INTRODUCTION

CHAPTER 1

1

1.1 CHEMICAL ROCKETS

One of the landmarks in the history of space science is the invention of

chemical rockets, which opened the universe to human exploration. The exact

origin of chemical rockets is lost in the shadow of time. Many authorities in the

field of rocketry believe that the formulation of black powder by the chinese,

the first rocket propellant, heralded their era (1,2). Indeed chemical rockets are

unique devices, which convert chemical energy into heat and eject "!he stored

matter, to derive momentum required for the propulsion. The energy thus

derived, is from the high pressure combustion reaction of rocket propellants.

1.2 TYPES OF ROCKET PROPELLANTS

The propellants used in rocket engines are classified as solid, liquid and

gaseous propellants according to their physical state. When both solid and

liquid are simultaneously employed, such systems are termed hybrid

propellants. As each propellant system mentioned above has specific merits,

they are chosen as per the requirements in the individual missions (3). Solid

and liquid propellants are, by far, the most widely used energy source for

rockets. In terms of specific impulse, which is the criterion most often used to

evaluate the relative worth of different propellants, liquid propellants have an

edge over the solid propellants (4) and hence they are the logical choice for

2

some applications. Likewise, the advanced nuclear, electrical or ion propulsion

engines, could deliver the highest specific impulses, but at present find only

their use in space, where long duration flights (5) are essential.

1.2.1 Solid propellant

Solid propellants are complex stable mixtures of oxidising and reducing

ingredients, in a plastic cake form, which when ignited, burn in a controlled

manner to result predominantly in high pressure, low molecular weight gases.

These gases at high temperature, when exhausted through the convergent­

divergent nozzle of the solid-rocket motor, provide the reaction force for the

rocket propulsion. The combustion of solid propellants is a self-sustaining,

exothermic, rapid oxidising reaction due to the presence of oxidizer and fuel

together. The solid propellants are classified as homogeneous or heterogeneous

based on their physical structure.

1.2. 1. 1 Homogeneous propellants

In the case of homogeneous propellant the oxidizer and fuel are linked

chemically in its structure, in contrast to heterogeneous propellant, wherein

they are physically mixed. Nitrocellulose, for instance, is a widely used

homogeneous propellant. Being a fibrous material, nitrocellulose is gelatinized

3

with other nitrated liquid materials, like nitroglycerin or trimethylol ethane

trinitrate and a small amount of stabilizer. These types of propellants are also

sometimes referred to as double base propellants (6).

1.2.1.2 Heterogeneous propellants

Modern heterogeneous propellants, termed composite propellants, are

constituted from several chemical ingredients like the polymeric binder,

oxidizer, metallic fuel, plasticizer, stabilizer, curing and crosslinking agents,

burning rate catalysts and other minor additives. Out of these, the principal

components are the oxidizer, the metallic fuel and the polymeric binder. Usually

the crystalline oxidizer is suitably dispersed in the polymeric binder.

1.3 OXIDIZERS AND METALLIC FUELS

1.3.1 Oxidizers

The major constituent of a composite propellant by weight is the

oxidizer. Hence the characteristics (7) of the oxidizer undoubtedly, would have

a profound influence on the ballistic and mechanical properties of the

propellant.

The oxidizer is also the source of oxygen in the composite propellant,

4

which when reacts exothermically with the polymeric binder and the metallic

fuel releases large quantities of hot exhaust gases (7).

The principal features of a good oxidizer are high oxygen content, high

density, low heat of formation, non-hygroscopicity, high flame temperature,

non-toxic, non-smoky exhaust gases and easy availability at relatively low cost

(8,9).

