Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determine the position of each atom. Molecular geometry influences several properties of a substance including its reactivity, phase of matter, color and biological activity; the angles between bonds that an atom forms depend only weakly on the rest of molecule, i.e. they can be understood as local and hence transferable properties. The molecular geometry can be determined by diffraction methods. IR, microwave and Raman spectroscopy can give information about the molecule geometry from the details of the vibrational and rotational absorbance detected by these techniques. X-ray crystallography, neutron diffraction and electron diffraction can give molecular structure for crystalline solids based on the distance between nuclei and concentration of electron density.
Gas electron diffraction can be used for small molecules in the gas phase. NMR and FRET methods can be used to determine complementary information including relative distances, dihedral angles and connectivity. Molecular geometries are best determined at low temperature because at higher temperatures the molecular structure is averaged over more accessible geometries. Larger molecules exist in multiple stable geometries that are close in energy on the potential energy surface. Geometries can be computed by ab initio quantum chemistry methods to high accuracy; the molecular geometry can be different as a solid, in solution, as a gas. The position of each atom is determined by the nature of the chemical bonds by which it is connected to its neighboring atoms; the molecular geometry can be described by the positions of these atoms in space, evoking bond lengths of two joined atoms, bond angles of three connected atoms, torsion angles of three consecutive bonds. Since the motions of the atoms in a molecule are determined by quantum mechanics, one must define "motion" in a quantum mechanical way.
The overall quantum mechanical motions translation and rotation hardly change the geometry of the molecule. In addition to translation and rotation, a third type of motion is molecular vibration, which corresponds to internal motions of the atoms such as bond stretching and bond angle variation; the molecular vibrations are harmonic, the atoms oscillate about their equilibrium positions at the absolute zero of temperature. At absolute zero all atoms are in their vibrational ground state and show zero point quantum mechanical motion, so that the wavefunction of a single vibrational mode is not a sharp peak, but an exponential of finite width. At higher temperatures the vibrational modes may be thermally excited, but they oscillate still around the recognizable geometry of the molecule. To get a feeling for the probability that the vibration of molecule may be thermally excited, we inspect the Boltzmann factor β ≡ exp , where Δ E is the excitation energy of the vibrational mode, k the Boltzmann constant and T the absolute temperature.
At 298 K, typical values for the Boltzmann factor β are: β = 0.089 for ΔE = 500 cm−1. When an excitation energy is 500 cm−1 about 8.9 percent of the molecules are thermally excited at room temperature. To put this in perspective: the lowest excitation vibrational energy in water is the bending mode. Thus, at room temperature less than 0.07 percent of all the molecules of a given amount of water will vibrate faster than at absolute zero. As stated above, rotation hardly influences the molecular geometry. But, as a quantum mechanical motion, it is thermally excited at low temperatures. From a classical point of view it can be stated that at higher temperatures more molecules will rotate faster, which implies that they have higher angular velocity and angular momentum. In quantum mechanical language: more eigenstates of higher angular momentum become thermally populated with rising temperatures. Typical rotational excitation energies are on the order of a few cm−1; the results of many spectroscopic experiments are broadened because they involve an averaging over rotational states.
It is difficult to extract geometries from spectra at high temperatures, because the number of rotational states probed in the experimental averaging increases with increasing temperature. Thus, many spectroscopic observations can only be expected to yield reliable molecular geometries at temperatures close to absolute zero, because at higher temperatures too many higher rotational states are thermally populated. Molecules, by definition, are most held together with covalent bonds involving single, and/or triple bonds, where a "bond
Dimethyl ether is the organic compound with the formula CH3OCH3, simplified to C2H6O. The simplest ether, it is a colorless gas, a useful precursor to other organic compounds and an aerosol propellant, being demonstrated for use in a variety of fuel applications, it is an isomer of ethanol. 50,000 tons were produced in 1985 in Western Europe by dehydration of methanol: 2 CH3OH → 2O + H2OThe required methanol is obtained from synthesis gas. Other possible improvements call for a dual catalyst system that permits both methanol synthesis and dehydration in the same process unit, with no methanol isolation and purification. Both the one-step and two-step processes above are commercially available; the two-step process is simple and start-up costs are low. A one-step liquid-phase process is in development. Dimethyl ether is a synthetic second generation biofuel, which can be produced from lignocellulosic biomass; the EU is considering BioDME in its potential biofuel mix in 2030. The Volvo Group is the coordinator for the European Community Seventh Framework Programme project BioDME where Chemrec's BioDME pilot plant is based on black liquor gasification in Piteå, Sweden.
