MDPE (Medium Density Polyethylene) has a little less branching then the HDPE. It is less notch sensitive then HDPE and has better stress cracking resistance. UHMWPE (Ultra High Molecular Weight Polyethylene) has extremely long chains, with molecular weight numbering in the millions (usually between 2 to 6 million). In general, HDPE molecules have between 700 and 1,800 monomer units per molecule, whereas UHMWPE molecules tend to have 100,000 to 250,000 monomers each. The chains of UHMW align in the same direction.