CHAPTER 1

POLYMER-METAL COMPLEXES AS CATALYSTS

Polymer-meld Complexes as Caldysts --

8

n a polvmeric- c-atalyst or polymer-supported catalyst a conventional

catal\,tic syccles IS attached to a macromolecular backbone. Natural and I synthetic polymers have been widely used as carriers of bioactive molecules

for modelling, e.g., enzyme catalysis.' Despite the' fact that they usually show

a much lower c,atalytic activity than a true enzyme,2 they proved to be

valuable in understanding some basic features of enzymic processes and in

the developing of other types of synthetic polymers. An important class of

these model compounds is that formed by transition rnptal ions or complr\

ions anchored to a polymer. When the matrix in polypeptide, the

corresponding polymer-supported complexes are supposed to mimic the

activity of metal lo enzyme^.^,^ The preparation and investigation of chemically

active species bound to polymeric supports continue to be an active

interdisciplinary field of research with applications in organic chemistry,

inorganic chemistry, chemical technology, biochemistry and biology." A

biopolymer catalyst, such as enzyme, exhibits high selectivity and activity for

several reactions. It is well known that their specificities are caused bv an

intramolecular k,ooperative catalysis between functional groups in the

enzyme. The use of polymer as supports for chelates and catalysts, as well as

synthetic reagents, has grown tremendously since Merrifield demonstrated

their use in peptide synthesis. The catalytic activities of metal complexes

bound on polymer matrices are entirely different from the corresponding

monomeric analogue due to the specificities induced by the macromolecular

ligands." a polymer-metal complex, the polymer support provide a specific

catalytic site and act as an effectivematrix for the formation of active centres9

The catalytic activity of a polymer-metal complex is generally lower

than the corresponding monomeric analogue due to steric effects. However,

the activity can sometimes be higher than the monomer complexes due to the

unsaturated coordination structure or by the strain in the formation of

inactive dimeric complex due to steric effect.10," Polymer-metal complexes

are widelv used as immobilised catalysts, in which a specific catalytic

behaviour was induced by the polymer matrix.12 In addition to this catalytic

activity, polymeric ligands have immense application in various areas like

cond~ct iv ih i ,~ '~~ ' modification of material surface by binding of metal

c~mplexes ,~"~^ separation of metal ions1820 and in the mimicking of

bio-inorgan~c s v s t e m ~ . ~ ~ ~ ~ ~

2.A. Polymer-metal Complexes: Synthesis and Classification

Recently there has been growing attention to the molecular function of

macromolecular complexes as a new frontier in material systems.

Macromolecular complexes are defined as complexes of macromolecules. A

typical example of macro molecular^ metal complexes is illustrated in

Figure 2.1. Their synthesis represents an attempt to give inorganic function to

organic polymer

Figure 2.1. Schematic illustration of macromolecular metal complex

Polymer-mefd Complexes as Ca'dysts 10

Intermacromolecular complexes and molecular complexes between

macromolecules and organic molecules are also included in the category of

macromolecular complexes. Recause the macromolecular complexes are

combination of macromolecules, small organic molecules and metal ions a t

the molecular level and because they are structurally labile, there are

unlimited possibilities for providing a wide variety of previously unknown

molecular funcbons.

In the macromolecular complexes, physicochemical properties and

chemical reactivities of the complex moieties are often strongly affected by

interactions with the surrounding polymer matrix. These interactions are

weak but significant and act multiply and dynamically. They not only

construct the rnacromolecular complexes themselves, but also control their

higher-order structure based on a dynamic conjugation between the complex

moieties and the polymer matrices. Thus a weak and soft profile in the

coordination reactions, a cooperative interactions behveen the complex

moieties and a multiplied or enhanced interactions are observed as

characteristic features of the macromolecular complexes.

Polymer-metal complex is formed by the interaction of a polymer

containing coordinating groups with metal ions. The polymeric ligand can be

obtained by the polymerisation of monomer containing coordinating sites or

by the reaction between a polymer and a low molecular weight compound

having coordinahng ability. The general methods for incorporating active

functional groups are classified into four different groups: (i) direct

polymerisation or copolymerisation of monomers containing the desired

functional groups; (ii) chemical modification of preformed polymer; (iii)

functional monomer idubition into or grafting onto the prepared polymer,

followed by polymerisation; and (iv) immobilisation of hydrophobic

chelating extractants during polymerisation or post-polymerisation.

, , Polymer-mefal Complexes as Caldysts i , . 1 1

, ' 7, 1 ,, T' , : .

