l l r e a c t i o n scheme i s p r e s e n t e d i n s c h e m e 1. Scheme

A similar phenolate

1

o x i d a t i o n by e l e c t r o n t r a n s f e r from a n i o n t o C u ( I I ) i s a l s o an i m p o r t a n t

step

in

2.

CHALLA E T A L .

Aminated

Polystyrene-Copper

Complexes

9

the p h e n o l o x i d a t i o n by c o p p e r - c o n t a i n i n g enzymes like l a c c a s e and t y r o s i n a s e ( 8 , 9 , 1 0 ) . Many important products l i k e l i g n i n s , tannins, pigments, antibiotics and a l k a l o i d s a r e p r o d u c e d t h r o u g h this step. Instead o f t h e b i o p o l y m e r i c l i g a n d s i n t h e e n z y m e s we introduced synthetic polydentates f o r copper complexation l i k e t h o s e l i s t e d i n s c h e m e 2. Scheme


Polymeric

2

ligands

Low m o l e c u l a r weight analogs

The d i m e t h y l a m i n o m e t h y l a t e d p o l y s t y r e n e ( I ) was prepared by c h 1 o r o m e t h y 1 a t i o n o f a t a c t i c polystyrene a c c o r d i n g t o G a l e a z z i (J_0 u s i n g m e t h y l a l and t h i o n y l c h l o r i d e i n s t e a d o f an e x c e s s o f t h e d a n g e r o u s chlorodimethyl ether: |

/

— v

C-Ôy

ZnCl +

MeOCH OMe 2

+ S0C1

2

?

^

|

>—,

CÔy"

C H

2

C 1

Me S0 2

+

3

+ HC1

The c h l o r o m e t h y 1 a t e d p o l y s t y r e n e was a m i n a t e d b y a l a r g e e x c e s s o f d i m e t h y 1 a m i n e d u r i n g 1 w e e k a t 20 C in dioxane:

MODIFICATION O F POLYMERS

10

-c-

-cHNMe

Me^H^Cl

2


NMe Copolymers o f s t y r e n e w i t h 4 - v i n y l p y r i d i n e ( I I ) a n d Nv i n y 1 i m i z a d o l e ( I I I ) were o b t a i n e d by c o p o l y m e r i z a t i o n f o r o n e d a y a t 6 0 ° C i n 25 w t % c o m o n o m e r s o l u t i o n s i n t o l u e n e , u s i n g AIBN as i n i t i a t o r . I n a l l cases t h e d e g r e e s o f s u b s t i t u t i o n , a, o f t h e f u n c t i o n a l i z e d polymers w i t h l i g a n d groups were d e r i v e d from t h e n i t r o g e n c o n t e n t s f o u n d by e l e m e n t a l a n a l y s e s . I n t h e f o l l o w i n g s e c t i o n s we s h a l l d i s c u s s : ( i ) the s t r u c t u r e and b e h a v i o u r o f t h e v a r i o u s copper c o m p l e x e s w i t h t h e l i g a n d s l i s t e d i n s c h e m e 2; ( i i ) t h e activities of the polymeric catalysts i n comparison with the low molecular weight analogs; ( i i i ) the e f f e c t o f t h e d e g r e e o f s u b s t i t u t i o n , a, o n t h e activities of the polymeric catalysts. Structure

and Behaviour

of t h e Copper

Complexes

The b a s i c s t u d y was p e r f o r m e d on c o p p e r complexes with N,N,N ,N -tetramethylethane-l,2-diamine (TMED), w h i c h were known t o be v e r y e f f e c t i v e oxidative c o u p l i n g c a t a l y s t s ( 7 , 1 2 ) . From o u r f i r s t kinetic s t u d i e s i tappeared that b i n u c l e a r copper complexes a r e the a c t i v e s p e c i e s a s i n some c o p p e r - c o n t a i n i n g e n z y m e s . B y a p p l y i n g t h e v e r y s t r o n g l y c h e l a t i n g TMED we w e r e a b l e t o i s o l a t e c r y s t a l s o f t h e c a t a l y s t a n d to d e t e r m i n e i t s s t r u c t u r e by X - r a y d i f f r a c t i o n ( 1 3 ) . F i g u r e 1 s h o w s t h i s s t r u c t u r e f o r t h e TMED c o m p l e x o f b a s i c c o p p e r c h l o r i d e C u ( 0 H ) C l p r e p a r e d from CuCl by oxidation i n moist pyridine. 1

T

X

X Figure 1. The structure of [TMED • Cu(OH) Cu • TMED] ' mined by x-ray diffraction 2

2

• 2CV as deter-

2.

