Hey ntripleb, first off, you may have noticed already, but it's spelled polyethylene
Anyway, so what exactly is the question you've been given, is that it word for word? Or are you just curious, based on perhaps what someone told you..
Basically you should know that polyethylene can come in the form of Low Density, where the hydrocarbon chains will have a lot of short alkyl branches every so often that disrupt the intermolecular dispersion forces (used to produce things like sandwich wrapping), or High Density if a metal oxide catalyst has been used with the reaction at a temperature somewhat higher than 300 degrees celcius, having very minimal branching and therefore packing the molecules together much more (heavier duty than LDPE, used for things like garbage bins).
So my understanding is due to polypropylene having a methyl group branching off every second carbon atom after the double bonds have broken, the intermolecular forces would be a little weaker than polyethylene, making the polymer a little less dense, and thus lighter, theoretically. But of course, there are so many different forms of polyethylene and polypropylene in general, in reality it very much depends on the process and conditions in which both of them were made in.
That is entirely different to molecular weight though, as yes if you were comparing the two with the same polymer lengths, polypropylene would have a higher molecular weight.
As for polyethylene being more responsive or the elasticity of it, I'm not exactly sure. I would be assuming polypropylene having slightly less density than polyethylene due to the methyl branches, to have slightly more freedom in movement, like the difference between LDPE and HDPE.
ChemBuddy | 
Chem Reddit | 
Topic: POLYETHENENE (Read 6479 times)