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Contents contributed and discussions participated by Becky Kriger

Becky Kriger

Chemistry - Condensation polymer - 0 views

  • Condensation polymers are any class of polymer formed through a condensation reaction, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymer include polyamides and polyesters.
  • The carboxylic acids and amines link to form peptide bonds, also known as amide groups. Proteins are condensation polymers made from amino acid monomers. Carbohydrates are also condensation polymers made from sugar monomers such as glucose and galactose.
  • Condensation Polymers, unlike Addition polymers are bio-degradable. The peptide or ester bonds between monomers can be hydrolysed by acid catalysts or bacterial enzymes breaking the polymer chain into smaller pieces.
Becky Kriger

Polypeptides - 0 views

  • Polypeptides are chains of amino acids. Proteins are made up of one or more polypeptide molecules.
  • One end of every polypeptide, called the amino terminal or N-terminal, has a free amino group. The other end, with its free carboxyl group, is called the carboxyl terminal or C-terminal.
  • he sequence of amino acids in a polypeptide is dictated by the codons in the messenger RNA (mRNA) molecules from which the polypeptide was translated. The sequence of codons in the mRNA was, in turn, dictated by the sequence of codons in the DNA from which the mRNA was transcribed. The schematic below shows the N-terminal at the upper left and the C-terminal at the lower right.
Becky Kriger

Polysaccharides - Chemistry Encyclopedia - 0 views

  • Polysaccharides are long polymers of monosaccharides and their derivatives. Unlike proteins or nucleic acids, these polymers can be either linear or branched, and they can contain only one type of monosaccharide (homopolysaccharides), or more than one (heteropolysaccharides)
  • Starch is a homopolysaccharide and has two forms: amylopectin and α-amylose. In nature, starch is approximately 10 to 30 percent α-amylose.
  • Starch is the main energy reserve in plants; glycogen is the main energy reserve in animals
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  • In contrast to amylopectin, which comprises 70 to 90 percent of natural starch, α-amylose is a branching polysaccharide.
  • Branches occur at every twelve to thirty residues along a chain of α (1→4) linked glucoses. As a result, amylopectin has one reducing end and many nonreducing ends.
  • Amylopectin and α-amylose are broken down by the enzyme amylase. In animals, salivary α-amylase begins the digestion process in the mouth. Pancreatic α-amylase continues the process in the intestine.
  • Glycogen is the energy storage carbohydrate in animals. Glycogen is found mainly in the liver (where it is responsible for up to 10 percent of liver mass) and skeletal muscle (1 to 2 percent of skeletal muscle mass)
  • However, glycogen branches more abundantly than amylopectin, with branches at every eight to twelve residues. As a result, it has many more nonreducing ends. Glycogen is broken down at these nonreducing ends by the enzyme glycogen phosphorylase to release glucose for energy.
  • The primary structural homopolysaccharides are cellulose and chitin. Cellulose, a major component of plant cell walls, is the most abundant natural polymer on Earth.
  • Like α-amylose, cellulose is a linear polysaccharide composed entirely of glucose. However, in cellulose the glucose residues occur in β(1→4) linkage rather than α (1→4) (see Figure 1).
  • In addition, individual cellulose strands can form hydrogen bonds with one another to provide additional strength. Most animals, including humans, lack the enzymes necessary to dissolve α(1→4) linkages and so cannot digest cellulose
  • The animals that can (such as ruminants) do so via a symbiosis with bacteria that secrete cellulose-degrading enzymes.
  • The second most abundant polymer on Earth is chitin. Chitin comprises much of the exoskeletons of crustaceans, insects, and spiders, as well as the cell walls of fungi. Structurally, chitin is very similar to cellulose, except that its basic monosaccharide is N-acetylglucosamine
Becky Kriger