CHALLA E T A L .

Aminated

Polystyrene-Copper

Complexes

11

The same c o m p l e x c o u l d b e o b t a i n e d s t a r t i n g f r o m CuCl and s u b s e q u e n t s u b s t i t u t i o n o f b o t h b r i d g e d c h l o r i d e s by a d d i n g h y d r o x y l i o n s . Scheme 3 d e s c r i b e s t h e f o r m a t i o n and i n t e r c o n v e r s i o n o f both b i n u c l e a r copper comp1exes . 2

Scheme

2 TMED,

2 Cu Cl Modification of Polymers Downloaded from pubs.acs.org by UNIV LAVAL on 07/11/16. For personal use only.

x

•CulU-TMED-complexô.

/ o 2

H^,

2 TMED ,

3

,0* r.

C l

Cu Cl .

H

\/

[

\ /

Q

Cu

N /\

H

/\

2

/

The b r i d g e d b i n u c l e a r s t r u c t u r e c o u l d b e c o r r o b o r a t e d by s e v e r a l t e c h n i q u e s : ( i ) infrared spectroscopy gave a b s o r p t i o n b a n d s f o r b r i d g e d OH a n d C u - 0 v i b r a t i o n s ; ( i i ) elemental analyses give the c a l c u l a t e d contents for hydroxo-bridged c o m p l e x ; ( i i i ) ESR m e a s u r e m e n t s d i d n o t produce s i g n a l s o f m o n o n u c l e a r C u ( I I ) ;( i v ) m a g n e t i c s u s c e p t i b i l i t y i n c r e a s e d w i t h t e m p e r a t u r e , an antiferromagnetic behaviour; (v) during oxidative c o u p l i n g 0« i s r e d u c e d t o H 2 O l i k e f o r b i n u c l e a r c o p p e r e n z y m e s , w h e r e a s ^2®! ^ u s u a l l y p r o d u c e d by mononuclear copper complexes. T i t r a t i o n o f C U C I 2 w i t h l i g a n d p r o d u c e d an i n c r e a s e d n e a r - i . r . a b s o r p t i o n a t 8 8 0 nm a s s h o w n i n figure 2. I t i s clear that the polymeric ligand (I) i s more e f f e c t i v e t h a n i t s l o w m o l e c u l a r w e i g h t analog DMBA. I t g i v e s t h e m a x i m u m a b s o r b a n c e e x a c t l y a t t h e t h e o r e t i c a l r a t i o N/Cu = 2 ( 1 4 ) , w h e r e a s a l a r g e e x c e s s o f DMBA i s n e e d e d t o a c h i e v e c o o r d i n a t i o n o f each C u ( I I ) w i t h two l i g a n d s . T h i s i s a good demonstration of t h e s o - c a l l e d p o l y c h e l a t e e f f e c t w i t h i n t h e seperate m a c r o m o l e c u l a r c o i l s . When t h e t i t r a t i o n w i t h polymeric l i g a n d was s t o p p e d h a l f - w a y , e . g . a t N / C u = 1, E S R signals revealed that part of the CuCl was s t i l l u n c h a n g e d a n d t h e o t h e r p a r t f o r m e d d i r e c t l y t h e ESR i n a c t i v e b i n u c l e a r c o m p l e x e s w i t h N/Cu = 2 . I n f a c t , this s i t u a t i o n appeared to y i e l d the highest r e a c t i o n rate f o rthe ch1oro-bridged c a t a l y s t because free CuCl could l i b e r a t e protons which a r e needed f o r t h e s

2

2


12


0)

O C

o

-Q i_ O I/) -Q O

I

0

1

1

I

2

I

t N/Cu

I

1

6

I

I

8

I

L_

10

Die Makromolekulare Chemie

Figure 2. Titration of a copper(II)chloride solution with DMBA (O) and polymer ligand (I) (Q). [CuCl ] = 4.46mM; solvent: 1,2-dichlorobenzene/methanol (13:2, v/v); room temperature. The curves are not corrected for dilution (14). 2 0

r e o x i d a t i o n o f C u ( I ) as shown i n scheme 1 ( 1 4 ) . W h e n we t i t r a t e d f r o m t h e o t h e r s i d e b y a d d i n g C11CI2 t o a d i l u t e s o l u t i o n o f p o l y m e r i c l i g a n d ( I ) , a n o t h e r phenomenon c o u l d be d e t e c t e d , v i z . a d e c r e a s e in reduced v i s c o s i t y o f t h e p o l y m e r s o l u t i o n (J_4) . T h i s points t ocontraction o f the separate coils of the p o l y m e r i c l i g a n d due t o i n t r a m o l e c u l a r c r o s s l i n k i n g v i a b i n u c l e a r c o p p e r c o m p l e x e s . We p r e v e n t e d g e l formation


2.

