Becky Kriger's List: Polymers and Plastics

• "Poly" means "many" and "mer"   means "parts.
• The parts are usually the same part used repeatedly in a chain-like manner.   Polymers are also referred to as plastics
• Nature has many examples of polymers. Cotton fibers are made   of sugar molecules that are repeated in a chain-like manner. Hair, wool,   and other natural fibers are polymers.
• Now be aware that one of the connecting sticks   can spring open. To see animation of this, move cursor   over the image. As long as cursor is over image, animation repeats
• 1) We start with several pairs of balls each with two connecting  sticks. 2) Something causes one of the connecting sticks to fling open. It then  connects to another ball, but in doing so, causes one of its connecting sticks  to fling open. 3) In moments all of the pairs of balls are now connected. Move cursor over image to see animation.
• Sometimes heat, high energy light, or something else causes   the double-bond to break and two of the middle 4 electrons split up and   end up on the two outer ends of the molecule. These electrons are unpaired,   which makes them eager to join with another electron.
• (place   cursor over the image to see animated version)
• The unpaired electron   triggers the ethylene molecule that bumps into it, to shift the inside electron   to pair with it. That then causes the newly unpaired   inner electron to move to the outside and it is now an unpaired   electron ready to cause the next ethylene molecule to repeat the process.
• The name of this polymer is appropriately   called, polyethylene.
• This is called high density polyethylene   (HDPE).
• High density polyethylene HDPE is used for bottles, buckets, jugs, containers,   toys, even synthetic lumber, and many other things.
• Sometimes the chains get up to 500,000 carbons long. Here   they are tough enough for synthetic ice, replacement joints and bullet-proof   vests. This is called Ultra High   Molecular Weight   PolyEthylene   or UHMWPE.
• low density polyethylene (LDPE).
• It is made by causing the long chains of ethylene to branch.   That way they cannot lie next each other, which reduces the density of   the polyethylene. This makes the plastic lighter and more flexible.
• Low density polyethylene is used to make plastic bags,   plastic wrap, and squeeze bottles, plus many other things.
• The favorite properties of plastics are that they are inert   and won't react with what is stored in them. They also are durable and won't   easily decay, dissolve, or break apart. These are great qualities for things   you keep, but when you throw them away, they won't decompose.
• The answer is to recycle the plastics. Here we see a bunch   of CDs getting recycled.
• Here are   two recycle code drawings. You already know about HDPE and LDPE.

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• 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.

• 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.
  

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• The chemical reaction in which high molecular mass molecules are formed from  monomers is known as polymerization. There are two  basic types of polymerization, chain-reaction (or addition) and step-reaction (or condensation) polymerization.

• One of the most common types of polymer reactions is chain-reaction  (addition) polymerization. This type of polymerization is a three step process  involving two chemical entities. The first, known simply as a monomer, can be regarded as one link in a polymer chain.  It initially exists as simple units. In nearly all cases, the monomers have at  least one carbon-carbon double bond. Ethylene is one example of a monomer used  to make a common polymer.
  
• The other chemical reactant is a catalyst. In chain-reaction polymerization, the  catalyst can be a free-radical peroxide added in  relatively low concentrations. A free-radical is a chemical component that  contains a free electron that forms a covalent bond with an electron on another  molecule. The formation of a free radical from an organic peroxide is shown  below:
• The first step in the chain-reaction polymerization process, initiation, occurs  when the free-radical catalyst reacts with a double bonded carbon monomer,  beginning the polymer chain. The double carbon bond breaks apart, the monomer  bonds to the free radical, and the free electron is transferred to the outside  carbon atom in this reaction.
• The next step in the process, propagation, is a repetitive operation in which  the physical chain of the polymer is formed. The double bond of successive  monomers is opened up when the monomer is reacted to the reactive polymer chain.  The free electron is successively passed down the line of the chain to the  outside carbon atom.
  
• Thermodynamically speaking, the sum of the energies of the polymer is less than  the sum of the energies of the individual monomers. Simply put, the single  bounds in the polymeric chain are more stable than the double bonds of the  monomer.

• Termination occurs when another free radical (R-O^.), left over  from the original splitting of the organic peroxide, meets the end of the  growing chain. This free-radical terminates the chain by linking with the last  CH₂^. component of the polymer chain. This reaction  produces a complete polymer chain. Termination can also occur when two  unfinished chains bond together. Both termination types are diagrammed below.  Other types of termination are also possible.
  
