Synthesis of Piperazine Quaternary Ammonium Alkali Catalyst and Its Application in Isocyanate Polymerization

Aliphatic isocyanate trimer has the advantage of excellent thermal stability, radiation resistance, corrosion resistance and flame retardancy. It is an important unit for various chemical products. Hexamethylene diisocyanate trimer (HDIT) is one of the popular trimers, which is mainly synthesized through the polymerization of monomers. A variety of catalysts have been applied in this procedure, including metal salts, phosphines, aminosilanes, N-heterocyclic carbenes and N-heterocyclic olefms. However, HDI monomer is usually expensive, while low catalyst activity, harsh preparation conditions, and easy-formation of polymer are often observed in the polymerization process. Pentamethylene diisocyanate (PDI), a cheap bio-based linear aliphatic polyisocyanate with similar structure to traditional HDI, has the potential to replace HDI. Since the activity of PDI is lower, it is found that the catalysts applicable for HDI work not well for PDI. Herein, we design and synthesize 1,4-bis(2-hydroxy-isopropyl)-1,4-dimethylpiperazine bisquaternary ammonium base (PQ-OH) as the catalyst for the production of both HDI trimer and PDI trimer (PDIT). Due to having two hydroxide ions, PQ-OH catalyst can attack the -NCO groups of two monomers at the same time, thus improving the reaction efficiency. Moreover, the beta-OH groups on the catalyst molecule also promote the polymerization process. The piperazine ring structure in the catalyst has obvious steric hindrance effect, which can enhance the reaction between small molecules, and inhibit the reaction between large molecules and small molecules. Thus, the product with high trimer content is obtained. A general synthesis procedure for PQ-OH is described as followed. Propylene epoxide (70.0 mL, 1.0 mol) is added to a mixture of 1,4-dimethylpiperazine (67.6 mL, 0.5 mol) and water (18 mL, 1.0 mol) dropwisely under argon. After stirring at room temperature for 4 h, the solution turns to brown and stratifies. The lower layer is separated and concentrated in vacua to remove excess propylene oxide, water and possible alcohol by-products. The obtained crude product is dissolved in methanol, followed by decolorizing with activated carbon and drying with molecular sieve. After filtering and concentration, the desired product is obtained as a white viscous liquid.