Polymers

An Introduction to Polymer Chemistry

• Polymer (Greek: poly=many; mer=part)

giant, complex molecules

• Made up by the linking together of large number of

small molecules (repeating units called monomers )

held together by covalent bonds (two or more bonding

sites)

Introduction

Monomers

Alkenes, vinyl chloride, adipic acid, glucose, amino acids, glycol

with two bonding sites act as monomers

C C

H

HH

H

E t h y l e n e

Aminoacidethane-1,2-diol

hexane-1,6-diamine

adipic acid

terephthalic acid

Isoprene

Butadiene, MW 54 Polybutadiene, MW 200000

Polymerization

MW 28 MW 28000

Monomer Polymer

Degree of Polymerization

• Number of repeating units in the polymer chain

formed is called the degree of polymerization

(n).

• Polyethene: (C2H4)n, where n stands for DP

• Molecular weight of PE, M = nMo,

where Mo is molecular weight of monomer

• Strength of the polymer can be increased by

increasing its DP

– High DP hard and heat resistant

– Low DP soft, gummy

• Data: If MW of PE, M = 28000,

MW of repeat unit, Mo = 28,

M = nMo

• Thus, n = M/Mo

= 28000/28

= 1000

Degree of Polymerization (n) = 1000

Degree of Polymerization

Classification

i) On the basis of origin– Natural

– Synthetic

ii) On the basis of nature of monomer• Homopolymers (comprise of monomers of the same type)

– Linear (homochain or heterochain)

– Branched

– Cross-linked

• Heteropolymers/ Copolymers (Different repeating units)

– Linear; Branched; Graft (regular/irregular); Block (regular/irregular)

iii) On the basis of chemical nature • Organic (polymer backbone chain made up of carbon atom)

• Inorganic (No carbon atoms in the backbone chain, eg., Silicone rubbers)

Homopolymer: Monomers of the

same type

Copolymer: Different repeating units

Random copolymer: Two or more different repeating units are

distributed randomly

Alternating copolymer: Alternating sequences of different monomers

Block copolymer: Long sequences of a monomer are followed by long

sequences of another monomer

Homopolymers can be linear,

branched or cross-linked

Nomenclature

Graft copolymer: Chain made from one type of monomer

with branches of another type

Tacticity: Orientation of monomeric units in polymer takes

place in orderly/disorderly fashion w.r.t main chain.

The difference in configuration affects their physical

properties

Isotactic: Head-to-tail configuration

Functional groups are all on the same side of the main

chain, FG= Y

Tacticity:

Syndiotactic: Functional groups occupy alternating position.

Atactic: Functional groups arranged in random manner

For example, atactic polypropylene is a gummy solid, while

isotactic version is highly crystalline & tough.

Tacticity:

• Polymerization

Fundamental process in which low molecular weight

compounds combine to form giant molecules/

macromolecules of high molecular weight

Three types of polymerization

• Addition

• Condensation

• Copolymerization

Types of Polymerization

Addition Polymerization

• formed from the monomer, without the loss of any

byproduct, like small molecules. Monomers with

double or triple bonds tend to polymerize without the

liberation of small molecules. Example: Polyethylene

(PE)

• It yields product that is an exact multiple of the original

monomer unit

Common Polyolefins

Monomer Polymer

Ethylene

H3CCH3

nRepeat unit

Polyethylene

CH3

CH3

n

CH3 CH3 CH3 CH3 CH3 CH3CH3Propylene

Polypropylene

PhCH3

n

Ph Ph Ph Ph Ph PhPhStyrene

Polystyrene

ClCH3

n

Cl Cl Cl Cl Cl ClClVinyl Chloride

Poly(vinyl chloride)

F2C CF2

Tetrafluoroethylene

F3C

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

CF3

nPoly(tetrafluoroethylene): Teflon

Condensation/Step Polymerization

• Formation of polymers from polyfunctional monomers

of organic molecules with elimination of small

molecules like water, HCl

• Functional group of one monomer unit reacts with

functional group of the other

• Eg- Nylon-66 (hexamethylene diamine + adipic acid)

