V . CONCLUSION AND OUTLOOK

The chemistry of crosslinked insoluble organic

polymeric reagents continues to be one of the active

areas of research in basic and applied organic chemistry.

Polymeric oxidising and halogenating reagents are one

among the most important classes of these reagents. In

the reactions and processes using crosslinked polymeric

reagents, advantage is made use of the insolubility of

the polymer-supported reagent and of its byproduct which

permits the easy removal of any excess reagent or spent

material from the desired product. This feature also

enables one to use a large excess of either the low-

molecular weight substrate or the insoluble polymeric-

reagent in order to increase the reaction rate and

yields. These systems can also be used for coll~mn

operations and batch processes and can be regenerated

several times. The attachment to the insoluble

macromolecular matrix can also solve the problems of

lability, toxicity or odour, often experienced with low-

molecular weight reagents. rn addition, the polymer

matrix can be so selected or tailor-made by adjusting the

macromolecular structural parameters of the three-

dimensional matrix. This can provide a specific

microenvironment that may induce certain selectivity or

specificity at the reaction site.

with a view to developing efficient and cost-

effective polymeric oxidising reagents and to analyse the

effect of the characteristic structural parameters of the

macromolecular matrix, different types of polymeric

supports which differ widely in their molecular character

and overall structure were investigated in the present

work. specific correlation between nature of the polymer

matrix and the reactivity of the attached oxidising

function forms the overall thrust of the study here.

In order to achieve these objectives a series of new

polymeric bromo derivatives based on linear and

differently crosslinked polyvinylpyrrolidones were

developed and their chemistry investigated. The

incorporation of bromo function in linear and crosslinked

polyvinylpyrrolidones, their application as solid phase

oxidising and halogenating reagents in synthetic organic

chemistry, investigation of the reactivity of the N-

halogeno function in different microenvironment arising

from the macromolecular structure and the correlation of

the reactivity with the nature and extent of crosslinking

are investigated.

The starting materials for the preparation of the

large number of polyvinylpyrrolidone-bromine complexes

designed and developed in these studies were either

commercially available samples of poly(N-vinyl-

pyrrolidone) or its crosslinked analogues prepared easily

by the free-radical suspension polymerisation of N-vinyl-

pyrrolidone (VP) with divinylbenzene (DVB), N,N1-

methylene-bis-acrylamide (NNMBA) or tetraethyleneglycol

diacrylate (TTEGDA). AIBN was used as the radical

initiator. These polymers on treatment with bromine in

CClq afforded the solid bromine complexes.

The polyvinylpyrrolidone-bromine complexes prepared

were characterised by IR spectroscopy, U V spectral

studies, N M R , thermogravimetry, elemental

analysis and scanning electron microscopy. The

percentage of bromine attached to the polymer could be

calculated from the thermoanalytical data. The bromine

capacities of the reagents were determined by iodometric

titration and the capacity varied in the range 1.5 to 3.8

mequiv of Br/g.

The polymeric reagents which could be obtained as

orange powders are stable under ordinary laboratory

conditions and can be stored indefinitely without

appreciable loss of capacity. The reagents are

noncorrosive and easy to handle. They can be recycled

and reused for further synthetic reactions. The

hygroscopic nature of the polyvinylpyrrolidone gets

reduced when it was subjected to bromination.

The PVP-Br resins were found to oxidise alcohols to

carbonyl compounds in 80-98% isolable yields. They were

also used for the double bond addition of unsaturated

compounds in 70-95% yields. But d - halogenation of ketones and N-halogenation of arnides were not possible

with these reagents. Thus ketones like acetophenone or

benzophenone and arnides like succinirnide or benzamide do

not undergo any change even after prolonged treatment

with the reagent. The oxidation conditions involved

stirring of the substrate with a five-fold molar excess

of the reagent in chloroform at room temperature.

Wetting of the reagent with water is necessary for the

effective reaction in the case of crosslinked PVP-Br.

Since linear PVP-Br is soluble in water, wetting with

water is not possible. The reactions with linear PVP-Br

were therefore carried out at refluxing temperature in

the absense of water.

