Polymer chemistry book

POLYMER CHEMISTRY
Alka L. Gupta

PRAGATI PUBLICATIONS


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Revised Edition: 2010
ISBN No. : 978-81-8398-998-5


IINTRODUCTION

1-10

BASICS
Importance of Polymers 11

Monomers and Repeat units 12
Degree of Polymerisation 14
Classification of Polymers 14
Linear Polymers 15
Branched Polymers 15
Cross-linked or Network Polymers 16
Addition Polymers 16
Condensation Polymers 16
Chain-Growth Polymers 17
Step-Growth Polymers 18
Elastomers 18
Fibres 20
Thermoplastics 20
Thermosetting Polymers 20
Nomenclature of Polymers 21
Polymerisation 23
Addition (chain) Polymerisation 23
Free-radical Addition Polymerisation 24
Ionic-Polymerisation 29
Cationic Polymerisation 29
Anionic Polymerisation 31
Coordination Polymerisation 32
Condensation (Step) Polymerisation 33
Polycondensation Polymerisation 34
Polyaddition Polymerisation 35
Ring-opening Polymerisation 36
Copolymerisation 37
Free-radical Copolymerisation 38
Monomer Reactivity Ratios 40
Reactivity-ratios and Copolymerisation Behaviour

Ionic Copolymerisation 46
Copolycondensation 47
Types of Copolymers 48
Polymer Reactions 50
Hydrolysis 50
Acidolysis 51
Aminolysis 51
Hydrogenation 51
Addition reactions 52
Substitution reactions 53
Reaction of Ketonic Groups 54
Reactions of Carboxylic Groups 54
Reactions of Aldehyde Groups 54

11-76

43


(vi)

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
Homogeneous System 61
Heterogeneous System 63
Suspension Polymerisation 63
Emulsion Polymerisation 63
Interfacial Polycondensation Polymerisation 65
Solid and Gas Phase Polymerisation 66
Miscellaneous Polymerisation . 67
Group Transfer Polymerisation 67
Metathetical Polymerisation 68
Electrochemical Polymerisation 69
Ziegler-Natta Polymerisation 71

fl POLYMER CHARACTERIZATION
Average Molecular Weight Concepts 77
Number-Average Concept 78
Weight-Average Concept 79
Viscosity-Average Molecular Weight 80
Polydispersity and Molecular Weight Distribution 81
The Practical Significance of Molecular Weight 82
Measurement of Molecular Weights 84
End-group Analysis 84
Viscometry 85
Light Scattering Method 88
Osmometry 89
Ultracentrifugation Method 94
Chemical Analysis of Polymers 97

Mass Spectrometry 98
Gas Chromatography 98
Spectroscopic Methods 98
1R 98
NMR 99
EPR 100
X-ray diffraction 100
Microscopy 101
Thermal Analysis 101
Differential Calorimetric ~alysis 102
Thermal Gravimetric Analysis 102
Physical Testing 102
Tensile Strength 102
Fatigue 103

61

77-103


(vii)
Impact 103
Teat Resistance 103
Harlmess 103
Abtasion Resistance 103

"

I

STRUCTURE AND PROPERTIES

104-139

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
128
Relationship between Tg and Tm
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

EM POLYMER PROCESSING
Plastics 140
Thermosetting Plastics 142
Elastomers 143
Fibres 145
Compounding 148
Processing Techniques 148
Calendering 149
Oiecasting 150
Rotational Casting 150
Film Casting 151
Injection Moulding 151
Blow Moulding 153
Extrusion Moulding 154
Compression Moulding 155
Thermoforming 155
Foaming 156
Reinforcing 157
Fiber Spinning 161

140-1641


~

"i

~ .

