Chemistry Reference
In-Depth Information
As a result, the formed polymer has the following structure:
n
Isomerization polymerizations by material transport
include propagation reactions that are
accompanied by hydride shifts. One example of this is a 1,2-hydride shift in the polymerization of
3-methylbutene-1 [
116
]:
hydrogen
migration
+
R
R
R
During propagation, a tertiary hydrogen atom migrates as a hydride ion to the second carbon. It
results in an energetically favored tertiary carbon cation. The final product is a 1,3-polymer [
116
]:
n
Another example of a hydride shift during propagation is a polymerization of vinyl cyclohexane
with the aid of AlCl
3
at temperatures ranging from
144 to 70
C[
120
,
121
]
R
R
hydru
de
shift
+
R
n
Successive hydride shifts are possible in propagation reactions where the structures of the
monomers favor them [
121
]:
another
hydride
hyddrid
e
H
+
shift
shift
The temperature of polymerization can have some effect. This was observed in the polymerization
of 3-methylbutene-1 [
122
]. At lower temperatures the amount of 1,3 vs., 1,2 placement apparently
increases rapidly. At
130
C an essentially pure crystalline 1,3 poly(methylbutene-1) forms. The
nature of the counterion also has an effect on the slope of the product composition curve. The polarity
of the solvent, however, apparently does not, nor does the monomer concentration [
122
].
Simultaneous migrations of hydrogen atoms and methyl groups take place in cationic polymeriza-
tion of 4,4-dimethyl-1-pentene (neopentylethylene) [
123
]. In the presence of AlCl
3
between
78 and
130
C the polymerization proceeds mainly as follows:
1
,4-placem
ent
n
n