2.A.I kndant complexes .- . .~~

In pendant complexes the metal ion is attached to the ligand function

which is appended on the polymer chain. Based on the chelating abilities it is

classified as monodentate and polydentate pendant complexes.

(a) Monodentate pedant contpkxes

A monodentate pendant complex is formed when a metal ion or a

stable metal complex in which central metal ion is already masked with low

molecular weight ligands except for one coordination site that remains vacant

is coordinated to a polymeric ligand. Such complexes have a simple structure

of the monodentate type as shown in Scheme 2.1. Even if the metal ion or the

metal complex has more than two labile ligands, it is often possible to form a

monodentate complex by selecting appropriate reaction conditions.

I I 1 &6pfJ Scheme 2.1. Monodentate pendant complexes

An example of the formation of a pendant monodentate complex is the

complexatton of < IS-[Co(en)zXz]X with poly(4-vinyl pyridine) (l).Li

Polymer-meld Complexes as Caldysls 12

(1) X = C1, Br, N3 en = ethylenediamim

Polydentate ligands often result in the formation of stable metal

complexes with bridged structures. Most of the chelating resins come under

this category. Pendant polymer.-metal complexes are characterised by their

relatively well-defined coordination structure. Structure of the polydentate

pendant metal complexes is given in Scheme 2.2.

Scheme 2.2. Polydentate pendnnt metal compleves

2.A.2 Inter- and intra-molecularly bridged cpmplexes

In the case of the complexation of a polymeric ligand with a metal ion,

which has four or six coordinating binding hands, the polymer-metal

complex formed may be of the intra-polymer chelate type or inter-polymer

chelate type (Scheme 2.3).

Pdymer-meld Complexes as Caldysfs 13

Scheme 2.3. (a) Intra-polymer-metal complexes; (b) Inter-polymer-metal complexes

The coordination structure in this type of -CH-CH2-CH-CH2- polymer-metal complex is not clear and it is often

difficult to distinguish between the inter- and 1 I

intra-molecular bridging. Thus it is :not easy to "\ / elucidate the polymer effect in studying the

0 characteristics ot the polymer-metal complexes.

/c\o

A typical example of this type of polymer-metal I

-CH-CH2-CH-CH2-

complex is the poly(viny1 alcohol)-Cu(I1) (2)

complex (2).24.

2.A.3 Polymerisation of coordinaled monomers

Metal complexes of low molecular weight monomers are poly merised

to give polymer-metal complexes. There are three basic polymerisation

techniques for the preparation of polymer-metal complexes from the

correspond~ng monomer complexes.

Pdyrner-metd Complexes as C&+sls 14

(a) Polyn~erisatiotr of vinyl cmirpounds cmitaitritig metal cmtrplexes

The vinyl monomers which can be polymerised are limited and the

polymerisation depends on the electrostatic interaction between the

coordinated monomers and the growing radical. This type of polymer-metal

complex is characterised by a clear coordination structure. The

polymerisation occurs by the radical of ionic initiation to form a polymer of

high molecular weight as depicted in Scheme 2.4.

Scheme 2.4. Polymerisation of coordinated vinyl monomcr.v

Some transition metal salts and model complexes act as inhibitors in

the polymerisation of vinyl monomers. Thus the polymerisation of 4-vinyl

pyridine is strongly inhibited in the presence of Cu(I1) or Fe(IIIJL5 But Zn(II),

Co(I1) or Cu(I)-4-vinyl pyridine complexes can be polymerised to form the

corresponding poly(4-vinyl pyridine)-metal complexes.26 Methacrylate

coordinated with amine-Co(I1I) c:omplexes to form methacrylato-pentamine-

Co(II1) perchlorate (3) and cis-din~ethacrylatotetramine-Co(II1) perchlorate (4).

(b) Polycortdeirsation of ~ le ta l compkxes corltairrirrg pee jurlrtiornl groups

'The prepdratlon of polymer-metal complex by condensation is

reported by Hartwell and Bailer (Scheme 2.5).27 Polyesters and polyamides

have been synthesised by interfacial polycondensation with aryl acid

chlorides.

