Isocyanate difunctional

There are many situations where isocyanates are derivatized before use, such as when MDI or TDI are reacted with high-molecular-weight, difunctional polyols to form low-NCO prepolymers used to make cast elastomers by the prepolymer method (Section 4.3.2.1). Because of their molecular weight, prepolymers can be of much higher viscosity than the starting isocyanates, and some are solid at room... 

Nitrile oxide precursors have been prepared by the reaction of an isocyanate and an alkyl nitroacetate. These precursors release alkanol and carbon dioxide when heated, to liberate the highly reactive nitrile oxide species. An improved synthetic procedure has been developed to afford novel cross-linking agents based on difunctional, trifunctional and aliphatic precursors. Application of these agents to polymer cross-linking has been demonstrated (527). 

It was found that whereas 2-fuiyl isocyanate 29 and its difunctional 2,5-homologue 30 are unstable and resinify on standing at room temperature even if kept under nitrogen, the corresponding urethanes formed with aliphatic and furanic alcohols are stable. In more general terms, both the kinetics and the mechanism of the formation of furanic urethanes and diurethanes could be rationalized on the basis of established criteria combined with the specific effects of the furan moiety (35). All the products were M y characterized and... 

The reaction of isocyanates with alcohol is strongly exothermic. A portion of this heat can be removed and better quality control achieved by forming a prepolymer with half of the alcohol and the diisocyanate. The new prepolymer has isocyanate terminals and can be used with the remaining quantities of the difunctional and/or the trifunctional alcohol in the propellant mix. In recent years isocyanate-terminated polymers... 

Figure 1. Two commercially available difunctional compounds featuring isocyanate and acrylate (vinyl) functionality.

The most practical laboratory methods involve the condensation of amines with either ureas, isocyanates, or isocyanate derivatives. The use of difunctional reactants yields polymers (Eq. 1). 

Some of these products are stable, while others can be decomposed easily to the starting material or other products. The reaction of isocyanate groups with alcohols is discussed in detail in this chapter. Reference is also made to the preparation of polymers using difunctional starting materials (Eq. 3). 

The polycyclotrimerization of difunctional isocyanates (or NCO-terminated prepolymers) produces polymer networks containing heterocyclic, thermostable per-hydro-1,3,5-triazine-2,4,6-trione (isocyanurate) rings as crosslinks ... 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

During the initial stages of cyclotrimerization of difunctional isocyanates, the formation of polyfunc-tional NCO-terminated oligomers takes place ... 

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. 

Step-growth polymerization proceeds via a step-by-step succession of elementary reactions between reactive sites, which are usually functional groups such as alcohol, acid, isocyanate, etc. Each independent step causes the disappearance of two coreacting sites and creates a new linking unit between a pair of molecules. To obtain polymers, the reactants must be at least difunctional monofunctional reactants interrupt the polymer growth. 

The urethane linkage is the fundamental group in polyurethane chemistry. The initial step in the preparation of a cast prepolymer is to react a suitable linear diol with a difunctional isocyanate, as illustrated in Figure 2.2. 

Aluminum isopropoxide has been used for the preparation of block copolyesters [147, 148]. Tri-block poly(e-CL-b-DXO-e-CL) was prepared by the sequential addition of different monomers to a living polymerization system initiated with aluminum isopropoxide in THF or toluene solution [95]. An alternative route for the preparation of the tri-block copolymer was to react the diblock poly(e-CL-b-DXO) containing an -OH functionality at the chain end using a difunctional coupling agent such as isocyanate or acid chloride (Scheme 18). However, the molecular weights were low and full conversion of monomers was not achieved. 

With preference to difunctional compounds, they have a molecular weight of about 2,000. The variety of the functional end-groups is impressive alcohols, amines, acids, esters and also isocyanate. 

The simplest case of a polymer molecule which can undergo either cyclization or chain extension is that in which the groups are situated at each end of the polymer chain (which can react together). An example of this type of polymer is a,hydroxy-terminated polystyrene which reacts with a difunctional isocyanate, and our first studies were carried out with this reaction system. Obviously, the chain extension reaction leads to an increased specific viscosity, and cyclization leads to the reverse. 

