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polymer(pŏl`əmər), chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds (see chemical bondchemical bond,
mechanism whereby atoms combine to form molecules. There is a chemical bond between two atoms or groups of atoms when the forces acting between them are strong enough to lead to the formation of an aggregate with sufficient stability to be regarded as an
..... Click the link for more information. ). The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer, and three monomers, a trimer. A structural unit is a group having two or more bonding sites. A bonding site may be created by the loss of an atom or group, such as H or OH, or by the breaking up of a double or triple bond, as when ethylene, H2C=CH2, is converted into a structural unit for polyethylenepolyethylene
, widely used plastic. It is a polymer of ethylene, CH2=CH2, having the formula (-CH2-CH2-)n, and is produced at high pressures and temperatures in the presence of any one of several catalysts, depending
..... Click the link for more information. , -H2C-CH2-. In a linear polymer, the structural units are connected in a chain arrangement and thus need only be bifunctional, i.e., have two bonding sites. When the structural unit is trifunctional (has three bonding sites), a nonlinear, or branched, polymer results. Ethylene, styrene, and ethylene glycol are examples of bifunctional monomers, while glycerin and divinyl benzene are both polyfunctional. Polymers containing a single repeating unit, such as polyethylene, are called homopolymers. Polymers containing two or more different structural units, such as phenol-formaldehyde, are called copolymers. All polymers can be classified as either addition polymers or condensation polymers. An addition polymer is one in which the molecular formula of the repeating structural unit is identical to that of the monomer, e.g., polyethylene and polystyrenepolystyrene
, widely used plastic; it is a polymer of styrene. Polystyrene is a colorless, transparent thermoplastic that softens slightly above 100°C; (212°F;) and becomes a viscous liquid at around 185°C; (365°F;).
..... Click the link for more information. . A condensation polymer is one in which the repeating structural unit contains fewer atoms than that of the monomer or monomers because of the splitting off of water or some other substance, e.g., polyesters and polycarbonatespolycarbonates,
group of clear, thermoplastic polymers used mainly as molding compounds (see plastic). Polycarbonates are prepared by the reaction of an aromatic difunctional phenol with either phosgene or an aromatic or aliphatic carbonate.
..... Click the link for more information. . Many polymers occur in nature, such as silk, cellulosecellulose,
chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose; other important natural sources are flax, hemp, jute, straw, and wood.
..... Click the link for more information. , natural rubberrubber,
any solid substance that upon vulcanization becomes elastic; the term includes natural rubber (caoutchouc) and synthetic rubber. The term elastomer is sometimes used to designate synthetic rubber only and is sometimes extended to include caoutchouc as well.
..... Click the link for more information. , and proteinsprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
..... Click the link for more information. . In addition, a large number of polymers have been synthesized in the laboratory, leading to such commercially important products as plastics, synthetic fibers, and synthetic rubber. Polymerization, the chemical process of forming polymers from their component monomers, is often a complex process that may be initiated or sustained by heat, pressure, or the presence of one or more catalysts.
a chemical compound of high molecular weight (from several thousand up to many millions), whose molecules (macromolecules) consist of a large number of repeating groups, or monomeric units. The atoms composing the macromolecules are bound on one another by regular and/or coordinate bonds.
Classification. Polymers are classified according to origin as natural polymers, or biopolymers (for example, proteins, nucleic acids, and natural resins), and synthetic polymers (polyethylene, polypropylene, and phenol-formaldehyde resins). The atoms or atomic groups may be arranged in an open chain or a sequence of consecutive rings (linear polymers, such as natural rubber), a branched chain (amylopectin), or a three-dimensional network (crosslinked polymers, such as solid epoxy resins). Polymers consisting of identical monomer units—for example, polyvinyl chloride, polycaproamide, and cellulose—are called homopo-lymers.
Macromolecules of the same chemical composition may be constructed of units with differing steric configuration. If the macromolecules consist of identical stereoisomers or of alternating unlike stereoisomers in a chain with a definite periodicity, the polymers are called stereoregular.
Polymers consisting of several types of monomeric units are called copolymers. Copolymers in which the units of each type form rather long continuous sequences that alternate within the macromolecule are called block copolymers. One or more chains of another structure may be bound to the inner (nonterminal) units of a macromolecule of a single chemical structure. Such copolymers are called graft copolymers.
