Chemistry Reference
In-Depth Information
9.4 Substitution Reactions
Many substitution reactions are carried out on polymers in order to replace atoms in the backbones or
in the pendant groups with other atoms or groups of atoms. These reactions do not differ much from
those of the small molecules.
9.4.1 Substitution Reactions of Saturated Polymeric Hydrocarbons
It is often desirable to replace hydrogens with halogens.
can be carried
out in the dark by simply exposing the polymer, either in sheet or in powder form, to fluorine gas. The
reaction is exothermic and it is best to dilute the gas with nitrogen 9:1 to allow a gradual introduction
of the fluorine and avoid destruction of the polymer [
132
,
133
]. In this manner, however, only the
surface layers are fluorinated and the substitutions occur only a few molecular layers deep.
In surface fluorination with vacuum ultraviolet glow discharge plasma, the photon component of
the plasma enhances the reactivity of the polymer surfaces toward fluorine gas [
134
]:
Fluorination of polyethylene
F
Gas phase
:
F
2
þ hnð
210
360 nm
Þ!
2
Solid Surface
~CH
2
−
CH
2
~ + hν (<160 nm)
e
+
CH
2
CH
e
CH
2
CH
The surface free-radicals can also cause elimination of hydrogen radicals and formation of double
bonds. Whether as free-radicals or through unsaturation, the units are now more reactive toward
fluorine.
A film that is 3 mil thick can be completely fluorinated on a 100-mesh phosphor bronze gauze, if
the reaction is allowed to proceed for several days [
135
]. Fluorination can also be carried out with
mercuric or cupric fluorides in hydrofluoric acid. The reaction must be carried out at 110
C for 50 h.
As much as 20% of fluorine can be introduced [
136
].
In direct fluorination of powdered high-density polyethylene with the gas, diluted with helium or
nitrogen, the accompanying exotherm causes partial fusion. In addition, there is some destruction of
the crystalline regions [
137
]. On the other hand, fluorination of single crystals of polyethylene can
result in fluorine atoms being placed on the carbon skeleton without disruption of the crystal structure.
The extent of cross-linking, however, is hard to assess [
138
]. The reaction has all the characteristics of
free-radical mechanism [
139
]:
F
F
2
!
2
þ F
!
R
þ
RH
HF
R
þ F
2
!
þ F
RF
can be carried out in the dark or in the presence of light. The two
reactions, however, are different, though both take place by free-radical mechanism. When carried
out in the dark at 100
C or higher, no catalyst is needed, probably because there are residual peroxides
from oxidation of the starting material. Oxygen must be excluded because it inhibits the reaction and
degrades the product [
140
]. The reaction is catalyzed by traces of TiCl
4
[
141
]. Such trace quantities
Chlorinations of polyethylene
