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Unit-I

POLYMER CHEMISTRY

1. Introduction

Polymers are macromolecules having high molecular weight. The macromolecule of polymers

are either biological on non-biological orgin. Biological polymers govern and control life; synthetic or

non-biological polymers such as plastics, fibres, elastromers are widely used in industry and everyday

life. Twentieth century is called as „‟Polymer Age” since many of the conventional materials made of

wood, iron and rubber are irrevocably replaced by polymers. In Greek „Poly‟ means many and „mer‟ unit

or part.

1.1 Terms used in polymer chemistry

Few terms are repeatedly used in polymer sciences which have got significance and specific

meaning. Some of them are,

1.1.1 Monomers and Polymers

Polymer is defined as a high molecular weight compound formed by the combination of small

repetitive units called monomers.

A monomer is a small chemical entity, which forms the backbone of the polymer. The following

table illustrates some monomers and repeating units of the polymer.

Table 1.1 Monomers, repeating units, Properties and applications of the polymers

Name of the

polymer Formula Monomer Properties Application Area

Polyethylene low density

(LDPE)

–(CH2-CH2)n– Ethylene CH2=CH2 soft, waxy

solid

Film wrap, plastic

bags

Polyethylene high density

(HDPE)

–(CH2-CH2)n– ethylene

CH2=CH2

rigid,

translucent

solid

Electrical

insulation

bottles, toys

Polypropylene (PP) different

grades

–[CH2-CH(CH3)]n– propylene

CH2=CHCH3

atactic: soft,

elastic solid

isotactic:

hard, strong

solid

Similar to LDPE

carpet, upholstery

2

Poly(ethylene

terephthlate)

(PET)

n

ethylene

CH2=CH2 & Terephthlic

acid

Transparent ,

solid

Bottles for soft

Drinks and other

Beverages.

Poly(vinyl

chloride) (PVC)

–(CH2-CHCl)n– vinyl chloride

CH2=CHCl

strong rigid

solid

Pipes, siding,

flooring

Poly(vinylidene

chloride) (Saran A)

–(CH2-CCl2)n– vinylidene chloride

CH2=CCl2

dense, high-

melting solid Seat covers, films

Polystyrene (PS)

–[CH2-CH(C6H5)]n– styrene

CH2=CHC6H5

hard, rigid,

clear solid

soluble in

organic

solvents

Toys, cabinets

packaging

(foamed)

Polyacrylonitrile (PAN, Orlon,

Acrilan)

–(CH2-CHCN)n– acrylonitrile

CH2=CHCN

high-melting

solid

soluble in

organic

solvents

Rugs, blankets

clothing

Polytetrafluoroe

thylene (PTFE, Teflon)

–(CF2-CF2)n– tetrafluoroethylene

CF2=CF2

resistant,

smooth solid

Non-stick surfaces

electrical insulation

Poly(methyl

methacrylate) (PMMA, Lucite,

Plexiglas)

–[CH2-C(CH3)CO2CH3]n– methyl methacrylate

CH2=C(CH3)CO2CH3

hard,

transparent

solid

Lighting covers,

signs skylights

Poly(vinyl

acetate) (PVAc)

–(CH2-CHOCOCH3)n– vinyl acetate

CH2=CHOCOCH3

soft, sticky

solid

Latex paints,

adhesives

cis-Polyisoprene natural rubber

–[CH2-CH=C(CH3)-

CH2]n–

isoprene

CH2=CH-C(CH3)=CH2

soft, sticky

solid

Requires

vulcanization

for practical use

Polychloroprene (cis + trans)

(Neoprene)

–[CH2-CH=CCl-CH2]n– chloroprene

CH2=CH-CCl=CH2

tough,

rubbery solid

Synthetic rubber

oil resistant

Polyamides(Nylo

n)

Tough, hard

engineering

plastic

Fibers, molded

objects

Polyesters

(Dacron, Mylar,

Fortrel)

Linear polyesters,

fibers, recording

tape

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Polyesters

(Casting resin)

Cross-linked with

styrene and

benzoylperoxide,

fiberglass boat

resin, casting resin

Phenol-

formaldehyde

(Bakelite)

Mixed with fillers,

Molded electrical

cases, adhesives,

laminates, varnishes

Cellulose acetate

(cellulose is a

polymer of

glucose)

Photographic film

Silicones

Water-repellent

coatings,

Temperature-

resistant

Fluids and rubber

1.7. CLASSIFICATION OF POLYMERS

Based on the source, polymers are broadly classified into two types natural polymers and synthetic

polymers.