1.3. 1. 1 Perchlorates and nitrates

Several crystalline oxidizers like KCI04 , NH4CI04 , LiCI04 , NaCI04 ,

N02CI04 , NaN03 , KN03 , NH4N03 are used in composite propellants. Crystalline

perchlorates and nitrates are the most widely studied oxidizers (10). The

oxidising potentials of perchlorates are known to be high and among them,

nitronium perchlorate is a superior energetic oxidizer, offering interesting

possibilities for the rocket propellant formulations (11). But it is extremely

hygroscopic and reacts with water to form a mixture of nitric and perchloric

acids. All perchlorates produce HCI and other chlorine compounds, which are

corrosive to many materials. Except ammonium, hydrazinium and nitronium

perchlorates, all form dense smoky exhaust due to the presence of KCI or NaCI

which are white powders. Among the nitrates, sodium and potassium nitrates

also produce undesirable smoke in the exhaust, although these nitrates are

relatively cheap and naturally available. By comparison, ammonium nitrate has

of this compound is its low oxidizing potential. However, it is being used for

low performance, low burning rate applications. Another difficulty with this

oxidizer is the polymorphic transformation of its crystals with temperature,

which leads to change in volume, resulting in the development of cracks in the

propellant grains. Out of the crystalline oxidizers, ammonium perchlorate is, by

far, the most preferred oxidizer (9), as others have one or more of the

drawbacks such as non-availability, high cost, hygroscopicity, thermal and

shock sensitivity and imparting inferior propellant mechanical properties.

1.3.2 Metallic fuels

Light metals or metal hydrides (12-14) in finely divided form are

thoroughly dispersed and distributed in the modern composite propellant to

enhance the rocket motor performance. These metallic fuels increase the

efficiency of the propellant, not only through their highly exothermic reaction

with the oxidizer, but also because they exclude water vapour and thereby

increase the hydrogen content in the exhaust gases. Metallic fuels thus prevent

the additional energy loss, due to the water-gas equilibrium and the dissociation

of the water molecules at the combustion temperature into various radicals.

The inclusion of metallic fuels increases the propellant density as well as the

combustion temperature, and suppresses certain types of combustion

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instabilities. Metal hydride addition to composite propellant is beneficial to

lower the propulsive gas molecular weight. Many of the hydrides like AIH3 ,

AI2Hs, Li3AIHs, BeH2 , Be(BH4 )2 are promising metallic fuels, although, at present

the uses are limited, due to several complications like their undesirable

reactions with other propellant ingredients. Among the metallic fuels Be, B, Mg

and AI have been thoroughly investigated in the past (15,16). Out of these, AI

is the most commonly used metallic fuel due to several advantages (1 7). Most

of the currently available propellants, in fact, are formulated with 10-20% AI,

to achieve a substantial increase in their ballistic properties.

1.4 POLYMERIC BINDERS

1.4. 1 Prerequisites for binders

The binder is the elastomeric matrix which contains the oxidizer and

metallic fuel particles in composite propellants (18,19). Since it is the

continuous phase of a solid propellant, the binder must serve a multitude of

functions. Thus, for a prepolymer to qualify as a good fuel binder it should

possess a number of special qualities. Ideally, it should be a liquid with

workable viscosity (20) and be able to accept high solid loading. It should also

be amenable for conversion to a crosslinked elastomeric network with high

tensile strength, hardness and elasticity (21). Unless the binder remains as a

7

good elastomeric matrix under severe thermal and mechanical stresses, a case

bonded propellant, ie., a solid propellant duly bonded to the rocket motor case,

would develop cracks (22). The polymeric binders thus contribute in a major

way to the structural integrity of the solid propellant grains developed out of

them. The binder should have acceptable low temperature properties (18,23)

and above all should function as a fuel having high H/e ratio yielding low

molecular weight species (24) during the combustion process. These

requirements are satisfied by several polymeric binders but to varying extent

(25).