CHALLA ET A L .

Aminated

Polystyrene-Copper

Complexes

13

due t o i n t e r m o l e c u l a r c r o s s l i n k i n g v i a c o m p l e x f o r m a t i o n by a p p l y i n g o n l y l o w p o l y m e r concentrations. F i n a l l y , we r e p o r t t h e e f f e c t o f t h e b r i d g e d l i g a n d on t h e s p e c i f i c i t y o f t h e c a t a l y s t s . I t c o u l d be shown t h a t t h e c h l o r o - b r i d g e d c a t a l y s t g e n e r a l l y p r o m o t e s C-C c o u p l i n g t o D P Q , w h e r e a s t h e h y d r o x o b r i d g e d c a t a l y s t i s s o m e w h a t s p e c i f i c f o r C-0 c o u p l i n g t o p o l y m e r PPO ( s e e s c h e m e 1 ) . T h i s t e n d e n c y i s c l e a r l y d e m o n s t r a t e d f o r TMED c o m p l e x i n F i g u r e 3 , w h e r e i n b o t h t h e f r a c t i o n DPQ f o r m a t i o n a n d t h e catalytic activity are plotted against the ratio NaOH/Cu; f o r NaOH/Cu = 1 a l l c h 1 o r o - b r i d g e s a r e s u b s t i t u t e d by h y d r o x o - b r i d g e s .

100-jr

NaOH / Cu

Figure 3. Effect of mineral base (NaOH) on the catalytic activity and specificity of the copper(II)-TMED complex. [CuCl ] = 3.33mM; [DMP] = 0.06'M; temp 25°C; solvent: 1,2-dichlorobenzene/methanol (9:1, v/v). The fraction DPQ was determined spectroscopically at 420 nm after 35% conversion. 2 0

0

In case o f t h e complexes w i t h p o l y m e r i c l i g a n d s I Iand I I I C-0 c o u p l i n g c o u l d b e f u r t h e r p r o m o t e d b y c h a n g i n g the s o l v e n t and i n c r e a s i n g t h e r a t i o 1igand/copper (JJ3,J_5) . B o t h f a c t o r s s e e m t o f o r c e t h e s u b s t r a t e s

14



to e n t e r t h e c a t a l y t i c complex as p h e n o l a t e a n i o n s by s u b s t i t u t i o n o f 0H~ a t t h e b r i d g e p o s i t i o n s a n d formation o f water. This mechanism l e a d i n g t o polymer formation i s q u i t e d i f f e r e n t f r o m t h a t f o r C-C c o u p l i n g w h i c h p r o b a b l y i n v o l v e s s u b s t r a t e c o o r d i n a t i o n t o Cu at t h e f r e e z - p o s i t i o n , s i n c e t h e p h e n o l s cannot substitute the strongly coordinating chloride bridges. T h i s s i t u a t i o n i s met f o r c o m p l e x e s w i t h t h e p o l y m e r i c l i g a n d ( I ) a n d i t s a n a l o g DMBA, b e c a u s e t h e y do n o t form s t a b l e h y d r o x o - b r i d g e d complexes. Both mechanisms a r e p r e s e n t e d i n F i g u r e 4.

Figure 4. Schematic of electron transfer processes for 2,6-disuhstituted phenol. The ligand groups are indicated as Am and the intermediate polymer chain segments as straight lines, (a) Hydroxo-bridged catalyst (b) chloro-bridged catalyst.

Activities

of Polymeric

Catalysts

and Analogs

We a l w a y s a p p l i e d t h e p o l y m e r i c ligands i n concentrations below those f o r homogeneous segmental d i s t r i b u t i o n . I n o t h e r w o r d s we d e a l t w i t h separate polymer c o i l s containing the a c t i v e centers, which


2.

CHALLA E T A L .

