Chemistry Reference
In-Depth Information
this rhodium catalyst under mild conditions [
53
]. The kinetics are consistent with a mechanism where
the active Rh-catalyst interacts with the unsaturation at the polymer in the rate-determining step.
The nitrile group, however, appears to also interact with the catalyst and inhibit the rates [
53
].
Hydrogenation of carbon-to-nitrogen double bonds in polymer backbones and in the pendant
groups can be carried out with lithium borohydride [
86
]:
THF
reflux
N
LiAlH
4
+
N
N
n
n
N
It was reported [
485
] that syndiotactic polystyrene can be hydrogenated over Ni/SiO
2
and
Pd/BaSO
4
catalysts. The Ni catalyst yields complete hydrogenenation when low molecular weight
polymer is used. Hydrogenation of high molecular weight polystyrene, however, is incomplete.
On the other hand, the Pd catalyst yields completely hydrogenating material. The hydrogenated
syndiotactic polystyrene is a crystalline material with good heat resistance [
485
].
9.2.3 Addition of Carbenes
Polyisoprenes and polybutadienes can also be modified by reactions with carbenes. Dichlorocarbene
adds to natural rubber dissolved in chloroform in a phase transfer reaction with aqueous NaOH [
54
].
A phase transfer reagent must be used with the aqueous NaOH. Solid sodium hydroxide can be used
without a phase transfer reagent. There is no evidence of
cis
-
trans
isomerization and the distribution
of the substituents is random [
54
].
Difluorocarbene, generated under mild neutral conditions, adds to 1,4-
-
polybutadienes to give materials containing cyclopropane groups [
55
]. The addition takes place
randomly, to give atactic stereo sequence distributions [
55
]:
cis
- and 1,4-
trans
F
F
+
CF
2
n
n
Fluorocarbene, formed from phenyl(fluoro,dichloromethyl)mercury by thermolysis in situ, also
adds to 1,4
cis
-and
trans
-polybutadienes. The carbene can add at various levels [
57
]. The addition
