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  • Intro 0:00
  • Polymers 0:05
    • Monomer to Polymer: Vinyl Chloride to Polyvinyl Chloride
  • Polymer Properties 1:32
    • Polymer Properties
  • Natural Polymers: Rubber 2:30
    • Vulcanization
  • Natural Polymers: Polysaccharides 4:55
    • Example: Starch
    • Example: Cellulose
  • Natural Polymers: Proteins 6:07
    • Example: Keratin
  • DNA Strands 7:15
    • DNA Strands
  • Synthetic Polymers 8:30
    • Ethylene & Polyethylene: Lightweight Insulator & Airtight Plastic
  • Synthetic Organic Polymers 12:22
    • Polyethylene
    • Polyvinyl Chloride (PVC)
    • Polystyrene
    • Polyamide
    • Polymethyl Methacrylate
    • Kevlar
    • Synthetic Material Examples
    • How are Polymers Made?
    • Chain-growth Polymers Additions to Alkenes can be Radical, Cationic or Anionic
  • Chain Branching 22:34
    • Chain Branching
  • Special Reaction Conditions Prevent Branching 24:28
    • Ziegler-Natta Catalyst
  • Chain-Growth by Cationic Polymerization 27:35
    • Chain-Growth by Cationic Polymerization
  • Chain-Growth by Anionic Polymerization 29:35
    • Chain-Growth by Anionic Polymerization
  • Step-Growth Polymerization: Polyamides 32:16
    • Step-Growth Polymerization: Polyamides
  • Step-Growth Polymerization: Polyesters 34:23
    • Step-Growth Polymerization: Polyesters
  • Step-Growth Polymerization: Polycarbonates 35:56
    • Step-Growth Polymerization: Polycarbonates
  • Step-Growth Polymerization: Polyurethanes 37:18
    • Step-Growth Polymerization: Polyurethanes
  • Modifying Polymer Properties 39:35
    • Glass Transition Temperature
    • Crosslinking
    • Copolymers
    • Additives: Stabilizers
    • Additives: Flame Retardants
    • Additives: Plasticizers
    • Additives: Colorants

Transcription: Polymers

Hi; welcome back to

Today, we are going to be talking about polymers.0002

One way to make a polymer is to start with a single unit (we call that a monomer--in this case, this is vinyl chloride); and if you imagine that this monomer reacts with another, and then that reacts with another, and so on, we can build up a dimer and a trimer and so on, until finally, we have a polymer (many, many, many repeating units).0006

And so, this is an example of a polymer; because the monomer is known as vinyl chloride, this polymer is called polyvinyl chloride (or PVC--this is a really common polymer that you may have heard of).0029

And where we find polymers in our everyday lives are, for example, many of the plastics that we have all around us--those are examples of polymers.0043

Now, not all polymers are all carbon chains like this; we will see that there are other polymers, as well.0053

But we have thousands of repeating units--tens of thousands of units--and depending on what these groups are hanging off of the polymer chain, it's going to dramatically affect the properties of our polymer, and therefore affect their uses and what kinds of materials we can use from them.0059

Now, notice I have put a bracket around here--this is showing the repeating unit.0076

It has a chlorine and two carbons, and a chlorine and two carbons; so one thing we can do when we look at a polymer is try and identify what the monomer component is, and what is the repeating unit that goes on through the polymer.0079

Now, how do polymers act--what are their properties like?0093

Well, they are very, very long chains; and so, if you have them in liquid form, those long chains tend to get tangled up--they don't flow very easily.0097

So, as a liquid, the polymer solutions are kind of thick, or gooey, or sticky--things that have those kinds of properties usually have some kind of polymer involved with them.0105

OK, as a solid, depending on the polymer, you could have ones that are very flexible, or spongy and pliable, others that are rigid and very durable.0116

They could be transparent; we have plastics that we use for those, where they can be opaque.0125

And what we find is a very, very wide range of properties; so we end up with very useful materials.0131

And again, the chain lengths will vary of a given polymer--we have varying lengths--but our molecular lengths can range anywhere from 10,000 to a million grams per mole--so these are just huge, huge molecules, extremely long chains (not necessarily carbon chains).0136

Now, nature provides some polymers: for example, rubber comes from the rubber tree.0151

If you make a cut on the bark of the tree, as a defense mechanism, it produces this rubber; and so you can tap into that, just like you would tap into a maple tree to get maple syrup.0157