Scheme 2.5. Polycondensation of low moleculur weight metul complexes

The polymer-metal complexes obtained by polycondensation and ring-

opening polymerisation are considered as coordination polymers since the

polymer chain is composed of coordinate bonds. This involves the

polymerisation of a metal complex containing a strained ring which is

capable of ring-opening polymerisation as shown in Scheme 2.6. An example

of this type of polymer-metal complex formation is the polymerisation of

beryllium complex of bis-P-carbonyl compound (Scheme 2.7).26

Polymer-metal Complexes as Ca(dys1s I6

Scheme2.6. Ring-opening polymerisafion of low molecular weight metal complexes

Scheme 2.7. Ring-opening polymerisation of Beryllium complex of

bis-gcarbonyl compound

2.B. Reaction of Peripheral Functional Groups in Metal Complexes with Polymers

This type of polymer-metal complex is formed by the chemical

reaction of neutral or functional polymers with peripheral functional groups

of metal complexes as shown in Scheme 2.8. These polymer-metai complexes

have definite coordination structures. DVB-crosslinked polystyrene and its

modified forms and silica are widely used as the insoluble polymer systems.

Some examples are given in Scherne 2.9.29

Scheme 2.8. Formation of polymer-metal complex by polymer analogous reacn'ons

Pdymermefal Complexes as Catalysts 17

Scheme 2.9. Formation of polystyrene- and silica-supported polymer-metul compleres by polymer-analogous reactions

The ~mmobilisat~on of metal-porphyrins into a polymer-mahix has

been achieved by the reaction of peripheral functional groups in the metal-

porphyrins with reactive polymers and they have been used as enzyme

mode1s.N'

2.C. Coordination Polymers

In coordinate polymers, the polymer chain is composed of coordinate

bonds and the l~gand is the bridging unit as shown in Schemr 2.10.

Muttidentate ligands capable of forming, this type of coordination polymers

are classified into: (a) linear coordinated polymers, and (b) network

coordinated polymers.

Scheme 2.10. Formation of coordination polymers by bridging of ligunds by metal ions

Powmer-meld Complexes as Cdalysts 18

2.C. 1 Linear coordinated polymers

Linear coordinated polymers can be prepared by two methods, from

bifunctional ligands and from simple compound or ion as ligands.

IIerr the 1)olvrner chain is composed of bifunrtional ligands and metal

ions. A typical example for the coordination complex formed between a

bifunctional ligand and metal ion is the complex between dithiooxamide and

metal ions (Scheme 2.11).31

Scheme 2.11. Iormurton cf net~vork-courdinuied polymer-tneful complex (4 dithiooxamide

(b) Sitrrple cmtrpouird or ion us ligands

A simple compound or ion can function as a bridging ligand giving

rise to a polymeric structure. Cupric chloride forms an associated structure in

which the chloride occupying as bridging ligands on both the axial and

equatorial coordination sites of <3u(II) (5).32

Polymer-meld Complexes as Caldysts I9

This type of polymer-metal complex is formed by a 'template reaction'

between two functional groups of the ligands induced by their coordination

to metal ion resulting in a chelate type metal complex as depicted in

Scheme 2.12. Poly(methy1 phthalocyanine) is an example for this type of

network-coordinated polymers.. It is formed by the reaction of

tetracyanobenzene with metal halides (Scheme 2.13).

L-L

Scheme 2.12. Schematic representation. of the formation (4 nehvork-coordinated polymers

Scheme 2.13. Formation of network-cwrdinated polymer-metul complex o j tetracyanobenzene

2.D. Catalytic Activities of Polymer-metal Complexes

Catalytically active polymers can be obtained by introducing a

catalytic centre to a polymer backbone and it is reasonable to assume that the

catalyst bound to the polymer will show specific catalytic activity, reflecting

the properties of polymer chain. In the case of metalloenzyme such as

oxidase and haemoglobin where a metal complex is the active site, the

macromolecular protein is that which plays a significant role. Polymeric

catalyst reduces the possibility of catalyst poisoning since the catalytic site is

somehow protected by the polymer matrix. In a polymer-metal complex,

aggregation is phvsically prevented by the rigidity of the polymer matrix and

has the advantage of maintaining its catalytic activity over a wide range of

concentration. In polymer-metal complexes, the selectivity arises from the

steric hindrance and/or chemical environment of the polymer matrix."

Polymer-metal complexes are markedly useful as immobilised catalyst

for practical use because it is more reactive than the corresponding monomer

analogous due to the specificities of their large ligand molecules. It is mainly

used in oxidations,j5 hydroge~~ation,~~ hydro ly~is ,~~ hydr~formylation,~

decomposi&n of H202," initiation of radical polymerisation,~l' asymmetric

synthesisll and optical resolution.~2

2.D.1 Catalytic activity of polymer-metal complex

The catalytic cycle of a metal complex catalysed reaction is

ML, + !j + L,.I M-S

M'L + S* * LI M'S*

M - metal ion; L - ligand; S - substrate

In the first step substrate coordinate to a metal catalyst and form

intermediate mixed complex (LMS). The substrate is activated by metal ion

and dissociates from the catalyst, having accomplished its purpose, is

regenerated to the original complex.

