CY6151 – ENGINEERING CHEMISTRY I

Unit – 1- Polymer Chemistry

The word polymer is derived from Greek words “Poly” means “many”

“mer” means “unit”.

Polymers:

Polymers are macromolecules formed by the combination of many small

molecules called monomers.

The molecular weight of polymer ranges from 500 to 106.

Examples: Poly vinyl chloride (PVC), polyethlyene

Monomers:

Monomer is a micro molecule which combines with each other to form a

polymer. These are the building blocks of polymer.

Examples: Vinyl chloride, ethylene

Requirements of a monomer:

a) The monomer must have multiple (double, triple) bonds or

b) The monomer must have two same or different functional groups.

Polymerisation:

Polymerisation is the process in which large number of monomers

combines to give a polymer with or without the elimination of small

molecules like water,HCl.

Example:

Polyethylene(polymer) is made of repeated linkages of ethylene

(monomer)

𝑛𝐶𝐻2 = 𝐶𝐻2𝐻𝑒𝑎𝑡/𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒,𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑡→ (−𝐶𝐻2 − 𝐶𝐻2 −)𝑛

Degree of polymerization (DP):

Degree of polymerization (DP) is the number of repeating units

(monomers) present in a polymer chain.

Degree of polymerization (DP) = 𝐌𝐨𝐥𝐞𝐜𝐮𝐥𝐚𝐫 𝐖𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐜 𝐧𝐞𝐭𝐰𝐨𝐫𝐤

𝐌𝐨𝐥𝐞𝐜𝐮𝐥𝐚𝐫 𝐖𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐫𝐞𝐩𝐞𝐚𝐭𝐢𝐧𝐠 𝐮𝐧𝐢𝐭

Example:

5𝐶𝐻2 = 𝐶𝐻2 → − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐶𝐻2 −

In this example, five repeating units are present in the polymer chain. So

the degree of polymerization is 5.

If molecular weight ranges from 500 to 5000 Dalton units, it is called

oligo polymer.

If molecular weight ranges from 10,000 to 2,00,000 Dalton units , it is

called high polymer.

Functionality:

The number of reactive sites or bonding sites or functional groups

present in a monomer is called its functionality.

Monomers which possess more than one reactive site alone can

undergo polymerization.

Functionality Examples

0 CH4 , CCl4 , C6H6

1 ROH , RCl , RCOOR, RNH2

2 Amino Acids , CH2 = CH2 (ethene),

CH2OH – CH2OH (glycol)

3 CH2OH – CHOH – CH2OH (glycerol)

C6H5OH (phenol)

4 CH ≡ CH(acetylene)

Significance of functionality:

Mono functional monomers:

Substances having only one bonding or reactive site are called “mono

functional monomers”.

Example: Acetic acid (CH3 – COOH).

They cannot undergo polymerization.

Bi functional monomers:

1. Bi functional monomers form linear or straight chain polymer.

2. Each monomeric unit in the linear chain is held together by strong

covalent bond.

3. The polymer chains are held together by weak Vander Waal’s forces.

4. Therefore, there is no restriction for the movement of one chain over

another.

5. These polymers are soft.

6. They possess less strength and low heat resistant .

7. They are soluble in organic solvents.

Tri functional monomers:

1. Tri functional monomers form branched chain polymer.

2. Therefore, the movement of polymer chain is more restricted than

that of straight chain polymer.

Poly functional monomers:

1. Poly functional monomers form cross-linked polymer.

2. The monomers in the polymer chain are held together by strong

covalent bonds.

3. Therefore, the movement of polymer chain is totally restricted.

4. They are very hard.

5. They possess very high strength and high heat resistance.

6. They are insoluble in organic solvents.

TACTICITY:

The orientation of monomeric units or functional groups in a polymer

molecule can take place in an orderly or disorderly manner with respect to

the main chain is called “tacticity”.

There are three types of tacticity.

a) Isotactic polymers

b) Syndiotactic polymers

c) Atactic polymers

a) Isotactic polymer:

If the functional groups are arranged on the same side of the main chain,

then, it is called isotactic polymer.

Examples : Cis polystyrene, Cis-isoprene

b) Syndiotactic polymer:

If the functional groups are arranged in alternative fashion, it is called

syndiotactic polymer.

Examples : Trans polystyrene, Trans-isoprene

c) Atactic polymer:

If the functional groups are arranged randomly, it is called atactic polymer.

Examples : Polystyrene, Polypropylene

Representation of tacticity:

Classification of polymers

Polymers are classified into two types

i) Natural Polymers.

ii) Synthetic polymers.

