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
In physical and organic chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have shape, or mass. A sample of objects that have an inconsistent size and mass distribution is called non-uniform; the objects can be in any form of chemical dispersion, such as particles in a colloid, droplets in a cloud, crystals in a rock, or polymer macromolecules in a solution or a solid polymer mass. Polymers can be described by molecular mass distribution. IUPAC has deprecated the use of the term polydispersity index, having replaced it with the term dispersity, represented by the symbol Đ which can refer to either molecular mass or degree of polymerization, it can be calculated using the equation ĐM = Mw/Mn, where Mw is the weight-average molar mass and Mn is the number-average molar mass. It can be calculated according to degree of polymerization, where ĐX = Xw/Xn, where Xw is the weight-average degree of polymerization and Xn is the number-average degree of polymerization.
In certain limiting cases where ĐM = ĐX, it is referred to as Đ. IUPAC has deprecated the terms monodisperse, considered to be self-contradictory, polydisperse, considered redundant, preferring the terms uniform and non-uniform instead. A uniform, polymer is composed of molecules of the same mass. Nearly all natural polymers are monodisperse. Synthetic near-monodisperse polymer chains can be made by processes such as anionic polymerization, a method using an anionic catalyst to produce chains that are similar in length; this technique is known as living polymerization. It is used commercially for the production of block copolymers. Monodisperse collections can be created through the use of template-based synthesis, a common method of synthesis in nanotechnology. A polymer material is denoted by the term disperse, or non-uniform, if its chain lengths vary over a wide range of molecular masses; this is characteristic of man-made polymers. Natural organic matter produced by the decomposition of plants and wood debris in soils has a pronounced polydispersed character.
It is the case of humic acids and fulvic acids, natural polyelectrolyte substances having higher and lower molecular weights. Another interpretation of dispersity is explained in the article Dynamic light scattering. In this sense, the dispersity values are in the range from 0 to 1; the dispersity the polydispersity index or heterogeneity index, is a measure of the distribution of molecular mass in a given polymer sample. Đ of a polymer is calculated: P D I = M w / M n, where M w is the weight average molecular weight and M n is the number average molecular weight. M n is more sensitive to molecules of low molecular mass, while M w is more sensitive to molecules of high molecular mass; the dispersity indicates the distribution of individual molecular masses in a batch of polymers. Đ has a value equal to or greater than 1, but as the polymer chains approach uniform chain length, Đ approaches unity. For some natural polymers Đ is taken as unity. Typical dispersities vary based on the mechanism of polymerization and can be affected by a variety of reaction conditions.
In synthetic polymers, it can vary due to reactant ratio, how close the polymerization went to completion, etc. For typical addition polymerization, Đ can range around 5 to 20. For typical step polymerization, most probable values of Đ are around 2 —Carothers' equation limits Đ to values of 2 and below. Living polymerization, a special case of addition polymerization, leads to values close to 1; such is the case in biological polymers, where the dispersity can be close or equal to 1, indicating only one length of polymer is present. The reactor polymerization reactions take place in can affect the dispersity of the resulting polymer. For bulk radical polymerization with low conversion, anionic polymerization, step growth polymerization to high conversion, typical dispersities are in the table below. With respect to batch and plug flow reactors, the dispersities for the different polymerization methods are the same; this is because while batch reactors depend on time of reaction, plug flow reactors depend on distance traveled in the reactor and its length.
Since time and distance are related by velocity, plug flow reactors can be designed to mirror batch reactors by controlling the velocity and length of the reactor. Continuously stirred-tank reactors however have a residence time distribution and cannot mirror batch or plug flow reactors, which can cause a difference in the dispersity of final polymer; the effects of reactor type on dispersity depend on the relative timescales associated with the reactor, with the polymerization type. In conventional bulk free radical polymerization, the dispersity is controlled by the proportion of chains that terminate via combination or disproportionation; the rate of reaction for free radical polymerization is exceedingly quick, due to the reactivity of the
Nylon is a generic designation for a family of synthetic polymers, based on aliphatic or semi-aromatic polyamides. Nylon is a thermoplastic silky material that can be melt-processed into films, or shapes, it is made of repeating units linked by amide links similar to the peptide bonds in proteins. Nylon polymers can be mixed with a wide variety of additives to achieve many different property variations. Nylon polymers have found significant commercial applications in fabric and fibers, in shapes, in films. Nylon was the first commercially successful synthetic thermoplastic polymer. DuPont began its research project in 1927; the first example of nylon was produced using diamines on February 28, 1935, by Wallace Hume Carothers at DuPont's research facility at the DuPont Experimental Station. In response to Carothers' work, Paul Schlack at IG Farben developed nylon 6, a different molecule based on caprolactam, on January 29, 1938. Nylon was first used commercially in a nylon-bristled toothbrush in 1938, followed more famously in women's stockings or "nylons" which were shown at the 1939 New York World's Fair and first sold commercially in 1940.