Polyesters & Amides

Monomer Polymer

CO2HHO2CHO

OH

O O

HO OH2C

H2C O

nTerephthalic

acidEthylene

glycol

Poly(ethylene terephthalate

H

Ester

HO OH

O O

4H2N NH24

Adipic Acid 1,6-Diaminohexane Nylon 6,6HO N

HNH

H

O O

4 4n

CO2HHO2C

Terephthalic acid

NH2H2N

1,4-Diamino benzene

Kevlar

O

HO

OHN

HN H

n

Amide

Natural PolymersMonomer Polymer

Isoprenen

Polyisoprene:Natural rubber

O

H

HO

H

HO

H

HOHH

OH

OH

Poly(ß-D-glycoside):cellulose

O

H

O

H

HO

H

HOHH

OH

OH

H

n

ß-D-glucose

H3N

O

O

R

Polyamino acid:protein

H3N

OHN

R1

OHN

Rn+1

O

OH

Rn+2n

Amino Acid

BaseO

OH

OP

O

O

O

oligonucleic acidDNA

NucleotideBase = C, G, T, A

BaseO

O

OP

O

O

O

DNA

DNA

Copolymerization

• Specific type of addition polymerization, without loss

of any small molecules

• Monomers of more than one type are involved thereby

giving variety of polymers

• Eg. Styrene-Butadiene rubber (Buna-S)

Mechanism

• Initiation: Free radicals formed from H2O2 through

the addition of heat

Free radical acts to open the C=C double bond by joining to one

side of the monomer.

This allows the monomers to react with other open monomers on

their other side.

• Propagation: Process continues with successive addition of

monomer units to the chains

• Termination: Once the desired polymer is obtained, the

polymerization reaction is terminated by combining two

chains

Mechanism

Polymer Crystallinity

• Polymers are never completely crystalline crystalline

regions with amorphous regions are together

• Polymer is crystalline if all molecules are arranged in an

orderly manner with symmetrical orientation

• Degree of crystallinity depends on DP & tacticity

• Crystalline polymers possess high density, sharp melting

points, strong, brittle and hard

• Amorphous polymers do not possess melting points,

but softening points

Melting and Glass transition temperatures

• Lowest temperature beyond which polymer becomes

hard, brittle, glass-like or temperature at which polymer

experiences transition from rubbery to rigid states is

called glass transition temperature (Tg)

In this state, solid tends to shatter if it is hit, since the

molecular chains cannot move easily.

• Temperature above which polymer turns out to be

flexible, elastic and rubbery (Tm)

Beyond glass transition temperature, crystalline and amorphous

polymer behaves differently as shown in the diagram below.

Effect of heat on polymer

Significance

• Tg and Tm are significant parameters

• Gives an indication of the temperature region at which

a polymeric material transforms from a rigid solid to a

soft viscous state

• Helps in choosing the right processing temperature in

which materials are converted into finished products

Factors affecting Tg

• Tg is directly proportional to the molecular

weight of the polymer.

• Greater the degree of cross-linking, higher the Tg.

• Polymers with strong intermolecular forces of

attraction have greater Tg.

• Side groups, especially benzene and aromatic

groups attached to main chain increases Tg.

Number average Molecular weight

• Consider a polymer sample in which n1, n2 and n3… are number

of molecules with molecular weights M1,M2,M3…respectively

then,

=

=

Where, ni is number of molecules of mass Mi

Molecular Weight

Number of Molecules, Ni Mass of Each Molecule, Mi

Total Mass of Each Type of

Molecule, NiMi

1 800,000 800,000

3 750,000 2,250,000

5 700,000 3,500,000

8 650,000 5,200,000

10 600,000 6,000,000

13 550,000 7,150,000

20 500,000 10,000,000

13 450,000 5,850,000

10 400,000 4,000,000

8 350,000 2,800,000

5 300,000 1,500,000

3 250,000 750,000

1 200,000 200,000

Total Mass = NiMi = 50,000,000

NiMi, where Ni is the number of molecules of weight Mi

Ni = 100

The number average molecular weight for this sample is then,

NiMi / Ni = 50,000,000/100 = 500,000

The number average molecular weight

Weight Average Molecular Weight

• If m1, m2, m3… are masses of the species with molecular weights

M1,M2, M3… resp. (m1=n1M1 where, n1 is number of molecules

with molecular weight M1)