• This exothermic reaction occurs extremely fast, forming individual chains of polyethylene often in less than 0.1 second.
• Step-reaction (condensation) polymerization is another common type of  polymerization. This polymerization method typically produces polymers of lower  molecular weight than chain reactions and requires higher temperatures to occur.  Unlike addition polymerization, step-wise reactions involve two different types  of di-functional monomers or end groups that react  with one another, forming a chain. Condensation polymerization also produces a  small molecular by-product (water, HCl, etc.).

• As indicated above, both addition and condensation polymers can be linear,  branched, or cross-linked. Linear polymers are made  up of one long continuous chain, without any excess appendages or attachments.  Branched polymers have a chain structure that  consists of one main chain of molecules with smaller molecular chains branching  from it. A branched chain-structure tends to lower the degree of crystallinity  and density of a polymer. Cross-linking in polymers  occurs when primary valence bonds are formed between separate polymer chain  molecules.  

  Chains with only one type of monomer are known as homopolymers. If two or more different type monomers are  involved, the resulting copolymer can have several  configurations or arrangements of the monomers  along the chain. The four main configurations are depicted below:
  
• They can be found in either crystalline or amorphous forms. Crystalline polymers are only possible  if there is a regular chemical structure (e.g., homopolymers or alternating  copolymers), and the chains possess a highly ordered arrangement of their  segments. Crystallinity in polymers is favored in symmetrical polymer chains,  however, it is never 100%. These semi-crystalline polymers possess a rather  typical liquefaction pathway, retaining their solid state until they reach their  melting point at T_m.
• Amorphous polymers do not show order. The molecular segments in amorphous  polymers or the amorphous domains of semi-crystalline  polymers are randomly arranged and entangled. Amorphous polymers do not have a  definable T_m due to their randomness
• At low temperatures, below their glass transition temperature (T_g), the segments are immobile and the  sample is often brittle. As temperatures increase close to T_g,  the molecular segments can begin to move. Above T_g, the  mobility is sufficient (if no crystals are present) that the polymer can flow as  a highly viscous liquid.
• Thermoplastics are generally carbon containing polymers synthesized by addition  or condensation polymerization. This process forms strong covalent bonds within  the chains and weaker secondary Van der Waals bonds  between the chains. Usually, these secondary forces can be easily overcome by  thermal energy, making thermoplastics moldable at high temperatures.
• Thermosets have the same Van der Waals bonds that thermoplastics do. They also  have a stronger linkage to other chains. Strong covalent bonds chemically hold  different chains together in a thermoset material. The chains may be directly  bonded to each other or be bonded through other molecules. This "cross-linking"  between the chains allows the material to resist softening upon heating.

• Transfer Molding  

  This process is a modification of compression molding. It is used primarily  to produce thermosetting plastics. Its steps are:
   

   
  1. A partially polymerized material is placed in a heated chamber.  
  2. A plunger forces the flowing material into molds.  
  3. The material flows through sprues, runners and gates.  
  4. The temperature and pressure inside the mold are higher than in the heated  chamber, which induces cross-linking.  
  5. The plastic cures, is hardened, the mold opened, and the part removed.  
   

  Mold costs are expensive and much scrap material collects in the sprues and  runners, but complex parts of varying thickness can be accurately produced.

• Blow Molding  

  Blow molding produces bottles, globe light fixtures, tubs, automobile  gasoline tanks, and drums. It involves:
   

   
  1. A softened plastic tube is extruded  
  2. The tube is clamped at one end and inflated to fill a mold.  
  3. Solid shell plastics are removed from the mold.
   

  This process is rapid and relatively inexpensive.

• Injection Molding  

  This very common process for forming plastics involves four steps:
   

   
  1. Powder or pelletized polymer is heated to the liquid state.  
  2. Under pressure, the liquid polymer is forced into a mold through an opening,  called a sprue. Gates control the flow of material.  
  3. The pressurized material is held in the mold until it solidifies.  
  4. The mold is opened and the part removed by ejector pins.
   

  Advantages of injection molding include rapid processing, little waste, and  easy automation.

