Isocyanates 2 molecules

Isocyanates are polar, reactive molecules. Isocyanates, either alone or in conjunction with certain resins, have been established as excellent primers for metal, glass, rubber, and fibers [44]. 

The covalent immobilization of enzymes usuahy begins with a surface modificahon or activation step. The immobilization of enzymes carried out by bifunchonal spedes proceeds by formation of inter- and intramolecular crosslinkages between the enzyme molecules. Isocyanate derivatives and glutaraldehyde (GA) are used as crosslinkers for the immobilization of enzymes in this reaction the amino groups of a protein form Schiff bases with glutaraldehyde [88, 89]. In experiments HMDI and GA served as achvating agents for immobilization of lipase 42. 

Mass spectral fragmentation patterns of alkyl and phenyl hydantoins have been investigated by means of labeling techniques (28—30), and similar studies have also been carried out for thiohydantoins (31,32). In all cases, breakdown of the hydantoin ring occurs by a-ftssion at C-4 with concomitant loss of carbon monoxide and an isocyanate molecule. In the case of aryl derivatives, the ease of formation of Ar—NCO is related to the electronic properties of the aryl ring substituents (33). Mass spectrometry has been used for identification of the phenylthiohydantoin derivatives formed from amino acids during peptide sequence determination by the Edman method (34). 

Asymmetric aryl isocyanate dimers, ia which the C=0 group of oae molecule reacts with the C=N group of another, have been postulated as labile iatermediates ia the formation of carbodiioiides (17) upoa heating isocyanates. 

Isocyanate. Lysine has two amino groups in the molecule and dHsocyanate is prepared by reaction with phosgene. Lysine trHsocyanate [69878-18-8] (LTI) is developing on a commercial scale in Japan (244). 

Uretha.nes. Urethane elastomers are prepared by the reaction of an isocyanate molecule with a high molecular weight ester or ether molecule. The result is either an elastomeric mbber form or a Hquid prepolymer that can be vulcanised with an amine or a hydroxyl molecule (see Urethane POLYAffiRS). 

It is possible to react an organic moiety to the hydroxyl groups on ceU waU components. This type of treatment also bulks the ceU with a permanently bonded chemical (68). Many compounds modify wood chemically. The best results are obtained by the hydroxyl groups of wood reacting under neutral or mildly alkaline conditions below 120°C. The chemical system used should be simple and must be capable of swelling the wood stmcture to facUitate penetration. The complete molecule must react quickly with wood components to yield stable chemical bonds while the treated wood retains the desirable properties of untreated wood. Anhydrides, epoxides, and isocyanates have ASE values of 60—75% at chemical weight gains of 20—30%. 

In these systems the prepolymer is prepared with an excess of isocyanate to give an isocyanate-terminated molecule. This is then reacted with unreacted glycol and other ingredients. The disadvantage of this system is that component streams are of similar volume and viscosity, this facilitating both metering and mixing. The system is used mainly for the manufacture of microcellular products. 

When this is carried out in suitable solvents at temperatures in the range 75-120°C, soluble products will be obtained. Polymeric MDI is usually used as the isocyanate component and this results in a stiff chain molecule. One such product is reported to have a of 200-220°C. 

Chain extenders are usually low molecular weight symmetrical diols or diamines. Chain extenders react with isocyanates in the same way as polyols do, but because they are low molecular weight, a high concentration of hydrogen-bonded molecules can associate and phase out of the polyol to form plastic-like domains called hard segments . Hard segments will be discussed in Section 4. Some of the more common diol and diamine chain extenders are shown in Table 3. 

The same ring expansion occurs on the reaction of 3,3-di-n-propyl-diaziridine with two molecules of phenyl isocyanate [Eq. (40)]. ... 

Alkyl uracyls have been known for some time to act as diuretic agents in experimental animals. The toxicity of these agents precluded their use in the clinic. Appropriate modification of the molecule did, however, yield diuretic agents with application in man. Reaction of allylamine with ethyl isocyanate affords the urea, 89 (the same product can of course be obtained from the same reagents with reversed functionality). Condensation with ethyl cyanoacetate affords aminotetradine (90). In... 

Two important methods by which isocyanates, polyols, and polyurethanes are rendered water dispersible both involve incorporation of internal emulsifiers, which serve to suspend the molecule in the aqueous phase.12c,f Ionic internal... 

This system was slightly modified by R J. Flory, who placed the emphasis on the mechanisms of the polymerisation reactions. He reclassified polymerisations as step reactions or chain reactions corresponding approximately to condensation or addition in Carother s scheme, but not completely. A notable exception occurs with the synthesis of polyurethanes, which are formed by reaction of isocyanates with hydroxy compounds and follow step kinetics, but without the elimination of a small molecule from the respective units (Reaction 1.3). 

Polyurethanes are thermoset polymers formed from di-isocyanates and poly functional compounds containing numerous hydroxy-groups. Typically the starting materials are themselves polymeric, but comprise relatively few monomer units in the molecule. Low relative molar mass species of this kind are known generally as oligomers. Typical oligomers for the preparation of polyurethanes are polyesters and poly ethers. These are usually prepared to include a small proportion of monomeric trifunctional hydroxy compounds, such as trimethylolpropane, in the backbone, so that they contain pendant hydroxyls which act as the sites of crosslinking. A number of different diisocyanates are used commercially typical examples are shown in Table 1.2. 

As explained in Chapter 1, the urethane group is the product of the reaction of a hydroxy compound with an isocyanate group (Reaction 4.8). This reaction occurs by step kinetics, yet is usually an addition process since no small molecule is lost as the reaction proceeds. 

The isocyanate group is reactive, a feature which leads to a large number of possible reactions when crosslinking is carried out. The essential feature of all the processes is that they involve reaction, initially at least, with an active hydrogen atom in the molecules of the co-reactant. For example, isocyanates will react with water, as illustrated in Reaction 4.10, to generate an unstable intermediate, a carbamic acid, which releases carbon dioxide to yield an amine. 

The treatment of isocyanates with 3-methyl-l-ethyl-3-phosphoIene-l-oxide (59) is a useful method for the synthesis of carbodiimides in good yields. The mechanism does not simply involve the addition of one molecule of isocyanate to another, since the kinetics are first order in isocyanate and first order in catalyst. The following mechanism has been proposed (the catalyst is here represented as... 

According to this mechanism, one molecule of isocyanate undergoes addition to C=0, and the other addition to C—N. Evidence is that labeling experiments have shown that each molecule of CO2 produced contains one oxygen atom derived... 

Waste PETP was depolymerised by glycolysis to give hydroxyl-terminated oligomers(DPET), which were used in the synthesis of urethane oils. The effect of depolymerisation temps., the type of glycol and the amount of catalyst on the yield and composition of the depolymerisation products was studied. The physical properties of the urethane oils were compared with those of a commercially-available product. The reaction of DPET with isocyanates produced random linkage between different molecules with or without terephthaloyl groups. 15 refs. 

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