chain growth polymersization

Dr. S.Ray_Chem Engg_NIT Agratala 4/12/2013

Polymer Processing Engineering 1

Chain growth Polymerization

Chain Polymerization



Chain Polymerization

• A monomer with double bond (vinyl, divinyl, 1,3 diene)

• Two growing chain can’t join together like in step poly.

• can be very high• Monomer consumed relatively slow but high MW

polymers are achieved quickly• Initiation and propagation mechanisms are

different.• Active centre always located at end of growing

chain

DP

Chain-growth Polymerization

Chain-Growth Polymerization (Addition) Processes

1. Free radical Initiation Processes

2. Cation Initiated Processes

3. Anion Initiated Processes

4. Group Transfer Polymerization

5. Coordination Polymerization



Free Radical Polymerization

• Free radical are independently-existing species tha t have unpaired electron. Normally they are highly reactive with short life time.

• Free radical polymerization’s are chain polymerization’s in which each polymer molecules grows by addition of monomer to a terminal free-radical reactive site known as active center.

• After each addition the free radical is transferred to the chain end.

• Chain polymerization is characterized by three distinct stages, Initiation, propagation and termination.


INITIATION

• This stage is a two steps stage• 1. The formation of free radicals from an initiator.• 2. The addition of one of these free radicals to a

monomer molecules.

• Free radical can be formed by two principal• 1. Homolytic scission (homolysis) or breakage of

a single bond.• 2. Single electron transferred to or from an ion or

molecule (redox reactions)

Free Radical Polymerization




Free radical Polymerization –Initiation Step

• Polymerization is initiated by formation of free radicals or initiators

• Free radicals are species containing an unpaired electron

• Denoted as R•

• Free radicals can generated by 1. thermal breakdown of organic peroxides, Hyperperoxides,

Azo, diazo compounds2. photochemical decompostion of metal iodides, metal alkyls

and azo compounds3. Redox reactions4. Persulphates – decomposition in aqueous phase5. Ionizing radiation such as α, β, γ or x-rays

Initiation - Free RadicalsA) Thermal Homolysis or decomposition• Useful for organic peroxides or azo compound like benzoyl peroxides

homolysis is chemical bond dissociation of a neutral molecule generating two free radicals. That is, two electrons that are involved in the bond are distributed one by one to the two species.A−B → A• + B•



Initiation –Free radicals

Initiation - Free Radicals

B) Photochemical Decomposition•Applicable to metal iodides, metal alkyls and azo compounds•Eg AIBN decomposed by radiation with wavelength 360 nm



REDOX REACTION• Redox reaction defined as the generation of free

radicals by electron transfer and it is use whenpolymerization performed at low temperature.

• Example� CH3 CH3

� -C-OOH + Fe2+` -C-O + OH + Fe3+

� CH3 CH3

� Cumyl Ferrous Cumyloyloxy

� hydroperoxide ion radical

� O O O O O O

�O-S-O-O-S-O + HO-S-O O-S-O + O-S-O + OH-S-O

� O O O O

�Presulphate Bisulphate Sulphate Sulphate Bisulphate� ion ion ion radical radical


C) Redox reactions



• An active center is crated when a free radical (R o)which is generated from an initiator attacks the ππππ-bond of the monomer molecules.

R + CH2 = CH R-CH2-CH or R-CH - CH 2

X X X

This is more This is morelikely stable

Sometime free radical react with each other such as:O O O

2 -C-0 -C-O- + C = O

OR

2


D) Persulphates• Useful in emulsion polymerisation• Decomposition occurs in aqueous phase• Radical diffuses into a hydrophobic monomer containing droplets• Eg

S2O82- � 2 SO4

• -

E) Ionizing Radiation • α, β, γ or x-rays can be used to initiate a polymerization• Cause the ejection of an electron, then dissociation and electron capture to produce radical



PROPAGATION• The addition of monomer molecules to the active center to

grow the polymer chain.• There are two modes of chain propagation

�Again mode (1) are more dominant.

�Time of addition for each monomer is of the order of a millisecond.�Thus several thousands of additions can take place in a few seconds.

