Engineering Chemistry S Datta

440 ENGINEERING CHEMISTRY
The next part of initiation involves the addition of this radical to the monomer molecule
(M) to initiate the chain.
2R • + M ⎯⎯→ M*
Free Monomer
radical
The M* is the initiator radical or primary radical.
Propagation. The mechanism of propagation is the reaction of the radical M* with its
own monomer M.
M* + CH 2
= CHY ⎯⎯→ M-CH 2
CHY*
Continuous addition of new monomer in this manner will finally produce a polymer
chain in which the substituents are located on alternate atoms.
Terminations. The most common terminations are the effect of
(i) Collision between two growing chains
H
⏐
i.e., —CH 2
— C
⏐
+
Y
• •
H
⏐
C ⎯CH 2 ⎯
⏐
Y
H H
⏐
⎯⎯→ —CH 2
— C ⎯ ⏐ C ⎯CH 2 ⎯
⏐ ⏐
Y Y
(ii) Collision of a growing chain with an initiator radical when the latter is proportionately
in excess.
(iii) Collision between a growing chain with impurities. Sometimes the chain length of
the polymer is controlled by deliberately adding inhibitors.
More rarely, by disproportionation an H atom of one radical is transferred to another
resulting in two polymers–one saturated and the other unsaturated.
H
⏐
—CH 2
— C +
⏐
Y
• •
H
⏐
C ⎯CH 2 ⎯
⏐
Y
H H H
⏐ ⏐ ⏐
⎯⎯→ CH 2
— CH + C = C ⎯
⏐ ⏐
Y Y
(b) Ionic polymerization is an important class of addition polymerization which also
involves the steps of initiation, propagation and termination but here, instead of free radicals,
the unstable intermediates are either cations or anions.
Cationic Mechanism
Initiation
X + + CH 2
= CH
⏐
Y
⎯⎯→ X – CH 2
⎯CH
⏐
Y
+
The initiators are Lewis acids like AlCl 3
,BF 3
, H 2
SO 4
etc., in presence of water.