1.4.2 Types of binders

1.4.2.1 Polysulphides

Polysulphide was one of the earliest entrants among the chemically

crosslinkable prepolymers, when at the Jet Propulsion Laboratory in 1946

scientists mixed potassium and/or ammonium perchlorate with Thiokol LP-3

liquid polymer to which was added a calculated quantity of the oxidative

curative, p-quinone dioxime. This polysulphide prepolymer as demonstrated by

Jorezak and Fettes (26) is synthesised by the reaction of dichloro ethyl formal

and trichloro propane with an excess of sodium polysulphide. Treatment of this

with sodium sulfide and sodium hydrosulphide, results in the controlled

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cleavage of the long chain polymers to relatively low molecular weight liquid

polymers, possessing terminal mercaptan functionality. The high molecular

weight rubber is generated by the oxidation of mercaptan groups to disulfides

by an oxidant like p-quinone dioxime as shown below.

H (SCH2CH20CH20CH2 CH2S h + HO-N=<:: >= N-OH

tE - (CH2. CH20 CH20 CH2 CH2S-S)y

+ H2N~NH2 + H2 0

E being the end groups of the cured polymer, which are in practice,

small in number and may be the residual hydroxyl functionality, resulted during

the manufacture. Alternatively, some other groups formed along with the last

stages of polymerization, effected through the process of cleavage or

substitution reactions. Further studies on polysulphides by researchers at the

Jet Propulsion Laboratories indeed turned a new leaf in the interdisciplinary

field of composite propellants, based on polymeric fuel binders and inorganic

oxidizers.

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1.4.2.2 Unsaturated polyesters

Soon better polymeric binders and propellants started to emerge. The

unsaturated polyesters which could be with in a desired molecular weight range

(27,28) were found to be good candidates, as these polymers possess higher

fuel values, than the conventional polysulphides. The unsaturated polyesters

are synthesised, generally through the condensation reaction of monomers

(polyhydric alcohol and polycarboxylic acid) bearing olefinic linkages. The

crosslinking of these polyesters during the propellant cure reaction, is achieved

with the help of a typical olefinic compound in presence of catalysts (29).

Styrene and methyl acrylate are compounds in point.

o

RICH1\CH2

o 0II II+ 2 ~O-C-CH-=CH-C-O-....A./\.fVV

! 0II 1\

~ O-C- CH 2- CH -C-O~

1CH-RICH2I

~O-C-CH2- CH-C-O~II 1\o 0

10

1.4.2.3 Polyurethanes

Several prepolymers with terminal hydroxyl groups have already been

synthesized, which could be cured with isocyanates to give urethane linkages.

Therefore, this class (of binder system) is termed polyurethanes (30,31). Many

of the hydroxyl terminated prepolymers are synthesized by polyesterification

(32) with an excess of difunctional alcohol or by polyetherification as

represented below:

o 0II II

n HO-C-R-C-OH+ (n+1)I

HO-R -OH

~o 0

I II II IHO-R -+ O-C-R-C~O-R +:- OH, n

R RKO H I I

----------;~~ HO --f- CH - C H2-0-t.:-CH -CH2-0HCH2- CH -R nI IOH OH

CR= H, alkyl)

11

Out of this, the polyester prepolymers are generally not preferred because of

the lower specific impulse they impart to the propellants by comparison with

polyethers at the same solid loadings. Another difficulty with polyester

prepolymers is the relatively high viscosity, caused by their broad IT'olecular

weight distribution. Polyethers are low viscosity prepolymers, available in large

quantities, possess proper rate of cure and also have the advantage of greater

aging stability. The most commonly used hydroxy terminated prepolymer being

poly (oxypropylene) glycol.

However, soon hydrocarbon based prepolymers with hydroxyl terminal

functionality were synthesized with better ballistic properties. HTPB belongs to

this category and is widely used as the prepolymer in high energy solid

propellants (33). HTPB when cured with isocyanate curing agents like 2,4

toluene diisocyanate (TOI), resulted in polyurethane as per the reaction shown

below:

HO +CH 2-· CH=CH- CH2 -1n OH + 0= C=N-R - N=C ==0

~ 0 H H 0II I I II

HO+ CH 2 -CH=CH-.-CH2-+n0-C-N-R-N-C-- 0-

12

Besides TDI, hexamethylene diisocyanate (HDI), 3-nitrazapentane diisocyanate

are also being used in high density propellants. The curing agents and the

hydroxy terminated prepolymer described above are essentially difunctional and

hence for crosslinking, a triol like glycerol or trimethylolpropane i5 also used.

