This is the process for the formation of polyethylene, polyproylene, polystyrene, polyvinyl chloride, natural rubber from isoprene, etc. These reactions require either an acid or radical group to initiate and then to terminate the process. The polymerization mixture consists mostly of the newly formed polymer and unreacted monomer. The reaction is usually exothermic. Usually as order increases, density and hardness increase also.
Atactic polypropylene is an elastomer.
Condensation polymerization occurs by joining two molecules with elimination of a small molecule like water. Products generally do not have as large molecular weights as by addition polymerization and reaction stops when one reagent is consumed. Properties of these reactions include The polymer chain forms slowly, sometimes over hours or days. Monomers are quickly converted to low molecular weight oligomers. High temperatures are often necessary.