[adianadiadi]

Why does PO₃^- ion undergo polymerization easily whereas NO₃^- does not?

I think the reason might be: stronger N=O bond. But P=O bond is also stronger. So any idea?

[rabolisk]

Probably because nitrogen, as a period 2 element, is incapable of forming more than 4 bonds, whereas phosphorus is.

[adianadiadi]

There is no question of forming 4 bonds during oligomerization.

[jcjlf]

There is no question of forming 4 bonds during oligomerization.

Indeed.
In the Lewis-structure of NO₃^- you have an electronic structure with one N=O bond, relatively stable because in period 2 the pi-pi orbitals do overlap pretty good, for the atoms are small.
In period 3 the atoms have a greater size, so these elements don't form strong double bonds, the P=O in a similar structure as in NO₃^- is weak or hardly existing because pi-pi interaction is weak. Thus PO₃^- is an unstable species on its own.
So PO₃^- easily adds up to PO₄^3- to form P₂O₇^4-, a more stable dimer with only single P-O bonds. And this can go on with the next PO₃^- attaching to form a trimer P₃O₁₀^5- etc. etc.

This explanation disregards p-d orbital interaction in phosphates, but it is of some help. There is a general tendency to form only single bonds in the third (and higher) period, e.g. compare CO₂ and SiO₂.