In condensation polymerization , a monomer with an exposed H hydrogen atom binds with a monomer with exposed OH oxygen-hydrogen atoms. The following 4-minute video discusses addition and condensation polymerization. In this video, you will learn how condensation polymers form, some examples of condensation polymers and the uses of these polymers.
Unlike addition polymers where monomers react to form a single product, in a condensation polymerization reaction not only is the polymer formed but also a small molecule is eliminated or lost, normally water. Polyesters and polyamides are the two types of condensation polymer we will look at.
We will first look at polyamides. This is known as the amide link. It is formed when a carboxylic acid reacts with an amine. In the formation of nylon 6,6, we react a molecule with an amine group on each side known as hexane-1, 6-diamine and a molecule with a carboxylic acid at each end, hexanedioic acid. Cellulose, the main component of wood and paper, also is a natural polymer.
Others include the starch molecules made by plants. Yet they have very different properties. Starch will dissolve in water and can be digested. The only difference between these two polymers is how the glucose monomers have been linked together. Living things build proteins — a particular type of polymer — from monomers called amino acids. Although scientists have discovered some different amino acids, animals and plants use only 20 of them to construct their proteins.
In the lab, chemists have many options as they design and construct polymers. Chemists may build artificial polymers from natural ingredients. Or they can use amino acids to build artificial proteins unlike any made by Mother Nature. More often, chemists create polymers from compounds made in the lab.
Polymer structures can have two different components. All start with a basic chain of chemically bonded links. This is sometimes called its backbone. One of these attachments may be as simple as a single atom. Others may be more complex and referred to as pendant groups. Many of the resulting polymers are largely isotactic in configuration, and have high degrees of crystallinity.
Species that have been used to initiate anionic polymerization include alkali metals, alkali amides, alkyl lithiums and various electron sources.
A practical application of anionic polymerization occurs in the use of superglue. When exposed to water, amines or other nucleophiles, a rapid polymerization of this monomer takes place. An efficient and stereospecific catalytic polymerization procedure was developed by Karl Ziegler Germany and Giulio Natta Italy in the 's. Their findings permitted, for the first time, the synthesis of unbranched, high molecular weight polyethylene HDPE , laboratory synthesis of natural rubber from isoprene, and configurational control of polymers from terminal alkenes like propene e.
In the case of ethylene, rapid polymerization occurred at atmospheric pressure and moderate to low temperature, giving a stronger more crystalline product HDPE than that from radical polymerization LDPE.
For this important discovery these chemists received the Nobel Prize in chemistry. Ziegler-Natta catalysts are prepared by reacting certain transition metal halides with organometallic reagents such as alkyl aluminum, lithium and zinc reagents. The catalyst formed by reaction of triethylaluminum with titanium tetrachloride has been widely studied, but other metals e. The following diagram presents one mechanism for this useful reaction.
Others have been suggested, with changes to accommodate the heterogeneity or homogeneity of the catalyst. Polymerization of propylene through action of the titanium catalyst gives an isotactic product; whereas, a vanadium based catalyst gives a syndiotactic product. The synthesis of macromolecules composed of more than one monomeric repeating unit has been explored as a means of controlling the properties of the resulting material.
In this respect, it is useful to distinguish several ways in which different monomeric units might be incorporated in a polymeric molecule. The following examples refer to a two component system, in which one monomer is designated A and the other B. Statistical Copolymers. Also called random copolymers. Most direct copolymerizations of equimolar mixtures of different monomers give statistical copolymers, or if one monomer is much more reactive a nearly homopolymer of that monomer.
The copolymerization of styrene with methyl methacrylate, for example, proceeds differently depending on the mechanism.
Radical polymerization gives a statistical copolymer. However, the product of cationic polymerization is largely polystyrene, and anionic polymerization favors formation of poly methyl methacrylate.
In cases where the relative reactivities are different, the copolymer composition can sometimes be controlled by continuous introduction of a biased mixture of monomers into the reaction.
Formation of alternating copolymers is favored when the monomers have different polar substituents e. For example, styrene and acrylonitrile copolymerize in a largely alternating fashion. Some Useful Copolymers Monomer A. A terpolymer of acrylonitrile, butadiene and styrene, called ABS rubber, is used for high-impact containers, pipes and gaskets.
Several different techniques for preparing block copolymers have been developed, many of which use condensation reactions next section. At this point, our discussion will be limited to an application of anionic polymerization. In the anionic polymerization of styrene described above , a reactive site remains at the end of the chain until it is quenched. The unquenched polymer has been termed a living polymer , and if additional styrene or a different suitable monomer is added a block polymer will form.
This is illustrated for methyl methacrylate in the following diagram. A large number of important and useful polymeric materials are not formed by chain-growth processes involving reactive species such as radicals, but proceed instead by conventional functional group transformations of polyfunctional reactants. These polymerizations often but not always occur with loss of a small byproduct, such as water, and generally but not always combine two different components in an alternating structure.
The polyester Dacron and the polyamide Nylon 66, shown here, are two examples of synthetic condensation polymers, also known as step-growth polymers. Although polymers of this kind might be considered to be alternating copolymers, the repeating monomeric unit is usually defined as a combined moiety. Formulas for these will be displayed below by clicking on the diagram. Condensation polymers form more slowly than addition polymers, often requiring heat, and they are generally lower in molecular weight.
The terminal functional groups on a chain remain active, so that groups of shorter chains combine into longer chains in the late stages of polymerization. The presence of polar functional groups on the chains often enhances chain-chain attractions, particularly if these involve hydrogen bonding, and thereby crystallinity and tensile strength.
