# In which type of polymerisation mechanism small molecules such as $H_2O,NH_3,HCl,ROH,.....$ are lost?

$\begin{array}{1 1}(a)\;\text{Free radical addition polymerisation}\\(b)\; \text{Step growth polymerisation}\\(c)\;\text{Cationic polymerisation}\\(d)\;\text{Anionic polymerisation}\end{array}$

Free radical,cationic & anionic polymerisation mechanisms are addition polymerisation mechanisms whereas step growth polymerisation is condensation polymerisation,only in case of step growth ,a large number of monomers combine with the loss of simple molecules($H_2O,NH_3,HCl,ROH$) to form a polymer.
Hence (b) is the correct answer.