You could tap into that and collect the rubber; and that material is useful, because it's waterproof; so it can be used for waterproofing, and that is what it was used for many, many years ago.0168

But, if you tried to use it...0179

And here is the structure of it--so the way rubber is made is: we take this isoprene unit--it's a 5-carbon diene; that is the monomer, and when this forms a chain, here we see our 5-carbon monomer again; and this is the structure of natural rubber.0182

It has the Z configuration, like the cis alkene here of those two priority groups are in the Z configuration; that is what natural rubber looks like.0200

Now, if we think of rubber like a rubber band, we know we can stretch that, and it goes back to its original shape.0209

Well, natural rubber doesn't do that; the polymer chains, if you tear at them--they will just tear apart; there is nothing holding them together.0215

But, in 1839, Charles Goodyear discovered that he could do a process called vulcanization, where he could modify the rubber by reacting it with sulfur.0225

And what is formed here are disulfide cross-linking: we get these bridges--you can see now that some of the double bonds used to be repeating double bonds, but some of the double bonds have been reacted and have added the disulfide bridge.0235

And what that does is: it holds one polymer chain...holds them all together.0255

And so now, we can stretch it, and it will conform to its original shape.0260

We describe such polymers as elastomers--ones that are capable of being stretched, and will come back.0266

That is what we recognize as...things like rubber bands...rubber that we use in our everyday lives.0274

Now, you might recognize the name Goodyear, because that is who created Goodyear tires, and so this is the material that was used for tires and other things that we use rubber for these days.0280

Polysaccharides are another example of natural polymers: this is when we have saccharide sugars linked together in very, very long chains.0296

A couple of examples of that: starch and cellulose--these are all glucose units, in both of these (in fact, I just noticed there is a typo--sorry about that); these are all glucose units; if the glucose units are hooked together where this oxygen, this glycosidic bond, is pointing down (that is the α linkage), that is what we know as starch.0305

That is the polymer that is found in potatoes, wheat, corn, rice--the things we know as kind of starchy materials; and we use that starch in cooking, as a thickening agent to make sauces and gravies thicker and so on.0327

There is some branching that can occur here, if it's a branched structure, as well.0341

And cellulose is the structure, when glucose has this glycosidic bond, where this oxygen is pointing in the up direction (the β linkage).0345

Cellulose is what is used for cell walls; so we can use that material for making paper or cotton or cardboard or something like that.0353

So, there are natural polymers that are used for materials, as well.0361

Some other examples of natural polymers include proteins; so a protein is what we get when we take an amino acid, and we link it together as an amide.0367

An amino acid has this...this was a carboxylic acid; this was an amine; and then, they hook together as this amide linkage.0379

And these R groups vary, depending on what amino acids you have used; so again, proteins can have a very wide variety of structures.0386

And they are not necessarily a single monomer, because every amino acid can be different; but again, we have these repeating units of very long chains; so that is why we describe them as a polymer.0397

And certain proteins also can be used for materials for structural things that nature builds.0408

For example, keratin is a protein that is used for hair; it's used to build horns, fingernails, claws, the scales that are in snakes, and bird feathers, turtle that is used in a wide variety of natural materials--some usually pretty durable materials.0415

And finally, DNA strands are also examples of polymers; a DNA strand has a sugar, phosphate backbone, and again, it varies.0436

We don't have identical monomer units, because the base--this is a ribo sugar or a deoxyribo sugar, depending on whether it's RNA or DNA--and the bases that we have here vary.0444

We could have adenosine or thymine and so on.0458

And it is those bases that hydrogen bond with other bases, and that is how we get the double-stranded DNA that goes to the helical shape, and so on.0462

So, this is also a polymer that has sugar and phosphate as its backbone, there.0473

Now, the way you can observe DNA as a polymer is: you can search on YouTube for some videos on how to isolate DNA from strawberries or from bananas; it's a really easy experiment you could do at home, where you denature the break down the cells, and then you can pull out these really, really long strands of stringy material, and that is actually the DNA.0480

You can see really, really long strands of the DNA that you can isolate; it's kind of a cool experiment to try.0503

Nature does provide us with certain polymers, but where it has really come into our everyday lives and really improved our lives through chemistry are looking at synthetic polymers.0512

That is where we develop polymers in the lab.0525

Now, this was actually an accidental discovery in 1933 at the Imperial Chemical Industries company in Great Britain.0528