Polymers in which all or several of the stereoisomers of a unit form rather long continuous sequences that alternate within the macromolecule are called stereoblock copolymers.
Polymers are divided into heteropolymers, in which the main chain contains atoms of various elements, most frequently carbon, nitrogen, silicon, and phosphorus, and homopolymers, in which the main chains consist of identical atoms. The most common homopolymers are carbon-chain polymers, in which the main chain consists only of carbon atoms—for example, polyethylene, polymethyl methacrylate, and polytetrafluoro-ethylene. Examples of heteropolymers are polyesters (polyethylene terephthalate and polycarbonates), polyamides, urea-formaldehyde resins, proteins, and some silicones. Polymers whose macromolecules contain atoms of inorganic elements along with hydrocarbon groups are called hetero-organic polymers. Inorganic polymers, such as plastic sulfur and poly-phosphonitryl chloride, form a separate group.
Properties and most important characteristics. Linear polymers have a specific set of physicochemical and mechanical properties. The most important properties are the ability to form high-strength anisotropic, highly oriented fibers and films; the capacity for large, slowly developing reversible deformations; the ability to swell in the hyperelastic state before dissolving; and the high viscosity of solutions. This set of properties results from the high molecular weight, the chain structure, and the flexibility of the macromolecules. In the transition from linear to branched, sparse three-dimensional networks, and finally to dense cross-linked structures, these properties become decreasingly pronounced. Strongly crosslinked polymers are insoluble, infusible, and incapable of hyperelastic deformations.
Polymers may exist in the crystalline and amorphous states. A necessary condition for crystallization is regularity of sufficiently long segments of the macromolecule. Various textures, such as fibrils, spheroidal aggregates, and single crystals, may arise in crystalline polymers, depending largely on the properties of the polymer material. Textures are less pronounced in amorphous polymers than in crystalline polymers.
Amorphous polymers may exist in three physical states: vitreous, hyperelastic, and viscous-flow. Polymers with a low temperature (below room temperature) for the transition from the vitreous to the hyperelastic state are called elastomers, and polymers with high transition temperatures are called plastics. The properties of polymers vary within a broad range, depending on chemical composition and the structure and mutual arrangement of the macromolecules. Thus 1,4-cis-polybutadiene, which is composed of flexible hydrocarbon chains, is elastic at about 20°C and undergoes transition to the vitreous state at – 60°C. Polymethyl methacrylate, which is composed of more rigid chains, is a hard, vitreous substance at about 20°C and undergoes transition to the hyperelastic state only at 100°C. Cellulose, which is a polymer with very rigid chains linked by intermolecular hydrogen bonds, cannot exist at all in the hyperelastic state at temperatures below its decomposition point. Great differences may be seen in the properties of polymers even if the differences in the macromolecular structures are not great at first glance. Thus, stereoregular polystyrene is a crystalline substance with a melting point of about 235°C, whereas its nonstereoregular (atactic) analogue is completely incapable of crystallizing and softens at about 80°C.
Polymers may enter into the following main types of reactions: (1) the formation of chemical bonds between the macromolecules (crosslinking), for example, in the vulcanization of rubbers and tanning of hides; (2) decomposition of the macromolecules into separate, shorter fragments (degradation); (3) reactions of the side functional groups of polymers with low-molecular-weight compounds not involving the main chain (polymer-analogue conversions); and (4) intramolecular reactions between the functional groups of a single macromolecule, for example, intramolecular ring closure. Crosslinking often proceeds simultaneously with degradation. An example of polymer-analogue conversion is the saponification of polyvinyl acetate, leading to the formation of polyvinyl alcohol.
The rate of reaction of a polymer with low-molecular-weight substances is often limited by the rate of diffusion of the latter into the polymer phase. This phenomenon is most clearly seen in the case of crosslinked polymers. The rate of reactions of macromolecules with low-molecular-weight substances often depends significantly on the nature and arrangement of the neighboring units relative to the reacting unit. This is also true for intramolecular reactions between functional groups belonging to a single chain.
Some properties of polymers, such as solubility, tendency to viscous flow, and stability, are very sensitive to the action of small quantities of impurities or additives that react with the macromolecules. Thus, one or two crosslinks are sufficient to convert a linear polymer from a soluble to an insoluble substance.
