Isocyanates oligomeric

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. 

These oligomerization steps result in a continuous increase in viscosity of the desired isocyanate and ultimately cause sohdification. 

Another example involved a batch of isocyanate crosslinker which was too tacky. Upon comparing the HPGPC trace of this sample with that of a control as shown in Figure 9, it is seen that the major difference between these two samples was the level of free caprolactam. The high content of free caprolactam in sample CX-006 depressed the glass transition temperature (Tg) of the sample to such an extent that CX-006 became too tacky. This method of analysis has proved to be a reliable and useful technique for detecting low levels of free caprolactam in this type of oligomeric crosslinker. 

Wood wafers were treated with mixtures of propylene oxide and oligomeric isocyanate (Guevera and Moslemi, 1983). The best treatment was found to be a mixture of 9 1 propylene oxide to isocyanate. In another study, Guevera and Moslemi (1984) studied the swelling properties of wood modified with propylene or butylene oxide and compared the data with modifications using a furan resin, or vinylpyrrolidinone. The best results were obtained by the use of alkylene oxides in combination with a cross-linking agent (trimethylol propane trimethacrylate). 

Preparation of coatings as free films. The oligomeric esterdlols were mixed with the crosslinkers HMMM or polyfunctional isocyanate. The molar ratio esterdlol/HMMM was 2 1 leading to an OH/OCH3 ratio of 4 6. The OH/NCO ratio was 1 1. Some 1 wt% diethanolamine salt of p-toluene sulphonlc acid, respectively 0.2 wt% Dabco were used as catalyst. The coatings were applied to Bonder 101 plates which had been sprayed with a thin layer (1-2 pm) of teflon. 

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. 

These two isocyanates are used individually in the monomeric form, or in blends (e.g., TM20, a TDI/MDI blend for automotive flexible foams), and are often preadvanced into oligomeric forms with very low molecular weight diols to form isocyanate-capped prepolymers. 

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. 

Hannig et al. (3) prepared carbodiimide- and uretoni mine-modified polyisocyanates using microwave radiation. These oligomeric isocyanates were subsequently used to prepare polyurethane articles. 

Most isocyanates are reacted with polyols to form polyurethanes. A significant green chemistry challenge is to manufacture commercially important polyurethanes in an environmentally and economically friendly manner. A promising new class of hybrid non-isocyanate polyurethanes (HNIPUs) eliminates the use of isocyanates and offers alternatives that are potentially safer than conventional polyurethanes [33]. HNIPUs are manufactured by reacting oligomeric cyclocarbonates with aliphatic amines (Scheme 5.1). 

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. 

The polyisocyanates which can be used for preparing isocyanate-based foams are mainly aromatic compounds and some aliphatic or aralkyl polyisocyanates. TDI (toluene diisocyanate) is widely used for flexible foams. Pure MDI (diphenylmethane diisocyanate) is used for elastomers and coatings. Modified TDI and modified MDI are used for high-resilience flexible foams. Polymeric isocyanates (polymeric MDI or oligomeric MDI) are mostly used for preparing rigid urethane and isocyanurate foams, and in part, for preparing flexible and semi-flexible foams. 

Aniline was converted into its novolak analogue by reacting with formaldehyde and hydrochloric acid in the presence of divalent metal cations such as Ca and Fe. The ratio of aniline/formaldehyde/hydrochloric acid was 7.5 1.0 0.3, respectively, using 0.00025 wt% metal ions. These oligomeric products are designed to be further modified to isocyanates by reacting with phosgene. 

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... 

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