And what a chemist did there was: he took a gas, like ethylene--I think he had the fluorinated version, tetrafluoroethylene--and he had it in a cylinder, a high-pressure amount of it in a cylinder, and put it under high pressure.0535

And he was going to use it for something, but when he let it sit overnight, he realized that in the morning...he looked at the pressure gauge, and there was no pressure left in the cylinder.0550

But the cylinder still weighed the same amount it did when it was full, so the material was still there; it was just no longer a gas.0560

He opened up the cylinder and found a powder in there instead of the gas; and so, he had discovered, accidentally, that this gas had polymerized (had formed these long, long, long chains) and made this material.0567

Now, he did the fluorinated version of that, so there were fluorines in all of these positions instead of hydrogens; and what he discovered was Teflon®.0584

And he tested this new material and found that it was unreactive; it wouldn't dissolve anything, wouldn't react with anything; you couldn't burn it; and so, what do you do with this?0593

But it turned out it had really cool applications: what we use Teflon® for...related materials are nonstick surfaces.0604

So, if we coat our cookware with that, it is very easy to cook, and things don't stick to it; so that was one example of one of the polymers that we use today.0611

If we use ethylene--if we look at the simplest polymer here, ethylene, where all it is is a 2-carbon chain--when it bonds together, we call that polyethylene.0621

Now again, this was discovered in the 1930s, and shortly thereafter some commercial uses were developed for it: because it's very lightweight, and it's hydrophobic, it was found that it could be a very good insulator.0632

So, by 1938, it was used to coat a telephone cable so that it could be laid underwater: that was the first time they were ever able to do that, because they didn't have a suitable material to waterproof it.0649

When World War II came around, they were able, using polyethylene plastic cases, to use that as an insulator around their radar units and radar devices, and they were able to install those in airplanes for the first time.0663

Radar is used to locate where submarines are under the water, and the British forces were able to install those in their aircraft for the first time ever, so they were able to take out all of the enemy subs, and that was a huge advantage in the war, and really made a big difference in their success in World War II.0679

And it turns out that it is a great insulator; it also can be made to be flexible and airtight.0701

So, in 1948, Earl Tupper from DuPont found some good uses for it, and you might recognize something like this: this is Tupperware®.0708

So, we could have flexible things that could keep out air and keep it airtight; so when it came to food storage, this really was a huge turning point, and really revolutionized the way we are able to keep our food safe and keep it lasting longer, thanks to plastic food storage.0716

And that is just the tip of the iceberg: really, there are so many more polymers, and just a few examples of that...0742

We talked about polyethylene; we saw some of the applications of that for Tupperware®; it is also what you have for plastic bags, Ziploc® bags, those sorts of things.0749

It's flexible and see-through and that kind of thing; Glad® Wrap, some of the wraps that we have, that we can put food in, as well, are polyethylene.0764

Polyvinyl is the structure of polyvinyl chloride; it has repeating chlorine units on there; that is called PVC.0775

Now, PVC, depending on how it is manufactured, can be either very rigid (so we could use that for sprinkler lines, to run water), or it can be very flexible (so if you have inflatable toys, those are also made from PVC, so that is another interesting use for that--a lot of use for toys) seats and that kind of thing...a lot of materials there...0783

Polystyrene is what we call it...styrene itself (this is styrene) is benzene with a double bond; so again, this is an ethylene derivative, but instead of a chlorine now, we have a benzene ring; so this is called styrene.0809

The polymer version of that is called polystyrene; polystyrene we use for foam cups or plastic cups, we can have some polystyrene that is light and flexible, like styrofoam cups; or we can have it more rigid to be utensils.0823

Those are all very good uses of polystyrene.0842

We can have polyurethane; polyurethane can be used (I'll have a structure of that down the road) for mattresses or the soles of your shoes; so that can be more of a foamy kind of flexible material.0846

The adhesives that we know of (like Scotch® tape, Post-It® notes)...the sticky part of that is a polymer material.0864

The synthetic fibers in fabrics--things like nylon, Dacron®, rayon, polyesters...those are all man-made materials.0874

Here is an example of a polyamide; so here is an amide functional group; and when you have many, many repeating units of that, that is an example of something like the fabric we use for nylon, that you might use for clothing, or you might use for carpeting and that sort of thing.0882

