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The Chemistry of Polymers

• Where does every thing start?

– Raw materials for the polymer industry

Monomers ( how monomers are produced).

– Polymer synthesis

Polymerization

Polymerization

Reactor

Monomer

Pellets/powder

Controlling parameters

•Ratio of reactants

•Special additives

•Temperature

•Pressure

Separation and

recovery

• Different grades of polymers can be produced

The process by which monomers react and form long chain molecules is called

Polymerisation.

Monomer

• Any material that is able to be polymerized, possesses a

unique chemical structure , which is termed

polyfunctionality.

• Having double bond.

• Paraffins can not be polymerized.

• Cyclic monomers

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Monomer • Small molecules having bifunctional groups can be

polymerized (an organic acid and an alcohol neutralize each

other but if having one functional group can not further react to

form long chain).

• Dicarboyxlic acid and glycol to give ester that have two

opposing groups at the ends, which enables more and more

reactions, thus building a large ester molecule- a polyester.

Formation of polyethylene terephthalate (PET)

Raw Materials

• Petrochemical Industry

Crude oil Coal

Anaerobic Cracking

Acetylene, methanol, phenol

Distillation

Gasoline and Kerosene

Ethylene, propylene and butylene Natural Gas Methane

Acetylene from coal

Vinyl chloride from Acetylene

Ethylene from Acetlyene

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• Methanol from Coke

Polymers derived from Formaldehyde

Polyacetal, phenol formaldehyde, urea-formaldehyde

Melamine-formaldeyhde

Natural gas to methanol

Methane to accetylene

Monomers from Ethylene

Vinyl chloride

Styrene

Vinyl Acetate

PVC

PVA

PS

Emulsified vinyl acetate is

used as glue, paste for

adhesion.

Cellulose acetate yarn + rayon fibres

Combination fabrics simulate of silk and

linen

Monomers from Propylene

Acrylonitrile Methyl methacrylate

Epichlorohydrin Polyol (glycerol)

PAN PMMA

epoxy

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Monomers for thermosets

Urea formaldehyde

Melamine formaldehyde

Epoxy

Phenol formaldehyde

Polymerization

• Chain (Addition polymerization)

• Stepwise polymerization (Condensation

polymerization)

Polymerization

Chain (Addition polymerization)

• Long chains appear at the early stage.

• Monomers are added to long chains, and steadily disappear

during the process.

• No elimination of small molecules.

• Quite high molecular weights may be obtained (105 to 2 x 106

g/mol)

• Extension of polymerization only increases conversion not the

chain length.

Polymerization

Examples of polymers produced via Addition mechanism

PE PP

PS PVC

PTFE

PMMA

PAN

PIB

PVA

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Polymerization

Stepwise polymerization (condensation polymerization)

• At the early stages dimers, trimers, and tetramer are formed.

• Monomers disappear in the early stage.

• Elimination of small molecules.

• Molecular weights are low to medium (<50,000 g/mol)

• Extension of the polymerization increases both conversion

and molecular weights.

Polymerization

Examples of polymers produced via Condensation

mechanism

Phenol formaldehyde, Nylon 6,6 (polyamide), urea-formaldehyde,

Melamine formaledhyde, polyester (PET), polyurethane,

polycarbonate, etc.

Chain polymerization

Free radical

Anion

cation

Ionic polymerization

Free radical

polymerization

Chemical nature of the monomer (characteristics

of the substituent groups) dictates the preferred

mechanism

Free radical Polymerization

• Mechanism

Initiation

Free radical

is not a

catalyst

Common Intiator

Benzoyl peroxide

85-95 ºC

Reaction between free

Radical and monomer

Azobiisobutylronitrile (AZBN)

50-70 ºC

Will remain with growing chain

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Propagation

Termination

Occur frequently

And leads to

longer chain

Monomer reaches the chain radical

by diffusion and is attached to

growing chain.

Reactions proceed in fractions of a

second and lead to a giant chain,

with a free radical at the end.

