A special family of polyesters are polycarbonates.
hydrocarbon backbones which contain ester linkages, hence the name.
The structure in the picture is called poly(ethylene terephthalate), or
PET for short, because it is made
up of ethylene groups and terephthalate groups (duh!). I realize that
terephthalate is not the kind of word most English-speaking mouths
are used to saying, but with practice you should be able to say it with
only a slight feeling of awkwardness when it rolls off your tongue.
The ester groups in the polyester chain are polar, with the carbonyl
oxygen atom having a somewhat negative charge and the carbonyl carbon
atom having a somewhat positive charge. The positive and negative
charges of different ester groups are attracted to each other. This
allows the ester groups of nearby chains to line up with each other in crystal
form, which is why they can form strong fibers.
The inventor who first discovered how to make bottles from PET was
Nathaniel Wyeth. He's the brother of Andrew Wyeth the famous painter.
But others had tried before. Go read this story of someone who may have
been the first person to
try to make a shatterproof bottle.
Now I'm sure everyone out there is just dying to have two questions
answered. The first one is:
PEN has a higher glass transition temperature than
PET. That's the temperature at which a polymer gets soft. The glass
transition temperature of PEN is high enough so
that it can withstand the heat of both sterilizing bottle washing and
hot strawberry jelly. PEN is so good at standing the heat that you don't
even have to make the bottle entirely out of it. Just mixing some PEN
in with the old PET gives a bottle that can take the heat a lot better
than plain old PET.
Ok here goes...
In the big plants where they make polyester, it's normal to start off
with a compound called dimethyl terephthalate. This is reacted with
ethylene glycol is a reaction called transesterification. The
result is bis-(2-hydroxyethyl)terephthalate and methanol. But if we heat
the reaction to around 210 oC the methanol will boil away and
have to worry about it anymore.
But in the laboratory, PET is made by other reactions. Terephthalic acid
and ethylene glycol can polymerize to make PET when you heat them with an
acid catalyst. It's possible to make PET from terephthoyl chloride and
ethylene glycol. This reaction is easier, but terephthoyl chloride is
more expensive than terephthalic acid, and it's a lot more dangerous.
Polymer Science Learning Center
Department of Polymer Science
The University of Southern Mississippi