**3. Chemistry of PU**

PU are carbonic acid derivatives. The older term for them is an ester of a substituted carbamic acid, polycarbamate, from carbamic acid. PU are formed by (i) the condensation polymerization reaction of bischloroformates with diamine (Scheme 3) and (ii) addition polymerization reaction of diisocyanates with di or polyfunctional hydroxy compounds, or other compounds having a plurality of active hydrogen atom (Scheme 4). The latter method is more important from the industrial point of view since in this method no by-product is formed.

**Scheme 3.** Reaction of bischloroformate with diamine

**Scheme 4.** Reaction of diisocyanate with di or poly hydroxy compound

The isocyanate reaction offers the possibility of producing tailor-made polymeric product ranging from fibres to rubber. Generally, the isocyanate reactions are divided into two classes, (a) addition (primary and secondary) reaction with compound containing active hydrogen (Schemes 5 and 6), (b) self-addition reaction (Scheme 7). In some of the reactions, CO2 is released which assists in the formation of PU foams.

8 Polyurethane

**3. Chemistry of PU** 

formed.

over tertiary amines because they are comparatively less volatile and less toxic. Metals catalyse the isocyanate-hydroxyl reaction by complex formation with both isocyanate and hydroxyl groups. The positive metal centre interacts with electron rich oxygen atom of both the isocyanate and hydroxyl groups forming an intermediate complex, which by further rearrangement results in the formation of urethane bonds. Difunctional low molecual weight diols (ethylene glycol, 1,4-butanediol, 1,6-hexanediol), cyclohexane dimethanol, diamines, hydroxyl-amines (diethanolamine and triethanolamine) are used as chain extenders in PU synthesis while those with functionality 3 or > 3 are used as crosslinkers. Since isocyanates are too sensitive to moisture or water even in traces, moisture scavengers, which react more readily with water than an isocyanate, are incorporated to cut off/eliminate the involvement of water during PU synthesis, e.g., oxazolidine derivatives, zeolite type molecular sieves. Blowing agents are used to produce PU foams with cellular

PU are carbonic acid derivatives. The older term for them is an ester of a substituted carbamic acid, polycarbamate, from carbamic acid. PU are formed by (i) the condensation polymerization reaction of bischloroformates with diamine (Scheme 3) and (ii) addition polymerization reaction of diisocyanates with di or polyfunctional hydroxy compounds, or other compounds having a plurality of active hydrogen atom (Scheme 4). The latter method is more important from the industrial point of view since in this method no by-product is

NH2

The isocyanate reaction offers the possibility of producing tailor-made polymeric product ranging from fibres to rubber. Generally, the isocyanate reactions are divided into two


C

N H R'

N H

O

n

R'

O

n

O

R

O

C

C

N H R

N H C

O

O

O

O

structures by foaming process (e.g., hydrocarbons, CO2, hydrazine).

H2N R'

C

**Scheme 3.** Reaction of bischloroformate with diamine

+

**Scheme 4.** Reaction of diisocyanate with di or poly hydroxy compound

O

R

Cl Cl

**Hydroxy compound wth excess of Phosgene**

OCN <sup>R</sup> NCO <sup>+</sup> R' HO OH

O

C

O

O

**Scheme 5.** Primary addition reactions of isocyanate with (a) amine, (b) water, (c) alcohol, (d) carboxylic acid, (e) urea.

Polyurethane: An Introduction 11

R N C O

3 R N C O

**Scheme 7.** Self -addition reactions of isocyanate

2

R N

**Pyridine**

**Strong base**

**e.g. NaOMe**

**Very strong base**

**Low temp.**

<sup>x</sup> N C <sup>O</sup> <sup>N</sup>

C

C

O

N R

**Uretidione**

<sup>R</sup> <sup>N</sup> <sup>C</sup> <sup>N</sup> <sup>R</sup> + CO 2

O

N

R

C

**Polyamide** 

x

O

**Isocyanurate**

O

R

O

N

**Carbodiimide**

C

C

N C R

O

**Scheme 6.** Secondary addition reactions of isocyanate with (a) polyurethane, (b) polyurea and (c) polyamide

Wurts in 1848 discovered the basic reaction of isocyanate (Scheme 4). He found that isocyanates having the structure R-N=C=O, where R= alkyl or aryl group, react rapidly at room temperature with compounds containing active hydrogen atoms, like amine, water, alcohol, carboxylic acid, urethanes and ureas (Scheme 8).

It is observed that a linear PU is formed when a diisocyanate react with diol whilst branched or cross-linked PU results with the reaction of polyhydric compound (polyol). The branched or cross- linked PU are also formed when a compound containing three or more isocyanate groups reacts with a diol; however, this approach is of limited commercial importance.

**Scheme 7.** Self -addition reactions of isocyanate

polyamide

importance.

R N C O +

N H

N

C

O

O

NH

O

H

+

alcohol, carboxylic acid, urethanes and ureas (Scheme 8).

+

N H

C

**Scheme 6.** Secondary addition reactions of isocyanate with (a) polyurethane, (b) polyurea and (c)

Wurts in 1848 discovered the basic reaction of isocyanate (Scheme 4). He found that isocyanates having the structure R-N=C=O, where R= alkyl or aryl group, react rapidly at room temperature with compounds containing active hydrogen atoms, like amine, water,

It is observed that a linear PU is formed when a diisocyanate react with diol whilst branched or cross-linked PU results with the reaction of polyhydric compound (polyol). The branched or cross- linked PU are also formed when a compound containing three or more isocyanate groups reacts with a diol; however, this approach is of limited commercial

O

R

**(c) Acylurea**

H

R

**(a) Allophanate**

**(b) Biuret**

R

H

N

H

N

C

C

O

N C

O

O

HN

O

N

O

O

N

C

O

N C

Polyurethane: An Introduction 13

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 order of reactivity of active hydrogen compounds with isocyanates in uncatalyzed

Aliphatic amines> aromatic amines> primary alcohols> water>secondary alcohol> tertiary

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

R N C

**R'' : N <sup>3</sup>**

H X R'

R N C O

**R'' : N <sup>3</sup>**

O

R

H

N

C

+

**R'' : N <sup>3</sup>**

O

X R'

R N C

**R'' : N <sup>3</sup>**

O

alcohol> phenol> carboxylic acid> ureas> amides>urethanes.

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

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

uncatalyzed reaction (Scheme 9).

R N C O

+

**R'' : N**

**3**

R' X H

**Scheme 10.** Tertiary amine catalysed reaction

+

the reactivity.

systems is as follows:

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