1.
Polymer
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A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Because of their range of properties, both synthetic and natural polymers play an essential and ubiquitous role in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure, Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. The units composing polymers derive, actually or conceptually, from molecules of low molecular mass. The term was coined in 1833 by Jöns Jacob Berzelius, though with a distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger, Polymers are studied in the fields of biophysics and macromolecular science, and polymer science. Polyisoprene of latex rubber is an example of a polymer. In biological contexts, essentially all biological macromolecules—i. e, proteins, nucleic acids, and polysaccharides—are purely polymeric, or are composed in large part of polymeric components—e. g. Isoprenylated/lipid-modified glycoproteins, where small molecules and oligosaccharide modifications occur on the polyamide backbone of the protein. The simplest theoretical models for polymers are ideal chains, Polymers are of two types, Natural polymeric materials such as shellac, amber, wool, silk and natural rubber have been used for centuries. A variety of natural polymers exist, such as cellulose. Most commonly, the continuously linked backbone of a used for the preparation of plastics consists mainly of carbon atoms. A simple example is polyethylene, whose repeating unit is based on ethylene monomer, however, other structures do exist, for example, elements such as silicon form familiar materials such as silicones, examples being Silly Putty and waterproof plumbing sealant. Oxygen is also present in polymer backbones, such as those of polyethylene glycol, polysaccharides. Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network, during the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization of PET polyester, the distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue. Laboratory synthetic methods are divided into two categories, step-growth polymerization and chain-growth polymerization. However, some methods such as plasma polymerization do not fit neatly into either category
2.
Polypropylene
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An addition polymer made from the monomer propylene, it is rugged and unusually resistant to many chemical solvents, bases and acids. Polypropylene has a relatively low energy surface that means that many common glues will not form adequate joints. Joining of polypropylene is often done using welding processes, in 2013, the global market for polypropylene was about 55 million tonnes. Polypropylene is the worlds second-most widely produced synthetic plastic, after polyethylene, Polypropylene is in many aspects similar to polyethylene, especially in solution behaviour and electrical properties. The additionally present methyl group improves mechanical properties and thermal resistance, the properties of polypropylene depend on the molecular weight and molecular weight distribution, crystallinity, type and proportion of comonomer and the isotacticity. In isotactic polypropylene, for example, the CH3 groups are oriented on one side of the carbon backbone and this creates a greater degree of crystallinity and results in a stiffer material that is more resistant to creep than both atactic polypropylene and polyethylene. The density of PP is between 0.895 and 0.92 g/cm³, therefore, PP is the commodity plastic with the lowest density. With lower density, moldings parts with lower weight and more parts of a mass of plastic can be produced. Unlike polyethylene, crystalline and amorphous regions differ only slightly in their density, however, the density of polyethylene can significantly change with fillers. The Youngs modulus of PP is between 1300 and 1800 N/mm², Polypropylene is normally tough and flexible, especially when copolymerized with ethylene. This allows polypropylene to be used as a plastic, competing with materials such as acrylonitrile butadiene styrene. Polypropylene has good resistance to fatigue, the melting point of polypropylene occurs at a range, so a melting point is determined by finding the highest temperature of a differential scanning calorimetry chart. Perfectly isotactic PP has a point of 171 °C. Commercial isotactic PP has a point that ranges from 160 to 166 °C, depending on atactic material. Syndiotactic PP with a crystallinity of 30% has a point of 130 °C. Below 0 °C, PP becomes brittle, the thermal expansion of polypropylene is very large, but somewhat less than that of polyethylene. Polypropylene is at room temperature resistant to fats and almost all organic solvents, non-oxidizing acids and bases can be stored in containers made of PP. At elevated temperature, PP can be dissolved in nonpolarity solvents such as xylene, due to the tertiary carbon atom PP is chemically less resistant than PE
3.
