Some Industrially Important Polymers - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 110

This reduces the ability of the polyethylene to pack closely together
and therefore lowers the density. Polyethylene of this type is called low
density polyethylene (LDPE) and has a density in the approximate range
of 0.910 - 0.930. The structure gives LDPE certain property advantages,
especially regarding rheological properties. LDPE is made to this day, but
now most polyethylene is made with organometallic catalysts and is more
linear.
Polyethylene made by organometallic catalysts such as Ziegler - Natta or
metallocene systems is linear and can be represented as follows.

A linear ethylene homopolymer can pack together more tightly and has
a higher density, somewhere around 0.95 g/cm 3 . High density polyethylene
(HDPE) is generally considered to have a density

0.940 g/cm 3 .The
density of ethylene homopolymer can be decreased by copolymerizing an
alpha-olefin with the ethylene. Major commercial polyethylene copolymers
are based upon using 1-butene, 1-hexene, or 1-octene. Of these three, the
lowest cost comonomer is 1-butene; 1-octene is the highest cost and has
the greatest effect. Many producers use 1-hexene which is a compromise
between cost and performance. The copolymer of ethylene with 1-hexene
can be represented as follows.

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