Polypropylene
Polypropylene is one of those rather versatile polymers out there. It serves
double duty, both as a plastic
and as a fiber. As a plastic it's
used to make things like dishwasher-safe food containers. It can do this because
it doesn't melt below 160oC, or 320oF.
Polyethylene, a more common plastic,
will anneal at around 100oC, which means that polyethylene dishes
will warp in the dishwasher. As a
fiber, polypropylene is used to make indoor-outdoor carpeting, the kind that
you always find around swimming pools and miniature golf courses. It works well
for outdoor carpet because it is easy to make colored polypropylene, and because
polypropylene doesn't absorb water, like
nylon does.
Structurally, it's a vinyl
polymer, and is similar to
polyethylene, only that on every other carbon atom in the backbone chain has
a methyl group attached to it. Polypropylene can be made from the monomer
propylene by Ziegler-Natta
polymerization and by metallocene
catalysis polymerization.
This is what the monomer propylene really looks like:
Wanna know more?
Research is being conducted on using metallocene catalysis polymerization
to synthesize polypropylene. Metallocene catalysis polymerization can do some
pretty amazing things for polypropylene. Polypropylene can be made with
different tacticities. Most
polypropylene we use is isotactic. This means that all the methyl groups
are on the same side of the chain, like this:
But sometimes we use atactic polypropylene. Atactic means that the
methyl groups are placed randomly on both sides of the chain like this:
However, by using special metallocene catalysts, it's believed that we can make
polymers that contain blocks of isotactic polypropylene and blocks of atactic
polypropylene in the same polymer chain, as is shown in the picture:
This
polymer is rubbery, and makes a good
elastomer. This is because the isotactic blocks will form
crystals by themselves. But
because the isotactic blocks are joined to the atactic blocks, the little hard
clumps of crystalline isotactic polypropylene are tied together by soft rubbery
tethers of atactic polypropylene, as you can see in the picture on the right.
To be honest, atactic polypropylene would be rubbery without help from the
isotactic blocks, but it wouldn't be very strong. The hard isotactic blocks hold
the rubbery isotactic material together, to give the material more strength.
Most kinds of rubber have to be
crosslinked to give them strength, but not polypropylene elastomers.
Elastomeric polypropylene, as this copolymer is called, is a kind of
thermoplastic elastomer. However,
until the research is completed, this type of polypropylene will not be
commercially available.
The polypropylene that you can buy off the shelf at the store today has
about 50 - 60% crystallinity, but this is too much for it to behave as an
elastomer.
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