Polymer chemistry book

• BASIC TERMS AND DEFINITIONS Addition chain polymerization: This occurs when small molecules join together under the stimulus of a catalyst, heat or radiation to form a linear polymer

POLYMER CHEMISTRY

Alka L Gupta

PRAGATI PUBLICATIONS

ISBN No : 978-81-8398-998-5

Addition (chain) Polymerisation 23

Free-radical Addition Polymerisation 24

Monomer Reactivity Ratios 40

Reactivity-ratios and Copolymerisation Behaviour 43

Reaction of Ketonic Groups 54

Reactions of Carboxylic Groups 54

Reactions of Aldehyde Groups 54

Reaction' of Nitric Group 55

Reaction of Amino Group 55

Reaction of Aromatic-Ring 55

Reaction of Amide Group 56

Cyclisation Reaction 56

Cross linking Reactions 57

Vulcanisation 58

Reaction with Ammonium acetate 59

Reaction with Ni-carbon compound 59

Reaction Introducing Aromatic group 60

Reaction with succinic anhydride 60

Nucleophilic substitution reaction 60

Polymerisation in Homogeneous and Heterogeneous Systems 61

Homogeneous System 61

Heterogeneous System 63

Suspension Polymerisation 63

Emulsion Polymerisation 63

Interfacial Polycondensation Polymerisation 65

Solid and Gas Phase Polymerisation 66

Polydispersity and Molecular Weight Distribution 81

The Practical Significance of Molecular Weight 82

Measurement of Molecular Weights 84

Thermal Analysis 101

Differential Calorimetric ~alysis 102

Thermal Gravimetric Analysis 102

Physical Testing 102

Tensile Strength 102

Fatigue 103

Morphology and Order in Crystalline Polymers 104

Configurations of Polymer chains 104 Crystal Structure of Polymers 111

Morphology of Crystalline Polymers 115

Strain-Induced Morphology 117

Polymer Structure and Physical Properties 120

Crystalline Melting Point (Tm) 120

Melting-Point of Homologous Series 121

Effect of Chain Flexibility and Other Sterle Factors 121

Chain Flexibility 121

Side-Chain Substitution 123

Glass Transition Temperature (Tg) 123

Experimental Demonstration of Tg 124

Glassy Solids and Glass Transition 124

Relationship between Tg and Tm 128 Effect of molecular weight on Tg 129 Effect of Plasticizers on Tg 129 Effect of copolymers on Tg 130 Effect of chemical structure on Tg 130 Effect of chain topology on Tg 131 Effect of chain branching and crosslinking on Tg 131

Factors Influencing Glass Transition Temperature 131

Determination of Glass Transition Temperature 135

Importance of Glass Transition Temperature 137

Property Requirements and Polymer Utilization 137

~ "i PROPERTIES OF COMMERCIAL POLYMERS

Conformation of the Nucleic Acids 251

Segments of RNA and DNA Polymers 252

Appendix 1: Polymer Degradation 254

Appendix 2: Photodegradation of Polymers 255 '(

Appendix 3 : Current and Promising Polymer Research Topics 256

of repeating structural units, derived from certain molecules of small units, i.e., monomers

The polymer is thus a long chain, in some polymer may coil, branch, cross-linked to other chains or take part in other orders or structural complexity

The chemical and physical interactions among the atoms of polymer are governed by the same laws that describe systems of small molecules, but extreme molecular size introduces a new realm of properties The diversity of macromolecular structure represented

by a given chemical composition increases with the number of monomeric units present and statistical considerations must enter the description of even the simplest polymer chain The extreme length of macromolecular chain inhibits their crystallization, hence diverse stable solid states occur which may be rubbery, glassy or semicrystalline New combinations of properties emerge, such as rubbery elasticity and strength, combined with flexibility and optical clarity Fabrication methods are found with polymers which facilitate their shaping into desired forms Polymers made by man, and their fabrication into finished products have become the basis of a major industry world wide Life itself is a basis of large molecules The remarkable adaptations of collagen and cellulose to structural functions, the specificity and efficiency of enzymes as catalysts, the binding and release of oxygen by hemoglobin and

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myoglobin, and the encoding of specific genetic information by the nucleic acids, all have

their origins in the polymeric nature of the molecules involved

Polymer study and research is thus interdisciplinary, with major contributions from

chemistry, physics, several branches of engineering, biomedical science and molecular

biology

Polymers are essential in fulfilling a broad range of national needs, present and

prospective, in such categories as energy, transportation, construction, agriculture and food

processing, medicine and national defense The history of polymer science and engineering

is replete with unforeseen discoveries of major consequence, and the future of this field is

bright one promising For example, the recent break throughs in understanding the structure

and invivo synthesis of biopolymers still have to make their major impact on synthetic

polymers, and the theory and application of composite materials based on polymers are still

in their infancy

Enormous studies have been made in the field of biopolymers, since the discovery of

the DNA double helical structure in 1953 This was followed by various advances: the

determination of the detailed sequence structure of nucleic acids and proteins, the

recognition of nucleic acids as the carriers of heredity and the solid-state synthesis of sizable

protein molecules Further progress has continued in many related areas, including such

vital aspects as the three-dimensional structure of enzymes, its connection to binding of

specific molecules, and thus its catalytic function

The production of polymers on a volume basis now exceeds that of steel, and its

growth rate (8.5% per year) is four times that of steel and nonferrous metals Polymer

industries add $ 90 billion per year of value added by manufacture and employ 3.4 million

people Polymers also have a high technology aspect which will be increasingly important in

the future and may have a critical impact on fulfilling national needs

• HISTORICAL BACKGROUND

The polymer science is a coherent subject since 1950 Prior to 1930, a number of

national products now recognized as polymers (e g , cellulose, starch, proteins, rubber) had

been studied with the relatively primitive instrumentation but highly ingenious methods of

chemical experimentation and reasoning then available Emil Fischer, after his classic

researches on the stereochemistry and synthesis of the sugars, turned in 1899 to the linkage

of the amino acids known to be combined in proteins He succeeded not only in getting two

amino acids to combine synthetically (as an amide), but by 1907 he had synthesized a

polypeptide chain containing as many as 18 amino acids residues, linked in known linear

sequence His synthetic polypeptide prove to behave in every respect by corresponding

natural intermediate products derived from the hydrolysis of proteins

Important synthetic derivatives of natural polymers had been discovered, among them

vulcanized rubber by C Good year in 1839, cellulose nitrate in 1870 by J.W Hyatt, cellulose

acetate by C and H Dreyfus in 1919, and even the first commercially successful class of

entirely synthetic polymers, the thermosetting phenolic resins by L.H Baeke land in 1909 The

structure of these amorphous, plastic, nonvolatile, slow diffusing materials was that they

consisted of micellar aggregates of small molecules, a colloidal state, cohering through

