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Chemistry: Organic Polymers

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  • Type: Video Tutorial
  • Length: 12:54
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 139 MB
  • Posted: 07/15/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Organic Chemistry (8 lessons, $12.87)
Chemistry: Organic Polymers (2 lessons, $2.97)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Thinkwell
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Polymers are a very important class of molecules that organic chemists and biochemists have to deal with. Lots of useful things are made out of polymers, for example, this Kevlar vest is bullet proof. This bulletproof Kevlar vest is made out of polymers. This is a very useful and important product in today's society. We have these rubber balls that are made out rubber. This one bounces really well, right? This is a normal rubber ball - a good bouncy ball. Here is an identical looking ball, doesn't bounce. So the difference is, this is ball is very bouncy and this ball is not. They are both made out of polymers, but the difference in the structure of the polymers is what gives the balls their different characteristics. Another important characteristic of these balls is that they roll at different rates. The bouncy ball rolls faster than the non-bouncy ball. The reason is the bouncy ball has less friction when it rolls across the surface.
Now if you are a tire manufacturer and you want to design a new rubber for a racing tire, what you want is a tire that creates a lot of friction. So that when the cars are going around the corners and so on, the friction holds the tire and the car to the road.
Now, what we have to understand, to understand organic polymers is, first of all, what polymers are? Second of all, how to make them? Third of all, give some examples of useful polymers and how to use them in industry.
So first of all, polymers are large molecules made of repeating units of smaller molecules. The smaller molecules are called monomers. So the monomer is just a small molecule, and then to make a polymer, you take the monomer and you basically string it along with a bunch of other monomers to make one big long chain of molecule. So here is the monomer, here is the polymer connected from monomer to the next and the next, and the next and so one. So that is what a polymer is. Poly means many. Mono means one. Polymer is basically a chain of monomers where the monomers have been linked one after the other.
There are two general kinds of polymers that we can talk about. There are homopolymers that contain one monomer and heteropolymers, which can contain two different kind of monomers. Here are two examples of heteropolymers. Here is a block co-polymer, what we have in this case is one block of one monomer and one block of another monomer and in a block co-polymer you have one monomer which is polymerized. The next monomer polymerized, the next one, so on and so on. They alternate between being, let's say, squares and circles and squares and circles. So you have a whole bunch of squares together and a whole bunch of circles together. For another kind of heteropolymer, where the squares and circles are alternating. Square, circle, square, circle and so on. This is also heteropolymer, but it is a different kind of heteropolymer than the block co-polymer.
Well how are polymers made? Two general methods. Addition, basically molecules are added to one another with no byproducts, and condensation. Where two molecules are added and a byproduct is formed. Oftentimes water, HCL or something else like that. So, in addition reaction, all the material goes into the polymer. You start with a monomer and it is completely consumed and goes into the polymer. In the condensation reaction, there is a byproduct and that byproduct is not a part of the polymer.
Here is an example of an addition reaction to make polyethylene. Polyethylene, as the name implies, is the polymer of the monomer ethylene. Now ethylene is the same as ethene. Ethene is the IU pack for a two-carbon alkene. Ethylene is the common name for two-carbon alkene. Polymer ethylene is called polyethylene, and sometimes in Europe, in fact, they call this polymer, polythene. In fact, there is the song on Abby Road, "Polythene Pan" by the Beatles. Those of you who were not born after the Beatles, might remember that. In any case, in an addition polymer you have monomer that starts off as so, and it adds another monomer and add the next one and so on and so on.
Now what I have tried to draw here is the movement of the electrons in the bonds. We start off with pi bonds and end up with only sigma bonds. What happens is that the green electrons that are in the pi bond are used to form the sigma bond between these two carbons. So you have the green electrons that forms the bond between there and there and that's the bond over there, let's say. Then you have this green pair of electrons that forms the bond from there to there and that's the bond over there. Then you have the green electrons over here that forms the bond over there, between these two molecules.
Now, what I have tried to do is, also indicate where one of the monomers would be in the finally polymer. Here is one molecule of ethylene, in red, and that is where it ends up being. So you can see what happens in this reaction, is that you start off with a monomer ethylene and then you polymerize it by taking advantage of the fact that the pi bonds can now form sigma bonds and form bonds between the individual monomer units.
There are a lot of very important homopolymers and here is a list of seven of them. One that you are almost certainly familiar with, is Teflon. Teflon is the polymer of perfluoroethylene or tetrafluoroethylene. So here is ethylene and here is the tetrafluoroethlyene and when you polymerize that you get fluorines at every site, at every carbon. The fact that you have fluorines here is very important in determining the structure and the properties, let's just say, of Teflon. Teflon is very slippery because of the fluorines there and because of the interaction that the fluorines have with other molecules. Polystyrene is the polymer of styrene. Here is the fennel group of styrene, here is the alkene. When it polymerized, it forms this kind of a species.
Now, I have abbreviated the benzene ring right here as fennel, PH. So PH stands for benzene, which stands for fennel. So instead of writing it over and over and over again, I just simply have it abbreviated as PH. You can see where one of these monomers would be, would be circled in red there. Here is polyvinyl chloride. Polyvinyl chloride is used to make PVC pipe. That is what PVC stands for, polyvinyl chloride. By the way, polystyrene is used to make styrofoam. Styrofoam cups are made out of polystyrene and they are blow with air inside of them to make them very light, so they are mostly air and some polystyrene. PVC, again polymerized polyvinyl chloride. Here is one of the vinyl chloride units. Natural rubber is a polymer of isoprene. Polyisoprene essentially. In isoprene, we have two pi bonds and what happens is, because we have two to begin with, there is one pi bond leftover in the final product. You only need one pi bond to polymerize and the other pi bond that is left, rearranges and ends up being over here in the final product.
