Industry expert Tony O'Lenick explores the difference between the molecular weight of a compound vs. a polymer...
Consider hexane: Hexane has a molecular weight of 86. In fact, every hexane molecule has a molecular weight of 86. Now, if another carbon is added to the chain, and the appropriate amount of hydrogen atoms are added as well, the molecular weight is increased to 100. Each compound has one molecular weight.
However, polymers are different: consider polyethylene. If a sample of polyethylene is analyzed, one, unique molecular weight will not be found, but a mixture of compounds having a range of molecular weights will. One molecule may be found in the distribution that has fifty thousand carbon atoms in it, and another that has fifty thousand and two carbon atoms in it. The molecular weight usually is a bell-shaped curve of a distribution of molecular weights.
Some of the polymer chains will be much larger than all the others at the high end of the curve, and some will be much smaller at the low end of the curve. The largest number will usually be clumped around a central point--the highest point on the curve. So, one has to discuss "average" molecular weights when discussing polymers.
Simple compounds have one molecular weight; polymers have a distribution. The distribution of molecular weight in a given polymer also will have an impact on performance in formulations.