• Compression Molding  

  This type of molding was among the first to be used to form plastics. It  involves four steps:
   

   
  1. Pre-formed blanks, powders or pellets are placed in the bottom section of a  heated mold or die.  
  2. The other half of the mold is lowered and is pressure applied.  
  3. The material softens under heat and pressure, flowing to fill the mold.  Excess is squeezed from the mold. If a thermoset, cross-linking occurs in the  mold.  
  4. The mold is opened and the part is removed.
   

  For thermoplastics, the mold is cooled before removal so the part will not  lose its shape. Thermosets may be ejected while they are hot and after curing is  complete. This process is slow

• Extrusion  

  This process makes parts of constant cross section like pipes and rods.  Molten polymer goes through a die to produce a final shape. It involves four  steps:
   

   
  1. Pellets of the polymer are mixed with coloring and additives.  
  2. The material is heated to its proper plasticity.  
  3. The material is forced through a die.  
  4. The material is cooled.

• In 1989, a billion pounds of virgin PET were used to make beverage bottles of  which about 20% was recycled. Of the amount recycled, 50% was used for fiberfill  and strapping. The reprocessors claim to make a high quality, 99% pure,  granulated PET. It sells at 35 to 60% of virgin PET costs.  

  The major reuses of PET include sheet, fiber, film, and extrusions.
• Of the plastics that have a potential for recycling, the rigid HDPE container is  the one most likely to be found in a landfill. Less than 5% of HDPE containers  are treated or processed in a manner that makes recycling easy.
• There is a great potential for the use of recycled HDPE in base cups, drainage  pipes, flower pots, plastic lumber, trash cans, automotive mud flaps, kitchen  drain boards, beverage bottle crates, and pallets.
• LDPE is recycled by giant resin suppliers and merchant processors either by  burning it as a fuel for energy or reusing it in trash bags. Recycling trash  bags is a big business.
• There is much controversy concerning the recycling and reuse of PVC due to  health and safety issues. When PVC is burned, the effects on the incinerator and  quality of the air are often questioned. The Federal Food and Drug  Administration (FDA) has ordered its staff to prepare environmental impact  statements covering PVC's role in landfills and incineration. The burning of PVC  releases toxic dioxins, furans, and hydrogen chloride.
• PVC is used in food and alcoholic beverage containers with FDA approval. The  future of PVC rests in the hands of the plastics industry to resolve the issue  of the toxic effects of the incineration of PVC. It is of interest to note that  PVC accounts for less than 1% of land fill waste.
• PS and its manufacturers have been the target of environmentalists for several  years. The manufacturers and recyclers are working hard to make recycling of PS  as common as that of paper and metals. One company, Rubbermaid, is testing  reclaimed PS in service trays and other utility items.
• Table 3: Major Plastic Resins and Their Uses 
         

  Resin Code	Resin Name	Common Uses	Examples of Recycled Products
  	Polyethylene Terephthalate (PET or PETE)	Soft drink bottles, peanut butter jars, salad dressing bottles, mouth wash jars	Liquid soap bottles, strapping, fiberfill for winter coats, surfboards, paint brushes, fuzz on tennis balls, soft drink bottles, film
  	High density Polyethylene (HDPE)	Milk, water, and juice containers, grocery bags, toys, liquid detergent bottles	Soft drink based cups, flower pots, drain pipes, signs, stadium seats, trash cans, re-cycling bins, traffic barrier cones, golf bag liners, toys
  	Polyvinyl Chloride or Vinyl (PVC-V)	Clear food packaging, shampoo bottles	Floor mats, pipes, hoses, mud flaps
  	Low density Polyethylene (LDPE)	Bread bags, frozen food bags, grocery bags	Garbage can liners, grocery bags, multi purpose bags
  	Polypropylene (PP)	Ketchup bottles, yogurt containers, margarine, tubs, medicine bottles	Manhole steps, paint buckets, videocassette storage cases, ice scrapers, fast food trays, lawn mower wheels, automobile battery parts.
  	Polystyrene (PS)	Video cassette cases, compact disk jackets, coffee cups, cutlery, cafeteria trays, grocery store meat trays, fast-food sandwich container	License plate holders, golf course and septic tank drainage systems, desk top accessories, hanging files, food service trays, flower pots, trash cans

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• 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
• 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.

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• 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.

• 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.
   

   <form>A water molecule is removed as the ester linkage is formed. Notice the  acid and the alcohol groups that are still available for bonding.</form>

• 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.)

