Why there isn't any unimolecular addition reaction?

[Tahmeed]

In my textbook, i have read about unimolecular elimination and nucleophilic unimolecular substitution reaction of organic compounds. In those reactions, firstly one bond breaks automatically then as soon as a carbocation forms then a nucleophile attacks the carbocation to complete the reaction.

Now, my question is, why the same doesn't happen in case of addition reaction of molecules having double bond? one bond breaking may cause a charge separation and then nucleophile/electrophile can attack the positive or negative charged side? I know molecules with charge separation aren't supposed to be much stable. But even in the case of a carbocation the same happens.

I have googled for unimolecular addition reaction but all the results were about unimolecular substitution/elimination.

 

[TeethWhitener]

"Unimolecular" and "bimolecular" refer to the kinetics of the reaction. In other words, in a unimolecular elimination/substitution, the reaction rate looks like:
$$\frac{d[A]}{dt} \propto k[A]$$
whereas a bimolecular reaction rate looks like:
$$\frac{d[A]}{dt} \propto k[A][ B] $$
How would you get a reaction rate that looks unimolecular for an electrophilic alkene addition reaction where two species have to come together?