The largest use of dimethyl ether is as the feedstock for the production of the methylating agent, dimethyl sulfate, which entails its reaction with sulfur trioxide: CH3OCH3 + SO3 → 2SO4Dimethyl ether can be converted into acetic acid using carbonylation technology related to the Monsanto acetic acid process: 2O + 2 CO + H2O → 2 CH3CO2H Dimethyl ether is a low-temperature solvent and extraction agent, applicable to specialised laboratory procedures. Its usefulness is limited by its low boiling point, but the same property facilitates its removal from reaction mixtures. Dimethyl ether is the precursor to trimethyloxonium tetrafluoroborate. A mixture of dimethyl ether and propane is used in some over-the-counter "freeze spray" products to treat warts, by freezing them. In this role, it has supplanted halocarbon compounds. Dimethyl ether is a component of certain high temperature "MAP-plus" blowtorch gas blends, supplanting the use of methyl acetylene and propadiene mixtures. Dimethyl ether is used as a propellant in aerosol products.
Such products include bug spray and some aerosol glue products. A major use of dimethyl ether is as substitute for propane in LPG used as fuel in household and industry, it is a promising fuel in diesel engines, gas turbines. For diesel engines, an advantage is the high cetane number of 55, compared to that of diesel fuel from petroleum, 40–53. Only moderate modifications are needed to convert a diesel engine to burn dimethyl ether; the simplicity of this short carbon chain compound leads during combustion to low emissions of particulate matter. For these reasons as well as being sulfur-free, dimethyl ether meets the most stringent emission regulations in Europe, U. S. and Japan. At the European Shell Eco Marathon, an unofficial World Championship for mileage, vehicle running on 100% dimethyl ether drove 589 km/liter, fuel equivalent to gasoline with a 50 cm3 displacement 2-stroke engine; as well as winning they beat the old standing record of 306 km/liter, set by the same team in 2007. Dimethyl ether is a refrigerant with ASHRAE refrigerant designation R-E170.
It is used in refrigerant blends with e.g. ammonia, carbon dioxide and propene. Unlike other alkyl ethers, dimethyl ether resists autoxidation. Dimethyl ether is relatively non-toxic, although it is flammable; the International DME Association NOAA site for NFPA 704
Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers trimers, longer oligomers and long chain polymers. Many occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters, polyurethanes, etc. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight; the easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain—each person has two hands. There is the possibility to have more than two reactive sites on a monomer: In this case branched polymers production take place. IUPAC deprecates the term step-growth polymerization and recommends use of the terms polyaddition, when the propagation steps are addition reactions and no molecules are evolved during these steps, polycondensation when the propagation steps are condensation reactions and molecules are evolved during these steps.
Most natural polymers being employed at early stage of human society are of condensation type. The synthesis of first synthetic polymeric material, was announced by Leo Baekeland in 1907, through a typical step-growth polymerization fashion of phenol and formaldehyde; the pioneer of synthetic polymer science, Wallace Carothers, developed a new means of making polyesters through step-growth polymerization in 1930s as a research group leader at DuPont. It was the first reaction designed and carried out with the specific purpose of creating high molecular weight polymer molecules, as well as the first polymerization reaction whose results had been predicted beforehand by scientific theory. Carothers developed a series of mathematic equations to describe the behavior of step-growth polymerization systems which are still known as the Carothers equations today. Collaborating with Paul Flory, a physical chemist, they developed theories that describe more mathematical aspects of step-growth polymerization including kinetics and molecular weight distribution etc.
Carothers is well 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; the distinction between "addition polymerization" and "condensation polymerization" was introduced by Wallace Hume Carothers in 1929, refers to the type of products, respectively: a polymer only a polymer and a molecule with a low molecular weight The distinction between "step-growth polymerization" and "chain-growth polymerization" was introduced by Paul Flory in 1953, refers to the reaction mechanisms, respectively: by functional groups by free-radical or ion This technique is compared with chain-growth polymerization to show its characteristics. Classes of step-growth polymers are: Polyester has high glass transition temperature Tg and high melting point Tm, good mechanical properties to about 175 °C, good resistance to solvent and chemicals.
