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Polymer
chemistry
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3.1.3 Elementary Reactions of
Radical Polymerization
• Radical polymerization is chain polymerization
• Composed by at lest three elementary reactions
– Chain initiation
– Chain propagation
– Chain termination
– Perhaps accompanied by chain transfer
reaction and so on
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（1）Chain Initiation Reaction
Initiator
Primary
Radical
heat absorption
heat liberation Monomer
Monomer
Radical
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Primary
Radical
Primary Radical
• The decomposition of initiators is a heat absorption reaction.
• With a relatively high activation emergy, about
100～170 KJ/mol
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• The reaction rate is relatively slow.
Decomposition rate constant is usually
10－4～10－6/s
Initiator decomposition reaction----controls the overall rate of the chain initiation.
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Monomer
Radical
Monomer
.
R
+ M
.
RM
.
RM
Monomer Radical
• The primary radicals add to monomers to
produce monomer radicals.
• The process of opening a π-bond, and
producing a σ-bond is a exothermal reaction.
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• With a low activation energy, about
20～34 kJ/mol
• And a great reaction rate constant.
This is a very fast reaction.
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(2) Chain Propagation Reaction
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Head-to-Tail structure
Head-to-Head structure
Tail-to-Tail structure
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（3）Chain Termination Reaction
Radical Exhaustion
Coupling Termination
Disproportionation Termination
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• Monoradical termination-----Exhausts one radical initiator
.
.
CH2 CH + R'
R
R
CH2 CH R'
X
X
• Biradical termination------coupling termination;
disproportionation termination
R
R
.
.
CH2 CH + CH CH2
X
X
.
.
CH2 CH + CH CH2
X
X
R
R
CH2 CH CH CH2
X
R
R
X
CH2 CH 2 + CH = CH
X
R
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X
R
Biradical termination -----coupling termination;
disproportionation termination
• Coupling Termination ---– The degree of polymerization is the sum of the
monomeric units of the two radical chains.
– The two ends of the molecule are the residues
of the initiators.
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Termination of disproportionation ---– The Degree of Polymerization is the number
of the monomeric units of the radical chain.
– Each molecule contains a initiator residues
end.
– However, the terminated chemical structures
of the two molecules are different, one
saturated, and the other unsaturated.
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• The modes of termination reactions
are determined by the monomer
structure and the polymerization
temperature.
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The modes of termination-----monomer structure
polymerization temperature
• With bulk substituents-----the possibility of
disproportionation termination increases.
• At low polymerization temperature---tending to coupling termination.
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The relative scale of coupling to
disproportionation termination is ：
The probabilit
coupling
The probabilit
disproport
y of
terminatio
n (c )

y of
ionation
k tc
k td
terminatio

A te
e
 ( E tc  E td ) / RT
A td
n (c )
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Table 3-3 Modes of some Monomer Radical
Termination
•
Monomers
Polymerization coupling Disproportionation
tempera termination
termination
0～60 ℃
P-chloro styrene
60、80
P- anisole styrene
60
80
Methyl methacrylate 0
25
60
Acrylonitrile
40，60
Vinyl acetate
90
Styrene
100 ％
100
81
53
40
32
15
92
0％
0
19
47
60
68
85
8
～100
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Table 3-4 Comparison between Chain Propagation
and Chain Termination Reaction
k p  10 [L/(mol·s)]
Propagation rate constant
Monomer concentration [M]:
10~10-1
[mol/L]
-4
6
Propagation rate
R p  k p [M][M·],10 ～10 [mol/(L·s)]
3
7
Rate constant of termination
[L/(mol·s)]
k t  10
Radical concentration [M·]:
10-7～10－10 [mol/L]
Termination rate R t  k t[M·]2, 10-8～10－10 [mol/(L·s)]
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（4）Chain Transfer Reaction
• To monomer
• To solvent
( or to chain transfer agent）
• To initiator
• To polymer
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[1] to monomer
.
CH2 CH + CH2 = C
.
CH2 CH + CH2 = CH
X
X
X
X
.
CH = CH + CH3 C H
X
X
[2] To solvent( or to chain transfer agent）
.
CH2 CH + YZ
X
CH2 CHY + Z
X
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[3] to initiator
.
CH2 CH + R
R
CH2 CH 2 + R.
X
X
[4] to molecule
.
CH2 CH +
X
CH2 CH
X
CH2 CH 2 +
X
.
CH2 C
X
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Characteristics of Radical
Polymerization
• From microscopic view, radical reaction
consists of four elementary reactions-initiation, propagation, termination, and
transfer.
• The rate of initiation is the lowest one, which
controls the overall rate of polymerization. In
conclusion, the characteristics of radical
polymerization are slow initiation, fast
propagation, fast termination, and easy
transfer.
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• The degree of polymerization increases
only in the propagation process.
• In the system there are only monomers
and polymers, no intermediate
compounds.
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• Prolonging the reaction time is mainly to
increase the conversion rate, with little
effects on the molecular-weight. However,
gel effect will increase the molecularweight.
• A small amount（0.01 % ～ 0.1 %）of
inhibitors is enough to terminate the
radical polymerization.
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Table 3-5 Comparison of the Characteristics of
Radical and Condensation Polymerization
Radical Polymerization
Condensation Polymerization
Elementary reactions’ rate
constants and activation
energy are different
Elementary reactions’ rate
constants and activation
energy are almost samely
Molecular weight increases
gradually and a fairly long
time is needed .
The polymer molecular wei
-ght is comparatively small
Polymerization process
involves all monomers
Degree of polymerization
does not change with
polymerization time
No intermediate products
whose degree of polymeri
-zation are increasing
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Radical Polymerization Condensation
Polymerization
No remarkable change Molecular weight of
of molecular weight
product increases with
with prolonged reaction polymerization time but
time and higher conver conversion increases
little
-sion.
exothermic reaction
Should be heated to high
tempareture
Accompanied with
robabilities of branch
No branching
-ing and even cross
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linking