Polymethyl methacrylate is what we use for Plexiglas®, contact lenses, if you want something that is very rigid and you can see through, very strong (think about fish tanks at the aquarium--when you have those huge fish tanks, those would be something like polymethyl methacrylate, Plexiglas®--that sort of use).0898

And even Kevlar®--Kevlar® is very, very strong, stronger than steel, and we can use those lightweight, durable plastics for motorcycle helmets, bulletproof vests, and things like tires.0925

If you think about it, hundreds of years ago, if you wanted to protect yourself (let's say you were going into battle and you wanted to protect yourself), what choices did you have--what options did you have?0940

The only options you had were things that were provided by nature; so you would maybe have an iron helmet, right?--or chain mesh armor, right?0952

Think of a suit of armor that you have with people who are jousting or something like that--very bulky; very heavy; not very practical.0963

And so, what do we have today when we want to protect our heads during sporting events?--we have hard plastic; we have foam; it's very lightweight, and very, very strong.0971

We have bulletproof vests that are very flexible, that can be worn under clothing very comfortably.0980

You don't get fatigued--so really great advances.0986

Another great advance are things like sporting equipment; it is also something that has really, really been revolutionized, thanks to synthetic polymers.0989

If you think about tennis--now, tennis has been played for hundreds of years, but if you wanted to make a tennis racket 100 years ago, what would you use for the strings--what options do you have?1000

I mean, you have cotton (that is not really strong enough), or twigs, or something like that (it would be pretty brittle).1015

Well, it turns out that they used...they called it catgut, but it was actually...I think it was usually sheep intestines.1021

They would take the intestines (because that is something that nature provides, and it's long's a natural-made polymer) and stretch those out and dry those out, and use that to string tennis rackets or violin strings (it used to be gut used for that, as well).1028

So nowadays, we have synthetic polymers for that--very strong, very durable, very lightweight.1047

We used to have wooden rackets; they would have to be wooden, because that was the only material you had; and those would warp, and they would be very heavy, and they would splinter.1052

So, there are really amazing things that we can do thanks to synthetic polymers.1059

If you think about really any sport...skis or roller blades or in-line skates or skateboards--all those materials are great; if you think about golf, golf used to know, irons and woods are the types of clubs we have in our golf bags, and that is because they used to be made of iron and wood.1064

Those were the only choices; but of course, nowadays, we have really great, strong synthetic polymers that are used for the shafts; very strong, but lightweight and very durable...all sorts of interesting composites that are made.1086

Not all of them are necessarily polymers, but again, a lot of great chemistry research is going into these sorts of materials.1102

And another place where life is much better thanks to chemistry is diaper technology.1109

Not only is the material on the outside made from nice, stretchy, breathable polymers, but inside we have a special powder--some beads in there...sodium polyacrylate is one example...called waterlock.1118

And so, that is something that, again, is a polymer; it has carboxylate groups on there; and it loves to grab onto water.1133

This can absorb so many times its weight in water, and really lock in and last overnight, if you have a baby that you want to keep dry overnight.1141

It is also used for artificial snow: if you have ever seen that little demonstration where you have a little bit of powder, and you put some water on it, and all of a sudden it just puffs up and becomes this fluffy material, that is the same thing we have in the diapers.1152

But if you use it just free-flowing, they can use that in movie productions to make it look like fake snow in the middle of the summer, and so on.1163

But naturally, before we had these polymers, we could have paper; we could have wood; we could have cotton.1172

The same thing if you wanted to be dressed: you could have cotton; you could have linen; you could have animal skins; so you could use leather...1182

And we still use those materials, certainly, but they are very limited in their flexibility and their durability; they can stain very easily; they don't hold up very well with washes.1191

And so, you can have...with synthetic materials, like rayon and polyester and so can have materials that are very long-lasting and stretchable, and have much better properties.1204

If you even think...I'm thinking back to...if you were a pirate, and you had some injuries, you would end up with a hook hand and a peg leg, right?--because those are the only materials available to you.1217

But nowadays, we have veterans returning from wars with injuries, and they have amazing prosthetic hands and legs and artificial hearts.1228

And if you think about the application for medical devices, as well, we wouldn't be able to do any of the things that we can do today without polymers.1240

So, polymers are exciting, because they really touch our everyday lives all around us.1248

Let's talk a little bit about how polymers are made and what synthetic polymers are like, and look at some of the functional groups involved.1253

If we go back to polyvinyl chloride and think about how they're made, they are an example of what is called a chain-growth polymer.1262

