Isocyanate dimerization with water

Silylene complexes show high reactivities toward nucleophiles such as water, alcohols, ketones, and isocyanates [3, 4]. Some typical reactions of (OC)4Fe=SiR2(HMPA) are summarized in Scheme 1 [2]. The dimerization forming the diferradisilacyclobutane is the only known [2+2]-cycloaddition starting from 1. The photochemical reaction of 1 with 2,3-dimetylbutadiene... 

The presently proposed mechanism of synergism implies that ligands other than amine which can coordinate with the tin ion should also synergize its catalytic activity toward alcohol-isocyanate reactions. This apparently is the case for triphenyl phosphine-DBTDL combination. When triphenyl phosphine was added to DBTDL, it accelerated the rate of reaction of isocyanate with alcohol and water (Figure 6). Although triphenyl phosphine is known to dimerize isocyanates, (12), the dimerization... 

Figure 22.1 Basic reactions of isoq nate with different reactants (a) hydroxyl compound to give urethanes, (b) amines to give ureas, (c) water to form an unstable compound, carbamic acid, which decomposes and produces an amine, (d) amines and excess isocyanate produces a urea linkage, (e) urea linkage and one molecule of isocyanate produces biuret, (f) urethane and asocyanate produce allophanates. Secondary reactions of isoc3canate self-condensation produce (g) uretidione ring (dimer-), (h) isocyanurate (trimer-), (j) carbodiimide (reprinted with permission of Chattopadh3ray et al., 2007, Elsevier [15]).

Residual isocyanate monomeric extracts from 19 commercial polyurethanes were analyzed as their 9-(methylaminomethyl)anthracene derivatives on a 45°C C,g column (2 = 254 nm, ex 412 nm, em). An 80/20 ac onitrile/water (3% TEA to pH 3 with H3PO4) mobile phase was used [1024]. Analytes included 2,4- and 2,6-toluenediisocyanate, 2,4-toluene diisocyanate dimer, diphenylmethane-4,4 -diiso-cyanate, hexamethylene diisocyanate, cyclohexyl-, phenyl-, and octadecylisocyanate and, isopherone diisocyanate. The effect of varying the level of acetonitrile (70-85%) on retention (as Id) was presented as a plot for six different C,g matoiafs. Detection limits were reported as 0.03 mg/kg. 

Step polymerization indicates a mechanism of growth where monomers combine with each other to form dimers, the dimers combine with each other or other monomer units to form tetramers or trimers, respectively, the process continuing until polymer is formed. While each coupling step in a step polymerization is often accompanied by the elimination of a small molecule (e.g., water), making it a condensation polymerization, this is not always the case (e.g., isocyanates and alcohols reacting to make polyurethanes). Furthermore, not all polymerizations in which a condensate is formed follow a stepwise mechanism. 

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