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• Polymers are made up of extremely large, chainlike molecules consisting of numerous, smaller, repeating units called monomers. Polymer chains, which could be compared to paper clips linked together to make a long strand, appear in varying lengths. They can have branches, become intertwined, and can have cross-links. In addition, polymers can be composed of one or more types of monomer units, they can be joined by various kinds of chemical bonds, and they can be oriented in different ways. Monomers can be joined together by addition, in which all the atoms in the monomer are present in the polymer, or by condensation, in which a small molecule byproduct is also formed.
• The importance of polymers is evident as they occur widely both in the natural world in such materials as wool, hair, silk and sand, and in the world of synthetic materials in nylon, rubber, plastics, Styrofoam, and many other materials.
• Polymers are extremely large molecules composed of long chains, much like paper clips that are linked together to make a long strand. The individual subunits, which can range from as few as 50 to more than 20,000, are called monomers (from the Greek mono meaning one and meros meaning part). Because of their large size, polymers (from the Greek poly meaning many) are referred to as macromolecules.
• Most synthetic polymers are made from the non-renewable resource, petroleum, and as such, the "age of plastics" is limited unless other ways are found to make them. Since most polymers have carbon atoms as the basis of their structure, in theory at least, there are numerous materials that could be used as starting points.
• Disposing of plastics is also a serious problem, both because they contribute to the growing mounds of garbage accumulating everyday and because most are not biodegradable. Researchers are busy trying to find ways to speed-up the decomposition time which, if left to occur naturally, can take decades.
• n order for monomers to chemically combine with each other and form long chains, there must be a mechanism by which the individual units can join or bond to each other. One method by which this happens is called addition because no atoms are gained or lost in the process. The monomers simply "add" together and the polymer is called an addition polymer.
• The simplest chemical structure by which this can happen involves monomers that contain double bonds (sharing two pairs of electrons). When the double bond breaks and changes into a single bond, each of the other two electrons are free and available to join with another monomer that has a free electron. This process can continue on and on. Polyethylene is an example of an addition polymer.
• The polymerization process can be started by using heat and pressure or ultraviolet light or by using another more reactive chemical such as a peroxide. Under these conditions the double bond breaks leaving extremely reactive unpaired electrons called free radicals. These free radicals react readily with other free radicals or with double bonds and the polymer chain starts to form.
• ifferent catalysts yield polymers with different properties because the size of the molecule may vary and the chains may be linear, branched, or cross-linked. Long linear chains of 10,000 or more monomers can pack very close together and form a hard, rigid, tough plastic known as high-density polyethylene or HDPE
• Shorter, branched chains of about 500 monomers of ethylene cannot pack as closely together and this kind of polymer is known as low-density polyethylene or LDPE.
• The ethylene monomer has two hydrogen atoms bonded to each carbon for a total of four hydrogen atoms that are not involved in the formation of the polymer. Many other polymers can be formed when one or more of these hydrogen atoms are replaced by some other atom or group of atoms.
• Natural and synthetic rubbers are both addition polymers. Natural rubber is obtained from the sap that oozes from rubber trees. It was named by Joseph Priestley who used it to rub out pencil marks, hence, its name, a rubber. Natural rubber can be decomposed to yield monomers of isoprene.
• It was sticky and smelly when it got too hot and it got hard and brittle in cold weather. These undesirable properties were eliminated when, in 1839, Charles Goodyear accidentally spilled a mixture of rubber and sulfur onto a hot stove and found that it did not melt but rather formed a much stronger but still elastic product. The process, called vulcanization, led to a more stable rubber product that withstood heat (without getting sticky) and cold (without getting hard) as well as being able to recover its original shape after being stretched. The sulfur makes cross-links in the long polymer chain and helps give it strength and resiliency, that is, if stretched, it will spring back to its original shape when the stress is released.
• A second method by which monomers bond together to form polymers is called condensation.
• Unlike addition polymers, in which all the atoms of the monomers are present in the polymer, two products result from the formation of condensation polymers, the polymer itself and another small molecule which is often, but not always, water.
• One of the simplest of the condensation polymers is a type of nylon called nylon 6.
• All amino acids molecules have an amine group (NH₂) at one end and a carboxylic acid (COOH) group at the other end. A polymer forms when a hydrogen atom from the amine end of one molecule and an oxygen-hydrogen group (OH) from the carboxylic acid end of a second molecule split off and form a water molecule. The monomers join together as a new chemical bond forms between the nitrogen and carbon atoms. This new bond is called an amide linkage.
• The new molecule, just like each of the monomers from which it formed, also has an amine group at one end (that can add to the carboxylic acid group of another monomer) and it has a carboxylic acid group at the other end (that can add to the amine end of another monomer). The chain can continue to grow and form very large polymers.
• Polymers formed by this kind of condensation reaction are referred to as polyamides.
• Nylon became a commercial product for Du Pont when their research scientists were able to draw it into long, thin, symmetrical filaments. As these polymer chains line up side-by-side, weak chemical bonds called hydrogen bonds form between adjacent chains. This makes the filaments very strong.
• Another similar polymer of the polyamide type is the extremely light-weight but strong material known as Kevlar. It is used in bullet-proof vests, aircraft, and in recreational uses such as canoes. Like nylon, one of the monomers from which it is made is terephthalic acid. The other one is phenylenediamine.
• Polyesters are another type of condensation polymer, so-called because the linkages formed when the monomers join together are called esters.
• Probably the best known polyester is known by its trade name, Dacron.
• Dacron is used primarily in fabrics and clear beverage bottles. Films of Dacron can be coated with metallic oxides, rolled into very thin sheets (only about one-thirtieth the thickness of a human hair), magnetized, and used to make audio and video tapes. When used in this way, it is extremely strong and goes by the trade name Mylar. Because it is not chemically reactive, and is not toxic, allergenic, or flammable, and because it does not promote blood-clotting, it can be used to replace human blood vessels when they are severely blocked and damaged or to replace the skin of burn victims.

