Chemistry Reference
In-Depth Information
8.7
Modes of Termination
It is believed that most macroradicals terminate in free-radical polymerizations
predominantly or entirely by combination. Experimental measurements of poly-
mer systems are scanty, however. It can be expected that disproportionation will
be more important for tertiary than secondary macroradicals. The former have
more
hydrogens available for transfer during disproportionation, and direct
coupling of tertiary radicals is more hindered sterically. Thus, the poly(methyl
methacrylate) radical terminates by combination and disproportionation while com-
bination is the only mode observed in polystyryl radical termination. The
k
tc
/t
td
ratio in polymers that terminate by both processes is temperature sensitive, with
higher temperatures apparently tending to encourage disproportionation.
β
8.8
Chain Transfer
In many free-radical polymerizations, the molecular weight of the polymer pro-
duced is lower than that predicted from
Eq. (8-64)
. This is because the growth of
macroradicals in these systems was terminated by transfer of an atom to the
macroradical from some other species in the reaction mixture. The donor species
itself becomes a radical in the process, and the kinetic chain is not terminated if
this new radical can add monomer. Although the rate of monomer consumption
may not be altered by this change of radical site, the initial macroradical will
have ceased to grow and its size is less than it would have been in the absence of
the atom transfer process. These reactions are called
chain transfer processes
.
They can be classified as varieties of propagation reactions (
Section 8.3.2
).
In general,
k
tr
M
n
H
1
T
(8-67)
Equation (8-67)
is written for transfer of a hydrogen atom between a macrora-
dical M
n
and a transfer agent TH. Other atoms, and particularly halogens, can be
transferred in place of hydrogen and TH can be monomer, solvent, initiator, poly-
mer, or any substance in the reaction mixture. The rate of transfer
R
tr
is
R
tr
5
M
n
1
TH
!
M
½
(8-68)
assuming as usual that the transfer rate constant
k
tr
is the same for all monomer-
ended radicals and taking [M
] to be the concentration of all such species. The
magnitude of
k
tr
will depend on the natures of M
n
and TH as well as the reaction
temperature.
The new radical T
can reinitiate. The rate constant for the addition of a par-
ticular monomer M to T
is
k
r
, so that the sequence of events in this process is
k
tr
½
TH
k
p
M
M
2
!
k
p
k
p
k
r
M
1
!
T
1
M
n
M
!
?!
(8-69)
where
k
p
is the propagation rate constant for this monomer.
