CHAIN POLYMERIZATION Free Radical Polymerization

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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 polymerizations are chain
  polymerizations 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.

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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 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).

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Example

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

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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.

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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

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

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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.

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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 to another

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Thank You

• See You Next Lecture