Polylactic acid (Polylactide)
Polylactide is thermoplastic. This means that once set it can be melted (for this plastic in particular at around 175o Celsius ) or made brittle. It can be reformed once formed and shows elastic properties above its glass transition temperature (for it at about 65o Celsius). That temperature is the temperature at which the molecules become more mobile. Polylactide is a strong rigid but resettable plastic because it is a thermoplastic.

lactic_acid.jpg
Above is a picture of it a lactic acid

The monomers of this polymer join in a very interesting way. The original monomers end in two hydroxides. This causes it to bond creating hydrogen dioxide (water). The water is released and the oxygen not used in the water bonds with the carbon of the next monomer. This type of monomer joining is known as dehydration synthesis. That is because it causes the monomers to lose what is the equivalent of water in joining. This synthesis follows the following equation: …C-O-H + H-O-C… à…C-O-C… + H-O-H.

lactide.jpg
Above is a picture of it as lactide

Unfortunately this way only ends it up with lactide. After this it has to be changed with ring opening polymerization. The ring opening polymerization breaks apart the lactide, which was automatically formed by the lactic acid, at the area it had automatically formed at. It then adds in more lactides to the chain making it look like it does below. This breaking can be done with a tin(II) chloride catalyast in experiments and a stannous octoate catalyst in building it. After that, it is complete. It does not crosslink or do anything else.

polylactide.jpg
Above is a picture of two monomers of polylactic acid. Note: The water molecules have not gone away or been taken by the molecule, I just did not show them.

It is a synthetic polymer based of natural things. It is made from fermented starches and sugars and therefore it is synthetic but it also is completely biodegradable. That fact and the fact that the lactic acid is found in nature is what makes it almost but not quite natural. For this reason, it is in fact partway in between the two. It is renewable, biodegradable and just generally not bad for the environment.

The polymer is used for many things. It can be used to create bio-plastic eating utensils. Along with that, it can make rods, plates, containers and other stuff to put into the body because it is biodegradable and because after biodegrading it becomes a molecule that the body makes regularly. It can even be used for clothing. It has many known uses and is also a very new molecule which has not really been explored much, meaning new uses might be found for it soon.

Note: this project is only on the polylactic acid made out of on l isomers. There is also a type made out of l and d isomers but we will not get into that here.

Sources

"Poly(lactic acid) (PLA)." The Gibson Group Research. The Gibson Group Research. 15 May 2008 <[[http://www.ch.ic.ac.uk/vgibson/research5.htm%3Cspan%3E.|http://www.ch.ic.ac.uk/vgibson/research5.htm.]]

"Polylactic acid." Plastics in Wiki. 24 July 2007. Media Wiki. 15 May 2008 <http://plastics.inwiki.org/Polylactic_acid<span>.

"Polylactic acid (Compostable plastic?)." ScienceBlogs.com. 26 Sept 2006. Seed Magazine. 16 May 2008 <http://scienceblogs.com/moleculeoftheday/2006/09/polylactic_acid_compostable_pl.php>.