Addition polymerization: It involves following three mechanisms:

1.                 Free radical polymerization

2.                 Ionic polymerization mechanism

3.                 Coordination polymerization mechanism

Free-radical polymerization: It involves following steps:

Initial step

1.                 Production of free radicals by the decomposition of compounds called initiators

R-R = 2R

Initiator free radicals

The decomposition of the initiator is splitting into two identical fragments called homiletic decomposition, where each fragment carrier's one unpaired electron with it called free radical.

Chain propagation step: The free radical (R) produced in the chain initiation step adds to an alkenes to form a new free radical, which then reacts with additional monomers in a chain reaction:

R + H₂C = RCH₂CH₂

RCH₂CH_{2 +}CH₂ = RCH₂CH_{2 :}CH₂