Page 205 - Carrahers_Polymer_Chemistry,_Eighth_Edition

168 Carraher’s Polymer Chemistry

(a) (b)

(c) (d)

(e)

FIGURE 5.2 Ball-and-stick models of HDPE (a), UHMWPE (b), LDPE (c), LLDPE (d), and ULPE (e).

products. They found that chromium trioxide supported on a silica–alumina catalyst produced a

hard solid rather than the usual waxy-like PE. They quickly looked at other olefins and soon discov-
ered a crystalline PP, namely a stereoregular PP, specifi cally iPP.
While the common name for the monomer is ethylene, the official name is ethane so that what

we know as PE is often referred to as polyethene or polythene. Even so, since the name PE is so
entrenched in our common vocabulary, it will be employed here. PE can be produced employing
radical, anionic, cationic, and ion-coordination polymerization. This is a result of the lack of substit-
uents on the ethylene monomer. Each of these different polymerizations result in a different type of
PE. Today there exists a wide variety of “polyethylenes” that vary in the extent and length of branch-
ing as well as molecular weight and molecular weight distribution and amount of crystallinity. Some
of these are pictured in Figure 5.2. Commercial low-density PE (LDPE) typically has between 40
and 150 short-alkyl branches for every 1,000 ethylene units. It is produced employing high pressure
o
(15,000–50,000 psi) and temperatures (to 350 C). It has a density of about 0.912–0.935. Because of
the branching, the LDPE is amorphous (about 50%) and sheets can allow the flow through of liquids

and gasses. Because of the branching and low amount of crystallinity, LDPE has a low-melting
o
point of about 100 C, making it unsuitable for uses requiring sterilization through use of boiling
water. LDPE has a combination of short to long branches, with long branches occurring at a rate of
about ten short branches to every long branch.

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