Chart 1.1. Classification of Polymers

1.7.1 Natural Polymers

1.7.1.1 Natural Organic Polymers

Polymer

s

Natural

Artificial

Organic

Inorganic Inorganic Organic

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The polymers obtained from nature are called natural polymers. Examples of natural polymers

are starch, cellulose, proteins, nucleic acids, natural rubber, jute fiber etc., Starch is a polymer of

glucose. It is a chief food reserve of plants. Cellulose is also a polymer of glucose. It is a chief structural

material of the plants, both starch and cellulose are produced by plant photosynthesis. Proteins are

polymers of α-amino acids. They have generally 20 to 100 α-amino acids joined together in a highly

organized arrangement. These are building block of animals. Nucleic acids are polymers of various

nucleotides.RNA and DNA are common nucleotides. Natural rubber is a polymer of unsaturated

hydrocarbon, 2-methyl-1,3-butadiene, called isoprene. It is obtained from latex of rubber trees.

nCH2=C-CH=CH2 -------CH2-C=CH-CH2----

׀ ׀

CH3 CH3

Isoprene ׀ polyisoprene (natural rubber)

1.7.1.2 Natural Inorganic Polymers

These are polymers containing no carbon atoms. the chains of these polymers are composed of

different atoms joined by chemical bonds. Examples are poly silicon dioxide and Poly phosphoric acid.

1.7.2 Synthetic polymers.

The polymers which are prepared in the laboratories are called synthetic polymers. These are

also called man-made polymers. A few examples are PVC, polyethene, nylon, Teflon, Teflon,

Bakelite, Terylene, etc.,

Type of synthetic polymers

(i) Organic polymers

These are polymers containing hydrogen, oxygen, nitrogen, sulphur and halogen atoms.

Polyethylene, Polyvinylalcohol, PVC, Epoxy polymers, polyurethane are a few illustrative examples.

(ii) Elemento-organic (or) hetero-organic polymers

These are polymers composed of carbon atoms and hetero-atoms (like N,S & O).The main chain

consists of carbon atoms and whose side groups contain hetero atoms linked directly to the C atoms in

the chain. For example Polysiloxanes and polytitoxanes.

Classification based on molecular weight

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Polymers classified in two types by its molecular weight or degree of polymerization. Polymers

with low degree of polymerization are known as oligomers, there molecular weight ranges from 500-

5000. Polymers with high degree of polymerization are known as high polymers, their molecular weight

ranges from 10,000- 2, 00,000.

1.6 FUNCTIOALITY AND ITS SIGNIFICANCE

The number of bonding sites or reactive sites or functional groups, present in a monomer is

known as its functionality.

Table 1.2 Functionality of monomers

S.No. Example Functionality

1

CH2= CH2

(Ethylene)

2(Two Bonding sites are due to the presence of one

double bond in the monomer. Therefore ethylene is a bi

functional monomer).

2 H2N-(CH2)6-NH2

Hexa methylene di amine

2 (this monomer contains two functional groups; hence

it is a bi functional monomer).

3 CH2-OH

CH2-OH

CH2-OH

(Glycerol)

3 (this monomer contains three functional groups; hence

it is a tri functional monomer).

1.6.1.SIGNIFICANCE OF FUNCTIONALITY

1.6.1.1. Bi functional monomers

Bi functional monomers (i.e., functionality of the monomer is 2) mainly form linear (or) straight

chain polymer. Each monomeric unit in the linear chain is linked by strong covalent bonds (primary

bonds), but the different chains are held together by weak Van der wall‟s forces of attraction (secondary

bonds). Therefore there is no restriction to movement of one chain over another. These types of

polymers are soft and flexible. These are soluble in organic solvents

× ×

× × × ×

× ×

× ×

××××

× ×

× ×

×

× ×

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Fig.1.1 Morphology of bi-functional polymers

1.6.1.2. TRIFUNCTIONAL MONOMERS

When a trifunctional monometer (i.e., functionality of the monometer is 3) is mixed in small

with a bifunctional monomer, they form branched chain polymer.

Fig.1.2. Morphology of tri-functional polymers

The movement of polymer chain in branched polymer is more restricted than that of straight

chain polymers.

1.6.1.3. Poly functional monomers

Poly functional monomers form cross-linked polymer (three-dimensional network polymer). All

the monomers in the polymer are connected to each other by strong covalent bonds. Therefore the

movement of polymer chain is totally restricted .This type of polymers are hard and brittle and possess

very high strength and heat resistance. They are insoluble in almost all organic solvents.

Fig.1.3. Morphology of poly-functional polymers

× × x

× × x × x ×

× × x

× × x

× × x

× × ×

× × x

× × x x

× × x x × x x

× × x x

× × x

× × x

× × ×

× × x

× × x x × × x x × x x

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1.2 POLYMERIZATION

Polymerization is a process in which a larger number of small molecules (called monomers)

combine to give a big molecule (called polymer) with or without the elimination of small molecules like

water, methanol, ammonia, HCl etc., Accordingly they are classified as addition or condensation

polymerization.