Kinetics of polymerization

Propagation step determines

Rate of the

polymerization

Chain length

Rate of propagation= Rate of polymerization

rp depends on the concentration of reactants and rate constant Kp

Free radicals reach a constant concentration ( steady-state assumption)

Rate of intiation= Rate of termination

Rate of polymerization α

Concentration of the monomer

and square root

of the concentration of the initiator

Arrhenius Relationship

T-1

lnk

Slope (–E/R) indicates the sensitivity to

the effects of temperature

The rate increases roughly two fold to three fold with a 10 ºC rise in temperature.

Chain addition mechanism leads to formation of linear polymers

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Chain Transfer Reaction Free radical passes from the growing chain to another

molecule (usually a small one like solvent, the

monomer itself, or even a polymeric “dead” chain)

leading to the termination of the one chain together

with the starting of a new one that is capable of

growing.

Branches are

formed

Shortening

of the

chain

LDPE

Branched chain polymers

Ionic polymerization

Uses specific catalysts to serve as electron withdrawing

agents (Lewis acid) or electron donor ( amines)

Cationic catalyst

Anionic catalyst

Propagation step is in

principle resembles

that of free radical

polymerization

But termination is

different and difficult

Two growing same ion chains repel, for termination it is required that

growing ion has to react with a counterion or in a transfer reaction

carbanion

Ionic polymerization

Cationic Polymerisation

• Small activation energy

• Polymerisation occur at low

temperature

Ionic polymerization

Anionic Polymerisation

• Monomers with electron

withdrawing groups

• Living polymer chains

• Deliberate termination

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Coordinative (stereospecific)

Polymerization

Based on Catalysts Ziegler

Natta Specific catalysts orient the mers in the

chain into a highly ordered configuration

HDPE

Isotactic, syndiotactic, atactic

Control of properties

Crystallinity,

mechanical properties,etc.

Organo-metallic

compounds

(trialkyl aluminium

and titanium

tetrachloride in

solvents)

Tacticity/Stereoisomerism

Step wise ( condensation ) Polymerization

Polyfunctionality = 2 Linear chain

Polyester

Thermoplastics

Step wise ( condensation ) Polymerization

Polyfunctionality = 2 Linear chain

Polyamide (nylon 6,6)

Thermoplastics

Gears (no lubricant needed),

bearing, electrical mountings,

wire insulation, filaments

Step wise (Condensation ) Polymerization

Polyfunctionality = 3 Reactive branches, 3D

structures, Crosslinked,

vulcanization

Thermosets

Melamine formaldehyde

Phenol formaldehyde

Resistant to heat and moisture (Dinnerware and

decorative table tops (formica)

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Step wise (Condensation ) Polymerization

Polyfunctionality = 3 Reactive branches, 3D

structures, Crosslinked,

vulcanization

Thermosets

Phenol formaldehyde

Step wise (Condensation ) Polymerization

Polyfunctionality = 3 Reactive branches, 3D

structures, Crosslinked,

vulcanization

Thermosets

Step wise (Condensation ) Polymerization

Polyfunctionality = 3 Reactive branches, 3D

structures, Crosslinked,

vulcanization

Thermosets

Melamine formaldehyde

Phenol formaldehyde

Step wise (Condensation ) Polymerization

Polyfunctionality = 3 Reactive branches, 3D

structures, Crosslinked,

vulcanization

Thermosets

Melamine formaldehyde

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•Phenol formaldehyde.

•Three reaction sites available resulting in thermoset.

• Low cost materials

• Resoles and Novolacs

• Resoles (excess of formaldehyde used in alkali solution) are uncured

liquid linear (non-cross linked) polymers

•Novolacs (insufficient formaldehyde used in acid solution) are powders.

•Novolacs require curing agent (hexa, hexamethylene tetramine) and

heat to form Bakelite.

•Molded phenolic parts are made from Novolacs. These have high

shrinkage and are brittle in nature.

•Novolacs are blended with fillers to improve their strength and toughness

(phenolic molding powders)

•Common filler is wood flour

Phenol formaldehyde (Phenolics)

•Phenol formaldehyde has excessive OH groups which impart adhesive

ability to it which is used to produce phenolic adhesives used for

plywood, printed circuit boards, foundry shells and cores, sandpaper,

brake linings and grinding wheels.

