While the abbreviations used to describe the make-up of polymers and rubbers is reasonably straightforward (PVA being probably the worst offender - is that polyvinyl acetate or polyvinyl alcohol?), the rubber EPDM is another nasty one. The first three letters are reasonable: "E" for ethylene, "P" for propylene and "D" for diene, which in theory could be any diene, but in practice is usually one of a few options. The "M" is the mystery however. People either have no idea or they think that it stands for "monomer". Huh?
The truth is well removed from that as it actually isn't part of an abbreviation at all. It is drawn from an ASTM standard, which is quite appropriate as the name ASTM isn't an abbreviation for anything either. (It used to stand for American Society for Testing and Materials, but it doesn't anymore). The standard D1418 is used to classify rubbers and the M-class is for rubbers that have saturated backbones. Why M? Heaven only knows. Other letters used to classify rubbers are N, for nitrogen-containing backbones and O for oxygen-containing backbones. So far so good. But then there is R for unsaturated backbones, Q for silicon- and oxygen-containing backbones (siloxane polymers have their own naming scheme, in which Q fits in quite nicely, but that is another story for another day), T for sulfur-containing backbones, U for carbon-, oxygen- and nitrogen-containing backbones and finally Z for phosphorus and nitrogen-containing backbones.
Given all of that, M isn't that bad of a choice, but since the letter S is available (since sulfur was bumped to T), why not go with S?
This all came about from a recent discussion with a friend over Superballs, the big black ones made by Wham-O. I had always thought that they were made from EPDM (with a pinch or two of carbon black), but it turns out that they are actually made from butadiene. You learn something new everyday.