But, mi=niMi

=

=

=

(mi=niMi where, ni is number of molecules with molecular weight Mi)

Number of

Molecules

Mass of Each

Molecule

Total Mass of

Each

Type of Molecule

Weight Fraction

(Wi)

Type of Molecule

(Ni) (Mi) (NiMi) (NiMi/ NiMi) (WiMi)

1 800,000 800,000 0.016 12,800

3 750,000 2,250,000 0.045 33,750

5 700,000 3,500,000 0.070 49,000

8 650,000 5,200,000 0.104 67,600

10 600,000 6,000,000 0.120 72,000

13 550,000 7,150,000 0.143 78,650

20 500,000 10,000,000 0.200 100,000

13 450,000 5,850,000 0.117 52,650

10 400,000 4,000,000 0.080 32,000

8 350,000 2,800,000 0.056 19,600

5 300,000 1,500,000 0.030 9,000

3 250,000 750,000 0.015 3,750

1 200,000 200,000 0.004 800

Weight Average Molecular Weight = WiMi = 531,600

The distribution of molecular weights in a polymer sample is often described by the ratio of the

weight average molecular weight to the number average molecular weight. In this case the ratio is

531,600/500,000 = 1.063. This ratio is the Polydispersity Index (or PDI).

Average molecular weight

• Viscosity Average Molecular weight

Where, ni is the number of individual molecules having

the molecular mass Mi .

Exponent a, between 0.6 and 0.8 for many

polymer/solvent systems

• In 1953, Hermann Staudinger formulated a macromolecular structure for rubber and received the Nobel Prize.

isoprene

based on the repeating unit 2-methylbuta-1,3-diene

Rubber

Elastomers/ Rubbers

• The polymers possessing elasticity to the extent of

nearly 200 to 300 percent are known as Elastomers

or Rubber

• Amorphous polymer with numerous cross linkages

and high degree of elasticity deformed by

stretching & regain original form when stretching

force is removed

• Rubber has no crystallinity. Their extension and contraction are

due to temporary movements of segments of polymer chain.

The chains do not slip past each other due to cross linkages

U n s t r e s s e d R u b b e r S t r e s s e d R u b b e rA p p l i e d r e l e a s e o f

S t r e s ss t r e s s

B a c k t o o r i g i n a l p o s i t i o n

Properties of Rubber

Important properties of rubber are its

• Flexibility

• Strength

• Impermeability to water

• High resistance to abrasion etc,

Due to these properties rubber is highly useful for industrial as well

as domestic purposes.

Types of Rubber

1. Natural Rubber- Obtained from natural sources

1. Synthetic Rubber- Made synthetically

• Raw material from rubber tree (Hevea brasiliensis) is tapped every second day for its

sap, known as latex, by making slanting cuts in

the bark of the tree.

• Latex is collected and acetic acid is added to it so

as to precipitate out the rubber, which then

hardens/coagulates.

• After being washed and dried, rubber is cured in

special smokehouses to protect it against

microbial attack.

• Purer the rubber, higher the grade – it is ready

for delivery to rubber companies worldwide.

Natural Rubber

• Natural Rubber

• Polymer of isoprene (2-methyl-1,3-butadiene)

• Low tensile strength, elasticity over a narrow

range of temperature

RubberDestructive

distillation

Isoprene

Dipentene

+

indicates number of isoprene units.

Molecular weight of rubber is very high of about 300,000.

Structure of Rubber

Rubber which is composed of all cis-linked isoprene units,

forms an amorphous structure that is highly elastic. On the

other hand, gutta percha, which is a polyisoprene compound

made of all trans-linked isoprene units, forms linear strands

which can interact into crystalline arrays that have plastic

properties, but are not elastic.