The polyvinylpyrrolidone-bromine complexes can be

used for the selective oxidation of alcohols. For this a

variety of diols were studied by taking equimolar

reagent-to-alcohol ratio and also with a five fold molar

excess of the reagent. Thus hydrobenzoin, a compound

containing two secondary alcoholic groups, gave benzoin

when equimolar reagent and substrate were used. But when

a five-fold m:lar excess of the reagent was used, both

the hydroxyl groups reacted simultaneously to give the

diketo compound, benzil. Thus it is possible to control

the oxidation of diols by adjusting the reagent-to-

substrate ratio. This selectivity appears to be

originated from the slow release of bromine from the PVP

support.

The influence of various reaction parameters like

the nature of the solvent, temperature, molar percentage

of crosslinks and concentration of the reagent function

on the course of the oxidation reaction was investigated.

For linear PVP-Br, as the polarity of the solvent

increases, the reactivity was also found to be

increasing. This is due to the effective swelling of the

hydrophilic polyvinylpyrrolidone matrix in polar

solvents. Out of the different solvents studied THF was

found to be the most suitable one.

The various crosslinking agents used for preparing

crosslinked PVP were NNMBA, DVB and TTEGDA. Polymeric

systems with varying molecular cha~acter and extent of

crosslinking were prepared and investigated to draw a

correlation between the variables of macromolecular

structure and the reactivity. The reactivity of the

reagent prepared from DVB-crosslinked polymer is less

compared to the more hydrophilic NNMBA-crosslinked

polymer. TTEGDA is a highly flexible and hydrophilic

crosslinking agent. The reactivity of the reagent

prepared from TTEGDA-crosslinked PVP is very high

compared to the other two crosslinking agents. The

enhanced reactivity is due to the hydrophilic flexible

crosslinking agent which acts as a spacer grouping

between the backbone macromolecular chain.

It was observed that by the incorporation of various

crosslinking agents and also by varying the amount of

crosslinking, the hydrophobic-hydrophilic balance of the

polymer backbone, which have a significant effect on

reactivity, is changed. Out of the various crosslinked

polymers used 10% TTEGDA-crosslinked polymer was found to

be the most reactive in terms of duration of reaction and

capacity. o r NNMBA- and Dvn-crosslinked polymers, it

was observed that as the crosslink density increases from

3% to 2 0 8 , the capacity of the reagent was decreasing.

This is due to the decreased swelling nature of the

NNMBA- and DVB-crosslinked polymers which prevents the

diffusion of the low molecular substrates into the

polymer matrix.

The mechanical stability and the hygroscopic nature

of the polymer was found to be dependent on the crosslink

density of the system. The lightly crosslinked polymers

are soft and sticky in nature and are highly hygroscopict

especially the one prepared from TTEGDA. They are

mechanically stable also. As the crosslink density

increases, the polymer becomes rigid and the hygroscopic

nature gets reduced. The highly crosslinked polymers are

somewhat fragile.

With each of the crosslinking agent, four types of

polymers with different molar percentage (3, 10, 15 and

20%) of crosslinks were prepared. The reactivity of the

corresponding bromo resins were studied following the

oxidation of benzoin to benzil spectrophotometrically.

For NNMBA-crosslinked PVP-Br, it was observed that 10%

crosslinked polymer was the most reactive interms of

yield and duration of reaction. with DVB-crosslinked

PVP-Br, it was tound that as the crosslink density

increases from 3% to 20%, the reactivity gradually

decreases. This is due to the rigid nature of DVB and

also due to the enhanced hydrophobicity induced to the

vinylpyrrolidone matrix from the incorporation of the

hydrophobic DVB-crosslinking agent. The order of

reactivity for TTEGDA-crosslinked PVP-Br is 15) 10) 3 >

20%. This shows that the increased hydrophilicity

enhances the reactivity of the polymeric reagent. The

hydrophilicity of the PVP matrix increases when it is

crosslinked with hydrophilic TTEGDA. The decreased

reactivity with 20% crosslinked polymer is due to the

decreased diffusion of low molecular weight reagents into

the polymer matrix, as it is highly crosslinked.

Out of the three crosslinking agents used to prepare

PVP-Br, the one which is prepared from TTEGDA was found

to be the most suitable for synthetic conversions. DVB-

and NNMBA-crosslinked PVP-Br needs long duration for the

completion of the reaction. But with TTEGDA-crosslinked

PVP-Br, the bromination and oxidation reactions were very

fast and the products were obtained in good yields.