The amount of triol added, is critical as the crosslink density (and hence the

mechanical properties) of the final propellant largely depends on this. Because

of the quantitative nature and the convenient rate (which could be controlled

by the selection of suitable catalysts) and the availability of several hydroxyl

compounds permitting the tailoring of propellant mechanical properties,

urethane reaction finds utility in many current propellant formulations.

However, the drawbacks of urethane reactions are the side reactions (30),

such as isocyanate dimerization and urea, biuret and allophanate formation.

Moreover, the moisture content of all ingredients used in propellant

formulations must be "kept as low as possible, to reduce the excessive

crosslinking and the carbon dioxide gas formation (34). The gas thus

generated, if not removed, would invariably result in void formation leading to

a spongy, undesirable propellant. One molecule of water would destroy two

isocyanate groups and generate a urea linkage as depicted below:

~N==C==O+ HOH

H 0, II

~N-C·-OH

13

HI

~N-H +C02

H 0 HI 'I I

~N- C - N....rJV'\./V'v

Further, the chemistry of urethane binders is made more complex by the

extremely high reactivity of isocyanates (30). Any active hydrogen such as

those found on isocyanate derived urethane and urea linkage, is another

potential reaction site. Consequent to the extremely high reactivity of the

isocyanate group, polyurethane propellants need much more sophisticated

processing techniques than that required for carboxyl terminated

polybutadiene, aziridine and/or epoxy - cured propellant systems. Moreover,

polyurethanes have inferior low temperature properties compared to

crosslinked systems formed from CTPBs (35).

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1.4.2.4 Polybutadienes

Higher fuel value requirements for the solid propellant binders, resulted

in the development of a few saturated as well as unsaturated hydrocarbon

based prepolymers, from time to time (36,37). The thorough acquaintance and

the voluminous studies available in literature, with respect to the elastomeric

1,4 butadiene and copolymers, led scientists to take up polymerization studies

on butadiene and related monomers, to prepare new prepolymers (38-40).

1.4.2.4.1 Synthesis through polymerization

Initially, a butadiene based system which found tremendous application

was the butadiene acrylic acid copolymer (PBAA) (41,42) which could be made

from the monomers, either through the bulk or emulsion polymerization method

(29) and of the two the latter is preferred by the propellant chemists in general.

This is so, because in the aqueous emulsion polymerization, due to the

difference in solubilities of the two monomers, namely, butadiene and acrylic

acid, the process permits to exercise control (43) over the introduction of

acrylic acid moieties into the synthesized copolymer chain. In fact, copolymers

with different average functionalities, molecular weights and molecular weight

distributions are easily achievable by this polymerization method (44) at

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permissible impurity levels. The copolymer PBAA with an average molecular

weight of 3000 and an average functionality of 2, prepared by the above

method, was widely used as a solid propellant binder. One of the major merits

of the PBAA prepolymer was that it permitted higher oxidizer solid loadings, as

its viscosity at the propellant processing temperature was appreciably low.

However, being a random copolymer, PBAA has certain inherent defects, like

the random nature of functional group distribution along the polymer chain and

also the wide variation of the number of functional groups per prepolymer

molecule. The prepolymer could thus be viewed as a mixture of nonfunctional,

monofunctional, difunctional and polyfunctional molecules exhibiting a range

of molecular weights. The uneven spacing of functional groups and also the

functionality distribution along the polymer chain, led to the poor reproducibility

of mechanical properties for the propellant developed from PBAA (17) and the

surface hardening of these propellants on storage.

The emulsion polymerization method was further extended to prepare

a terpolymer of polybutadiene acrylic acid and acrylo nitrile (PBAN), another

principally polybutadiene based copolymer system, prepared employing azobis

isobutyronitrile initiator as shown below.

