Polymers If different ages of the past – Stone Age, Iron Age, Bronze Age – have been designated by the break-through material which enabled humankind to progress, then what Age do we live in today? It could be argued that for the past 150 years, we have progressed as far as we have due largely to our use of polymers. Thus the age in which we live could rightfully be dubbed “the Age of Polymers.” From carpets to clothing to cars to cell phones, it would be hard to imagine life today without polymers. Polymers are essentially long-chain molecules made up of small molecules called “monomers”. These monomers get linked together through a process called “polymerization.” We, ourselves are largely made up of proteins which are polymers made out of amino acid monomers – more on that later! Right now, it is important to consider that there are basically just two methods of polymerization: addition and condensation.

Polymers Let’s start off with addition polymerization. Addition polymerization starts with a free radical and a bunch of double bonded monomers. Recall that a free radical is simply some molecule with an open bonding site: a single unpaired electron. There are several different free radicals that can be used, so we will just represent this free radical as a big blue sphere with an unpaired electron: Think of this free radical as a particle that is “looking” for electrons to complete its octet: essentially, it’s hungry to form a bond! The simplest double-bonded organic molecule is ethene (also known as “ethylene”), which you will recall is made up of two carbon atoms and four hydrogen atoms: Although we have written it like this, we should realize that it actually has more of a bow-tie shape: Which we will tilt back to emphasize the double bond:

Polymers Make a guess what you think will happen when the free radical encounters the double bond between the two carbon atoms. (Write this down in your notes.) Then click to see if you were correct. Was your prediction correct? Write down what you just saw happen. What you should have noticed is that the free radical broke apart the double bond and formed a single bond to one of the carbon atoms. But you also should have noticed that there is now a new unpaired electron on the other carbon atom. In other words… we still have a free radical. Predict what you think will happen when another ethene molecule comes along. Do you see where this is headed? Were you correct? And how will this end??? (Keep in mind that several billion free radicals were added to a container holding perhaps a few trillion ethene molecules.) Write down your prediction first.

Polymers That’s right, when two of these polymerizing free radical chains meet up, they bond to each other and the polymerization stops. With that in mind, write down your thoughts on how chemists can control to some extent the average chain length for this type of free radical initiated polymerization. Does it make sense that adjusting the initial concentration of the free radicals would affect the resulting chain length. So, which would lead to a longer chain length: a low concentration of free radicals (like on the left) or a high concentration (on the right)?

Polymers Is this what you were thinking – that the lower the free radical concentration, the longer the average chain length? Note: since the polymer is made of many repeating ethylene units all linked together into one, it is known as “polyethylene,” and it is by far the most commonly used synthetic polymer in the world. Also note: in the diagrams below, the polyethylene chains are only a few dozen ethylene units in length at the longest. Actual polyethylene molecules are often thousands to hundreds of thousand of units long! And also note: the chains are far more curvy and twisted and rope-like than the diagrams below show.

Polymers So ethene (ethylene) is only one monomer that can undergo addition polymerization. Really, any molecule with a double bond can do it. What if one of the hydrogen atoms were replaced with a chlorine atom? This would make chloroethene (AKA: “ethylene chloride” or, by an even older naming system: “vinyl chloride.”) Can you visualize what would happen if vinyl chloride were polymerized? In your notes, draw a picture of what it would look like. Is this what you drew? And what would this polymer be called? Polyvinyl chloride, of course – or PVC for short. And hopefully you can see that with all those chlorine atoms, polyvinyl chloride would have very different properties than polyethylene.

or simply n CH2=CHOH  ( CH2-CHOH )n Polymers Similarly, if one of the hydrogen atoms on an ethylene molecule were exchanged for an alcohol (-OH) group… instead of polyvinyl chloride, it would create polyvinyl alcohol (or PVA). This polymerization reaction could be written as a chemical equation in the following way: CH2=CHOH + CH2=CHOH + CH2=CHOH + …  CH2-CHOH-CH2-CHOH-CH2-CHOH-… or simply n CH2=CHOH  ( CH2-CHOH )n Try writing the reaction for the polymerization of vinyl chloride described on the previous slide. Where n may be greater than 100,000! Is this what you came up with? n CH2=CHCl  ( CH2-CHCl )n

Polymers And if all four of the hydrogen atoms on an ethylene molecule were swapped out for fluorine atoms… Tetrafluoroethylene would polymerize into _______???__________ Were you thinking polytetrafluoroethylene? That’s a mouthfull! Sometimes it’s called PTFE. More often it goes by the name “Teflon.” Teflon is one of the few polymers that contains no hydrogen at all: Try writing the reaction for the polymerization of tetrafluoroethylene Is this what you came up with? n CF2=CF2  ( CF2-CF2 )n

Polymers Let’s try another addition polymer: polypropylene. Right now, draw a sketch of what you think it looks like. As you should be able to tell by now, it starts with propylene, which is just another name for propene which looks like this: But if you think about it, propylene is just an ethylene molecule with one of the hydrogens replaced by a methyl group (-CH3) So… now can you visualize what polypropylene would look like? … like polyethylene with one methyl branching off every other carbon. And how would you represent its polymerization reaction? Is this what you came up with? n CH2=CHCH3  ( CH2-CHCH3 )n

