Homolysis

There are two types of covalent bond breaking: namely homolysis and heterolysis.

In homolysis, two free radicals are formed.
X-Y → X• + Y•

In heterolysis, a cation and an anion are formed.
X-Y → X+ + Y-

In a polar solvent, heterolysis is favoured. This is because solvolysis (the ion is surrounded by solvent molecules and is attracted by the oppositely-charged ends of solvent molecules) stabilizes the ions formed.

Besides, free radicals are very reactive, which can react with some polar solvents (especially protic solvents).

呢條問題好好
關於solvent effect