1 polymer chemistry polymer chemistry 2 chapter 3 radical polymerization 3.1 mechanism of radical...
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Polymer Polymer chemistrychemistry
Polymer Polymer chemistrychemistry
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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 u
npaired 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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①Radical Addition Reaction
...
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②Radical Coupling Reaction
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③Radical Disproportionation Reaction
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④Radical Dissociation Reaction
..CO
O+ CO2
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⑤Radical Transfer Reaction
.RR'
.+ R R R' +R
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• Most of the mono olefin, conjugated diolefin, alkyne, and carbonyl compounds, and some of the heterocyclic compounds can be polymerized from the thermodynamic viewpoint.
3.1.2 Monomer Structure and Polymerization Types
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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
• CH2 = CH - X, the electronic effect of the s
ubstituents X involves the inductive or reson
ance 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 CH2 = CH - 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 pr
opagating 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, aci
d, 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 π-bond, are easy to be induced and polarized, thus, can undergo all of the four modes polymerization mentioned above.
CH2 =CH-CH=CH2
CH2=C-CH=CH2
CH3
CH2=CH
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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 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 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.