The main characteristics of polymer supported catalysts are:

(1) Polymers can be used as insoluble carriers for catalyst. The domain in

the solid phase or the boundary of its surface provides a specific

catalytic stte. The isolation of the products from the catalyst is

markedly facilitated and the catalyst can be used repeatedly by a

simple procedure.

(2) Though the activities of the polymer-metal complexes are sometimes

lower than those of corresponding monomeric analogous, due to steric

lundrances, they can sometimes be higher than the monomer

complexes. For instance, unsaturated coordination structures or sterical

prevention of the formation of an inactive dimeric complex will

enhance the catalytic activity.

(3) Selectivities of the reaction by polymer complexes are often different

from the monomer complexes.

(4) Polymer can be used as an effective matrix for the formation of active

metal parbcles.

2. D.2 Application of polymer-metal complexes as catalysts

Oxidation of hydroquinone catalysed by C u ( v complex of

poly(1-vinylimidazole), vinylamine-vinyl acetarnide copolymers and

polyethylene imine were s t~died.4~ The poly(1-vinyl imidazole) complex

showed catalytic activity 20 times higher than cupric ions. The oxidation

mechanism can be represented as:

Polymermeld Complexes as Calalysls 22

Oq f u(II) complex JCH H2Q

0 2 C u(1) complex % Q

(H2Q: hydroquinone; Q: quinone)

Dehydrogenation of alcohols catalysed by the solid phase copper

complex of poly(e carbobenzoxy-L-lysine) was carried out at 150-250°C.44

The advantages of polymer complexes were:

Selectivity for the substrate due to steric effect of polymer ligand was

observed.

The metal ions are monodispersed monoatomically giving high

catalybc effic~ency.

The synthetic procedure is excellent on account of the insolubility of

the polvmer catalyst.

In the autooxidation of cumene45 H catalysed by the metal complex of ,,,,,,,

polymer-supported Schiff base, the

activity decreased in the order: Mn > co > . Ni > Cu > Zn and decreased with

increasing molecular weight of the Schiff base. In this example, the polymer

complexes showed lower activity than the complexes of the monomeric Schiff

bases, probably due to steric reasons.

Polymer complexes having more than two kinds of active sites are

found to have high catalytic activity. In this example, polymer-bonded

metalloporphyrin whch contains both an oxidation site (Co) and a proton

acceptor site (amine) are effective catalysts for the oxidation of thiol to

disulfides.46

Since the catalytic hydrogenation proceeds via a mixed complex of

catalyst and substrate, at least one coordination site of the catalyst must be

free or labile in order that the substrate be coordinated to the catalyst Such a

free or labile coordination site, so called "unsaturation" can be realised in a

polymer-metal complex due to steric effects. The Pd complex of the diphenyl

phosphinated polystyrene showed

remarkably high catalytic activity for CH2 P(Pb- -h--..PdC12

the hydrogenation of the methyl

ester of soybean fatty acid and the

monomer~r complex PdC12(PPh7)2.47 The polymer metal complexes mainly

used for hydrogenation are Rh, Ru or Ni carbonyl complexes of biphenyl

phosphonated supported on polystyrene.

The polvmer complex of titanocene A <

dichloride showed 15-120 times high

catalytic activity for the homogeneous

hydrogenation of olefins than the

monomeric titanocene dichloride.*"

Selective hydrogenation of olefins

catalysed by polymer-metal complexes PPh2-----RuClz(CO)z

was observed. The diphenyl

phosphinated polystyrene-Ru complex or

diacetyl iminomethylated polystyrene-Pd CH2N(CH2COOhPd

complex catalysed the hydrogenation

of octadiene giving selectively

cy c lo~c tene .~~

TIus is because the rate of hydrogenation of the diene to rnonoene is

much higher than that of the morloene cyclooctane.

Recent interesting works are the use of polymer as matrix for the

dispersion of metal particles. A colloid has been prepared by reduction of

Rh(III)-chloride with methyl alcohol in presence of PVA.9 The colloidal

dispersion of (Rh) in methyl alcohol-water solution was effective fur Lhe

hydrogenation of olefins at 30°C: under an atmospheric hydrogen pressure.