16

CH3 C H3I I

N= C - C- N=N- C-C=NI ICH3 CH3

t~~2CH3I

2 .C- C=NI

. CH3

+ YCH 2=CHICN

-t-C H2- CHiy CH.. I I

CN C OOH

~ termination

PBAN

17

The interjection of acrylonitrile moieties in the polymer structure resulted

in the better spacings of carboxyl groups along the prepolymer chain, which

in turn bestows PBAN propellants with much better reproducible mechanical

properties (45). Moreover, the propellants formulated and developed from

PBAN prepolymers exhibited far lesser tendency to surface harden. The

undesirable surface hardening trends, consequent to the attack of oxygen at

the polymer double bonds are generally suppressed in the case of nitrile

rubbers (46). Thus, laboratory simulated experiments with other non-nitrile

polybutadiene based prepolymers like carboxy terminated polybutadienes

(CTPBs) (29) had demonstrated that, by comparison with them, PBAN

propellants substantially resist surface hardening, when exposed to oxygen

even at elevated temperatures. Although first synthesized more than two

decades ago, PBAN based propellants, even today, find use in large booster

rocket motors, carrying several tons of propellants. The free radical initiated

polymerizations carried out in the emulsion system, could be used to synthesise

PBAN or PBAA. The PBAA thus prepared, has a molecular weight range of

2500-4000, viscosity 275-325 poise at 25°C, density 0.90- 0.92 glee, and a

heat of combustion of 10.2-10.4 Kcal/g. The corresponding values for PBAN

are, molecular weight 3000- 4500, viscosity at 25°C, 300-350 poise, density

0.93-0.94 glee and heat of combustion 9.9-10.1 Kcal/g respectively.

In general, polybutadienes of intermediate molecular weight range with

terminal functionality could be synthesized by the free radical or anionic

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polymerization methods (38-40). The free radical polymerization, usually gives

rise to prepolymers with branched structures and with broad molecular weight

range, in contrast to the anionic method, which results in prepolymers with

narrow molecular weight distributions.

The free radical polymerization mentioned above, is effected through the

use of peroxide as the initiator. The scheme could be outlined as:

o 0II II

HOOC-R-C-O-O- C-R- COOH

t.6• R-COOH +-RCOOH + 2 C02

Inc H2=C H - CH= CH 2

•2 CH2-CH=CH-CH2~R- COO H

lCoupling

CTPB

19

The prepolymer could be prepared by using azo initiators also as

indicated below.

CN CNI I

HOOC - CH2- CH2- C -N=N-C-C H2 - CH2- COO HI ICH3 CH3

C H3I

+-C- CH2-CH2-COOH + N2ICN

~ n CH2=CH-CH=CH2

CN- .1

2 C H2 - CH=CH -C H2'V'VVVV\./ C- C H2 -C H2 - CO 0 HI \C H3

J Coupling

CTP B

The anionic polymerization employs, on the other hand lithium initiation

technique. Organolithium compounds are preferred in many cases. The scheme

could be represented as:

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x CH2=CH -CH== CH2

lLi-R-Li

L i -( CH2 -CH= CH-CH2 -+-)n-+( CH2 - CH -t=- LiI m

11. C02 CH= CH22. Acid

HOOC -( CH2-CH=CH-CH2J:::-t CH2 -CH~ COOHn I m

CH=CH 2

eTPS

The above synthetic routes led to CTPB prepolymers with an average

molecular weight of 3500-5000. Still lower molecular weight polybutadiene

(M'11 = 2800) prepolymers are also available, possessing hydroxyl functionality.

Several attempts have been made to synthesize polybutadienes and

other rubbers of intermediate molecular weight range with terminal functionality

(47) through the various degradation techniques also.

