The Foundations of Chemistry

Addition Polymerization

Polymerization is an important addition reaction (Section 27-17) of the alkenes. Polymers

formed by this kind of reaction are called addition polymers.The formation of poly-

ethylene is an important example. In the presence of appropriate catalysts (a mixture of

aluminum trialkyls, R 3 Al, and titanium tetrachloride, TiCl 4 ), ethylene polymerizes into

chains containing 800 or more carbon atoms.

catalyst

nCH 2 UCH 2 888888nX(CH 2 XCH 2 X)n

ethylene polyethylene

The polymer may be represented as CH 3 (CH 2 XCH 2 )nCH 3 , where nis approximately

400. Polyethylene is a tough, flexible plastic. It is widely used as an electrical insulator
     and for the fabrication of such items as unbreakable refrigerator dishes, plastic cups, and
     squeeze bottles. Polypropylene is made by polymerizing propylene, CH 3 XCHUCH 2 , in
     much the same way. Teflon is made by polymerizing tetrafluoroethylene in a similar
     reaction.
     catalyst
     nCF 2 UCF 2 888888nX(CF 2 XCF 2 X)n
     heat
     tetrafluoroethylene Teflon

The molecular weight of Teflon is about 2 

106. Approximately 20,000 CF 2 UCF 2 mole-

cules polymerize to form a single giant molecule. Teflon is a very useful polymer. It does

notreact with concentrated acids and bases or with most oxidizing agents. It does not

dissolve in most organic solvents.

Natural rubber is obtained from the sap of the rubber tree, a sticky liquid called latex.

Rubber is a polymeric hydrocarbon formed in the sap by the combination of about 2000

molecules of 2-methyl-1,3-butadiene, commonly called isoprene. The molecular weight

of rubber is about 136,000.

When natural rubber is warmed, it flows and becomes sticky. To eliminate this problem,

vulcanizationis used. This is a process in which sulfur is added to rubber and the mixture

is heated to approximately 140°C. Sulfur atoms combine with some of the double bonds

in the linear polymer molecules to form bridges that bond one rubber molecule to another.

This cross-linking by sulfur atoms converts the linear polymer into a three-dimensional

polymer. Fillers and reinforcing agents are added during the mixing process to increase

the durability of rubber and to form colored rubber. Carbon black is the most common

reinforcing agent. Zinc oxide, barium sulfate, titanium dioxide, and antimony(V) sulfide

are common fillers.

Some synthetic rubbers are superior to natural rubber in some ways. Neoprene is a

synthetic elastomer (an elastic polymer) with properties quite similar to those of natural

rubber. The basic structural unit is 2-chloro-1,3-butadiene, commonly called chloroprene,

which differs from isoprene in having a chlorine atom rather than a methyl group at carbon

2 of the 1,3-butadiene chain.

CH

chloroprene neoprene (a synthetic rubber)

nCH 2 CH 2

Cl

C

Cl

(CH 2 CH C CH 2 )n

polymerization

isoprene natural rubber

2 nCH 2 CH 2

CH 3

CCH (CH 2 CH 2

CH 3

CCH CH

CH 3

CH 2 C CH 2 )n

Teflon is a trade name owned by
DuPont, a company that has developed
and manufactured many fluorinated
polymers.

1092 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 11.9, Addition Polymerization.

Numerous other polymers are elastic
enough to be called by the generic
name “rubber.”

Many cooking utensils with
“nonstick” surfaces are coated with a
polymer such as Teflon.