As with any group of measurements, it's helpful to know how widely distributed the individual values really are. Some chains in the material will be longer (and heavier) and some chains in the material will be shorter (and lighter). What if something goes wrong with one of the growing chains, and it can no longer add new monomers? That chain experienced an early death, and it will never grow as long as the others.Īs a result, when we are speaking about the molecular weight of a polymer, we are always talking about an average value. What if one monomer starts reacting, forming a growing chain, before any of the others get started? With its head start, this chain will become longer than the rest. It requires monomers to come together and react. Why the difference? First of all, polymer growth is a dynamic process. In any given material, there will be some chains that have added more monomers and some chains that have added fewer. Notice that key point: it is just an average. The number of enchained monomers in an average polymer chain in a material is called the degree of polymerization (DP). Because polymers are assembled from smaller molecules, the length (and consequently the molecular weight) of a polymer chain depends on the number of monomers that have been enchained into the polymer. That variation introduces some unique aspects of polymer molecular weight. That's because a polymer is a large molecule made up of repeating units, but how many repeating units? Thirty? A thousand? A million? Any of those possibilities might still be considered a representative of the same material, but their molecular weights would be very different, and so would their properties. In polymers, molecular weight takes on added significance. It might seem obvious that molecular weight is an essential property of any molecular compound. Of course, all molecules have molecular weights of their own. Molecular weight is one of the most central aspects of polymer properties.
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