If both the isocyanate and the hydroxyl-containing material are difunctional, if the mixture is made up to have one isocyanate per hydroxyl, if there are no side reactions, and if the reaction can be driven to completion, a single linear thermoplastic polymer should result. If one or more of the reactants is more than difunctional, it is possible to create an infinite three-dimensional network,... 

A wide range of materials is included in this class. The common feature is the use of chain extension reactions to provide products with acceptable commercial properties. The chain extension reaction effectively reduces the actual number of chain ends, thereby eliminating the generally poor properties observed when very low-molecular-weight polymers are cross-linked. The chain extension step involves the reaction of a difunctional polymeric polyol with difunctional organic isocyanates to give the polyurethane ... 

The creation of difunctional or multifunctional isocyanate molecules entirely based on renewable resource chemistry. 

These new systems open up applications such as the difunctional compounds (used in polycondensation) which exhibit new reactive groups (amine or isocyanate) or the pseudo living polymers which are able to initiate new polymaizations for obtaining triWockcopolymers. 

The structural variations possible in R and R make it possible to vary the toughness and elasticity of the polyurethane adhesive. Polyesters or polyethers are prepared with terminal hydroxyl groups that can then be reacted with difunctional or polyfunctional isocyanates. Polyurethane prepolymers can be formed in Reaction 1, with the desired terminal group produced by using one or other of the diol and diisocyanate reactants in excess. 

Reaction with difunctional amines (see the next paragraph) allows the formation of poly (urethane-co-urea) and further extends the versatility of the segment architecture. For further control of the structure, a prepolymer is formed. The reaction, such as the polyurethane reaction shown above, is carried out with excess di-isocyanate so that an isocyanate-terminated prepolymer is obtained. The isocyanates used are typically aromatic, such as toluene di-isocyanate. This prepolymer is then reacted with a short-chain diol or diamine (for a polyurea) to form the final polymer. 

The earlier example of the formation of a poly(urethane-co-urea) showed the change in properties made possible by including a comonomer (in that case a difunctional amine) together with the usual diol for reaction with a di-isocyanate. This can be extended to a wide range of step polymerizations where an additional reactant is added. Examples could be the use of two AB-type monomers (e.g. amino acids) or two AA (e.g. diacids) to react with one BB (a diamine) to form co-polyamides. Seveml features of step polymerization help in understanding the resultant copolymer. For example, since high-molar-mass polymer is formed only late in the reaction, the composition of the copolymer will be that of the feed ratio of the monomers. 

Yet another polymer synthesis strategy is to react the difunctional dimer molecules with prepolymers. Equation 7.5 shows an example of this technique.5 In this instance, the prepolymer is one of the Hypo polymers sold by W.R. Grace. Analysis of the prepolymer sample showed it contained, on average, three tolyl isocyanate end groups Mn was about 2000. 

Devise a copolymer containing blocked isocyanate groups, such that on reaction with a difunctional reagent to cure it, no reblocking would occur, and no volatile compound would come off. 

Methyl, ethyl, propyl, and butyl isocyanates react quickly with dry wood to give good substitution of cell wall component hydroxyl groups. Phenyl and p-tolyl isocyanates and difunctional isocyanates tend to polymerize in the lumen with much lower... 

Polycocyclotrimers were prepared by polycocyclotrimerization of difunctional isocyanates of variable chain length or difunctional isocyanate with monofunctional isocyanate. The stress-strain and viscoelastic properties of resulting polymers were determined. It was found that co-polycyclotrimers prepared from diisocyanates of the variable chain length had typical properties of phase separated block copolymers. 

The isocyanurate rings are thermally stable (decomposition started at temperatures above A00°C (5)) and therefore cyclotrimerization of difunctional isocyanates can be utilized for preparation of thermally stable polymers. (2 J3)... 

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