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76-446: In polymer chemistry , polymerization ( American English ), or polymerisation ( British English ), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them. In chemical compounds , polymerization can occur via a variety of reaction mechanisms that vary in complexity due to

152-441: A digital micromirror device . Polymer chemistry The work of Henri Braconnot in 1777 and the work of Christian Schönbein in 1846 led to the discovery of nitrocellulose , which, when treated with camphor , produced celluloid . Dissolved in ether or acetone , it becomes collodion , which has been used as a wound dressing since the U.S. Civil War . Cellulose acetate was first prepared in 1865. In years 1834-1844

228-424: A free radical , cation , or anion . Once the growth of a chain is initiated by formation of an active center, chain propagation is usually rapid by addition of a sequence of monomers. Long chains are formed from the beginning of the reaction. Chain-growth polymerization (or addition polymerization) involves the linking together of unsaturated monomers, especially containing carbon-carbon double bonds . The pi-bond

304-408: 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

380-420: A radical , also known as a free radical , is an atom , molecule , or ion that has at least one unpaired valence electron . With some exceptions, these unpaired electrons make radicals highly chemically reactive . Many radicals spontaneously dimerize . Most organic radicals have short lifetimes. A notable example of a radical is the hydroxyl radical (HO · ), a molecule that has one unpaired electron on

456-550: A Nobel Prize for their discovery of catalysts for the polymerization of alkenes . Alan J. Heeger , Alan MacDiarmid , and Hideki Shirakawa were awarded the 2000 Nobel Prize in Chemistry for the development of polyacetylene and related conductive polymers. Polyacetylene itself did not find practical applications, but organic light-emitting diodes (OLEDs) emerged as one application of conducting polymers. Teaching and research programs in polymer chemistry were introduced in

532-460: A group that is instead electron-withdrawing, the SOMO then interacts with the empty π* orbital. There are no electrons occupying the higher energy orbital formed, while a new SOMO forms that is lower in energy. This results in a lower energy and higher stability of the radical species. Both donating groups and withdrawing groups stabilize radicals. Another well-known albeit weaker form of delocalization

608-581: 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. Radicals are either (1) formed from spin-paired molecules or (2) from other radicals. Radicals are formed from spin-paired molecules through homolysis of weak bonds or electron transfer, also known as reduction. Radicals are formed from other radicals through substitution, addition , and elimination reactions. Homolysis makes two new radicals from

684-490: A key role in these defense mechanisms. These are often the three vitamins, vitamin A , vitamin C and vitamin E and polyphenol antioxidants . Furthermore, there is good evidence indicating that bilirubin and uric acid can act as antioxidants to help neutralize certain radicals. Bilirubin comes from the breakdown of red blood cells ' contents, while uric acid is a breakdown product of purines . Too much bilirubin, though, can lead to jaundice , which could eventually damage

760-492: A lifetime that is less than a few nanoseconds. To avoid confusion, particularly for carbon-centered radicals, Griller and Ingold introduced the following definitions: "Stabilized should be used to describe a carbon-centered radical, R · , when the R−H bond strength is weaker than the appropriate C−H bond of alkane." "Persistent should be used to describe a radical that has a lifetime that is significantly greater than methyl [radical] under

836-413: A new lower-energy filled bonding-orbital and a singly-filled new SOMO, higher in energy than the original. While the energy of the unpaired electron has increased, the decrease in energy of the lone pair forming the new bonding orbital outweighs the increase in energy of the new SOMO, resulting in a net decrease of the energy of the molecule. Therefore, electron-donating groups help stabilize radicals. With

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912-464: A normal organic compound. Organic radicals are inherently electron deficient thus the greater the electronegativity of the atom on which the unpaired electron resides the less stable the radical. Between carbon, nitrogen, and oxygen, for example, carbon is the most stable and oxygen the least stable. Electronegativity also factors into the stability of carbon atoms of different hybridizations. Greater s-character correlates to higher electronegativity of

988-456: A photosensitizer. Typical chemical transformations with this singlet dioxygen species involve, among others, conversion of cellulosic biowaste into new poylmethine dyes. In chemical equations, radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows: Radical reaction mechanisms use single-headed arrows to depict the movement of single electrons: The homolytic cleavage of