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intermolecular forces of non-chemical origin There was a prejudice against believing that stable molecules of indefinitely large dimensions could exist

The clear concepts of macromolecules, are attributed to Hermann Staudinger In 1920's, Staudingers did work on styrene, convinced that the amorphous material readily forms on standing or heating consists of styrene units covalently bonded in long chains through a chemical reaction involving opening of the vinylic double bond He succeeded in preparing

of polystyrenes of varying degrees of polymerization (i.e., number of styrene units per chain)

as reflected in their average molecular weight and molecular weight distributions He demonstrated a corelation of molecular size with the viscosity of their dilute solutions in suitable solvents The fundamental principles of vinyl polymerization were outlined by J.F Paul in 1937 in terms of chain reaction sustained by a free-radical mechanism

A landmark in polymer science and engineering was the commercial development in

1939 of nylon 66, discovered by carothers This entirely synthetic aliphatic polyamide, resembling natural silk but with controllable structural regularity and attendant desirable physical properties, proved to be a product for which a demand rapidly became evident C.S Marvel pioneered in the organic chemistry of polymers and made outstanding contributions to polymer synthesis His research on polymers stable on high temperatures and on polymers with heterocyclic structures has led to concepts and materials of major commercial significance

An unexpected break through in polymer research was achieved in 1955, when Karl

Ziegler discovered polymerization catalysts based on various coordination compounds of

transition metals With such a catalyst, Ziegler found that polyethylene could be synthesized from ethylene rapidly at ambient temperature and pressure Furthermore, this polyethylene was almost entirely linear unlike the branched low density polyethylene known since 1935, which is produced only at elevated temperatures and pressure in excess of a thousand atmospheres

Giulio Natta then succeeded with catalysts of this type in polymerizing propylene and discovered the first synthetic stereospecific polymerization Natural stereospecific reactions occur in the formation of proteins and other polymer of biologic origin such as rubber and Gutta-percha (which are stereoisomers of each other) In polypropylene, all the propylene units are aligned 'head to tail', i.e., pendant methyl groups occur at the same end of each unit, an orientation of the chain which as favoured by the reaction kinetics, but they may assume either of two different mirror-image-configurations depending on the orientation of the pendent methyl group about the chain The Ziegler-Natta catalyst permits the synthesis

of stereoregular polymers, in which the monomeric units have either the same or regularly alternating configurations, or the synthesis of isomeric randomly oriented polymers, which have quite different physical properties Some stereoregular synthetic polymers may occur

in a semicrystalline state, the randomly oriented polymers are always amorphous

"'" The Ziegler-Natta catalysts were used primarily to produce new forms of earlier

polymers rather than polymers of new chemical composition This has been a major trend in more recent developments, i.e., existing polymer types have been vastly improved by chemical and physical modification For example, modification of polymer glasses to produce tough, impact-resistant materials, increase in the elastic modulus of polyethylene

produced by extrusion, additives to preserve or fire-retard polymers, immobilization of

enzymes by attachment to inert polymers, and many others

The development of molecular biology was greatly stimulated by the discovery of the

double-helical structure of the DNA molecule G.D Watson and F.H Crick, 1953) Molecular

biology deals with the biopolymers, macromolecules of biological origin and Significance

General polymer science has contributed particularly in the understanding of the structures

and function of biological polymers, and has been enriched by consideration of the problems

and achievements in that field

Aside from the intrinsic interest of biological macromolecules as polymers, interest

exist in a wide variety of biological and biomedical problems where the specific chemical

and physical properties of polymeric materials are adaptable to solutions Biomedical

implants, prostheses and artificial organs come to mind, as well as polymeric or polymer

bound pesticides and drugs, and polymeric films, used in agriculture to protect young plants

and to conserve moisture

• BASIC TERMS AND DEFINITIONS

Addition (chain) polymerization: This occurs when small molecules join together under the

stimulus of a catalyst, heat or radiation to form a linear polymer usually without the

elimination of a small molecule This can be of the following three types (a)-{c) :

(a) Free radical addition polymerization: In this type, chains are initiated by a free-radical

such as phenyl

(b) Cationic addition polymerization: The active species which initiates the addition

polymerization is a cation such as a proton

(c) Anionic addition polymerization: The initiating species in this case is an anion such as

NH~-)

Coordination polymerization : There are a number of coordination catalysts such as a

combination of aluminium trialkyl and titanium or vanadium chloride, which will

polymerize olefenic compounds to yield a stereospecific polymer, e.g., isostatic

polypropylene from propylene

Adhesive: Material that binds and holds the surfaces together

Amorphous: A non-crystalline polymer or non-crystalline areas in a polymer

Atactic: Polymer in which there is a random arrangement of pendant groups on each side

of the chain

Biopolymer: A naturally occurring polymer such as cellulose

Block-eo-polymer: The repeating unit consists of segments or blocks of similar monomers

tied together along the macro-molecular chain

Branched chain polymer: A polymer having extensions of polymer chain attached to the

polymer backbone

Calendering: A process of making polymeric sheets by means of a machine containing

counter-rotating rolls

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Compression moulding: A fabrication technique of moulding, a thermosetting polymer

by means of heating and applying pressure

Chain-transfer: A reaction in which a free-radical abstracts an atom or group of atoms from a solvent, initiator, monomer or polymer

Chain-polymerization: See addition polymerization

Colligative properties: Properties of a solution which are dependent on the number of solute molecules present

Co-polymers: A long chain polymer composed of at least two different monomers, joined together in an irregular sequence

Critical chain length: The minimum chain-length required for the entanglement of the polymer chains

Cross-links: Covalent bonds between two or more polymer chains

Crystalline polymers: A polymer with an ordered structure, which has been allowed to disentangle and form a crystal

Condensation (step) polymerization: The polymers are formed by various organic condensation reactions with the elimination of small molecules such as water

Crystalline melting point (T m): This is the range of melting temperature of the crystalline domain of a polymer sample and is accompanied by change in polymer properties It is also the first-phase transition when the solid and liquid phases are in equilibrium Degree of polymedzation (DP or P): It is the average number of repeating units in a macromolecule The degree of polymerization is obtained by dividing the (average) molecular weight by the molecular weight of the monomer

Elastomer : These are the non-crysalline high polymers or rubbers that have three-dimensional space network structure (e.g., that produced by vulcanization), which improves stability or resistance to plastic deformation Normally, elastomers exhibit long range elasticity at room temperature