Polystyrene
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Polystyrene /ˌpɒliˈstaɪriːn/ is a synthetic aromatic polymer made from the monomer styrene. Polystyrene can be solid or foamed, general-purpose polystyrene is clear, hard, and rather brittle. It is an inexpensive resin per unit weight and it is a rather poor barrier to oxygen and water vapor and has a relatively low melting point. Polystyrene is one of the most widely used plastics, the scale of its production being several million tonnes per year, Polystyrene can be naturally transparent, but can be colored with colorants. Uses include protective packaging, containers, lids, bottles, trays, tumblers, as a thermoplastic polymer, polystyrene is in a solid state at room temperature but flows if heated above about 100 °C, its glass transition temperature. It becomes rigid again when cooled and this temperature behavior is exploited for extrusion and also for molding and vacuum forming, since it can be cast into molds with fine detail. Polystyrene is very slow to biodegrade and is therefore a focus of controversy among environmentalists, Polystyrene was discovered in 1839 by Eduard Simon, an apothecary from Berlin. From storax, the resin of the American sweetgum tree Liquidambar styraciflua, he distilled an oily substance, several days later, Simon found that the styrol had thickened into a jelly he dubbed styrol oxide because he presumed an oxidation. By 1845 Jamaican-born chemist John Buddle Blyth and German chemist August Wilhelm von Hofmann showed that the transformation of styrol took place in the absence of oxygen. They called the product metastyrol, analysis showed that it was identical to Simons Styroloxyd. In 1866 Marcelin Berthelot correctly identified the formation of metastyrol/Styroloxyd from styrol as a polymerization process, about 80 years later it was realized that heating of styrol starts a chain reaction that produces macromolecules, following the thesis of German organic chemist Hermann Staudinger. This eventually led to the substance receiving its present name, polystyrene, the company I. G. Farben began manufacturing polystyrene in Ludwigshafen, about 1931, hoping it would be a suitable replacement for die-cast zinc in many applications. Success was achieved when they developed a reactor vessel that extruded polystyrene through a tube and cutter. In 1941, Dow Chemical invented a Styrofoam process, before 1949, the chemical engineer Fritz Stastny developed pre-expanded PS beads by incorporating aliphatic hydrocarbons, such as pentane. These beads are the raw material for moulding parts or extruding sheets, BASF and Stastny applied for a patent that was issued in 1949. The moulding process was demonstrated at the Kunststoff Messe 1952 in Düsseldorf, the crystal structure of isotactic polystyrene was reported by Giulio Natta. In 1954, the Koppers Company in Pittsburgh, Pennsylvania, developed expanded polystyrene foam under the trade name Dylite, in 1960, Dart Container, the largest manufacturer of foam cups, shipped their first order. In 1988, the first U. S. ban of general polystyrene foam was enacted in Berkeley, in chemical terms, polystyrene is a long chain hydrocarbon wherein alternating carbon centers are attached to phenyl groups
4.
Step-growth polymerization
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Many naturally occurring and some synthetic polymers are produced by step-growth polymerization, e. g. polyesters, polyamides, polyurethanes, etc. Due to the nature of the mechanism, a high extent of reaction is required to achieve high molecular weight. The easiest way to visualize the mechanism of a polymerization is a group of people reaching out to hold their hands to form a human chain — each person has two hands. There also is the possibility to have more than two sites on a monomer, In this case branched polymers are produced. Most natural polymers being employed at early stage of society are of condensation type. The synthesis of first truly synthetic polymeric material, Bakelite, was announced by Leo Baekeland in 1907, through a typical step-growth polymerization fashion of phenol and formaldehyde. The pioneer of synthetic science, Wallace H. Carothers. Carothers developed a series of equations to describe the behavior of step-growth polymerization systems which are still known as the Carothers equations today. Collaborating with Paul J. Carothers is also known for his invention of Nylon. Step growth polymerization and condensation polymerization are two different concepts, not always identical, in fact polyurethane polymerizes with addition polymerization, but its reaction mechanism corresponds to a step-growth polymerization. It can exist as fibers and films, the former is used in garments, felts, tire cords, etc. The latter appears in magnetic recording tape and high grade films, polyamide has good balance of properties, high strength, good elasticity and abrasion resistance, good toughness, favorable solvent resistance. The applications of polyamide include, rope, belting, fiber cloths, thread, substitute for metal in bearings, polyurea shows high Tg, fair resistance to greases, oils, and solvents. It can be used in truck bed liners, bridge coating, caulk, polysiloxane are available in a wide range of physical states—from liquids to greases, waxes, resins, and rubbers. Uses of this material are as antifoam and release agents, gaskets, seals, cable and wire insulation, hot liquids and gas conduits and they possess properties like crystalline thermoplasticity, high impact strength, good thermal and oxidative stability. They can be used in machinery, auto-industry, and medical applications, for example, the cockpit canopy of F-22 Raptor is made of high optical quality polycarbonate. Polysulfides have outstanding oil and solvent resistance, good gas impermeability, good resistance to aging, However, it smells bad, and it shows low tensile strength as well as poor heat resistance. It can be used in gasoline hoses, gaskets and places that require solvent resistance, polyether shows good thermoplastic behavior, water solubility, generally good mechanical properties, moderate strength and stiffness
5.