1064-430: A prevalent radical, O 2 reacts with many organic compounds to generate radicals together with the hydroperoxide radical. Drying oils and alkyd paints harden due to radical crosslinking initiated by oxygen from the atmosphere. The most common radical in the lower atmosphere is molecular dioxygen. Photodissociation of source molecules produces other radicals. In the lower atmosphere, important radical are produced by

1140-824: A series of 13-hydroxyoctadecadienoic acid and 9-hydroxyoctadecadienoic acid products that serve as signaling molecules that may trigger responses that counter the tissue injury which caused their formation. ROS attacks other polyunsaturated fatty acids, e.g. arachidonic acid and docosahexaenoic acid , to produce a similar series of signaling products. Reactive oxygen species are also used in controlled reactions involving singlet dioxygen 1 O 2 {\displaystyle {}^{1}\mathrm {O} _{2}} known as type II photooxygenation reactions after Dexter energy transfer ( triplet-triplet annihilation ) from natural triplet dioxygen 3 O 2 {\displaystyle {}^{3}\mathrm {O} _{2}} and triplet excited state of

1216-540: A spin-paired molecule by breaking a covalent bond, leaving each of the fragments with one of the electrons in the bond. Because breaking a chemical bond requires energy, homolysis occurs under the addition of heat or light. The bond dissociation energy associated with homolysis depends on the stability of a given compound, and some weak bonds are able to homolyze at relatively lower temperatures. Some homolysis reactions are particularly important because they serve as an initiator for other radical reactions. One such example

1292-400: A very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (i.e., >95%). Solid state polymerization to afford polyamides (e.g., nylons) is an example of step-growth polymerization. In chain-growth (or chain) polymerization, the only chain-extension reaction step is the addition of a monomer to a growing chain with an active center such as

1368-444: Is hyperconjugation . In radical chemistry, radicals are stabilized by hyperconjugation with adjacent alkyl groups. The donation of sigma (σ) C−H bonds into the partially empty radical orbitals helps to differentiate the stabilities of radicals on tertiary, secondary, and primary carbons. Tertiary carbon radicals have three σ C-H bonds that donate, secondary radicals only two, and primary radicals only one. Therefore, tertiary radicals are

1444-409: Is α-tocopherol ( vitamin E ). The tocopherol radical itself is insufficiently stable for isolation, but the parent molecule is a highly effective hydrogen-atom donor. The C−H bond is weakened in triphenylmethyl (trityl) derivatives. A large variety of inorganic radicals are stable and in fact isolable. Examples include most first-row transition metal complexes. With regard to main group radicals,

1520-498: Is an exceptionally reactive electrophile it allows nucleophilic addition of hemiacetal intermediates, which are in general short-lived and relatively unstable "mid-stage" compounds that react with other non-polar molecules present to form more stable polymeric compounds. Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very hazardous. This phenomenon is known as autoacceleration , and can cause fires and explosions. Step-growth and chain-growth are

1596-456: Is an important example of a stable diradical. Singlet oxygen , the lowest-energy non-radical state of dioxygen, is less stable than the diradical due to Hund's rule of maximum multiplicity . The relative stability of the oxygen diradical is primarily due to the spin-forbidden nature of the triplet-singlet transition required for it to grab electrons, i.e., " oxidize ". The diradical state of oxygen also results in its paramagnetic character, which

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1672-541: Is an important source of radicals (see eq. 1 below). These reactions give the chlorine radical, Cl , which catalyzes the conversion of ozone to O 2 , thus facilitating ozone depletion ( eq. 2.2 – eq. 2.4 below). Such reactions cause the depletion of the ozone layer , especially since the chlorine radical is free to engage in another reaction chain; consequently, the use of chlorofluorocarbons as refrigerants has been restricted. Radicals play important roles in biology. Many of these are necessary for life, such as

1748-476: Is available. For combustion to occur, the energy barrier between these must be overcome. This barrier can be overcome by heat, requiring high temperatures. The triplet-singlet transition is also " forbidden ". This presents an additional barrier to the reaction. It also means molecular oxygen is relatively unreactive at room temperature except in the presence of a catalytic heavy atom such as iron or copper. Combustion consists of various radical chain reactions that

1824-514: Is conferred to the radical anion when the charge can be delocalized . Examples include alkali metal naphthenides , anthracenides , and ketyls . Hydrogen abstraction generates radicals. To achieve this reaction, the C-H bond of the H-atom donor must be weak, which is rarely the case in organic compounds. Allylic and especiall doubly allylic C-H bonds are prone to abstraction by O 2 . This reaction

