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Hi! I’m Deboki Chakravarti and welcome to Crash Course Organic Chemistry!
Necessity may be the mother of invention,
but, sometimes, complete chance can play a part, too.
Not all chemistry discoveries were meticulously planned.
For example, the creation of the material that coats saucepans and stops food sticking to them was a fortuitous accident.
When working in a team trying to develop non-toxic refrigerants in the 1930s at chemical company DuPont,
Dr. Roy Plunkett was using a gaseous compound called tetrafluoroethylene.
One day, he opened a cylinder and found that the gas had disappeared.
Instead, there was a white solid.
That white solid was actually polytetrafluoroethylene (PTFE for short),
which formed when the tetrafluoroethylene polymerized.
Plunkett decided to test it and found that most substances wouldn’t stick –
perfect for non-stick cooking pans!
And Teflon was created.
PTFE is just one kind of polymer, a class of chemical compound that we're gonna learn more about today!
[Theme music]
So far, the molecules we’ve encountered have had tens of atoms within them.
But polymers are huge molecules, made up of hundreds, thousands,
or even millions of small, identical subunits called monomers.
Some of the most important polymers are made by nature.
Your DNA, RNA and proteins are all examples of biological polymers –
we’ll devote a whole episode to these later on, because they're a big part of organic chemistry.
But there are lots of human-made polymers, too.
Plastics are polymers, like polyethylene bags and polystyrene packaging.
Both of these are addition polymers, which are formed by adding the same monomers together,
over and over.
They're both really simple examples, because there's just one type of monomer.
Naming simple addition polymers is really straightforward,
generally you just stick "poly" in front of the monomer’s name.
So polystyrene is made up of thousands of styrene monomers joined together.
Drawing out the structure of styrene is simple.
So is drawing a couple styrenes bonded together –
the double bond between the two carbons breaks to form the bonds between monomers.
But drawing the full structure of a polystyrene molecule would be near-impossible...
by monomer 100 I'd have a hand cramp and wasted paper.
Happily, there’s a simple abbreviation we can use. Instead of drawing out all of the monomers,
we can simply draw one of them and indicate how it joins onto the rest of the polymer chain.
We put brackets around the molecular motif that repeats through the polymer structure,
which is logically called the repeat unit.
The “n” to the bottom right of the brackets simply means that we’re dealing with a lot of repeats!
We can draw polytetrafluoroethylene, the non-stick compound we met in the introduction,
like this too.
It's also a simple addition polymer!
Now, there are also addition polymers formed from more than one type of monomer,
called copolymers.
Instead of using "n" next to all the brackets,
we use different letters to distinguish different repeat units.
One example is the copolymer in Lego bricks, made from a mix of acrylonitrile,
1,3-butadiene and styrene monomers.
The monomer's initials make up the polymer name: ABS.
Forming addition polymers isn’t as simple as throwing the monomers together in a beaker and hoping for the best.
We have to use an addition polymerization reaction –
sometimes called chain growth polymerization.
And these reactions can happen in three ways:
using radicals, cations, or anions.
Free radical polymerization uses a radical, which is a molecule with an unpaired electron,
to kick off the reaction.
This "kickoff" step is called initiation, and in this case, the radical is our initiator.
Let's say we wanted to make polyethylene from some ethylene monomers,
which is just the common name for the IUPAC name ethene.
We can use an organic peroxide to get things started,
because oxygen-oxygen single bonds can easily break to form free radicals.
When a free radical approaches the ethylene double bond,
it grabs one of the electrons from the carbon-carbon pi bond to form a bond between itself and one of the carbon atoms.
This leaves an unpaired electron on the other carbon, so we’ve still got a radical.
This radical can then join on to another ethylene molecule, and so on,
until we form longer and longer chains.
One way this process comes to a stop is by two free radicals colliding with each other,
producing a final molecule in a process called termination.
Termination is random and happens at different points for different chains,
so polymer chains can vary in length.
Cationic polymerization uses a positively charged cation as an initiator,
which transfers a positive charge to the monomer.
As an example, we can protonate an alkene,
breaking the double bond and leaving a positive charge on one carbon.
Another monomer’s double bond can then make a new bond, lengthening the chain,
and so on as the polymer grows.
Unlike free radical polymerization,
two growing chains colliding can’t cause termination here because both chains are positively charged.
Instead, they undergo chain transfer reactions.
These involve one of the hydrogens on the carbon next to the carbocation,
which can be yanked off in a number of ways to reform the double bond.
Finally, anionic polymerization uses a negatively charged anion as an initiator.
This type of reaction requires a monomer with a group that can stabilize a negative charge.
Cyanoacrylates, like the polymers in superglue or liquid stitches used in medicine,
undergo this mechanism.
So if you’ve ever accidentally stuck your fingers together with superglue,
you've done anionic polymerization!
We just need a little bit of water to kickstart the reaction, like the moisture on your hands,
or even just in the air!
The water molecule attacks the double bond and forms an anion really easily,
because the anion has resonance with both the nitrile and the ester.
The anion repeats this process with another monomer, lengthening the chain again and again.
Termination happens when a proton transfers from another reaction participant –
the solvent, another monomer, or another polymer chain.
All the addition polymerization reactions we just talked about make addition polymers.
As we saw, they link monomers together without producing any side products.
But there's also step-growth polymers, also called condensation polymers,
which can be made from condensation polymerization reactions.
This type of polymerization kicks out a small molecule, often water,
when two monomers join together.
A great example of a condensation polymer
(and another fortuitous polymer discovery)
is Kevlar.
In 1965, Stephanie Kwolek was working at the chemical company, DuPont,
trying to develop a fiber for car tires,
when she noticed a mix of some of the polymers she was working with had formed a thin, cloudy solution.
