Chemistry Reference
In-Depth Information
This was felt to be true even in polar media [
21
]. Sigwalt and coworkers, however, disputed that [
28
].
They based their opinions on cationic dimerizations of 1,1-diphenylethylene with AlCl
3
H
2
O in the
presence of 2,6-di-
-butyl-4-methylpyridine. The results suggested to them that sterically hindered
amines do not inhibit co catalytic initiation by AlCl
3
H
2
O. Also, that the free hindered amine is a
powerful terminating agent and that the sterically hindered pyridine forms a strong complex with
AlCl
3
. When engaged in a complex with AlCl
3
the reactivity of the hindered amine in termination is
much reduced [
28
].
Alkyl halide solvents in the polymerization reaction mixture may cause an initiation to result from
a transfer of AlX
2
L
or (RX)AlX
2
L
to the more basic monomer [
23
]:
t
Þ
2
þ
A1X
4
2AX
3
þ
2RX
Ð
AlX
2
ð
RX
Þ
2
A1X
2
RX
X
2
A1M
þ
ð
RX
þ
M
!
RX
ð
monomer
Þ
where X
alkyl.
In summarizing the initiation mechanism by complexed Lewis acids [
24
], the catalysts must be
present in the reaction mixture in three forms: (1) as ionized molecules, (2) as ion pairs, and (3) as free
ions. All three forms are in equilibrium. When the monomer is introduced, it complexes with some of
the ionized molecules and a new equilibrium is established. Additional ion pairs form by a slow
process. This depends upon a change in the monomer concentration [
25
].
Some strong Lewis acids are actually capable of initiating carbon cationic polymerizations without
a co-catalyst. As a result, many mechanisms were offered to explain the reaction paths of initiations
by Lewis, like aluminum halides and some others, without the aid of any co-reactant. Following are
some more prominent ones.
One mechanism [
26
] was based on an observation that olefins with allylic hydrogens like isoprene,
methyl styrene, indene, and cyclopentadiene can be polymerized by super dry, pure Lewis acids,
alone. This led to a suggestion that the process may involve an allylic self-initiation [
26
]:
¼
halogen; R
¼
+
MnX
n
MnX
n
H
The above suggested mechanism can only apply to polymerizations of monomers with allylic
hydrogens. Also, it is contradicted by the ability of 1,1-diphenylethylene to dimerize in a super dry
system in the presence of aluminum halides. Yet, this compound lacks allylic hydrogens [
27
].
Two other very similar mechanisms [
28
] are based on a concept that initiation takes place by a
process of halometalation:
R
MeX
n-1
R
X
+
MeX
n
R'
R'
Lewis acid
Depending upon the substituents on the olefins, the metal organic compound may ionize in the
presence of a second molecule of a Lewis acid, or it may eliminate HX:
R
-HX
MeX
n
R
X
R
MeX
n-1
MeX
n-1
MeX
n-1
MeX
n+1
R'
R'
R'
