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Chapter 26: Q19P (page 1337)

Explain why the addition of a small amount of glycerol to the polymerization mixture gives a stiffer urethane foam.

Short Answer

Expert verified

Stiffer urethane foam is formed due to the formation of cross-linking polymer of urethane. Cross linking between chains is possible since glycerol (most basic triol) is used. Cross linking helps to obtain a thermosetting polymer.

Step by step solution

01

Urethanes (or carbamate ester)

A urethane is an ester of a carbamic acid (R-NH-COOH) , a half amide of carbonic acid. They are commonly made by treating an isocyanate with an alcohol or a phenol.

02

Thermosetting polymers

They are semifluid substances with low molecular weights which on heating on a mould undergo a change in chemical composition to yield a hard, infusible and insoluble mass. This hardening on heating is due to excessive cross linking between different polymer chains.

03

The reaction

Stiffer urethane foam is formed due to the formation of cross-linking polymer of urethane. Cross linking between chains is possible since glycerol (most basic triol) is used. Cross linking helps to obtain a thermosetting polymer.

Formation of stiffer urethane foam upon addition of glycerol

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Most popular questions from this chapter

Poly (vinyl alcohol), a hydrophilic polymer used in aqueous adhesives, is made by polymerizing vinyl acetate and then hydrolyzing the ester linkages.

(a) Give the structures of poly (vinyl acetate) and poly (vinyl alcohol).

(b) Vinyl acetate is an ester. Is poly (vinyl acetate) therefore a polyester? Explain.

(c) We have seen that basic hydrolysis destroys the Dacron polymer. Poly (vinyl acetate) is converted to poly (vinyl alcohol) by a basic hydrolysis of the ester groups. Why doesn鈥檛 the hydrolysis destroy the poly (vinyl alcohol) polymer?

(d) Why is poly (vinyl alcohol) made by this circuitous route? Why not just polymerize vinyl alcohol?

Draw the structure of isotactic poly(acrylonitrile) and syndiotactic polystyrene.

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes syrupy, and then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, and then show how further condensation leads to cross-linking. (Hint: The carbonyl group lends acidity to the N - Hprotons of urea. A first condensation with formaldehyde leads to an imine, which is weakly electrophilic and reacts with another deprotonated urea.)

. (a) Draw the structure of gutta-percha, a natural rubber with all its double bonds in the trans configuration.

(b) Suggest why gutta-percha is not very elastic, even after it is vulcanized.


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(a) Draw the structure of cholic acid, showing the rings in their chair conformations, and label each methyl group and hydroxy group as axial or equatorial. (Making a model may be helpful.)

(b) Cholic acid is secreted in bile as an amide linked to the amino group of glycine. This cholic acid-amino acid combination acts as an emulsifying agent to disperse lipids in the intestines for easier digestion. Draw the structure of the cholic acid-glycine combination and explain why it is a good emulsifying agent.
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