**2.4. Catalyst**

The isocyanates especially the aromatic ones are highly reactive, so catalyst is generally not needed in such cases. However, catalyst is required for synthesis of aliphatic isocyanate containing SMPU and at low temperature. Also, the reaction rate rapidly decreases when the steric hindrance increases, mainly on the substitute in higher alcohols. In such cases, catalyst is required. These catalysts are mainly divided into two categories viz. amino (basic) and organometallic compounds. The commonly used amine catalysts are triethyl amine, triethylenediamine (TEDA), dimethylethanolamine (DMEA), dimethylcyclohexylamine (DMCHA), and so on. Similarly, organometallic catalysts are mercury, lead, tin, bismuth, and zinc-based organo-complexes. However, these catalysts are toxic and hence they have disposal problems which hinder their uses. Nowadays, bismuth and zinc carboxylates, alkyl tin carboxylates, oxides and mercaptide oxides such as dibutyl tin dilaurate (DBTDL), dioctyltin mercaptide, stannous octoate, dibutyltin oxide, and so on are used successfully [21].