1.3 DEGREE OF POLYMERIZATION(DP)

Polymerization reaction results of polymer chains of different length is specified by number of repeating

units in the polymer chain. This is called as degree of polymerization(DP). It is represented by the

following relationship,

𝐷𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑝𝑜𝑙𝑦𝑚𝑒𝑟𝑖𝑧𝑎𝑡𝑖𝑜𝑛 =𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔𝑕𝑡 𝑜𝑓 𝑡𝑕𝑒 𝑝𝑜𝑙𝑦𝑚𝑒𝑟 𝑐𝑕𝑎𝑖𝑛

𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔𝑕𝑡 𝑜𝑓 𝑡𝑕𝑒 𝑚𝑜𝑛𝑜𝑚𝑒𝑟𝑖𝑐 𝑢𝑛𝑖𝑡

𝐷𝑝 =𝑀𝑝

𝑀𝑚

For example

The molecular weight is the product of molecular weight of repeating unit and DP. Using

polyethylene is an example, a polymers of DP 1000 has molecular weight of 28 X 1000 = 28000.

1000CH2 = CH2 -CH2-CH2-CH2 -CH2-CH2-CH2-CH2-CH2-CH2 -CH2-

In this example 1000 repeating units are present in the polymer chain. So, the degree of

polymerization is 1000.

1.8. Polymerization Reactions

Polymers are made or “polymerized” by chemical reactions. Small simple undergo chemical

bond formation in the presence of catalysts to form very large molecule.

1.8.1. Addition (Chain Growth) Polymerization

Addition polymerization reactions involves a rapid “chain reaction” of chemically activated

monomers. Each reaction sets up the condition for another to proceed. Each step needs a reactive site a

double carbon bond or an unsaturated molecule. The three stages are: initiation, propagation,

termination In the case of the polymerization of polyethylene, initiation can come from a free radical – a

single unit that has one unpaired electron such as an OH molecule. H2O2 can break up into 2 OH¯

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molecules. Each can act to initiate and to terminate the reaction. The termination here would be called

recombination. The composition of resultant molecule is a multiple of the individual monomers. These

reactions most commonly produce linear structures but cannot produce network structures.

1.8.2. Condensation (Step Growth or Stepwise) Polymerization

As the term condensation involves two molecules fairly different functional groups reacts to

give a polymer product. In these reactions between reactive monomers that occur in one step at a time.

They are slower than addition polymerization needs reactive functional groups. In all condensation

polymerizations reactions involves the elimination of byproduct such as water, methanol, HCl etc., No

reactant species has the chemical formula of monomer repeating unit. Most commonly produce network

products and sometimes linear structures also. Detailed mechanisms of these reactions are discussed in

the following sections.

Table.1.3. Difference between addition polymerization and condensation polymerization

S.No. Addition Polymerization Condensation polymerization

1. The monomer must have at least

one multiple bond

Example : Ethylene ; CH2=CH2

Vinyl chloride ; CH2=CHCl

The monomer must have two identical or

different reactive functional groups

Example : Ethylene glycol; HO-CH2-CH2-OH

Adipic acid ; HOOC-CH2-CH2-CH2-CH2-

COOH

2 Monomers add on to give

polymer and no other byproduct

is forms

Polymer is formed by the condensation of

reactive functional groups with the elimination

of small molecules like water, methanol, etc.,

3 Number of monomeric unit

decrease steadily throughout the

reaction

Monomers disappear at the initial stage of the

reaction

4 Molecular weight of the polymer

is an integral multiple of a

monomer.

Molecular weight of the polymer need not to be

an integral multiple of a monomer.

5 High molecular weight

compounds formed at once

Molecular weight of the polymer increases

steadly throughout the reaction.

6 Longer reaction time gives higher

yield, but have a little effect on

molecular weight.

Longer reaction is required to obtain high

molecular weight polymer.

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7 Homo-chain polymer is obtained Hetero –chain polymer is obtained.

8 Thermo plastics are produced Thermosetting polymers are produced.

Synthesis of Addition Polymers

All the monomers from which addition polymers are made are alkenes or functionally substituted

alkenes. The most common and thermodynamically favored chemical transformations of alkenes are

addition reactions. Many of these addition reactions are known to proceed in a stepwise fashion by way

of reactive intermediates, and this is the mechanism followed by most polymerizations. A general

diagram illustrating this assembly of linear macromolecules, which supports the name chain growth

polymers, is presented here. Since a pi-bond in the monomer is converted to a sigma-bond in the

polymer, the polymerization reaction is usually exothermic by 8 to 20 kcal/mol. Indeed, cases of

explosively uncontrolled polymerizations have been reported.

Scheme1.1.General addition polymerization reaction.

It is useful to distinguish four polymerization procedures fitting this general description.

• Radical Polymerization The initiator is a radical, and the propagating site of reactivity (*) is a carbon

radical.

• Cationic Polymerization The initiator is an acid, and the propagating site of reactivity (*) is a

carbocation.

• Anionic Polymerization The initiator is a nucleophile, and the propagating site of reactivity (*) is a

carbanion.

