Callister - An introduction - 8th edition

608 • Chapter 15 / Characteristics, Applications, and Processing of Polymers
Three distinct stages—initiation, propagation, and termination—are involved in
addition polymerization. During the initiation step, an active center capable of propagation
is formed by a reaction between an initiator (or catalyst) species and the
monomer unit.This process has already been demonstrated for polyethylene (Equation
14.1), which is repeated as follows:
H
H
H
H
R· C
C
R
C
C·
(15.5)
H
H
H
H
Again, R represents the active initiator, and is an unpaired electron.
Propagation involves the linear growth of the polymer chain by the sequential
addition of monomer units to this active growing chain molecule. This may be represented,
again for polyethylene, as follows:
H
H
H
H
H
H
H
H
R
C
C· C
C
R
C
C
C
C·
(15.6)
H
H
H
H
H
H
H
H
Chain growth is relatively rapid; the period required to grow a molecule consisting
of, say, 1000 repeat units is on the order of 10 2 to 10 3 s.
Propagation may end or terminate in different ways. First, the active ends of
two propagating chains may link together to form one molecule according to the
following reaction: 6
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
R
( C
C ) m C C· ·C C ( C C ) n R
R ( C C ) m C C C C ( C R
(15.7)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
R
( C
C ) m C C· ·C C ( C C ) n R
R
( C
C ) m C C H C C ( C C ) n R
H
H
H
H
H
H
H
H
H
H
H
H
H
C ) n
(15.8)
The other termination possibility involves two growing molecules that react to form
two “dead chains” as 7
H
H
thus terminating the growth of each chain.
Molecular weight is governed by the relative rates of initiation, propagation,
and termination. Ordinarily, they are controlled to ensure the production of a polymer
having the desired degree of polymerization.
6 This type of termination reaction is referred to as combination.
7 This type of termination reaction is called disproportionation.