Polymer Structures - 0 views

    • Becky Kriger
       
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  • Although the fundamental property of bulk polymers is the degree of polymerization, the physical structure of the chain is also an important factor that determines the macroscopic properties.
  • Configuration refers to the order that is determined by chemical bonds. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed. Conformation refers to order that arises from the rotation of molecules about the single bonds. These two structures are studied below.
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  • The two types of polymer configurations are cis and trans. These structures can not be changed by physical means (e.g. rotation). The cis configuration arises when substituent groups are on the same side of a carbon-carbon double bond. Trans refers to the substituents on opposite sides of the double bond.
  • Three distinct structures can be obtained. Isotactic is an arrangement where all substituents are on the same side of the polymer chain. A syndiotactic polymer chain is composed of alternating groups and atactic is a random combination of the groups. The following diagram shows two of the three stereoisomers of polymer chain.
  • The ability of an atom to rotate this way relative to the atoms which it joins is known as an adjustment of the torsional angle. If the two atoms have other atoms or groups attached to them then configurations which vary in torsional angle are known as conformations.
  • different conformation may represent different potential energies of the molecule. There several possible generalized conformations: Anti (Trans), Eclipsed (Cis), and Gauche (+ or -). The following animation illustrates the differences between them.
  • The geometric arrangement of the bonds is not the only way the structure of a polymer can vary. A branched polymer is formed when there are "side chains" attached to a main chain. A simple example of a branched polymer is shown in the following diagram.
  • One of these types is called "star-branching". Star branching results when a polymerization starts with a single monomer and has branches radially outward from this point. Polymers with a high degree of branching are called dendrimers Often in these molecules, branches themselves have branches.
  • A separate kind of chain structure arises when more that one type of monomer is involved in the synthesis reaction. These polymers that incorporate more than one kind of monomer into their chain are called copolymers. There are three important types of copolymers. A random copolymer contains a random arrangement of the multiple monomers. A block copolymer contains blocks of monomers of the same type. Finally, a graft copolymer contains a main chain polymer consisting of one type of monomer with branches made up of other monomers. The following diagram displays the different types of copolymers.
  • In addition to the bonds which hold monomers together in a polymer chain, many polymers form bonds between neighboring chains. These bonds can be formed directly between the neighboring chains, or two chains may bond to a third common molecule.
  • Polymers with a high enough degree of cross-linking have "memory." When the polymer is stretched, the cross-links prevent the individual chains from sliding past each other. The chains may straighten out, but once the stress is removed they return to their original position and the object returns to its original shape.
  • In vulcanization, a series of cross-links are introduced into an elastomer to give it strength. This technique is commonly used to strengthen rubber.
  • Elastomers,or rubbery materials, have a loose cross-linked structure. This type of chain structure causes elastomers to possess memory. Typically, about 1 in 100 molecules are cross-linked
  • Natural and synthetic rubbers are both common examples of elastomers. Plastics are polymers which, under appropriate conditions of temperature and pressure, can be molded or shaped (such as blowing to form a film). In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity.
Becky Kriger

Condensation Polymerization - 0 views

  • The monomers that are involved in condensation polymerization are not the same as those in addition polymerization. The monomers for condensation polymerization have two main characteristics:. Instead of double bonds, these monomers have functional groups (like alcohol, amine, or carboxylic acid groups). Each monomer has at least two reactive sites, which usually means two functional groups. Some monomers have more than two reactive sites, allowing for branching between chains, as well as increasing the molecular mass of the polymer.
  • Let's look again at the functional groups on these monomers. We've seen three: The carboxylic acid group The amino group The alcohol group
  • You might have learned in chemistry or biology class that these groups can combine in such a way that a small molecule (often H2O) is given off. The Amide Linkage:When a carboxylic acid and an amine react, a water molecule is removed, and an amide molecule is formed. Because of this amide formation, this bond is known as an amide linkage. The Ester Linkage:When a carboxylic acid and an alcohol react, a water molecule is removed, and an ester molecule is formed. Because of this ester formation, this bond is known as an ester linkage.
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  • Example 1:A carboxylic acid monomer and an amine monomer can join in an amide linkage. As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer. Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide.
  • Example 2:A carboxylic acid monomer and an alcohol monomer can join in an ester linkage. A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding.
  • Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)
Becky Kriger