Extrusion moulding: A fabrication technique by which a heat softened polymer is forced continuously by a screw through a die

Fibers: A fiber is a thread or thread like structure composed of strings or filaments of linear macromolecules that are cross-linked in such a manner as to give rise to an assemblage of molecules having a high ratio of length to width

Functionality: The number of reactive groups in a molecule

Glass transition temperature (T g): This is the temperature at which an amorphous polymer starts exhibiting the characteristic properties of the glassy state, (because of the onset of segmental motion) stiffness, brittleness and rigidity

Graft co-polymer: When a monomer is polymerized onto the primary high polymer chain obtained by the polymerization of another kind of monomer, a graft polymer results Inhibitor: An additive-which reacts with a chain-forming radical to produce non-radical products or radicals of low reactivity, incapable of adding fresh monomer units Injection moulding: A fabrication process in which a heat-softened polymer is forced continuously through a die by means of a piston

Intermolecular forces: Secondary valency forces among different molecules

Intramolecular forces: Secondary valency forces within the same molecule

Isostatic polymers: A polymer in which all the pendant groups are arranged on the same

side of the polymer backbone

Kinetic chain-length : It is defined as the average number of monomer molecules

contained per radical which initiates a polymer chain

Linear chain polymer: It consists of a linear polymer chain without any branching

Macromolecules: See polymers

Mer: The repeating unit in a polymer chain

Micelle: It is an aggregation of crystallites of colloidal dimensions and exists either in solid

state or in solution

Molecular weight: Most polymer are poly-disperse or mixtures containing polymer

molecules of different molecular weights Different measures of molecular weight are

defined as:

Number average molecular weight (Mn): The arithmetical mean value obtained by

dividing the sum of molecular weights by the number of molecules

Weight average molecular weight (Mw): The second power average of molecular weight

in a polydisperse polymer

Z-average molecular weight (Mz): The third power average of molecular weight in a

polydisperse polymer

Monomer: All high polymers are formed by the joining together of many molecular units

of groups of molecular units The number of units or mer of a polymer is the unit of the

molecule which contains the same kind of and number of atoms as the real or

hypothetical repeating unit

Oligomer: A polymer containing very few repeating units, usually between 2 and 10

Osmotic pressure: The pressure that, a solute would exert in solution if it were an ideal

gas at the same volume

Pendant groups: Groups attached to the main polymer chain or backbone An example is

the methyl groups in polypropylene

Plastics: A group of artificially prepared substances usually of organic origin which

sometimes during their stage of manufacture have passed through a plastics condition

Plasticizer: An additive which reduces the inter-molecular forces between polymer chains

and thus acts as an intemallubricant

Polymer or macromolecule: A giant molecule made up of a large number of repeating

units such as polyethylene which may contain 100 or more ethylene monomer units

Polymerization: It is the process of formation of large molecules from small molecules

with or without the simultaneous formation of byproducts such as water A classical example is the formation of polystyrene from styrene molecules

Polydispersed: A polymer containing molecules of different ~olecular weight~

Rayon: Regenerated cellulose in the form of a filament; used as a fiber

Retarder: An agent which acts as a chain-transfer agent to produce less reactive free-radicals

Rheology: The science of flow

Ring opening polymerization: Formation of polymers by the opening of rings such as those of ethers or lactams The formation of Nylon-6 from caprolactam is an example Spinneret: A metal plate with many small holes of uniform size used for spinning Step polymerization: See condensation polymerization

Stereoselective polymerization: In this type of polymerization one type of ordered structure is preferentially formed in contrast to the other

Stereospecific polymerization: Polymerizations which yield ordered structures (isostatic

or syndiotactic)

Syndiotactic: A polymer in which pendant groups are arranged alternatively on each side

of the polymer backbone

Tacticity: The arrangement of pendant groups in space

Thermoplastic: These soften in a reversible physical process under the influence of heat and sometimes of pressure and can be moulded into different shapes under this condition They retain their shapes on cooling

Thermosetting resins: These soften under the influence of heat and pressure and can be moulded into different shapes They become hard and infusible on account of chemical change and cannot be remoulded

Theta temperature: A temperature at which a polymer of infinite molecular weight starts

to precipitate from a solution

Viscosity: Resistance to flow

Viscosity, intrinsic [Tll: The limiting viscosity number obtained by the extrapolation of relative viscosity to zero concentration

Viscosity, reduced: The specific viscosity divided by concentration

Viscosity, relative: The ratio of the viscosities of the solution and the solvent

Viscosity, specific: The difference between the relative viscosity and one

Vulcanization: This is a process by which cross-links between linear elastomer chains are introduced An example is the introduction of sulphur cross-link in natural rubber by heating it with sulphur

Ziegler-Natta catalyst: A catalyst with the composition TiCl3 - AIR3, obtained from titanium tetrachloride and aluminium trialkyl

While "polymer" in popular usage suggests "plastic", the term actually refers to a large class of natural and synthetic materials with a variety of properties and purposes Natural polymer materials such as shellac and amber have been in use for centuries Biopolymers such as proteins and nucleic acids

play crucial roles in biological processes A

variety of other natural polymers exist, such as

cellulose, which is the main constituent of

wood and paper Some common synthetic

polymers are Bakelite, neoprene, nylon, PVC

(polyvinyl chloride), polystyrene

poly-acryonitrile and PVB (polyvinyl butyral)

Polymers are studied in the fields of polymer

chemistry, polymer physics, and polymer

In the intervening century, synthetic polymer materials such as Nylon, polyethylene, Teflon, and silicone have formed the basis for a burgeoning polymer industry These years have also shown significant developments in rational polymer synthesis Most commercially important polymers today are entirely synthetic and produced in high volume, on appropriately scaled organic synthetic techniques

Synthetic polymers today find application in nearly every industry and area of life Polymers are widely used as adhesives and lubricants, as well as structural components for products ranging from children's toys to aircraft They have been employed in a variety of biomedical applications ranging from implantable devices to controlled drug delivery Polymers such as poly (methyl methacrylate) find application as photoresist materials used

in semiconductor manufacturing and low-k dielectrics for use in high-performance microprocessors Recently polymers have also been employed in the development of flexible polymer-based substrates for electronic displays

• POLYMER SYNTHESIS

Polymer synthesis is the process of combining many small molecules known as monomers into a covalently bonded chain During the polymerization process, some chemical groups may be lost from each monomer The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue

