Isocyanates oligomerization reactions

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

For methylene diphenyl diisocyanate (MDI), the initial reaction involves the condensation of aniline [62-53-3] (21) with formaldehyde [50-00-0] to yield a mixture of oligomeric amines (22, where n = 1, 2, 3...). For toluene diisocyanate, amine monomers are prepared by the nitration (qv) of toluene [108-88-3] and subsequent hydrogenation (see Amines byreduction). These materials are converted to the isocyanate, in the majority of the commercial aromatic isocyanate phosgenation processes, using a two-step approach. 

The two key isocyanates that are used in the greatest volumes for polyurethane polymers are toluene diisocyanate (TDl) and methylene diphenyl diisocyanate (MDl). Both isocyanates are produced first by nitration of aromatics (toluene and benzene, respectively), followed by hydrogenation of the nitro aromatics to provide aromatic amines. In the case of MDl, the aniline intermediate is then condensed with formaldehyde to produce methylene dianiline (MDA), which is a mixture of monomeric MDA and an oligomeric form that is typical of aniline/formaldehyde condensation products [2]. The subsequent reaction of phosgene with the aromatic amines provides the isocyanate products. Isocyanates can also be prepared by the reaction of aromatic amines with dimethylcarbonate [3, 4]. This technology has been tested at the industrial pilot scale, but is not believed to be practiced commercially at this time. 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

Urethanes Precursors. Polyurethanes can be produced by the reaction of oligomeric diols with diisocyanates. The properties of the polyurethanes are intimately related to the chemicals contained in the starting materials. Specifically, the molecular weight distribution of the diols and the functionality of the isocyanates affect the properties. We have found SFC useful for characterizing the building blocks of polyurethanes, namely diols and isocyanates. 

Heating of isocyanates above 150 °C slowly produces carbodiimides. For example, heating of hexamethylene diisocyanate at 189-195 °C for 20 hr produced 4-6 % of oligomeric isocyanate terminated carbodiimides, but in addition 18-20 % of isocyanate terminated isocyanurates were formed. The reaction is facilitated if a slow stream of nitrogen is passed through the boiling isocyanate. The unsymmetrical isocyanate dimer 47 was proposed as an intermediate in this transformation. 

The reaction of sterically hindered aromatic diisocyanates with bases or phospholene oxide catalysts afford oligomeric carbodiimides having terminal isocyanate groups. If the catalytic conversion of 4,4 -diphenylmethane diisocyanate (MDI) is conducted in the... 

Functionalized polymers of the type B and C (Scheme 1) can be formed via polyaddition processes of bifunctional reactants, without splitting off of low molecular weight compounds. Most syntheses of stabilizers have been based on reactions of bis-isocyanates with H-acid nucleophiles. Some reactions of oxiranes may be listed here too (syntheses involving oxiranes are listed in Sect. 3.1.1.2 if polymerization aspects are more evident syntheses of stabilizers formed via reactivity of oxirane moieties attached to an oligomeric or polymeric chain are classified as reactions on polymers. Sect. 3.2.2.1... 

While molecule 6a is stable in solution for weeks, it is found that the appropriate isocyanate-substituted derivative 6b rearranges to form the l,3,5-triphenyl-l,3,5-hexahydrotriazine 7, along with oligomeric [(2-Me2NCH2C6H4)(H2C=CH)SiO] (3). A possible reaction mechanism for the formation of 3 and 7 from 6b is presented in Scheme 1. 

The carbodiimide can react with another molecule of an isocyanate the resulting highly strained four-membered ring intermediate undergoes further reactions that are not detailed here. Suffice it to say that these reactions lead to an oligomerization of isocyanates. 

In principle, any cross-linking reaction can be employed in oligomeric coatings, but the desire to reduce energy requirements has spurred interest in the perfection of novel curing mechanisms such as moisture curing. One such system, based on isocyanate-oxazolidine chemistry (27). has been successfully developed. The simplified chemistry involves the reactions... 

LC is used in production facilities to characterize MDI and its precursor, methylene dianiline (MDA). Either normal-phase or reversed-phase conditions can be used. The isocyanates must first be derivatized by reaction with an alcohol. The isomer composition is determined by LC, as well as the distribution of monomeric and 2- and 3-ring oligomeric components. LC complements SEC for MDI/MDA characterization. 

A stoichiometric mixture of an oligomeric diol and diisocyanate will react to give a polyurethane. However, if there is double the quantity of diisocyanate, the reaction product will be an ohgomer with isocyanate end groups. Such adhesives cure by the ingress of moisture by the following sequence of reactions. (Moisture cure of adhesives). 

Desmodur N3200 [34], an oligomeric form of hexamethylene di-isocyanate (HDI). Heating causes reaction of the isocyanates with surface amines as shown in Scheme 15.2B. The length of the polyurea cross-links depends on both the di-isocyanate concentration in the soak solution and the amount of residual water from the hydrolysis left in the gels. This is because isocyanate reacts with water to generate an amine that can react with other isocyanates to cause chain extension. 

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