Carbohydrates and Polysaccharides - 0 views

  • Disaccharide Monosaccharides sucrose from α-glucose + α-fructose maltose from α-glucose + α-glucose α-lactose * from α-glucose + β-galactose * Lactose also exists in a beta form, which is made from β-galactose and β-glucose
  • A condensation reaction takes place releasing water. This process requires energy. A glycosidic bond forms and holds the two monosaccharide units together.
  • Carbohydrates (also called saccharides) are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules.
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  • a source of energy
  • building blocks for polysaccharides (giant carbohydrates
  • components of other molecules eg DNA, RNA, glycolipids, glycoproteins, ATP
  • Monosaccharides are the simplest carbohydrates and are often called single sugars.
  • Monosaccharides have the general molecular formula (CH2O)n, where n can be 3, 5 or 6.
  • n = 3 trioses, e.g. glyceraldehyde n = 5 pentoses, e.g. ribose and deoxyribose ('pent' indicates 5) n = 6 hexoses, e.g. fructose, glucose and galactose ('hex' indicates 6)
  • Molecules that have the same molecular formula but different structural formulae are called structural isomers.
  • Monosaccharides containing the aldehyde group are classified as aldoses, and those with a ketone group are classified as ketoses. Aldoses are reducing sugars; ketoses are non-reducing sugars.
  • in water pentoses and hexoses exist mainly in the cyclic form, and it is in this form that they combine to form larger saccharide molecules.
  • There are two forms of the cyclic glucose molecule: α-glucose and β-glucose.
  • Two glucose molecules react to form the dissacharide maltose. Starch and cellulose are polysaccharides made up of glucose units.
  • Galactose molecules look very similar to glucose molecules. They can also exist in α and β forms. Galactose reacts with glucose to make the dissacharide lactose.
  • However, glucose and galactose cannot be easily converted into one another. Galactose cannot play the same part in respiration as glucose.
  • Fructose reacts with glucose to make the dissacharide sucrose.
  • Ribose and deoxyribose are pentoses. The ribose unit forms part of a nucleotide of RNA. The deoxyribose unit forms part of the nucleotide of DNA.
  • Monosaccharides are rare in nature. Most sugars found in nature are disaccharides. These form when two monosaccharides react.
  • The three most important disaccharides are sucrose, lactose and maltose.
  • Disaccharides are soluble in water, but they are too big to pass through the cell membrane by diffusion.
  • This is a hydrolysis reaction and is the reverse of a condensation reaction. It releases energy.
  • Monosaccharides are converted into disaccharides in the cell by condensation reactions. Further condensation reactions result in the formation of polysaccharides. These are giant molecules which, importantly, are too big to escape from the cell. These are broken down by hydrolysis into monosaccharides when energy is needed by the cell.
  • Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers. The properties of a polysaccharide molecule depend on: its length (though they are usually very long) the extent of any branching (addition of units to the side of the chain rather than one of its ends) any folding which results in a more compact molecule whether the chain is 'straight' or 'coiled'
  • Starch is often produced in plants as a way of storing energy. It exists in two forms: amylose and amylopectin
  • Amylose is an unbranched polymer of α-glucose. The molecules coil into a helical structure. It forms a colloidal suspension in hot water. Amylopectin is a branched polymer of α-glucose. It is completely insoluble in water.
  • Glycogen is amylopectin with very short distances between the branching side-chains.
  • Inside the cell, glucose can be polymerised to make glycogen which acts as a carbohydrate energy store.
  • Cellulose is a third polymer made from glucose. But this time it's made from β-glucose molecules and the polymer molecules are 'straight'.
  • Cellulose serves a very different purpose in nature to starch and glycogen. It makes up the cell walls in plant cells. These are much tougher than cell membranes. This toughness is due to the arrangement of glucose units in the polymer chain and the hydrogen-bonding between neighbouring chains.
  • Cellulose is not hydrolysed easily and, therefore, cannot be digested so it is not a source of energy for humans.
Becky Kriger