Laboratory Synthesis

Laboratory synthetic methods are generally divided into two categories, condensation polymerization and addition polymerization However, some newer methods such as plasma polymerization do not fit neatly into either category Synthetic polymerization reactions may be carried out with or without a catalyst Efforts towards rational synthesiS of biopolymers via laboratory synthetic methods, especially artificial synthesis of proteins, is

an area of intense research

Biological Synthesis

There are three main classes of biopolymers : polysaccharides, polypeptides, and polynucleotides In living cells they may be synthesized by enzyme-mediated processes,

such as the formation of DNA catalyzed by DNA polymerase The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA and suhsequently translate that information to synthesize the specified protein from amino acids The protein may be modified further following translation in order to provide appropriate structure and functioning

Modification of Natural Polymers

Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers Prominent examples include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of sulphur

• POLYMER STRUCTURE

The structural properties of a polymer relate to the physical arrangement of monomer residues along the backbone of the chain Structure has a strong influence on the other properties of a polymer For example, a linear chain polymer may be soluble or insoluble in water depending on whether it is composed of polar monomers (such as ethylene oxide) or nonpolar monomers (such as styrene) On the other hand, two samples of natural rubber may exhibit different durability even though their molecules comprise the same monomers Polymer scientists have developed terminology to precisely describe both the nature of the monomers as well as their relative arrangement

Monomer Identity

The identity of the monomers comprising the polymer is generally the first and most important attribute of a polymer The repeat unit is the constantly repeated unit of the chain, and is also characteristics of the polymer Polymer nomenclature is generally based upon the type of monomers comprising the polymer Polymers that contain only a single type of monomer are known as homopolymers, while polymers containing a mixture of monomers are known as copolymers Poly(styrene), for example, is composed only of styrene monomers, and is therefore is classified as a homopolymer Ethylene-vinyl acetate, on the other hand, contains more than one variety of monomer and is thus a copolymer Some biological polymers are composed of a variety of different but structurally related monomers, such as polynucleotides composed of nucleotide subunits

A very common error is to use the term "monomer" to refer to the repeating units of the polymer In fact, these two things are different The monomer is the stable molecule that will

be used as the polymerization reaction starts Then, a loss of a minimum of two chemical groups of the monomer forms the repeating unit A simple example is polyethylene The monomer is the ethylene (ethene) molecule, while the repeating unit is -C-C-

A polymer molecule containing ionizable subunits is known as a polyelectrolyte An ionomer is a subclass of polyelectrolyte with a low fraction of ionizable subunit

• CHAIN LINEARITY

The simplest form of polymer molecule is a straight chain or linear polymer, composed of',a single main chain The flexibility of an unbranched chain polymer is characterized by its persistence length A branched polymer molecule is composed of a main chain with one or

more substituent side chains or branches Special types of branched

polymers include star polymers, comb polymers, and brush

polymers If the polymer contains a side chain that has a different

composition or configuration than the main chain, the polymer is

called a graft or grafted polymer A cross-link suggests a branch

point from which four or more distinct chains emanate A polymer

molecule with a high degree of crosslinking is referred to as a

polymer network Sufficiently high crosslink concentrations may

lead to the formation of an 'infinite network', also known as a 'gel',

in which networks of chains are of unlimited extend, essentially all

chains have linked into one molecule

Chain Length

Polymer bulk properties may be strongly dependent on the

size of the polymer chain Like any molecule, a polymer molecule's

size may be described in terms of molecular weight or mass In

polymers, however, the molecular mass may be expressed in terms

Flg-1 Appearance of real

linear polymer chains as recorded using an atomic force microscope

on surface under liquid medium Chain contour length for this polymer

-0.4 nm

of degree of polymerization, essentially the number of monomer units which comprise the polymer For synthetic polymers, the molecular weightis expressed statistically to describe the distribution of molecular weights in the sample This is because of the fact that almost all industrial processes produce a distribution of polymer chain sizes Examples of such statistics include the number average molecular weight and weight average molecular weight The ratio of these two values is the polydispersity index, commonly used to express the "width" of the molecular weight distribution

The maximum length of a polymer chain is its contour length

Monomer Arrangement in Copolymers

Monomers within a copolymer may be organized along the backbone in a variety of ways

• Alternating copolymers possess regularly alternating monomer residues

• Periodic copolymers have monomer residue types arranged in a repeating sequence

• Random copolymers have a random sequence of monomer residue types

• Statistical copolymers have monomer residues arranged according to a known statistical rule

• Block copolymers have two or more homopolymer subunits linked by covalent bonds Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively

'part') All the substances referred to as polymers, are big molecules with molar masses ranging from several thousands to several millions

The term polymer is not new for human beings, infact it is 4 billion years ago since the formation of the earth was over The origin of life was occurred by a polymer 'protein' Protein is a complex molecule formed by the combination of elements like carbon, hydrogen, oxygen and nitrogen which were present on the earth Almost the whole human body was built around the same polymer

Polymers had also appeared in their other natural forms like wood, cellulose, starch, cotton, glue, rubber etc The term rubber was coined by Joseph Priestley who discovered

that this material would rub out or erase pencil marks Although rubber was probably used

as early as the eleventh century and the rapid growth of rubber industry led to the development of plantation rubber in 1876

A Swiss scientist Christian Schonbeiri discovered a nitro derivative of a naturally

occurring polymer 'cellulose' while working with a mixture of nitric acid and sulphuric acid

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During the experiment the glass beaker broken down, he mop up the mixture with a cotton cloth and left it for drying near a fire-place The cotton cloth soon caught fire because of formation of 'gun cotton', a nitro derivative of cellulose

In nineteenth century, elephant tusk, i.e., ivory was used for making billiard balis An American scientist John Wesley Hyatt invented a new polymer 'celluloid', resembled with ivory This discovery has become a major invention contributing to our present 'plastic age'

In 1909, Leo Baekeland developed a resin from phenol and formaldehyde which was named as 'Bakelite'

In 1912, Jacques Brandenburger discovered a transparent polymer 'cellophane' Within a decade, several polymers were started appearing in newer forms with increasingly advanced properties Most of the synthetic polymers are of a relatively recent origin

Polymers are the chief products of modern chemical industry which form the backbone

of present society They have become so much a part of our daily life that it appears almost impossible that we could ever do without them The materials made of polymers find multifarious uses and applications in all walks of our society Common examples of these include plastic dishes, cups, non-stick pans, kitchen utensils, plastic pipes and fittings, plastic bags, rain coats, automobile tyres, seat covers, TV, radio, computer, transistor, cabinets, synthetic fibres, flooring materials, materials for biomedical and surgical operations, synthetic glues, telephone, mobile and other electrical components, light elegant plastic luggage, colourful plastic chairs and tables, etc

