I do not know if the following is the correct answer.
But ... is there any meaning to talk of the molecular weight of a polymer since the chains will be of varied, uncertain, length?
Is your original question about high-density polymers?
http://en.wikipedia.org/wiki/HDPE
Clive
I found the high and low molecular weight word through here:
Molecular weight, for the purposes of this discussion, may be pictured as representing polymer chain length. The greater the M.W., the greater the chain length and the greater the flocculating ability. Longer chain length allows bridging, or attaching to, greater numbers of particles. However, coagulant polymers have relatively low molecular weights and flocculating ability when compared to the high molecular weight cationic, nonionic and anionic flocculant polymers. Therefore, polymeric coagulants have the ability to enhance flocculation only to a limited degree.
from this source:http://www.tramfloc.com/tf29.html
Thats why i am curios about the high and low molecular weight thing