Polymerisation of lactic acid
This procedure is based on one developed by Horn, Bader and Buchholz (tested by David Read (School Teacher Fellow, School of Chemistry, University of Southampton) and Rachel Hadi-Talab (Chemistry Technician, Science Learning Centre London)). The original documentation can be downloaded from:
In the plastics industry today, there are two significant goals. One is to develop plastics from renewable starting materials, and the other is to develop plastics which are biodegradable. Some plastics are able to satisfy both demands. You are going to make one example.
1. Why is it desirable to produce plastics from renewable starting materials?
2. Why is it desirable to produce plastics which are biodegradable?3. What sort of items would it be particularly useful to make from biodegradable plastics?
3. Can you think of any items that are not suited to being manufactured from biodegradable plastics?
Polylactic acid or polylactide (PLA) is a thermoplastic aliphatic polyester derived from renewable resources, such as corn starch (in the United States), tapioca roots, chips or starch (mostly in Asia), or sugarcane (in the rest of the world).
You are probably familiar with the fact that lactic acid (3-hydroxypropanoic acid) is produced in your muscles when you do vigorous exercise (anaerobic respiration). Lactic acid can be produced on an industrial scale using a type of fermentation. The lactic acid can then be polymerised by dehydration in an esterification process:
As the scheme above shows, the reaction can be reversed, with poly(lactic acid) being hydrolysed back to the monomer. This is the basis of the biodegradability of the polymer.
This practical takes very little time – you should be able to complete the task in 5 minutes but you need to do it in school because you need a fume cupboard.
Boiling tube, test tube holder, Bunsen burner, heat mat, anti-bumping granules, 10 cm^3 measuring cylinder, Petri dish lined with aluminium foil, 5 cm^3 lactic acid (available as an 85 % aqueous solution or as the neat acid), micro-spatula load of tin (II) chloride (anhydrous).
Wear safety glasses and apron/labcoat at all times. There is a risk of the solution spitting when heated, so it is also sensible to wear gloves. Carry out the heating process in a fume cupboard. Ensure that the room is well ventilated.
Eye or skin contact may cause severe irritation or burns. Materials are stable but combustible. Materials are incompatible with strong oxidizing agents.
Materials are harmful if swallowed, inhaled or absorbed through the skin.
1) Measure out 5 cm^3 of lactic acid using the measuring cylinder. Pour this into the boiling tube.
2) Add a small amount of tin (II) chloride to the lactic acid and drop in an anti-bumping granule.
3) Light a Bunsen burner in the fume cupboard and set it to a roaring blue flame.
4) Using the test tube holder, hold the boiling tube so its bottom is in the top of the flame. Gently move the tube around so that the same spot isn’t exposed to the hottest part of the flame for the whole time.
5) Try to keep the tube pointing towards the back of the fume cupboard – hot vapours will be given off.
6) When the mixture turns a yellow/brown colour, quickly pour it into the Petri dish lined with aluminium foil. Place the hot boiling tube on the heat mat to cool down.
You may wish to keep some samples of the plastic to demonstrate their biodegradability relative to other plastics.
7) The aluminium foil covered in the polymer can be disposed of with normal household waste.
a. What are the properties of the polymer you have made?
b. What could you do to get the polymer into a form that would be useful to you?
c. The polymer is biodegradable, but how do you think you could accelerate the breakdown of the material in the lab?
d. Look at the website below. Why is this method of producing lactic acid particularly in keeping with the aims of ‘Green Chemistry’?