We call it that because it is a chain reaction that gives rise to its structure.1269

Now, we start with alkenes; and the addition reactions can be either radical or with cations or anions.1275

We'll look at examples of all of those.1282

PVC can be made with a radical reaction: if we start with some kind of initiator--something that is a radical source--radicals can add to alkenes; and the reaction that happens is: it steals one electron from the double bond to form a σ bond, and one electron stays behind.1285

And so, we end up, now, with a new radical; and if we have plenty of this monomer around, of the vinyl chloride, then this radical can add another unit of a monomer, and then we would get a dimer.1305

We just formed a new bond; and then we have two more carbons and a chlorine and another radical.1321

We would call this a dimer; and this could add again, and so on; etc.1327

So, many, many times--thousands of times--and we end up with this polymer called the polyvinyl chloride.1333

OK, so radical reactions are really great for a chain reaction, because in their nature, they are chain reactions.1339

Every addition to an alkene results in a new radical, which can continue adding to another alkene, and so on.1345

That is a really great strategy.1351

OK, now one drawback with using radicals is that, besides just adding to alkenes, they can also do atom abstractions.1354

What we end up with (sometimes) is some chain branching.1364

We have this growing polymer chain (this is just polyethylene), and we have this radical here; and rather than adding to a new alkene, it can interact--it can instead bump into one of the existing polyethylene polymer chains that is in the reaction mixture, and it can abstract a hydrogen atom.1368

And what that means is: it plucks a hydrogen off of the chain.1387

So, just like the reaction we have seen in free radical halogenation, radicals can pluck off a hydrogen, and that leaves a radical behind.1391

OK, now that gives us a radical that is within the carbon chain--within the polymer--somewhere in the middle.1399

And that radical can now continue on and do an addition reaction; and now, all of a sudden, we have created a branching point; and this radical can continue, and so on.1407

It is possible to have some branch points when we do these radical reactions, and when we do that with ethylene, the resulting product is described as low-density polyethylene, or LDPE.1421

OK, now it is called low density, because if you have branched chains, they don't pack very well; and that makes them less dense; that makes them soft, and that makes the material flexible.1437

The material we have in our plastic bags, like a Ziploc® bag, is going to be something that would be a low-density branched chain.1448

OK, it's kind of like if you are trying to pack sticks, and all of the branches have branches on them; they don't pack very tightly--they would make a very loose pile of sticks.1456

Now, there are certain things we can do--depending on your reaction conditions, you can control whether or not you have branching.1469

And one example of that is to use the Ziegler-Natta catalyst.1476

This was such a significant discovery that it earned these two gentlemen the Nobel Prize in chemistry in 1963; and it is an aluminum catalyst.1481

If you mix this with titanium tetrachloride (I think I have a...this is just a + sign here; this kind of looks almost like a carbon chain--sorry about that) mix these two reagents, and that creates this: this is called the Ziegler-Natta catalyst.1497

And (there is my giant + sign again) now, if we take this catalyst and react it with ethylene, what happens is: the titanium kind of grabs onto the ethylene--it holds onto the ethylene--and then, it inserts it into this titanium-carbon bond and extends the chain.1513

Now, there are four carbons attached here.1532

And then, it grabs onto another ethylene molecule, and it inserts it again.1534

Now, we have a 6-carbon chain, and so on.1540

So, one by one, it adds a single ethylene molecule; and in doing so, because there is no radical mechanism, there is no way you can get branching to occur.1542

So, this is a way of forming only long carbon chains.1553

Now we have linear chains; and when these linear chains pack (now again, imagine packing sticks that have no branches on them--when you pack those sticks, you can pack them very tightly together, and so) we are going to get a high-density material.1557

This would be called high-density polyethylene, or HDPE; that is going to be a stronger material, and less flexible; so that is what we have in something like milk jugs.1572

It is going to be still polyethylene, but it is going to have different properties.1581

Now, this catalyst is also extremely useful because it will also control the stereochemistry of the polymer, if we create chiral centers.1586

And again, this affects how the material is going to pack; and therefore, it affects the properties of that material.1596

So, there are certain ways that the catalyst will act, where, if you are creating a chiral center, all of the chiral centers will be pointing in the same direction; we call that isotactic.1602

Here, they are all wedges.1612

Or, you could have ones where they are alternating: so we have a dash, wedge, dash, wedge; that is called syndiotactic.1614