Examples for natural polymers:

1. Starch:

i) It is a polymer of glucose.

ii) It is a chief food reserve of plants.

2. Cellulose:

i) It is also a polymer of glucose.

ii) It is a chief structural material of the plant.

3. Proteins:

i) These are polymers of α-amino acids.

ii) They are building block of animals.

4. Nucleic acids:

i) They are polymers of various nucleotides.

ii) RNA and DNA are common nucleotides.

Examples for synthetic polymers:

1. PVC

2. Poly ethylene

3. Nylon

4. Teflon

5. Bakelite

Types of synthetic polymers:

1. Organic polymers:

i) These polymers contain hydrogen, oxygen, nitrogen,

halogen atoms apart from carbon atoms.

ii) Example: Polyethylene, Poly vinyl alcohol.

2. Hetero chain polymers:

i) These polymers contain carbon atoms and hetero atoms

like nitrogen, sulphur, and oxygen.

ii) Example: Polyester, polyamides.

3. Inorganic polymers:

i) These polymers consist of different atoms joined by

chemical bonds.

ii) Example: Poly siloxanes, Poly phosphoric acid.

PLASTICS

Definition:

Plastics are high molecular weight organic materials that can be

moulded into any desired shape by the applications of heat and pressure in

presence of a catalyst. Example: PVC , PET

Advantages of plastics:

1. They are light in weight.

2. They possess low melting point.

3. They can be easily moulded.

4. They are corrosion resistant.

5. They are chemically inert.

6. They have adhesive property.

7. They possess good shock absorption capacity.

Disadvantages of plastics:

1. They have high softness.

2. They undergo deformation under load.

3. They undergo embrittlement under low temperature.

4. They possess low heat resistance.

Classification of plastics:

1. Thermoplastics

2. Thermosetting plastics

Differences between Thermoplastics and thermosetting plastics

S.No Thermoplastics Thermosetting plastics

1 They are formed by addition

polymerization.

They are formed by condensation

polymerization

2 Example: PVC , Polyethylene Example: Polyester, Bakelite

3 They are linear long chain

polymers.

They consist of three dimensional

network structure.

4

All the polymer chains are held

together by weak Vander

Waals forces.

All the polymer chains are linked by

strong covalent bonds

5 They soften on heating, and

harden on cooling.

They do not soften on heating

because the covalent bonds retain

their strength on heating

6 They can be remoulded into

any number of times They cannot be remoulded

7 They are soft, weak and less

brittle

They are hard, strong and more

brittle

8 They have low molecular

weight.

They have high molecular weight.

9 They are soluble in organic

solvents.

They are insoluble in organic

solvents.

10 They are prepared by injection

moulding

They are prepared by compression

moulding.

TYPES OF POLYMERISATION:

1. Addition polymerization.

2. Condensation polymerization.

3. Co-polymerization.

Addition polymerization:

Monomers having multiple bonds (double or triple bond) undergo

addition polymerization. Monomers combine to give polymer through

addition reaction without elimination of any smaller molecules like H2O,

HCl

Example:

Ethylene polymerizes to give polyethylene.

nCH2 = CH2Heat/pressure,Catalyst→ (−CH2 − CH2 −)n

Condensation polymerisation:

Monomers having same or different types of functional groups undergo

condensation polymerization. The polymerization proceeds by step wise

reaction between reactive functional groups and small molecules are

eliminated.

Example:

Hexa methylene diamine and adipic acid condense to form nylon 6,6.

Co-polymerisation:

1. It is a special kind of polymerisation, otherwise known as “Joint

polymerisation”.

The product is known as ‘co-polymers’. This is superior to other

polymerization because it is used to alter the hardness, strength, rigidity

and crystallinity of the monomers. e.g SBR

SBR or styrene Butadiene rubber

CH2H2N NH26

Hexamethylene diamine Adipic acid

+ CH2HOOC COOH4

n n CH2NH NH6

CH2C C4

O On

Nylon 6,6

+ 2n-1 H2O

Differences between Addition polymerisation and Condensation

polymerisation

S.No Addition Polymerisation Condensation

Polymerisation

1 Eg. PVC Eg. Nylon 6,6

2 Otherwise known as “Chain growth

Polymerisation”.

Otherwise known as “Step

wise Polymerisation”.

3 Monomers are adding together to form

polymers.

Monomers are condensed

to form polymer.

4 No elimination of other molecules. Elimination of smaller

molecules occur.

5 At least one multiple bond presence is

essential condition.

