Chemistry Reference
In-Depth Information
O
+
N
Br
O
Br
MgBr
The bromination reaction is accompanied by shifts of the double bonds that are coupled with the
sites of substitution. Several different substituents can form. The polymers may contain pentane
diylidene, hexane diylidene, and heptane diylidene units [
46
,
47
].
By contrast, chlorination of polybutadiene in benzene is a straightforward addition reaction of the
halogens to the double bonds [
48
,
49
]:
+
Cl
2
n
Cl
n
Cl
Cl
n
Cl
Very little HCl is liberated until all the double bonds are consumed. When CCl
4
is used in place of
benzene, some substitutions occur during the latter stages of the reactions. If cross-linking occurs at
the same time, the substitutions may not be extensive. The cross-linking reactions are believed to
involve carbocationic intermediates.
Polybutadiene can be halogenated readily in tetrahydrofuran with iodine chloride or bromine [
49
].
The products are glassy polymers. These products dehalogenate in reactions with organolithium
compounds, like
-butyllithium, and polystyryllithium in tetrahydrofuran solution.
Dehalogenation of poly(iodo-chlorinated butadiene) with
n
-butyllithium,
sec
n
-butyllithium yields product with different
cis
ratios. Also, this is accompanied by partial cross-linking. The reactions may involve [
49
]
halogen-metal exchanges that are followed by intra- and intermolecular elimination of lithium halide.
In brominations of polybutadienes, both couplings and eliminations take place. Both iodo-chlorinated
and brominated polybutadienes form graft copolymers when reacted with polystyryllithium in
tetrahydrofuran [
50
]. Gel formation, however, accompanies the grafting reaction.
/
trans
9.2.2 Hydrogenation
Atactic 1,4-polybutadiene and syndiotactic 1,2-polybutadiene can be hydrogenated at 100
C and
50 bar pressure of hydrogen with a soluble catalyst {[(Ph)
3
-P]
3
RhCl}. Complete saturation of double
bonds results [
52
]. Butadiene acrylonitrile copolymers can also be hydrogenated quantitatively with
