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.
Branching does not occur unless the monomer has several functional groups. These reactions are used to make polyesters and polyamides by using molecules with more than one functional group. Strong hydrogen bonding create strong films of these materials as well. Crosslinking can make polymers stronger by introducing covalent chemical bonds between polymer chains. Sulfur is a good crosslinking agent because it wants to be two-coordinate.