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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

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.

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

①Radical Addition Reaction

．．．

②Radical Coupling Reaction

③Radical Disproportionation Reaction

④Radical Dissociation Reaction

..CO

O+ CO2

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.