1900-400: Is demonstrated by its attraction to an external magnet. Diradicals can also occur in metal-oxo complexes , lending themselves for studies of spin forbidden reactions in transition metal chemistry. Carbenes in their triplet state can be viewed as diradicals centred on the same atom, while these are usually highly reactive persistent carbenes are known, with N-heterocyclic carbenes being

1976-678: Is lost by formation of a new sigma bond. Chain-growth polymerization is involved in the manufacture of polymers such as polyethylene , polypropylene , polyvinyl chloride (PVC), and acrylate . In these cases, the alkenes RCH=CH 2 are converted to high molecular weight alkanes (-RCHCH 2 -) n (R = H, CH 3 , Cl, CO 2 CH 3 ). Other forms of chain growth polymerization include cationic addition polymerization and anionic addition polymerization . A special case of chain-growth polymerization leads to living polymerization . Ziegler–Natta polymerization allows considerable control of polymer branching . Diverse methods are employed to manipulate

2052-591: Is not initiated because each growth step requires the assistance of light. Photopolymerization can be used as a photographic or printing process because polymerization only occurs in regions which have been exposed to light. Unreacted monomer can be removed from unexposed regions, leaving a relief polymeric image. Several forms of 3D printing —including layer-by-layer stereolithography and two-photon absorption 3D photopolymerization —use photopolymerization. Multiphoton polymerization using single pulses have also been demonstrated for fabrication of complex structures using

2128-442: Is the basis of drying oils , such as linoleic acid derivatives. In free-radical additions , a radical adds to a spin-paired substrate. When applied to organic compounds, the reaction usually entails addition to an alkene. This addition generates a new radical, which can add to yet another alkene, etc. This behavior underpins radical polymerization , technology that produces many plastics . Radical elimination can be viewed as

2204-537: Is the basis of the radical chain reaction . The art of polymerization entails the method by which the initiating radical is introduced. For example, methyl methacrylate (MMA) can be polymerized to produce Poly(methyl methacrylate) (PMMA – Plexiglas or Perspex) via a repeating series of radical addition steps: Newer radical polymerization methods are known as living radical polymerization . Variants include reversible addition-fragmentation chain transfer ( RAFT ) and atom transfer radical polymerization ( ATRP ). Being

2280-433: Is the homolysis of halogens, which occurs under light and serves as the driving force for radical halogenation reactions. Another notable reaction is the homolysis of dibenzoyl peroxide, which results in the formation of two benzoyloxy radicals and acts as an initiator for many radical reactions. Classically, radicals form by one-electron reductions . Typically one-electron reduced organic compounds are unstable. Stability

2356-427: The aging process itself. Similarly, the process of mito hormesis suggests that repeated exposure to radicals may extend life span. Because radicals are necessary for life, the body has a number of mechanisms to minimize radical-induced damage and to repair damage that occurs, such as the enzymes superoxide dismutase , catalase , glutathione peroxidase and glutathione reductase . In addition, antioxidants play

Polymerization - Misplaced Pages Continue

2432-789: The functional groups present in the reactants and their inherent steric effects . In more straightforward polymerizations, alkenes form polymers through relatively simple radical reactions ; in contrast, reactions involving substitution at a carbonyl group require more complex synthesis due to the way in which reactants polymerize. As alkenes can polymerize in somewhat straightforward radical reactions, they form useful compounds such as polyethylene and polyvinyl chloride (PVC), which are produced in high tonnages each year due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. In general, polymers such as PVC are referred to as " homopolymers ", as they consist of repeated long chains or structures of

2508-401: The lung . This process promotes the development of emphysema . Oxybenzone has been found to form radicals in sunlight, and therefore may be associated with cell damage as well. This only occurred when it was combined with other ingredients commonly found in sunscreens, like titanium oxide and octyl methoxycinnamate . ROS attack the polyunsaturated fatty acid , linoleic acid , to form

2584-456: The main classes of polymerization reaction mechanisms. The former is often easier to implement but requires precise control of stoichiometry. The latter more reliably affords high molecular-weight polymers, but only applies to certain monomers. In step-growth (or step) polymerization, pairs of reactants, of any lengths, combine at each step to form a longer polymer molecule. The average molar mass increases slowly. Long chains form only late in

