Polymer chemistry

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Polymer chemistry

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Chapter 3 RADICAL POLYMERIZATION

• 3.1 Mechanism of Radical Polymerization
• 3.2 Initiators and Initiation
• 3.3 Rate of Radical Polymerization
• 3.4 Molecular Weight and Chain
• Transfer Reaction
• 3.5 Thermodynamics of Polymerization
• 3.6 Methods of Polymerization

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3.1 Radical Polymerization Mechanism

• 3.1.1 The activity and the reaction of the free
• radical
• 3.1.2 Monomer structure and types of polymer-
• ization
• 3.1.3 Elementary reactions of the radical
  polymer-
• ization
• 3.1.4 Characteristics of the radical
  polymerization
• reaction

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3.1.1 The activity and the reaction of the free
radical

• Free radical can be formed if there are
  unpaired electron or lone electron.
• The electron is called monoradical if it is the
  only unpaired electron.
• If there are only two unpaired electrons, they
  are called diradical.

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Free Radicals

• Atomic radicals
• Molecular radicals
• Ionic radicals
• Electroneutral
• compound residue

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Generation of Free Radicals

• Thermal decomposition

• Photochemical decomposition
• Oxidation-Reduction reaction
• High energy particle radiation

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(1)Activity of The Free Radical

• The activity of a free radical is determined by
  its structure.
• The stronger the conjugative effect of a free
  radical, the more stable it is.
• Polar group lessens the activity of the free
  radical.
• Bulky group lessens the activity of reaction,
  because it prevents the nearing of the reagent.

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The Order of the Relative Activity of Radicals
The Radicals in the last line are the inert
radicals that have no ability of initiating
olefinic monomers polymerization
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(2)Reactions of Radicals

• The Radical addition reaction
• The Radical coupling reaction
• The Radical disproportionation reaction
• The Radical dissociation reaction
• The Radical transfer reaction

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? Radical Addition Reaction
...
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? Radical Coupling Reaction
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? Radical Disproportionation Reaction
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? Radical Dissociation Reaction
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? Radical Transfer Reaction
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3.1.2 Monomer Structure and Polymerization
Types

• Most of the mono olefin, conjugated diolefin,
  alkyne, and carbonyl compounds, and some of the
  heterocyclic compounds can be polymerized from
  the thermodynamic viewpoint.

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• However, the selectivity of the various
  monomers to different polymerization mechanisms
  varies greatly.

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Examples

• Vinyl chloride only can undergo radical poly-
• merization.
• Isobutylene only can undergo cationic polymer-
• ization.
• Methyl methacrylate can undergo radical as well
• as anionic
  polymerization.
• Styrene can undergo radical, anionic, cationic,
• and coordination polymerization.

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What makes the differences is mainly decided by
the structure of the substitu-ent on the
carbon-carbon double bond, and is also decided by
the electronic effect and the steric effect of
the substituent.

• Ethylene, the most simple alkene, with a
  symmetric structure, can undergo radical
  polymerization under high pressure, and
  coordination polymerization by particular
  initiator systems.

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Monosubsitituted Alkene Double Bond Monomers

• CH2CH-X, the electronic effect of the
  substituents X involves the inductive or
  resonance effect.
• The effect of substituent manifests itself by its
  alteration of electron-cloud density on the
  double bond and it has the ability to affect the
  stability of the active center.

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• Whether an alkene polymerizes by radical,
  anionic, or cationic initiators depends on the
  inductive and resonance characteristics of the
  substituents present.

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To CH2CH-X, when X is electron-pushing
substituent

• It increases the electron-cloud density,
  facilitating its bonding to a cationic species.
• Further, these substituents stabilize the
  cationic propagating species by resonance, and
  decrease the activation energy of the reaction.
• Thus, electron-pushing substituents facilitate
  the monomers to cationic polymerization.

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Electron-pushing substituents such as alkyl,
alkoxy, phenyl, and alkenyl

• The effect of alkyl groups in facilitating
  cationic polymerization is weak,
• And it is only the 1, 1-disubstituted alkenes
  which undergo cationic polymerization.
• CH3
• CH2C CH2CH
• CH3
  OR

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To CH2CH-X, when X is electron- withdrawing
substituent

• It lowers the electron-density,
• and stabilizes the propagating anionic species by
  resonance.
• And, thus, it facilities anionic polymerization
  of the monomers.

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Electron-withdrawing substituents cyano and
carbonyl ( aldehyde, ketone, acid, or ester)

• Radical polymerization is somewhat similar to
  anionic polymerization.
• Electron-withdrawing substituents facilitate the
  attack of an anionic species by decreasing the
  electron-density on the double bond.
• They stabilize the propagating of anionic species
  by resonance, which weakens the activation energy
  of the reaction.

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Strong electron-withdrawing substituents
facilitate the monomers to anionic polymeri-
zation with weaker ones inclining to radical
polymerization

• Monomers with substituents between the two can
  undergo either anionic or radical polymerization.

• Halogen substituents, although electron-
  withdrawing inductively, can resonance stabilize
  the anionic propagating species, however, both of
  the effects are weak.

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Conjugated Alkene

• Styrene, butadiene, isoprene, and other
  conjugated alkene, because of its strong
  delocalization of the p-bond, are easy to be
  induced and polarized, thus, can undergo all of
  the four modes polymerization mentioned above.

CH2 CH-CHCH2 CH2C-CHCH2 CH3
CH2CH
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Steric Effect of the Substituent

• Steric Effect-----the volume, amount, and
  location
• of the
  substituent.
• In kinetics----- It produces a noticeable
  effect on the capability of polymerization.
• However, it usually doesnt contain the
  selectivity to different active centers.
• Steric effects of monosubstituents are not obvious

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1,1-disubstituted alkene monomers

• Steric effects usually being ignored, the
  activity and selectivity of the monomers are only
  thought to be decided by the electron-effect of
  both substituents.
• However, when both of the substituents are phenyl
  groups, because of its large bulk, monomers can
  only form dipolymer.

R
CH2C R
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1,2-disubstituted monomers

• Owing to strong steric effect, this kind of
  monomers are usually hard to polymerize.
• For example, maleic anhydride is hard to
  homopolymerize, but can copolymerize with
  styrene or vinyl acetate.

CHCH R R
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Tri or tetrasubstituted ethylene

• They ususlly cannot polymerize.
• But, there are an exception when the
  substituent is fluorin.
• Owing to the small radius of the fluorin, all of
  them , from mono to tetrasubstituted
  fluoroethylene, can polymerize well.