Diisocyanate—diol reaction

Epoxy resins may be produced which contain tertiary carbamates by the use of glycidol as the chain-terminating alcohol in a diisocyanate - diol reaction. For example, a stepwise reaction has been used in which one equivalent of glycidol has been reacted with 1,6-hexamethylene diisocyanate to produce a mixture represented ideally by the reaction ... 

Figure 7.6 Schematic representation of hard clusters in (a) a polyurethane network composed of a long diol, a triol, and a diisocyanate (polyaddition reactions, Chapter 2). (b) a hybrid inorganic-organic network composed of a silane end-capped long diol (polycondensation reactions, Chapter 2).

The prepolymer is formed by addition polymerization, and there are no by-products formed that have to be removed. The prepolymer chains extend by the reaction of terminal isocyanate groups (-NCO) with the diol. The mole reaction ratio of the diol to diisocyanate is normally kept in the range of one mole diol to 1.6 to 2.25 moles of the diisocyanate. A reaction ratio of one mole diol to one mole diisocyanate will produce a linear material. This type of polyurethane is normally thermoplastic. 

Condensation polymers by the above delinition are usually produced by step-growth polymerizations but not all step-growth syntheses are condensation reactions. Thus there is no elimination product in polyurethane synthesis from a diol and a diisocyanate (cf. reaction (1-12)) ... 

Polyurethane pre-polymer or thermoplastic polyurethanes are prepared by the reaction of polyester or polyether diols with diisocyanates. The reaction can be conducted in bulk, in a solvent, or in an aqueous solution or dispersion. Unless the reaction is carried out at an elevated temperature a catalyst is required. Organotin compounds are the prime catalyst for this reaction. 

Polymer formation can occur through a variety of reactions. Metallocene polyamides, polyethers, polyesters, polyhydrazides and polyurethanes have been synthesized through condensation reactions of 1,1 -difunctional metallocenes (typically ferrocene derivatives) with diacid halides (if the metallocene derivative contains functional electron-pair donor bases) and electron-pair donor bases (as diamines, hydrazines, diisocyanates, diols) if the metallocene derivative contains functional electron-pair acceptor acids . 

There are step-growth polymerization reactions in which a small molecule is not produced (e.g., the reaction between a diol and a diisocyanate) these reactions are considered irreversible and are usually very fast, leading to high degrees of polymerization. 

A stoichiometric mixture of an oligomeric diol and diisocyanate will react to give a polyurethane. However, if there is double the quantity of diisocyanate, the reaction product will be an ohgomer with isocyanate end groups. Such adhesives cure by the ingress of moisture by the following sequence of reactions. (Moisture cure of adhesives). 

Instead of the diols above, Maji et al. [56] evaluated a hydroxyl functional polyester-polyurethane prepolymer prepared by the ester formation of diethylene glycol with adipic acid and subsequent urethane formation with the 2,4-toluene diisocyanate (TDI) reaction with the excess hydroxyls on the polyester (Urepan 600 prepared by M/s Rhein... 

PU foams are prepared by reactions of diols with toluene 2,4-diisocyanate the reactions are catalyzed by bases, including tertiary amines. We have investigated reactions of mustard with primary and secondary amines. We have also explored the catalytic activity of the resulting amines for PU production. 

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). 

Step-growth polymerization is characterized by the fact that chains always maintain their terminal reactivity and continue to react together to form longer chains as the reaction proceeds, ie, a -mer + -mer — (a + )-mer. Because there are reactions that foUow this mechanism but do not produce a molecule of condensation, eg, the formation of polyurethanes from diols and diisocyanates (eq. 6), the terms step-growth and polycondensation are not exactly synonymous (6,18,19). 

No by-product is formed from this reaction. Toluene diisocyanate (Chapter 10) is a widely used monomer. Diols and triols produced from the reaction of glycerol and ethylene oxide or propylene oxide are suitable for producing polyurethanes. 

A urethane is typically prepared by nucleophilic addition reaction between an alcohol and an isocyanate (R—N = C=0), so a polyurethane is prepared by reaction between a cliol and a diisocyanate. The diol is usually a low-molecular-weight polymer (MW 1000 amu) with hydroxyl end-groups the diisocyanate is often toluene-2,4-diisocyanate. 

Step-growth polymers, the second major class of polymers, are prepared by reactions between difunctional molecules, with the individual bonds in the polymer formed independently of one another. Polycarbonates are formed from a diester and a diol, and polyurethanes are formed from a diisocyanate and a diol. 

Polyurethane (Section 31.4) A step-growth polymer prepared by reaction between a diol and a diisocyanate. 

Hie most representative member of this class of polyesters is the low-molar-mass (M 1000-3000) hydroxy-terminated aliphatic poly(2,2/-oxydiethylene adipate) obtained by esterification between adipic acid and diethylene glycol. This oligomer is used as a macromonomer in the synthesis of polyurethane elastomers and flexible foams by reaction with diisocyanates (see Chapter 5). Hydroxy-terminated poly(f -caprolactonc) and copolyesters of various diols or polyols and diacids, such as o-phthalic acid or hydroxy acids, broaden the range of properties and applications of polyester polyols. 

Polyurethanes (PURs) are usually described as being prepared by the reaction of diols with diisocyanates. However, this is an oversimplification because often water is deliberately added in the production of flexible polyurethane foams. Unreacted isocyanate groups react with water to form carbon dioxide and urea groups in the polymer chain. The carbon dioxide acts as a blowing agent in the production of PUR foams. Also, polyurethanes can be formed by the reaction of bischloroformates with diamines. 

To obtain the polyurethanes, typically a prepolymer was first prepared by reacting the diisocyanate with various diols in dimethylformamide or dimethylacetamide in a two to one molar ratio at 100-110°C for two hours under nitrogen atmosphere. A solution of chain extenders, such as BEP, was then added to the prepolymer reaction mixture and further reacted another three hours. The polymer was isolated by quenching the reaction mixture in cold water. Fine white powder was obtained with a typical yield of around 90%. 

The above prepolymer on treatment with 2 as the chain extender in dry DMF did not proceed at ambient temperature. The mixture had to be heated to 60°C for 3 h before the reaction was complete. After curing at 60°C for 24 h, the yellow, translucent block polyurethane film (BPUR2) again showed the absence of the —NCO peak in the IR spectrum indicating that curing had been complete. The fact that a higher temperature had to be used in the case of 2 as the chain extender compared to 1 is in keeping with the lower order of reactivity of diols with diisocyanates as compared to diamines with diisocyanates. 

Step polymerization is used to synthesize multiblock copolymeric elastomers (referred to as segmented elastomers). An example is the polyester-polyurethane system produced by the reaction of a diisocyanate with a mixture of macro diol and smallsized diol (Eq. 14). The macro diol (usually referred to as a... 

---