Chemistry Reference
In-Depth Information
more quickly than paired carbocations. The rate of polymerization will
then
increase as the reaction temperature is lowered.
The activation energy for degree of polymerization
E
X
n
is always negative
and the degree of polymerization decreases as the temperature is raised. This is
because
E
tr
is greater than
E
p
(as would be expected if
E
p
is zero for propagation
with free ions). Thus, low-molecular-weight grades of polyisobutene are made by
AlCl
3
or BF
3
initiation at
40
C while high-
temperatures between 0 and
2
100
C. High-molecular-weight
butyl rubber can only be made at low temperatures, because the comonomer iso-
prene is also a chain transfer agent.
molecular-weight polymers are made at
85 to
2
2
11.4.7
Polymerization of Vinyl and Cyclic Ethers
The ether oxygen is a Lewis base (electron donor), and polymerization of vinyl
and cyclic ethers can be initiated by reaction with an ion pair comprising an
acidic cation and a weakly nucleophilic base. These monomers do not polymerize
by free-radical or anionic processes. Thio ethers behave similarly.
The general structure of cyclic ethers may be written as 11-17 with
x
2:
$
(CH
2
)
O
X
11-17
Molecules with these structures polymerize because this process involves a
ring-opening reaction that relieves strain. Four-member rings (oxetanes,
x
3) are
subject to angle strain and to crowding of hydrogens on adjacent carbons.
Tetrahydrofuran (
x
5
4) also polymerizes because of steric repulsion between
adjacent hydrogens. Six-membered ring ethers like tetrahydropyran (
x
5
5) do not
polymerize. However, trioxane (the cyclic trimer of formaldehyde) can be reacted
cationically to produce polyformaldehyde. The reactivity of the trioxane ring is
perhaps a result of the stability of the propagating carbenium ion 11-18 in which
the charge is delocalized onto the adjacent oxygen:
5
CH
2
CH
2
OO
H
O
O
HO
CH
2
O
CH
2
O
CH
2
CH
2
CH
2
CH
2
CH
2
BF
3
OH
(11-58)
O
+
O
HO
CH
2
O
CH
2
O
CH
2
BF
3
OH
H BF
3
OH
11-18