The colloidal dispersion contains rhodium particles of remarkably small size

of 8 and 40 A.51 The reactions were considered as like this:

RhCL A Rh(II1)-PVA complex CHJOH (reductiotl)

Rh particles (diameter: 8 A) (growk!) +

Rh particles (diameter: 40 .A)

Porphyrin containing polymers were found to be remarkably effective

as a catalyst in photo redox systems.52 In such systems, various porphyrin

groups such as protoporphyrins and chlorine as well as metalloporphyrin

Polymermet# Complexes 6s Caldysts 25

groups such as Mg(II)- and Cu(Q-chlorophyllin and Fe(IIl) and Co(1I)

protoporphyrin were incorporated into vinyl polymers.

(c) Hydrolysis

The hydrolysis of sodium pyrophosphate was effected by using

somemetal complexes of poly(methacry1acetone) as catalysts.53 The

catalytic activity of the polymer complexes declined in the following order:

Zr(IV) 0 > U(V1) 0 2 > Cr(m) 2 Ce(1II) E Cu(II). The enhancement of activity

was ascribed not only due to the electrostatic effect of the polymer complexes

but also to a possible multiplying effect by ZrOL+ and ZrOL(0H) along the

polymer-ligand chains. The catalytic

hydrolysis of oligophosphates by

poly(L-Lysine)-Cu(Q complexes has

been reported." Breslow et ~ 1 . ~ ~ \ ' ~i

attached the Ni(1I) complex to , . ,

cyclohexaamylose, which formed a

hydrophobic cavity, as shown as (6), @:"-

and studied the hydrolysis of p-nitro ( 6 )

phenyl acetate. The reaction was

accelerated by a factor of about 103 over the uncatalysed system, the increased

reactivity being the result of binding of the substrate in the hydrophobic

cavity.

Hydroformylation is the reaction of olefins, carbonmonoxide and

hydrogen producing saturated aldehyde or alcohols. Insoluble catalysts of

polymer-metal complexes are remarkably useful for this reactions, since they

are easily separated from the products and used repeatedly. They sometimes

show selectivities for the reaction due to the polymeric structure.

Polymer-metal Complexes as Catalysts 26

L-+R& I C,

CHO

The complex of [Rh(C0)2Cl]z bound to the crosslinked polystyrene

containing the -P(Ph)z, -N(CIt)z or -CHzN(CEt)z group catalysed the

hydroformylation of 1-hexane.M The polymeric phosphine-Rh complex

showed selectivity for the aldehyde synthesis and the polymeric amine-Rh

complex, selectivity for the alcohol synthesis. The complex of RhCI(C0)

bound to phosphinated silica gel coated with polystyrene was a stable

heterogeneous catalyst for the hydroformylation of ethylene.57

The cobalt resin derived from a crosslinked polv(chloromethylated

styrene) is a good hydroformylation catalyst for 1-pentene and cyclohexene

under mild condition.%

Catalyst recovery can be achieved by simple filtration and its catalytic

activity is retained after prolonged exposure of the resin to air. Such

polymer-supported insoluble catalysts are especially suitable for commercial

process, because expensive catalyst can be recovered and used repeatedly by

simple procedures.

(e) ltiitiation of radical polyiletisation

Polymer-metal complexes initiate radical polymerisation and often

show higher activity than the corresponding monomeric analogous.

Nylon-Cu(1I) systems initiate the radical polymerisation of methyl

methacrylate (MMA) in presenre of CQ and water.59 The study of

Polymer-metal Complexes as Cefdysfs 27

polymerisation by Cu(Il) complex of (E amino caproic acid) and other

oligomeric nvlon showed the increase of the catalytic activity with the

increase of the degree of polymerisation of the nylon. This in fact, the low

activity of the N-substituted oligomer, and the importance of the end amino

group for this catalysis suggested the mechanism given below."

Natural productsb' such as cellulose and soluble starch were found to

initiate the polymerisation of MMA in aqueous solution in presence of CuZ'

ions, probably through the complex formation of CuZ+ with the natural

polymer.

(f) Asymrtretric synthesis and optical resolutiorr

Steric hndrance of polymer ligand can provide T

asymmetric structure whch bring about asymmetric

reaction.