It can exist as films. The former is used in garments, tire cords, etc; the latter appears in 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, fiber cloths, substitute for metal in bearings, jackets on electrical wire. Polyurethane can exist as elastomers with good abrasion resistance, good resistance to grease and good elasticity, as fibers with excellent rebound, as coatings with good resistance to solvent attack and abrasion and as foams with good strength, good rebound and high impact strength. Polyurea shows high Tg, fair resistance to greases and solvents, it can be used in truck bed liners, bridge coating and decorative designs. Polysiloxane are available in a wide range of physical states—from liquids to greases, waxes and rubbers. Uses of this material are as antifoam and release agents, seals and wire insulation, hot liquids and gas conduits, etc.
Polycarbonates are transparent, self-extinguishing materials. They possess properties like crystalline thermoplasticity, high impact strength, good thermal and oxidative stability, they can be used in machinery, auto-industry, 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 and ozone. However, it smells bad, it shows low tensile strength as well as poor heat resistance, it can be used in gasoline hoses and places that require solvent resistance and gas resistance. Polyether shows good thermoplastic behavior, water solubility good mechanical properties, moderate strength and stiffness, it is applied in sizing for cotton and synthetic fibers, stabilizers for adhesives and film formers in pharmaceuticals. Phenol formaldehyde resin have good heat resistance, dimensional stability as well as good resistance to most solvents, it shows good dielectric properties.
This material is used in molding applications, radio and automotive parts where their good dielectric properties are of use. Some other uses include: impregnating paper, decorative laminates for wall coverings. Poly-Triazole polymers are produced fr
Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus; the higher the associated electronegativity number, the more an atom or a substituent group attracts electrons towards itself. On the most basic level, electronegativity is determined by factors like the nuclear charge and the number/location of other electrons present in the atomic shells; the opposite of electronegativity is electropositivity: a measure of an element's ability to donate electrons. The term "electronegativity" was introduced by Jöns Jacob Berzelius in 1811, though the concept was known before that and was studied by many chemists including Avogadro. In spite of its long history, an accurate scale of electronegativity was not developed until 1932, when Linus Pauling proposed an electronegativity scale, which depends on bond energies, as a development of valence bond theory.
It has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed, although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trends between elements; the most used method of calculation is that proposed by Linus Pauling. This gives a dimensionless quantity referred to as the Pauling scale, on a relative scale running from around 0.7 to 3.98. When other methods of calculation are used, it is conventional to quote the results on a scale that covers the same range of numerical values: this is known as an electronegativity in Pauling units; as it is calculated, electronegativity is not a property of an atom alone, but rather a property of an atom in a molecule. Properties of a free atom include ionization electron affinity, it is to be expected that the electronegativity of an element will vary with its chemical environment, but it is considered to be a transferable property, to say that similar values will be valid in a variety of situations.
Caesium is the least electronegative element in the periodic table, while fluorine is most electronegative. Francium and caesium were both assigned 0.7. However, francium's ionization energy is known to be higher than caesium's, in accordance with the relativistic stabilization of the 7s orbital, this in turn implies that francium is in fact more electronegative than caesium. Pauling first proposed the concept of electronegativity in 1932 as an explanation of the fact that the covalent bond between two different atoms is stronger than would be expected by taking the average of the strengths of the A–A and B–B bonds. According to valence bond theory, of which Pauling was a notable proponent, this "additional stabilization" of the heteronuclear bond is due to the contribution of ionic canonical forms to the bonding; the difference in electronegativity between atoms A and B is given by: | χ A − χ B | = − 1 / 2 E d − E d + E d 2 where the dissociation energies, Ed, of the A–B, A–A and B–B bonds are expressed in electronvolts, the factor −1⁄2 being included to ensure a dimensionless result.