The most important characteristics of polymers are chemical composition, molecular weight and molecular weight distribution, degree of branching and macromolecular flexibility, and stereoregularity. The properties of polymers depend significantly on these characteristics.
Production. Natural polymers are formed in the cells of living organisms during biosynthesis. They may be isolated from plant and animal raw material by extraction and selective precipitation. Synthetic polymers are produced by polymerization and polycondensation. Carbon-chain polymers are usually produced by polymerization of monomers with one or more carbon-carbon multiple bonds or of monomers containing unstable carbo-cyclic groups (for example, cyclopropane derivatives). Heteropolymers are produced by polycondensation, as well as by polymerization of monomers containing carbon-element multiple bonds (for example, C=0, G≡N, and N=C=0) or unstable heterocycles (for example, in olefin epoxides and lactams).
Use. Because of their mechanical strength, elasticity, electrical insulation, and other valuable properties, articles made from polymers are used in various branches of industry and in the household. The main types of polymer materials are plastics, rubbers, fibers, lacquers and varnishes, paints, adhesives, and ion-exchange resins. The significance of biopolymers is that they form the basis of all living organisms and participate in virtually all vital processes.
History. The term “polymerism” was introduced by J. Ber-zelius in 1833 to describe a special type of isomerism, in which substances (polymers) of identical composition have various molecular weights—for example, ethylene and butylene; oxygen and ozone. Thus, the meaning of this term did not correspond to the modern concept of polymers. “True” synthetic polymers were unknown at that time.
Several polymers apparently were prepared in the first half of the 19th century. However, chemists at that time usually tried to inhibit polymerization and polycondensation, which led to the “resinification” of the products of the main chemical reaction— that is, in effect, to the formation of polymers (to this day, polymers are often called resins). The first references to synthetic polymers date to 1838 (polyvinylidene chloride) and 1839 (polystyrene).
Polymer chemistry developed in the early 1860’s with A. M. Butlerov’s formulation of the theory of chemical structure. But-lerov studied the relationship between the structure and relative stability of molecules that is evident in polymerization reactions. The subsequent development of the science of polymers (until the late 1920’s) was largely due to an intensive search for methods of synthesizing rubber. Outstanding scientists from many countries (G. Bouchardat, W. Tilden, the German scientist K. Garries, I. L. Kondakov, and S. V. Lebedev) took part in this effort. In the 1930’s, the free-radical mechanism of polymerization was demonstrated by H. Staudinger, and the ionic mechanism by the American scientist F. Whitmore. The work of W. Carothers played a great role in the development of concepts of polycondensation.
Since the early 1920’s, theoretical concepts of the structure of polymers have also been developed. It was assumed at first that such biopolymers as cellulose, starch, rubber, and proteins, as well as some synthetic polymers that have similar properties (for example, polyisoprene), consist of small molecules that have an unusual capacity for aggregation in solution into complexes of a colloid nature because of noncovalent bonds (the “small block” theory). Staudinger was the author of the fundamentally new concept of polymers as substances consisting of macromolecules (species of unusually high molecular weight). The triumph of this concept in the early 1940’s dictated consideration of polymers as a qualitatively new object of chemical and physical investigation.
REFERENCESEntsiklopediia polimerov, vols. 1–3. Moscow, 1972–77.
Strepikheev, A. A., V. A. Derevitskaia, and G. L. Slonimskii. Osnovy khimii vysokomolekuliarnykh soedinenii, 2nd ed. [Moscow, 1967.]
Losev, I. P., and E. B. Trostianskaia. Khimiia sinteticheskikh polimerov, 2nd ed. Moscow, 1964.
Korshak, V. V. Obshchie metody sinteza vysokomolekuliarnykh soedinenii. Moscow, 1953.
Kargin, V. A., and G. L. Slonimskii. Kratkie ocherki po fiziko-khimii polimerov, 2nd ed. Moscow, 1967.
Odian, G. Osnovy khimii polimerov. Moscow, 1974. (Translated from English.)
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V. A. KABANOV
polymer(1) Meaning "many parts," it is a material constructed of smaller molecules of the same substance that form larger molecules. For example, plastic is a synthetic polymer, while protein is a natural polymer. See polymer semiconductor.
(2) (Polymer) A toolkit for designing websites based on Web Components and Google's user interface design. For more information, visit www.polymer-project.org. See Web Components and Material Design.