The chemistry of_polymers.

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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
NattaSpecific 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