Hence, the difference in Pauling electronegativity between hydrogen and bromine is 0.73 As only differences in electronegativity are defined, it is necessary to choose an arbitrary reference point in order to construct a scale. Hydrogen was chosen as the reference, as it forms covalent bonds with a large variety of elements: its electronegativity was fixed first at 2.1 revised to 2.20. It is necessary to decide which of the two elements is the more electronegative; this is done using "chemical intuition": in the above example, hydrogen bromide dissolves in water to form H+ and Br− ions, so it may be assumed that bromine is more electronegative than hydrogen. However, in principle, since the same electronegativities should be obtained for any two bonding compounds, the data are in fact overdetermined, the signs are unique once a reference point is fixed. To calculate Pauling electronegativity for an element, it
Diethyl ether, or ether, is an organic compound in the ether class with the formula 2O, sometimes abbreviated as Et2O. It is a colorless volatile flammable liquid, it is used as a solvent in laboratories and as a starting fluid for some engines. It was used as a general anesthetic, until non-flammable drugs were developed, such as halothane, it has been used as a recreational drug to cause intoxication. Most diethyl ether is produced as a byproduct of the vapor-phase hydration of ethylene to make ethanol; this process uses solid-supported phosphoric acid catalysts and can be adjusted to make more ether if the need arises. Vapor-phase dehydration of ethanol over some alumina catalysts can give diethyl ether yields of up to 95%. Diethyl ether can be prepared both in laboratories and on an industrial scale by the acid ether synthesis. Ethanol is mixed with a strong acid sulfuric acid, H2SO4; the acid dissociates in the aqueous environment producing hydronium ions, H3O+. A hydrogen ion protonates the electronegative oxygen atom of the ethanol, giving the ethanol molecule a positive charge: CH3CH2OH + H3O+ → CH3CH2OH2+ + H2OA nucleophilic oxygen atom of unprotonated ethanol displaces a water molecule from the protonated ethanol molecule, producing water, a hydrogen ion and diethyl ether.
CH3CH2OH2+ + CH3CH2OH → H2O + H+ + CH3CH2OCH2CH3This reaction must be carried out at temperatures lower than 150 °C in order to ensure that an elimination product is not a product of the reaction. At higher temperatures, ethanol will dehydrate to form ethylene; the reaction to make diethyl ether is reversible, so an equilibrium between reactants and products is achieved. Getting a good yield of ether requires that ether be distilled out of the reaction mixture before it reverts to ethanol, taking advantage of Le Chatelier's principle. Another reaction that can be used for the preparation of ethers is the Williamson ether synthesis, in which an alkoxide performs a nucleophilic substitution upon an alkyl halide, it is important as a solvent in the production of cellulose plastics such as cellulose acetate. Diethyl ether has a high cetane number of 85–96 and is used as a starting fluid, in combination with petroleum distillates for gasoline and Diesel engines because of its high volatility and low flash point.
Ether starting fluid is sold and used in countries with cold climates, as it can help with cold starting an engine at sub-zero temperatures. For the same reason it is used as a component of the fuel mixture for carbureted compression ignition model engines. In this way diethyl ether is similar to one of its precursors, ethanol. Diethyl ether is a common laboratory aprotic solvent, it has limited solubility in water and dissolves 1.5 g/100 g water at 25 °C. This, coupled with its high volatility, makes it ideal for use as the non-polar solvent in liquid-liquid extraction; when used with an aqueous solution, the diethyl ether layer is on top as it has a lower density than the water. It is a common solvent for the Grignard reaction in addition to other reactions involving organometallic reagents. Due to its application in the manufacturing of illicit substances, it is listed in the Table II precursor under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances as well as substances such as acetone and sulfuric acid.
William T. G. Morton participated in a public demonstration of ether anesthesia on October 16, 1846 at the Ether Dome in Boston, Massachusetts. However, Crawford Williamson Long, is now known to have demonstrated its use as a general anesthetic in surgery to officials in Georgia, as early as March 30, 1842, Long publicly demonstrated ether's use as a surgical anesthetic on six occasions before the Boston demonstration. British doctors were aware of the anesthetic properties of ether as early as 1840 where it was prescribed in conjunction with opium. Diethyl ether supplanted the use of chloroform as a general anesthetic due to ether's more favorable therapeutic index, that is, a greater difference between an effective dose and a toxic dose. Diethyl ether increases tracheobronchial secretions. Diethyl ether could be mixed with other anesthetic agents such as chloroform to make C. E. mixture, or chloroform and alcohol to make A. C. E. Mixture. In the 21st century, ether is used; the use of flammable ether was displaced by nonflammable fluorinated hydrocarbon anesthetics.