Hateno (1969, 1970) and Nozawa (1971)

succeeded in carrying out an asymmetrically selective H---.

oxidation of 3,4-dihydroxy phenyl alanine DOPA by

poly(l1ysine) (PLL)-Cu(I1) complex."@ D-DOPA was

6" &H

Ph2P PPh2 found to be oxidised by the PLI,Cu(II) complex more

rapidly than L-DOPA. Such an asymmetric oxidation

was ascribed to this asymrnelrically selective interaction of DOPA with PLL-

Cu(II) complex. The combination of two or more asymmetric cupric complex

units held on the a-helical structure of PLL was considered to cause the

asymmetric catalysis.

Rh complex of diphosphine compounds bound to polystyrene

catalysed the asymmetric hydrogenation of ketones such as acetophenone,

methyl benzyl ketone with the optical yield of 58%.62

Asymmetr~c structure provided by polymer-metal complexes makes

optical resolution of the adsorbed organic molecules. The Cu2+ complex of

the polymer containing N,N-dibenzyl leucine adsorbed D-isomer of amino

acids more than L- i~omer .~~ The skreoselectivity was opposite to the

complex of the monomeric analogue. The crowded structure composed of

isobutyl and benzyl groups would cause selective adsorption of D-isomer

(Scheme 2.14).

Scheme 2.14. Selective adsorption of D-valine by ihe crowded copper complex of N, N ' -d iben~l leucine polymer

The decomposition of Hz02 is effected using various metal complexes

as catalysts. 'The catalysis has its origin in redox action of a metal complex. A

transient intermediate is formed between the metal catalyst and the substrate,

at least in the initial step of the reaction, but the reaction also proceeds

through chain decomposition. 'The decomposition of hydrogen peroxide is

often employed as a standard reaction to determine the catalytic activity of

polymer-metal complexes because the experimental technique is simple.

Polymer metal complexes exhibit high efficiency in the catalytic

decomposition of H202 because:

(i) Some polymer-metal complex contain incomplete complexes due to

steric hindrance and increases catalytic activity.M.65

(ii) In other polymer complexes, the coordinate bond between polymer

ligand and metal ion is relatively weak and substrate coordinate with

high frequencv."

(iiif Chain decomposition of substrate proceeds rapidly because

concentration of catalytic site is locally enhanced.fJh9

The decomposition of Hz02 have been studied as a catalase model

rather than as a catalyst of practical use. Polymer-metal complexes showed

much higher catalytic activity for the decomposition of H202 than the

corresponding monomeric analogous."R

The catalybc activity of Fe(1II)-triethylene tetramine (trien) complex

was found to be lo7 times higher than that of ferric aqueous in the

decomposition of H202.70 The mechanism was proposed where the OOH- ion

coordinates to ferric ions as a bidentateligand, resulting in the cleavage of the

(0-0) bond with the evolution of oxygen.

Poly(P-diketone)-Cu complex and -CH- CH2-

poly(hydroxamic acid)Cu complex showed higher

catalytic activity than the monomeric catalyst.71 I

NH \

The high activity of polymer complexes is // ascribed to the weak coordination bonds between

\ the polymeric ligands and metal ions. A higher

"\ /" molecular weight of the polymer ligand gave higher /""\ activity. Polymethacrylate acetone-Cu, polyglycine- \ P Cu72 and ferric complexes of polyacrylic acidn also \

NH- //

C showed h g h catalytic activity for the decomposition I of H202.

-CH2- CH- '

It was postulated based on the action of catalase that Hz02 should be

activated prior to being decomposed by the enzyme's hemine.74 According to

this mechanism, one carbonyl and one amino group of the apoenzyme were

believed to play a role of the cofactor. These groups forms hydrogen bonds

with one molecule of HzOz which is thereby activated and the second

molecule of Hz02 forms a complex with the first one giving rise to the fission

of 0-0 bond. As for the model of such mechanism the decomposition of

H202 by F++ complex of poly(acrylic acid), which is partly amidated by

diethylene triamine (diene) was studied."

Potfmwmefsl Complexes as Ca!@sts 11 (>> 'd

H.. - P A - - 4

2.E. Polymer-metal Complexes as Heterogeneous Biologically Active Catalysts

When a homogeneous catalyst such as a transition metal complex,

transition metal or an enzyme is covalently bound or strongly absorbed on a

polymer matrlx we have a polymer-supported catalyst. The polymer can be

organic or inorganic in nature and be soluble or insoluble in the reaction

medium. Insoluble supports are preferred because the main purpose of

supporting a catalyst is to be able to recover and recycle it easily. The

advantages apart from recovery and &use are: (a) isolation of catalytic sites

and prevention of agglomeration leading to inactivation, and (b) coordinate

unsaturation introduced by the polymeric matrix resulting in enhanced

specificity.

