Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

ADDITION POLYMERS Addition polymers

are formed in addition reactions a

nd include such familiar materials as

polyethylene, Teflon

®, polyvinyl chloride (PVC), and polystyrene. Most addition

polymers are derived from alkene monomers (Figure 13.30). The polymers differ in the identity of the R groups attached to the alkene. The properties of the polymers are dictated by both the length of the chain, ‘x’ in Figure 13.30, and the identity of the R groups attached to the alkene. Table 13.2 shows some common addition polymers and their uses. POLY(ISOBUTYLENE) One way in which an addition polymer of an alkene can be formed is by initiating the reaction with a small amount of hydrohalic

acid. The mechanism is very similar to the

addition of the hydrogen halide to a double bond discussed in Section 13.5 and shown in Figure 13.24. The C=C

bond is a weak base, and it can donate a pair of electrons to the π

strong acid (H

1+) to form a C-H bond. The resulting carbocation is a very strong Lewis

acid and reacts with any base present. In

the hydrohalogenation reaction, the halide ion

serves as the base. In the polymerization reaction, however, only a small amount of hydrohalic acid is used to initiate the reacti

on, so very little halide ion is available. As

shown in Figure 13.31, it is the C=C

bond of the alkene monomer, which is a far more π

plentiful base, that attacks the carbocation. The resulting carbocation reacts with yet another alkene. The polymer is formed by successive reactions of C=C

bonds with the π

carbocation at the end of the chain. Each su

ccessive reaction lengthens the polymer chain.

The carbocation is formed much more easily

on carbons with alkyl groups than on carbons

with hydrogen atoms. As a result, the carbocation forms on the carbon atom with the fewest hydrogen atoms bound to it.

Natural rubber is a polymer of isoprene (Figure 13.32) that can now be made in the

chemistry laboratory. Butyl rubber is a polym

er made from isobutylene and isoprene. The

United States produces over 500 million pounds of butyl rubber each year. CONDENSATION POLYMERS Condensation polymers are produced in condensation reactions. The requirement for forming condensation polymers is that

each of two reacting monomers must have two

reactive centers.

The mechanisms are identical to tho

se of the other condensation reactions

(esters and amides) we have already discussed. The only difference is that, in polymer chemistry, the products continue to react to form larger molecules.

R^1 R 3

R^2 R^4

R^2

R^1

R^2

R^2

R^4

R^4

R^4

R^3

R^1

R^3

x

Figure 13.30 Alkene polymerization Alkene monomers can polymerize to give the corresponding polymer. X is an integer that can be in the

millions

.

Table 13.2

Some common addition polymers and their uses

The numbering on the R-groups is shown in Figure 13.30. R^1

R

R 2

R 3

(^4)
Polymer Name Common Uses
H H H H
polyethylene
plastic bottles and bags
H H H CH
polypropylene 3
carpets
H CH
H CH 3
poly(isobutylene) tires 3
H H H Cl
poly(vinyl chloride) plumbing and hoses
H H H C
H 6
polystyrene 5
insulation
F F F F polytetrafluoro- cooking utensils ethylene (teflon)
x

...

H

Figure 13.31 Polymerization of isobutylene “x” is an integer that can be as high as 20,000 in a commercial polymer.

Fig

ure 13.32 Iso

prene

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