Foam peanut
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Foam peanuts, also known as packing peanuts, styrofoam popcorn or packing noodles are a common loose-fill packaging and cushioning material used to prevent damage to fragile objects during shipping. They are shaped to interlock when compressed and free flow when not compressed and they are roughly the size and shape of an unshelled peanut and commonly made of expanded polystyrene foam. Two to three inches of peanuts are used for cushioning and void filling packaging applications. The original patent was filed for by Robert E. Holden in 1962 and was granted in 1965, polystyrene-based packing peanuts were developed and patented by Tektronix Inc. They were made available circa 1965 by Dow Chemical. Originally made from 100% virgin polystyrene resin, peanuts made from 100% recycled polystyrene have been available since the mid-90s. The color and shape sometimes indicate what it is made of, often green is 70% or more recycled polystyrene, white is 70% or more virgin resin and pink means an antistatic agent has been applied, although there are some variations. The most common shapes are similar to a S, figure 8 or W. Foam peanuts are very light, polystyrene peanuts may be used and reused many times with little or no loss in protection for the product shipped. They may be reused and recycled at many packing and shipping stores, because of their build-up, polystyrene peanuts may also be used for various methods of home insulation, although it is not recommended because they are not flame retardant. In the early 1990s, starch-based packing peanuts were developed as a more environment-friendly alternative, the starch in the peanuts comes from crop-based sources rather than petroleum-based polystyrene, and is non-toxic. One of the first brands of biodegradable peanuts, Biofoam, is made from the grain sorghum, biodegradable foam peanuts have no electrostatic charge, another benefit over polystyrene. Being biodegradable and nontoxic, they are safe for humans. However, they are not produced in conditions, and are not recommended for eating. Also, during the process, the nutritional value is removed from starch-based packing peanuts. This removes edible components, such as sugars, that would otherwise attract rodents and their main drawbacks compared with polystyrene are lower resilience, higher weight, dust creation, and higher price. Starch-based peanuts are soluble in water, and polystyrene peanuts are soluble in acetone, starch based products can be disposed with down the sink, dissolving on contact with the water. The invention of packing peanuts is often attributed to Robert E. Holden, to facilitate the creation of the packing peanut, Holden also invented an apparatus that would help to cut and expand plastic members through a steaming process. By feeding plastic strips through the device, it would create the foamy cellular structure that is the trademark of the classic packing peanuts and it also allowed the plastic to be created in such a way that they would interlock, providing a more secure shipping solution
6.
Carbocation
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A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are methenium CH+3, methanium CH+5, some carbocations may have two or more positive charges, on the same carbon atom or on different atoms, such as the ethylene dication C 2H2+4. Until the early 1970s, all carbocations were called carbonium ions and this nomenclature was proposed by G. A. Olah. One textbook retains the name of carbonium ion for carbenium ion to this day. The history of carbocations dates back to 1891 when G. Merling reported that he added bromine to tropylidene and then heated the product to obtain a crystalline, water-soluble material, C 7H 7Br. He did not suggest a structure for it, however, Doering and this ion is predicted to be aromatic by Hückels rule. In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gives deep-yellow solutions in concentrated sulfuric acid, triphenylmethyl chloride similarly formed orange complexes with aluminium and tin chlorides. In 1902, Adolf von Baeyer recognized the character of the compounds formed. He dubbed the relationship between color and salt formation halochromy, of which green is a prime example. Carbocations are reactive intermediates in organic reactions. This idea, first proposed by Julius Stieglitz in 1899, was developed by Hans Meerwein in his 1922 study of the Wagner–Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction, the E1 reaction, the chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them. The first NMR spectrum of a carbocation in solution was published by Doering et al. in 1958. It was the ion, made by treating hexamethylbenzene with methyl chloride. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960 by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C, the NMR spectrum established that it was non-classically bridged. In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a species on dissolving tert-butyl fluoride in magic acid. The NMR of the norbornyl cation was first reported by Schleyer et al. the charged carbon atom in a carbocation is a sextet, i. e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability. Therefore, carbocations are often reactive, seeking to fill the octet of electrons as well as regain a neutral charge
7.
