In organic chemistry, an “olefin” or “alkene” is a molecule containing at least one double bond between carbon atoms. Polyethylene (PE) and polypropylene (PP) are “poly” olefins. The double bond is the means by which these molecules are joined together in a long polymer chain.
Ethylene “mers” can be strung together like beads on a string to form polyethylene. Different kinds of polyethylene can be made, depending on the number of “mers” in the “polymer.” A single polyethlylene chain may contain thousands or tens of thousands of carbon atoms.
[As an aside, The prefix “poly-” derives from Greek word polus, meaning “much”; or from the Greek word polloi, meaning “many.” Furthermore, the suffix “-mer” is derived from the Greek word meros, meaning “part.”]
There are other tricks that can be played in the production of polyethylene. These mostly have to do with how the hydrogen atoms are lined up along the chain of carbon atoms.
The properties of PP are distinctly different from those of PE. That’s because PP does not grow like a long string of beads but branches can shoot out in every direction. The “middle” carbon atom of the starting olefin (propene, or propylene) can attach to another carbon atom, resulting in four carbon atoms that are not on a straight line. Thus, polypropylene does not grow neatly along a straight line but rather grows into a kind of three-dimensional web on the molecular level.
Consequently, PP has quite different mechanical properties compared to PE. If PE is one-dimensional then PP is three-dimensional.
As it turns out, the properties of PP are more suitable for underlayment than those of PE. PP does not have problems with stress-cracking and it has excellent chemical resistance at higher temperatures. PP is similar to PE, but there are specific differences, including a lower density, higher rigidity and hardness, higher softening point (PP doesn't melt below 160 degrees C, while PE anneals at around 100 degrees C).