2660-539: The 1940s. An Institute for Macromolecular Chemistry was founded in 1940 in Freiburg, Germany under the direction of Staudinger. In America, a Polymer Research Institute (PRI) was established in 1941 by Herman Mark at the Polytechnic Institute of Brooklyn (now Polytechnic Institute of NYU ). Polymers are high molecular mass compounds formed by polymerization of monomers . They are synthesized by

2736-399: The 20th century the word "radical" was used in chemistry to indicate any connected group of atoms, such as a methyl group or a carboxyl , whether it was part of a larger molecule or a molecule on its own. A radical is often known as an R group . The qualifier "free" was then needed to specify the unbound case. Following recent nomenclature revisions, a part of a larger molecule is now called

2812-493: The average length of the polymer, the progress of reactions, and in what ways the polymer branches. Polymers can be classified in many ways. Polymers, strictly speaking, comprise most solid matter: minerals (i.e. most of the Earth's crust) are largely polymers, metals are 3-d polymers, organisms, living and dead, are composed largely of polymers and water. Often polymers are classified according to their origin: Biopolymers are

2888-448: The balanced equations. Radicals are important in combustion , atmospheric chemistry , polymerization , plasma chemistry, biochemistry , and many other chemical processes. A majority of natural products are generated by radical-generating enzymes. In living organisms, the radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and thus blood pressure. They also play

2964-512: The breaking bond is drawn with a "fish-hook" arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow. The second electron of the breaking bond also moves to pair up with the attacking radical electron. Radicals also take part in radical addition and radical substitution as reactive intermediates . Chain reactions involving radicals can usually be divided into three distinct processes. These are initiation , propagation , and termination . Until late in

3040-449: The carbon atom (due to the close proximity of s orbitals to the nucleus), and the greater the electronegativity the less stable a radical. sp-hybridized carbons (50% s-character) form the least stable radicals compared to sp -hybridized carbons (25% s-character) which form the most stable radicals. The delocalization of electrons across the structure of a radical, also known as its ability to form one or more resonance structures, allows for

3116-840: The central nervous system, while too much uric acid causes gout . Reactive oxygen species or ROS are species such as superoxide , hydrogen peroxide , and hydroxyl radical , commonly associated with cell damage. ROS form as a natural by-product of the normal metabolism of oxygen and have important roles in cell signaling. Two important oxygen-centered radicals are superoxide and hydroxyl radical . They derive from molecular oxygen under reducing conditions. However, because of their reactivity, these same radicals can participate in unwanted side reactions resulting in cell damage. Excessive amounts of these radicals can lead to cell injury and death , which may contribute to many diseases such as cancer , stroke , myocardial infarction , diabetes and major disorders. Many forms of cancer are thought to be

Polymerization - Misplaced Pages Continue

3192-911: The degree of branching , by its end-groups , crosslinks , crystallinity and thermal properties such as its glass transition temperature and melting temperature. Polymers in solution have special characteristics with respect to solubility , viscosity , and gelation . Illustrative of the quantitative aspects of polymer chemistry, particular attention is paid to the number-average and weight-average molecular weights M n {\displaystyle M_{n}} and M w {\displaystyle M_{w}} , respectively. The formation and properties of polymers have been rationalized by many theories including Scheutjens–Fleer theory , Flory–Huggins solution theory , Cossee–Arlman mechanism , Polymer field theory , Hoffman Nucleation Theory , Flory–Stockmayer theory , and many others. The study of polymer thermodynamics helps improve

3268-425: The electron deficiency to be spread over several atoms, minimizing instability. Delocalization usually occurs in the presence of electron-donating groups, such as hydroxyl groups (−OH), ethers (−OR), adjacent alkenes, and amines (−NH 2 or −NR), or electron-withdrawing groups, such as C=O or C≡N. Delocalization effects can also be understood using molecular orbital theory as a lens, more specifically, by examining

3344-485: The first synthetic rubber called neoprene in 1931, the first polyester , and went on to invent nylon , a true silk replacement, in 1935. Paul Flory was awarded the Nobel Prize in Chemistry in 1974 for his work on polymer random coil configurations in solution in the 1950s. Stephanie Kwolek developed an aramid , or aromatic nylon named Kevlar , patented in 1966. Karl Ziegler and Giulio Natta received