Aminated

Polystyrene-Copper

Complexes

15

were d e s c r i b e d as " c o o p e r a t i v e m i c r o p h a s e s " by W i l l i a m s (J_6) . T h e c r o w d i n g o f t h e l i g a n d groups i n the i n t e r i o r o f a polymer c o i l leads t oan enhanced l o c a l c o n c e n t r a t i o n and t os t r o n g e r s t e r i c interaction. The l a t t e r m i g h t s t a b i l i z e b i n u c l e a r c o m p l e x e s , w h i c h are t h e r e a l c a t a l y s t s . The e f f e c t o f t h e enhanced l o c a l c o n c e n t r a t i o n was a l r e a d y i n d i c a t e d i n t h e p r e v i o u s s e c t i o n when d e a l i n g w i t h t h e h i g h e r c o o r d i n a t i n g e f f i c i e n c y o f t h epolymeric l i g a n d ( I ) as c o m p a r e d t o a n e q u i v a l e n t a m o u n t o f t h e a n a l o g DMBA. So, b o t h e f f e c t s m a i n t a i n a n e n l a r g e d local c o n c e n t r a t i o n o f a c t i v e c a t a l y t i c centers and cause t h e rate ofoxidative coupling with polymeric catalysts t o be h i g h e r t h a n w i t h e q u i v a l e n t a m o u n t s o f l o w m o l e c u l a r weight analogs, e s p e c i a l l y f o rlow 1igand/copper r a t i o s . This r a t e enhancement i s c l e a r l y demonstrated in Figure 5 f o r polydentates ( I ) v s . DMBA (J_7) , a n d was a l s o f o u n d f o r p o l y d e n t a t e ( I I ) v s .p y r i d i n e ( 1 8 ) .

1

o

15


Figure 5. Initial rate of oxygen consumption R vs. initial DMP concentration for various ligands. (X) DMBA; (Q) polymer ligand (I) with a = O.JO and fa] = 1.00; (%) polymer ligand (I) with a = 0.10 and fa] = 0.17; (O) polymer ligand (I) with a = 0.18 and fa] = 0.17. [CuCl ] = 3.33mM; N/Cu = 1; temp 20°C; solvent: 1,2-dichlorobenzene/methanol (13:2, v/v) (17). 0

2 0

MODIFICATION OF

16

POLYMERS

An e s t i m a t i o n o f t h e l o c a l l i g a n d concentration, [N] Q , c o u l d be a c h i e v e d by a s s u m i n g f r e e m o v e m e n t of the l i g a n d s i n the i n t e r i o r of a sphere w i t h radius 5, t h e r o o t mean s q u a r e r a d i u s o f g y r a t i o n of the polymer chain: C

O

;

lOOOPa

F n 1


[

N

]

c o i r

=

i

n A

—

, 4 3

1

2 n

( 1 )

H e r e P d e n o t e s t h e d e g r e e o f p o l y m e r i z a t i o n and a t h e degree of substitution. T h i s e x p r e s s i o n i s r e l a t e d to eq. (2) u s e d by M o r a w e t z f o r t h e e f f e c t i v e l o c a l c o n c e n t r a t i o n o f one chain end i n t h e n e i g h b o u r h o o d o f t h e o t h e r , w h i c h i s relevant for ring closure kinetics: C

eff.

_ 1000 " ~N— A

, ,2 I 3 (

^, 2

w n

}

.

( 2 )

2 T h e m e a n s q u a r e e n d - t o - e n d d i s t a n c e o f a freely jointed chain without excluded volume i s known t o be e q u a l t o 6 < s 2 > . The r a d i u s of g y r a t i o n can be d e r i v e d from l i g h t s c a t t e r i n g or from the intrinsic viscosity (19): [] n

=

( 3 )

M n (b is a universal constant and M = m.P. Under the . n conditions applied in Figure 5 with a constant overall l i g a n d c o n c e n t r a t i o n o f 3.3 mM, we f o u n d , indeed: 1

[N]

. coil

»