Halothane was the first such anesthetic developed and other used inhaled anesthetics, such as isoflurane and sevoflurane, are halogenated ethers. Diethyl ether was found to have undesirable side effects, such as post-anesthetic nausea and vomiting. Modern anesthetic agents reduce these side effects. Prior to 2005 it was on the World Health Organization's List of Essential Medicines for use as an anesthetic. Ether was once used in pharmaceutical formulations. A mixture of alcohol and ether, one part of diethyl ether and three parts of ethanol, was known as "Spirit of ether", Hoffman's Anodyne or Hoffman's Drops. In the United States this concoction was removed from the Pharmacopeia at some point prior to June 1917, as a study published by William Procter, Jr. in the American Journal of Pharmacy as early as 1852 showed that there were differences in formulation to be found between commercial manufacturers, between international pharmacopoeia, from Hoffman's original recipe. The anesthetic and intoxicating effects of ether have made it a recreational drug.
Diethyl ether in anesthetic dosage is an inhalant which has a long history
Polyethylene glycol is a polyether compound with many applications, from industrial manufacturing to medicine. PEG is known as polyethylene oxide or polyoxyethylene, depending on its molecular weight; the structure of PEG is expressed as H−n−OH. PEG is the basis of a number of laxatives. Whole bowel irrigation with polyethylene glycol and added electrolytes is used for bowel preparation before surgery or colonoscopy. PEG is used as an excipient in many pharmaceutical products; when attached to various protein medications, polyethylene glycol allows a slowed clearance of the carried protein from the blood. The possibility that PEG could be used to fuse nerve cells is being explored by researchers studying spinal cord injury; because PEG is hydrophilic molecule, it has been used to passivate microscope glass slides for avoiding non-specific sticking of proteins in single-molecule fluorescence studies. Polyethylene glycol is used in a variety of products; the polymer is used as a lubricating coating for various surfaces in aqueous and non-aqueous environments.
Since PEG is a flexible, water-soluble polymer, it can be used to create high osmotic pressures. It is unlikely to have specific interactions with biological chemicals; these properties make PEG one of the most useful molecules for applying osmotic pressure in biochemistry and biomembranes experiments, in particular when using the osmotic stress technique. Polyethylene glycol is commonly used as a polar stationary phase for gas chromatography, as well as a heat transfer fluid in electronic testers. PEG has been used to preserve objects that have been salvaged from underwater, as was the case with the warship Vasa in Stockholm, similar cases, it replaces water in wooden objects, making the wood dimensionally stable and preventing warping or shrinking of the wood when it dries. In addition, PEG is used when working with green wood as a stabilizer, to prevent shrinkage. PEG has been used to preserve the painted colors on Terracotta Warriors unearthed at a UNESCO World Heritage site in China; these painted artifacts were created during the Qin Shi Huang Di dynasty.
Within 15 seconds of the terra-cotta pieces being unearthed during excavations, the lacquer beneath the paint begins to curl after being exposed to the dry Xian air. The paint would subsequently flake off in about four minutes; the German Bavarian State Conservation Office developed a PEG preservative that when applied to unearthed artifacts has aided in preserving the colors painted on the pieces of clay soldiers. PEG is used in mass spectrometry experiments, with its characteristic fragmentation pattern allowing accurate and reproducible tuning. PEG derivatives, such as narrow range ethoxylates, are used as surfactants. PEG can be reacted with an isocyanate to make polyurethane. PEG has been used as the hydrophilic block of amphiphilic block copolymers used to create some polymersomes. PEG is used as a crowding agent in in vitro assays to mimic crowded cellular conditions. PEG is used as a precipitant for plasmid DNA isolation and protein crystallization. X-ray diffraction of protein crystals can reveal the atomic structure of the proteins.