(viii)

165-2341

PROPERTIES OF COMMERCIAL POLYMERS
Polyethylene 165
Polyvinyl chloride 169
Polyarnides 171
Polyesters 173
'phenolic Resins 175
Epoxy ~esins 178
Silicone Polymers 179
Electrically Conducting Polymers
Functional Polymers 183
Fire-Retardmg Polymers 184
Biomedical Polymers 188
Contact Lens 196
Dental Polymers 206
Artificial Heart 218
Artificial Kidney 220
Artificial Skin 225

Artificial Blood 230

180

Ii POLYMER ADDITIVES

235-2431

Types of Fillers 236
Miscellaneous Mineral Fillers ' 237
Plasticizers 238
Properties of Plasticizers 238
Importamt Plasticizers 238
Antioxidants 239
UV-Stabilizers and Absorbers 242
Fire Retardants 243
Colourants 243

I' NATURAL POLYMERS

244-2531

Polysaccharides and Lignin 244
Reactions of Cellulose 245
Starch 246
Lignin 247
Glycogen 248
Proteins 248
Nucleic Acids 250
Conformation of the Nucleic Acids 251

Segments of RNA and DNA Polymers 252

IAPPENDICES
Appendix 1: Polymer Degradation 254
Appendix 2: Photodegradation of Polymers 255
.
Appendix 3: Current and Promising Polymer Research Topics

ISUGESTED READINGS
ISUBJECT INDEX

254-2571
'(

256

258-260
(i)-(iv)

I,·
I


INTRODUCTION
PROLOGUE
• Historical Background
• Basic Tenns and Definitions
• Polymer
• Polymer Synthesis
• Polymer Structure

• Chain Linearity

Polymers science and engineering deal with the chemistry, molecular stru~ture, physical
properties, the applications and processing in the useful forms and the biological
significance of materials. It is the chemistry of large molecules-macromolecules each
containing from thousands to millions of atoms. The atoms are typically linked in a sequence
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
(1)


2•

POLYMER CHEMISTRY

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

I

1
I


INTRODUCTION.

"'"

3

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


4 • POLYMER CHEMISTRY
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.

l


INTRODUCTION •

5

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 (Tg): 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.


6 •

POLYMER CHEMISTRY

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.

..

INTRODUCTION •

7

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.

• POLYMER
A polymer is a large molecule (macromolecule) composed of repeating structural
units connected by covalent chemical bonds. The word is derived from the Greek word
1tOAU (poly), meaning "many"; and Il&POS (meros), meaning "part". Well known examples of
polymers include plastics, DNA and proteins. A simple example is polypropylene whose
repeating unit structure is shown below.


8 •

POLYMER CHEMISTRY

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
Polypropylene
play crucial roles in biological processes. A
variety of other natural polymers exist, such as
CH3

CH3
CH3
cellulose, which is the main constituent of
I
I
I
-C-CH2 -C-CH2 -C-CH2 wood and paper. Some common synthetic
I
I
I
polymers are Bakelite, neoprene, nylon, PVC
H
H
H
(polyvinyl chloride), polystyrene polyacryonitrile and PVB (polyvinyl butyral).
IUPAC
Poly(l-methylethylene)
Polymers are studied in the fields of polymer
name
chemistry, polymer physics, and polymer
science.
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,


INTRODUCTION •

9

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


10 •

POLYMER CHEMISTRY

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 Flg-1 Appearance of real
linear polymer chains as

chains have linked into one molecule.

Chain Length

recorded
using
an
atomic force microscope
on surface under liquid
medium. Chain contour
length for this polymer
Is -204 nm, thickness Is
-0.4 nm

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
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.

•••


BASICS
•
•
•
•
•
•
•
•
•
•

PROLOGUE
Importance of Polymers
Monomers and Repeat Units
Degree of Polymerisation
Classification of Polymers
Polymerisation

Co-polymerisation
Types of Co-polymers
Polymer Reactions
Polymersation in Homogeneous and
Heterogeneous systems
Miscellaneous Polymerisation

• IMPORTANCE OF POLYMERS
The polymers are complex, giant, high molecular weight macromolecules formed by
the combination of a large number of one or more type of small molecules of low molecular
weight. The word polymer implies 'many part' (Greek: poly means 'many' and mer means
'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.
(11)


12 •

POLYMER CHEMISTRY

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:


BASICS.

13

(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.

