MadSci Network: Chemistry

Re: What stops a polymer growing linearly and Is a bowling ball 1 molecule?

Area: Chemistry
Posted By: Werner Sieber, Research Scientist,Ciba Specialty Chemicals Corp.
Date: Tue Jul 15 03:53:54 1997
Area of science: Chemistry
ID: 868653866.Ch

Dear Teacher -

Most polymerizations in the world are done by the RADICAL mechanism, so I shall take this as the exemplary case. For other mechanisms like ionic, or condensation polymerization, similar principles can be derived.

As you probably know, for chemists, radicals are molecules bearing lonely electrons, which strife to find a partner. Some radicals are less stable (i.e. more reactive, more likely to give off the electron) than others. The unpaired electron may sit at the end of a molecule or somewhere else. Reacting partners are either a) molecules bearing double bonds (or triple bonds), b) other radicals or c) molecules even without multiple bonds, acting as "radical transfer agents".

In case a) we have the normal linear growth of a chain. It proceeds as long as the radical encounters molecules with double bonds during its lifetime. If it encounters a radical transfer agent (case c)) the lone electron jumps over, the chain growth is stopped, and the radical transfer agent starts a new chain. If the growing radical encounter another of its kind (case b)), this is called recombination, and is another way to stop chain growth. Since oxygen is in fact a di-radical, it is a very efficient chain stopper, and it must be kept out of polymerization reactions.

Elastomers, the materials generally used to make balls, consist of crosslinked chain molecules, i.e. some the starting monomer molecules bear more than one double bond. A little thought will convince you that in this case each polymer chain will have connections with others, so that you can well say that your bowling ball (like your diamond) is a single molecule.

Best Regards
Werner Sieber

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