Synthetic rubber
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A synthetic rubber is any artificial elastomer. These are mainly polymers synthesised from petroleum byproducts, about 15 billion kilograms of rubbers are produced annually, and of that amount two thirds are synthetic. Global revenues generated with synthetic rubbers are likely to rise to approximately US$56 billion in 2020, synthetic rubber, like natural rubber, has uses in the automotive industry for tires, door and window profiles, hoses, belts, matting, and flooring. The expanded use of vehicles, and particularly motor vehicle tires, starting in the 1890s. In 1909, a team headed by Fritz Hofmann, working at the Bayer laboratory in Elberfeld, Germany, succeeded in polymerizing Isoprene, the first rubber polymer synthesized from butadiene was created in 1910 by the Russian scientist Sergei Vasiljevich Lebedev. This form of synthetic rubber provided the basis for the first large-scale commercial production by the tsarist empire and this early form of synthetic rubber was again replaced with natural rubber after the war ended, but investigations of synthetic rubber continued. Russian American Ivan Ostromislensky who moved to New York in 1922 did significant early research on synthetic rubber, political problems that resulted from great fluctuations in the cost of natural rubber led to the enactment of the Stevenson Act in 1921. By 1925 the price of rubber had increased to the point that many companies were exploring methods of producing synthetic rubber to compete with natural rubber. K. Neoprene is highly resistant to heat and chemicals such as oil and gasoline, the company Thiokol applied their name to a competing type of rubber based on ethylene dichloride which was commercially available in 1930. The first rubber plant in Europe SK-1 was established by Sergei Lebedev in Yaroslavl under Joseph Stalins First Five-Year Plan on July 7,1932, in 1935, German chemists synthesized the first of a series of synthetic rubbers known as Buna rubbers. These were copolymers, meaning the polymers were made up from two monomers in alternating sequence, other brands included Koroseal, which Waldo Semon developed in 1935, and Sovprene, which Russian researchers created in 1940. B. F. Goodrich Company scientist Waldo Semon developed a new, Ameripol made synthetic rubber production much more cost effective, helping to meet the United States needs during World War II. The production of rubber in the United States expanded greatly during World War II. Military trucks needed rubber for tires, and rubber was used in almost every other war machine, the U. S. government launched a major effort to improve synthetic rubber production. A large team of chemists from many institutions were involved, including Calvin Souther Fuller of Bell Labs, the rubber designated GRS, a copolymer of butadiene and styrene, was the basis for U. S. synthetic rubber production during World War II. By 1944, a total of 50 factories were manufacturing it and it still represents about half of total world production. The Ferrara, Italy, synthetic rubber factory was bombed August 23,1944, three other synthetic rubber facilities were at Ludwigshafen/Oppau, Hanover/Limmer, and Leverkusen. A synthetic rubber plant at Oświęcim in Nazi-occupied Poland, was construction on March 5,1944
8.
Resin identification code
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It was developed originally by the Society of the Plastics Industry in 1988, but has been administered by ASTM International since 2008. The Society of the Plastics Industry introduced the Resin Identification Code system in 1988 as a number of communities were implementing recycling programs. Plastics must be recycled separately, with materials, in order to preserve the material’s value. In its original form, the used as part of the RIC consisted of arrows that cycle clockwise to form a triangle that encloses a number. When a number is omitted, the arrows arranged in a form the universal Recycling Symbol. In 2008, ASTM International took over the administration of the RIC system, in 2013 this standard was revised to change the graphic marking symbol of the RIC from the chasing arrows of the Recycling Symbol to a solid triangle instead. Since its introduction, many have used the RIC as a signifier of recyclability, when many plastics recycling programs were first being implemented in communities across the United States, only plastics with RIC Codes 1 and 2 were accepted to be recycled. The list of acceptable plastic items has grown since then, and in some areas municipal recycling programs can collect and successfully recycle most plastic products regardless of their RIC Code. This has led some communities to instruct residents to refer to the form of packaging when determining what to include in a recycling bin. Modifications to the RIC are currently being discussed and developed by ASTMs D20.95 subcommittee on recycled plastics. In the U. S. the Sustainable Packaging Coalition has also created a How2Recycle label in an effort to replace the RIC with that more closely with how the public currently uses the RIC. Rather than indicating what type of plastic resin a product is made out of, the How2Recycle labels also encourage consumers to check with local facilities to see what plastics each municipal recycling facility can accept. Your Recycling Quandaries Information from Co-op America about what happens when plastic is recycled. Resin Codes from the American Chemistry Council
9.