3420-451: The four methyl groups to impede the way of a reacting molecule so the structure is unstable. The stability of many (or most) organic radicals is not indicated by their isolability but is manifested in their ability to function as donors of H . This property reflects a weakened bond to hydrogen, usually O−H but sometimes N−H or C−H. This behavior is important because these H donors serve as antioxidants in biology and in commerce. Illustrative

3496-661: The gasoline-air mixture, tetraethyl lead was once commonly added to gasoline. This prevents the combustion from initiating in an uncontrolled manner or in unburnt residues ( engine knocking ) or premature ignition ( preignition ). When a hydrocarbon is burned, a large number of different oxygen radicals are involved. Initially, hydroperoxyl radical (HOO ) are formed. These then react further to give organic hydroperoxides that break up into hydroxyl radicals (HO ). Many polymerization reactions are initiated by radicals. Polymerization involves an initial radical adding to non-radical (usually an alkene) to give new radicals. This process

3572-518: The historical definition of radicals contends that the molecules have nonzero electron spin. However, in fields including spectroscopy and astrochemistry , the definition is slightly different. Gerhard Herzberg , who won the Nobel prize for his research into the electron structure and geometry of radicals, suggested a looser definition of free radicals: "any transient (chemically unstable) species (atom, molecule, or ion)". The main point of his suggestion

3648-537: The initiation, propagation, and termination rates during chain polymerization. A related issue is temperature control, also called heat management , during these reactions, which are often highly exothermic. For example, for the polymerization of ethylene, 93.6 kJ of energy are released per mole of monomer. The manner in which polymerization is conducted is a highly evolved technology. Methods include emulsion polymerization , solution polymerization , suspension polymerization , and precipitation polymerization . Although

3724-425: The intracellular killing of bacteria by phagocytic cells such as granulocytes and macrophages . Radicals are involved in cell signalling processes, known as redox signaling . For example, radical attack of linoleic acid produces a series of 13-hydroxyoctadecadienoic acids and 9-hydroxyoctadecadienoic acids , which may act to regulate localized tissue inflammatory and/or healing responses, pain perception, and

3800-513: The intramolecular interaction of the unpaired electron with a donating group's pair of electrons or the empty π* orbital of an electron-withdrawing group in the form of a molecular orbital diagram. The HOMO of a radical is singly-occupied hence the orbital is aptly referred to as the SOMO, or the Singly-Occupied Molecular Orbital. For an electron-donating group, the SOMO interacts with the lower energy lone pair to form

3876-437: The material properties of various polymer-based materials such as polystyrene (styrofoam) and polycarbonate . Common improvements include toughening , improving impact resistance , improving biodegradability , and altering a material's solubility . As polymers get longer and their molecular weight increases, their viscosity tend to increase. Thus, the measured viscosity of polymers can provide valuable information about

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3952-630: The modern chemical sense, as they are permanently bound to each other, and have no unpaired, reactive electrons; however, they can be observed as radicals in mass spectrometry when broken apart by irradiation with energetic electrons. In a modern context the first organic (carbon–containing) radical identified was the triphenylmethyl radical , (C 6 H 5 ) 3 C . This species was discovered by Moses Gomberg in 1900. In 1933 Morris S. Kharasch and Frank Mayo proposed that free radicals were responsible for anti-Markovnikov addition of hydrogen bromide to allyl bromide . In most fields of chemistry,

4028-539: The most abundant radical in the universe is also the most abundant chemical in the universe, H . Most main group radicals are not however isolable , despite their intrinsic stability. Hydrogen radicals for example combine eagerly to form H 2 . Nitric oxide (NO) is well known example of an isolable inorganic radical. Fremy's salt (Potassium nitrosodisulfonate, (KSO 3 ) 2 NO) is a related example. Many thiazyl radicals are known, despite limited extent of π resonance stabilization . Many radicals can be envisioned as

4104-499: The most common example. Triplet carbenes and nitrenes are diradicals. Their chemical properties are distinct from the properties of their singlet analogues. A familiar radical reaction is combustion . The oxygen molecule is a stable diradical , best represented by O–O . Because spins of the electrons are parallel, this molecule is stable. While the ground state of oxygen is this unreactive spin-unpaired ( triplet ) diradical, an extremely reactive spin-paired ( singlet ) state