00000000000

----+

Beads of same kind (representing
monomers of same chemical)

Representing homo-polymer
molecule

000000000000
Beads of different kind (representing

=

----+

Representing co-polymer
molecule

monomers of more than one chemicals)

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.
Addition

I

I

3CH2=CH2 -----)-) -CH 2-CH 2+CH2-CH2+CH2-CH 2Ethylene
monomer

Polymerisation

POlyethy\ene molecule coritaining
3 repeat units of -CH2-CH2-

or

-{CH2-CH2~

Condensation

I

I

I

3HO-R-COOH ---~) HO-R-COO-l-R-COO-!-R-COO-!-H
Hydroxy acid
monomer

Polymerisation

: : :

or
HO-[R-C00h-H
Polyester molecule containing
3 repeat units of -R-COO-

+

2H20
2 molecules of water eliminated


14 •

POLYMER CHEMISTRY

• 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 polymerisation'. 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.


BASICS.

15

(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)

~#
(Linear)
(Branched)
Isolated random coils
(Dilute Polymer Solution)

Interconnected network structure
(Cross-Linked Polymers)

Fig. 1. Representative conformations of polymer molecules under different situations.

(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
Fig. 2. Linear chain.
these type polymers are polyethylene, polyester and nylon etc.
(b) Branched Chain Polymers: In this type of polymers, the ~
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.


16 •

POLYMER CHEMISTRY

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:
(i) nCH2=CH2 ---+ (-CH2-CH2-)n
Ethylene

Polyethylene or Polyethene

(ii) nCH2=CH ---+ (-CH2-CH-)n

I
CI

I
CI

Vinyl chloride

(iii) nCH2 =CH

I
CH3

Polyvinyl chloride(PVC)

---+ (-CH2- CH-)n
I
CH 3

Propylene

Polypropylene

(iv) nCH2 =CH ---+ (-CH2 - CH)n
I
I
CN
CN

Acrylonitrile

Orlon

(b) Condensation Polymers: In this type of polymers, the monomers react together
with the elimination of a simple molecule like H20, NH3 or ROH, etc. The reaction is called
cond.ensation and the product formed is called condensation polymer. As the process
involves the elimination of by-product molecules, the molecular mass of the polymer is not
the integral multiple of the monomer units. For example, nylon-66 is a condensation
polymer of hexamethylene diamine and adipic acid.
nH2N-{CH2)6-NH2 + nHOOC-{CH 2)4-COOH---+
Hexamethylene diamine

Adipic Acid

H

H

I

I

(-N - (CH2)6 - N - C - (CH2)4 - C-)n

II
o

Nylon-66

II

0

+ 2nH20


BASICS.

17

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.
it is called decron.
CH300C-Q-' COOCH3 + 2HOCH2-CH20H
Dimethyl terephthalate

Ethylene glycol

f

~

OCH2-CH2-00C-Q-' coln + OHCH2-CH20H

I

Terylene

Ethylene glycol

Bakelite: It is a polymer of phenol and formaldehyde.

OH

OH

OH

OH

OH

n~
~CH,+CH,+CH'~
lSJ +3/4 nHCHO- lSJ
lSJ
lSJ
lSJ +nH,O
Formaldehyde

Phenol

Bakelite

• 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:
(i)

Initiator-~~

A

Free radical

•

•

(ii) A + CH2=CH2 ~ A - CH2 - C H2
Monomer

Intermediate species

•

(iii)A-CH2-CH2 + CH2=CH2
Monomer

~

•

A-CH2-CH2-CH2-CH2
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.


18 •

POLYMER CHEMISTRY

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 :

A+B
Monomer

A-B+A

Condense

) A-B

Step 1
Condense

) A-B-A

Step 2

A-B-A+B

--~

Step 3

A-B-A-B

The stepwise process of chain growth thus goes on. This process can also occur in
another way :
Condense

A+B

) A-B
Step 1

A-B

A-B

) A-B-A-B..... {A-B)n
Step 2

Polymer

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
polymerisa 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:
(a) Elastomers
(b) Fibers
(c) Thermoplastics
(d) Thermosetting Polymers
(a) Elastomers: These are the polymers in which the polymer chains are held up by
weakest attractive forces. They are amorphous polymers having high degree of elasticity.