• Its plasticity is greater than elasticity. It can’t

sustain stress. Thus when stretched to a great

extent, it undergoes deformation permanently.

• It has large water absorption tendency, which

make it week

• Limitations of natural rubber: It softens at high

temperature and becomes brittle at low

temperature.

• Natural rubber is attacked by acids, oxidizing

agents, non-polar solvents and oxidized by air.

To overcome these limitations rubber is

vulcanized.

Drawbacks of Natural rubber

Vulcanization

• Vulcanization is a process which is essentially

compounding rubber with different chemicals like

sulphur, H2S, benzoyl chloride etc.

• Heating raw rubber with sulphur at 100 -140 ˚C.

Sulphur enters the double bonds of rubber and forms

cross-linkages. Excellent changes in properties,

resistance to changes in temperature, increased

elasticity, tensile strength, durability, chemical resistance

• Brings about stiffening of rubber by anchoring &

restricting intermolecular movement by providing

cross-linkages between chains.

Vulcanization

• The toughness or stiffness of vulcanization

depends on the amount of sulphur included.

• For flexible tyre rubber, sulphur content is

from 3-5% whereas for tougher variety like

ebonite, content of sulphur is 32 %.

• Ebonite is so tough that it can be machined

and has very good electrical insulation property.

Property Raw Rubber Vulcanized Rubber

Elasticity Very high Low, depending on % S

Tensile Strength 200 kg/cm2 2000 kg/cm2

Chemical resistance Very poor Higher

Durability Less Higher

Quality Inherent Can be controlled by

vulcanization

Applications of rubber

• Due to remarkable resistance to electricity, it is used as

an insulating coating on wires and cables, used for

electric power transmission.

• Due to its elasticity, it is used to fabricate rubber bands,

rubber goods, golf balls, tubes for automobiles, etc

• It acts as an excellent adhesive

• Foam-rubber is used for making pillows, cushions,

mattresses, automotive pads, etc.

• Polysulfide rubber is used as a solid-propellant fuel for

rocket motors.

Preparation, Properties and Uses of

Commercial Rubbers

Copolymerization of butadiene & styrene carried out at 5oC

Good and tough mechanical properties

Easily attacked by oxidizing agents, mainly ozone, organic solvents

Uses: Manufacture of tyres, insulating wires and cables, adhesives,

lining of vessels

Buna-S Rubber/Cold Rubber

Synthetic rubber is any vulcanizable man-made rubber-like polymer

which can be stretched to twice its length and on releasing the stress, it

returns to its original shape and size.

Polyurethane (Isocynate) Rubber

• Ethylene glycol polymerizes with ethylene diisocyanate to form

polyurethane rubber.

• Highly resistant to oxidation

• Resistant to organic solvents, attacked by acids and alkali

Uses: surface coatings and manufacture of foams and fibers

The world consumption of synthetic polymers

: 150 million metric tons per year.

1) Plastics : 56%

2) Fibers : 18%

3) Synthetic rubber : 11%

4) Coating and Adhesives : 15%

Industrial Polymers

Plastics

ThermoPlastics/Thermosoftening Polymers

• Some polymers when heated become soft and can be

moulded into any shape that can retain on cooling

• PVC, PE, nylon sealing wax, etc

Thermosetting polymers

• On heating, polymers undergo a chemical change and

become an infusible mass which cannot be reshaped

• Bakelite, polyester, resins

The polymeric materials, which are rigid, dimensionally

stable and usually brittle are known as plastic.