[DMBA] =

3.3

mM

S u b s t i t u t i o n of e q . ( 3 ) i n eq. (1) r e v e a l s t h a t [ N ] i i s h o u l d be p r o p o r t i o n a l t o a / [ r | ] f o r t h e polymeric c a t a l y s t s . H o w e v e r , t h e a c t i v i t i e s o f two polymeric c a t a l y s t s w i t h t h e same v a l u e o f a b u t a n e a r l y 6 - f o l d difference in [n], i.e. a 10-fold difference in M , w e r e p r a c t i c a l l y e q u a l t o e a c h o t h e r i n F i g u r e 5. This m e a n s t h a t t h e l o c a l c o n c e n t r a t i o n c o n c e p t d o e s no longer s u f f i c i e n t l y apply to comparison of activities of d i f f e r e n t p o l y m e r i c c a t a l y s t s (see l a s t section). I n F i g . 5 we a l s o saw that the i n i t i a l rates,R ,of o x i d a t i v e c o u p l i n g showed a l i m i t i n g v a l u e with increasing substrate concentration, which resembles the c

v

Q

o

2.

CHALLA E T A L .

Aminated

Polystyrene-Copper

Complexes

17

the s a t u r a t i o n e f f e c t o c c u r r i n g i n enzyme k i n e t i c s . T h i s prompted us t o d e s c r i b e o u r k i n e t i c s f o r medium substrate concentrations a l s o i n terms o f the M i c h a e l i s - M e n t e n s c h e m e a s T s u c h i d a e t a l . (2J3) d i d before f o ro x i d a t i v e coupling with the e l e c t r o n transfer i n the polymeric C u ( I I ) - s u b s t r a t e complex as rate-determining s t e p ( s e e a l s o scheme 1 and F i g . 4 ) :

"1


Cu(II)

R

o

+ DMP«

V k

K

(k

+ DPQ/PPO

(4)

V [DMP] s o

s

2

Cu(I)

[Cu(II)-DMP]

(5)

[Cu(II)comp1ex]

1

k )/k 2

(6)

1

wherein R = i n i t i a l reaction rate, V = l i m i t i n g rate f o r [ D M P ] = oo a n d K = M i c h a e l i s c o n s t a n t . Eq. ( 4 ) denotes the s o - c a l l e l Lineweaver-Burk plot o f reciprocal rate vs. reciprocal substrate concentration. T h i s k i n d o f a n a l y s i s was s u c c e s s f u l l y a p p l i e d t o b o t h 2 , 6 - d i m e t h y l p h e n o l (DMP) a n d 2 , 6 - d i p h e n y l p h e n o l ( D P P ) and t o t h e p o l y m e r i c l i g a n d s ( I ) , ( I I ) and( I I I ) l i s t e d i n s c h e m e 2 (J_5 *J_7 ,J_8) . G o o d L i n e w e a v e r - B u r k p l o t s d e r i v e d f r o m F i g u r e 5 a r e s h o w n i n F i g u r e 6.

20 40 [DMP] /(dm mo~l ) 1

3

1


Figure 6.

O «=

Lineweaver-Burk

plots derived from Figure 5 for polymer ligands (I).

0.18, [r,] = 0.17; (%)a = 0.10, [r,] = 0.17; (0)a

= 0.10, [rj] = 1.00

(17).

18


F r o m t h e i n t e r c e p t s a n d s l o p e s V , k2 a n d K could, be c a l c u l a t e d . F o r t h e a n a l o g DMBA t h e v a l u e s o f V a n d k2 w e r e d i r e c t l y t a k e n f r o m t h e c o n s t a n t m a x i m u m r a t e as s h o w n i n F i g u r e 5. M o s t o f o u r r e s u l t s a r e g a t h e r e d i n T a b l e I (J_5 ,2J_,2_2) , w h i c h d e m o n s t r a t e s t h a t t h e o b s e r v e d i n c r e a s e i n r a t e w i t h a i s governed by an increase of the electron transfer rate constant, k2, whereas t h e M i c h a e l i s c o n s t a n t , K , changed i n t h e w r o n g way c o n s i d e r i n g e q . (4). g

g

s

g

I:


Table

10 a

ligand P

t

v

K i n e t i c r e s u l t s on o x i d a t i v e c o u p l i n g o f 2,6-disubstituted p h e n o l s a t 25°C i n t h e solvent mixture 1,2-dichlorobenzene/methanol (13:2 v/v). 2

e

10 k -1 s 3

1

1

2

K j 3 ,-1 dm m o l g

s u b s t r a t e 2,6 - d i m e t h y l p h e n o l DMBA pol. pol . pol. pol.

