Chemistry Reference
In-Depth Information
These differ from conventional free-radical polymerizations in that the growing
macroradicals can be considered (somewhat inexactly) to be living. True “living”
polymerizations are those without transfer or termination reactions. (They are dis-
cussed in Chapter 11, in connection with the ionic initiation systems for which
they were first developed.) This implies that the end of the macrospecies is still
active for the addition of more monomer and affords a means for the synthesis of
block copolymers (Section 1.5.4) by the addition of another monomer after the
consumption of the preceding monomer.
Polymerization in the presence of stable nitroxi
de
fr
ee
radicals under specific
conditions yields polymers with polydispersities (
M
w
=
M
n
ratios) less than the
theoretical values given in
Section 8.14
for low-conversion polymerizations
[20]
.
The polymerization kinetics differ from those summarized above because the
nitroxide radicals form stable adducts with the macroradicals. “Pseudoliving”
polymerization schemes may vary. One process consists first of forming an
initiator-nitroxide adduct, which can be isolated, purified, and added to monomer
to initiate polymerization by thermal homolysis of the adduct. A useful advan-
tage of the method is the ability to produce AB block copolymers of controlled
structure by adding preformed stable B macroradicals to designed, purified A-
type macroradicals. An alternative process with the same objectives is “degener-
ative chain transfer”
[21]
in which conventional free-radical termination pro-
cesses (
Section 8.3.3
) are swamped by chain transfer to an appropriate transfer
agent. The growing macroradicals are capped by halogen atoms which are sup-
plied by the transfer agent and removed by primary initiator radicals. The
method differs from telomerization, reactions (8-70) and (8-71), in the choice of
lower initiator levels and less reactive transfer agents. The techniques described
in this paragraph use more expensive reactants and the reactions are slower than
in conventional free-radical polymerizations. They are useful primarily for syn-
theses of polymers with structures that cannot be produced by more established
processes.
8.16
Effects of Temperature
8.16.1
Rate an
d D
egree of Polymerization
R
p
(
Eq. 8-29
) and DP
n
(
Eq. 8-63
) each depend on a combination of three rate
constants:
k
d
,
k
p
, and
k
t
. [Actually
k
t
may itself be the sum of
k
tc
and
k
td
,
Eq. (8-
20)
, but experimental data on the temperature dependencies of each of these ter-
mination rate constants are scanty.] The influence of temperature on an individual
rate constant can be expressed by an Arrhenius-type expression:
k
Ae
E
=
RT
(8-115)
5
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