How are polymers made? : Scientific American - 0 views

  • Synthetic polymers are produced by chemical reactions, termed "polymerizations."
  • but such reactions consist of the repetitive chemical bonding of individual molecules, or monomers. Assorted combinations of heat, pressure and catalysis alter the chemical bonds that hold monomers together, causing them to bond with one another. Most often, they do so in a linear fashion, creating chains of monomers called polymers.
  • The monomer ethylene is composed of two carbon atoms, each bonded to two hydrogen atoms and sharing a double bond with one another. Polyethylene consists of a chain of single-bonded carbon atoms, each still carrying its two hydrogen atoms.
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  • One way to produce polyethylene is called "free radical polymerization." As in other polymerizations, the process has three stages, known as initiation, propagation and termination. To begin, we need to add a catalyst to our supply of ethylene. A common catalyst is benzoyl peroxide, which when heated has the habit of splitting into two fragments, each with one unpaired electron, or free radical. These fragments are known as initiator fragments.
  • The unpaired electron naturally seeks another and finds a convenient target in the double bond between the carbon atoms in the ethylene molecule. Taking an electron from the carbon bond, the initiator fragment bonds itself to one of the monomer's carbon atoms.
  • The new radical also seeks a partner. And so ethylene monomers begin attaching themselves in a chain, creating new radicals each time and lengthening the chain. This stage is called propagation.
  • Eventually, free radical polymerization stops due to what are called termination reactions. For example, instead of stealing an electron from double-bonded carbons or a nearby propagating chain, the carbon atom with the free radical sometimes steals an entire hydrogen atom from another chain end. The polymer end--robbed of its hydrogen--easily forms a double bond with its adjacent carbon atom, and polymerization stops.
  • Because every part of the ethylene monomer is included in the finished polymer, the free radical polymerization of polyethylene is referred to as an addition polymerization
  • Polymerizations that use only portions of a monomer, however, are known as condensation polymerizations. The monomers that condense with each other must contain at least two reactive groups in order to form a chain.
  • For example, poly(ethyleneterepthalate), a polyester known as PET that is commonly found in soda bottles, forms from a reaction of two monomers: ethylene glycol and terephthoyl chloride. At the reaction's end, an atom of hydrogen and an atom of chlorine are left out of each PET molecular junction, resulting in a by-product of hydrogen chloride (HCl) gas.
Becky Kriger

The Science of Nylon - Spinning the Elements - 0 views

  • Nylon was developed as a synthetic substitute for silk.
  • Silk is a protein. Like all proteins, it is a polypeptide, and it has a structure something like this:
Becky Kriger

How Is Nylon Made? - 0 views

  • nylon was used to make parachutes, clothes, military uniforms, tires, machine parts and other necessary items
  • Nylon is made through a complex chemical reaction known as ring opening polymerization. In this reaction, a molecule with a ring shape such as hydrocarbons found in petroleum are submitted to various types of acids and bases. The ensuing chemical reactions cause the ring-shape molecular structure to flatten and lengthen. These molecules are caused to connect with one another to form molecular chains by being heated well above 600 degrees Fahrenheit. When done, what you have is a liquid with a high surface tension. If it cools down it will harden into a solid useless mass, so while it's still a liquid it is extruded through a hole with a diameter slightly greater than that of a human hair.
  • There is one problem, however, with this is process called hydrolysis. It's a chemical reaction during which the oxygen and hydrogen molecules in nylon's molecular chain can be broken away from the chain to produce water. This is the primary means by which nylon decays. It does not happen over time, but is instead a reaction to contact with certain caustic materials such as sulfuric or hydrochloric acid
Becky Kriger

On oil and plastic - 0 views

  • Manufacturers take simple hydrocarbons from whatever source material they're using -- commonly crude oil, but also natural gas, corn, and other biomass -- and turn them into polymers
  • In the case of crude oil, they do this by heating it to more than 750 degrees Fahrenheit, then separating its components. The polymers usually travel onward in life in the form of pellets, ending up at one plastic factory or another to be molded into familiar shapes
  • Polyethylene (HDPE or LDPE) is the soft one you likely encounter most, in milk jugs, shampoo bottles, plastic bags, and so forth. Polystyrene (PS) is the hard plastic that makes casings for computers and other appliances
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  • call Styrofoam. Polypropylene (PP) is used in dishwasher-safe containers
  • You'll often find polyethylene terephthalate (PET) in soda bottles, and it is sometimes recycled into fleece, upholstery fabrics, and other useful materials. And then of course there's polyvinyl chloride (PVC)
  • about 4 percent of the world's annual oil production of some 84.5 million barrels per day is used as feedstock for plastic, and another 4 percent or so provides the energy to transform the feedstock into handy plastic.
  • drilling. Recycling, however, does cut into energy use. According to the U.S. EPA, manufacturing new plastic from recycled plastic requires two-thirds of the energy used in virgin plastic manufacture. I have more numbers, too: one ton of recycled plastic saves 685 gallons of oil.
Becky Kriger

