Chemistry Reference
In-Depth Information
Acids such as H
2
SO
4
, HClO
4
, and H
3
PO
4
are better initiators than hydrogen
halides because their anions are larger and less nucleophilic.
Initiation by protonic acids is relatively inexpensive. The major applications
of such processes are for reactions of simple olefins like propylene or butenes or
olefinic derivatives of aromatics such as coumarone (11-15) and indene (11-16).
The products of these reactions have very low molecular weights because of
transfer reactions discussed below (
Section 11.4.4
). The low polymers of olefins
are used as lubricants and fuels while the coumarone-indene polymers are
employed in coatings and as softeners for rubbers and bitumens.
O
11-15
11-16
Heterogeneous acidic materials are used widely as cracking and isomerization
catalysts in the petroleum industry. These materials consist of acidic clays or syn-
thetic, porous, crystalline aluminosilicates (molecular sieves). They appear to
function as protonic acids. The counterion in these cases is the catalyst surface,
which is a very poor nucleophile.
Initiation by proton addition is usually accomplished by generating the protons
from the interaction of Lewis acids and auxiliary, hydrogen-donating substances.
These processes are discussed in the following section.
11.4.2.2
Lewis Acids
These are halides and alkyl halides of Group III metals and of transition metals in
which the
d
electron shells are incomplete. This is the most generally useful
group of initiators and includes compounds like BF
3
, SnCl
4
, AlCl
3
, AlR
2
Cl,
SbCl
5
, and so on.
Initiation is facilitated by interaction of the Lewis acid with a second com-
pound (called a
cocatalyst
) that can donate a proton or carbenium ion to the
monomer. Typical cocatalysts are water, protonic acids, and alkyl halides.
Examples of Lewis acid
cocatalyst interactions are given in reactions (11-29)
and (11-32). The general reaction for
(11-33)
BF
3
+
H
2
O
H
BF
3
OH
2
initiation by a Lewis acid ZX
n
and a cocatalyst BA is
(11-34)
ZX
n
+
BA
ZX
n
BA
R'
R
(11-35)
ZX
n
BA
+
CH
2
C
A
CH
2
C
ZX
n
B
R'
R
Here Z is a metal, X is generally a halogen or an organic entity, and BA is an
ionizable compound. The anionic fragment of the complex between the Lewis
