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The physical properties of these three polymeric substances differ from each other, and of course from their monomers. Rubber and cellulose molecules have similar mass ranges, but fewer monomer units because of the monomer's larger size. As noted above, synthetic HDPE macromolecules have masses ranging from 10 5 to 10 6 amu (LDPE molecules are more than a hundred times smaller). Some Common Addition PolymersĪ comparison of the properties of polyethylene (both LDPE & HDPE) with the natural polymers rubber and cellulose is instructive. A listing of some important addition polymers and their monomer precursors is presented in the following table. Polymers formed by a straightforward linking together of monomer units, with no loss or gain of material, are called addition polymers or chain-growth polymers. Many polymeric materials having chain-like structures similar to polyethylene are known. The influence of different mass distributions on M n and M w may be examined with the aid of a simple mass calculator. As the weight dispersion of molecules in a sample narrows, M w approaches M n, and in the unlikely case that all the polymer molecules have identical weights (a pure mono-disperse sample), the ratio M w / M n becomes unity. Since larger molecules in a sample weigh more than smaller molecules, the weight average M w is necessarily skewed to higher values, and is always greater than M n. Two experimentally determined values are common: M n, the number average molecular weight, is calculated from the mole fraction distribution of different sized molecules in a sample, and M w, the weight average molecular weight, is calculated from the weight fraction distribution of different sized molecules. Because of this, polymer molecular weights are usually given as averages. The HDPE molecules, for example, are all long carbon chains, but the lengths may vary by thousands of monomer units. Unlike simpler pure compounds, most polymers are not composed of identical molecules. The synthetic methods used to prepare this and other polymers will be described later in this chapter. The two open bonds remaining at the ends of the long chain of carbons (colored magenta) are normally not specified, because the atoms or groups found there depend on the chemical process used for polymerization. This polymer is called polyethylene rather than polymethylene, (-CH 2-) n, because ethylene is a stable compound (methylene is not), and it also serves as the synthetic precursor of the polymer. If Y and Z represent moles of monomer and polymer respectively, Z is approximately 10 -5 Y. HDPE is composed of macromolecules in which n ranges from 10,000 to 100,000 (molecular weight 2*10 5 to 3 *10 6 ). Here ethylene (ethene) is the monomer, and the corresponding linear polymer is called high-density polyethylene (HDPE). For polyethylene, arguably the simplest polymer, this is demonstrated by the following equation. The repeating structural unit of most simple polymers not only reflects the monomer(s) from which the polymers are constructed, but also provides a concise means for drawing structures to represent these macromolecules. Writing Formulas for Polymeric Macromolecules Some important examples of these substances are discussed in the following sections.Ģ. Indeed, applications of these materials as fibers, flexible films, adhesives, resistant paints and tough but light solids have transformed modern society. Recognition that polymeric macromolecules make up many important natural materials was followed by the creation of synthetic analogs having a variety of properties. The terms polymer and monomer were derived from the Greek roots poly (many), mono (one) and meros (part). For his contributions to chemistry, Staudinger received the 1953 Nobel Prize. He formulated a polymeric structure for rubber, based on a repeating isoprene unit (referred to as a monomer). In contrast to the prevailing rationalization of these substances as aggregates of small molecules, Staudinger proposed they were made up of macromolecules composed of 10,000 or more atoms. This limiting view was challenged by Hermann Staudinger, a German chemist with experience in studying natural compounds such as rubber and cellulose. Prior to the early 1920's, chemists doubted the existence of molecules having molecular weights greater than a few thousand.
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