Halogenated polymers

Polyvinyl chloride (PVC) became well known in the 1960's. It was then first used as a material for plastic raincoats and simulated leather goods. It is manufactured in a similar way to polystyrene. To make it soft enough for cloth­ing and simulated leather, it has to be mixed with a plasticizer or softener. In its harder form, it is used in pipes, panels, and other molded parts. Its use has declined slightly since the monomer, chloroethene (vinyl chlo­ride), was shown to cause a rare form of liver cancer if inhaled. A well-known fluorine-con­taining polymer is Teflon (polytetrafluoroethy-lene or PTFE). It is a very low friction, slippery material. This makes it ideal for coating cook­ing pans to keep food from sticking to the sur­faces.

Natural and synthetic rubbers

Rubbers are special polymers with elastic properties. This means that they return to their original shape after being deformed or stretched. Natural rubber is obtained from Hevea trees. It is used in cements, adhesives, tape for insulating electrical equipment, and for cable wrapping. It becomes more useful when vulcanized. Vulcanization is a process in which sulfur reacts with the carbon-carbon double bonds to form "bridges" of sulfur atoms that cross-link the molecules. Vulcan­ized rubber is principally made into tires. It is also used in hoses, footwear, and a variety of other items. There are also a wide variety of

Monomer

CH₂= CH

H

Ethene

CH₂= CH

CI

Chloroethene

CH₂= CH

CN

Propenonitrile

Phenylethene CH₂= CH

OCOCH.

Ethenyl ethanoate

CH₂=CH

CH,

Propene

CH,

CH

CH = CH,

Butadiene

Polymer

+-CH₂— CH-

H

Polyethylene

f-CH₂ — CH-

Cl

Polyvinyl chloride

\

CN

Acrylic polymer

Polystyrene

■CH,

CH----

Quot;

0COCH₃

Polyvinyl acetate
---- CH₂—CH-j-

CH,

Polypropylene

CH₂—CH

CH—CH₂-

Synthetic rubber -1"

Polyethyleneis made by polymerizing ethene or eth­ylene. It was one of the first polymers to be synthesized. Many others can be best understood by considering their monomers as deriva­tives of ethene. In this dia­gram, the chemical formu­lae of the monomers (left-hand column! have been presented so as to empha­size this common factor. The double-bonded "eth­ene" parts of the molecules are on pink panels. After polymerization, this element of the molecule (but now only singly bonded) forms the long repeating back­bone or chain of the poly­mer. These single-bonded "ethene" parts are shown on yellow panels in the right-hand column.

Rubberis valued for its re­silience and elasticity, two essential properties for the inflatable children's "castle." Once only a natural prod­uct, most types of rubber are now synthetic polymers.

102 Organic chemistry: Polymers

Date: 2015-12-11; view: 975