How can one maintain a specific chain length in free radical propagation?

Answer 1

It is difficult to control precisely, but in order to make polymers of a specific length using a free radical chain reaction, there are three steps: initiation, propagation and termination. To stop the chain reaction (termination) you need something to quench the free radicals so that the reaction cannot continue. This can be done by either adding a radical-quencher -- often a large molecule that can stabilize a free radical thus removing its high reactivity -- or the reaction can also be terminated because a limiting reactant has been consumed. While these methods will stop the reaction at a specific point in time, they don't guarantee that all of the polymer chains will be of equal length (there will be a distribution of lengths around some average value, although that distribution might be wide or narrow depending on numerous factors). Generally to obtain a polymer chain of one specific length, further purification and separation techniques are required.

Answer 2

To maintain a specific chain length in free radical propagation, a chain transfer agent can be used to transfer the growing radical to another species without adding or removing carbons. Another method is to carefully control the concentration of the reactants to limit the number of propagation steps that occur. Finally, terminating the reaction before the chain length grows beyond the desired length can also help maintain a specific chain length.