Radical polymerization
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Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules, following its generation, the initiating free radical adds monomer units, thereby growing the polymer chain. Free radical polymerization is a key route for obtaining a wide variety of different polymers. In 2001,40 billion of the 110 billion pounds of polymers produced in the United States were produced by radical polymerization. Free radical polymerization is a type of chain growth polymerization, along with anionic, cationic, initiation is the first step of the polymerization process. During initiation, a center is created from which a polymer chain is generated. Not all monomers are susceptible to all types of initiators, Radical initiation works best on the carbon-carbon double bond of vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones. In the first step, one or two radicals are created from the initiating molecules, in the second step, radicals are transferred from the initiator molecules to the monomer units present. Several choices are available for these initiators, thermal decomposition, The initiator is heated until a bond is homolytically cleaved, producing two radicals. This method is used most often with organic peroxides or azo compounds, photolysis, Radiation cleaves a bond homolytically, producing two radicals. This method is used most often with metal iodides, metal alkyls, photoinitiation can also occur by bi-molecular H-abstraction when the radical is in its lowest triplet excited state. An acceptable photoinitiator system should fulfill the requirements, High absorptivity in the 300-400 nm range. Efficient generation of radicals capable of attacking the double bond of vinyl monomers. Adequate solubility in the binder system, should not impart yellowing or unpleasant odors to the cured material. The photoinitiator and any byproducts resulting from its use should be non-toxic, redox reactions, Reduction of hydrogen peroxide or an alkyl hydrogen peroxide by iron. Other reductants such as Cr2+, V2+, Ti3+, Co2+, persulfates, The dissociation of a persulfate in the aqueous phase. This method is useful in emulsion polymerizations in which the radical diffuses into a hydrophobic monomer-containing droplet, ionizing radiation, α-, β-, γ-, or x-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical. Electrochemical, Electrolysis of a solution containing both monomer and electrolyte, a monomer molecule will receive an electron at the cathode to become a radical anion, and a monomer molecule will give up an electron at the anode to form a radical cation
10.
Cationic polymerization
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Cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers charge to a monomer which then becomes reactive. This reactive monomer goes on to react similarly with other monomers to form a polymer, the types of monomers necessary for cationic polymerization are limited to olefins with electron-donating substituents and heterocycles. Similar to anionic polymerization reactions, cationic polymerization reactions are very sensitive to the type of solvent used, specifically, the ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain. Cationic polymerization is used in the production of polyisobutylene and poly, monomer scope for cationic polymerization is limited to two main types, olefins and heterocyclic monomers. Cationic polymerization of both types of monomers occurs only if the reaction is thermally favorable. In the case of olefins, this is due to isomerization of the double bond, for heterocycles. Monomers for cationic polymerization are nucleophilic and form a stable cation upon polymerization, cationic polymerization of olefin monomers occurs with olefins that contain electron-donating substituents. These electron-donating groups make the olefin nucleophilic enough to attack electrophilic initiators or growing polymer chains, at the same time, these electron-donating groups attached to the monomer must be able to stabilize the resulting cationic charge for further polymerization. Some reactive olefin monomers are shown below in order of decreasing reactivity, note, however, that the reactivity of the carbenium ion formed is the opposite of the monomer reactivity. Heterocyclic monomers that are cationically polymerized are lactones, lactams, upon addition of an initiator, cyclic monomers go on to form linear polymers. The reactivity of heterocyclic monomers depends on their ring strain, monomers with large ring strain, such as oxirane, are more reactive than 1, 3-dioxepane which has considerably less ring strain. Rings that are six-membered and larger are less likely to due to lower ring strain. Initiation is the first step in cationic polymerization, during initiation, a carbenium ion is generated from which the polymer chain is made. The counterion should be non-nucleophilic, otherwise the reaction is terminated instantaneously, there are a variety of initiators available for cationic polymerization, and some of them require a coinitiator to generate the needed cationic species. Strong protic acids can be used to form a cationic initiating species, high concentrations of the acid are needed in order to produce sufficient quantities of the cationic species. The counterion produced must be weakly nucleophilic so as to prevent early termination due to combination with the protonated olefin, common acids used are phosphoric, sulfuric, fluro-, and triflic acids. Only low molecular weight polymers are formed with these initiators, Lewis acids are the most common compounds used for initiation of cationic polymerization. The more popular Lewis acids are SnCl4, AlCl3, BF3, although these Lewis acids alone are able to induce polymerization, the reaction occurs much faster with a suitable cation source
11.