Polymers are the compounds of light weight, high strength, flexible, chemical resistant with special electrical properties Polymers can be converted into an attractive choice of wide variety of colours, strong solid articles, transparent glass like sheets, flexible rubber-like materials, soft foams, smooth and fine fibres, jelly-like food materials etc Polymers can be used to seal joints, bear loads, fill cavities, jerk resistant in between glasswares, and bond objects Today the polymers are enriching the quality of human life

• MONOMERS AND REPEAT UNITS

A polymer is made up of many small molecules which have combined to form a single large molecule The individual small molecules which constitute the repeating units in a polymer are known as monomers (means, 'single parts') The process by which the monomer molecules are linked to form a big polymer molecule is called 'polymerisation' For example, polyethylene is a polymer which is obtained by the polymerisation of ethylene The ethylene molecules are referred to as monomer units

nCH2 =CH2 ~ (-CH 2-CH 2 -}n

Ethylene Polyethylene

Similarly, Butadiene is a gaseous compound, with a molecular weight of 54 It

combines about 4000 times and forms a polymer known as polybutadiene, with about 2,00,000 molecular weight

n Butadiene ~ Polybutadiene

(4,000 times) (Synthetic rubber)

Polymers are divided into two broad categories depending upon the nature of the repeating units These are:

(1) Homopolymers

(2) Copolymers (mixed polymers)

The polymer formed from one kind of monomers is called homopolymers For example,

polyethylene is an example of homopolymer

The polymer formed from more than one kind of monomer units is called co-polymer or

mixed polymer For example, Buna-S rubber which is formed from 1, 3-butadiene (CH2=CH-CH=CH2) and styrene (C6HSCH=CH2) is an example of co-polymer

Beads of same kind (representing Representing homo-polymer

000000000000 Beads of different kind (representing + =

monomers of more than one chemicals)

Representing co-polymer molecule

As it is stated earlier that polymerisation is possible with molecules of same or of different monomeric compounds When molecules just add and form the polymer, the process is called addition polymerisation In this case the monomer units retain their structural identity when it gets transformed into a polymer For example, the molecule of ethylene monomer can undergo addition polymerisation and form polyethylene, in which the structural identity of ethylene is retained

When the two monomers (of the same or different molecules) link with each other by the elimination of a small molecule, such as water or methyl alcohol, as a by-product to form

a polymer, the process is called condensation polymerisation The condensation takes place between two reactive functional groups, like the carboxyl group (-COOH) of an acid and the hydroxy group (-OH) of an alcohol It is, therefore, observed that in 'addition polymerisation' the molecular weight of the polymer is almost equal to that of all the molecules which combine to form the polymer, while in 'condensation polymerisation' the molecular weight of the polymer is lesser than the weight of the simple molecules eliminated during the condensation process

3 repeat units of

-R-COO-+ 2H20

2 molecules of water eliminated

• DEGREE OF POLYMERISATION

In polymerisation reactions, the polymer molecule formed contains a structural identity, repeating itself several times These repeating entities are called the repeat units of the polymer molecule The size of the polymer molecule is decided by the number of repeat units present in it This number is called the 'degree of polymer is at ion' For example, ~ above case (last page), 3 monomers of ethylene molecule can add onto each other to form a single molecule of polyethylene Here the polyethylene molecule contains 3-CH2-CH2-repeat units, hence, the degree of polymerisation is 3

Similarly, in other case, 3 molecules of a hydroxy acid (HO-R-COOH) undergo condensation polymerisation reaction and form a polyester molecule Here, the 3-repeat units will be -R-COO- Hence, the degree of polymerisation is 3

• CLASSIFICATION OF POLYMERS·

Polymers are classified in a number of ways:

(1) On the basis of source or origin

(2) On the basis of structure

(3) On the basis of mode of synthesiS

(4) On the basis of interparticle forces

(1) Classification of Polymers Based upon Origin or Source

On the basis of origin or source, the polymers are classified into two types:

(a) Natural Polymers (b) Synthetic Polymers

(a) Natural Polymers: The polymers, which are isolated from natural materials, mostly plants and animal sources, are called natural polymers A few examples are: (i) Polysaccharides : Starch and cellulose are very common examples of polysaccharides They are the polymers of glucose Starch is a chief food reserve of plants while cellulose is chief structural material of plants

(m Proteins: These are polymers of a-amino acids They are building blocks of animal cells They constitute indispensable part of our food Natural protein, wool, leather, etc., are proteins

(iii) Nucleic Acids: These are the polymers of various nuc1eotides RNA and DNA are common examples

(iv) Natural Rubber: Substance obtained from latex is known as natural rubber It is

a polymer of 2-methyl-1, 3-butadiene (isoprene)

Biopolymers: It may be noted that polymers like polysaccharides, proteins nucleic acids, etc., which control different life processes in plants and animals are also called biopolymers

(b) Synthetic Polymers: The polymers which are prepared in the laboratory are referred to as synthetic polymers or man-made polymers Some examples of synthetic polymers are polyethylene, polystyrene, teflon, PVC, synthetic rubber, nylon, bakelite, orlon, polyester, terylene etc

(2) Classification of Polymers Based on Structure

This classification of polymers is based upon how the monomeric units are linked

together Based on their structure, the polymers are classified as :

(a) Linear Polymers

(b) Branched Chain Polymers

(c) Cross-linked Polymers or Network Polymers

Interpenetrating random coils

(Solid Polymers-Amorphous)

Folded chains (Solid Polymers-Crystalline)

Spiralled or helical chains (Polypeptides or Proteins)

(a) Linear Polymers: These are the polymers where monomeric units are linked

together to form long straight chains The polymeric chains are

stacked over one another to give a well packed structure As a

result of close packing, such polymers have high densities, high

tensile strength and high melting points Common examples of

these type polymers are polyethylene, polyester and nylon etc Fig 2 Linear chain

monomeric units are linked to constitute long chains, which are

also called main-chain There are side chains of different lengths

which constitute branches Branched chain polymers are Fig 3 Branched chain

irregularly packed and thus, they have low density, lower tensile strength and lower melting points as compared to linear polymers Amylopectin and glycogen are common examples of such type

(c) Cross-Linked Polymers or Network Polymers: In this

type of polymers, the monomeric units are linked together to

constitute a three dimensional network The links involved are called

cross links Cross-linked polymers are hard, rigid and brittle because

of their network structure Common examples of this type of Fig 4 Cross-linked polymers are bakelite, formaldehyde resin, melamine, etc chain

