Transcript Note - kau.edu.sa

Steady-state Polymerization Reaction
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1. At the start of the polymerization reaction the rate of
formation of free radical is greater than rate at which they lost by
termination.
2. [M ] increase rapidly and so the rate of loss of radicals by
termination fast step increases.
3. [M ] will reach stage at which the rate of loss of free radical
by termination is exactly equal the rate of formation.
4. The net rate of change in [M] is the equal to zero.
5. At this stage the reaction is said to be under steady-state
conditions S.S.C.
Normally all free radical polymerization operate under S.S.C.
All the time but the first few seconds.
If the reaction is not at S.S.C. Then [M] will increase
continuously and the reaction will go out of control and could
lead to an explosion.
AUTOACCELERATION
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This phenomenon is observed when,
 [M] is high a sharp increases in the rate of propagation as the
conversion of monomer increases.
 This will lead to increase in the viscosity media and the large
molecules become slower and the termination rate drops because the
mobility of the long chain radical reduced and they begin to have
difficulty in moving into close for termination to occurs.
 This will result in a dramatic decrease in kt for termination and an
increase in rate of propagation.
 The initiation and propagation reaction will not be affected because
the monomers molecules are small and have high mobility even in
high viscous media.
 Free radical polymerization are exothermic reaction and energy is
increases as autoacceleration begins ==> if the dissipation of energy
is poor there may be an explosion.
 This can be avoid by
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(1) stop the reaction before chain diffusion becomes difficult.
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(2) Use a dilute solution of monomer.
Degree Of Polymerization
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The number-average degree of polymerization ,
Xn of polymer produced is given by
moles of monomer consumed in unit time
Xn =
______________________________________________________
moles of polymer consumed in unit time
kp [M][M ]
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Xn =
2 Ktc [M]2 + 2ktd[M]2
at S.S.C.
d[R ]
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dt
- [dM ]
=
______________
dt
from equation 1 and 3
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- d[M]
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Ri = ___________ = 2 kt [M ]2
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dt
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Ri
==> [M ] = ( _______)1/2
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2kt
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kp [M]
 Xn = _______________________________
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Ri
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(1 + q) kt 1/2 (____)1/2
Where
2
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ktd
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q = _______ = fraction of termination reaction.
q =0
Kt
 when termination occur by combination only.
 q =1
 when termination occur by Disproportionation only.
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CHAIN TRANSFER
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Xn are found to be experimentally < then those calculated
using the equation.
 This is due to the premature termination of the growing
chain
 This reaction known as chain transfer.
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M  i + T-A = Mi - T + A
 where
 T and A are fragments linked by single bond in a
hypothetical molecules.
 A is capable of initiating another active center such as
Free radical with a monomer.
 T and A (are often H2 or halogen atom).
 M + A
AMi
 Chain transfer to polymer may result in the formation of
branched polymer molecules. This is known as
intramolecular or back-biting reaction.
Example
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Polymerization of ethylene.
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H
----CH
CH
2
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CH2
----CH-CH2CH2CH2CH3
CH2
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CH2
CH2 = CH2
CH2C  H2
------CH-CH2-CH2-CH2-CH2-CH2-CH3
Inhibition And Retardation
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Some substances react with the active centers to produce
radical or non-radical which are incapable of re-initiating
polymerization.
 If the reaction polymerization is stopped the substance
called inhibitor.
 If the reaction polymerization is slowed down the
substance called retarden.
 Inhibitors usually add to monomer to prevent
polymerization during storage. Therefore it should be
clean before uses.
 Oxygen can act as a retarder or an inhibitor and must not
included in the polymerization, therefore free radical
polymerization are performed under an inert atmosphere.
Thank You
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