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Comparatively Speaking: The Meaning of Subscripts
By: Anthony J. O'Lenick, Jr., Siltech LLC
Posted: March 11, 2009
In this look at chemical structures, Tony O'Lenick reviews the meaning of subscripts in the context of polymers. In a related previous feature, the author compared the number of molecules in compounds with compositions, while another article examined the general molecular notation of compounds vs. compositions.
What do subscripts really mean?
Consider a composition made by adding ethylene oxide (EO) and propylene oxide (PO) to lauryl alcohol. The generic structure used in our industry is shown in Figure 1.
This molecule is a polymer and a composition; that is, it contains an oligomer distribution in which the value of “a” and “b” vary. The reported number for “a” and “b” is the average. Does the structure in Figure 1 require that all of “a” is added first--that is, all ethylene oxide is added before any “b” or propylene oxide is added? Or would a chemist of ordinary skill in the polymer arts realize that the ethylene oxide and propylene oxide could be added by mixing oxides before reacting?
The answer is that the structure collects the average number of “a” and “b” in the molecule, sums them, and lists them as shown in Figure 1. There is no implication that all “a” goes on first before any “b”; blending the ethylene oxide and propylene oxide first would also be covered by the same structure.