高分子化学(浙江大学) polymer2-1 Radical Chain Polymerization

3.3 Reactions of Radical Polymerization
Initiation (Thermal 热分解) Ea =105-150 KJ/mol
AIBN
fast
Ea =20-34 KJ/mol
Initiator breaks down smoothly to generate a low, steady-state (稳态) concentration of radicals.
B. 1,2-disubstituted disfavor for polymerization (electronic and steric)
C. tri- and tetra-substituted vinyl monomers normally impossible, except fluoroalkene Polymerization ability is mainly determined by the electron effect, steric effect should be also considered when the substitute is large.
The Steady State （稳态）
• Initiation is relatively slow but continuous. • Termination speeds up as active radical concentration builds. • Termination removes (kills) active radicals.
– if both X and Y are electron donor/acceptor, CH2=CH(CN)2 (anion), CH2=CH(CH3)2 (cation) – if X and Y are different, determined by stronger group
CH3 CH2 C COOCH3
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(4) X=H, Y= Cl , F inductive and conjunctive effect , radical polymerization only (5) X=H, Y=aromatic or conjunctive group radical/anion/cation, ca styrene, butadiene (6) X=R, Y=R’, 1,1-disubstituted
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Propagation (增长)
• Monomer adds quickly to the small number of growing chains present at any given time. • Exothermic (55 - 95 kJ/mol); low Ea(20 -30 kJ/mol )
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Disproportionation Vs Coupling
• Termination mode depends largely on the structure of the monomer unit, the reaction temperature and pressure. • The mode of termination has no influence on the rate of radical polymerization process • The molecular weight distributions are strongly influenced by the termination mode.
A + CH2 CH OR A CH2 CH OR
cation(阳离子)
（3）X=H, Y=CN, COOH, COOR (electron withdrawing)
B + CH2 CH CN B CH2 CH CN
anion(阴离子)/radical
* CH2=CHNO2 can be only polymerized by anion
COOCH3>CH3, radical and anion
Monomer for coordination polymerization
Ti
R
•
Steric effect of substitute
A. 1,1-disubstituted vinyl monomer
CH2
C
R is too large to be polymerized
Kinetics
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-
• However, in more highly substituted systems, there is steric crowding in the polymer chains, and this decreases the driving force.
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2.1 Monomer vs Mechanism
Ionic Polym. Radical Polym.
Heterolysis (异裂)
O
O
Homolysis (均裂) ring opening
X'
X
Vinyl monomers (CH2=CHXY) • Electron Effect
（1）X=Y = H radical polymerization （2）X=H, Y=OR (electron donating)
3）Coupling (偶合）
CF3 + CH4
CF3H + CH3
R + R
R
R
4）Disproportionation （歧化）
. CH3 + CH3CH2
5）Fragmentation （分解）
CH3CO
CH4 + CH2
CH2
CH3 + CO
3
Reactions of Polymerization
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Essence of Chain Polymerization
R* = R• radical polymerization R* = Ranion polymerization R* = R+ cation polymerization e.g.coordination polym.
R*
+
R
C
C*
Problem with Alkyl Groups
• The allyl radical (lower structure) is very stable, and forms preferentially. • Attempted free radical polymerization of propylene fails！
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Thermodynamics
General for All Chain Polymerizations • Vinyl polymerizations work in general because converting a double bond into two single bonds is exothermic(放热).
stable
Reactivity of radicals
H > CH 3 > C6H5 > RCH2 > R 2CH > CH2=CH-CH2 > CH2
>
C
very stable
3.2 Elementary Reactions of Radicals
1）Addition(加成) CH3 + CH2 CH2
.
Stereoselectivity(立体选择性)
.
+
isotactic
head to tail
+
?
.
syndiotactic
head to head
• Monosubstituted and 1,1-disubstituted vinyl groups present a choice to the incoming radical • Attack almost always occurs at the least substituted carbon atom, mostly for steric reasons, but also because of the stabilizing effect of the substituent(s). • The result is that head-to-tail addition predominates. The exceptions are small substituents like chlorine or especially fluorine.
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Chain Transfer (链转移)
X-Y can be monomer; solvent; polymer; other chemicals
Chapter 2
Radical/Chain Polymerization
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1 Reactions of Radical Polymerization
2
Elementary Reactions of Radicals
1）Addition(加成) CH3 + CH2 CH2
CH3 CH2CH2
.
2 ) Transfer(转移)
3. Mechanism of Radical Polymerization
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3.1 Features of Radical
Cl2 + H2
hv
HCl 2Cl HCl + H HCl + Cl
Chain reaction