The formula in parentheses is the monomer unit. The monomer unit has a head (the CHOH group) and a tail (the CH2 group). This formula does not indicate how adjacent monomer groups are oriented with respect to each other. One might suppose that they form regular chains, with the head of one monomer bonded to the tail of its neighbor, and so on, down the chain. Nothing is that perfect, so the chains often contain runs of regularly arranged monomers, with occasional head-to-head defects. One finds chains of the sort shown below, where the central linkage is a head-to-head, 1,2-glycol structure.