• Coordination Catalytic Polymerization The initiator is a transition metal complex, and the

propagating site of reactivity (*) is a terminal catalytic complex.

1. Radical Chain-Growth Polymerization

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Virtually all of the monomers described above are subject to radical polymerization. Since this can

be initiated by traces of oxygen or other minor impurities, pure samples of these compounds are often

"stabilized" by small amounts of radical inhibitors to avoid unwanted reaction. When radical

polymerization is desired, it must be started by using a radical initiator, such as a peroxide or certain

azo compounds. The formulas of some common initiators, and equations showing the formation of

radical species from these initiators are presented below.

Scheme.1.2. Mechanism of formation of free radical initiators

By using small amounts of initiators, a wide variety of monomers can be polymerized. One example

of this radical polymerization is the conversion of styrene to polystyrene, shown in the following

diagram. The first two equations illustrate the initiation process, and the last two equations are examples

of chain propagation. Each monomer unit adds to the growing chain in a manner that generates the

most stable radical. Since carbon radicals are stabilized by substituent of many kinds, the preference for

head-to-tail regioselectivity in most addition polymerizations is understandable. Because radicals are

tolerant of many functional groups and solvents (including water), radical polymerizations are widely

used in the chemical industry.

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Scheme.1.3.Mechanism of initiation of polymerization reaction

Termination may be happened by combination or disproportionation, in both types of termination

two reactive radical sites are removed by simultaneous conversion to stable product(s). Since the

concentration of radical species in a polymerization reaction is small relative to other reactants (e.g.

monomers, solvents and terminated chains), the rate at which these radical-radical termination reactions

occurs is very small, and most growing chains achieve moderate length before termination.

Scheme.1.4.Mechanism of termination of polymerization reaction

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2. Cationic Chain-Growth Polymerization

Polymerization of isobutylene (2-methylpropene) by traces of strong acids is an example of

cationic polymerization. This process is similar to radical polymerization, as demonstrated by the

following equations. Chain growth ceases when the terminal carbocation combines with a nucleophile or

loses a proton, giving a terminal alkene (as shown here).

Scheme.1.5.Mechanism of cationic polymerization reaction

Monomers bearing cation stabilizing groups, such as alkyl, phenyl or vinyl can be polymerized

by cationic processes. These are normally initiated at low temperature in methylene chloride solution.

Strong acids, such as HClO4 , or Lewis acids containing traces of water (as shown above) serve as

initiating reagents. At low temperatures, chain transfer reactions are rare in such polymerizations, so the

resulting polymers are cleanly linear (unbranched).

3. Anionic Chain-Growth Polymerization

Treatment of a cold THF solution of styrene with 0.001 equivalents of n-butyllithium causes an

immediate polymerization. This is an example of anionic polymerization, the course of which is

described by the following equations. Chain growth may be terminated by water or carbon dioxide, and

chain transfer seldom occurs. Only monomers having anion stabilizing substituents, such as phenyl,

cyano or carbonyl are good substrates for this polymerization technique. Many of the resulting polymers

are largely isotactic in configuration, and have high degrees of crystallinity.

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Scheme.1.6.Mechanism of cationic polymerization reaction

Species that have been used to initiate anionic polymerization include alkali metals, alkali

amides, alkyl lithiums and various electron sources. A practical application of anionic polymerization

occurs in the use of superglue. This material is methyl 2-cyanoacrylate, CH2=C(CN)CO2CH3. When

exposed to water, amines or other nucleophiles, a rapid polymerization of this monomer takes place.

1.9. PLASTICS

Plastics are high molecular weight organic materials that can be moulded into any desired

shape by the application of heat and pressure in the presence of a catalyst. Originally, plastics were

discovered and then developed based on trial and error method. The present time is sometime referred as

plastic age, because the use of polymeric material is percolated in large variety of applications. This

rapid growth has taken place only in the last 50 years. It was man‟s desire to replace, glass, metal,

ceramic, wood and other materials of constructions. Since plastics possess the following advantages,

they have wider applications.

1.9.1.Advantages of plastics over other materials

Advantages of plastics over other materials are

i) They are light in weight.

ii) They possess low melting point.

iii) They can be easily moulded and have excellent finishing

iv) They possess very good strength and toughness.

v) They possess good shock absorption capacity.

vi) They are corrosion resistant and chemically insert.

vii) They have low co-efficient of thermal expansion and possess good thermal and electrical

insulating property.

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viii) They are very good water-resistant and possess good adhesiveness.

1.9.2 Classification of plastics

Based on the structure and type of resin used for used for the manufacture of plastics,

plastics are classified into two main types thermoplastics and thermosetting plastics. A resin is a basic

binding material, present in a plastic which undergoes polymerization reaction during moulding.

