What are the molecular requirements for condensation polymerization?
Monomers must contain a functional group at each side (at least bifunctional) and these functions  have to react with each other (like amine and carboxylic acid to yield amide group or alcohol and carboxylic acid to yield ester group,...) .


  
  We can start with two types of monomers that hold identical functional groups at each side.
  




One example of that is the reaction of 1,6-diaminohexane with hexandioic acid to yield Nylon-6,6.


  
  We can also start from one type of monomers with two different functional groups at each side.
  




One example of that is the reaction of 11-aminoundecanoic acid to yield Nylon-11.

The condition that the reaction should give off a small molecule is no longer valid. The counter example is the formation of polyurethane by reaction of diisocyanates and diols without giving off  small molecules. We prefer name this type of polymerization "a step growth polymerization" in opposite to "chain growth polymerization" (known before as polyaddition).