Level 3: Advanced. Course rewards. Free statement of participation on completion of these courses. Create your free OpenLearn profile. Course content Course content. Introduction to polymers Start this free course now. Free course Introduction to polymers. Figure 19 Geometrical isomerism in polyisoprene.
Cis- polyisoprene in the main polymer occurs in natural rubber, trans -polyisoprene in gutta percha. Previous 2. Next 2. Skip Your course resources Your course resources As you work through this course you will need various resources to help you complete some of the activities. Download Resource. Print Print. Take your learning further Making the decision to study can be a big step, which is why you'll want a trusted University.
OpenLearn Search website Back to top. Our partners OpenLearn works with other organisations by providing free courses and resources that support our mission of opening up educational opportunities to more people in more places. Only monomers having anion stabilizing substituents, such as phenyl, cyano or carbonyl are good substrates for this polymerization technique.
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. They may be colored as desired and reinforced by glass or carbon fibers, and some may be expanded into low density foams. Many modern adhesives involve the formation of a plastic bonding substance. Plastics have replaced an increasing number of natural substances. In the manufacture of piano keys and billiard balls plastics have replaced ivory, assisting the survival of the elephant.
It is noteworthy that a synthetic fiber manufacturing facility occupies a much smaller area of ground than would be needed to produce an equal quantity of natural fibers, such as cotton, wool or silk.
With all these advantages it is not surprising that much of what you see around you is plastic. Indeed, the low cost, light weight, strength and design adaptability of plastics to meet a variety of applications have resulted in strong year after year growth in their production and use, which is likely to continue. Indeed, many plastics are employed in disposable products meant only for a single use.
Successful solutions to technological projects are often achieved by focusing on a limited set of variables that are directly linked to a desired outcome. However, nature often has a way of rewarding such success by exposing unexpected problems generated "outside the box" of the defined project.
In the case of plastics, their advantageous durability and relative low cost have resulted in serious environmental pollution as used items and wrappings are casually discarded and replaced in a never ending cycle. We see this every day on the streets and fields of our neighborhoods, but the problem is far more dire.
Charles Moore, an American oceanographer, in discovered an enormous stew of trash, estimated at nearly million tons, floating in the Pacific Ocean between San Francisco and Hawaii.
The information provided here, and the illustration on the left, come from an article by Susan Casey in BestLife Clock-wise circulation of currents driven by the global wind system and constrained by surrounding continents form a vortex or gyre comparable to a large whirlpool.
The larger North Pacific Subtropical Gyre, referred to as the doldrums, is the convergence zone where plastic and other waste mixes together. Aside from its disgusting aesthetic presence, the garbage patch is representative of serious environmental and health problems. No one knows how long it will take for some of these plastics to biodegrade, or return to their component molecules.
Persistent objects such as six-pack rings and discarded nets trap sea animals. Smaller plastic scraps are mistaken for food by sea birds; and are often found undigested in the gut of dead birds. Nurdles, lentil-size pellets of plastic, found in abundance where plastics are manufactured and distributed, are dispersed by wind throughout the biosphere. They're light enough to blow around like dust and to wash into harbors, storm drains, and creeks. Escaped nurdles and other plastic litter migrate to the ocean gyre largely from land.
At places as remote as Rarotonga, in the Cook Islands they're commonly found mixed with beach sand. Once in the ocean, nurdles may absorb up to a million times the level of any organic pollutants found in surrounding waters. Nurdles in the sea are easily mistaken for fish eggs by creatures that would very much like to have such a snack. Wonder what the ride would be like in a truck going 60 with steel wheels?
So science came to the rescue! A massive government funded effort was mounted in the US that very quickly lead to synthetic rubber. First out of the block was polyisobutylene. It was a pretty good rubber and still is but mainly for the unusual fact that it doesn't pass gas sorry. It's the only commercial polymer that will hold air inside a tire for weeks, even months, at a time. Natural rubber can't do this, so one outcome of the war effort was a solution to the major problem of having to air up your tires every week.
Ok, the war ended, natural rubber was available to the entire world again. Production zoomed!
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