The following examples of condensation polymers are illustrative. Note that for commercial synthesis the carboxylic acid components may actually be employed in the form of derivatives such as simple esters. Also, the polymerization reactions for Nylon 6 and Spandex do not proceed by elimination of water or other small molecules. Nevertheless, the polymer clearly forms by a step-growth process.
Some Condensation Polymers Formula. The high T g and T m values for the amorphous polymer Lexan are consistent with its brilliant transparency and glass-like rigidity. Kevlar and Nomex are extremely tough and resistant materials, which find use in bullet-proof vests and fire resistant clothing. Many polymers, both addition and condensation, are used as fibers The chief methods of spinning synthetic polymers into fibers are from melts or viscous solutions.
Polyesters, polyamides and polyolefins are usually spun from melts, provided the T m is not too high. Polyacrylates suffer thermal degradation and are therefore spun from solution in a volatile solvent. Cold-drawing is an important physical treatment that improves the strength and appearance of these polymer fibers. At temperatures above T g , a thicker than desired fiber can be forcibly stretched to many times its length; and in so doing the polymer chains become untangled, and tend to align in a parallel fashion.
This cold-drawing procedure organizes randomly oriented crystalline domains, and also aligns amorphous domains so they become more crystalline. In these cases, the physically oriented morphology is stabilized and retained in the final product. This contrasts with elastomeric polymers, for which the stretched or aligned morphology is unstable relative to the amorphous random coil morphology.
By clicking on the following diagram , a cartoon of these changes will toggle from one extreme to the other. This cold-drawing treatment may also be used to treat polymer films e. Step-growth polymerization is also used for preparing a class of adhesives and amorphous solids called epoxy resins. Here the covalent bonding occurs by an S N 2 reaction between a nucleophile, usually an amine, and a terminal epoxide.
In the following example, the same bisphenol A intermediate used as a monomer for Lexan serves as a difunctional scaffold to which the epoxide rings are attached. Bisphenol A is prepared by the acid-catalyzed condensation of acetone with phenol. Most of the polymers described above are classified as thermoplastic.
This reflects the fact that above T g they may be shaped or pressed into molds, spun or cast from melts or dissolved in suitable solvents for later fashioning.
Because of their high melting point and poor solubility in most solvents, Kevlar and Nomex proved to be a challenge, but this was eventually solved.
Another group of polymers, characterized by a high degree of cross-linking, resist deformation and solution once their final morphology is achieved. Such polymers are usually prepared in molds that yield the desired object. Because these polymers, once formed, cannot be reshaped by heating, they are called thermosets. Partial formulas for four of these will be shown below by clicking the appropriate button. The initial display is of Bakelite, one of the first completely synthetic plastics to see commercial use circa A natural resinous polymer called lignin has a cross-linked structure similar to bakelite.
Lignin is the amorphous matrix in which the cellulose fibers of wood are oriented. Wood is a natural composite material, nature's equivalent of fiberglass and carbon fiber composites.
A partial structure for lignin is shown here. Historically, many eras were characterized by the materials that were then important to human society e. The 20th century has acquired several labels of this sort, including the nuclear age and the oil age ; however, the best name is likely the plastic age.
During this period no technological advancement, other than the delivery of electrical power to every home, has impacted our lives more than the widespread use of synthetic plastics in our clothes, dishes, construction materials, automobiles, packaging, and toys, to name a few.
The development of materials that we now call plastics began with rayon in , continuing with Bakelite in , polyethylene in , Nylon and Teflon in , polypropylene in , Kevlar in , and is continuing.
The many types of polymers that we lump together as plastics are, in general, inexpensive, light weight, strong, durable and, when desired, flexible. Plastics may be processed by extrusion, injection-moulding, vacuum-forming, and compression, emerging as fibers, thin sheets or objects of a specific shape. Monomers join together to make polymer chains by forming covalent bonds—that is, by sharing electrons. Other bonds then hold the groups of chains together to form a polymer material.
When most people think of polymers, the first things that jump to mind tend to be man made—Aussie banknotes, for instance, or weird seventies PVC raincoats. But there are also many polymers that occur in nature. The starches found in corn and potatoes are polysaccharides sugar polymers. Silk and hair are polymers known as polypeptides. Cellulose, which makes up the cell wall of plants, is another natural polymer. DNA is a naturally occuring polymer.
The first man-made polymers were actually modified versions of these natural polymers. Celluloid, the stuff from which silent-movie film was made, was a plastic created from chemically modified cellulose.
The first completely synthetic polymer that is, made by people through chemical synthesis , invented in the early years of the twentieth century, was Bakelite: a plastic made by reacting phenol and formaldehyde under pressure at high temperatures. It was discovered when its inventor, Leo Baekeland, was trying to find a replacement for shellac, a natural polymer made from the shells of Asian lac beetles. Different combinations of monomers in these long polymer chains result in polymers with different properties more about how and why a bit later on , so polymers can be created depending on what kind of characteristics you need your material to have—strength, durability, flexibility, and so on.
In other words, they can be moulded—using heat, for example. Many plastics are synthesised from hydrocarbon-containing oil or petroleum though not all plastics are: bioplastics , for example, can be made from plants or even bacteria. The process by which oil is turned into plastic typically goes something like this.
First, an oil refinery cracks the oil into small hydrocarbons GLOSSARY hydrocarbons an organic compound made up of only hydrogen and carbon the monomers. Finally, the polymers, in the form of a resin a mass of polymer chains go to a plastics factory, where additives give the plastic the desired properties. In addition polymerisation—you guessed it—monomers are simply added together in a repeating pattern.
This results in no other, additional, substance being created. The other way in which polymers can be created is called condensation polymerisation. In this process, when each monomer is added to the chain, an additional, small molecule—such as water—is created as a by-product. Nylon and polyester are made this way.
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