And without the catalyst, you get just random orientations; so some are dashes; some are wedges; they just kind of add on any way that they want--that is called atactic.1621

So, just like if you have all right-handed gloves, you can pack those differently than if you had a mixture of right-handed gloves and left-handed gloves; the stereochemistry affects the shape of the molecule, and therefore it affects the packing.1630

There are ways that you can control the stereochemistry of certain polymers, as well, by varying the catalyst that is being used.1645

Now, besides radicals, we can also use cationic polymerization.1657

Now, that is only going to be useful for stable carbocations; so things that are tertiary, or maybe benzylic--those would be good polymers that can be made by this cationic mechanism.1661

So, what we would do is: we would start with a monomer--for example, this is isobutylene; and if we react this with an acid, the alkene can get protonated.1674

Now, of course, it would protonate on the end carbon--remember Markovnikov's Rule?--we would add the proton to the carbon with more carbons to get this tertiary carbocation.1683

Of course, that is a very stable carbocation; and if you have more of this monomer around--if you have plenty of this monomer around--then, after you form this carbocation, a second alkene can react.1694

Just like we attacked a proton, an alkene can also attack a carbocation.1706

And so, that is going to result in...we have our two methyls; this was the new bond that we just formed, and it is going to form in this direction, so that the resulting carbocation is again tertiary, and so on.1711

And so, we can build this carbon chain: this would be called polyisobutylene.1726

And again, we see our repeating unit, these four carbons: two carbons with the two methyls.1731

And polyisobutylene is this kind of segment of the chain; it's found in a polymer called butyl rubber, and butyl rubber is actually a copolymer, where you mix both isobutylene and isoprene together, so you get different patches of polymers.1736

Some parts of the polymer look like this; some parts of the polymer have isoprene units built into it.1754

But that synthetic rubber is very useful for things like basketballs and rubber gloves and even chewing gum.1760

So, the polymer that we chew (again, kind of stretchy and gooey--it must be some kind of polymer) is usually butyl rubber.1767

And then, finally, we can have anionic polymerization; so that is where you have an anion being formed as an intermediate.1777

Now, that would be...again, for that mechanism, we would need to have some way to stabilize the anion; and the way we usually do that is: we have an electron withdrawing group on there--something like a cyano group or an ester group or a carbonyl, and so on.1782

So, in this case, we would initiate that reaction by adding some anion in (like butyllithium, let's say), and this is going to attack the alkene.1796

We would describe this as a conjugate addition, a 1,4 addition; we have seen this before, when we have electron withdrawing groups; that makes this carbon partially positive.1807

It has resonance to make this carbon partially positive; that is where the anion would add.1817

And why can we form this anion?--we can form this anion because of the presence of both of these electron withdrawing groups.1821

The cyano is an electron withdrawing group; the carbonyl and the ester is the electron withdrawing group; and so, that gives us resonance.1830

We can have resonance with the carbonyl; so this is a stable intermediate.1837

And we could also have resonance with the cyano; let's show it coming back down here.1845

You can come back down here, and then, instead, go to the cyano.1849

There is a lot of resonance stabilization here, and that is why this would be a suitable monomer to undergo an anionic polymerization reaction--chain growth.1858

This monomer is called cyanoacrylate; and after it adds again, we can bring in another monomer; this can do another 1,4 addition.1871

And there is the new bond that we formed; and then, this has, again, a cyano and the ester and the new anion--the new resonance-stabilized anion...and so on, etc.1889

Oh, I'm sorry; I drew it down here; and so this can continue, etc.1903

OK, so here is our polymer; this polymer is called a cyanoacrylate polymer, and this is used as an adhesive; you have used this if you have ever used superglue or crazy glue--one of those instant glues.1909

They will polymerize; they will harden when they are exposed to air and they are exposed to a catalyst.1924

So, this would be a very good adhesive that we can use.1931

Now, instead of a chain the mechanisms we have seen up until now are called chain growth polymers: we start with one chain--one end of the polymer--and it keeps adding to that same end to make chains, either with a radical intermediate or an anion or a cation intermediate.1938

OK, another type of polymer, and another type of polymer growth is called step growth.1955

This is the opposite of chain growth, where what we are doing is: we are reacting two functional groups.1960

And so, for example, if we have a dicarboxylic acid and a diamine, then what happens when an amine reacts with a carboxylic acid?--this nitrogen can attack the carbonyl and replace that OH, and we can form an amide.1967