This solution was usually thrown away, but she decided to send a sample for tests,
and it was found to be much stronger than other similar polymers.
This discovery was Kevlar!
Which is now used in a wide range of applications including bulletproof vests and fighter plane panels.
To form Kevlar, an amine group in 1,4-diaminobenzene can attack the carbonyl carbon in terephthaloyl dichloride.
This is a nucleophilic acyl substitution reaction.
They combine with the loss of a molecule of hydrochloric acid to form the polymer chain.
Kevlar is super strong because the polymer chain can form strong hydrogen bonds with a neighboring chain.
Another condensation polymer is Lexan,
a polycarbonate plastic used for food packaging and some plastic drink bottles.
Polycarbonates are also used for the ‘glass’ in glasses to correct vision,
and have even been used in some cell phone casings.
Commercial syntheses of condensation polymers have used phosgene,
a corrosive and highly toxic gas.
But there are friendlier syntheses out there, so let’s check out one of those.
One of our monomers is a type of carbonate,
a functional group that reacts twice since it contains two phenol leaving groups.
In this polymerization, the other monomer is bisphenol A, also known as BPA.
If we look at the mechanism, we can see this condensation polymerization is very similar
to the addition-elimination mechanisms we learned for esters and other carboxylic acid derivatives in episode 32.
The two monomers are mixed together at high temperature with a small amount of basic catalyst,
kicking off a transesterification reaction.
The anion we form from BPA attacks the carbonyl carbon in the carbonate,
kicking out a phenoxide leaving group.
Then, a similar reaction repeats with another BPA molecule,
kicking out the second phenoxide group.
A good reminder that the small molecule we lose during condensation polymerization isn’t always water!
Also, because the molecule we end up with isn't a radical, cation, or anion,
we don't have a termination step.
The reaction just ends when the conditions aren't favorable or we're out of stuff to react.
Now, the different polymers we’ve seen so far have lots of different uses.
But the properties of the same polymer can vary too.
Polyethylene is a great example of this.
While we introduced it as the flexible plastic that is used to make most plastic bags,
it’s also the harder plastic sometimes used to make plastic toys, water pipes, and containers.
The reason comes down to polymer morphology,
the study of how the same polymer can be structured in different ways.
Polyethylene has two forms:
high density polyethylene (or HDPE) and low density polyethylene (or LDPE).
When we zoom in on the structure of HDPE, we can see that it’s very linear –
there’s one main chain, with very few branches coming off of it.
Because of this, the polymer molecules can pack closely together,
so the intermolecular forces between the molecules are stronger, resulting in a harder, less flexible plastic.
By contrast, LDPE has a very branched structure –
there are lots of smaller chains coming off of the main chain.
As a result, the polymer molecules can’t pack together very tightly,
so the intermolecular forces are weaker, resulting in a softer, more flexible plastic.
There are other structure types too, which have different effects on polymer properties!
So far, we've been describing the structure of individual polymer molecules with words like "linear" or "branched."
But even a tiny piece of plastic will contain a huge number of molecules,
so we also need words to describe how polymer strands pack together:
crystalline or amorphous.
Like, imagine spaghetti, where each noodle represents a polymer molecule.
Uncooked spaghetti noodles line up with each other in a very orderly way,
kinda like a crystalline polymer.
But if we throw our spaghetti in a pan and cook it,
the noodles will tangle up with each other, kinda like an amorphous polymer.
The spaghetti analogy isn't perfect because polymers aren't perfectly straight lines like uncooked spaghetti –
they fold over on themselves and each other in orderly or not-so-orderly ways.
So when all this folding is neat and structured, the polymer structure is crystalline.
When it’s messy and tangled, the polymer structure is amorphous.
Also, real polymers can have both crystalline and amorphous regions.
This is a good thing, because they balance each other out.
While crystalline regions give a polymer strength and rigidity, they can also make it more brittle.
We can use a polymer’s degree of crystallinity to describe where it sits on this spectrum,
with 0% being entirely amorphous, and 100% being entirely crystalline.
Going back to polyethylene as an example,
HDPE is highly crystalline because its molecules pack closely together.
LDPE is highly amorphous, and the branches prevent the individual polymer molecules from stacking together in an orderly way.
Polymers can also behave differently at different temperatures.
When we heat a polymer, at some point it will turn rubbery, soft, and flexible.
We call this point the glass transition temperature, Tg.
And if we heat a polymer to a point beyond Tg,
it will melt completely into a liquidy goop.
Some polymers, such as polystyrene, have a Tg above room temperature.
These polymers are hard and brittle at normal temperatures,
because they aren't warm enough to be flexible.
Other polymers, such as polyisoprene, have a Tg below room temperature.
So they're soft and flexible at normal temperatures and would need to be cooled down to get brittle.
How a polymer behaves based on its glass transition temperature can have significant consequences,
as demonstrated by the Challenger shuttle disaster.
The booster rockets of the Challenger shuttle had polymer seals called O rings on the joints between their segments.
The night prior to the launch, the temperature at the launchpad dropped below the Tg of the polymer.
So the O rings lost their elasticity and became brittle.
This change in polymer structure led to fuel escaping during the launch,
which led to the tragic explosion.
Polymer chemistry is really important to understand,
because we use organic polymers in so many different ways.
In this episode, we:
Met examples of addition and condensation polymers,
Learned different types of polymerization, and
Saw how the morphology of polymers affects their properties.
In the next episode, we’ll look closer at benzene and aromatic compounds,
and how we can use NMR to decipher their structures.
Until then, thanks for watching this episode of Crash Course Organic Chemistry.
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