When ethene polymerises two things can happen: the reaction can join the molecules together to produce long unbranched chains or the conditions can be changed to produce more branched chains. This has a profound effect on the final product.
Unbranched chains can nestle together very closely and this allows the molecules to experience a lot of intermolecular attraction. They pack closely together and the result is HDPE (high density polyethene). HDPE is rigid as a result.
Branched chains, of course, cannot pack so closely together that intermolecular forces are weaker and result in LDPE (low density polyethene). LDPE is weaker and more flexible than HDPE as a result.