1.5 DEGRADATION OF POLYMERS

It was found that degradation of high molecular weight polymers,

results in the formation of polymers with either intermediate (48) or low

21

molecular weight products, depending on the extent of polymer backbone

chain scission (49,50) and it could be brought about by a host of physical or

chemical agencies (51-56) or a combination of them (57). Thus, a reduction in

polymer molecular weight could be effected either through mechanical,

thermal, or chemical means or by employing specific agencies like the

ultraviolet rays, as in the case of photodegradation (58). Ultrasonics (59) could

also be used to bring down the polymer molecular weight. Besides these, high

energy radiations like gamma, alpha and beta rays are frequently utilized to

bring forth degradation (60,61) of a large number of polymers.

Polymer chains under mechanical stresses (62-71) could be ruptured to

smaller fragments, where by macroradicals are produced, the presence of

which have been confirmed by the ESR spectral studies (72,73). Examples of

such macroradicals generated from polymers like polyethylene and

polypropylene are given below:

~CH2-CH2-CH2-CH2~

/'\./\./VV'\rv-CH2-CH2- + -CH2 -CH2~

CH3 CH3I I

~CH-CH2-CH- CH2~

1CH3I

-C- CH2 ..rvvvvv­IH

22

The radicals are often generated at 77 K by ball-milling or grinding in the

absence of oxygen, although many times it becomes impossible to identify,

either one or both the primary radicals thus produced (as a result of the main

chain scission) because they are rather very unstable even at 77K. Thus, the

ESR spectrum of polybutadiene which underwent mechanical degradation,

could not detect the presence of the following primary radical (I).

H H H, I I

• C-C=C-C-C­I I I I IH H H H H

H HI • I

H-C-C-C=C-C­I I I I IH H H H H

IT

as it gets transformed into a secondary radical (II) by the hydrogen atom shift

which could be detected (72). Macroradicals are also suitable for preparing

block copolymers (70,74).

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Some high molecular weight polymers like polyisobutene (75), when

repeatedly extruded in a capillary rheometer at 80aC, undergo shear

degradation and consequently the average molecular weight decreases under

the influence of the mechanical stress. It is thus concluded that chemical bonds

are ruptured under the influence of stress. In some specific cases, u!trasonic

waves are frequently preferred to induce polymer degradation, as in the case

of polystyrene, in a nitrogen saturated cyclohexanone solution, for the

mechanical degradation of this polymer (76). Ultrasonic degradation studies on

several polymers (77,78) have revealed its effectiveness in conveniently

dispersing the mechanical energy (79) in polymer solutions.

1.5.1 Mastication of rubber

Degradation studies with respect to elastomers like natural rubber,

which essentially is cis 1,4 polyisoprene, dates back to 19th century (80).

Mastication is a term coined especially for the mechanical treatment of natural

rubber. Notable reductions in average molecular weights have been achieved,

in the case of rubbers, when the mastication process was carried out in the

presence of air (81). During mechanical degradation of rubbers, allyl type

radicals are produced by the homolytic bond scission (82,83).

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CH3 CH3I I

~C=CH-CH2-CH2-C=CH~

CH3I

~C=CH-CH2· +

Intermolecular crosslinking will occur if the mechanical stress takes

place in an inert atmosphere. This will be reflected as an increase in the

average molecular weight. Thus when natural rubber is masticated in an argon

atmosphere, the weight average molecular weight is found to increase with

time (84). Ultimately an infusible and insoluble three dimensional network

polymer results due to intermolecular crosslinking, consequent to the

macroradicals formed reacting with the double bonds of other macromolecules.

Hence, if a reduction in molecular weight is aimed, the mastication should be

carried out in presence of a radical scavenger. Generally, the oxygen molecules

present in air can react with the macroradicals and there by generate peroxy

radicals. The peroxyl radicals thus formed are capable of abstracting hydrogen

atoms.

25

The mastication technique for the reduction of polymer molecular

weight is not restricted to natural rubber alone, infact it has successfully been

applied to butyl rubbers, EPDM, polychloroprene, styrene nitrile and acrylic

rubbers as well. Since during the mechanical degradation macroradicals are

generated, chemists have used this method for the synthesis of block and graft

copolymers also (85).

