Common terminology for the Chain Extender or Cross Linking Agent, which when mixed with the Prepolymer results in the chemical reaction that produces the Polyurethane.  Diamines, especially Moca have proven to be good general purpose curative.  Other aromatic amines such as MDA, MPD or Caytur can be used, as can Glycols (E.g. 1,4 Butane-diol).