How are these compounds made? Well many of them are made by a radical synthesis. In this context, radical does not mean hip or groovy or whatever. What it means is, a molecule that has an odd number of electrons. Whenever we have a species with an odd number of electrons, we call that a radical. So in this case, we can take benzyl peroxide, and benzyl peroxide is the active ingredient in zit medicine, it is an antibiotic and that is how it works to cure zits. When we heat this up, we lose two molecule of CO[2] and generate two of these benzene radicals, where there is no longer a CH bond in that carbon and we just have an odd electron sitting right there. These radicals are very reactive species. They will react with almost anything and they react very quickly with alkenes. What they do is add to alkenes. So in an initiation step, the radical from the benzene molecule adds to the alkene to form this new radical. This new radical is also very reactive. It is now going to react with more alkene to give you this product. So the first step is the initiation, that gets the reaction started, the second step is called propagation step, this happens to give you this product. Now this product also has a radical, it can react with another alkene and it does that. And it keeps on and on and on until you have incorporated many molecules of the alkene into your own polymer.
Ultimately you end up with a molecule that looks like this, where N is a large number and we have many alkane units incorporated into this growing polymer. At the very end we have this highly reactive radical. Once all the ethylene is consumed or as the reaction is proceeding this very reactive radical can sometimes distract the hydrogen atom from the someplace in the reaction mixture and that terminates the end of the reaction with the CH[3] group.
So what we have is the following. We have a polyethylene unit where end we have the initiator that we used and the other end we have the terminator, which is oftentimes the hydrogen atom. Now the presence of the initiator and the terminator are really inconsequential because there is a large number of carbons in between and their presence is not really felt that much because they are present in very small amounts.
Here is an example of a condensation polymer. This is the reaction to make a polyester. We talked about the synthesis of esters before, when we talked about organic functional groups. And esters can be made from alcohol and carboxylic acid. In this reaction what we have is a molecule that contains two alcohols and two carboxylic acids. So you can see what can happen is that this action can react with that carbon to form, let's say, that bond over there. And this carbon can react with another molecule of ethylene glycol, which is what this is called to form that bond over there. And then the other option, the ethylene glycol can react with another molecule to form that bond and so on. We can keep forming bonds and as they alternate between the alcohol and the carboxylic acid. So by doing this we form a polymer that is a polyester that has many ester functionalities in it and that is why it is called polyester. This is actually the material that was used to make the polyester clothing in the 70's, maybe 60's and 80's and so on. In fact, it is also the material that is used to make two liter bottles. This reaction is a condensation reaction, because the byproduct of the reaction is water. So when you mix this and that together with a little bid of acid, water condenses out and that is the byproduct, hence the name condensation.
Nylon is another very important polymer. Nylon was invented in the olden days, when we wanted a substitute for silk. This functional group is called and amid, we haven't talked about this, but it is a very important functional group for biochemistry. We will talk about it when we talk about biochemistry. Silk is a naturally occurring amid. And chemists though in the old days, that perhaps we could mimic the properties of silk by making a synthetic polyamide. And the way the made it was. They started with this, acid chloride. This is called acid chloride and it is a very functional group. It reacts with this amine to form this amide over here. So by mixing this molecule, which has these two reactive ends to it and to this molecule, which has these two reactive ends to it, we can make a polyamide, where we have amide functionalities here, here and they repeat and go on for a very long array.
Kevlar is another polyamide. Only this time the monomers are benzene derived instead of being alkane derived. So in this molecule we have this benzene group with two amino and this molecule we have this benzene group with these two acid chloride units and they can condense to give HCL, to provide you with this amide and that amide and this can then repeat and do it again and for a long ascended chain. This material is very strong. It is used in tires. Instead of using steel, the belts are made of Kevlar, oftentimes. It is also used in bulletproof vests.
Lexan is another polymer and it is made from this molecule, which has two alcohol groups on a benzene ring. This kind of unit we saw earlier is called phenol. Whenever you have a benzene with an OH next to it, it is called a phenol. This can condense with phosgene. Phosgene was used as a chemical warfare agent. When you breath it, it blisters your lungs and liberates HCL. Back in World War I, this was commonly used. But when you mix these two together your form this ester unit over here. You form HCL and you form another ester unit on the other side. Phosgene has two of these chlorines, so it can actually react on either side of the C double bond O. And that is what forms this functional group over here. This is called a carbonate. When you have a carbonyl, with an oxygen on either side, that is a carbonate. And lexan is a polycarbonate.
Finally the dead ball demonstration that I did before. Here is the bouncy ball, here is the dead ball. The dead ball is made of this polymer. It is a hydrocarbon polymer that has the cyclopentane rings in the middle between alkenes. So I am not going to talk about why it is that this is a dead ball, but the bottom line is that the structure of this polymer gives it the properties that it has for being dead and not bouncing.
So the bottom line is that you have to understand that polymers are compounds that have properties that are determined by their structure. And by varying the structure of the polymer, we can get different properties, very useful properties. There are two kinds of polymers. There are homopolymers and heteropolymers. Heteropolymers contain different monomer units. Homopolymers are made of the same monomer unit. Polymers can be made by addition reaction or by condensation reactions. And a condensation reaction there is a byproduct. The byproduct is either water or HCL or some other molecule.
Organic Chemistry
Organic Polymers
Organic Polymers Page [1 of 3]

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