Expanding monomers are sometimes used in step-growth polymerization, a type of polymerization that results in long chain polymers.
In step-growth polymerization (condensation reaction) small molecules are eliminated, which are also contributing to shrinkage when removed. Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. 
