Organic Chemistry

Section 28.7 Physical Properties of Polymers 093

Polyurethanes
A urethane—also called a carbamate—is a compound that has an OR group and
an NHR group bonded to the same carbonyl carbon. Urethanes can be prepared by
treating an isocyanate with an alcohol.

Polyurethanesare polymers that contain urethane groups. One of the most common
polyurethanes is prepared by the polymerization of toluene-2,6-diisocyanate and
ethylene glycol. If the reaction is carried out in the presence of a blowing agent, the prod-
uct is a polyurethane foam. Blowing agents are gases such as nitrogen or carbon dioxide.
At one time, chlorofluorocarbons—low-boiling liquids that vaporize on heating—were
used, but they have been banned for environmental reasons (Section 9.9). Polyurethane
foams are used for furniture stuffing, carpet backings, and insulation. Notice that
polyurethanes prepared from diisocyanates and diols are the only step-growth polymers
that we have seen in which a small molecule is notlost during polymerization.

One of the most important uses of polyurethanes is in fabrics with elastic properties,
such as spandex (Lycra®). These materials are block copolymers in which some of the
polymer segments are polyurethanes, some are polyesters, and some are polyamides. The
blocks of polyurethane are soft, amorphous segments that become crystalline on
stretching (Section 28.7). When the tension is released, they revert to the amorphous state.

PROBLEM 19

If a small amount of glycerol is added to the reaction mixture of toluene-2,6-diisocyanate

and ethylene glycol during the synthesis of polyurethane foam, a much stiffer foam is

obtained. Explain.

28.7 Physical Properties of Polymers

The individual chains of a polymer such as polyethylene are held together by van der
Waals forces. Because these forces operate only at small distances, they are strongest if
the polymer chains can line up in an ordered, closely packed array. The regions of the
polymer in which the chains are highly ordered with respect to one another are called
crystallites (Figure 28.4). Between the crystallites are amorphous, noncrystalline
regions in which the chains are randomly oriented. The more crystalline—the more

OH OH OH

CH 2 CH CH 2

glycerol

+

O

RN CO ROH RNH C OR

an isocyanate an alcohol a urethane

3-D Molecules:

Toluene-2,6-diisocyanate;

Ethylene glycol

n

O

C

O CN N CO

CH 3

+

toluene-2,6-diisocyanate

ethylene glycol

NH

O

NH C OCH 2 CH 2 O

CH 3 O

C NH

O

NH C OCH 2 CH 2 O

CH 3

a polyurethane

O

C

HOCH 2 CH 2 OH

Figure 28.4

In the circled regions, called

crystallites, the polymer chains are

highly ordered, similar to the

ordering found in crystals.

Between the circles are

noncrystalline regions in which the

polymer chains are randomly

oriented.