Chart 1.2.Classification of Plastics

Example are Examples are

i. Polyethylene i. Bakelite

ii. polyvinyl chloride ii. Polyester

iii. Polystyrene iii. Urea-formaldehyde

iv. Poly acrylic nitrile iv. Epoxy-resin

1.9.2.Thermoplastics

Thermoplastics are prepared by addition polymerization. they are straight chain or a slightly

branched polymers and various chains are held together by weak Van der Waal‟s forces of attraction.

Thermoplastics can be softened on heating and hardened on cooling. They are generally soluble in

organic solvents.

1.9.4.Thermosetting plastics

Thermosetting plastics are made up by three dimensional networks. They are heat resistant and

could not be remoulded by heating. They are known for toughness. Usually they are manufactured by

condensation polymerization reactions

Table.1.4. Difference between thermal plastics and thermosetting plastics

Plastics

Thermoplastics Thermosetting plastics

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S.No. Thermal Plastics Thermosetting Plastics

1 They are formed by addition

polymerization

Example: Polyethylne, PVC etc.,

They are formed by condensation

polymerization

Bakelite, Nylons, epoxy resins

2 They are long linear chain

polymers

They consist of three dimentional cross

linked network

3 They soften on heating and

harden on cooling

They do not soft on heating

4 They are weak, soft and less brittle They are strong, hard and more brittle

5 All the chains of the polymer are

held by hydrogen bonding or Van

dar Walls forces.

All the polymers chains are cross linked

by strong covalent bond.

6 They can be remoulded They cannot be remoulded

7 They have low molecular weights They have high molecular weights

8 They are soluble in organic

solvents

Ther are insoluble in organic solvents

1.10. Properties of Polymers

1.10.1.The Glass Transition Temperature, Tg

Glass transition temperature is defined as the temperature bellow which a polymer is hard and

above which it is soft. The hard brittle state of a polymer is known as the glassy state and the soft

flexible state is rubbery of viscoelastic state. On further heating, the polymer (if it is un-cross linked)

becomes a highly viscous liquid and starts flowing; this state is termed as viscofluid state, the transition

takes place at its flow temperature.

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Fig.1.4. Change of state with temperature of polymeric materials

When an amorphous material is cooled, there is no abrupt change in volume. In the case of a

crystalline material there is sudden change in volume. At the glass transition temperature, Tg, there is

change in slope of the curve or specific volume Vs temperature is moving form a low value in a glassy

state to a higher volume in the rubbery state over a range of temperature as shown in figure.1.4.

Fig.1.5.Change in volume of a polymer as a function of temperature

The following features are known to influence the glass transition temperature:(a) The presence of

groups pendant to the polymer backbone, since they increase the energy required to rotate the molecule

about primary bonds in the main polymer chain. This is especially true of side chains or branches.

( b ) The presence of inherently rigid structures in the backbone of the molecule, e.g. phenylene groups.

(c) Cross linking.

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( d ) Hydrogen bonds between polymer chains. ( e ) Relative molar mass, which influences 7; because

higher molar mass polymers have less ease of movement and more restrictions in their overall molecular

freedom than polymers of lower molar mass. (f) The presence of plasticizers. The effects of these

different factors can be seen in the Tg values of some typical polymers. A number of these values are

particularly contributes to the relative level of the glass transition temperature.

1.10.2.POLYMER STEREOCHEMISTRY

Mono substituted vinyl monomers form polymers containing a series of asymmetric carbon

atoms along the molecule. The precise arrangement of these asymmetric carbons gives rise to three

different possible stereo chemical arrangements. Firstly, where all asymmetric carbon atoms adopt

identical configurations, the resulting polymer is described as isotactic. Secondly, where there is a

regular alternating arrangement of asymmetric carbon atoms, the polymer is described as syndiotactic.

Lastly,

where there is no regularity at all in the arrangement of asymmetric carbon atoms, the resulting structure

is known as atactic. These three steric arrangements are illustrated in Figure 1.2.

Figure 1.2 Steric arrangements of polymers

Scheme.1.7.Tacticity of polymer

1.11.Molecular weight of a polymer

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The molecular weight is single digit for simple chemical entity but it is in the order of millions for

polymer. For example molecular weight of ethylene is 28, each of its molecules has the same chemical

structure [CH2=CH2] . But ethylene is polymerized the structure is indefinite

Wherein n can change from one polyethylene to another polyethylene in the same sample. This is

because, the number of polymer chains start growing at any instant, but all then do not get terminated

after growing the same time. The chain termination is random process, and so polymer molecules formed

can have different number of monomer units and thus different molecular weights. A polymers sample is

a mixture of molecules of same chemical type but different molecular weight. It is necessary, therefore,

to take an average molecular mass in these substances. Two types of molecular mass as reckoned with.

These are number average molecular mass(𝑀 n) and weight average molecular mass(𝑀 w).

1.11.1Number average concept

Consider a basket containing four vegetables: onion, brinjals, cabbages and cauliflowers. Just for

ease of understanding, let each onion of the onion lot weigh the same and so also of the other vegetables.