There are three distinct stages in the preparation of a supported

catalyst. The first is the choice of suitable support material. The second is the

introduction of suitable functional groups on the support which would

interact with the catalytic species in the loading process. The choice of such

functional groups depends to a large extent on the catalyst chosen. The

incorporation of the catalyst on the support is usually carried out under mild

conditions so as to retain the activity of the catalyst and finally, the extent of

the loading acheved must be ascertained by chemical analysis.

Metal ions have an important role in the activity of bioinorganic

materials in whch metal ions are bound to proteins, nucleic acids and related

ligands. Metalloenzymes are generally formed between a polypeptide and

metal ion. The coordination structure of the complex, the conformation of the

polypeptide which is dependent on the sequence of the amino acids in the

polypeptide, stiffness of the backbone and the interaction between the

pendant groups cause specificities in metalloenzymes.76 The presence of

many kind of b~ological substances having coordinating ability around the

metal ion and the occurrence of reaction by metalloenzymes via the mixed

complex with the substrate requires to study of fundamental coordination

chemistry of mixed ligand complexes in order to elucidate the coordination

behaviour of blood plasma, the complex formation of Mg(II), Cu(II), Mn(II),

Fe(III), Co(II), Zn(II) and Pb(II) ions with amino acids and other organic acids

are s i m ~ l a t e d . ~

In metallic enzymes, the large protein ligand decides the character and

the activity of the central metal ions by providing a selective binding site for

the substrate. This selectivity is generated by the long chain of apoproteins.

A distorted coordination structure was developed for the complex by the

hydrophobic environment created by the protein molecule surrounding the

central metal ion. In blue-copper proteins the coordination geometry of the

central copper ion is not square planar but shows a distorted tetrahedral

structure which is caused by steric strain of the peptide chain. The

coordination structure of its oxidised and reduced states are very similar due

to the rigid stereostructure around the Cu(1I) ion. Such a rigid structure is

retained by a specific configuration of the polypeptide chain. The Cu(1I) ion

itself is surrounded by hydrophobic groups. Such a distorted structure and

hydrophobic environment caused by the polypeptide chain are largely

responsible for the anomalous characteristics of blue-copper proteins.

The chem~cal approach to peptide synthesis has yielded impressive

successes in recent years in the preparation of several biologically active

pep tide^.'^ With the development of new reagents and techniques the

synthesis of most small peptides has been placed within easy reach.

Insoluble polymeric supports enable separations of synthetic reactions

mixtures by simple filtrati0n.~9 They permit use of large excess of reagents in

peptide synthesis to drive the coupling.reactions to completionsU and provide

easy separation of troublesome byproducts from reactions mixtures. Another

advantage of polvmeric supports sometimes cited is "site isolation" by which

supposed that the rigid polymer chains prevent polymer-bound species from

reacting with one another, thus favouring either intramolecular reaction or

reactions with reagents in solution rather than interchain reactions within the

polymer m a t r i ~ . ~ '

Considerable attention has been focused on the preparation of

immobilised enzymes in the past decades, and many different kinds of

support matrices and techniques for enzyme immobilisation have been

d e v e l ~ p e d . ~ ~ - ~ ~ Immobilised enzyme systems can have a wide range of

potential applications in enzyme engineering, among which are enzyme

bioreactors and enzyme electrodes. Such systems have several advantages .. .

over the free enzyme including their ease of recovery and reusability,

operational simplicity and thermal and storage stability. Among the support

matrices used, polvmeric materials have been used extensively since they can

have various functional groups and can be easily modified chemically.

The immobilisation of enzyme on a macromolecular support can in

principle mimc the above conditions to some extent. The irnmobilisation of

enzymes on insoluble polymeric supports facilitates their recovery from the

reaction mixture, but it can also slow down enzyme deactivation and make

possible the exploitation of local concentration of active site.%,S9 The

characteristic effect during immobilisation of enzymes on crosslinked

~nsoluble polymer~c support is the steric effect imposed by the three-

dimensional crosslinked matrix.*,9' This often reduces the overall activity of

the enzyme. The influence of three-dimensional macromolecular matrix on

enzyme activity can be reduced by incorporating a spacer arm between

polymer matrix and en~yme.~z The immobilisation also provides an

opportumty of reutilisation. The long term stability and simple recovery of

immobilised enzymes guarantee their multiple reuse. The autodigestion of

proteolytic enzymes often leads to inactivation. In such a system,

immobilisation Increases stability as the covalent binding of enzyme

molecules to the carrier can prevent their inactivation with each other.

