Polymer
chemistry
1
Chapter 3
RADICAL POLYMERIZATION
•
•
•
•
3.1
3.2
3.3
3.4
Mechanism of Radical Polymerization
Initiators and Initiation
Rate of Radical Polymerization
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 polymerization
3.1.3 Elementary reactions of the radical polymerization
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
（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
（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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① Radical Addition
Reaction
．．
．
② Radical
Coupling Reaction
③ Radical
Disproportionation
Reaction
④ Radical
Dissociation Reaction
O
C
.
O
.
+ CO2
⑤ Radical Transfer Reaction
.
R' + R
R
R'
.
R +R
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 polymerization.
Isobutylene only can undergo cationic polymerization.
Methyl methacrylate can undergo radical as well
as anionic polymerization.
Styrene can undergo radical, anionic, cationic,
and coordination polymerization. 17
Ethylene, the most simple alkene, with a
symmetric structure, can undergo radical
polymerization under high pressure, and
coordination polymerization by particular
initiator systems.
What makes the differences is mainly
decided by the structure of the substituent on the carbon-carbon double bond,
and is also decided by the electronic effect
and the steric effect of the substituent.
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Monosubsitituted Alkene Double
Bond Monomers
• CH2＝CH－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. 19
– 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 CH2＝CH－X, when X is electronpushing 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
CH2=C
CH2=CH
CH3
OR
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To CH2＝CH－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 polymerization with weaker ones inclining to radical
polymerization
• Monomers with substituents between the
two can undergo either anionic or radical
polymerization.
• Halogen substituents, although electronwithdrawing 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 π-bond, are easy to be
induced and polarized, thus, can undergo all
of the four modes polymerization mentioned
above.
CH2=CH
CH2 =CH-CH=CH2
CH2=C-CH=CH2
CH3
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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 doesn’t 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 electroneffect of both substituents.
• However, when both of the substituents are
phenyl groups, because of its large bulk,
monomers can only form dipolymer.
R
CH2=C
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.
CH=CH
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.
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