How can ethylene molecules be joined
together to form polyethylene polymer?
Many different kinds of monomers have
been employed in polymerization reactions. We will focus our
attention on what is perhaps the simplest type of monomer subunit
alkenes or "olefins". The smallest olefin is
ethylene, C2H4, which consists of two
carbon atoms that are each bonded to two hydrogen atoms and
linked by a double bond. The double bond is composed of a strong s-bond and a weaker p-bond that results from overlap of p
orbitals on the carbon centers. Ethylene is a two-dimensional
(flat) molecule, and the atoms around each carbon center are
trigonally oriented (120o apart). When ethylene
monomers polymerize, the weak p-bonds are destroyed and a long chain
of CH2CH2 units is produced (Scheme 1).
All of the carbon-carbon bonds in the
resulting polymer ("polyethylene") are single bonds and
the groups around each carbon center are tetrahedrally oriented (109.5o apart). Hence, as shown in
Figure 1, the chain has three-dimensional structure. The value of
n (i.e., the length of the chain) varies with the reaction
conditions but is generally 500 or more. Furthermore, a typical
synthetic polymer does not have a single value for n. Rather, it
consists of a mixture of individual polymer molecules with
different lengths. Therefore, it is meaningful to discuss the average
chain length or molecular weight for a polymer sample.
A portion of linear polyethylene, showing the three-dimensional structure.
Carbon labels are omitted for clarity.
|A 3D stick
representation of linear polyethylene. The hydrogens are shown in white and the
carbons are in green. The polymer is terminated by
To download a pdb file for an 8-mer polyethlyene, please
click on the 2D representation, the stick representation,
or the words polyethylene.
Polyethylene can be produced in several
different ways. One approach involves relatively high pressure
(2000 atm) and high temperature (200 oC) conditions and the use of a
"free radical" initiator. A free radical is a molecule
with at least one unpaired electron. This synthetic approach
leads to polyethylene which is highly branched, i.e., instead of
a single long strand, a typical molecule has many side branches
(see Figure 2). This open structure results in a polymer with a
relatively low density (usually around 0.91 gr/cm3) and it is, therefore,
referred to as low density polyethylene or LDPE. LDPE melts at
about 120 oC and is soft, stretchy,
transparent, and not very strong. It is used in products such as
flexible squeeze bottles or wash bottles and clear plastic
Two-dimensional representation of
a portion of branched polyethylene. Hydrogen labels are omitted for clarity.
|A 3D stick representation of branched
hydrogens are shown in white and the carbons are in
green. The polymer is terminated by hydrogens.
To download a pdb file for a branched polyethlyene,
please click on the 2D representation, the stick
representation, or the words branched polyethylene.
Prior to the 1960's, LDPE was the only
form of polyethylene available. However, in the late '50's and
early '60's, Karl Ziegler in Germany and Giulio Natta in Italy
discovered that the use of metal-based catalysts, particularly
titanium/aluminum systems, led to a new type of polyethylene.
These new catalysts allowed Ziegler and Natta to make very long (>10,000 monomer units) linear
polyethylene molecules under very mild conditions (1-10
atmospheres, 50 o-100
oC). Because these
chains did not possess side branches, they could line up parallel
to one another in an ordered structure. This efficient packing of
molecules gave rise to a higher density material (density between
0.97 and 0.99 gr/cm3). In addition, this high
density polyethylene or HDPE had greater strength and a higher
melting point (130 oC) than the LDPE discussed above.
Furthermore, it was opaque, rather than transparent like LDPE.
Today, HDPE is used in applications that require strength and
rigidity, e.g., the one gallon plastic milk jugs that must retain
their shape when filled with milk and stack on top of one
another. The plastic grocery bags that are often used as
substitutes for paper are also made of HDPE.
Using Cochrane's Molecular Models,
build a model of linear polyethylene.
Designer Plastics Module Table of Contents