Trimerisation isocyanates

The underlying reaction for polyisocyanurate formation is the trimerisation of an isocyanate under the influence of specific catalysts (Figure 27.9). 

Amongst the catalysts used or the polymerisation-trimerisation reactions are alkali metal phenolates, alcoholates and carboxylates and compounds containing o-(dimethylaminomethyl)phenol subgroups. Fluorocarbons such as trichloro-fluoromethanes are used as the sole blowing agents in the absence of any isocyanate-water reaction. 

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

Besides trimerisation, leading to the production of polyisocyanurates, isocyanates can react with each other to form polycarbodi-imides with the simultaneous evolution of carbon dioxide ... 

The nickel-catalysed co-trimerisation of acetylenes with isocyanates gave good yields of uracils 46, depending on the isocyanate substituents, pyridones being unwanted by-products... 

Isocyanates have some important reactions without the participation of active hydrogen compounds. These reactions, of real importance in polyurethane chemistry are dimerisation, trimerisation, formation of carbodiimides and reaction with epoxides and cyclic anhydrides. 

We have confirmed that the cause of crosslinking and branching, in the synthesis of linear polyoxazolidones by an epoxide/isocyanate reaction, is the trimerisation of the isocyanate to form isocyanurate (Reaction 5). Alternative synthetic routes to polyoxazolidones, avoiding the use of diisocyanates and so overcoming the problem of crosslinking during their formation have been reported and include the reaction of equimolar mixtures of diamin-imides and diepoxides and the reaction between diepoxides and diurethanes... 

It appears, however, that although trimerisation is accompanied by a low conversion of epoxy and isocyanate to oxazolidone, the major part of this latter process occurs at much higher temperatures. 

A major feature of the chemistry of the oxazolidone formation reaction, emerging from the of epoxy and isocyanate model compound study, was the competition to that reaction provided by the trimerisation of the isocyanate. This process was a complicating factor in the preparation of linear... 

Catalyst From Peak 1, Fig. 1 Trimerisation of isocyanate to isocyanurate (IV) From Peak 2, Fig. 1 Oxazolidone formation (I)... 

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