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• 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.

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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.

• 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.

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• The major practical problem, however, is that homopolymers  blend together with difficulty and even where blends are possible, as in some  thermoplastics, phase separation can occur readily.

   

  This problem is often overcome by polymerizing a mixture of  monomers, a process known as copolymerization.
• It gives a much greater range of structures than is possible by mixing  homopolymers because of the possibility of branching, structural isomerism  within a single monomer, and the way in which the different repeat units can be  added together.
• suppose that two monomers, A and B, are copolymerized. The chain could start  with either a molecule of A or a molecule of B, and at each successive addition  there are always two possibilities as to which monomer molecule will be  attached. As shown in Table 7, the number of possible  chain structures grows rapidly as n increases. Since the number of  possible structures is proportional to 2ⁿ, it is easy to see  that even for low degrees of polymerization the number of possible copolymers is  very large indeed. Some of these molecules are identical however
• Table 7 shows that the composition varies from chains  of only monomer A (homopolymer A) to chains containing only monomer B  (homopolymer B).

• In many of the structures no regularity can be detected,  although there will be short sequences of one type of unit, and the copolymer  can be regarded as completely random; such copolymers are usually said to be  ideal copolymers. These possible copolymer structures are shown  schematically in Table 7.

• It can be shown that the rate of change of monomer  concentration in any copolymerization is given by the equation

   

  

   

  

   

   

  where [M₁] and [M₂] are the  concentrations of monomers 1 and 2 at any instant and r₁ and  r₂., are reactivity ratios. The reactivity ratios represent  the rate at which one type of growing chain end adds on to a monomer of the same  structure relative to the rate at which it adds on to the alternative monomer.  The copolymer equation can be used to predict chain structure in the  three different ways, already mentioned.

• An ideal copolymer will tend to form when each type of  chain end shows an equal preference for adding on to either monomer. In this  case,

   

  

   

  

   

   

  and the copolymer equation becomes

   

  

   

  

   

   

  Hence composition depends on the relative amounts of  monomer present at any time and the relative reactivities of the two  monomers.
• Step growth copolymerizations produce ideal (random) copolymers since in this  special case r ₁ = r ₂ = 1.
• The main reason for copolymerizing different monomers is to adjust the physical  properties of a given homopolymer to meet a specific demand. SBR elastomer, for  example (Table 1), based on 24 wt%  styrene monomer shows better mechanical properties and better resistance to  degradation than polybutadiene alone
• A second reason for copolymerization is to enhance the chemical reactivity of a  polymer, particularly to aid crosslinking. Conventional vulcanization in rubbers  is brought about by forming sulphur crosslinks at or near double bonds in the  chain

• To show the dramatic effect of copolymer structure on  physical properties, consider the change from random SBR copolymer to a block  copolymer of exactly the same chemical composition but where the styrene and  butadiene parts are effectively homopolymer chains linked at two points:

   

  

   

  The material behaves like a vulcanized butadiene rubber  without the need for chemical crosslinking since the styrene chains segregate  together to form small islands or domains within the structure. Such so-called  thermoplastic elastomers (TPEs) today form an important growth area for  new polymers because of the process savings in manufacture that can be achieved  with their use.