Assume that the number of each vegetable and its weight are tabulated.

Vegetable entity Number of units

in each entity, n

Weight of each

unit, M(g)

Total weight of each entity

W= nM(g)

Onions 2 10 20

Brinjals 4 20 80

Cabbages 6 100 600

Cauliflowers 3 250 750

Total 15 1450

Now, we have to find out the averages weight of the vegetables present in the basket. To work this

out, one might assume that that the individual vegetable entity contributes to the average weight in the

proportion of its numbers. What we can then get is the number average weight , arrived at as follows:

Total number of vegetables contained in the basket = 15

Number of onions present in the basket = 2

Therefore, the number fraction of onions = 2/15

Similarly,

Number fractions of brinjals = 4/15

Number fractions of cabbages = 6/15

Number fraction of cauliflowers = 3/15

Contribution made by 2 onions towards average weight of vegetables in the basket :

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Number fraction of onion X weight of each onion =(2/15) X 10

= 1.33 g

Similarly

Contribution made by 4 brinjals towards average weight of vegetables in the basket = (4/15) X 20

= 5.33 g

Contribution made by 6 cabbages towards average weight of vegetables in the basket = (6/15) X 100

= 40.00 g

Contribution made by 4 brinjals towards average weight of vegetables in the basket = (3/15) X 250

= 50.00 g

Summing up the contributions made by each vegetable variety, we get the number average weight of the

total vegetables as

1.33+5.33+40+50 = 96.66

1.11.2.Weight average concept

The other method of calculation the average weight is based on the assumption that the individual

vegetable variety contributes to the total weight in the proportion not its number but its weight. What we

then get is the „weight average‟ is arrived as follows:

Total weight of all vegetables in the basket = 1450 g

Weight of onions present in the basket = 20 g

Therefore,

Weight fraction of onions = 20/1450

Similarly, weight fraction of brinjals, cabbages and cauliflowers are 80/1450, 600/1450 and 750/1450,

respectively.

Next, contribution made by onions towards average weight of the vegetables in the basket = weight

fraction of onions X average weight of onions = (20/1450) X 10 = 0.14 g

Similarly,

Corresponding contribution of brinjals = (80/1450) X 50

= 1.10 g

Corresponding contribution of cabbages = (600/1450) X 100

= 41.38 g

Corresponding contribution of cauliflowers = (7500/1450) X 250

= 129.31g

Summing up the contribution made by the each vegetable variety, we get the weight-average weight of

total vegetables as

20

0.14 + 1.10 + 41.38 + 129.31 = 171.93 g

1.11.3.Generalization of the foregoing concepts

In the calculation of molecular weights of a polymer, either we can use number-average (𝑀 n) or

weight-average (𝑀 w) molecular weights. The method of calculating (𝑀 n) and (𝑀 w) can now be easily

generalized, using simple mathematics. Assume that there is n number of polymers in a polymers sample

and n1 of them have M1 molecular weight; n2 have M2 molecular weight and so on till we get ni and Mi

molecular weight(see Fig.)

Fig.1.6.Polymer sample with different molecular weight

Now we have total number molecules (n) is given by n = n1+ n2 +n3+…….+ni = Σni

Number of molecules in fraction 1 = n1

Number fraction if fraction 1 = 𝑛1

𝑛 =

𝑛1

𝛴𝑛 𝑖

Molecular weight contribution by fraction 1 = 𝑛1𝑀1

𝛴𝑛 𝑖

Similarly molecular weight contribution by other fractions will be as follows:

𝑛2𝑀2

𝛴𝑛𝑖 ,𝑛3𝑀3

𝛴𝑛𝑖,𝑛4𝑀4

𝛴𝑛𝑖,…… . . ,

𝑛𝑖𝑀𝑖

𝛴𝑛𝑖

Number average molecular weight of whole polymer is given by

𝑀 𝑛 = 𝛴𝑛𝑖𝑀𝑖

𝛴𝑛𝑖=𝑛1𝑀1

𝛴𝑛𝑖+𝑛2𝑀2

𝛴𝑛𝑖+

𝑛3𝑀3

𝛴𝑛𝑖, +

𝑛4𝑀4

𝛴𝑛𝑖+ ⋯… . . +

𝑛𝑖𝑀𝑖

𝛴𝑛𝑖

Similarly total weight of the polymer = W = ΣniMi.