Living polymerization
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In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways, chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the chains grow at a more constant rate than seen in traditional chain polymerization. Living polymerization is a method for synthesizing block copolymers since the polymer can be synthesized in stages. Additional advantages are predetermined molar mass and control over end-groups, Living polymerization is desirable because it offers precision and control in macromolecular synthesis. This is important since many of the properties of polymers result from their microstructure. While these polymerization reactions are similar, there is a distinct difference in the definitions of these two reactions. However, this distinction is still up for debate in the literature, after initial addition of monomer to the initiator system, the viscosity would increase, but eventually cease after depletion of monomer concentration. However, he found that addition of more monomer caused an increase in viscosity, indicating growth of the polymer chain and this was a major step in polymer chemistry, since control over when the polymer was quenched, or terminated, was generally not a controlled step. With this discovery, the list of potential applications expanded dramatically, today, living polymerizations are used widely in the development of various types of polymers or plastics. This is because chemists now have control of the chemical makeup of the polymer and, thus. This level of control rarely exists in non-living polymerization reactions, leaving this method as the synthetic route if attainable. The most common types of living polymerization reactions are anionic, cationic, free-radical, or ring-opening in nature, Fast Rate of Initiation Another key characteristic is that the rate of initiation must be much faster than the rate of chain propagation. This would allow all of the species to form before chain propagation begins so all of the chains grow at the same rate. Low Dispersity Dispersity or polydispersity index is an indication of the broadness in the distribution of polymer chains, Living polymers tend to have low Đ due to the absence of chain termination pathways as well as the rate of initiation being much faster than the rate of propagation. If chain termination are present then chains will die or become inactive at various times during the polymerization which leads to polymer chains with varying xn. As stated in the section, if the rate of propagation is much slower than the rate of initiation then all of the initiators will form the active species before the onset of propagation. When both of these characteristics are considered it becomes apparent that the concentration of active chains, that is those undergoing polymerization, predictable Molecular Weight Since termination and chain transfer are absent in living polymerization then each initiator that generates an active species will be responsible for one chain
12.
Radical (chemistry)
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In chemistry, a radical is an atom, molecule, or ion that has an unpaired valence electron. Most radicals are reasonably stable only at low concentrations in inert media or in a vacuum. A notable example of a radical is the hydroxyl radical. Two other examples are triplet oxygen and triplet carbene which have two unpaired electrons, free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules. The latter can be affected by any process that puts energy into the parent molecule, such as ionizing radiation, heat, electrical discharges, electrolysis. Radicals are intermediate stages in many chemical reactions, free radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes. In living organisms, the free radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and they also play a key role in the intermediary metabolism of various biological compounds. Such radicals can even be messengers in a process dubbed redox signaling, a radical may be trapped within a solvent cage or be otherwise bound. The qualifier free was then needed to specify the unbound case, following recent nomenclature revisions, a part of a larger molecule is now called a functional group or substituent, and radical now implies free. However, the old nomenclature may still appear in some books, the term radical was already in use when the now obsolete radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase radical in 1785 and the phrase was employed by Antoine Lavoisier in 1789 in his Traité Élémentaire de Chimie, a radical was then identified as the root base of certain acids. Historically, the radical in radical theory was also used for bound parts of the molecule. These are now called functional groups, for example, methyl alcohol was described as consisting of a methyl radical and a hydroxyl radical. In a modern context the first organic free radical identified was triphenylmethyl radical and this species was discovered by Moses Gomberg in 1900 at the University of Michigan USA. In 1933 Morris Kharash and Frank Mayo proposed that free radicals were responsible for anti-Markovnikov addition of hydrogen bromide to allyl bromide. It should be noted that the electron of the breaking bond also moves to pair up with the attacking radical electron. Free radicals also take part in addition and radical substitution as reactive intermediates. Chain reactions involving free radicals can usually be divided into three distinct processes and these are initiation, propagation, and termination