4180-537: The most stable and primary radicals the least stable. Most simply, the greater the steric hindrance the more difficult it is for reactions to take place, and the radical form is favored by default. For example, compare the hydrogen-abstracted form of N -hydroxypiperidine to the molecule TEMPO . TEMPO, or (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, is too sterically hindered by the additional methyl groups to react making it stable enough to be sold commercially in its radical form. N -Hydroxypiperidine, however, does not have

4256-414: The oxygen atom. Two other examples are triplet oxygen and triplet carbene ( ꞉ CH 2 ) which have two unpaired electrons. Radicals may be generated in a number of ways, but typical methods involve redox reactions , ionizing radiation , heat, electrical discharges, and electrolysis are known to produce radicals. Radicals are intermediates in many chemical reactions, more so than is apparent from

4332-408: The persistency. This distinction is necessary because these two types of stability do not always correlate with each other. For example, benzylic radicals, which are known for their weak benzylic C−H bond strength, are thermodynamically stabilized due to resonance delocalization. However, these radicals are kinetically transient because they can undergo rapid, diffusion-limited dimerization, resulting in

4408-468: The photodissociation of nitrogen dioxide to an oxygen atom and nitric oxide (see eq. 1.1 below), which plays a key role in smog formation—and the photodissociation of ozone to give the excited oxygen atom O(1D) (see eq. 1.2 below). The net and return reactions are also shown ( eq. 1.3 and eq. 1.4 , respectively). In the upper atmosphere, the photodissociation of normally unreactive chlorofluorocarbons (CFCs) by solar ultraviolet radiation

4484-414: The polymer dispersity and molecular weight may be improved, these methods may introduce additional processing requirements to isolate the product from a solvent. Most photopolymerization reactions are chain-growth polymerizations which are initiated by the absorption of visible or ultraviolet light. Photopolymerization can also be a step-growth polymerization. The light may be absorbed either directly by

4560-434: The polymerization process and can be modified by the additive of monomers. The additives of monomers change polymers mechanical property, processability, durability and so on. The simple reactive molecule from which the repeating structural units of a polymer are derived is called a monomer. A polymer can be described in many ways: its degree of polymerisation , molar mass distribution , tacticity , copolymer distribution,

4636-406: The possibility of any covalent molecule exceeding 6,000 daltons. Cellophane was invented in 1908 by Jocques Brandenberger who treated sheets of viscose rayon with acid . The chemist Hermann Staudinger first proposed that polymers consisted of long chains of atoms held together by covalent bonds , which he called macromolecules . His work expanded the chemical understanding of polymers and

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4712-549: The products of breaking of covalent bonds by homolysis . The homolytic bond dissociation energies , usually abbreviated as "Δ H  °" are a measure of bond strength. Splitting H 2 into 2 H , for example, requires a Δ H  ° of +435 kJ/mol , while splitting Cl 2 into two Cl requires a Δ H  ° of +243 kJ/mol. For weak bonds, homolysis can be induced thermally. Strong bonds require high energy photons or even flames to induce homolysis. Diradicals are molecules containing two radical centers. Dioxygen (O 2 )

4788-412: The products of organisms , a thermosetting phenol - formaldehyde resin called Bakelite . Around the same time, Hermann Leuchs reported the synthesis of amino acid N-carboxyanhydrides and their high molecular weight products upon reaction with nucleophiles, but stopped short of referring to these as polymers, possibly due to the strong views espoused by Emil Fischer , his direct supervisor, denying

4864-694: The proliferation of malignant cells. Radical attacks on arachidonic acid and docosahexaenoic acid produce a similar but broader array of signaling products. Radicals may also be involved in Parkinson's disease , senile and drug-induced deafness , schizophrenia , and Alzheimer's . The classic free-radical syndrome, the iron-storage disease hemochromatosis , is typically associated with a constellation of free-radical-related symptoms including movement disorder, psychosis, skin pigmentary melanin abnormalities, deafness, arthritis, and diabetes mellitus. The free-radical theory of aging proposes that radicals underlie

4940-404: The properties of rubber ( polyisoprene ) were found to be greatly improved by heating with sulfur , thus founding the vulcanization process. In 1884 Hilaire de Chardonnet started the first artificial fiber plant based on regenerated cellulose , or viscose rayon , as a substitute for silk , but it was very flammable. In 1907 Leo Baekeland invented the first polymer made independent of

