Types of Vinyl Polymerization

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Types of Vinyl Polymerization
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Fate of Initiator Radicals

• Radical reactions
• Recombination in solvent cage

Reaction with polymer radicals (kt)--primary
termination
Reaction with initiator (MIH)
Hydrogen abstraction from polymer chains (chain
transfer to polymer)
Reaction with solvent or inhibitor

• Chain initiation, Ri 2 f kd I

• Efficiency factor, f 0.1 - 0.9

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Radical Initiators

• Azo Initiators

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Decomposition of Azo Initiators

• 2- bond cleavage to liberate nitrogen

Cage Recombination ---Side reaction- irreversible
coupling of succinonitrile radicals, efficiency
decreases at high conversion
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Peroxy Initiators

• High temperature initiators

Moderate temperature initiators
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Peroxy Initiators

• Low temperature initiators, 35-60 C

3-bond cleavage process?
b-cleavage to carbon centered radical
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Redox Initiation
0-5 C in water
0-5 C in organic/aqueous phase
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Decomposition of Peroxy Initiators

• 1-bond cleavage process

If R aryl, acyl radical initiates alkyl,
CO2 lost before initiation occurs
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Reaction of benzoyloxy radicals with styrene
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Chain Transfer

• Hydrogen transfer to growing polymer chain

• Reinitiation of growing chain using transferred
  radical

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Effect of Chain Transfer on Rp and DP
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Control by Chain Transfer

• Chain transfer depends upon nature and
  concentration of chain transfer agent.

Where Ctr is the chain transfer constant that
includes the rate constants for hydrogen
abstraction and re-initiation of a new chain
Ctr is specific for a given monomer at a given
temperature
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Common Chain Transfer Agents
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Additional Chain Transfer Processes

• Chain transfer to monomer, Ctr x 104
• Ethylene, 0.4- 4.0 Styrene, 0.3-0.6

Vinyl acetate, 1.75-2.8 Vinyl chloride,
10.8-16 Allyl systems, 50-100
Chain transfer to polymer--branching Polyethylene
Vinyl acetate Vinyl chloride
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Transfer to Polymer

• Polyethylene branching

Long branches
Short branches
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Inhibition of Radical Polymerization

• Must stop oxygen- and carbon centered radicals

Radicals generated by auto oxidation
May be metal catalyzed

• Oxygen centered radicals stopped by hydrogen
  transfer

Carbon centered radicals stopped by addition
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Critical Inhibitor Properties
An inhibitor should not add to, abstract from or
otherwise reach with monomer or solvent
Inhibitors should not undergo self reaction or
unimolecular decomposition
Inhibitors must react rapidly with the
propagating and/or initiator derived radicals to
terminate polymer chains
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Trapping Oxygen Centered Radicals
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Trapping carbon centered radicals

• Carbon centered radicals stopped by addition to
  oxygen or carbon

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Typical Inhibitors
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Stable Radical Inhibitors