Chemistry Reference
In-Depth Information
The half-lives were reported to be in the range of minutes to hours. Induction periods were observed.
Formation of trimethylsilyl iodide was proposed to be the cause of an induction period [
399
]. The
polymerization follows first-order kinetics with respect to the concentrations of the initiator and the
catalyst. With respect to concentration of the monomer the reaction is described as being first-order
internally. It follows, however, an external reaction order of 1.52 due to the higher polarity of the
reaction medium at higher monomer concentrations. The authors tentatively propose that the active
species are formed from initiator, catalyst and trimethylsilyl iodide [
399
]:
I
I
OSi(CH
3
)
3
HgI
HgI
2
Hg
O
OSi(CH
3
)
3
OCH
3
OCH
3
OCH
3
OHgI
+
(CH
3
)
3
SiH
OCH
3
Part of the proposed mechanism is that TMSI forms fromMTS and iodine. It is assumed, therefore,
that TMSI forms a complex with HgI
2
, which in turn activates MTS to form an active center capable
of initiating the polymerization [
399
]:
k
1
+
(CH
3
)
3
SiI
(CH
3
)
3
Si
I
HgI
2
HgI
2
I
(CH
3
)
3
Si
I
Hg
I
OSi(CH
3
)
3
k
2
O
Si(CH
3
)
3
+
(CH
3
)
3
Si
I
HgI
2
OCH
3
OCH
3
Because group transfer polymerization is another form of a living polymerization, attempts have
been made to write kinetic equations to include all forms of such polymerizations in one unified
scheme. Livinenko and Muller [
400
] carried out a general kinetic analysis and compared molecular
weight distributions for various mechanisms of activity exchange in living polymerizations. They
concluded that molecular weight distributions in many living, e.g. anionic, group transfer, cationic,
and radical polymerizations strongly depend on the dynamics of various equilibria between chain
ends of different reactivity [
400
]. They also concluded that a very important special case is the
equilibria between active and dormant centers [
400
]. Mechanisms that include uni- and bimolecular
isomerizations (or activations/deactivations), aggregation, direct bimolecular activity exchange, and
for both fast and slow monomer addition can be unified. The averages of the molecular weight
distributions and polydispersity indexes,
P
n
were derived by them and the dependencies of these
averages on three universal parameters were analyzed: (1) on the reactivity ratio of the two species,
l ¼ k
0
p
=k
p
, (2) on the fraction of the more active species,
P
w
/
I
0
, which is determined by the initial
concentrations of reagents, and (3) on a generalized exchange rate parameter,
a ¼ P
*/
b
, which quantifies the
rate of exchange relative to that of propagation. The dependence of
on the initial concentrations of
reagents is defined by the mechanism of exchange and can be used as a mechanistic criterion to
distinguish between various possible mechanisms. Livinenko and Miller concluded that for the cases
where
b
1, the polydispersity indexes decrease with monomer conversions. This is a common
observation in many living polymerizations where, 10
b >
< b <
100. At full conversions, a simple
relation,
P
w
=P
n
1
þ Y=b
, is valid, where
Y
depends on
a
and
l
. For the common cases where one