PEG is used to fuse two different types of cells, most B-cells and myelomas in order to create hybridomas. César Milstein and Georges J. F. Köhler originated this technique, which they used for antibody production, winning a Nobel Prize in Physiology or Medicine in 1984. Polymer segments derived from PEG polyols impart flexibility to polyurethanes for applications such as elastomeric fibers and foam cushions. In microbiology, PEG precipitation is used to concentrate viruses. PEG is used to induce complete fusion in liposomes reconstituted in vitro. Gene therapy vectors can be PEG-coated to shield them from inactivation by the immune system and to de-target them from organs where they may build up and have a toxic effect; the size of the PEG polymer has been shown to be important, with larger polymers achieving the best immune protection. PEG is a component of stable nucleic acid lipid particles used to package siRNA for use in vivo. In blood banking, PEG is used as a potentiator to enhance detection of antibodies.
When working with phenol in a laboratory situation, PEG 300 can be used on phenol skin burns to deactivate any residual phenol. In biophysics, polyethylene glycols are the molecules of choice for the functioning ion channels diameter studies, because in aqueous solutions they have a spherical shape and can block ion channel conductance. PEG is the basis of personal lubricants. PEG is used in a number of toothpastes as a dispersant. In this application, it binds water and helps keep xanthan gum uniformly distributed throughout the toothpaste. PEG is under investigation for use in body armor, in tattoos to monitor diabetes. In low-molecular-weight formulations, it is used in Hewlett-Packard designjet printers as an ink solvent and lubricant for the print heads. PEG is one of the main ingredients in paintball fills, because of its thickness and flexibility. However, as early as 2006, some paintball manufacturers began substituting cheaper oil-based alternatives for PEG. PEG is used as an anti-foaming agent in food – its INS number is 1521 or E1521 in the EU.
A nitrate ester-plasticized polyethylene glycol is used in Trident II submarine-launched ballistic missile solid rocket fuel. Dimethyl ethers of PEG are the key ingredient of Selexol, a solvent used by co
A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles 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 and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers, their large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, a tendency to form glasses and semicrystalline structures rather than crystals. The terms polymer and resin are synonymous with plastic; the term "polymer" derives from the Greek word πολύς and μέρος, refers to a molecule whose structure is composed of multiple repeating units, from which originates a characteristic of high relative molecular mass and attendant properties. The units composing polymers derive or conceptually, from molecules of low relative molecular mass.
The term was coined in 1833 by Jöns Jacob Berzelius, though with a definition distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger, who spent the next decade finding experimental evidence for this hypothesis. Polymers are studied in the fields of biophysics and macromolecular science, polymer science. Products arising from the linkage of repeating units by covalent chemical bonds have been the primary focus of polymer science. Polyisoprene of latex rubber is an example of a natural/biological polymer, the polystyrene of styrofoam is an example of a synthetic polymer. In biological contexts all biological macromolecules—i.e. Proteins, nucleic acids, polysaccharides—are purely polymeric, or are composed in large part of polymeric components—e.g. Isoprenylated/lipid-modified glycoproteins, where small lipidic 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: occurring and synthetic or man made. Natural polymeric materials such as hemp, amber, wool and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, the main constituent of wood and paper; the list of synthetic polymers in order of worldwide demand, includes polyethylene, polystyrene, polyvinyl chloride, synthetic rubber, phenol formaldehyde resin, nylon, polyacrylonitrile, PVB, many more. More than 330 million tons of these polymers are made every year. Most the continuously linked backbone of a polymer used for the preparation of plastics consists of carbon atoms. A simple example is polyethylene. Many other structures do exist. Oxygen is commonly present in polymer backbones, such as those of polyethylene glycol, DNA. 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 happens in the polymerization of PET polyester. The monomers are terephthalic acid and ethylene glycol but the repeating unit is —OC—C6H4—COO—CH2—CH2—O—, which corresponds to the combination of the two monomers with the loss of two water molecules; the distinct piece of each monomer, 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; the essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only, such as in polyethylene, whereas in step-growth polymerization chains of monomers may combine with one another directly, such as in polyester. Newer methods, such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out without a catalyst. Laboratory synthesis of biopolymers of proteins, is an area of intensive research. There are three main classes of biopolymers: polysaccharides and polynucleotides.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids; the protein may be modified further following translation in order to provide appropriate structure and functioning. There are other biopolymers such as rubber, suberin and lignin. Occurring polymers such as cotton and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on the market. Many commercially important polymers are synthesized by chemical modification of occurring polymers. Prominent examples inclu