Polymerization isomerism is a type of isomerism involving coordination complexes of transition metals.
Polymerization isomers have the same empirical formula but molecular formulas that are integral multiples of the empirical formula.
Thus, complexes (ii), (iii), and (iv) are polymerization isomers of complex (i).
They all have the empirical formula
Complexes (ii) and (iii) appear to be dimers of complex (i), and complex (iv) appears to be a pentamer of complex (i).
However, none of these is a real polymer or even an oligomer.
I suspect that the term "polymerization isomer" was coined before the nature of the complexes was completely understood.