Synthetic Polymers

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  • Topic: Polymer, Monomer, Polymer chemistry
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INTERCHAPTER S

Synthetic Polymers

The formation of nylon by a condensation polymerization reaction at the interface of water and hexane, two immiscible solvents. The lower water layer contains the compound hexanedioyl dichloride, Cl C(CH2)4C O O Cl

hexanedioyl dichloride

The reaction produces nylon and HCl(aq). The polymer forms at the interface between the two solutions and is drawn out as a continuous strand.

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S. SyNTHETIC PolymERS

S1 HOCH2CH2 + H2C=CH2 → HOCH2CH2CH2CH2

Some molecules contain so many atoms (up to tens of thousands) that understanding their structure would seem to be an impossible task. By recognizing that many of these macromolecules exhibit recurring structural motifs, however, chemists have come to understand how these molecules are constructed and, further, how to synthesize them. These molecules, called polymers, fall into two classes: natural and synthetic. Natural polymers include many of the biomolecules that are essential to life: proteins, nucleic acids, and carbohydrates among them. Synthetic polymers—most of which were developed in just the last 60 or so years—include plastics, synthetic rubbers, and synthetic fibers. We shall study synthetic polymers in this Interchapter and natural polymers in the next one. Enormous industries have been built around synthetic polymer chemistry, which has profoundly changed the quality of life in the modern world. It is estimated that about half of all industrial research chemists are involved in some aspect of polymer chemistry. Few of us have not heard of nylon, rayon, polycarbonate, polyester, polyethylene, polystyrene, Teflon®, Formican®, and Saran, all of which are synthetic polymers. The technological impact of polymer chemistry is immense and continues to increase.

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The product of this step is also a free radical that can react with another ethylene molecule according to HOCH2CH2CH2CH2 + H2C=CH2 → ̣ HOCH2CH2CH2CH2CH2CH2 The product here is a reactive chain that can grow longer by the sequential addition of more ethylene molecules. The chain continues to grow until some termination reaction, such as the combination of two free radicals, occurs. The polyethylene molecules formed in this manner typically contain thousands of carbon atoms. The polymerization of ethylene can be written schematically as nH2C=CH2 → — CH2CH2— ( )n monomer polymer

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S-1. Polymers Are Composed of many molecular Subunits Joined End to End The simplest very large molecule, or macromolecule, is polyethylene. Polyethylene is formed by joining many ethylene molecules, H2C=CH2, end to end. The repeated addition of small molecules to form a long, continuous chain is called polymerization, and the resulting chain is called a polymer (poly = many; mer = unit). The small molecules or units from which polymers are synthesized are called monomers. The polymerization of the monomer ethylene ̣ can be initiated by a free radical, such as HO , the hydroxyl radical. (Recall from Section 7-7 that a free radical is a species having one or more unpaired electrons.) The first step in the polymerization of ethylene is the reaction described by

The notation — CH2CH2— means that the group ( )n enclosed in the parentheses is repeated n times; it also serves to identify the monomer unit. The free radical that initiates the polymerization reaction is not indicated because n is large and thus the end group constitutes only a trivial fraction of the large polymer molecule. The precise number of monomer molecules incorporated into a polymer molecule is not important for typically large values of n. Polymer syntheses generally produce polymer molecules with a range of n values. The polymer properties are described in terms of the average value of n. It makes little difference whether a polyethylene molecule consists of 5000 or 5100 monomer units, for example. Polyethylene is a tough, flexible...
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