The average molecular weight or molar mass average of a polymer is the average molar mass of the polymer chains.
In polymers, the individual polymer chains rarely have exactly the same chain length and molar mass. There is always a distribution of molar masses.
There are two common types of molar mass average.
Number Average Molar Mass (
"Weight" Average Molar Mass (
The molar mass distribution of a polymer is measured by its dispersity, Đ (pronounced D stroke).
If the molecules all have the same mass, the distribution is uniform. If they have different masses, the distribution is nonuniform.
Biological polymers are often uniform. They may have Đ ≈ 1, indicating that all the polymers are about the same length.
For typical addition polymers, Đ often ranges from 10 to 20.
With some synthetic polymers, you can control the reaction conditions such as temperature, initiation rate, reactant ratios; limit termination steps, etc. so that the chains are similar in length.
You can often get dispersities close to 1.
The video below shows how to calculate