In polymer chemistry, in situ polymerization means "in the polymerization mixture." There are numerous unstable oligomers (molecules) which must be synthesized in situ (i.e. in the reaction mixture but cannot be isolated on their own) for use in various processes.
Example of this method include urea-formaldehyde (UF) and melamine formaldehyde (MF) encapsulation systems. In such type of in situ polymerization a chemical encapsulation technique is involved very similar to interfacial coating. The distinguishing characteristic of in situ polymerization is that no reactants are included in the core material. All polymerization occurs in the continuous phase, rather than on both sides of the interface between the continuous phase and the core material.