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Lecture 5 Polymerization Mechanism(cont.)

Lecture 5 Polymerization Mechanism(cont.). 2- Addition mechanism. Polymerization –The mechanism of the process. 3-Chain Polymerization represented by the addition reaction. Long chains appear at the early stage. There is no elimination of small molecules.

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Lecture 5 Polymerization Mechanism(cont.)

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  1. Lecture 5Polymerization Mechanism(cont.) 2- Addition mechanism

  2. Polymerization –The mechanism of theprocess • 3-Chain Polymerization • represented by the addition reaction. • Long chains appear at the early stage. • There is no elimination of small molecules. • Quite high molecular weights may be obtained ( lo5to 2 x lo6) 4.Step wise polymerization • represented by the condensation reaction • There is (usually) an elimination of a small molecule. • Molecular weights are low to medium (below 50,000 normally) .

  3. Chain Mechanism Free Radical Polymerization

  4. Stages of Free Radical Polymerization • initiation (start) • propagation (growth) • chain transfer (stop/start) • termination (stop) • During initiation active centers are being formed. • During termination active centers disappear. • Growth rate of a chain is very high (103 - 104 units/s). • Chains with a degree of polymerization of 103 to 104 are being formed in 0.1 to 10 s.

  5. Initiation Kinetics The rate of radical production is then given by:

  6. The actual rate of initiation Ri is expressed in terms of the rate of radical production that leads to actual polymer chains growing!: where f is the efficiency factor: the fraction of radicals that really leads to initiation. The rate constant ki is NOT used in the mathematical description of the polymerization.

  7. Propagation This reaction is responsible for the growth of the polymer chain. It is the reaction in which monomer is added at the active center: The rate of this reaction Rp can be expressed as:

  8. Termination • Chain growth stops by bimolecular reaction of two growing chain radicals: • termination by combination (ktc) • termination by disproportionation (ktd) The general kinetic equation reads:

  9. Polymerization Kinetics The rate of polymerization in a chain growth polymerization is defined as the rate at which monomer is consumed. Since for the production of high molar mass material Rp » Ri this equation can be re-written as: • From the beginning of the polymerization: • increasing number of radicals due to decomposition of the initiator • increasing termination due to increasing radical concentration (Rtµ [M·]2) • eventually a steady state in radical concentration:

  10. This steady state assumption leads to: From which the differential rate equation is derived: At low conversion this means: log(Rp) vs log[M] yields a slope = 1 log(Rp) vs log[I] yields a slope = 0.5

  11. This steady state assumption leads to: From which the differential rate equation is derived: At low conversion this means: log(Rp) vs log[M] yields a slope = 1 log(Rp) vs log[I] yields a slope = 0.5

  12. Example of initiators :

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