The carbonyl now has a nitrogen attached; and what did we lose at the same time?--we lost this OH and this H: we lost a molecule of water.1988

These reactions are described as condensation reactions; whenever you lose a small molecule in the process of a mechanism, we describe it as a condensation.1998

And so, we can have both ends--both ends of the amine will react with carboxylic acids, and will make amides and grow on.2008

So, in a step growth, we can have reaction at either end, and they are growing kind of one step at a time; they will build together to make a dimer and then a trimer and then a tetramer and then a "polygomer" (that is what we call it when we have kind of multiples), and then those oligamers can come together to form the polymers, and so on.2018

It kind of can grow from both ends at the same time.2037

This is an example of a polyamide, because we have the amide functional groups; and it is called nylon--this is called nylon 6,6, because the diamine had 6 carbons and the diacid had 6 carbons.2042

So, this is called nylon 6,6; it is a very common nylon that is used for synthetic fabric in carpeting.2055

And there are many other functional groups you can make; we can also make polyesters--you have probably heard of polyester clothing, a really great way for fabrics to maintain their color, and they are long-lasting.2064

The way we make an ester is: again, we have a carboxylic acid; we have a leaving group; but now, we are reacting it with an alcohol--a diol.2078

So now, it's the oxygen that attacks the carbonyl, and so the resulting product is going to be an ester; the resulting functional group is an ester.2086

Here, if we use this dicarboxylic acid and ethylene glycol as our diol, the product we get--the polymer--is called polyethylene terephthalate, or PET; this is what is used in plastic soda bottles--it is one of the types of recycling that we have.2096

Another big area of polymers are the processes we can do to recycle them and break them down and reuse them in other applications.2112

And so, if we wanted to pick out the repeating unit here, we could start with is our diacid, and then it hooks up with a diol, and so it would be these two carbons, and then those two oxygens.2122

That would be an example of a polyester.2141

And this is another example of a condensation reaction, because this gives off a molecule of water as the ester is formed.2145

We could also have polycarbonates; a carbonate is what we call what kind of looks like an ester, but you have an O-R group on both sides; that is described as a carbonate.2158

And so, how do we make a carbonate?--we need phosgene or some other related derivative that has leaving groups on both sides.2167

If you have a leaving group on both sides, then when you react it with an alcohol, each alcohol will replace one of the chlorines; so we'll end up with this carbonate structure there.2175

This particular polymer is called LexanTM; it is used for bike helmets, bulletproof glass, polycarbonate lenses you can get in your eyeglasses (they are stronger and lighter weight and thinner)...2188

And again, this is another example of a condensation reaction; what is the molecule that is lost when we form this ester?2201

We lose the chlorine, and we lose the hydrogen from each of these alcohols, so we are losing two molecules of HCl for every carbonate functional group that is made.2213

This is another example of a condensation reaction; most step-growth polymerizations are condensations.2227

OK, and then, another example is called a polyurethane; and a urethane is the functional group when you have a carbonyl with an O-R on one side and a nitrogen on the other side, so it's like an ester and an amide both sharing the same carbonyl.2240

We call those urethanes.2256

So, the way we make a urethane is kind of interesting: we start with this functional group--it's called an isocyanate, and if you react that with an alcohol (this is our nucleophile), if it attacks the carbon and breaks this nitrogen bond, you can see how we can form this urethane.2258

We have this O-R group attached to a carbonyl and then attached to a single bond, N-R.2274

That is kind of the general mechanism on how we get urethanes from isocyanates.2280

The monomers we would need then, again, would be a diol (like ethylene glycol) and some diisocyanate; so notice that all of our monomers are bifunctional--they need a functional group on each end so that they can grow in both directions--that is how we form the chain, like linking paper clips together.2285

And so, if we mix these two, and if we throw in some butane when we do this process (butane is a very small molecule--it's very volatile), this reaction produces heat.2304

And so, what happens is: the butane forms bubbles as a gas; and so, the product we get out is going to be a foam, like we would use in foam mattresses or the squishy part in car seats, shoe soles--we need something that can compress and be a shock absorber.2317

This would be a great way--polyurethanes are very useful in that application.2335

So again, where we used to have the isocyanate, that has now been converted to a carbonyl; and then, here is the diol group that we added in.2342

So, the repeating unit doesn't matter which side you go on; here I see my diol component, and then here is my isocyanate component, and then we go to the other carbonyl; so there is our repeating unit for the polyurethane.2351

