Polymers

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Polymers
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Poly(ethene)
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One of the most common and useful reaction for
making polymers is free radical polymerization.
It is used to make polymers from alkene (vinyl)
monomers, that is, from small molecules
containing carbon-carbon double bonds.
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The whole process starts off with a molecule
called an initiator. When they split, the pair
of electrons in the bond which is broken, will
separate (homolytic fission) . This is unusual as
electrons like to be in pairs whenever possible.
When this split happens, we're left with two
fragments, called initiator fragments, of the
original molecule, each of which has one unpaired
electron. Molecules like this, with unpaired
electrons are called free radicals.
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Initiation
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Propagation
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where I the initiator molecule and R any
organic group, e.g. H, CH3, Cl, C6H5.
Propagation
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Propagation
where R any organic group, e.g. H, CH3, Cl,
C6H5
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Chain transfer to polymer can also occur as a
propagation step in polymerisation. This is the
process where a growing radical is transferred
from the end of a chain to the middle of another
polymer chain,


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Termination
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poly(propene)
Polypropene 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 is
used to make things like dishwasher-safe food
containers. It can do this because it doesn't
melt below 160 oC, or 320 oF. Polyethene, a more
common plastic, will anneal at around 100 oC,
which means that polyethene dishes will warp in
the dishwasher. As a fibre, polypropene 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 coloured
polypropylene, and because polypropylene doesn't
absorb water, like nylon does.
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Structurally, it is an addition polymer, and is
similar to polyethene, 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.
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Propene
Poly(propene)
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Reasearch 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
Poly(propene)
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But sometimes we use atactic polypropylene.
Atactic means that the methyl groups are placed
randomly on both sides of the chain like this
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When all the R groups lie on one side of the
carbon chain the polymer is described as
isotactic
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If the R groups show no regular pattern along the
polymer chain then the polymer is called atactic

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When all the R groups lie on alternating sides of
the carbon chain the polymer is described as
syndiotactic
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However, using special metallocene catalysts it
is believed that we can make polymers which
contain blocks of isotactic polypropylene and
blocks of atactic polypropylene in the same
polymer chain, as is shown in the picture
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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, each little hard clump of crystalline
isotactic polypropylene will be tied together by
soft rubbery tethers of atactic polypropylene, as
you can see in the picture.
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Most kinds of rubber have to be cross-linked to
give them strength, but not polypropene
elastomers.
other polymers
http//www.psrc.usm.edu/macrog/pp.htm
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Metallocene polymerization is making a big stir
in the plastics business. It's making a stir
because it's the hottest thing to hit vinyl
polymers since the invention of Ziegler-Natta
polymerization. So what's all the song and dance
surrounding this stuff about? The reason for all
the fuss is that metallocene catalysis
polymerization allows one to make polyethylene
that can stop bullets! This new polyethylene is
better than Kevlar for making bullet proof vests.
It can do this because it has a much higher
molecular weight than polyethylene made by the
Zieglar-Natta recipe. How high, you ask? Up to
six or seven million, that's how high!
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There's more here than high weights. This also is
good for making polymers of very specific
tacticities. It can be tuned to make isotactic
and syndiotactic polymers, depending on what you
need.
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Big deal. What's a metallocene? I figured you'd
want to know that. Again, I could give simple
answer that a metallocene is a positively charged
metal ion sandwiched between two negatively
charged cyclopentadienyl anions. Big deal.
What's a cyclopentadienyl anion?
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You may notice the on carbon atom with two
hydrogens, whereas the rest have one. These two
hydrogens are acidic, that is, they fall off very
easily. When this happens, it leaves its bonding
electrons behind. So the carbon it left now has
an extra pair of electrons.
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Don't you just hate it when you've got a pair of
extra electrons and nothing to do with them?
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But this is not the case with cyclopentadiene,
fear not! See those two double bonds in the
molecule? Each of those has two electrons,
remember, making four in all. Add those two extra
electrons on the carbon that lost the hydrogen,
and we have six. This is important. Six
electrons in a ring molecule like this will make
the ring aromatic. If you've had enough organic
chemistry to know what this means, great! If you
haven't just know that it means the ring in this
anionic form will be very stable.
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These cyclopentadienide ions have a charge of -1,
so when a cation comes along, like Fe with a 2
charge, two of the anions will form an iron
sandwich. That iron sandwich is called ferrocene.