�Therefore polymer structure are like�-----CH2-CH-CH 2-CH-CH 2-CH-CH 2-CH------

X X X X

1. Head to Tail

R-CH2-CH + CH2=CH R-CH2-CH-CH 2-CH

X X X X

2. Head to Head

R-CH2-CH + CH2=CH R-CH2-CH-CH-CH 2

X X X X

Free Radical Polymerization: Termination

• Termination of chains can take place in several ways

1. Interaction between two active chain ends

2. Reaction of an active chain end with an initiator radical

3. Termination by transfer of active centre to another molecule (may be solvent, initiator or monomer )

4. Interactions with impurities (e.g. oxygen) or inhibitors

• Most important termination reaction

– Combination : Process of Combining two chains, coupling together at their ends (e.g. Polystyrene for temp > 333K)

– Disproportionation: involves the abstraction of a hydrogen atom from one end to give an unsaturated group and two dead polymer chain (e.g. PMMA for T > 333K)



Free Radical Polymerization: Termination

Termination: Chain Transfer

• Cases of premature termination.• Chain transfer is essential to continue chain reaction




Termination : InhibitorObjective: To suppress polymerization



Termination: Inhibitor

•ATRP involves the chain initiation of free radical polymerization by a halogenated organic species in the presence of a metal halide species. •Transfer of Cl atom prevent termination

Variation: Free radical polymerization



Methods of Free Radical Polymerization

There are four commonly used methods for freeradical polymerization

• 1. Bulk polymerization - With monomer only• 2. Solution polymerization – in a solvent• 3. Suspension polymerization - With monomer

dispersed in an aqueous phase

• 4. Emulsion polymerization – as emulsion

Free radical polymerization

BULK POLYMERIZATION• The simplest method of polymerization where the rea ction

mixture contains only the monomer and a monomer sol uble initiator.

• Example: PMMA

• Advantage of the high concentration of monomer result in

• 1. High rates of polymerization• 2. High degree of polymerization• 3. High purity of product• 4. High molar mass polymer are

produce

• Disadvantage• Increase in the reaction

viscosity with conversion.• *difficulty in removing heat.• *autoacceleration• *if the polymer formed is

insoluble in the monomer (such as acrylonitrile, vinyl chloride) �� polymer separate by precipitating �� kinetics does not apply




SOLUTION POLYMERIZATION

• This method is used to solve the problems associate d with the bulk polymerization because the solvent is employed to lower the viscosity of the reaction, thus help in the hea t transfer and reduce autoacceleration.

• BUT•• The disadvantage of solvent presence is• 1. Reduce monomer concentration which results in

decreasing the rate of the reaction and the degree of polymerization.

• 2. Solvent may cause chain transfer.• 3. Clean up the product with a non solvent or evapor ation of

solvent.


SUSPENSION POLYMERIZATION

• This method is used also to solve the problem ofheat transfer.

• It is similar to bulk polymerization where the reactionmixture is suspended as droplets in an inert medium.

• Monomer, initiator and polymer must be insoluble inthe suspension media such as water.




EMULSION POLYMERIZATION

• This is similar to suspension polymerization exceptthat the initiation is soluble in suspension media andinsoluble in the monomer.

• The reaction product is colloidally stable dispersionknown as latex.

• The polymer particles have diameter in the range of(0.05 - 1 µµµµ m) smaller than suspension.


Techniques of Free Radical Polymerizations



IONIC POLYMERIZATION

• Chain polymerization of olefinic monomers which canoccur with charged radicals. For example styrene.

�Cationic polymerization when the radical is positively cha rge.

�Anionic polymerization when the radical is negatively char ge

• This polymerization are more monomer specific than freeradical polymerization and will proceed only with monomersthat have substituent groups which can stabilize the activecenter.

• For cationic polymerization the active center will proceed if Xgroup is able to donate electrons or delocalize the positivecharge.

• For anionic polymerization the active center will proceed i f Xgroup is able to withdraw electrons or delocalize the negati vecharge.

Kinetics and Mechanism



Factors affecting the kinetics of chain growth polymerization



Chain Growth Polymerization Step Growth Polymerization

The only growth reaction is addition of monomer to a growing chain with a reactive terminus

Reaction can occur independently between any pair of molecular species

The reaction mixture consists of high polymer and unreacted monomers, with very few actively growing chains

The reaction mixture consists of oligomers of many sizes, in a statistically calculable distribution

Monomer concentration decreases steadily as reaction time increases

Monomers disappear early, in favor of low oligomers

High polymer appears immediately, average molecular weight doesn't change much as reaction proceeds

Oligomers steadily increase in size, polymer average molecular weight increases as reaction proceeds

Increased reaction time increases overall product yield, but doesn't affect polymer average molecular weight

Increase T, increase reaction rate, decrease molecular mass

Long reaction times are essential to produce polymer with high average molecular weight

Reaction rate slow at Tambient but increase in T has little effect on chain length of polymer

chain growth polymersization

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