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Introduction:
Polymers are long chain giant organic molecules are
assembled from many smaller molecules called monomers.
Polymers consist of many repeating monomer units in long
chains. A polymer is analogous to a necklace made from many small
beads (monomers).
Many monomers are alkenes which react by addition to
their unsaturated double bonds. The formation of polyethylene
from ethylene (ethene) may be illustrated in the graphic on the
left as follows:
The electrons in the double bond are used to bond two monomer
molecules together. This is represented by the red arrows moving
from one molecule to the space between two molecules where a
new bond is to form.
Note that in the complete polymer, all of the double bonds
have been turned into single bonds. No atoms have been lost and
you can see that the monomers have just been joined in the process
of addition.
A simple representation is -[A-A-A-A-A]-.
Polyethylene is used in plastic bags, bottles, toys, and electrical
insulation.
LDPE - Low Density Polyethylene: The first commercial
polyethylene process used peroxide catalysts at a temperature
of 500 C and 1000 atomspheres of pressure. This yields a transparent
polymer with highly branched chanis which do not pack together
well and is low in density. Today most LDPE is used for blow-molding
of films for packaging and trash bags.
HDPE - High Density Polyethylene: An alternate method
is to use Ziegler-Natta aluminum titanium catalysts to make HDPE
which has very little branching, allows the strands to pack closely,
and thus is high density. It is 3 times stronger than LDPE and
more opaque. About 45% of the LDPE is blow molded in milk and
squeezable bottles.
Link to Polyethylene
- with Chime molecule - Macrogalleria at U. Southern Mississippi
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