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CHAIN POLYMERIZATION

Free Radical Polymerization Free radical are independently-existing species that have

unpaired electron. Normally they are highly reactive

with short life time.

Free radical polymerizationsare chain polymerizationsin 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.

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Example

The formation of polyvinyl monomer.

CH2= CHX

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INITIATION

This stage is a two steps stage

1. The formation of free radicals from aninitiator.

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 ofa single bond.

2. Single electron transferred to or from an ionor molecule (redox reactions)

Homolytic can be achieved by heat (thermolysis)

or by light such as U.V. (photolysis).

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Example

Sometimes the radicals undergo further breakdown (-

scissions) such as

O O O

-C-0-0-C- 2 -C-O

Benzoly peroxide Benzolyooxy radicals

(CH3)2C-N = N- C (CH3)2 2(CH3)2C + N2

CN CN CN

Azobisisobutyronitril 2-Cyanopropyl radicals

O O

-C-0 + C = O

(CH3)2 - C-O CH3 + (CH3)2-C = 0

CH3 Methyl acetone

radical

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REDOX REACTION

Redox reaction defined as the generation of freeradicals by electron transfer and it is use when

polymerization performed at low temperature. Example

CH3 CH3

-C-0-0H + 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 0-S-O + O-S-O + OH-S-O

O O O O

Presulphate Bisulphate Sulphate Sulphate Bisulphate

ion ion ion radical radical

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An active center is crated when a free radical (Ro)which is generated from an initiator attacks the -bondof the monomer molecules.

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

X X X

This is more This is more

likely stable

Sometime free radical react with each other such as:

O O O

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

OR

2

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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-CH2-CH-CH2-CH-CH2-CH------

X X X X

1. Head to Tail

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

X X X X

2. Head to Head

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

X X X X

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TERMINATION

The last stage of chain reaction in which the growth of

the polymer chain terminated (or stopped).

There are two mechanisms of termination

1. Combination

Coupling together of two growing chains to form a

single polymer molecules. polystyrene

2. Disproportionation

when a hydrogen atom move from one growing chain toanother

--CH2-CH + CH - CH2--- ---- CH2-CH- CH-CH2---

X X X X

H H

--CH2-CH + C - C--- ----CH2-CH2+ CH=CH2---

X X H X X

Saturated end Unsaturated end

group polymer group polymer

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Thank You

See You Next Lecture