Polymerisation joins many small molecules (monomers) into one long chain, by one of two routes: addition, where double bonds open and link end to end with nothing lost, or condensation, where each link squeezes out a small molecule such as water. Starch, cellulose, proteins, rubber and shellac are natural polymers; plastics are the same idea made deliberately. Each glucose link in starch or cellulose weighs 162 mass units, the glucose mass of 180 minus the 18 of water lost.
Part of Organic Chemistry
How small organic molecules are joined into the long chains that make up fibres, rubbers, glues and plastics.
Two Routes To A Long Chain
Addition polymerisation. The monomer has a double bond (see Bonding Rules). One of the two bonds opens and attaches to the next monomer, again and again. Ethylene becomes polyethylene this way. Nothing is lost, so the polymer’s mass equals the sum of the monomers. Rubber latex is a natural addition polymer built from a diene.
Condensation polymerisation. The monomers carry reactive groups at both ends (see Functional Groups). Each time two ends meet they form a link and release a small molecule, usually water. This is esterification repeated: a diacid and a diol give a polyester, an amine and an acid give an amide, as in protein. The carrier’s functional-group table already shows the pieces.
Natural Polymers The Corpus Already Uses
- Starch and cellulose, chains of glucose. Cellulose is the fibre in wood and cotton.
- Proteins, chains of amino acids, such as casein (see Casein Plastic) and the collagen behind Hide Glue.
- Rubber, the addition polymer of latex; see Rubber and Polymers.
- Shellac, a resin polymer from insects; see Shellac.
The plastics that are made, not harvested, are covered in the plastics articles: Plastics from Scratch and Plastics Manufacturing, with the two behaviour classes in Thermoplastics and Thermosets. The early plastics Celluloid and Bakelite have their own pages. This article deliberately gives no nitration of cellulose and no phenol-resin recipe; those belong to the plastics articles and are written there as chemistry without a procedure.
Method: Reading A Polymer
Step 1: Ask which route made it. If the chain contains ester, amide or ether links in the backbone, it is a condensation polymer; a carbon-only backbone is an addition polymer.
Step 2: Ask whether it softens on heating. Chains that are not cross-linked soften and can be remelted; cross-linked networks char instead. That is the thermoplastic and thermoset split.
Step 3: Choose by use: fibre, flexible film, adhesive, or rigid solid.
Worked Example: Mass Of A Starch Chain
Glucose is C6H12O6: 6 × 12 + 12 × 1 + 6 × 16 = 180 g per mole (rounded atomic masses).
- Joining two glucose units releases one water, 18 g. Each unit that joins therefore adds 180 − 18 = 162 g to the chain.
- A chain of 1,000 glucose units (an illustrative length) is 1,000 × 162 + 18 for the one water kept at the ends = 162,018 g per mole, about 162,000.
- Check by adding: 1,000 glucose = 180,000 g; 999 water lost = 999 × 18 = 17,982 g; 180,000 − 17,982 = 162,018 g.
Run in reverse, each link broken by acid and heat takes one water back in and returns one glucose, which is how starch is converted to fermentable sugar.
Common Mistakes
- Expecting every polymer to melt. Cross-linked ones do not.
- Handling monomers as if they were the harmless polymer. The monomers of several common plastics are toxic or irritating even when the finished plastic is inert. Treat unreacted monomer as a hazard.
- Ignoring the heat. Many polymerisations release heat and can run away; see Heat Management.
Safety
Unreacted monomers and runaway heat are the two hazards
Do not run any polymerisation in a sealed vessel. Keep fume and ventilation outdoors, avoid skin contact with monomers and solvents, and do not attempt to nitrate cellulose: nitrocellulose at high nitrogen content is an explosive, and the margin between plastic-grade and explosive-grade product is narrow and cannot be controlled without instruments. The plastics articles explain this as chemistry and history; none of them is a recipe.
What’s Next
- Learn next: Plastic Alternatives — natural polymers that work as plastics
- Combine with: Adhesives and Glues — polymers used as glue