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.
The reason for this is two-fold. First, the exact location of a particular ethylene oxide segment or propylene oxide segment on a backbone in a complex mixture is an analytical nightmare to determine. Second, even if it were possible, there would be an endless number of names for these polymers depending on their exact structure.
Assume the EO and PO were mixed then added to the alcohol, and assume there is no difference in reactivity between EO and PO, which is a faulty assumption. If a = 1 and b = 1, then Figures 2 and 3 show the two possible products.
As the chemists moves to a = 2 and b = 2, things get complicated there are now more possible products:
The possibilities then grow exponentially as the number of “a” and “b” are added.
One needs to be careful in looking at complex copolymers so as not to read too much into the names and structures. Bear in mind that polymers are complex mixtures and that the exact nature of the mixture might well have an impact on performance in formulations.