The structure of Propene (C3H6) is

           H    H      H
           |    |     /
      H -- C -- C == C
           |    |     \
           H    H      H
                     
and so the general form of a propene polymerisation
reaction forming polypropylene (or poly(propene)) is

    [        |     /]        [   |     /]
  n [-- C -- C == C ]  ----> [-- C == C ]
    [        |     \]        [   |     \] n