2.F. Macromolecular Effects on Catalytic Activities of Polymer-metal Complexes

2.F.7 Porosity and surface area

During the last few decades, heterogeneous catalytic processes have

been developed to such an extent that they constitute the main process routes

to the bulk of the output of the chemical industry. The pore-size and surface

area of the polymers varied with the nature and extent of crosslinking agent.

Optimum crosslinked polyme~ic ligands should have large surface area and

average pore-radius. Considerable insight . . has been obtained on the chemical . r

characteristics of catalytically active species and it has been well correlated

that the reaction rate increases with the exposed area of the catalytic species.

In order to bbtain reasonable reaction rates, such area has to be increased to

an adqua& level per unit volume and this is possible only by making the

catalyst htghly porous and spreading the active species over the entire surface

whch would be largely in theinterior of the catalyst pellet. Thus the internal

areas are accessible to the reactants by diffusion only.

In the case of catalytic reaction taking place in the interior surfaces of a

porous catalyst, frequently, gradients in reactant concentration and

temperature may be present inside the catalyst making the interpretation of

experimental or operational data difficult The development of a catalyst

formulation should therefore take into account not only by the chemical

nature and dispersion of the active species but the design of the catalyst

should also consider factors such as the average pore-size, the variation in

pore-size and adequate mechanical strength.

The characteristics of the pore structure also influence greatly the

response of catalysts to poisons. In general, poisons are chemical constituents

which are very strongly absorbed by the active catalytic surface. Certain

catalytic transformations are associated with consecutive reactions or side

reactions and a catalyst with high selectivity is essential to minimise

undesired products. In the hydrogenation of butadiene to butane, and the

oxidation of o-xylene to anhydride, further degradative reactions can occur

on the primary product which is the desired one. However, the diffusion

limitation during regeneration can be minimised by having macro-micro

combinations of pore-sizes. Such a catalyst will have a network of larger

sized pores which have a lower active surface but facilitate rapid diffusion of

the gas inside catalytic part as well as a network of smaller sized pores of

much shorter d~mensions which incorporate the bulk of the active catalvtic

species.

LF.2 Natum of support and extent of cmsslinking

The complexation behaviour of ligand function supported on polymer

is generally Wfeqent from the corresponding low molecular weight

.. analogous.y3 This variation is dependent on the polymer matrix to which the

ligand function is attached." Complexing groups in a macromolecular ligand

may adopt conformation different from that required by the preferred

stereochemistry of a given metal ion. Functional groups are strategically

distributed over a macromolecule so that both the more favourable and the

less favourable conformation will occur. The characteristic factors like matrix

structure, matrix configuration, matrix nature, spacer between the crosslinked

matrix and the iigand function and extent of crosslinking has a significant

effect on complexahon. The complexation characteristics of insoluble

macromolecules are strongly influenced by the nature and extent of

crosslinking in the polymer l i g a ~ ~ d . ~ ~ A high degree of crosslinking results in

a low degree of a metal ion intake and a lower stability of the resulting metal

complex by mak~ng the polymer chain more rigid. In the case of poly(4-vinyl

pyridine)-Cu complexes with increasing divinyl benzene content, conversion

of planar coordination centres to tetrahedral ones occurs. This change in

stereostructure results from an increase in steric hindrance to complex

formation wlth a polymeric ligand.

2.F.3 Spacer groups

The complexing power of macromolecular ligand depends upon the

arrangement of functional groups relative to the main chain. The shorter

distance between them, the lower the efficiency of complex formation because

of steric hndrance. By introducing a spacer to polymer matrix the less

reactive functional groups can make more reactive cause high catalytic

activity.

Functional groups directly attached to the backbone chain of high

polymers are often less reactive than if these groups were not attached to a

polymer. Ths feduced reactivity is caused by steric hindrance or by the fact

that a 1 or part of the polymer chains interferes with the reactivity of the

individual Gnctional group. Spacer groups could be designed to be flexible

or stiff, a most important capability to design macromolecular architecture by

modern polymer synthesis. The commonly used spacers are hexamethylene

dimnine,% polyethylene glycol" and a,0-dibromoalkane.9~ If the functional

groups initially present in the polymer are not sufficiently isolated, the spaces

may become doubly coupled with the polymer and is effectively lost. Such

reactions have been used to study site isolation.

Polymer-mefd Complexes as Celaph 37

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