I I I I

6. 5 10. 3 18 39

2.5 3.1 12.6 39.5 88 . 3

I I I I

2 7 19 32

5 1 2 18 39

ine II II II

3. 6 7.2 12. 7

3.9 6.0 11.5 15.4

E f f e c t of t h e Degree Ac t i v i t y

10.2 8.4 7.0 5.6

AS* TT» 1 -, -1 JK mol

(DMP) 20 1 1 26 36 52

-

228 259 197 151 96

27 44 73 87

+

209 138 33 21

1 3 29 49 11 1

+

267 198 120 126

(DPP)

4.2 3. 1 1 .5 0.8

s u b s t r a t e 2,6 - d i m e t h y l p h e n o l pyr i d pol. pol. pol.

T

32.3 11.4 4.5 1 .7 1. 1

s u b s t r a t e 2,6 - d i p h e n y l p h e n o l pol . pol. pol. pol .

AH* , , -1 kJmol

(DMP)

o f S u b s t i t u t i o n on

Catalytic

The i n c r e a s e i n r a t e o f o x i d a t i v e c o u p l i n g when applying polymeric l i g a n d s w i t h h i g h e r a i s once more presented i n F i g u r e 7 f o rd i f f e r e n t s u b s t r a t e s and polymeric l i g a n d s (J_5, 2J_, 2_2) . S o , w h i l e k e e p i n g a l l o v e r a l l c o n c e n t r a t i o n s and c o n d i t i o n s u n a l t e r e d , t h e r a t e c a n be enhanced s i m p l y by c o n c e n t r a t i n g t h e c a t a l y t i c s i t e s i n a s m a l l e r number o f p o l y m e r i c m i c r o p h a s e s . S i n c e t h i s e n h a n c e m e n t d i d n o t a r i s e when the number o f m i c r o p h a s e s was l o w e r e d b y i n c r e a s i n g


2.

CHALLA ET A L .

Aminated Polystyrene-Copper

19

Complexes

Figure 7. Initial rate of oxygen consumption R vs. degree of substitution a. Reaction conditions: temp 25° C; [CuCl ] = 3.3mM; N/Cu = 1; [substrate] = 0.06M. (%) substrate DPP, polymer ligand (I); (Q) substrate DMP, polymer ligand (II); (O) substrate DMP, polymer ligand (I). 0

2 0

0

the m o l e c u l a r w e i g h t o f t h e polymer l i g a n d w i t h c o n s t a n t a, i t m u s t b e c o n c l u d e d that a decreasing intermediate chain length between neighbouring ligand groups e x e r t s an e x t r a p o s i t i v e e f f e c t on c a t a l y t i c activity. I n o r d e r t o a n a l y z e t h i s k i n e t i c e f f e c t o f a , we determined t h e a c t i v a t i o n parameters of k2 from t h e temperature dependencies of V as d e r i v e d from Lineweaver-Burk plots at different temperatures. In F i g u r e 8 t h i s p r o c e d u r e i s s h o w n f o r DPP a s s u b s t r a t e and p o l y m e r i c l i g a n d ( I ) w i t h a = 0.39. The f i n a l l y r e s u l t i n g values o f t h e a c t i v a t i o n enthalpy A H 2 and a c t i v a t i o n entropy AS* were a l r e a d y p r e s e n t e ^ i n T a b l e I . I t i s p e c u l i a r t o n o t e t h a t b o t h AH2 and A S 2 i n c r e a s e w i t h a, i . e . s h o r t e r i n t e r m e d i a t e chain length between neighbouring l i g a n d g r o u p s . T h i s means that t h e i n c r e a s e o f k 2 i s caused by t h e r e l a t i v e l y stronger increase of AS* which compensates t h e r e t a r d i n g e f f e c t o f i n c r e a s i n g AH*. m

a

x

+


20


0

10

[DPP] ~ 0

1

3

20 1

(dm mol" )

>

Figure 8. Lineweaver-Burk plots for oxidative coupling of DPP catalyzed by copper complexes of polymer ligand (I) with a =0.39 at 5 different temperatures. [CuCl ] = S.3mM; N/Cu = 1; solvent: 1,2-dichlorobenzene/methanol (13:2, v/v). 2 0

