Commercial Polyisocyanates
Two types of diisocyanates are employed in polymer preparations, aromatic and aliphatic ones. The most commonly used aromatic diisocyanates are toluene diisocyanate and 4,4'-diphenylmethane diiso- cyanate. Commercial toluene diisocyanate often comes as a mixture of 2,4 and 2,6 isomers in ratios of 80/20 or 65/35. When the reaction takes place at room temperature, the four position is 8-10 times more reactive than two. At elevated temperatures, however, this difference in reactivity decreases, and at 100°C the reactivity of the isocyanate groups in both positions is approximately equal.
Among other aromatic diisocyanates that are in commercial use are p-phenylene diisocyanate, m phenylene diisocyanate, 1-chloro-2,4-phenylene diisocyanate, 3,30-dimethyl-4,40-bisphenylene diisocyanate, 4,40-bis(2-methylisocyanophenyl)-methane, and 4,40-bis(2-methoxyisocyanophenyl) methane. The common aliphatic diisocyanates are hexamethylene diisocyanate, hydrogenated (H12) 4,40 dipenylmethane diisocyanate, isophorone diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate. Other aliphatic diisocyanates that are commercially used are lysine diisocyanate, methyl cyclohexyl diisocyanate, isopropyl dine bis-(4-cyclohexyl isocyanate), and tetramethylene diisocyanate. Many additional polyfunctional isocyanates are described in the literature.