The reactivity of the polymeric reagent was found to

be dependent on the nature of crosslinking agent and also

molar percentage of crosslinks. The three types of

crosslinking agents used differ widely in their polarity.

Thus the polymers prepared from these three types of

crosslinking agents differ in the hydrophobic/hydrophilic

nature. This affects their swelling characteristics

which in turn affects the reactivity of the attached

function. Thus by the introduction of divinylbenzene as

crosslinks, the hydrophobicity and rigidity again

increases and the reactivity of the corresponding

brominated resin decreases. Introduction of the

hydrophilic TTEGDA-crosslinking agent increases the

flexibility and hydrophilicity of the crosslinked polymer

system. From the synthetic reactions carried out with

PVP-Br which contain varying amounts of TTEGDA-

crosslinking, it was observed that 15% crosslinked

polymer was most reactive. This shows that increased

hydrophilicity enhances the reactivity of the PVP-Br

reagent. NNMBA-crosslinked PVP-Br has a reactivity

intermediate between DVB- and TTEGDA-crosslinked PVP-Br.

In order to study the solvation properties of

crosslinked PVP-Br, solvents of varying polarity were

selected and oxidation of benzoin were carried out with

each of these solvents. Benzene was found to be the most

suitable solvent for the hydrophobic DVB-crosslinked PVP-

Br. For NNMBA-crosslinked polymer, the most effective

solvent is chloroform. When the crosslinking agent is

TTEGDA, the most suitable solvent is dichloromethane.

These studies show that the nature of the polymer

backbone dictates the most suitable solvent for the

effective reaction.

The effect of concentration of the reagent during

oxidation was studied by taking the oxidation of benzoin

to benzil as the model reaction with different reagent-

to-substrate ratio (1:1, 2:lr 3 : l r 4:lr 5:l). In all the

cases a higher excess was found to result in an enhanced

rate of reaction.

Another type of reagent developed was based on t-

butyl hypochlorite. These reagents can also be used for

the oxidation and halogenation of' organic substrates.

The two supports used for the immobilization of this

function were PS-PEG graft copolymer and TTEGDA-

crosslinked polystyrene. Polystyrene-PEG graft

copolymers were prepared from chloromethylated

polystyrene (2% DVB-crosslinked) and t-butyl hypochlorite

function was introduced into it by a series of polymer-

analogous reactions. Here the t-butyl hypochlorite

function was separated from the polymer matrix by a long

amphiphilic polyoxyethylene chain. In TTEGDA-crosslinked

polystyrene, the ethyleneoxide linkage is in the

crosslinks and not on the pendant side chain as in the

case of PS-PEG grafts. The reactivities of the reagents

prepared from these two types of supports were compared

and it was observed that TTEGDA-crosslinked polystyrene

supported hypochlorite was more reactive than PS-PEG

supported t-butyl hypochlorite. By the introduction of

PEG300 into chloromethylated polystyrene, the pore

dimension of the original polymer will change and it will

become difficult for the substrate molecules to diffuse

into the polymer matrix. This non-availability of active

function could be the reason for the slow reactivity of

PEG-bound hypochlorite function.

Investigation of the nature of the polymer matrix on

the reactivity of the attached oxidising function depends

on a number of factors characteristics of the

macromolecular matrix. Thus the reactivity of the PVP-Br

complexes were found to be considerably increased by

increasing the hydrophilic nature of the support. By the

selection of appropriate crosslinking agent and adjusting

the crosslink density it is possible to design polymer

matrices with desired flexibility/rigidity, mechanical

integrity and optimum hydrophobic-hydrophilic balance.

Solvent effects modify the diffusion of substrates and

products inside the resin. These studies also indicated

that linear and crosslinked PVP-Br complexes could be

developed as efficient solid phase polymeric reagents for

oxidation and bromination of organic substrates. From

the studies carried out with t-butyl hypochlorite reagent

functions on different types of polymeric supports, it

was observed that the ethyleneoxide linkage in the

crosslinks facilitates the efficiency of the synthetic

reagent. The macromolecular structural parameters

operating on the synthetic efficiency could also be

delineated from these studies.

R E F E R E N C E S