**4. Mechanism**

The reaction of an isocyanate with active hydrogen compounds is carried out with or without a catalyst. The self-addition reactions of isocyanates do not usually proceed as readily as reactions with active hydrogen compounds.

#### **4.1. Reaction in the absence of a catalyst**

The active compound itself acts catalytically in the reaction as follows (Scheme 9).

**Scheme 9.** Isocyanate reaction in the absence of a catalyst

As given in Scheme 9, in the reactions proceeding in the absence of a catalyst, the electrophilic carbon of the isocyanate is attacked by the nucleophilic centre of the active hydrogen compound; hydrogen is added to –NCO group. The reactivity of the –NCO groups is increased due to the presence of the electron withdrawing groups, and decreases by the electron donating groups. While the aromatic isocyanates are more reactive than the aliphatic isocyanates, steric hindrance at –NCO or HXR' groups reduce the reactivity.

The order of reactivity of active hydrogen compounds with isocyanates in uncatalyzed systems is as follows:

Aliphatic amines> aromatic amines> primary alcohols> water>secondary alcohol> tertiary alcohol> phenol> carboxylic acid> ureas> amides>urethanes.

## **4.2. Reaction in the presence of a catalyst**

12 Polyurethane

**4. Mechanism** 

R N C O + R' X H

**Scheme 8.** Reaction of isocyanate with active hydrogen compound

readily as reactions with active hydrogen compounds.

**4.1. Reaction in the absence of a catalyst** 

R N C O

R' X H

+

R' X H

+

The reaction of an isocyanate with active hydrogen compounds is carried out with or without a catalyst. The self-addition reactions of isocyanates do not usually proceed as

The active compound itself acts catalytically in the reaction as follows (Scheme 9).

R N C

H X R'

R' X H

R N C O

**Scheme 9.** Isocyanate reaction in the absence of a catalyst

O

X H

R'

As given in Scheme 9, in the reactions proceeding in the absence of a catalyst, the electrophilic carbon of the isocyanate is attacked by the nucleophilic centre of the active hydrogen compound; hydrogen is added to –NCO group. The reactivity of the –NCO

R

H

N

C

R' X H

+

O

X R'

R N C

O

X H

R'

R

H

N

C

O

X R'

The isocyanate reactions of class (a) are also extremely susceptible to catalysis. The various isocyanate reactions are influenced to different extents by different catalysts. Many commercial applications of isocyanates utilize catalysed reactions. Tertiary amines, metal compounds like tin compounds (as mentioned earlier in the chapter) are most widely used catalysts for the reaction (Schemes 10 and 11). The mechanisms are similar to that of the uncatalyzed reaction (Scheme 9).

The tertiary amines and metal salts catalyse the reaction as follows:

**Scheme 10.** Tertiary amine catalysed reaction

Polyurethane: An Introduction 15

Networks PU, hybrids and composites are used in various applications such as paints and coatings, adhesives, sealants, foams, absorbents, flame retardants, fuel binders, in automobiles, in biomedical applications (urological stenting practices, carriers of antituberculosis drugs, orthopaedics), extraction of metals, grouting technologies, crashworthiness, treatment of industry wastewater, cast elastomers, and others as also

Dr Fahmina Zafar (Pool Officer) and Dr Eram Sharmin (Pool Officer) acknowledge Council of Scientific and Industrial Research, New Delhi, India for Senior Research Associateships against grant nos. 13(8385‐A)/2010‐POOL and 13(8464‐A)/2011‐10 POOL, respectively. They are also thankful to the Head, Department of Chemistry, Jamia Millia Islamia (A Central

Chattopadhyay D.K., Raju K.V.S.N. Structural Engineering of Polyurethane Coatings for High Performance Applications. Progress in Polymer Science 2007; 32: 352–

Desroches M., Escouvois M., Auvergne R., Caillol S., Boutevin B. From Vegetable Oils to Polyurethanes: Synthetic Routes to Polyols and Main Industrial Products. Polymer

Lligadas G., Ronda J.C., Galia`M., Cadiz V. Plant Oils as Platform Chemicals for Polyurethane Synthesis:Current State–of–the–Art. Biomacromolecules 2010; 11: 2825–

Malcolm P S. Polymer Chemistry An Introduction. 3rd Edn. New York: Oxford University

Nylen P., Sunderland E. Modern Surface Coatings. London: John Wiley & Sons;

Paul, S. Surface Coating–Science and Technology. New York: John Wiley & Sons;

Petrović Z. S. Polyurethanes from Vegetable Oils. Polymer Reviews 2008; 48:109–

discussed in proceeding chapters.

Eram Sharmin and Fahmina Zafar\*

Reviews 2012; 52 (1): 38–79.

Press, Oxford; 1999.

*Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi, India* 

University), for providing support to carry out the work.

**Author details** 

**Acknowledgement** 

**7. References** 

418.

2835.

1965.

1985.

155.

Corresponding Author

 \*

**Scheme 11.** Metal salts catalysed reaction

The catalytic activity of amines closely parallels to the base strength of the amines except when steric hindrance becomes pronounced. This catalyst is also effective for self-addition reactions while metal salt compounds generally have less influence; tin compounds are particularly poor catalysts in these reactions.

### **5. Hazards**

Although PU are chemically inert in their fully reacted form , the risks of asthmatic symptoms arise on human exposure even in smaller concentrations due to the volatility associated with isocyanates arise the risk of asthmatic symptoms on 12 human exposure, even in smaller concentrations. On exposure to flames, hazards of ignition are feared. Isocyanates may also be sensitive on our skin. Some isocyanates may also be anticipated as carcinogens. Thus, persons working with isocyanates must be equipped with proper protection devices such as gloves, masks, respirators, goggles, and others, as precautionary measures.

## **6. Conclusion**

PU are thermoplastic and thermoset in nature. The type, position, and structure of both the isocyanate and polyol determine the progress of PU forming reactions as well as their properties and end-use applications. Hydrogen bonding also plays a key role in determining the properties of final PU product. Due to the associated health hazards, complete precautions are necessary while working with isocyanates. PU are available as one-pack or two-pack PU. PU dispersions, waterborne PU, PU Interpenetrating Networks PU, hybrids and composites are used in various applications such as paints and coatings, adhesives, sealants, foams, absorbents, flame retardants, fuel binders, in automobiles, in biomedical applications (urological stenting practices, carriers of antituberculosis drugs, orthopaedics), extraction of metals, grouting technologies, crashworthiness, treatment of industry wastewater, cast elastomers, and others as also discussed in proceeding chapters.
