Reactions of the Isocyanates

These reactions can be divided into two categories. They are additions to compounds with active hydrogens and self-condensations. The reactions are well described in organic chemistry textbooks, so there is little reason to describe them here. It is noteworthy, though, that the uncatalyzed reactions of isocyanates with various active hydrogen compounds are probably broadly similar. Among them, the most investigated reactions are those of alcohols with isocyanates 

Electron-withdrawing substituents increase the positive charge on the isocyanate carbon and move the negative charge further away from the site for the reaction  

This makes an attack by an electron donor easier and yields a faster reaction. Electron-donating groups therefore have an opposite effect. 

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In these reactions the active acylating agent is the carbamyl chloride, formed by the reaction of the isocyanate with hydrogen chloride (137) ... 

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

Experimental Materials. All the data to be presented for these illustrations was obtained from a series of polyurethane foam samples. It is not relevant for this presentation to go into too much detail regarding the exact nature of the samples. It is merely sufficient to state they were from six different formulations, prepared and physically tested for us at an industrial laboratory. After which, our laboratory compiled extensive morphological datu on these materials. The major variable in the composition of this series of foam saaqples is the aaK>unt of water added to the stoichiometric mixture. The reaction of the isocyanate with water is critical in determining the final physical properties of the bulk sample) properties that correlate with the characteristic cellular morphology. The concentration of the tin catalyst was an additional variable in the formulation, the effect of which was to influence the polymerization reaction rate. Representative data from portions of this study will illustrate our experiences of incorporating a computer with the operation of the optical microscope. 

The rate-determining first step is the reaction of the isocyanate with phosphane oxide, resulting in the formation of isocyanate and CO2. Once the isocyanate is formed, it reacts with additional isocyanate to give carbodi-imide and regenerate the catalyst. A more detailed investigation of the reaction mechanism (Scheme 25) followed years later (69ACSA2697 72ACSA1777). 

Differences In Crosslink Chemistry. The main crosslinking reaction In Isocyanate-polyol coatings Is the reaction of the Isocyanate group with hydroxy groups to form a urethane crosslink. 

Phenanthridone has been obtained by the action of carbon dioxide on 2-lithioaminobiphenyl49 and by cyelizing 2-biphenylyl isocyanate with aluminum chloride,50 the latter reaction appearing to be a general one. Thus, cyclization of 5-bromo-2-biphenylyl isocyanate gives 2-bromophenanthridone,51 while reaction of the isocyanate (17) with trifluoroacetic acid is a key stage in the synthesis of dihydroxy-crinene (18).52... 

The degree of cure is reported as a ratio of the isocyanate peak area (between 2251 and 2100 cm-1) to the internal standard methylene peak area (between 3040 and 2700 cm-1). Testing of control samples has determined this ratio to be 0.39 0.05 (2a) at 72h for a conventional topcoat dry enough to fly. At 72 h there was still a significant amount of isocyanate, but the topcoat properties were sufficient to fly therefore, the project goal was to obtain a 0.39 ratio at less than 72 h. Complete reaction of the isocyanate took 300 h using the conventional formulation. 

The preparation of metal nitrides with N3 reagents typically employs d° metal complexes as starting materials. However, the reactions of r-butyl isocyanate with metal-oxo complexes of OsVI and RuVI represent rare examples of the use N3- reagents with d2-metals. It has been postulated that reaction of the isocyanate with metal-oxo 3 affords a four-membered ring intermediate 4, followed by the extrusion of carbon dioxide to yield r-butyl metal imide 5 (Scheme 1). Elimination of isobutylene from this complex then produces the metal nitride and the isobutylene. 

Some of the more interesting and innovative work has occurred in areas combining aspects of more than one chemistry type. For instance, moisture-curable thermoplastic adhesives have received much attention. Hot melt adhesives have been developed that contain active, moisture-curable isocyanate groups. The compositions provide rapid processing on assembly lines because a reasonable bond is formed as soon as the thermoplastic adhesive cools from the melt. However, bond strength and performance improve with time as the composition is slowly crosslinked to a thermoset by reaction of the isocyanates with atmospheric moisture.96,97... 

If it is assumed that the reaction of the isocyanate with the complexes is slow compared with the decomposition rate of the complexes, then the following reaction equation results ... 

Electron transfer polymers were prepared from p-benzoquinone-diols and diisocyanates, in the presence of DBTDL. At room temperature, no reaction of the isocyanate with the benzoquinone took place and the polymers were not cross-linked. 

Polymers in a foamed or cellular state have many useful properties. Energy-efficient buildings have sheets (or sprayed foams) of polystyrene foam insulation in their construction, sheets made by compounding CFC-12 under pressure with liquid polymer followed by release to atmospheric pressure via extrusion. Polyurethane foams were produced using CFC-11, which was vaporized by the heat of reaction of the isocyanate with the polyol. The foaming mass was usually heat-treated to complete the process. 

Polyurethane foam was made by stepwise polymerization of methylenediphenyl isocyanate CH2(C6H4NCO)2 and 2-hydroxymethyl-2-ethyl-l,3-propanediol in the mole ratio of 1 0.7. Calculate the weight fraction solubles and the total crosslink density in the network if the extent of reaction of the isocyanate is 90%. 

Catalyst Selection. The choice of catalysts determines not only the rate of PUR formation by means of the fundamental reaction between isocyanates and their partner reactants, but also the contribution of some of the more exotic reactions of the isocyanates. The rate of PUR formation is, of course, of critical importance to the reaction-injection molding and reinforced-reac-tion-injection molding technology. Among the exotic reactions of the isocyanates the following may be worthy of note. 

Most investigators agree that the strength and durability of isocyanate-bound wood panels are due to the chemical reaction of the isocyanate group with wood hydroxyls as illustrated by the above equation. Thus, the multifunctional isocyanate molecule forms a chemically bonded bridge between two or more adjacent wood particles. This reaction is only one of several involving isocyanates that can and probably do occur in a hot press during formation of particleboards when isocyanate binder is used. A very important reaction is that of isocyanate with water to produce a very unstable carbamic acid which immediately decomposes to form a primary amine and COg ... 

By proper selection of the radicals Rj and R2t the rate of the Isocyanate-oxazolidine reaction can be made much faster than that of the hydrolysis reaction of the Isocyanate alone. 

Another important use for polyisocyanates is in adhesives, usually as additives to rubber cements. For example, in rubber-to-metal adhesion, a chemical bond probably forms between the rubber and metal by reaction of the isocyanate groups with active hydrogens in the rubber and with the hydrated oxide layer on the metal surface. 

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