5016-521: The reactant monomer ( direct photopolymerization), or else by a photosensitizer which absorbs the light and then transfers energy to the monomer. In general, only the initiation step differs from that of the ordinary thermal polymerization of the same monomer; subsequent propagation, termination, and chain-transfer steps are unchanged. In step-growth photopolymerization, absorption of light triggers an addition (or condensation) reaction between two comonomers that do not react without light. A propagation cycle

5092-799: The reaction. Step-growth polymers are formed by independent reaction steps between functional groups of monomer units, usually containing heteroatoms such as nitrogen or oxygen. Most step-growth polymers are also classified as condensation polymers , since a small molecule such as water is lost when the polymer chain is lengthened. For example, polyester chains grow by reaction of alcohol and carboxylic acid groups to form ester links with loss of water. However, there are exceptions; for example polyurethanes are step-growth polymers formed from isocyanate and alcohol bifunctional monomers) without loss of water or other volatile molecules, and are classified as addition polymers rather than condensation polymers. Step-growth polymers increase in molecular weight at

5168-516: The result of reactions between radicals and DNA , potentially resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy. Some of the symptoms of aging such as atherosclerosis are also attributed to radical induced oxidation of cholesterol to 7-ketocholesterol. In addition radicals contribute to alcohol -induced liver damage, perhaps more than alcohol itself. Radicals produced by cigarette smoke are implicated in inactivation of alpha 1-antitrypsin in

5244-448: The reverse of radical addition. In radical elimination, an unstable radical compound breaks down into a spin-paired molecule and a new radical compound. Shown below is an example of a radical elimination reaction, where a benzoyloxy radical breaks down into a phenyl radical and a carbon dioxide molecule. The generation and reactivity of organic radicals are dependent on both their thermodynamic stability and kinetic stability, also known as

5320-456: The same condition." While relationships between thermodynamic stability and kinetic persistency is highly case-dependent, organic radicals can be generally stabilized by any or all of these factors: the presence of electronegativity, delocalization, and steric hindrance. The compound 2,2,6,6-tetramethylpiperidinyloxyl illustrates the combination of all three factors. It is a commercially available solid that, aside from being magnetic, behaves like

5396-566: The same monomer unit, whereas polymers that consist of more than one monomer unit are referred to as copolymers (or co-polymers). Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (ca. −80 °C) to form trimers ; molecules consisting of 3 monomer units, which can cyclize to form ring cyclic structures, or undergo further reactions to form tetramers , or 4 monomer-unit compounds. Such small polymers are referred to as oligomers . Generally, because formaldehyde

5472-490: The singlet radical can initiate. The flammability of a given material strongly depends on the concentration of radicals that must be obtained before initiation and propagation reactions dominate leading to combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out. As indicated, promotion of propagation or termination reactions alters flammability. For example, because lead itself deactivates radicals in

5548-444: The structural and functional materials that comprise most of the organic matter in organisms. One major class of biopolymers are proteins , which are derived from amino acids . Polysaccharides , such as cellulose , chitin , and starch , are biopolymers derived from sugars. The poly nucleic acids DNA and RNA are derived from phosphorylated sugars with pendant nucleotides that carry genetic information. Synthetic polymers are

5624-632: The structural materials manifested in plastics , synthetic fibers , paints , building materials , furniture , mechanical parts, and adhesives . Synthetic polymers may be divided into thermoplastic polymers and thermoset plastics . Thermoplastic polymers include polyethylene , teflon , polystyrene , polypropylene , polyester , polyurethane , Poly(methyl methacrylate) , polyvinyl chloride , nylons , and rayon . Thermoset plastics include vulcanized rubber , bakelite , Kevlar , and polyepoxide . Almost all synthetic polymers are derived from petrochemicals . Free radical In chemistry ,

5700-403: Was followed by an expansion of the field of polymer chemistry during which such polymeric materials as neoprene, nylon and polyester were invented. Before Staudinger, polymers were thought to be clusters of small molecules ( colloids ), without definite molecular weights , held together by an unknown force . Staudinger received the Nobel Prize in Chemistry in 1953. Wallace Carothers invented

5776-470: Was then identified as the root base of certain acids (the Latin word "radix" meaning "root"). Historically, the term radical in radical theory was also used for bound parts of the molecule, especially when they remain unchanged in reactions. These are now called functional groups . For example, methyl alcohol was described as consisting of a methyl "radical" and a hydroxyl "radical". Neither are radicals in

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