Steady-state Polymerization Reaction Steady-state Polymerization Reaction 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 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 (1) stop the reaction before chain diffusion becomes difficult. (2) Use a dilute solution of monomer.

Degree Of Polymerization 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  ] Xn = _____________________________ 2 K tc [M  ] 2 + 2k td [M  ] 2 at S.S.C. d[R  ] - [dM  ] _________ = ______________ dt dt

q = 0 when termination occur by combination only. q =1 when termination occur by Disproportionation only. from equation 1 and 3 - d[M  ] R i = ___________ = 2 k t [M  ] 2 dt R i ==> [M  ] = ( _______ ) 1/2 2k t k p [M] Xn = _______________________________ R i (1 + q) k t 1/2 ( ____ ) 1/2 2 Where k td q = _______ = fraction of termination reaction. K t

CHAIN TRANSFER 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. M  i + T-A = M i - 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 H 2 or halogen atom). M + A  AM i  Chain transfer to polymer may result in the formation of branched polymer molecules. This is known as intramolecular or back-biting reaction.

Example Polymerization of ethylene. H ----CH  CH  CH-CH 2 CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 CH 2 = CH 2 CH 2 C  H CH-CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

Inhibition And Retardation 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.

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