Chain polymerization free radical polymerization
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CHAIN POLYMERIZATION Free Radical Polymerization PowerPoint PPT Presentation


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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.

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CHAIN POLYMERIZATION Free Radical Polymerization

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Chain polymerization free radical polymerization

CHAIN POLYMERIZATIONFree Radical Polymerization

  • Free radical are independently-existing species that 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.


Example

Example

  • The formation of polyvinyl monomer.

  • CH2 = CHX


Initiation

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)

  • Homolytic can be achieved by heat (thermolysis) or by light such as U.V. (photolysis).


Example1

O O O

  • -C-0-0-C- 2 -C-O

    Benzoly peroxide Benzolyooxy radicals

  •  (CH3)2 C-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

Example

  • Sometimes the radicals undergo further breakdown (-scissions) such as


Photolysis

PHOTOLYSIS

  • Photolysis is the second principle of free radical formation.

  • The advantage of this method is that the formation of free radicals begins at the instant of exposure and cases as soon as the light source is removed.


Redox reaction

  • 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

REDOX REACTION

  • Redox reaction defined as the generation of free radicals by electron transfer and it is use when polymerization performed at low temperature.

  • Example


Chain polymerization free radical polymerization

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

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


Propagation

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

  • Therefore polymer structure are like

  • -----CH2-CH-CH2-CH-CH2-CH-CH2-CH------

    X X X X

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.


Termination

  • --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

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 to another


Thank you

Thank You

See You Next Lecture


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