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• 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.

• 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)  

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• why these polymers, these macromolecules, act differently from small  molecules.
•  
  Chain entanglement  
  Summation of intermolecular forces  
  Time scale of motion
• Remember now that most polymers are linear polymers;  that is, they are molecules whose atoms are joined in a long line to form a huge  chain. Now most of the time, but not always, this chain is not stiff and  straight, but is flexible. It twists and bends around to form a tangled mess.
• when a polymer is molten, the chains will act like spaghetti tangled up on a  plate. If you try to pull out any one strand of spaghetti, it slides right out  with no problem. But when polymers are cold and in the solid state, they act  more like a ball of string.
• intermolecular forces affect polymers just like small molecules. But with  polymers, these forces are greatly compounded. The bigger the molecule, the more  molecule there is to exert an intermolecular force. Even when only weak Van der  Waals forces are at play, they can be very strong in binding different polymer  chains together. This is another reason why polymers can be very strong as  materials. Polyethylene, for example is very nonpolar. It  only has Van der Waals forces to play with, but it is so strong it's used to  make bullet proof vests.

• This is a fancy way of saying polymers move more slowly than small  molecules do. Imagine you are a first grade teacher, and it's time to go to  lunch. Your task is to get your kids from the classroom to the cafeteria,  without losing any of them, and to do so with minimal damage to the territory  you'll have to cover to get to the cafeteria. Keeping them in line is going to  be difficult. Little kids love to run around every which way, jumping and  hollering and bouncing this way and that. One way to put a stop to all this  chaotic motion is to make all the kids join hands when you're walking them to  lunch. This won't be easy rest assured, as there's always going to be a lot of  little boys who are too macho to hold the hands of the girls next to them in  line, and some who are too insecure in their manhood to hold anyone's hand. But  once you get them to do this, their ability to run around is severely limited.  Of course, their motion will still be chaotic. The chain of kids will curve and  snake this way and that on its way to eat soybean patties disguised as who knows  what. But the motion will be a lot slower. You see, if one kid gets a notion to  just bolt off in one direction, he or she can't do it because he or she will be  bogged down by the weight of all the other kids to which he or she is bound.  Sure, the kid can deviate from the straight path, and make a few other kids do  so, but the deviation will be far less than you'd bet if the kids weren't all  linked together.  

  It's the same way with molecules.
• So then how does this make a polymeric material different from a material made  of small molecules? This slow speed of motion makes polymers do some very  unusual things. For one, if you dissolve a polymer in a solvent, the solution  will be a lot more viscous than the pure solvent.

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• 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

• 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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• The most common polymers are polyolefins, especially polyethylene (better  known as Polythene, although this is a trade name owned by DuPont) and  polypropylene. However, efficient ways of producing these vital materials are  only the result of recent discoveries and have been dependant on the chemistry  of catalysts.
• Since the 1950s, the production of polyolefins has depended on the use of  Ziegler-Natta catalysts.
• Ziegler-Natta catalysts are based on a mixture of a transition metal, commonly a  titanium compound, and an alkali metal, most commonly aluminium oxide.
• their products have variable physical properties. To this day, the systems are  little understood, but the monomers (polymer starting materials) react through a  number of reaction sites on the catalyst. Unfortunately, this means the polymer  can grow from many sites and at different rates, leading to a very wide  distribution in the molecular weight, based on the polymer chain length.
• Modern life has demanded more of the humble polymer. We want polymers that are  stronger than steel, lighter than aluminium, and can be dyed any colour  imaginable
• Metallocenes are positively charged metal ions, most commonly Titanium or  Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see  fig). Their big advantage over the Ziegler-Natta systems is that they catalyse  the reaction of olefins through only one reactive site. Due to this “single  site” reaction, the polymerization continues in a far more controllable fashion,  leading to polymers with narrow ranges of molecular weight and, more  importantly, predictable and desirable properties.
• it has been found that changing the ligands (functional groups attached to the  metal) upon the metallocene molecule can controllably affect the properties of  the polymer.
• The drawback of metallocene catalysts is that they are unable to polymerize  polar molecules, such as common acrylics or vinyl chloride. This is due to the  metallocenes’ oxophilicity – their propensity for binding to oxygen.
• Catalysts using late transition metals – those metals from groups 6 and higher  in the Periodic Table – have become increasingly utilized. These compounds have  good polymerization activity, although slightly less than metallocenes. However,  crucially they can polymerize reactions with polar monomers.
• The most commercially advanced of this type of catalysts are the Brookhart  catalysts [6], which are diimine complexes of palladium or nickel (see fig).