Figure 9 demonstrates this compensation e f f e c t by t h e l i n e a r r e l a t i o n s h i p between AS^ and AH2. This indicates t h a t b o t h a c t i v a t i o n p a r a m e t e r s depend e q u a l l y on a and t h a t t h e i s o k i n e t i c t e m p e r a t u r e , i . e . t h e s l o p e o ft h e l i n e , amounts t o 256°K. T h u s , a t -17°C t h e r a t e would become i n d e p e n d e n t o f a, whereas i ti n c r e a s e s w i t h a at h i g h e r temperatures. For a p o s s i b l e q u a n t i t a t i v e d e s c r i p t i o n of t y p i c a l p o l y m e r e f f e c t s we m a d e t h e a s s u m p t i o n t h a t t h e v a l u e s o f A H 2 and A S 2 found f o r t h e low m o l e c u l a r weight c a t a l y s t s stand f o r the a c t i v a t i o n process of the naked c a t a l y s t - s u b s t r a t e complex and a r e i n d e p e n d e n t o f a. So, a f t e r s u b t r a c t i n g t h e s e v a l u e s t h e s e p a r a t e p o l y m e r e f f e c t s a r e f o u n d . T h e n we h a v e t o e x p l a i n why more e n t r o p y i s g a i n e d and more e n t h a l p y is needed f o r a d a p t a t i o n o f t h e i n t e r m e d i a t e chains t o

2.

CHALLA E T A L .

Aminated

Polystyrene-Copper

Complexes

21


150-

Figure 9.

Compensation plot of activation parameters for the electron transfer rate constant k taken from Table I 2

the t r a n s i t i o n s t a t e , when t h o s e c h a i n s become s h o r t e r . I n p r i n c i p l e , s u c h t r e n d s w e r e f o u n d i n t h e same w a y by S i s i d o e t a l . f o r i n t r a m o l e c u l a r h y d r o l y s i s between two c h a i n ends o f p o l y s a r c o s i n e ( 2 3 ) . Interpretations of electron transfer reactions w i t h i n n o r m a l t r a n s i t i o n m e t a l complexes a r e b a s e d on the F r a n c k - C o n d o n p r i n c i p l e , thus i n d i c a t i n g t h a t t h e metal-substrate complex has t o be deformed b e f o r e electron transfer takes place (2^_) • T h i s m e a n s that in o u r case t h ewhole polymeric catalyst-substrate complex i s deformed i n t o a t r a n s i t i o n s t a t e which r e s e m b l e s more o r l e s s t h e f i n a l c o n f i g u r a t i o n o f a t e t r a h e d r a l C u ( I ) complex. B u i l d i n g m o l e c u l a r models of o c t a h e d r a l and t r i g o n a l b i p y r a m i d a l copper complexes we n o t i c e d t h a t t h e t e r t i a r y NMe2 g r o u p s o f p o l y m e r ligand ( I ) a r e almost f i x e d i n the former case, p r e d o m i n a n t l y d u e t o s t e r i c i n t e r a c t i o n o f t h e Me groups. I n a t r i g o n a l b i p y r a m i d , however, these amine groups c a n r o t a t e almost f r e e l y ( F i g u r e 10). The consequences f o r t h e chains between adjacent aminated styrene units a r ed r a s t i c . I t follows that p r i o r to a c t i v a t i o n the end-to-end distances f a l l within a very s m a l l r a n g e , 3, w h e r e a s i n t h e a c t i v a t e d s t a t e they


22

MODIFICATION

O F POLYMERS

Figure 10. Schematic of the proposed catalyst-substrate complex, before and in the activated state. The increase of the chain end-to-end distance possibilities is represented by the bases of the cones.

can t a k e a n y v a l u e i n a much l a r g e r i n t e r v a l , A . Hence t h e number o f c o n f o r m a t i o n s increases with factor X given by: X

- / W(h)dh A

/ J 8

a

(7)

W(h)dh

where W(h) d e n o t e s t h e e n d - t o - e n d d i s t r i b u t i o n . This procedure i s i l l u s t r a t e d i n Figure 11. C l e a r l y , X i s an i n c r e a s i n g f u n c t i o n o f a, f o r not t o o s m a l l v a l u e s o f a. T h i s i s i n c o n f o r m i t y with the i n c r e a s e o f t h e a c t i v a t i o n e n t r o p y A S w i t h a. Generally, the conformational energy increases with d e c r e a s i n g e n d - t o - e n d d i s t a n c e (25). As most o ft h e additional conformations of t h e a c t i v a t e d s t a t e have s h o r t e r e n d - t o - e n d d i s t a n c e ( s e e F i g u r e 11), t h e c o n t r i b u t i o n t o t h e a c t i v a t i o n enthalpy AH* i s a l s o p o s i t i v e . Moreover, i tappears that t h i s c o n t r i b u t i o n ^ i n c r e a s e s w i t h a. The s t r o n g e r i n c r e a s e o f AS* a n d A H 2 w i t h 0( f o r DMP l o c a t e d a t t h e b r i d g e p o s i t i o n o f t h e c a t a l y t i c complex of polymer l i g a n d ( I I ) (see Table I and F i g u r e 4 ) , i s i n l i n e w i t h t h e a b o v e v i e w s , since one s h o u l d e x p e c t a d d i t i o n a l s t e r i c i n t e r a c t i o n i n that case. 2

2.