Thermoplastic polymers Thermosetting polymers

They soften on heating and harden on

cooling

They are fusible on initial heating, but

turn into hard infusible mass on heating

further

Can be reshaped and recycled Cannot be reshaped and recycled

Formed by addition polymerization Formed by condensation polymerization

Linear in structure Three dimensional in structure

They are soluble in some organic solvents Insoluble in organic solvents

Moulded articles are taken out after cooling

the mould to avoid deformation of the

article

Moulded articles are taken out from the

mould even when they are hot.

e.g. Polyethylene, polystyrene, PVC, PVA e.g. Nylon 6:6, Phenolformaldehyde,

ureaformaldehyde,

Comparisons

Compounding of Plastics

• Unusual for a finished high polymeric articles to solely

consist of high polymers alone

• Mixed with ingredients known as additives resulting in

useful functions and imparts useful properties to the

finished products

• Main types of compounding ingredients are

– Resin: Binder, which holds different constituents/additives

together. Natural or synthetic resins used in this case

• Plasticizers: Low MW organic liquids added to polymer to

improve its flexibility; Added 8-10% of total bulk of plastics

(oils, camphor, dioctyl phthalates) The small molecules penetrate

into the polymer matrix and neutralize a part of intermolecular

forces of attraction between macromolecules and increase

mobility of polymer segments so that chains can slide over each

other. Hence, plasticizers act as an internal lubricant

• Stabilizers: Most polymers do not possess chemical

stability change colors & decompose

– Stabilizers are additives which chemically stabilize the polymer

and thus arrest degradation

– Organic, inorganic, organometallic compounds like CaO, BaO,

Organo-tin compounds

• Fillers/Extenders: Inert material added to enhance

mechanical strength-- asbestos powder, saw dust, cotton

pulp, clay, etc

• Lubricants: Glossy finish to product, Prevents plastics

from sticking to fabrication equipments; oils, waxes,

soaps, etc

• Catalysts

Antioxidants like H2O2, benzoyl peroxide, ZnO, NH3,

Ag, Pb, are added to the polymeric matrix to accelerate

the cross linking in thermosetting plastics while

moulding process

• Coloring materials

Organic dyes and pigments impart desired color for

aesthetic appeal of the finished polymeric material.

Some colors are added to impart UV protection to the

finished products.

Preparation, Properties and Uses of

Commercial Plastics

Phenol Formaldehyde Resin

Acid or a base as a catalyst to undergo condensation polymerization

product nature depends on the catalyst and ratio of phenol and formaldehyde.

Novolac resin is a linear thermoplastic polymer, whereas

Bakelite is a cross-linked thermosetting polymer.

Properties and Uses

• Phenolic resins are rigid, hard, water resistant

• Resistant to acids, salts, organic solvents

• Easily attacked by alkalies due to the presence of free

hydroxy groups

• Possess electrical insulating properties due to low

thermal conductivity

• Uses:

– Used to fabricate insulators, plugs, switches

– Used as cation-exchanger resin in water softening

– Adhesives in paints and varnishes

– Propellar shafts for paper industry and mills

Poly(methyl methacrylate) (PMMA)

• Poly(methyl methacrylate) (PMMA) is a transparent

thermoplastic often used as a lightweight or shatter-

resistant alternative to glass.

• Although it is not technically a type of glass, the

substance has sometimes historically been called acrylic

glass.

• Chemically, it is the synthetic polymer of methyl

methacrylate.

Property

Applications of PMMA

Safety glass such as Plexiglass and Lucite – uses

range from windows for aquariums and under-

water restaurants to safety shields at hockey

rinks to skylights in your home to simple

paperweights

Used as bone cement for use in arthroplastic

procedures of the hip, knee, and other joints for

the fixation of polymer or metallic prosthetic

implants to living bone

Used in Pacemakers

Artificial eye lenses used for cataract surgery

Urea formaldehyde resin

Monomethylol urea on polymerization, yields a linear UF resin polymer

• Urea-formaldehyde resin yields clear, water-white

products.

• Hardness, tensile strength is comparatively better than

phenolic resins

• Better heat & moisture resistance

Uses:

• Adhesives for plywood, decorative laminates- surface

coatings

• Due to their colorability, solvent, grease resistance

cosmetic containers

• Electrical insulation

Conducting Polymers

• Polymers are poor conductors of electricity, due to non-

availability of large number of free electrons

• The polymeric material which possess electrical

conductivities on par with metallic conductors known as

conducting polymer.