Effects of Temperature on Polymers - 0 views

  • Many polymers have a mixture of ordered (crystalline) regions and random (amorphous) regions.  In the glassy state the tangled chains in the amorphous region are frozen so movement of chains is not possible.  The polymer is brittle.
  • If the glassy material is heated, the chains reach a temperature at which they can move.  This temperature is called the glass transition temperature Tg.  Above this temperature the polymer is flexible. 
  • The glass transition temperature of a polymer can be changed by two different ways: Copolymerisation.  Ethene can be polymerised with propene to give a new polymer with different properties. Plasticisers.  PVC is quite brittle.  Its Tg can be lowered, making it less brittle, by introducing a substance between the polymer chains, allowing the chains to slide over each other more easily.  Such a substance is called a plasticiser.
Becky Kriger

Sugars & Polysaccharides - 0 views

shared by Becky Kriger on 08 Dec 08 - Cached
  • D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehyde. 
  • D & L sugars are mirror images of one another.
  • Monosaccharides - simple sugars,  with multiple hydroxyl groups. Based on the number of carbons (e.g., 3, 4, 5, or 6) a monosaccharide is a triose, tetrose, pentose, or hexose, etc. Disaccharides - two monosaccharides covalently linked Oligosaccharides - a few monosaccharides covalently linked. Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units.
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  • For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers.
Becky Kriger

Rubber Polymers - 0 views

  • Rubber is an example of an elastomer type polymer, where the polymer has the ability to return to its original shape after being stretched or deformed.
  • The elastic properties arise from the its ability to stretch the chains apart, but when the tension is released the chains snap back to the original position.
  • Natural rubber is an addition polymer
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  • Natural rubber is from the monomer isoprene (2-methyl-1,3-butadiene). Since isoprene has two double bonds, it still retains one of them after the polymerization reaction. Natural rubber has the cis configuration for the methyl groups.
  • Charles Goodyear accidentally discovered that by mixing sulfur and rubber, the properties of the rubber improved in being tougher, resistant to heat and cold, and increased in elasticity. This process was later called vulcanization
  • Vulcanization causes shorter chains to cross link through the sulfur to longer chains.
  • Some of the most commercially important addition polymers are the copolymers. These are polymers made by polymerizing amixture of two or more monomers. An example is styrene-butadiene rubber (SBR) - which is a copolymer of 1,3-butadiene and styrene which is mixed in a 3 to 1 ratio, respectively.
  • More than 40% of the synthetic rubber production is SBR and is used in tire production. A tiny amount is used for bubble-gum in the unvulcanized form.
  • . At the nipple end of the balloon, there is lots of rubber and therefore many, many polymer chains - still loosely coiled. These chains can be pierced without popping the balloon because the the chains can still be stretched. This is because they allow the skewer in between the chains without breaking the chains or the bonds that connect them. But on the sides of the balloon, these chains are stretched almost to their limit and very far apart. The piercing is too much for the stretched chains and they break apart., and the balloon pops.
Becky Kriger

