Reactive isocyanate

The polyaddition reaction is influenced by the stmcture and functionaHty of the monomers, including the location of substituents in proximity to the reactive isocyanate group (steric hindrance) and the nature of the hydroxyl group (primary or secondary). Impurities also influence the reactivity of the system for example, acid impurities in PMDI require partial neutralization or larger amounts of the basic catalysts. The acidity in PMDI can be reduced by heat or epoxy treatment, which is best conducted in the plant. Addition of small amounts of carboxyHc acid chlorides lowers the reactivity of PMDI or stabilizes isocyanate terrninated prepolymers. 

Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

Thermoplastic polyurethanes have no reactive isocyanate groups and cannot crosslink. 

Such reactive isocyanates always contain about 1% by weight of free diisocyanate monomer which is highly toxic, therefore when in use ventilation... 

Processing of rigid foams from two part formulations involves combining measured quantities of the polyisocyanate with a polyhydroxyl such that there are no or limited reactive isocyanate functional groups. Moisture is not required to complete the cure. Once the reactants are combined the mixture is poured into a form where expansion and polymerization take place simultaneously. Cure times are usually very fast, on the order of minutes. 

The partially aromatic PAs are exclusively made of die diamine-diacid type and not die amine-acid type. The aromatic diamines, similar to phenylene diamines, color easily and dieir polymers are conjugated, having a golden brown color. The aromatic diacids used in the formation of partially aromatic PAs are mainly terephthalic and isophthalic acids. Starting with the diacids, the PA salt is made first and with this the salt prepolymers are prepared. The prepolymerization is usually carried out in an autoclave to prevent die sublimation of the reactants. In a laboratory synthesis it would be preferable to avoid this autoclave step as one is not always available. It is possible to start with the more reactive esters, such as diphenyl isophtiialate, or with the acid chlorides starting with the reactive isocyanates is, in principle, also possible. The terephthalic and isophthalic acids are also used to modify PA-6,6 and PA-4,6 to more dimensionally stable copolymers.6,18... 

Lastly, of course, the main reaction of interest is the formation of urethane groups by reaction of isocyanate groups and hydroxy-groups of the polyester or polyether. Even these reactions do not exhaust the possibilities available to the highly reactive isocyanate group. It will then go on to react with the urethane links to form a structure known as an allophanate (see Reaction 4.13). 

C in air showed the above absorption. Thus in all cases, the prepolymer possessed reactive isocyanate end groups. 

Extrinsic fluorescence is used whenever the natural fluorescence of a macromolecule is inadequate for accurate fluorescence measurement. In this case, one can attach a fluorescent reporter group by using the reactive isocyanate or isothiocyanate derivatives of fluorescein or rhodamine, two intensely fluorescent molecules. One can covalently also label a protein s a- and e-amino groups with dansyl chloride (/.e., A,A-dimethylaminonaphtha-lenesulfonyl chloride). Another useful reagent is 8-ani-lino-l-naphthalenesulfonic acid (abbreviated ANS). This compound is bound noncovalently by hydrophobic interactions in aqueous solutions, ANS is only very fluorescent, but upon binding within an apolar environment, the quantum yield of ANS becomes about 100 times greater. 

Boeniger ME. 1991. Air concentrations of TDl and total reactive isocyanate group in three flexible polyurethane manufacturing facihties. Applied Occupational and Environmental Hygiene 6(10) 53-63. 

The second approach uses the more reactive isocyanates, which can be prepared from the corresponding primary amines by treatment with phosgene [254], Oligomers of hydrazine-derived ureas have been prepared as peptide mimetics on PEG in solution [255]. For this purpose, protected pentafluorophenyl carbazates have been used to convert support-bound hydrazines into ureas (Figure 16.25). 

When the carbons in reactants that end up at C-3 and C-5 in the ring are both in oxidation state 4+, then oxadiazolidinediones may be formed. For example,130 the very reactive isocyanate 40 reacts with hydroxylamine to give an intermediate (41) that cyclizes in alkaline solu-... 

The polyaddition reaction is influenced by the structure and functionality of the monomers, including the location of substituents in proximity to the reactive isocyanate group (steric hindrance) and the nature of the hydroxy] group (primary or secondary). Impurities also influence the reactivity of tlie system. 

Fully cured polyurethanes present no health hazard they are chemically inert and insoluble in water and most organic solvents. Dust can be generated in fabrication, and inhalation of the dust should be avoided. Polyether-based polyurethanes are not degraded in the human body, and are therefore used in biomedical applications. Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

Foams. Polyurethane foams are prepared by the polymerization of polyols with isocyanates. One of the most commonly used reactive isocyanates toluenediisocyanate, TDI. It is made from toluene by nitration and then reduction followed by treatment with phosgene. The isocyanate residue reacts readily with alcohols to give carbamates (urethanes) or amines to give ureas. 

The most reactive isocyanate group (-NCO) in the structure will react with the hydroxyl (-OH) group first. The reaction will tend to favor the most reactive site. The reaction is allowed to proceed until almost all the diisocyanate is used up. The reaction is stopped before the viscosity increases too much. The temperature and speed of the reactions are such that catalysts are normally not used in these reactions. 

Diisocyanate A chemical compound, usually organic, containing two reactive isocyanate groups. Isocyanate groups consist of a nitrogen atom bonded to a carbon atom bonded to an oxygen atom, -N=0=O. It is... 

The reaction between toluene 2,4-diisocyanate and carboxylated MWCNTs afforded amido-functionalized nanotubes containing highly reactive isocyanate groups on their surface (Scheme 1.4). The amount of the isocyanate groups was determined by chemical titration and thermogravimetric analysis (TGA) [105]. The modified tubes may constitute promising components to prepare polymer-nanotube composites and coatings [106]. 

Scheme 1.4 Amido-functionalized CNTs containing highly reactive isocyanate groups at the tube surface.

Basic hydrolysis of the reactive isocyanate 4-41 leads to an intermediate that tautomerizes under the reaction conditions to give 4-42, which spontaneously decarboxylates. The irreversible decarboxylation yields the amine, which contains one less carbon (lost as CO2) than the starting material. 

An alternative route to chiral p-lactams was provided by reactions of electron-deficient isocyanates with chiral nucleophilic alkenes such as vinyl ethers or vinyl acetates. Chlorosulfonyl isocyanate (CSI), a commonly used reactive isocyanate [34], undergoes stereospecific 5-yn-addition to alkenes. The chlorosulfonyl group can subsequently be reductively removed from the nitrogen atom. It has been shown that reactions between CSI and (Z)- and ( )-alkenyl ethers stereoselectively give cA-3,4-disubstituted azetidinones from (Z)-olefins and tmns-3,4-... 

The demonstrated utility of these high-reactive isocyanates prompted us to search for a phosgenation process more economical than the oxalyl chloride route. Our attempts succeeded in the development at a laboratory scale of a method based on the reaction of phosgene with N,N-disilyl amides as depicted in scheme 161 (Ref. 215). 

Isocyanates, the initial products of the Hofmann rearrangement, usually undergo solvolysis to give amines, as described earlier (Section 4.4.1.1). However, a two-phase Hofmann reaction using phase transfer catalysts allows isolation of reactive isocyanates (equation 12). The phase transfer catalyst is essential for the formation of primary and secondary isocyanates, while tertiary isocyanates are formed without the catalyst. Shorter reaction times at room temperature as well as high dilution conditions minimize the formation of by-products such as dialkylureas or acylalkylureas. Chlorine in place of bromine considerably decreases the yield of isocyanates. 

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