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History of the Polymer Concept

 
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"Drop the idea of large molecules, organic molecules with a molecular weight higher than 5000 do not exist. Purify your rubber; then it will crystallise."
- H.Wieland, to H.Staudinger, reported in
"Polymers: The Origins and Growth of a Science", by Herbert Morawetz

Early in the 20th century, the notion of single molecules with molecular weights in the tens of thousands or greater was met with derision. After a lecture by Hermann Staudinger, one of the pioneers of the concept of polymers, he was compared by a learned member of the audience to an African explorer who claimed to have seen a zebra 400 feet long.

This is not a story of heroes and villains - skepticism and imagination are both scientific virtues.

A new idea may be beautiful, but until it is proved by experiment it is not science. Both the brilliant new theory and the ruthless derision with which it is often met are necessary for the growth of science.

One early indication of the very high molecular weight of polymers was what are known as 'colligative properties'. Colligative properties are properties of a solution that depend only on the number of particles in solution, not on their chemical nature or identity. These include depresssion of freezing point, elevation of boiling point, and osmotic pressure. If the total mass of dissolved material is known, these properties give an easy way to estimate the molecular weight of a substance.

At a very high molecular weight, all of these effects are vanishingly small. For a long time, the prevailing wisdom was that small molecules of monomer associated into 'colloids' which did not interact with the solvent and therefore did not have any colligative properties - the mysterious behaviour of these materials was seen as a new kind of chemistry, giving clues to the nature of life.

Eventually, the tiny osmotic pressures and freezing point depressions seen in polymer solutions could no longer be ignored, or attributed to small amounts of a low molecular weight impurity.

The methods we now use to analyse polymers were not available in the 1920s, and the polymer concept was only accepted after Staudinger carried out painstaking experiments on the synthesis of oligomers (very small polymers, up to about 20 monomer units) and showed that their properties converged towards those of the large polymer.

Interestingly enough, Staudinger himself was equally dismissive some years later of the first claims that ethene had been polymerised...