Weight of fraction1 = W1 = n1M1

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Weight fraction of 1 = 𝑛1𝑀1

𝑊=

𝑛1𝑀1

𝛴𝑛 𝑖𝑀𝑖

Molecular weight contribution of the fraction 1 is given by

𝑊1𝑀1

𝛴𝑛𝑖𝑀𝑖=𝑛1𝑀1𝑀1

𝛴𝑛𝑖𝑀𝑖=

𝑛1𝑀12

𝛴𝑛𝑖𝑀𝑖

Similarly the molecular contributions of other fraction will be

𝑛2𝑀22

𝛴𝑛𝑖𝑀𝑖,

𝑛3𝑀32

𝛴𝑛𝑖𝑀𝑖,……… .,

𝑛𝑖𝑀𝑖2

𝛴𝑛𝑖𝑀𝑖

The weight average molecular weight of the whole polymer will be

𝑀 𝑤 = 𝛴𝑛1𝑀1

2

𝛴𝑛𝑖𝑀𝑖=

𝑛1𝑀12

𝛴𝑛𝑖𝑀𝑖+𝑛2𝑀2

2

𝛴𝑛𝑖𝑀𝑖+

𝑛3𝑀32

𝛴𝑛𝑖𝑀𝑖+ ⋯…… . +

𝑛𝑖𝑀𝑖2

𝛴𝑛𝑖𝑀𝑖

For synthetic polymers, 𝑀 𝑤 is greater than 𝑀 𝑛 . It they were be equal , the polymer sample may be

considered as perfectly homogeneous (i.e., each molecule has same molecular weight); but this does not

happen.

1.11.4.Poly dispersity index.

Simple chemical molecules like water, all molecules are in the same size and molecular weight, so

they are monodisperse but polymers molecules are in different size and hence different molecular mass

so they are polydisperse. Monodispersity and polydispersity is represented in the fig.1.7.

All molecules are in same size Molecules are in different size

Fig.1.7. Dispersity of polymers

The heterogeneity of the polymer sample is called polydispersity. The weight average molecular

mass (𝑀 𝑤 ) is always greater than number average molecular mass (𝑀 𝑛 ),(Figure 1.) unless the polymer

sample is monodisperse i.e., 𝑀 𝑛 = 𝑀 𝑤 .

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Fig.1.8.Ditribution of molecular weight of a polymer.

The ratio of weight average molecular mass and number average molecular mass( 𝑀 𝑤 /𝑀 𝑛 ) is

called the polydispersity index(P.D.I) of a polymer sample.

𝑃𝐷𝐼 = 𝑀 𝑤

𝑀 𝑛

1.12. PRACTICAL METHODS OF CHAIN POLYMERISATION

Chain reactions are used to prepare a variety of high molecular mass polymers of commercial

importance and in practice may take one of four forms, namely bulk, solution, suspension, and emulsion

methods. These four methods are described in the sections that follow, together with the „loop‟

modification which has become of commercial importance recently in producing latexes by emulsion

polymerization for the paint industry.

1.12.1 Bulk Polymerisation.

Bulk polymerization is the simple technique for the preparation of high molecular weight polymer,

which involves the addition reactions. The starting material consist pure polymer with only traces of

initiator and chain transfer agent. For the laboratory preparation of vinyl polymers this method is used.

In industry polyethylene, polystyrene and poly methyl methacrylate are prepared by using this

technique. Polyethylene is produced from gaseous monomers under pressure either high of low. In the

case of poly (styrene), bulk polymerisation is nonetheless used for the commercial production of the

polymer.

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Fig.1.8.Bulk polymerization

Manufacture of the polymer takes place in discrete stages in different parts of the plant. The reaction

is initiated in a tank which is heated to a temperature of 80 ᵒC; styrene undergoes self-initiation on

heating, so that no extra initiator is required for this step, which is allowed to continue until about 35%

conversion to polymer. At this conversion the mixture still has a sufficiently low viscosity to enable

fairly easy stirring and transport. From this stage of 35% polymerisation the mixture is passed down a

tower in an atmosphere of nitrogen; there is a thermal gradient throughout the tower from 100 ᵒC at the

top to 200 ᵒC at the bottom. This gradient is maintained by a complicated arrangement of heaters and

coolers which compensate for the exotherm that the polystyrene undergoes itself as increasing

proportions of monomer are converted to polymer. At the bottom of the tower, the high molar mass

poly(styrene) is extruded, granulated, and cooled prior to packaging.

Some disadvantages of bulk polymerization are

i. Viscosity of polymeric material increases as polymerization occurs. These results the

difficulties in handling the polymer.

ii. Since chemical reactions are generally exothermic and the increasing viscosity inhibits the

dissipation of heat, there can be localized overheating leading to the charring and possible

degradation of the product.

1.12.2. Solution Polymerisation

In solution polymerization technique the monomer is dissolved in an appropriate solvent. It

eliminates the difficulties associated with the exotherm on polymerization may be overcome since

temperature can be more readily controlled than in the bulk technique. If the right solvent is chosen the

product may form to give a solution suitable for casting or spinning.