During World War II all nylon production was diverted to the military for use in parachutes and parachute cord. Wartime uses of nylon and other plastics increased the market for the new materials. DuPont, founded by Éleuthère Irénée du Pont, first produced gunpowder and cellulose-based paints. Following WWI, DuPont produced other chemicals. DuPont began experimenting with the development of cellulose based fibers producing the synthetic fiber rayon. DuPont's experience with rayon was an important precursor to its marketing of nylon. DuPont's invention of nylon spanned an eleven-year period, ranging from the initial research program in polymers in 1927 to its announcement in 1938, shortly before the opening of the 1939 New York World's Fair; the project grew from a new organizational structure at DuPont, suggested by Charles Stine in 1927, in which the chemical department would be composed of several small research teams that would focus on “pioneering research” in chemistry and would “lead to practical applications”.
Harvard instructor Wallace Hume Carothers was hired to direct the polymer research group. He was allowed to focus on pure research, building on and testing the theories of German chemist Hermann Staudinger, he was successful, as research he undertook improved the knowledge of polymers and contributed to science. In the spring of 1930, Carothers and his team had synthesized two new polymers. One was neoprene, a synthetic rubber used during World War II; the other was a white elastic but strong paste that would become nylon. After these discoveries Carothers’ team was made to shift its research from a more pure research approach investigating general polymerization to a more practically-focused goal of finding “one chemical combination that would lend itself to industrial applications”, it wasn't until the beginning of 1935 that a polymer called "polymer 6-6" was produced. The first example of nylon was produced by Wallace Carothers on February 28, 1935, at DuPont's research facility at the DuPont Experimental Station.
It had all the desired properties of strength. However, it required a complex manufacturing process that would become the basis of industrial production in the future. DuPont obtained a patent for the polymer in September 1938, achieved a monopoly of the fiber. Carothers died 16 months before the announcement of nylon, therefore he was never able to see his success; the production of nylon required interdepartmental collaboration between three departments at DuPont: the Department of Chemical Research, the Ammonia Department, the Department of Rayon. Some of the key ingredients of nylon had to be produced using high pressure chemistry, the main area of expertise of the Ammonia Department. Nylon was considered a “godsend to the Ammonia Department”, in financial difficulties; the reactants of nylon soon constituted half of the Ammonia department's sales and helped them come out of the period of the Great Depression by creating jobs and revenue at DuPont. DuPont's nylon project demonstrated the importance of chemical engineering in industry, helped create jobs, furthered the advancement of chemical engineering techniques.
In fact, it developed a chemical plant that provided 1800 jobs and used the latest technologies of the time, which are still used as a model for chemical plants today. The ability to acquire a large number of chemists and engineers was a huge contribution to the success of DuPont's nylon project; the first nylon plant was located at Seaford, beginning commercial production on December 15, 1939. On October 26, 1995, the Seaford plant was designated a National Historic Chemical Landmark by the American Chemical Society. An important part of nylon's popularity stems from DuPont's marketing strategy. DuPont promoted the fiber to increase demand. Nylon's commercial announcement occurred on October 27, 1938, at the final session of the Herald Tribune's yearly "Forum on Current Problems", on the site of the approaching New York City world's fair; the “first man-made organic textile fiber”, derived from “coal and air” and promised to be “as strong as steel, as fine as the spider’s web” was received enthusiastically by the audience, many of them middle-class women, made the headlines of most newspapers.