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• 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  T_g.  Above this temperature the polymer is flexible. 

• The glass transition temperature of a  polymer can be changed by two different ways:

   
   
  1.  

     Copolymerisation.  Ethene can be  polymerised with propene to give a new polymer with different properties.

      
  2.  

     Plasticisers.  PVC is quite  brittle.  Its T_g 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.  

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• The highest quality glass has the chemical formula SiO₂. But this is  misleading. That formula conjures up ideas of little silicon dioxide molecules,  analogous to carbon dioxide molecules. But little silicon dioxide molecules  don't exist.
• Instead, in nature SiO₂ is often found as a crystalline solid, with a  structure like you see on your right. Every silicon atom is bonded four oxygen  atoms, tetrahedrally, of course; and every oxygen atom is bonded to two silicon  atoms. When SiO₂ is in this crystalline form we call it  silica.
• But this silica isn't glass. We have to do something to it first to make it into  glass. We have to heat it up until it melts, and then cool it down really fast.  When it melts, the silicon and oxygen atoms break out of their crystal  structure. If we cooled it down slowly, the atoms would slowly line back up into  their crystalline arrangement as they slowed down.
• As you can see, there is no order to the arrangement of the atoms. We call  materials like this amorphous. This is the glass that is used for  telescope lenses and such things. It has very good optical properties, but it's  brittle. For everyday uses, we need something tougher. Most glass is made from  sand, and when we melt down the sand, we usually add some sodium carbonate. This  gives us a tougher glass with a structure that looks like this:
• So is this a polymer or not? Usually it isn't considered as such. Why? Some may  say it's inorganic, and polymers are usually organic. But there are many inorganic polymers out there. For example, what about polysiloxanes? These linear, and yes, inorganic  materials have a structure very similar to glass, and they're considered  polymers. Take a look at a polysiloxane:
• So glass could be considered a highly crosslinked  polysiloxane. But we usually don't think of it that way. Why not? Probably  because even in a highly crosslinked system, you could still trace a polymer  chain and see where the crosslinks are. But with glass, it'd be tough to do  that.

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• 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
• 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 a
  mixture 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.

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• 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

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• The red lines connecting the 4th carbon atom  of one isoprene unit with the 1st carbon of the next indicate that  latex is an addition polymer that results from the 1,4-addition of  one isoprene unit to the next. Note the head-to-tail pattern in  which the isoprene units are connected. Note, too, that the  stereochemistry is the same at each double bond, namely  cis.

•    

  Synthetic  Rubber

     

  In 1839 Charles Goodyear discovered,  literally by accident, that heating natural rubber with elemental  sulfur altered the properties of the polymer, most notably making  it tougher and more elastic. Goodyear's discovery led to the  development of synthetic rubber, a material that found its most  profitable application in the manufacture of automobile tires.  Investigation of the structure of synthetic rubber revealed that  the sulfur had formed disulfide bonds that linked one polyisoprene  chain to the next. As Figure 2 demonstrates, these cross-links  serve to restore the polymer to its original shape after it has  been deformed by the application of a force.

     

  Figure  2

     

  Bouncing  Back

         
• Not surprisingly, the desireable properties  of natural as well as synthetic rubber led to investigations of  the polymerization of structural analogs of isoprene. One notable  success came from the polymerization of 2-chloro-1,3-butadiene,  sometimes called chloroprene. Polychloroprene is known  commercially as neoprene rubber. It is widely used in the  automotive industry for the manufacture of oil-resistant hoses.  Neoprene that contains entrapped air has good insulating  properties and is used in the production of wet suits.

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• 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.

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