Oh, I'm sorry, I wanted to point out that that particular reaction did not produce a small molecule when it did the reaction of the isocyanate with the alcohol; no small molecule is ejected, so that is actually not called a condensation reaction.2375

So, even though it is step growth, not all step growths are condensations; that is one exception--the formation of the polyurethanes.2390

Sometimes they are used synonymously, and I just wanted to point out that that is not entirely correct.2398

OK, finally, let's talk about how you can maybe modify the polymer properties.2404

So, for example, one of the properties that we would measure if you were doing polymer chemistry and characterizing a new polymer after you made it--you would be measuring things like its glass transition temperature.2411

That is the temperature at which the polymer softens; so you might have a rigid polymer that is a solid, that is hard; and then, as you start to heat it up, it starts to soften.2425

The temperature at which it softens and can become pliable is known as the glass transition temperature.2435

And we can modify properties such as that by doing things like cross-linking (we already talked about this) with the disulfide...cross-linkage: you can do that sort of thing, like we saw with the vulcanized rubber.2442

You can make copolymers, like we saw with the chewing gum (the butyl rubber).2459

That is where you have, not just a single monomer where you get all of the same units repeating all throughout, but you throw in a mixture of monomers.2466

And so, you can get some copolymers that end up alternating, depending on the monomers, depending on your reaction conditions, and so on.2478

So, you can get some copolymers that alternate, or, like in the case of butyl rubber, you can get some that have chunks of A and then chunks of the B and so on.2491

You can have kind of a random grouping.2503

And then, of course, you can have other ones where it's just a total mess; but those are kind of two of the more common types of copolymers that you can have.2505

And again, by adding in different monomers, by changing the structure of the polymer chain, you are getting some different properties; and we can design our polymers to have exactly the properties that we want.2513

You can also add in a variety of...typically small organic molecules that will be kind of interdispersed throughout the polymer.2528

You have things like stabilizers, so we can add things that might act as antioxidants or protect the material against UV, if this is a know that if you have plastic materials that are out in the sun for a long time, they usually turn yellow; they usually turn brittle; they crack.2539

They don't last very long: OK, well, you can improve their longevity if you are making a product that is going to be outside--going to need to be UV-resistant.2558

You can add in components that will absorb the UV or scavenge the radicals that to help slow the decomposition of the plastic and make it last longer.2569

Those are some things that you might be able to add.2580

You can also add flame retardant; so, for making a material that is going to be a couch in someone's house, we don't want it to burn easily.2583

If you add a flame retardant in with the polymer materials into the foam, into the fabric, then that is something that will hinder the possibility of it catching fire, or cause it to be much slower-burning.2592

And of course, the slower-burning something is, the better chance that anyone who is inside the house has a chance of escaping harm.2606

So, that is a really important additive that we can have to make polymers safer.2615

You can also add things called plasticizers, and plasticizers are things that make materials more flexible.2622

A lot of polymers, by nature, end up being kind of rigid or brittle, and they just break easily; so those wouldn't have many useful commercial applications.2629

But, if you could add something to make it more pliable, then we have all of the fun and interesting materials that we have.2638

Now, some of the controversy--some of the problems that we have in the polymer industry is: some of the materials that have been added (let's say as a flame retardant, or something to make it more pliable) sometimes can leach out and enter our systems, and they might have some detrimental health effects.2644

There are some plastics where they are outlawing certain plastics in baby toys, and that sort of thing, and so they are trying to be as safe as possible; you are trying to do this trade-off between making something safer versus something that might solve one problem, but maybe cause another.2664

That is a very interesting...a lot of media, a lot of interesting articles to read about those various additives to polymers.2686

And of course, you can add colorants, because if you open up a toy chest, or you go to Toys R Us, and you see all of the toys on the shelves--of course, they are all beautifully colored, long-lasting colors.2694

And those are typically not natural properties of the carbon chains; the carbon-chain polymers are usually colorless, just white; and so, we can add in colorants to give them all of the vibrant colors that we have in our fabrics and the latex paints, and that kind of thing--we have colorants in there, as well.2707

Hopefully, this has given you a brief introduction into how polymers are made--a little bit of the polymer terminology that exists--and gives you a better appreciation for how we truly have better living through chemistry, especially when it comes to polymers.2728

Thanks very much, and I hope to see you again soon.2745