Emulsion polymerization - Wikipedia, the free encyclopedia - 0 views

  • Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water.
  • Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in water.
  • A dispersion resulting from emulsion polymerization is often called a latex (especially if derived from a synthetic rubber) or an emulsion (even though "emulsion" strictly speaking refers to a dispersion of a liquid in water). These emulsions find applications in adhesives, paints, paper coating and textile coatings.
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  • Advantages of emulsion polymerization include:[1] High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate. The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate. Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight. The final product can be used as is and does not generally need to be altered or processed. Disadvantages of emulsion polymerization include: Surfactants and other polymerization adjuvants remain in the polymer or are difficult to remove For dry (isolated) polymers, water removal is an energy-intensive process Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.
  • The first "true" emulsion polymerizations, which used a surface-active agent and polymerization initiator, were conducted in the 1920s to polymerize isoprene.[6][7]
  • The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps: A monomer is dispersed or emulsified in a solution of surfactant and water forming relatively large droplets of monomer in water. Excess surfactant creates micelles in the water. Small amounts of monomer diffuse through the water to the micelle. A water-soluble initiator is introduced into the water phase where it reacts with monomer in the micelles. (This characteristic differs from suspension polymerization where an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves.) This is considered Smith-Ewart Interval 1. The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet. Monomer in the micelle quickly polymerizes and the growing chain terminates. At this point the monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system, this is considered Smith-Ewart Interval 2. More monomer from the droplets diffuses to the growing particle, where more initiators will eventually react. Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. This is considered Smith-Ewart Interval 3. Depending on the particular product and monomer, additional monomer and initiator may be continuously and slowly added to maintain their levels in the system as the particles grow. The final product is a dispersion of polymer particles in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'.
  • Both thermal and redox generation of free radicals have been used in emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50°C, providing a thermal source of initiation.
  • Emulsion polymerizations have been used in batch, semi-batch, and continuous processes. The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product. Modern process control schemes have enabled the development of complex reaction processes, with ingredients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.
  • Colloidal stability is a factor in design of an emulsion polymerization process. For dry or isolated products, the polymer dispersion must be isolated, or converted into solid form. This can be accomplished by simple heating of the dispersion until all water evaporates. More commonly, the dispersion is destabilized (sometimes called "broken") by addition of a multivalent cation. Alternatively, acidification will destabilize a dispersion with a carboxylic acid surfactant. These techniques may be employed in combination with application of shear to increase the rate of destabilization. After isolation of the polymer, it is usually washed, dried, and packaged.
  • Ethylene and other simple olefins must be polymerized at very high pressures (up to 800 bar).
  • Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in radical polymerization operate in emulsion polymerization.
  • Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility.
  • Ethylene and other olefins are used as minor comonomers in emulsion polymerization, notably in vinyl acetate copolymers.
  • Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50°C.
  • Selection of the correct surfactant is critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as tensile strength, gloss, and water absorption
  • Anionic, nonionic, and cationic surfactants have been used, although anionic surfactants are by far most prevalent.
  • Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha olefin sulfonate.
  • Some grades of poly(vinyl alcohol) and other water soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants (for example, they do not lower surface tension). It is believed that these polymers graft onto growing polymer particles and stabilize them.[12]
  • Other ingredients found in emulsion polymerization include chain transfer agents, buffering agents, and inert salts. Preservatives are added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization, however.
  • Polymers produced by emulsion polymerization can be divided into three rough categories. Synthetic rubber Some grades of styrene-butadiene (SBR) Some grades of Polybutadiene Polychloroprene (Neoprene) Nitrile rubber Acrylic rubber Fluoroelastomer (FKM) Plastics Some grades of PVC Some grades of polystyrene Some grades of PMMA Acrylonitrile-butadiene-styrene terpolymer (ABS) Polyvinylidene fluoride PTFE Dispersions (i.e. polymers sold as aqueous dispersions) polyvinyl acetate polyvinyl acetate copolymers latexacrylic paint Styrene-butadiene VAE (vinyl acetate - ethylene copolymers)
Becky Kriger

Turning Plastics Back to Oil - 0 views

  • Key to GRC's process is a machine that uses 1200 different frequencies within the microwave range, which act on specific hydrocarbon materials. As the material is zapped at the appropriate wavelength, part of the hydrocarbons that make up the plastic and rubber in the material are broken down into diesel oil and combustible gas.
  • "Anything that has a hydrocarbon base will be affected by our process," says Jerry Meddick, director of business development at GRC, based in New Jersey. "We release those hydrocarbon molecules from the material and it then becomes gas and oil."
  • Whatever does not have a hydrocarbon base is left behind, minus any water it contained as this gets evaporated in the microwave.
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  • Take a piece of copper wiring," says Meddick. "It is encased in plastic - a kind of hydrocarbon material. We release all the hydrocarbons, which strips the casing off the wire." Not only does the process produce fuel in the form of oil and gas, it also makes it easier to extract the copper wire for recycling.
  • Autofluff is the stuff that is left over after a car has been shredded and the steel extracted. It contains plastics, rubber, wood, paper, fabrics, glass, sand, dirt, and various bits of metal. GRC says its Hawk-10 can extract enough oil and gas from the left-over fluff to run the Hawk-10 itself and a number of other machines used by Gershow.
  • Because it makes extracting reusable metal more efficient and evaporates water from autofluff, the Hawk-10 should also reduce the amount of end material that needs to be deposited in landfill sites.
Becky Kriger

Polynucleotides - 0 views

  • A polymer of mononucleotides is called a polynucleotide. In polynucleotides, only one phosphoric acid is present on each ribose sugar so hydrolysis of polynucleotides produces equimolar solutions of nitrogenous base, ribose sugar, and phosphate. The phosphoric acid component of polynucleotides readily loses a proton and so polynucleotides are also called nucleic acids.
  • Polynucleotides, both DNA and RNA, are the information carriers of living organisms and play the central role in reproduction.
Becky Kriger