Polymers OK, one final addition polymer: polyacetylene. Acetylene you may recall is another name for ethyne: C2H2. Instead of a double bond, it has a triple bond. So draw a sketch right now of what you think this will polymerize into. Here’s a hint: the free-radical only has one unpaired electron… So it can only break open one of the three bonds between the carbons. Is this what you were thinking: a polymer chain with alternating single and double bonds? And how would you represent this polymerization reaction? Is this what you came up with? n CH≡CH  ( CH=CH )n

Polymers Now let’s look at how condensation polymers are formed: It is a very different process than addition polymerization, and as you might have guessed from the name, it almost always involves water. Usually condensation polymerization involves monomers with easily removed H atoms on one end and easily removed –OH’s on the other. One example of this was mentioned earlier: the polymerization of amino acids into proteins. Amino acids, as the name implies are organic molecules with an amine functional (-NH2) group on one end and a carboxylic acid (-COOH) on the other. When two of these come together – the amino end of one and the acid end of the other… An interesting rearranging of bonds occurs: C O N H C O N H

Polymers Now let’s look at how condensation polymers are formed: It is a very different process than addition polymerization, and as you might have guessed from the name, it almost always involves water. Usually condensation polymerization involves monomers with easily removed H atoms on one end and easily removed –OH’s on the other. One example of this was mentioned earlier: the polymerization of amino acids into proteins. Amino acids, as the name implies are organic molecules with an amine functional (-NH2) group on one end and a carboxylic acid (-COOH) on the other. When two of these come together – the amino end of one and the acid end of the other… An interesting rearranging of bonds occurs: C O N H H C O N H O H

Polymers Now let’s look at how condensation polymers are formed: It is a very different process than addition polymerization, and as you might have guessed from the name, it almost always involves water. Usually condensation polymerization involves monomers with easily removed H atoms on one end and easily removed –OH’s on the other. One example of this was mentioned earlier: the polymerization of amino acids into proteins. Amino acids, as the name implies are organic molecules with an amine functional (-NH2) group on one end and a carboxylic acid (-COOH) on the other. When two of these come together – the amino end of one and the acid end of the other… An interesting rearranging of bonds occurs: Then it happens again… and again… C O N H H O C O N H H O C O N H H O So… in condensation polymerization, each time a polymer link is made, a water molecule is “condensed out.” C O N H

Polymers Cellulose – the main biopolymer of which plant fibers are made – is a condensation polymer made from glucose (C6H12O6) monomers: Watch how it undergoes condensation polymerization.

Polymers Cellulose – the main biopolymer of which plant fibers are made – is a condensation polymer made from glucose (C6H12O6) monomers: Watch how it undergoes condensation polymerization.

Polymers Cellulose – the main biopolymer of which plant fibers are made – is a condensation polymer made from glucose (C6H12O6) monomers: Watch how it undergoes condensation polymerization.

Polymers Cellulose – the main biopolymer of which plant fibers are made – is a condensation polymer made from glucose (C6H12O6) monomers: Watch how it undergoes condensation polymerization.

Polymers Cellulose – the main biopolymer of which plant fibers are made – is a condensation polymer made from glucose (C6H12O6) monomers: Watch how it undergoes condensation polymerization.

Polymers Cellulose – the main biopolymer of which plant fibers are made – is a condensation polymer made from glucose (C6H12O6) monomers: Watch how it undergoes condensation polymerization. Notice all the water molecules that get “condensed” out as these glucose monomers join together.

Polymers Proteins and cellulose are both naturally occurring biopolymers. One synthetic condensation polymer that you have probably heard about is nylon. (Its full name is 6-6 nylon; see if you can figure out why.) Unlike all the polymers that have been discussed so far, nylon is a “copolymer” since it is made with more than one type of monomer. The two monomers that make up nylon are hexanedioic acid: And 1,6-hexanediamine: When one of the amines on the diamine reacts with one of the acids on the dioic acid, they link together as you would expect: Notice the dioic acid has a carboxylic acid functional group on each end… … and the diamine has an amine functional group on each end.

Polymers Proteins and cellulose are both naturally occurring biopolymers. One synthetic condensation polymer that you have probably heard about is nylon. (Its full name is 6-6 nylon; see if you can figure out why.) Unlike all the polymers that have been discussed so far, nylon is a “copolymer” since it is made with more than one type of monomer. The two monomers that make up nylon are hexanedioic acid: And 1,6-hexanediamine: When one of the amines on the diamine reacts with one of the acids on the dioic acid, they link together as you would expect: And so on and so on… Because the dioic acid (A) has an acid on each end, and the diamine (B) has an amine on each end, they cannot polymerize with their own kind (no -A-A-A-A- … nor -B-B-B-B-…), but instead they must form an “alternating copolymer” (-A-B-A-B-A-B-…) Ingenious!

Polymers So, hopefully you’ve learned quite a bit from watching this power-point animation about what polymers are and the two main ways polymers form from their monomer building blocks.