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Ferrocene
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Ferrocene, or his (Cyclopentadieny) iron, is an
organometallic compound. Formula Fe (C5H6)2. It
is orange-yellow flake crystals with the smell of
camphor at room temperature. Melting point 172
174 C. Insoluble in water. Soluble in benzene,
ether, gasoline, diesel and other organic
solvents. It is chemically stable and does not
react with acid, base and ultraviolet. It does
not decompose below 400C and is harmless to man.

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Sometimes a metal with a bigger charge is
involved, like zirconium with a 4 charge. To
balance the charge, the zirconium will bond to
two chloride ions, -1 charge on each, to give a
neutral compound.
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Zirconocenes are a little different from
ferrocene. You see, those extra ligands, the
chlorines, take up space. It's hard for them to
squeeze in between the cyclopentadienyl rings. So
to make room for the chlorines, the rings become
tilted with respect to each other, opening like a
clam shell. This gives the chlorines space to
breathe. Take look at the picture showing this
tilt
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We can use some derivatives of bis-chlorozirconoce
ne to make polymers. Take this one for example
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It's different from bis-chlorozirconocene in that
each cp ring has a six-carbon aromatic ring fused
to it, shown in red. This two-ring system made of
a cp ring fused to a phenyl ring is called
indenyl ligand. Plus, there is a ethene bridge,
drawn in blue, which links the top and bottom cp
rings. These two features make this compound a
great catalyst for making isotactic polymers.
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You see, the big bulky indenyl ligands, pointed
in opposite directions as they are, guide the
incoming monomers, so that they can only react
when pointed in the right direction to give
isotactic polymers. That ethene bridge holds the
two indenyl rings in place. Without the bridge,
they could swivel about and might not stay
pointed in the right way to direct isotactic
polymerization.
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The Polymerization To make our zirconocene
complex catalyze a polymerization, the first
thing we have to do is add a pinch of something
called MAO. This compound was not discovered by
the late Chinese dictator Mao Zedong as some of
you might be guessing. Rather, MAO is short for
methyl alumoxane. Wouldn't you know it, MAO is
itself a polymer, with a structure like this
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It's an unusual polymer because it has metal
atoms in the backbone. But we're more interested
in what it does than what it is. To get our
catalyst to work, we need to use a whole bunch of
MAO, almost 1000 times the amount of catalyst.
The MAO is going to do something with the
chlorines of our zirconocene. You see, those
chlorines are what we call labile. That is to
say, they like to fall off of the zirconocene. So
MAO can replace them with some of its methyl
groups. We're left with a catalyst that looks
like this
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Wouldn't you know it, the methyl groups can fall
off, too. When one of them falls off, we get a
complex that looks like this
You'll notice in the picture that the
positively-charged zirconium is stabilized
because the electrons from the carbon-hydrogen
bod are shared with the zirconium. This is called
a-agostic association. But still, the zirconium
is lacking in electrons. It needs more than just
a wimpy agostic association to satisfy it. That's
where our alkene monomer comes in. Imagine an
alkene like propene. It's carbon-carbon double
bond is loaded with electrons to share. So it
shares a pair with the zirconium, and, at least
for now, everyone will be satisfied.
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The Polymerization
Ok folks, this brings up a question. Knowing why
this catalyst gives isotactic polypropylene, what
kind of catalyst would give syndiotactic
polypropylene?
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Have you figured it out yet? It's a catalyst such
as this bad boy, which was investigated by Ewen
and Asanuma in 1988
I think you can figure out why we get
syndiotactic polymerization from this catalyst.
Succesive monomers approach from opposite sides
of the catalyst, but they're always pointing
their methyl groups up. This way, the methyl
groups end up on alternating sides of the polymer
chain.