It is known that the degree of polymerisation, which is the average number of monomer units in each polymer chain formed, increases as the reaction proceeds for step-growth polymerisation as longer and longer chains are formed throughout the reaction. However, the degree of polymerisation supposedly remains approximately the same through the reaction for chain-growth polymerisation. Why is this the case? Shouldn't the degree of polymerisation also increase throughout the reaction?

More interestingly, it is also mentioned in Introduction to Polymer Chemistry (3rd ed.) that:

Also, unlike the stepwise process, average chain length decreases with increase in reaction temperature.

If this is the case, wouldn't the degree of polymerisation decrease as reaction proceeds?

I was thinking, could it be because in the chain-growth process, polymer chains of roughly the same length are formed as the reaction proceeds. For example, at the start, a chain is initiated and the chain propagates until the number of monomer units it contains is $\ce {100}$. Once it has reached this length, the chain terminates. As the reaction proceeds, more chains are initiated but all of which terminate when the chain length is about $\ce {100}$ monomers. Is this what actually happens?


Carraher Jr., C. E. (2003). Introduction to Polymer Chemistry (3rd ed.). CRC Press

  • $\begingroup$ Mhm, pretty much this. $\endgroup$ – Mithoron Mar 27 at 20:17

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