Monomers must have two

or more functional groups.

6 Homo polymers are formed. Hetero polymers are

formed.

7 Thermoplastics are formed. Thermosetting plastics are

formed.

8 Molecular weight of the polymer is the

integral multiple of monomers.

Need not be so.

9 Monomers disappear slow and

steadily.

Monomers disappear at the

initial stage of the reaction.

10 Longer processing time is needed to

increase yield, but have a little effect

on molecular weight.

Longer time is essential for

increasing molecular

weight.

ANIONIC ADDITION POLYMERISATION:

i) When the polymerisation is initiated by negatively charged ion (anion),

the reaction is known as anionic polymerization.

ii) Monomers with electron withdrawing groups like Clˉ, CNˉ follow this

mechanism. These groups stabilise the carbanion formation.

iii) Catalysts (Initiators): Lewis bases like KNH2, NaNH2, LiNH2

iv)Monomers: Vinyl chloride, acrylonitrile undergoes this kind of

polymerization mechanism.

MECHANISM:

Anionic polymerisation occurs in three steps.

1. Initiation

2. Propagation

3. Termination

1. Initiation:

The catalyst initiates polymerisation by the addition of NH2ˉ ion to the

monomer to form chain initiating species (CIS). The monomer taken is

acrylonitrile.

2. Propagation:

It involves the growth of chain initiating species by the successive addition

of large number of monomers.

In this step, the transfer of negative charge to the newly added monomers

takes place. It produces living polymer.

3. Termination:

Termination of the growing chain occurs by the addition of suitable

terminating agents like ammonia. It produces dead polymer.

CATIONIC ADDITION POLYMERISATION:

i) When the polymerisation is initiated by positively charged ion (cation),

the reaction is known as cationic polymerization.

ii) This type of polymerisation takes place when electron donating groups

like CH3, C6H5 are present in a monomer. These groups stabilise the

carbonium ion formation.

iii) It is carried out at low temperature and in a non-polar solvent

iv) Catalysts: The Lewis acids like AlCl3, BF3, TiCl4 are generally acting as

catalysts or initiators. Water, HCl acts as co-catalyst.

v) Monomers: Isoprene, Styrene and isobutylene undergo this kind of

polymerization mechanism.

Example: Formation of polystyrene

1. Initiation:

The catalyst initiates polymerisation by the addition of H+ ion to the

monomer to form chain initiating species (CIS)

2. Propagation:

It involves the growth of chain initiating species by the successive addition

of large number of monomers.

In this step, the transfer of positive charge to the newly added monomer

takes place. It produces living polymer.

3. Termination:

Termination of the growing chain involves removal of proton from the

polymer chain. It produces dead polymer.

CH

H

H

C

H

C6H5

.AlCl3 OH + C

H

H

C

H

C6H5

n CH

H

H

C

H

C6H5

Cn

H

H

C

H

C6H5

.AlCl3 OH

PREPARATION, PROPERTIES AND USES OF NYLON -6, 6

Preparation:

Hexa methylene diamine and adipic acid condenses to give nylon 6,6

Properties:

1. Nylon 6,6 has good flexibililty.

2. Nylon 6,6 possess remarkable elasticity

3. It absorbs only little moisture.

4. It has translucent nature.

5. It has high impact strength.

6. It is insoluble in common organic solvents except phenol, formic acid.

CH2H2N NH26

Hexamethylene diamine Adipic acid

+ CH2HOOC COOH4

n n CH2NH NH6

CH2C C4

O On

Nylon 6,6

+ 2n-1 H2O

Uses:

1. It is used as tooth brush bristles.

2. Nylon can also be used in automobile gears – Nylon bearings work

without any lubrication.

3. Nylon is used to make fabrics in textile industry.

4. Nylon fibres are used to make ropes, socks and carpets.

PREPARATION, PROPERTIES AND USES OF EPOXY RESINS

Epoxy resins are cross linked thermosetting resins.

They are poly ethers because of their R-O-R skeleton.

Preparation:

In presence of sodium hydroxide, Bisphenol – A and epichloro hydrin

combine together to form Epoxy resin. The reaction is carried out in basic

medium. The value of n ranges from 1 to 20.

Properties:

i) They are flexible and tough.

ii) They possess very good heat resistance.

iii) They possess excellent adhesion property.

iv) They possess high chemical resistance.

Applications:

1. They are used as surface coatings.

2. They are used to control shrinkage.

3. They are used as laminating materials in electrical components.

4. They are used to produce components for automobiles.

5. They are used as adhesives like araldite.