Nucleotides, Polymerization of DNA - 0 views

  • Nucleic acids are linear, unbranched polymers of nucleotides
  • Nucleotides consist of three parts:
  • two purines, called adenine (A) and guanine (G) two pyrimidines, called thymine (T) and cytosine (C) RNA contains: The same purines, adenine (A) and guanine (G). RNA also uses the pyrimidine cytosine (C), but instead of thymine, it uses the pyrimidine uracil (U).
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  • A five-carbon sugar (hence a pentose). Two kinds are found: Deoxyribose, which has a hydrogen atom attached to its #2 carbon atom (designated 2') Ribose, which has a hydroxyl group atom there
  • A nitrogen-containing ring structure called a base. The base is attached to the 1' carbon atom of the pentose. In DNA, four different bases are found:
  • The combination of a base and a pentose is called a nucleoside.
  • One (as shown in the first figure), two, or three phosphate groups. These are attached to the 5' carbon atom of the pentose.
  • The nucleic acids, both DNA and RNA, consist of polymers of nucleotides. The nucleotides are linked covalently between the 3' carbon atom of the pentose and the phosphate group attached to the 5' carbon of the adjacent pentose.
  • Most intact DNA molecules are made up of two strands of polymer, forming a "double helix". RNA molecules, while single-stranded, usually contain regions where two portions of the strand twist around each other to form helical regions.
Becky Kriger

Addition Polymers - 0 views

  • Addition polymers are usually made from molecules that have the following general structure: Different W, X, Y, and Z groups distinguish one addition polymer from another.
  • In the first stage, a substance is split into two identical parts, each with an unpaired electron. (Peroxides, which contain an O-O bond, are often used in this role.) A molecule with an unpaired electron is called a free radical. The free radical then initiates the reaction sequence by forming a bond to one of the carbon atoms in the double bond of the monomer. One electron for this new bond comes from the free radical, and the second electron for the new bond comes from one of the two bonds between the carbon atoms. The remaining electron from the broken bond shifts to the carbon atom on the far side of the molecule, away from the newly formed bond, forming a new free radical. Each half-headed arrow indicates the shift of one electron.
  • The chain begins to grow--propagate, stage two--when the new free radical formed in the initiation stage reacts with another monomer to add two more carbon atoms. This process repeats over and over again
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  • It can be terminated--stage three--when any two free radicals combine, thus pairing their unpaired electrons and forming a covalent bond that links two chains together.
  • Polyethylene molecules made with the free radical initiation process tend to form branches that keep the molecules from fitting closely together. Techniques have been developed that use catalysts, like Cr2O3, to make polyethylene molecules with very few branches.
  • yielding a high-density polyethylene, HDPE, that is more opaque, harder, and stronger than the low-density polyethylene, LDPE, made with free radical initiation.
Becky Kriger

What are Ziegler-Natta Catalysts? - 0 views

  • It was discovered that Group IV metals, especially titanium, were effective polymerization catalysts for ethylene. Following Ziegler’s successful preparation of linear polyethylene in 1953, Giulio Natta prepared and isolated isotactic (crystalline) polypropylene at the Milan Polytechnic Institute. This was immediately recognized for its practical importance. Ziegler and Natta shared the Nobel Prize in Chemistry in 1963.
  • A Ziegler-Natta catalyst is composed of at least two parts: a transition metal component and a main group metal alkyl compound. The transition metal component is usually either titanium or vanadium. The main group metal alkyl compound is usually an aluminum alkyl. In common practice, the titanium component is called "the catalyst’ and the aluminum alkyl is called "the co-catalyst".
  • In some instances, especially for catalyzing the polymerization of propylene, a third component is used. This component is used to control stereoregularity
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  • Today, Ziegler-Natta catalysts are used worldwide to produce the following classes of polymers from alpha olefins: High density polyethylene (HDPE) Linear low density polyethylene (LLDPE) Ultra-high molecular weight polyethylene (UHMWPE) Polypropylene (PP)--homopolymer, random copolymer and high impact copolymers Thermoplastic polyolefins (TPO’s) Ethylene propylene diene monomer polymers (EPDM) Polybutene (PB)
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