Chemistry Reference
In-Depth Information
can even be residual titanium halide from a Ziegler-Natta catalyst left over in the polymer from the
polymerization reaction. When it is carried out at 50
C in chlorobenzene, -CHCl- groups form [
142
]:
Cl
2
+
Cl
This slows the chlorination of adjacent groups.
Trace amounts of oxygen catalyze chlorinations in the presence of visible light [
140
]. The same
reaction in ultraviolet light is accompanied by cross-linking. The photochemical process can be
illustrated as follows:
1.
Cl
2
+
hν
2Cl
+
Cl
2.
+
HCl
Cl
+
Cl
2
3.
+
Cl
etc.
Chlorination of polyethylene can result in varying amounts of hydrogen atoms being replaced by
chlorine. It is possible to form a product that contains 70% by weight of chlorine. The amount of
chlorination affects the properties of the product. At low levels of substitution, the material still
resembles the parent compound. When, however, the level of chlorine reaches 30-40%, the material
becomes an elastomer. At levels exceeding 40%, the polymer stiffens again and becomes hard.
Commercial chlorinations of polyethylene are usually conducted on high-density (
0.96)
linear polymers. The molecular weights of the starting materials vary. High molecular weight
polymers form tough elastomers. Low molecular weight materials, however, allow easier processing
of the products. The reactions are carried out in carbon tetrachloride, methylene dichloride, or
chloroform at reflux temperatures of the solvents and at pressures above atmospheric to overcome
poor solubility. The solubility improves with the degree of chlorination. Industrially, two different
procedures are used. In the first one, the reactions are conducted at 95-130
C. When the chlorinations
reach a level of about 15%, the polymers become soluble and the temperatures are lowered
considerably [
143
]. In the second one, the reactions are conducted on polymers suspended in the
solvent. When the chlorine content reaches 40% and the polymers become soluble, chlorinations are
continued in solution. By continuing the reaction, a chlorine content of 60% can be reached. The
products from the two processes differ. The first one yields a homogeneous product with the chlorine
atoms distributed uniformly throughout the molecules. Chlorination in suspension, on the other hand,
yields heterogeneous materials with only segments of the polymeric molecules chlorinated. Some
commercial chlorinations are conducted in water suspensions. These reactions are carried out at 65
C
until approximately 40% levels of chlorine are achieved. The temperatures are then raised to 75
Cto
drive the conversions further. In such procedures, agglomerations of the particles can be a problem.
To overcome that, water is usually saturated with HCl or CaCl
2
[
144
]. Problems with agglomeration
are also encountered during suspension chlorinations in solvents, like CCl
4
. Infra-red spectra of
chlorinated polyethylenes show presence of various forms of substitutions. There are -CHCl-CHCl-
as well as -CCl
2
- groups present in the materials [
145
,
146
]. Surface photo chlorination of polyolefin
films [
146
] considerably improves the barrier properties of the films to permeations of gases.
Chlorinations of polypropylene
D
>
usually result in severe degradations of the polymer. When TiCl
4
is the chlorination catalyst, presumably, less degradation occurs [
140
]. Studies of
bromination of
(atactic) show that when the reaction is carried out in the dark, in CCl
4
at 60
C, the
substitution reactions proceed at the rate of 0.5%/h [
147
].
polypropylene
