Chemistry Reference
In-Depth Information
Cp
*
5
-pentamethylcyclopentadienyl) are reacted with B(C
6
F
5
)
3
, a methyl cation is abstracted and
a complex forms, as shown below:
¼ Z
MeB(C
6
F
5
Þ
3
Cp
TiMe
3
þ
Cp
TiMe
2
þ
½
B(C
6
F
5
Þ
3
!½
Evidence has now been presented that
indicates that
the above compound behaves as a
carbocationic polymerization initiator for styrene,
-vinylcarbazole, vinyl ethers, and isobutylene.
The mechanism of initiation and polymerization of these monomers by such metallocene complexes
is still being investigated. It was suggested by Wang et al. [
53
], that the mechanism of carbocationic
polymerization of such olefins by the above complex would involve coordination of the olefins, as
shown below, in a nonclassical
N
1
-fashion, with the metal-olefin. This interaction is stabilized by a
complementary borate-olefin interaction. The next step in the polymerization process by this
mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin
molecules by secondary olefin monomers, followed by chain growth [
53
]:
Z
monom
er
Ti
H
Ti
H
Ti
H
H
C
6
F
5
H
H
H
B
H
H
H
C
6
F
5
C
6
F
5
H
C
6
F
5
C
6
F
5
C
6
F
5
H
H
B
polymerization initiator
monomer
coordination
H
chain growth
4.3.2 One Electron Transposition Initiation Reactions
Some
radical sources
, will, in the presence of oxidizing agents, or light, or heat energy, initiate
cationic polymerizations of monomers, like
-butyl vinyl ether. Those that are most readily oxidized
are carbon atom centered radicals that have substituents like benzyl, allyl, alkoxy, or structures with
nitrogen or sulfur. Also, radicals that are formed by addition of other radicals to alkyl vinyl ethers are
particularly reactive.
Oxidants that can be used in these reactions are salts, like (C
6
H
5
)
2
I
+
PF
6
. Such salts oxidize the
radical and also supply the counter-ions, as shown below [
54
]:
n
HO
OH
HO
2
PF
6
I
2
Δ
80
o
C
2
2
2
2
O
+
2
2H PF
6
+
+
2
2
I