CHALLA ET AL.

Aminated Polystyrene-Copper Complexes

23


W(h)

6

—»h

A

Figure 11. Illustration of Equation 7 for the calculation of the increase in nu of intermediate chain conformations accompanying deformation and activ the polymeric catalyst-substrate complexes Anyhow, our study has demonstrated the benefit of " s t r a i n e d " polymeric catalyst-substrate complexes, phenomenon well-known in enzymology (26) and once indicated by the term "entatic state" (16). Literature

a

Cited

1. Morawetz, H. "Macromolecules in S o l u t i o n " , John Wiley, New York, 1965, 1975, ch. IX. 2. Overberger, C.G. J. Polym. Sci., Polym. Symp. E d . , 1975, 50, 1. 3. Kunitake, T . ; Okahata, Y. Adv. Polym. S c i . , 1976, 20, 159. 4. Ise, N. in "Reactions on Polymers", Moore, J . A . , E d . , Reidel, Dordrecht-Holland, 1973, p. 27. 5. Tsuchida, E . ; N i s h i d e , H. Adv.Polym.Sci., 1977, 24, 1. 6. Kabanov, V . A . , Intern. Symp. Macromolecules, Dublin, 1977. 7. Hay, A . S . Polym. Eng. Sci., 1 976, 16, 1. 8. Malkin, R.; Malmström, B. G. in "Advances in Enzymology", Nord, F . F . , E d . , Interscience, New York, 1970, p. 177.


24

MODIFICATION OF POLYMERS

9. Brown, B.R. in "Oxidative Coupling of Phenols", Taylor, W.I.; Battersby, A.R., Eds., Marcel Dekker, New York, 1967, p. 167. 10. Ochiai, E.I. "Bioinorganic Chemistry", Allyn & Bacon, Boston, 1977, ch. 9. 11. Galeazzi, L . , Ger. Pat. 2,455,946, June 1975. 12. Kevelam, H . J . ; de Jong, K.P.; Meinders, H.C.; Challa, G. Makromol. Chem., 1975, 176, 1369. 13. Meinders, H.C.; van Bolhuis, F . ; Challa, G. J. Mol. Catal., 1979, 5, 225. 14. Schouten, A . J . ; Wiedijk, D.; Borkent, J.; Challa, G. Makromol. Chem., 1977, 178, 1341. 15. Meinders, H.C.; Challa, G. J. Mol. Catal., submitted. 16. Williams, R.J.P. Pure and Appl. Chem., 1974 , 38, 249. 17. Schouten, A . J . ; Prak, N . ; Challa, G. Makromo1. Chem., 1977, 178, 401. 18. Meinders, H.C., thesis, Groningen, 1979. 19. Flory, P.J. "Principles of Polymer Chemistry", Cornell University Press., Ithaca, 1953, p. 661, 616. 20. Tsuchida, E . ; Kaneko, M; Nishide, H. Makromol. Chem., 1972, 151, 221. 21. Schouten, A . J . ; Noordegraaf, D.; Jekel, A . P . ; Challa, G. J. Mol. Catal., 1979, 5, 331. 22. Breemhaar, W.; Meinders, H.C.; Challa, G., to be published. 23. Sisido, M.; Mitamura, T; Imanishi, Y . ; Higashimura, T. Macromolecules 1976, 9 , 316. 24. Tsuchida, E . ; Nishide, H . ; Nishiyama, T. Makromo1. Chem., 1974, 175, 3047. 25. Primilat, S.; Hermans, J. J. Chem. Phys. 1973, 59, 2602. 26. Jenks, W.P. "Catalysis in Chemistry and Enzymology", McGraw-Hill, New York, 1969, ch. 5. RECEIVED

July 12, 1979.

Aminated Polystyrene-Copper Complexes as Oxidation Catalysts: The

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