Isocyanates reaction with hydroxyl groups

Urethane adhesives take their name from the product of the most common step-growth polymerization reaction used to generate the adhesive polymers. Isocyanates react with hydroxyl groups to create urethane (or carbamate) linkages ... 

The carboxylic groups of mesoporphyrin-lX (15 b) and mesohemin-EX (15 c) were converted into isocyanate groups (15 d, c)66. The reaction with hydroxyl group containing crosslinked polymers like poly(2-hydroxyethylmethacrylate) or sephadex (with hydroxy-propyl groups) in pyridine at room temperature led to fixed porphyrins (20 a, b). 

As often happens with polymerization reactions, simple rate laws can seldom describe the whole course of reaction because of catalysis or inhibition by the urethane groups formed or by the initial reagents. Self-association of metallic catalysts, or their loose complexation by products or reagents, also prevents correlation of rates of reaction by simple proportionality or even power-law relations. Catalysis of isocyanate reaction with hydroxyls [250, 251] is by far the best understood. 

Isocyanates react with hydroxyl groups in a rearrangement reaction to form a urethane group. In the simple case, this can be described by an alcohol reacting with an isocyanate, as indicated below. 

These reactions are usually undesirable, since they lead to a decrease in the amount of isocyanate available for reaction with hydroxyl groups and produce carbon dioxide, which can cause bubbles in the polyurethane if it is formed late in the polymer s formation. 

The reaction of hydroxyl group on inorganic particles with reactive polymers having isocyanate (27.28), alkoxysilane (29,30), and chlorosilane (29) groups efficiently gave the polymer-grafted particles [Reactions (9) and (10)]. 

Since the early days of polyurethane discovery, the technology has focused on isocyanate reactions with polyesters or polyethers. The differences will be discussed in later sections. These reactions are responsible for the growth of the polyurethane industry. The polyesters of interest to polyurethane chemists terminate in hydroxyl groups and are therefore polyols produced by the polycondensation of dicarboxyhc acids and polyols. An example is a polyol with a polycarbonate structure (Figure 2.3). 

The most common reactions are those between isocyanate and alcohol group [Figure 2.17(a)] or amine group [Figure 2.17(b)]. Carboxylic acids also react readily with isocyanates through the hydroxyl group [Figure 2.17(c)] but are somewhat less reactive than are the primary alcohols and... 

Polyurethanes are formed when a diisocyanate (or polyisocyanate) is reacted with hydroxyl groups at a molar ratio of 2 or higher (isocyanate hydroxyl). When the polyol and polyisocyanate are combined in the presence of a suitable catalyst, the exothermic polymerization reaction begins spontaneously. This type of synthesis is an addition polymerization. Most polyols and polyisocyanates used for manufacturing PUs are liquid at standard room temperature. Industrially, the PU synthesis reaction is rapid, and the product is a solid polymer. The reaction rate can be varied significantly by changing the catalyst type and concentration, facilitating the use of PUs in a variety of applications. ... 

The modification of a filler surface with isocyanates is a simple process which involves the reaction of hydroxyl groups on the filler surface with monomeric isocyanate. 2,4-toluene diisocyanate or hexamethylene diisocyanate are commonly used. Since isocyanates are bifunctional they can be further reacted with polyols to form a coating on the surface or they can be used for the reinforcement of polyurethane. A strong covalent bonding can be verified by controlled extraction with the solvent. Bound material will not be removed from the fillefs surface. 

Polymerization in such systems is based on the reaction of isocyanate with hydroxyl groups to form the urethane linkage-OrganometaI Iic compounds (especially organotin) are often used to catalyze this reaction in commercial applications such as Reaction Injection Molding. Formation of elastomers with good mechanical properties is dependent on both reaction kinetics and development of two phase morphology. 

Nonetheless, for the more than 50 years since the first publication in this field, NIPUs still do not have sufficiently broad application. This can be explained by certain features of these materials. Cyclic carbonate (CC) groups interact with aliphatic and cycloaliphatic polyamines at ambient temperatures more slowly than isocyanates with hydroxyl groups. The rate of this reaction is comparable to the rate of curing epoxy resins (ER) with amines. At the same time, the CCs react only with primary amino groups, in contrast to the ERs, which react with primary and with secondary amino groups. This results in a decrease in cross-linking density of the polymer network. 

The polyurethanes, although not used in large volumes, have specific applications of value for aerospace. Accordingly, a brief discussion of the chemistry and the material applications is in order. There are three chemical reactions of importance the reaction of hydroxyl groups with isocyanates to form urethanes (Reaction 1), the reaction of isocyanates with water to form amines (Reaction 2), and the reaction of amines with isocyanates to form ureas (Reaction 3). 

Hydroxyl groups of biomolecules are less reactive with isothiocyanates and active esters, but reactions often can be completed under appropriate conditions, which include a longer reaction time. Sulfonyl chlorides and isocyanates are more reactive with hydroxyl groups. 

The reaction between the isocyanate and the hydroxyl groups results in 100% cross-linking if the reactants are converted in stoichiometric proportions. Optimum paint films are generally formed when NCO OH equals 1. However, it may be advantageous to deviate from this ratio (under- or over-cross-linking). With aliphatic polyisocyanates, a catalyst (accelerator) is generally necessary. Small amounts (0.05-0.5%) of tertiary amines or metal-containing compounds are normally added... 

First, UV irradiation initiates, with the aid of an appropriate initiator, the polymerization of acrylate groups, and then heating causes the isocyanate groups to react with hydroxyl groups. The latter reaction results in the formation of urethane linkages (see Scheme 11.3). 

Scheme 11.3 Formation of urethane linkages by the reaction of isocyanate with hydroxyl groups.

Cellulose carbamate was discovered and patented by Hill and Jacobson [77]. A polymer usually can be formed by reacting cellulose with isocyanic acid (lA) (Figure 10.47). In most reactions, the hydroxyl group present at C6 position in the repeating cellulose unit reacts with... 

The isocyanate will also react with hydroxyl groups in the EPI adhesive as shown in Fig. 6. The reaction between isocyanate and hydroxyl gives urethane linkage. 

From this emphasis on hydroxyl groups, it might be thought that the hardening process consists solely of urethane formation between resin and polyisocyanate. This is obviously not so, since water vapour is usually present and the isocyanate reactions with water must proceed in competition with the reaction with hydroxyl. This will lead to consumption of isocyanate groups and linking of isocyanate molecules by urea linkages. With a difunctional isocyanate this tends to space the resin chains farther apart,... 

In the course of their work, operators need not be exposed to isocyanate vapours by inhalation if adequate precautions are taken. The vapour pressure of the isocyanate, the temperature resulting from the exothermic reaction of the isocyanate with hydroxyl groups and the elevated processing temperature of the components which is frequently necessary, all affect the concentration of the vapour. 

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