1.5.2 Copolymers

A good deal of studies in the past had been devoted to the syntheses

of block copolymers with various elastomers (86), as scientists find a special

delight in combining the properties of different polymers in a single system.

This led to the development of vast and diverse varieties of copolymers with

suitable combinations of the desired properties (87-90). Conversion of

chemically inert and unvulcanisable polyisobutene to vulcanisable rubbers by

the incorporation of polyisoprene unsaturated blocks, into the polymer chain

is a classical example (91). Later, different possibilities have been

demonstrated by researchers for the syntheses of new combinations,

employing mechanochemical syntheses (54,92-95) of various polymers and

also their utility for the preparation of many commercial polymers. Very exotic

block and graft hydrocarbon based copolymers have been synthesised through

the mechanochemical synthetic routes. These polymers were possessing many

26

useful properties in a single macromolecule, which were otherwise

unattainable. Several publications had appeared, over a period of time, in this

regard (85,96). It has become apparent that many interesting block and graft

copolymers could be synthesized, by subjecting polymer-polymer or

polymer-monomer systems to high speed stirring or ultrasonic irradiation (97).

The block copolymer synthesis on a laboratory scale, through ultrasonic

treatment had been demonstrated already by many scientists (98-101).

1.5.3 Functionalization of polymers

Degradation,as has already been mentioned,is an effective method for

preparing polymers with low or intermediate molecular weights, starting from

high molecular weight polymers (102) and in many cases it could be made an

effective route for the introduction of various functional groups also.

Khodzhemirov, Sorokina and Kosolova had successfully demonstrated (103) the

preparation of intermediate molecular weight range high cis content polymers,

through the degradation of (high molecular weight) conjugated dienes in

presence of tungsten based catalysts. In another approach Mori, Fuji (104) and

Kempermann (105) had studied the thermal degradation of natural rubber in

presence of catalysts employing phenyl hydrazone. Similarly, the sensitizing

effect of various catalysts in the photodegradation studies of neoprene and

natural rubber had been evaluated in detail by Ranby and Rabek (106) and the

27

obvious effect of hydrogen peroxide and p-toluene sulfonic acid redox couples

in the depolymerisation process were evaluated by Pautrat and Marteau (107).

Palit, Mukherjee and Konar (108) could successfully introduce terminal carboxyl

functionality, after subjecting natural rubber to mastication. In many cases,

mastication to obtain polymers with intermediate molecular weight range is a

nonrandom degradation process, because the rupture according to Angier (109)

and Brislow (110) is not at random, but occurs mainly in molecules whose size

exceeds the critical chain length. It was further shown (37) that terminal

hydroxyl functionality could be introduced in natural rubber of useful molecular

weight range and distribution, prepared through the mastication process, by

the subsequent treatment of it with a 30-40% hydrogen peroxide solution. This

has resulted in hydroxyl terminated telechelics of relatively lower molecular

weights and the studies with these and similar functionalized polymers have

established their utility as (solid propellant) binders (111), sealants (112),

coating materials (113) and adhesives (114).

1.5.3.1 Oxidative degradation

An attractive alternative for the preparation of ((, (,.) functionalized

polymers of suitable molecular weight range, is the controlled degradation of

high molecular weight polymers possessing cleavable bonds. This strategy has

been employed in the past to prepare telechelic oligobutenes by the oxidative

degradation method (115).

28

Ebdon, Dix et al. (116,117) had shown the effectiveness of oxidative

degradation for the syntheses of various terminally functionalized oligomers.

Some of them could also be synthesized, by the degradation of high

molecular weight diene polymers, using the appropriate reaction of ozone with

the double bond. Ozone first forms a five membered ozonide (118) and finally

cleaves the polymer chain (119), as shown below.

J'VVV'\/'\./"V CH.::=CH-vvvvv-

~CH-O-CH~

\ 1° 0

~H20~CHO +OHC~

+ H202

Oxidative degradation could thus be used to synthesize macromolecules with

terminal functionality.