(3) Classification of Polymers Based on Synthesis

On the basis of the mode of synthesiS, the polymers are classified as :

(a) Addition Polymers (b) Condensation Polymers

(a) Addition Polymers: When the monomer units are repeatedly added to form long chains without the elimination of any by-product molecules, the product formed is called addition polymer and the process involved is called addition polymerisation The monomer units are unsaturated compounds and are usually of alkenes The molecular formula and hence the molecular mass of the addition polymer is an integral multiple of that of the monomer units A few examples of addition polymerisation are:

Some other examples of condensation polymers are:

Dacron or Terylene (Polyester>: It is a polyester fibre, made by the esterification of terephthalic acid with ethylene glycol In England, it is known as terylene, whereas in U.S.A

Bakelite: It is a polymer of phenol and formaldehyde

• CHAIN GROWTH AND STEP GROWTH POLYMERS

Many times, it becomes difficult to find out whether the polymerisaiton has occurred through condensation or through addition Therefore, a more rational classification has recently been proposed according to the mechanism of combination of monomer units According to this system, there are two types of polymers observed which are :

(a) Chain Growth Polymers (b) Step Growth Polymers

(a) Chain Growth Polymers: Chain growth polymerisation is a process of successive addition of monomer units to the growing chain by a chain mechanism The monomer unit gets converted to some active intermediate species by a small amount of initiator such as organic peroxide or an acid or a base Depending upon the conditions, the intermediate species may be free radical or an ion, and it reacts with other monomer unit to form still bigger intermediate species The monomer units are, thus, successively added to intermediate species by a chain process The chain growth polymerisation of ethene involving free radical initiation is given below:

Monomer Bigger intermediate species

A chain propagation is, thus, set up which results in the growth of the chain by repeated addition of monomer units The polymers formed by chain growth polymerisation are called chain growth polymers Addition polymers are generally formed by this process

Some examples of chain growth polyiners are:

Polyethylene, polyisoprene; polypropylene, teflon, etc

(b) Step-Growth Polymers: As the name suggests, the step growth polymerisation involves stepwise intermolecular condensation, taking place through a series of independent reactions Each reaction involves a condensation process involving the loss of a simple molecule like H20, NH3, HeI, ROH etc This type of polyinerisation occurs if the monomer molecules have more than one similar or dissimilar functional groups The step growth polyinerisation starting with two monomers A and B as :

Condense

Step 1 A-B

The polymers formed by step growth polymerisation are called step growth polyiners The condensation polyiners like nylon, bakelite, darron are formed by this type of pol ymerisa tion

(1) Classification of Polymers Based on Inter Particle Forces

The mechanical properties of polymers such as elasticity, tensile strength, toughness, etc., depend upon intermolecular forces like Vanderwaal's force and hydrogen bonds existing in the macromolecules Although these intermolecular forces are found in simple molecules also, but their effect is less significant in them as compared to that in macromolecules It is because of the fact, that in polymers there is combined effect of these forces all along the long chains ObViously, longer the chain, more intense is the effect of intermolecular forces

On the basis of the magnitude of intermolecular forces, the polymers have been classified into the following four categories:

r

The weak forces permit the polymer to be stretched out about ten times their normal length but they return to their original position when the stretching forces is withdrawn In fact, these polymers consist of randomly coiled molecular chains having few cross links When the stress is applied, these randomly cross chains straighten out and the polymer gets stretched As soon as the stretching force is released, the polymer regain the original shape because weak forces do not allow the polymer to remain in the stretched form

The best known elastomer is rubber, whether synthetic or natural The elasticity of such polymers can be further modified by introducing few cross-links between the chains For example, natural rubber, a gummy material, has a poor elasticity It is a polymer of isoprene

Sulphur crosslink (Unstretched)

(b) Fibers: These are the polymers which have quite strong interparticle forces such

as Hydrogen-bonds They have high modulus and high tensile strength These are thread-like polymers and can be woven into fabrics Silk, terylene, nylon, etc., are some common examples of such types of polymers The H-bonds in nylon-66 are shown below:

Plasticizers: Some plastics do not soften to workable extent on heating Such plastics easily softened by the addition of some organic compounds which are known as plasticizers For example PVc, i.e., poly vinyl chloride is extremely stiff even while hot However, addition of di-n-butylphthalate, a plasticizer, makes it soft and workable

(d) Thennosetting Polymers: These are the polymers which become hard and infusible on heating They are normally made from semi-fluid substances with low molecular masses, by heating in mould Heating results in excessive cross-linking between the chains forming three dimensional network of bonds as a consequence of which a

• NOMENCLATURE OF POLYMERS

Standardized Polymer Nomenclature

There are multiple conventions for naming polymer substances Many commonly used polymers, such as those found in consumer products, are referred to by a common or trivial name The trivial name is assigned based on historical precedent or popular usage rather than a standardized naming convention Both the American Chemical Society and IUPAC have proposed standardized naming conventions; the ACS and IUPAC conventions are similar but not identicaL Examples of the difference between the various naming conventions are given in the table below:

Common Name ACS Name IUPAC Name Poly (ethylene oxide) or (PEO) poly(oxyethylene) poly(oxyethylene)

Poly (ethylene terephthalate) poly (oxy-l, poly

or (PET) 2-ethanediyloxycarbonyl-l, ( oxyethyleneoxyterephth=aloyl)

4-phenylenecarbonyl) Nylon poly[imino(l-oxo-l, 6-hexanediyl)] poly[imino(l-oxohexane-l, 6-diyl)]

For naming a polymer, a wide variety of trade names are commonly used which are based on its source In addition (polymers, the prefix poly is attached to the name of the monomer, and so polyethylene, polystyrene, polyacrylonitrile denote polymers prepared from these single monomers

When the monomer has a substituted parent name or has a multi-worded name then this is enclosed in parentheses and prefixed with poly, e.g., poly (methylmethacrylate), poly(ethylene oxide), poly (vinyl chloride), etc

H the polymer is prepared by self-condensation of a single monomer such as ro-amino lauric acid, are named in a similar way, but this polymer, poly (ro-amino lauric acid), can also

be prepared by a ring-opening reaction using lauryllactum and then be called poly (lauryl lactam) Both the names are correct

IUP AC has suggested the nomenclature of single-stranded, regular, organic polymers and has proposed the procedures as follows:

(1) In first step, select a constitutional repeat unit, CRU, which may consist of one or more subunits The name of the polymer is then the name of the CRU, in parentheses prefixed by poly

