Isocyanate group, reactivity

The isocyanate group reactivity is based on the polarisation of these groups represented by the following resonance limit structures [1] (equation 1.1) ... 

The isocyanates used with rigid foam systems are either polymeric MDI or specialty types of TDI. Both contain various levels of polymerized isocyanate groups which contribute to molecular weight per cross-link and also may affect reactivity due to steric hindrance of some isocyanate positions. 

Polyurethane Formation. The key to the manufacture of polyurethanes is the unique reactivity of the heterocumulene groups in diisocyanates toward nucleophilic additions. The polarization of the isocyanate group enhances the addition across the carbon—nitrogen double bond, which allows rapid formation of addition polymers from diisocyanates and macroglycols. 

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. 

This process is based on the very high reactivity of the isocyanate group toward hydrogen present ia hydroxyl groups, amines, water, etc, so that the chain extension reaction can proceed to 90% yield or better. Thus when a linear polymer is formed by chain extension of a polyester or polyether of molecular weight 1000—3000, the final polyurethane may have a molecular weight of 100,000 or higher (see Urethane polymers). 

Thermoplastic polyurethanes have no reactive isocyanate groups and cannot crosslink. 

From these, prepolymers are prepared where the diisocyanates may be completely reacted as in the case of the urethane oils which resemble the oil-modified alkyds but have urethane (—NHCOO—) links in place of the ester (—COO—) links of the alkyds, or where one only of the isocyanate groups is combined, leaving the other to participate in crosslinking reactions. Such a reactive prepolymer is the biuret that may be prepared from hexamethylene diisocyanate, has the following structure ... 

The isocyanate group is more reactive than the epoxy group in that it will react at room temperature with water and hydroxyl groups as well as with amine groups. However, the latter reaction is too fast to be practicable so the standard two-pack coatings are based on isocyanate and polyhydroxyl prepolymers such as hydroxyl terminated polyesters or polyethers as in the last example given in the section on epoxy resins. 

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. 

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). 

The reaction rates of diisocyanates are strongly influenced by their molecular structure. The reactivity of isocyanate groups is enhanced by adjacent electron-withdrawing substituents. Aromatic rings are very effective electron withdrawing groups, and it is for this reason that the majority of commercial diisocyanates are aromatic. Many of the diisocyanates used commercially consist of mixtures of isomers. Some of the more important commercial diisocyanates are illustrated in Fig. 25.6. Diisocyanates must be handled carefully to avoid exposing workers to their hazardous vapors. 

In preparing elastomers by the two-step procedure, the entire quantities of LHT-240 and TDI were mixed in the resin flask under high-purity nitrogen for 2 hours at 50°C. Even without catalyst, this period was sufficient to obtain complete reaction, or essentially so, because of the relatively high reactivity of the para isocyanate group in the TDI. The... 

In a wider sense, functionality distribution also means combination of reactive groups of one kind (for instance, hydroxy, or carboxyl, or isocyanate groups) of higher and lower reactivity in one precursor molecule. By this combination, the network build-up can be effectively controlled. For instance, one less reactive A group out of three in a trifunctional monomer in a RA3 + R B2 system promotes chain extension in contrast to branching and shifts the gel point to higher values [30],... 

Organic isocyanates (R-N=C=0) are xenobiotics used extensively in the manufacture of paints, pesticides, and polyurethanes. The reactivity of the isocyanate group can also underlie the toxic reactions observed in patients that have been exposed to organic isocyanate monomers. Furthermore, the isocyanates formed in the body can be metabolites of various other xenobiotics, such as ... 

Polyurethanes are useful in numerous applications such as reaction injection molding, rigid and flexible foams, coatings and adhesives. However, due to the high reactivity of the isocyanate group [96], yielding either dimers, via self-condensation or a carbamate via the reaction with an alcohol, the A,jB-monomers have to be produced in-situ in the reaction vessel. 

Compatibility. Owing to the high reactivity of the isocyanate group, polyurethane propellants require a more sophisticated processing technique than the rather foolproof, carboxy-terminated polybutadiene aziri-dine and/or epoxy-cured propellant systems. Processing is even more complicated if bonding agents (see below) are present, which are used to bolster mechanical properties in practically any modern propellant. 

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. 

In the reaction of 2,4-toluene diisocyanate the 4-position isocyanate group reacts first and then the one at the 2-position [12]. Some representative examples of the reactivity of diisocyanates with 2-ethylhexanol are shown in Fig. 2. 

Aromatic polyisocyanates Aromatic polyisocyanates are primarily used for a wide variety of PU foamed plastics, elastomers, and adhesives. More than 90% of PUs are produced from aromatic polyisocyanates. The isocyanate group bonded to an aromatic ring is more reactive toward water or hydroxyl groups than that bonded to an aliphatic compound. The most important aromatic diisocyanates are also given in Figure 4.7. 

Since all proteins contain both of these reactive groups, if there were a possibility of producing a polyurethane, and particularly a reticulated polyurethane, with excess isocyanate groups, it would be possible to produce an enzymatically active surface on a high-surface-area, high-void-volume reticulated struemre. This is possible and in fact is easier than the most common methods used currently to immobilize enzymes. 

H.F. Bluhm, USP 3256214(1966) CA 65, 9132(1966) [Prepn of explosive polyurethane resins, which are suitable for use as sensitive coatings for AN blasting compns and for prepn of foamed resin expls is described. The resins, which can be cured at RT, are prepd by reaction of an org poiyiso-cyanate with a nitric acid partial ester of a polyhydroxy compd including a component contg > OH groups reactive with the isocyanate. For example, equal parts by wt of an-hydroenneaheptitol nitrate (13.77 wt %N as... 

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. 

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