Nylon was introduced as part of "The world of tomorrow" at the 1939 New York World's Fa
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity can be conceptualized as quantifying the frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a fluid is forced through a tube, it flows more near the tube's axis than near its walls. In such a case, experiments show; this is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity. A fluid that has no resistance to shear stress is known as an inviscid fluid. Zero viscosity is observed only at low temperatures in superfluids. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. A fluid with a high viscosity, such as pitch, may appear to be a solid; the word "viscosity" is derived from the Latin "viscum", meaning mistletoe and a viscous glue made from mistletoe berries.
In materials science and engineering, one is interested in understanding the forces, or stresses, involved in the deformation of a material. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time; these are called. For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation. Although it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow. In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u.
If the speed of the top plate is low enough in steady state the fluid particles move parallel to it, their speed varies from 0 at the bottom to u at the top. Each layer of fluid moves faster than the one just below it, friction between them gives rise to a force resisting their relative motion. In particular, the fluid applies on the top plate a force in the direction opposite to its motion, an equal but opposite force on the bottom plate. An external force is therefore required in order to keep the top plate moving at constant speed. In many fluids, the flow velocity is observed to vary linearly from zero at the bottom to u at the top. Moreover, the magnitude F of the force acting on the top plate is found to be proportional to the speed u and the area A of each plate, inversely proportional to their separation y: F = μ A u y; the proportionality factor μ is the viscosity of the fluid, with units of Pa ⋅ s. The ratio u / y is called the rate of shear deformation or shear velocity, is the derivative of the fluid speed in the direction perpendicular to the plates.
If the velocity does not vary linearly with y the appropriate generalization is τ = μ ∂ u ∂ y, where τ = F / A, ∂ u / ∂ y is the local shear velocity. This expression is referred to as Newton's law of viscosity. In shearing flows with planar symmetry, it is what defines μ, it is a special case of the general definition of viscosity, which can be expressed in coordinate-free form. Use of the Greek letter mu for the viscosity is common among mechanical and chemical engineers, as well as physicists. However, the Greek letter eta is used by chemists and the IUPAC; the viscosity μ is sometimes referred to as the shear viscosity. However, at least one author discourages the use of this terminology, noting that μ can appear in nonshearing flows in addition to shearing flows. In general terms, the viscous stresses in a fluid are defined as those resulting from the relative velocity of different fluid particles; as such, the viscous stresses. If the velocity gradients are small to a first approximation the v
Stoichiometry is the calculation of reactants and products in chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among quantities of reactants and products form a ratio of positive integers; this means that if the amounts of the separate reactants are known the amount of the product can be calculated. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined the amount of the other reactants can be calculated; this is illustrated in the image here, where the balanced equation is: CH4 + 2 O2 → CO2 + 2 H2O. Here, one molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water; this particular chemical equation is an example of complete combustion. Stoichiometry measures these quantitative relationships, is used to determine the amount of products and reactants that are produced or needed in a given reaction.
Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry. In the example above, reaction stoichiometry measures the relationship between the methane and oxygen as they react to form carbon dioxide and water; because of the well known relationship of moles to atomic weights, the ratios that are arrived at by stoichiometry can be used to determine quantities by weight in a reaction described by a balanced equation. This is called composition stoichiometry. Gas stoichiometry deals with reactions involving gases, where the gases are at a known temperature and volume and can be assumed to be ideal gases. For gases, the volume ratio is ideally the same by the ideal gas law, but the mass ratio of a single reaction has to be calculated from the molecular masses of the reactants and products. In practice, due to the existence of isotopes, molar masses are used instead when calculating the mass ratio; the term stoichiometry was first used by Jeremias Benjamin Richter in 1792 when the first volume of Richter's Stoichiometry or the Art of Measuring the Chemical Elements was published.
The term is derived from the Ancient Greek words στοιχεῖον stoicheion "element" and μέτρον metron "measure". In patristic Greek, the word Stoichiometria was used by Nicephorus to refer to the number of line counts of the canonical New Testament and some of the Apocrypha. A stoichiometric amount or stoichiometric ratio of a reagent is the optimum amount or ratio where, assuming that the reaction proceeds to completion: All of the reagent is consumed There is no deficiency of the reagent There is no excess of the reagent. Stoichiometry rests upon the basic laws that help to understand it better, i.e. law of conservation of mass, the law of definite proportions, the law of multiple proportions and the law of reciprocal proportions. In general, chemical reactions combine in definite ratios of chemicals. Since chemical reactions can neither create nor destroy matter, nor transmute one element into another, the amount of each element must be the same throughout the overall reaction. For example, the number of atoms of a given element X on the reactant side must equal the number of atoms of that element on the product side, whether or not all of those atoms are involved in a reaction.