(2) Before naming CRU, it must be orientated correctly This involves placing the constituent parts in order of seniority with the highest to the left In descending order this would be heterocyclic rings, chains with hetero atoms, carbocyclic rings and chains with only carbon atoms, if such an order is possible chemically Thus

+O-CH2 -CH2 + would be poly (oxyethylene) rather t!tan

+CH2 -CH2 - 0 + poly (ethylene oxy)

(3) If there is a substituent on part of the CRU, then orientation will place the

substituent closest to the left of the substituted portion, and thus

+ 0 - CH- CH2+, poly (oxy I-methyl ethylene) is preferred, rather than

I CH3 +0-CH 2-CH+-

I CH3 The nomenclature of some common polymers is given below:

2-;-I CH3

Polyhexamethylene adipamide

Polyethylene Polyisoprene

•

-Poly (l-methoxy carbonyl)

I-methyl-ethylene)

CH3

I +C-CH2~

I COOCH3 Poly (l-methoxcarbonyl) ethylene +CH-CH2-t.;

Polyethylene terephthalate Poly(vinyl butyryl)

Polymerisation is a process which allows simple low molecular weight compounds to combine and forms a complex high molecular weight compound For this process, each molecule of the compound should have the capability to react at least with two other molecules of the same or some other compound In other words, they should have a functionality of at least two

The functionality of a compound depends on the number of its reactive sites Due to the presence of the reactive functional groups, a compound assumes its functionality These groups are such as -OH, -COOH, -NH2,-SH,-NCO, etc The number of these functional groups per molecule of the compound defines its functionality

As described earlier, the polymerisation is effected by two processes, i.e., addition and condensation polymerisation These terms were based on the conventional classification by

Carothers (1929) and have since been modified by H.F Mark (1950) as chain polymerisation and step polymerisation

(1) ADDITION (CHAIN) POLYMERISATION

This type of polymerisation is characterised by a self-addition of the monomer molecules, rapidly through a chain reaction The product has the same composition as that

of the monomer molecules In this reaction no byproduct is formed Since the compounds containing reactive double bonds, therefore, can proceed by a chain reaction mechanism

Typical examples are :

(a) Olefines (CH2=CHR)

Ethylene Propylene Isobutylene (b) Dienes (CH2=CR-CH=CH2)

Isoprene

1, 3-Butadiene Chloroprene

CH2=C-CH3- CH=CH2 CH2=CH-CH=CH2 CH2-CH-CCl=CH2

CH2=CHCONH2 CH2=CHCN CH2=CHCOOH CH2=CHC6HS

CF2=CF2

CH2=CHCI CH2=CCl2

Addition polymerisation reactions consist of three important steps : (i) initiation (ii)

propagation and (iii) termination The entire process can be brought about by a free radical,

ionic (carbonium ion or carbanion) or coordination mechanism Depending on the

mechanism, we will discuss here three types of addition chain polymerisation :

(A) Free-Radical Addition Polymerisation

The polymer chain is initiated by free radicals produced by the decomposition of

compounds called initiators:

Initiators : The initiators are thermally unstable compounds When energy is

supplied, they decompose into two identical fragments by 'homolytic decomposition' Each

fragment carries one unpaired (lone) electron with it; and called free radicals

If R-R is an initiator, it may split into two symmetrical components at its bond

between the two R-R as :

I R+R~ R + R

: Free radicals Initiator

The low molecular weight compounds mainly azo, peracids, peroxides, peresters,

hydroperoxides are useful as initiators Initiators can be decomposed and form free radicals

,

-whiie induced by heat energy, light energy or catalysts The initiators can also be decomposed by using ultraviolet light and form the same free radicals as those formed by its thermal decomposition Free-radicals can be also produced by direct excitation of the monomer molecules in UV light as :

Azo bis isobutyro nitrile Free radicals

The initiators decomposed into free-radicals by thermal energy can be illustrated by the following examples:

The rate of decomposition of these initiators depends on the reaction temperature, solvents used, and the intensity and wavelength of the radiations

Initiation: A free-radical contains an unpaired electron; which always search a lone electron to couple with and get stabilised itself Free-radical is, therefore, highly reactive species which attacks the double bond in the monomer molecule in such a manner:

A double bond formed between two carbon atoms, C=C, one pair of electrons is called

dose to nucleus, therefore, they are susceptible to attack by other reactive species By the homolytic decomposition of the initiator free-radicals are produced, which combine with one

of the n electrons and forms a normal pair of electrons at the sigma level, and the other electron of the n pair is transferred to the other end of the molecule as given below:

R·~ CH2 • • JcH ~ R: -CH2-CH or R-CH2-CH

Now, the monomer unit is linked to the free-radical unit through a sigma bond forming

a single molecule The other electron of the original n electron pair becomes unpaired and

deprived of a new partner Thus, this whole sequence, in which one free-radical attacks a monomer molecule and adds with it, simultaneously transfers the free-radical site from itself

to the monomer unit is called the initiation step

• PROPAGATION

After initiation step, the propagation step get started, where the free-radical site at the first monomer unit attacks the double bond of a fresh monomer unit Thus, the second monomer molecule links with the first and the transfer of the radical site from the first monomer molecule to the second takes place by the unpaired electron transfer process as shown below:

•

R- [CH2 -~ ]-CH2-~

X n X

where n denotes the monomer molecules added in the chain growth In the growing chain,

the mode of the addition of monomer molecule can be of the head-to-tail, head-to-head, tail-to-head or tail-to-tail type Suppose, the -CH2 - is the head and -CHX-is tail part of

a monomer unit, the four types of propagation can be shown as :

by coupling' because this process involves the coupling of the one lone pair of electrons

In the reaction (2), one H-atom from one chain is abstracted by the other chain, and utilised unpaired electron, whereas the chain which has donated the H-atom, gets stabilised and form a double bond In this reaction, two polymer molecules are formed Both the molecules consist of shorter chain length as against a single molecule of a longer chain length Such type of termination is known as 'termination by disproportionation.'

