Chemistry Reference
In-Depth Information
C(Ph)S-R] [
277
] They found that the effec-
tiveness of these agents also depends upon the nature of the monomer and on the polymerization
conditions. For the polymerization of styrene, methyl methacrylate, butyl acrylate, and methyl
acrylate at 60
C, the effectiveness of R decreases in the following order [
277
]:
These RAFT agents are based on the structure [S
¼
R = C(alkyl
Þ
2
CN
C(CH
3
Þ
2
Ar
>
C(CH
3
Þ
2
C( = O)O(alkyl)
>
C(CH
3
Þ
2
C( = O)NH(alkyl)
>
r
C(CH
3
Þ
2
CH
2
C(CH
3
Þ
3
C(CH
3
)HPh
>
C(CH
3
Þ
3
CH
2
Ph
¼
C(CH
3
)
2
CN or C(CH
3
)
2
Ph did these
thiocarbonylthio compounds yield polymers with narrower polydispersities in batch polymerizations.
Also, only these compounds allowed molecular weight control that may be expected from a living
polymerization. The reaction mechanism was proposed by Moad and coworkers as follows [
277
]:
In addition, among the above compounds, only when R
initiation
M
M
initiator
I
P
n
chain transfer
P
n
K
K
addit
β
+
S
S
R
P
S
S
R
P
n
S
S
+
n
R
K
-addit
K
−
β
2
3
4
reinitiation
M
M
R
R
M
P
m
K
i
K
p
chain equilibration
+
P
m
S
S
P
n
P
S
S
P
P
m
S
S
+
m
n
P
n
M
M
termination
K
t
P
n
+
P
m
dead polymer
Moad and coworkers concluded [
277
] that a major factor that determines the transfer coefficient of
dithiobenzoate derivatives is the way the intermediate 3 (see the above in equation) partitions
between starting materials and products. This in turn is determined by the relative ability of the
leaving group R and by the propagating radical. Steric factors and radical stability of R are also
important. They conclude that more stable, more electrophilic, and bulkier radicals are better leaving
groups. The partitioning of R between the monomers (to reinitiate) or by adding to polymeric RAFT
can also have a significant effect on the rate of consumption of RAFT agent [
277
].
Vana and coworkers [
278
] studied conversion vs. time and molecular weight distributions in
conjunction with the kinetic scheme for the RAPT process. In particular, conditions leading to inhibition