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Fig.1.9.Solution polymerization

Disadvantages with solution polymerization are

i. Reaction temperature is limited by the boiling point of the solvent used which in turn restricts the

rate of reaction that may be achieved.

ii. It is difficult to free the product of the last traces of the solvent.

iii. Selection of a completely inert solvent cannot be done, of means that there is possible chain transfer

to the solvent and hence a restriction on the molar mass of the product. This last point is particularly

important and is the one that is primarily responsible for the rarity of solution techniques in the

manufacture of commercially important polymers.

Suspension Polymerization

Only water-insoluble monomers can be polymerized bb this technique. The monomer is

suspended in water, in the form of fine droplets, which are stabilized and prevented from coalescing

by using suitable water protective colloids, surface active agents by stirring. The size of monomer

droplet formed depend upon monomer water ratio, concentration of stabilizing agent and agitation

speed.

The initiators are monomer soluble. Since each monomer is independent of other droplets and

it will act as individual bulk polymerization nucleus. Aqueous phase separating monomers and

polymers act as heat transfer medium and exothermicity is well controlled.

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Fig.1.10.suspension polymerization

Polymerization proceeds to 100 % conversion and the product obtained is spherical beads [For this

reason, this polymerization is called as bead or pearl polymerization] . Isolation of product is very easy

only water wash of beads remove all the unwanted materials. Polystyrene beads, styrene-divinyl benzene

copolymer beads, PVA beads are the main products of this technique.

1.12.3. Emulsion Polymerisation

Emulsion polymerization technique is a versatile and widely used method of polymerization. In

this technique droplets of monomer are dispersed in water with the aid of an emulsifying agent, usually a

synthetic detergent. The detergent forms small micelles 10-100 pm in size, which is much smaller than

the droplets that can be formed by mechanical agitation in suspension polymerisation. These micelles

contain a small quantity of monomer, the rest of the monomer being suspended in the water without the

aid of any surfactant. Emulsion polymerisation is initiated using a water-soluble initiator, such as

potassium per sulfate. This forms free radicals in solution which may initiate some growing chains in

solution. These radicals or growing chains pass to the micelles and diffuse into them, which causes the

bulk of the polymerisation to occur in these stabilized droplets.

As emulsion polymerisation proceeds, like the suspension technique but unlike either the bulk or

the solution techniques, there is almost no increase in viscosity. The resulting dispersed polymer is not a

true emulsion any more, but instead has become latex. The particles of the latex do not interact with the

water; hence viscosity is not found to change significantly up to about 60% solids content. Emulsion

polymerisation is used in the commercial production of synthetic diene elastomer and also to produce

commercial latexes of the type used in paints.

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Figure. 1.1. Emulsion Polymerization

Fig.1.11.Emulsion polymerization

1.13.Nylons 6,6

The aliphatic polyamides are generally known as Nylons. There are different types of nylons

usually indicated by number system. Nylon 66 is one of the important member of the family of

Nylons. Nylon 66 is a polyamide prepared by polycondensation of hexamethylene diamine and

adipic acid, monomers having six carbon atoms each, so it is named as Nylon 66.

Scheme1. Preparation of Nylon 66

Scheme.1.8.Preparation of Nylon 66

Properties of Nylon 66

Nylon 66 is used as plastic as well as fibre. It has good tensile strength, abrasion resistance and

toughness upto 150 ºC. Also, it offer resistance to many solvents. However, phenols, cresols and formic

acid dissolve this polymer. Nylon 6,6 is very stable in nature. Nylon 6,6 is very difficult to dye but once

it is dyed it has a high colorfastness and is less susceptible to fading

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Applications of Nylon 66

1. A large quantity of Nylon 66 is used to produce tyre cord.

2. Nylon 66 is used to prepare monofilaments and ropes.

3. It is used to prepare textile fibre for the use of dress

4. It is used as substitute for metal gears and bearings.

5. Nylon 6,6 is waterproof in nature so it is also used to make swimwear.

6. Other popular applications are: carpet fibres, apparel, airbags, zip ties, ropes, conveyor belts, hoses

and the outer layer of turnout blankets. Nylon 6-6 is also a popular guitar nut material

1.14.Epoxy resin

The epoxy polymers are generally poly ethers .One type of epoxy polymers is prepared from

epichlorohydrin and bisphenol-A. The reaction is carried out with the excess of epichlorohydrin. The

scheme is as follows.

Scheme.1.9.Preparation of epoxy resin

Properties of epoxy resins.

Epoxy resins possess remarkable chemical resistance and good adhesion because of the presence

of chemically inert ether linkage. They are flexible and possess heat resistivity. Epoxy resin contain

polar groups so they possess good adhesive properly.

Applications of epoxy resins

1. Epoxy resins are used as structural adhesives like araldite.

2. Epoxy resins are used in surface coatings, glass fibre reinforced plastics[FRP]

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3. They are applied over cotton, rayon and bleached fabrics to impart crease-resistance and shrinkage

control.

4. These are used as electrical insulators and laminating materials.

5. Epoxy resin compounds are used in the production of light weight components for automobiles and

aircrafts.