Isocyanates chemistry

Whilst rigid closed-cell polyurethanes are excellent thermal insulators they do suffer from a limited and often unsatisfactory level of fire resistance, even in the presence of phosphorus-containing and halogen-containing fire retardants. Considerable promise is now being shown by the polyisocyanurates, which are also based on isocyanate chemistry. 

In analogous fashion to isocyanate chemistry, isothiocyanates like 125 can be utilized to produce the corresponding quinazoline-2-thioxo-4-ones. Makino and co-workers reported the solid phase synthesis of quinazoline derivatives 126 through the reaction of polymer-bound primary amines 124 with isothiocyanates 125 <00TL8333>. 

Isocyanate chemistry is of itself a very complex field of chemistry, and even a brief description of the extent is outside the scope of this article. For a comprehensive description of isocyanate chemistry and its applications to polyurethane polymers, readers are directed to a handbook that is the standard in the industry [1]. A brief description follows for the purpose of introducing the role of isocyanates in polyurethane chemistry. 

Recent Advances in Isocyanate Chemistry S. Ozaki, Chem. Rev., 1972, 72, 457-498. 

The study of industrial applications of isocyanate chemistry and polymers derived from isocyanates received much attention in Europe, particularly Germany, in the 1930s and during World War II. Patents on aspects of urethane chemistry appeared as early as 193755 jhg effort going into understanding isocyanate chemistry and commercializing urethane products continues.56-60... 

Worthy, W. (1985). Bhopal report - methyl isocyanate chemistry of a hazard. Chem. Eng. News 63(6) 27-36. 

Catalysis in isocyanate chemistry is of broad interest from a theoretical viewpoint. It is possible to vary the structure of each of the reactants and of the catalyst and to observe profound effects. As for the catalyst, it has been established that both basicity and steric properties are of fundamental importance. Kinetic measurements have proved most useful in investigations, carried out for the most part since 1945. Knowledge gained has permitted consideration of the steric and electronic factors of the active reactant-catalyst complex, key to the catalytic mechanisms. 

The rapid industrial growth of polyurethanes and related polymers in the last 20 years has provided a surge of research dealing with both practical and theoretical aspects. The chemistry of isocyanates has been reviewed by Saunders and Slocombe (1) and by Arnold et al. 2). The extensive developments in isocyanate chemistry will be again brought up to date in a forthcoming book by Saunders and Frisch (3). The study of the mechanism of reaction has been emphasized particularly by Baker and co-workers (4) and by Dyer and co-workers (5). Detailed aspects of polyurethane foam formations are given comprehensive treatment by Dombrow 6) and by Saunders (7). 

The reaction of isocyanates with hydroxyl compounds is probably the most widely studied field of isocyanate chemistry. The reaction rate in this system depends on the nature of the isocyanate, of the alcohol, the solvent used and, of course, on the catalyst. 

We have recently been examining chemistry that allows the preparation of RIM derived polyamides from isocyanates. We reported sane initial results during a previous RIM symposium held in Atlanta. (2-3) We reported the use of enamine isocyanate chemistry to give amide products (equation 1). 

Inspired by the seminal report of Nagel [34], which described a very active and selective Rh-pyrphos catalyst attached covalently to sihca gel, Pugin and colleagues have developed the modular toolbox which his depicted schematically in Figure 12.4. The main elements of their system are functionalized chiral diphosphines, where three different hnkers are based on isocyanate chemistry and various carriers [37, 45, 58]. This approach allows for a systematic and rapid access to a variety of immobilized chiral catalysts, with the possibility of adapting their catalytic and technical properties to specific needs. 

MA Thomson, PJ MeUing, and AM Slepski. Real Time Monitoring of Isocyanate Chemistry Using a Fiber-Optic FTIR Probe. Polymer Preprints 42 310-311, 2001. 

Isocyanate chemistry was also explored in order to graft long alkyl chains on the cellulose fibres and use the modified fibres as sorbent for organic pollutant... 

The versatility of isocyanate chemistry provides many avenues for synthetic manipulation and product development. Just a few of many examples are mentioned below. 

Real-Time Monitoring of Isocyanate Chemistry Using a Fiber-Optic... 

Mid-IR fiber optics provides a convenient way to monitor many kinds of chemical reactions in situ and in real time. Isocyanate chemistry, both in the solution phase and in the form of urethane foams and binders, lends itself well to this approach. It is shown in this paper that the calibration of mid-IR fiber optics is simple and transferable between probes and probe heads, and that useful information about reaction rates and extent of cure can be obtained by simple peak area measurements in many cases. 

The diversity of isocyanate chemistry, combined with the availability of selective catalysts, has made it possible to select reactions which fit desired modes and rates of processing. Table 2.1 lists some of the known polymer-forming reactions of the isocyanates. 

Even though the area of isocyanates chemistry becomes more and more wide, the largest majority of applications still remains based on the classic addition reactions leading in general to the appearance of urethane or urea groups. Numerous works were dedicated to the elucidation of the mechanism of these fundamental reactions. However, many fundamental problems are yet unclear so far. 

From the perspective of macromolecular science and especially for the isocyanate chemistry, the main problem resides in the elucidation of the hydrogen bond associates structure especially in the case of diols and polyols as in fact they finally represent the products involved directly in the macromolecular synthesis. Certainly a similar question is also risen in the case of diamines leading to the urea groups formation. 

Also in this time frame, wire insulation producers began producing polyamide-imides using lower-cost isocyanate chemistry. This technology largely supplanted the acid chloride route for the wire enamel market. 

In isocyanate chemistry many unintended side reactions can occur. The amine formed from the unstable carbamic acid can react with isocyanate, forming a urea. This secondary nitrogen urea or any other amide nitrogen may react further with another isocyanate, forming a cross-link at this new biuret functional group. The amine may also react with the anhydride on TMA, forming the amic acid. And, depending on temperature, the isocyanate may react with the solvent itself. All these side reactions and their influences on stoichiometry must be fuUy understood and controlled. 

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