Polyurethanes three-step reaction

Toluene diisocyanate (a) is produced industrially together with its isomer 2,6-toluene diisocyanate in a three-step reaction sequence First toluene nitration is carried out and from the obtained mixture of ortho-, meta-, and para-isomers only the ortho/para mixture is retained for further nitration. The resulting 2,4- and 2,6-dini-trotoluenes are then hydrogenated using nickel or palladium catalysts to the corresponding aromatic diamines. The latter are converted with phosgene into the technical TDI mixture, which is almost exclusively applied in combination with diols and triols to form polyurethane foam materials. 

PEO and PPO, are mixed to create Type III segmented polyurethanes, three different diblock sequences of equal probability must be obtained PEO-PEO, PEO-PPO, and PPO-PPO are the principal species. The same principles that govern the formation of l-,3-,5-,-7... unit combination of diisocyanates and diamines in the hard segment, also predict that the reaction product of 2 mol of a, co diol with 1 mol of diisocyanate after Step 1 of the three-step process (Figure 2) will consist mostly of the monomer (the diol), some of the desired trimer (diol -TDI-diol), but also some of the pentamer (diol-TDI-diol-TDI-diol), etc. 

Important polymers that are produced by polyaddition are polyamide 6 (nylon) and all kinds of polyurethanes. In polycondensation one mol of a small molecule (typically H2O) is liberated per step of chain growths, important polymers that are produced by polycondensation are polyamide 6.6, poly(ethylene terephthalate) (PET), polycarbonate, polyarylate, and polysulfide. Step growth polymerization is usually slow, equilibrium limited and isothermal to slightly exothermic. Polyaddition and polycondensation reactions of monomers with three or more reactive end groups lead to three-dimensionally crosslinked resins. 

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