Hydroxyl groups reactivity with isocyanates

As an immediate consequence, the oligo-polyols containing terminal primary hydroxyl groups are more reactive in the reaction with isocyanates than the oligo-polyols having only secondary hydroxyl groups. The primary hydroxyl content of an oligo-polyol is an important characteristic, because it is possible to determine the potential reactivity with isocyanates. 

All the oligo-polyols are used to build the polyurethane high MW structure in a reactive process, as a consequence of the oligo-polyols terminal hydroxyl group reaction with polyisocyanates. The reactivity of oligo-polyols in polyurethane fabrication is a very important practical characteristic. Reactivity is a measure of the reaction rate of an oligo-polyol with an isocyanate in order to make the final polyurethane polymer. One practical method is the measurement of viscosity, in time, by Brookfield Viscosity Test (BVT), especially used to determine the reactivity of ethylene oxide capped polyether polyols. Figure 3.12 shows the effect of the primary hydroxyl content upon the reactivity of ethylene-oxide capped polyether triols of MW of 5,000 daltons. 

Isothiocyanates are widely used for derivatizing primary amines. They possess higher selectivity and are less reactive towards water, hydroxyl and carboxylic groups compared with isocyanates [42-44]. Labelling with phenyl isothiocyanate (PITC) or naphthyl isothiocyanate (NITC) has been used for the determination of C3—Cjo alkylamines in ethanol [45]. 

Reactive polyurethane adhesives polyurethane prepolymers with terminal hydroxyl groups or terminal isocyanate groups. Uses plastics, metals, silicate-containing materials. 

Following this work, the y -12F-diol was used for the direct reaction with hexamethylene-1,6-diisocyanate in the presence of dibutyltin dilaurate to produce a cross-linked elastomer or a reactive prepolymer which was terminated with either isocyanate or hydroxyl groups, depending on which reactant was in excess (142,143). 

The isocyanate group is more reactive than the epoxy group in that it will react at room temperature with water and hydroxyl groups as well as with amine groups. However, the latter reaction is too fast to be practicable so the standard two-pack coatings are based on isocyanate and polyhydroxyl prepolymers such as hydroxyl terminated polyesters or polyethers as in the last example given in the section on epoxy resins. 

Reactive organic chemicals can be bonded to cell wall hydroxyl groups on cellulose, hemicelluloses, and lignin. Much of our research has involved simple epoxides (1 3) and isocyanates (4), but most of our recent effort has focused on acetylation. Acetylation studies have been done using fiberboards (5f6), hardboards (7 11) particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2 257, liquid phase acetylation (, ), or reaction with ketene (28). 

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)]. 

The functionalization of SAMs via ruthenium-catalyzed cross metathesis of vinyl-terminated SAMs has been reported by Lee et al.76 to install a variety of acrylic derivatives on SAMs bearing vinyl groups on their outer surface. The major drawback of this approach is the intra-SAM metathesis which causes the formation of a mixture of surface-bound products, limiting the reproducibility of the method. The formation of urethanes by the reaction of diisocyanates77 or isothiocyanates78 with hydroxyl- and amino-terminated SAMs has been reported as well. The reaction of hydroxyl-terminated SAMs with diisocyanates, allowed the preparation of isocyanate SAMs that proved to be reactive towards amines, alcohols, and water, displaying the standard chemistry of the isocyanate groups.77... 

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