• Polymers with polyconjugated structures are insulators in

pure state, but when treated with oxidizing or reducing

agents can be converted into polymer salts with electrical

conductivities comparable to metals.

What makes the material conductive?

Diamond, which contains only σ bonds, is an insulator and its

high symmetry gives it isotropic properties.

Graphite and acetylene both have mobile π electrons and are,

when doped, highly anisotropic metallic conductors.

Conducting polymers are classified as

• π-electron conducting polymers: In these polymers, backbone of the

polymer is made up of molecules that contain conjugated π-electrons which

extend the entire polymer and make the polymer conducting.

• Conducting element-filled polymer: Here, polymer acts as a binder that

binds the conducting elements like carbon black, metal oxides, metallic fibres

that conduct electricity.

• Inorganic polymer: A metal atom with polydentate ligand, which is a charge

transfer complex is bound to the polymer to make it conducting.

• Doped- conducting polymer: Polymer is made conducting by exposing the

surfaces to charge transfer agents in gas or in solution phase.

• Blended conducting polymer: This polymer is made by blending a

conventional polymer with a conducting polymer.

Types of conducting polymers

Conditions

• Polymer should consist of alternating single and

double bonds called conjugated double bonds

• Polymer matrix has to be disturbed –

– Either by removing electrons from them (oxidation),

– Or inserting electrons into the material (reduction).

The process is known as doping

– By doping with electron donor like alkali-metal ion

or electron acceptor like AsF5, Iodine, etc polymers

turn conductive materials

Fist conditions to become conductive:

1-The first condition for this is that the polymer consists of

alternating single and double bonds, called conjugated

double bonds.

In conjugation, the bonds between the carbon atoms are

alternately single and double. Every bond contains a

localised “sigma” (σ) bond which forms a strong chemical

bond. In addition, every double bond also contains a less

strongly localised “pi” (π) bond which is weaker.

2-The second condition is that the plastic has to be disturbed

- either by removing electrons from (oxidation), or

inserting them into (reduction), the material. The process is

known as Doping.

• There are two types of doping:

1-oxidation with halogen (or p-doping).

2- Reduction with alkali metal (called n-doping).

xNaCHxNaCH

x

nn

3

2

3ICHI

xCH nn

2nd conditions to become conductive:

• Iodine molecule attracts an electron from polyacetylene chain and

becomes I3-

• Polyacetylene molecule, now positively charged, is termed a radical

cation, or polaron• Lonely electron of the double bond, from which an electron was

removed, can move easily.

• As a consequence, double bond successively moves along the

molecule– Conducting Polymers

Polypyrrole

•Hetero atomic polymers •More stable•Easy to prepare•Greater opportunity to functionalize

Polypyrole

Conducting

PolymersTelecommunication

Aerospace

Battery

technology

Smart

Materials

Applications

Self healing polymers

• Inspired from biological systems ‘Wound healing’

• Inherent ability of polymers to repair damage caused by

mechanical usage over time

• Terminator Polymers

• Chemistry World posted a video of the product in action,

showing someone cutting a piece of the polymer in two with a

scalpel, pressing the pieces back together and leaving it on a table

for two hours at room temperature. The person is unable to pull

the material apart with their hands upon returning.

• This is the next generation breakthrough in polymers.

• Autonomic healing: A propagating crack ruptures the

microcapsules, releasing the healing agent into the crack

plane by capillary action. Polymerization is initiated by

contact with the embedded catalyst or initiator, bonding

the crack faces, and restoring structural continuity.

• Non-autonomic healing: Partially self-

contained; healing capability is designed into the

material, but additional external stimuli such as

heat or UV-radiation is required for the healing

to occur.

Applications

• Nissan Motor Co. Ltd has commercialized

world’s first self-healing clear coat for car

surfaces-trade name of this product is ‘Scratch

Guard Coat’

• Self healing concretes– in progress

• Self-healing materials are now used as composites

in aircrafts.

Thank you