Under appropriate conditions, hemoglobin dissociates into four subunits

The oxygen saturation curve for the isolated alpha subunit is hyperbolic. What does that mean? That means that there is no cooperativity from its other subunits. Well, that makes sense since it's an isolated chain! Nevertheless, you will also notice that the oxygen saturation curve for myoglobin (the single chain oxygen carrying protein in muscle tissue) is also hyperbolic. So, there is a similarity here. Myoglobin is also a single chain protein showing no cooperativity from any form of quaternary structure, just like isolated alpha chain.

In addition, the isolated chain of hemoglobin is not affected by acid, carbon dioxide or BPG. Once again, this tells us that the source of cooperativity in hemoglobin arises from its quaternary structure, i.e. from the other subunits. Another way to put it is this: cooperativity arises from the interaction among the subunits. Without any interaction from the subunits, there is no cooperativity for oxygen binding.

Positive cooperativity is a property that only multimeric proteins possess.