• Mold releases are required to avoid problem of stickiness.

•Phenolics are nonflammable (char rather than melt or burn) and due to

the ability to form char they are used as coating for rocket nozzles.

•Low thermal conductivity (pan handles, bases for toasters, knobs for

appliances and motor housings)

•To improve resistance to UV dark pigment is usually added in phenolics.

•High electrical resistance (electrical switches, circuit breakers,

connectors, automotive electrical parts).

•Solvent sensitivity is good for organic solvents but is poor for acids and

bases.

•Compression molding is the technique for making products.

Phenol formaldehyde (Phenolics)

Unsaturated polyester

Linear polyester +

Ethylene glycol Maleic acid

MEKP (peroxide curing agent)

is used for room temperature curing,

Cobalt naphthanate is used as activator

Differences between Addition and

Condensation Polymerisation

• Polymer growth mechanism

• Dependence on previous step

• Initiator needed

• Type of monomer

• Number of active sites (functional groups per monomer)

• Number of different types of monomers needed to form polymer

• By-product formed

• Basis (polymer repeat unit representation)

• Polymer chain characteristics

• Branching

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Two or more monomers polymerize in situ

Process controlled by tendency to

homopolymerize and cross polymerize

Polymers formed by condensation polymerization are not copolymers

Copolymerization

Kinetic constants

Homopolymerization

Cross polymerization

Copolymers

Alternating copolymer

Random Chain copolymer

Block copolymer

Graft copolymer

Examples PVC-PVA, SBR, ABS, etc.

Example Problem

A copolymer consists of 15 wt.% polyvinyl acetate (PVA) and 85 wt.%

polyvinyl chloride (PVC). Determine the mole fraction of each

component.

Basis: 100 g

Small amount of PVA in PVC

reduces rigidity of PVC .

Example Problem

Determine the mole fractions of vinyl chloride and vinyl acetate in a

copolymer having a molecular weight of 10,250 g/mol and a degree of

polymerisation (DP) of 160.

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Techniques of Polymerization

Mass/Bulk Polymerization

Autoclave or reactor

Thermometer

Monomer and

initiator

•Entire batch is single

phase.

•Styrene to polystyrene

•Stirring required as

reaction mass becomes

viscous

•Batch process

•Process is used

extensively for

condensation

polymerisation (due to

low heat of reaction)

Water is circulated ( Ist steam

then cooling water)

Pros Cons

Purest products Heat removal problem for exothermic

reactions is difficult

Gives largest possible yield per

reaction volume

As concentration of reaction mass

increases , viscosity also increases,

agitation becomes difficult

No subsequent separation, purification

and recovery steps

Unreacted monomer traps

Mass Polymerization

Solution Polymerization

•Monomer is dissolved in an inert solvent

together with initiator.

•Can be batch or continuous

•Also called solvent polymerization

Heat removal and control easier

Tracking reaction easier because

rate of reaction are lower

A readymade solution of polymer is

directly available from the reactor. It

is useful in applications such as

lacquers and adhesives

Rate of polymerization is low, and it lowers the

average chain length of the polymer

Solvents toxicity and flammability increase cost

Recovery of used solvent required addition

technology increasing cost of production

Yield per reactor volume is less than that in mass

polymerization

Suspension Polymerization

•Monomer is mixed with a catalyst and then

dispersed as a suspension in water.

•Heat released by the reaction is absorbed by

the water.

•After polymerisation, the product is separated

and dried.

•Production of vinyl type polymers.

10-500 µm

Emulsion Polymerization

What is an emulsion?

A stable colloidal solution, e.g, Milk, consist of an

immiscible liquid dispersed and held in another liquid

by a substance called emulsifier

Soaps and surfactants have two ends of different solubility

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Monomer

Monomer takes position

in the micelle and

emulsion form. • The initiator is water soluble and

free radicals and the monomers

diffuse into the hydrophobic interior of

micelles while water is attracted to

hydrophilic exterior zone.

•Emulsion polymerization is suitable

for Addition reaction systems

R = 0.2-10 nm

Micelles serves as core for

the growing polymer chain

Good thermal control,

high production rate,

high molecular

weight, narrow MWD