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MOLECULAR WEIGHT INTRODUCTION
Molecules such as ethene (ethylene) and propene (propylene) have specific molecular weights (28.05 and 42.08 respectively).
Polymers such as polyethylene and polypropylene do not have a single molecular weight, but rather, a distribution of molecular weights. This will be explained shortly, but first the definition of degree of polymerization is needed.
A molecule of polyethylene with a molcular weight of 2805 is comprised of 100 repeat units. It is said to have a Degree of Polymerization, DP, of 100. For this discussion, it is easier to think of polymers in terms of their degree of polymerization, than in molecular weight.
If a polymer has an average DP of 100, there are polymer chains with DP values such as 99, 98, 97, etc..., and polymers with DP values on the other side of the average, 101, 102, 103, etc., as is illustrated below:
So when the question is asked, what is the molecular weight?, the question is really what is the average molecular weight?
There are different methods for measuring average molecular weight, and they don't all give the same answer.
Number average molecular weight, Mn, is the definition of molecular weight that would be expected from the common definition for "mean" or "average".
Weight average molecular weight, Mw, gives an answer that is greater than Mn, but this is of importance because Light Scattering experiments give results that follow the formula for Weight Average Molecular Weight, rather than Number Average Molecular Weight.
Molecular Weight Definitions
Mark Houwink
Light Scattering
Osmotic Pressure
