Chain growth or addition polymer (and polyolefin).
Polypropylene (the name "polypropene" is never used outside of a school classroom) is an addition polymer, formed by the chain growth polymerisation of propylene (
The polymer chain forms as follows:
1) An initiator starts the process. A typical initiator would be some organic compound with a labile group, commonly peroxides, although disulphides can also be used.
2) Initiator reacts with the double bound in the propylene to create a free radical centre.
3) Free radical centre reacts with another propylene molecule to create a dimer unit which is also a free radical centre. This reacts with more propylene molecules to give longer and longer chains, which still have free radical centres.
4) Chain termination by which the radical centre is eliminated by combination or disproportionation. Polymer chain then stops "growing".
Because the polymer chain effectively forms by adding on multiple monomer units the process is known as "addition polymerisation". Because of the molecules forming as a growing chain of monomer units, the term "chain growth polymerisation" is also used.
(This is a simplified description - in reality, chain branching would also occur via transfer of the radical centre to different parts of the chain or different parts of different chains).