Thus, in above cases, the product molecule is formed known as polymer This product does not consist of any free-radical site and, hence, cannot grow any further The process of termination thus causes in deactivation of growing chain The groups which are present at the end of the chain are called the 'end groups' For example, in above cases, these groups are -R, and -CH2=CHX

• CHAIN TRANSFER

There is another method of chain termination, which takes place by the 'transfer reaction' In this reaction, the growth of one polymer chain is deactivated and gets stopped, meanwhile, there is a generation of a new free-radical which starts a new polymer chain growth The transfer reaction occurs by the abstraction of a H-atom or some other atom present in the system This process follows as:

• INHIBITORS

In the polymer industry, several inhibitors are used which are capable of inhibiting or killing the chain growth by uniting with the active free-radicals and producing inactive free-radicals A few examples of these inhibitors are nitrobenzene, hydroquinone, benzothiazine, dinitrobenzene, etc The inhibiting action of these chemical substances can be represented as :

(Inhibitor) stabilised end-group

•

In this reaction the inhibitor (nitrobenzene) adds on the growing chain C and forms a polymer chain with nitrobenzene resonance stability end group and carrying a radical site The free-radical nature of the end group is strong enough to reunite with the radical of another growing chain and terminate the growth of it as follows:

In this case, it is observed that a single inhibitor molecule has killed two growing chains

The atmospheric oxygen is a good inhibitor, therefore, the free-radical polymerisation

is generally- carried out under an atmosphere of nitrogen Due to the biradical nature of oxygen atom, the inhibiting action takes place in a powerful way as represented below:

The ionic polymerisation is divided into two categories:

(i) Cationic polymerisation

(i) Cationic Polymerisation

In this type of polymerisation, initiators and monomers are used during the chain growth; however, the initiation is done by a proton and the propagation carried out by a carbonium ion

In cationic polymerisation, the initiators used are strong Lewis acids such as BF3, SnCl4, TiCl4, AlCl3 and are called 'catalysts'

The mCimomers, which can undergo for this purpose are styrene, methyl styrene, many vinyl ethers and isobutylene

The mechanism of this polymerisation involves an attack on the 1t electron pair of the monomer molecule In the chain reaction, first a proton is introduced into a monomer The proton attracts the 1t electron pair towards it and a positive charge of the proton is transferred to the end of the monomer molecule; thus forms a carbonium ion In this reaction, a sigma bond is formed between the proton and monomer molecule and the polymer chain growth started This is an 'initiation' process; where the carbonium ion attacks on the 1t electron pair of the second monomer molecule and attracts it over The positive charge is transferred to the far end of the second monomer molecule Thus a chain reaction begins, where only a displacement of the electron pair and the formation of a carbonium ion takes place The whole process can be represented in the following manner:

+~

H ~+ CH27 CH - + Proton", -", I

The above process occurs in the presence of a 'catalyst' (e.g., BF3) and a 'co-catalyst' (e.g., water or methanol), and form hydrates as follows:

]-F-B

F

1 , +H-O-H ~ F-BF',-OH - - - - H+

Carbonium ion (Cation) Counter-ion

CH3-CH-CH2-CH-CH2-CH+ <J

Since, the addition of monomer units are increased, the chain keeps on growing, the 1t

electron pairs of the monomer molecules are pulled in opposite direction to the growth of the chain

After propagation process, termination starts when a collision between carbonium ion and an anion takes place Termination occurs by following two steps:

(r.) The termination process causes the arrest of the chain growth, where donation of a proton to the counter-ion resulting in the formation of a double bond ~t the end of the growing polymer molecule:

[BF30Hr

CH3-CH rv"V'V' CH2-CH+ • CH3-CH rv"V'V' CH==CH + [BF30Hr H+

In this process, a proton is donated and BF3 hydrate is re-formed It is called 'ion-pair precipitation'

(b) In this process, termination occurs by simple 'coupling', when a covalent bond is

formed between carbonium ion (C+) and the counter-ion It is represented as follows :

+

CH3-CH ~ CH2-CH[BF30Hr - - +

(ii) Anionic Polymerisation

In anionic polymerisation, a negatively charged ion attacks on 1t electron pair of the monomer molecule and pushing it as far away as possible, i.e., to the end of the molecule Simultaneously it forms a sigma bond with the monomer unit

At the same time, a carbanion is also formed This is represented as follows:

RQH2-RH + R-CH2-CH

Monomer molecule

Carbanion

Now the propagation process starts It is initiated by newly formed carbanion, which attacks on the second monomer unit Thus the 1t electron pair pushed away to the end of the molecule Again a new sigma bond is formed between the carbanion and monomer unit In this electron pair displacement process, the negatively charged ion pushes the 1t electron pair

of the monomer double bond down to sigma electron level

In termination process, some strong ionic substances are added deliberately In ionic polymerisation, termination is not usually a spontaneous process If some substances are not added, the reaction proceeds till all the monomer molecule is consumed Thus, if there is no more monomer is left for polymerisation, the carbanions at the chain ends remain active which may initiates the polymerisation when a fresh monomer is added Studies have shown that by adding a fresh quantity of monomers, the anionic polymerisation can be restarted, even after weeks The polymers formed by this manner are called 'living polymers' and such technique is known as 'living polymerisation technique' Example of this type of polymer is 'block copolymer'

The anionic polymerisation is represented as :

The monomer molecules, which are used to undergo anionic polymerisation include styrene, butadiene, acrylonitrile and isoprene

The fundamental difference between cationic and anionic polymerisation is that cationic process consists of the movement of the nelectron pair in a opposite direction to that

of the chain growth, while in latter case, it is in the same direction as that of the chain growth

(C) Coordination Polymerisation

In coordination polymerisation reactions the monomer molecules used are generally dienes and olefines These reactions are catalysed by organo-metallic compounds In such polymerisation, a monomer-catalyst complex is formed between the monomer and organometallic compound A coordination bond is used between a carbon atom of the monomer and the metal atom of the catalyst, thus formation of monomer-catalyst complex takes place The polymerisation process proceeds as follows:

where, M = Transition metals such as Ni, Cr, Mo, V, Ti or Rh

A coordinated metal-carbon bond is formed in the monomer-catalyst complex This is known as the active centre The propagation process starts at the active centre site where the chaIn growth begins as :

Here, M=metal ion, and Ml,M2,M3,M4, etc., are the first, second, third and fourth monomer units which are added to the polymer growing chain

In this polymerisation, the metal counter-ion is placed in a specific special arrangement with respect to the anion In some catalyst systems, the spatial arrangement has a large effect

on spatial orientation of the incoming monomer and also on the manner in which the monomer is inserted into the growing chain and this imparts stereo-regularity to the polymer formed A highly stereo-regular polymer can be formed by using a proper catalyst (e.g., Ziegler-Natta catalyst) and solvent system

Thus, the coordination polymerisation is characterised by the initiation, propagation and termination reactions as given below:

Here, M = Transition metals [Termination by spontaneous internal transfer]

(2) Condensation (Step) Polymerlsation

In condensation or step polymerisation reaction, the reaction takes place in a step-wise manner and the polymer is formed through a reaction between functional groups of the