Chemical reactions, as macroscopic unit operations, consist of a large number of elementary reactions, where a single molecule reacts with another molecule. As the reacting molecules consist of a definite set of atoms in an integer ratio, the ratio between reactants in a complete reaction is in integer ratio. A reaction may consume more than one molecule, the stoichiometric number counts this number, defined as positive for products and negative for reactants. Different elements have a different atomic mass, as collections of single atoms, molecules have a definite molar mass, measured with the unit mole. By definition, carbon-12 has a molar mass of 12 g/mol. Thus, to calculate the stoichiometry by mass, the number of molecules required for each reactant is expressed in moles and multiplied by the molar mass of each to give the mass of each reactant per mole of reaction; the mass ratios can be calculated by dividing each by the total in the whole reaction. Elements in their natural state are mixtures of isotopes of differing mass, thus atomic masses and thus molar masses are not integers.
For instance, instead of an exact 14:3 proportion, 17.04 kg of ammonia consists of 14.01 kg of nitrogen and 3 × 1.01 kg of hydrogen, because natural nitrogen includes a small amount of nitrogen-15, natural hydrogen includes hydrogen-2. A stoichiometric reactant is a reactant, consumed in a reaction, as opposed to a catalytic reactant, not consumed in the overall reaction because it reacts in one step and is regenerated in another step. Stoichiometry is not only used to balance chemical equations but used in conversions, i.e. converting from grams to moles using molar mass as the conversion factor, or from grams to milliliters using density. For example, to find the amount of NaCl in 2.00 g, one would do the following: 2.00 g NaCl 58.44 g NaCl mol − 1 = 0.034 mol In the above example, when written out in fraction form, the units of grams form a multiplicative identity, equivalent to one, wit
Degree of polymerization
The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the number-average degree of polymerization is given by D P n ≡ X n = M n M 0, where Mn is the number-average molecular weight and M0 is the molecular weight of the monomer unit. For most industrial purposes, degrees of polymerization in the thousands or tens of thousands are desired; this number does not reflect the variation in molecule size of the polymer that occurs, it only represents the mean number of monomeric units. Some authors, define DP as the number of repeat units, where for copolymers the repeat unit may not be identical to the monomeric unit. For example, in nylon-6,6, the repeat unit contains the two monomeric units —NH6NH— and —OC4CO—, so that a chain of 1000 monomeric units corresponds to 500 repeat units; the degree of polymerization or chain length is 1000 by the first definition, but 500 by the second.
In step-growth polymerization, in order to achieve a high degree of polymerization, Xn, a high fractional monomer conversion, p, is required, according to Carothers' equation X ¯ n = 1 1 − p For example, a monomer conversion of p = 99% would be required to achieve Xn = 100. For chain-growth free radical polymerization, Carothers' equation does not apply. Instead long chains are formed from the beginning of the reaction. Long reaction times increase the polymer yield, but have little effect on the average molecular weight; the degree of polymerization is related to the kinetic chain length, the average number of monomer molecules polymerized per chain initiated. However it differs from the kinetic chain length for several reasons: chain termination may occur wholly or by recombination of two chain radicals, which doubles the degree of polymerization chain transfer to monomer starts a new macromolecule for the same kinetic chain, corresponding to a decrease of the degree of polymerization chain transfer to solvent or to another solute (a modifier or regulator decreases the degree of polymerization Polymers with identical composition but different molecular weights may exhibit different physical properties.
In general, increasing degree of polymerization correlates with higher melting temperature and higher mechanical strength. Synthetic polymers invariably consist of a mixture of macromolecular species with different degrees of polymerization and therefore of different molecular weights. There are different types of average polymer molecular weight, which can be measured in different experiments; the two most important are the weight average. The number-average degree of polymerization is a weighted mean of the degrees of polymerization of polymer species, weighted by the mole fractions of the species, it is determined by measurements of the osmotic pressure of the polymer. The weight-average degree of polymerization is a weighted mean of the degrees of polymerization, weighted by the weight fractions of the species, it is determined by measurements of Rayleigh light scattering by the polymer. Anhydroglucose unit