while polypropylene has the repeat unit
The subscript "n" denotes the degree of polymerisation, that is, the number of units linked together. The molecular mass of the repeat unit, MR, is simply the sum of the atomic masses of the atoms within the repeat unit. The molecular mass of the chain is just the product nMR. Other than monodisperse polymers, there is normally a molar mass distribution caused by chains of different length.
More complex repeat units occur in vinyl polymers, where one of the hydrogen atoms is substituted by another different atom, or by a larger fragment. So PVC for example possesses a chain in which the hydrogen atom is substituted by a chlorine atom: