Chemistry Reference
In-Depth Information
Chapter 6
Common Chain-Growth Polymers
6.1 Polyethylene and Related Polymers
Polyethylene is produced commercially in very large quantities in many parts of the world.
The monomer can be synthesized from various sources. Today, however, most of ethylene comes
from petroleum by high temperature cracking of ethane or gasoline fractions. Other potential sources
can probably be found, depending upon the availability of raw materials.
Two main types of polyethylene are manufactured commercially. These are low (0.92-0.93 g/cm
3
)
and high (0.94-0.97 g/cm
3
) density polymers. The low-density material is branched while the high-
density one is mostly linear and much more crystalline. The most important applications for the
low-density polyethylene are in films, sheets, paper, wire and cable coatings, and injection molding.
The high-density material finds use in blow molded objects and in injection molding.
6.1.1 Preparation of Polyethylene by a Free-Radical Mechanism
Up to the late 1960s, most low-density polyethylene was produced commercially by high-pressure
free-radical polymerization. Much of this has now been replaced by preparation of copolymers of
ethylene with
a
-olefins by coordination polymerization. These preparations are discussed further in
this chapter. High-pressure polymerizations of ethylene, however, might still practiced in some places
and it is, therefore, discussed here. The reaction requires a minimum pressure of 500 atm [
1
]to
proceed. The branched products contain long and short branches as well as vinylidene groups. With
an increase in pressure and temperature of polymerization, there is a decrease in the degree of
branching and in the amount of vinylidene groups [
2
,
3
].
Free-radical commercial polymerizations are conducted at 1,000-3,000 atm pressure and
80-300
C. The reaction has two peculiar characteristics: (1) a high exotherm and (2) a critical
dependence on the monomer concentration. In addition, at these high pressures oxygen acts as an
initiator. At 2,000 atm pressure and 165
C temperature, however, the maximum safe level of oxygen
is 0.075% of ethylene gas in the reaction mixture. Any amount of oxygen beyond that level can cause
explosive decompositions. History of polyethylene manufacture contains stories of workers being
killed by explosions. Yet, the oxygen concentration in the monomer is directly proportional to the
