Video Transcript
In this video, we will learn how to
describe the formation of different types of polymers and list common examples and
uses.
A polymer is a large molecule made
of many repeating subunits. The poly- in polymer means many,
like a polygon with many sides. The -mer in polymer means parts, so
a polymer is a molecule with many parts that fit together like links in a chain. We call the repeated subunits of
the polymer or, in other words, the individual links that make up the chain
monomers. In the simplified example we’ve
drawn here, the orange circles that make up the chain are its monomers. And we can refer to them as
monomers whether they are a part of the chain or separate from it. When we build a polymer, we add
monomers to the end of the chain, making it longer. It’s worth noting that polymers can
have many different lengths. They can be 10 units long, 1000
units long, or even 1000000 units long.
Later in the video, we’ll learn
more about the precise chemical structure of these building blocks. For now, let’s take a look at the
different kinds of polymer that can form. The main way to categorize polymers
splits them into two broad groups. For some polymers, no by-product is
made during their formation, like in the upper example drawn here. For other polymers, their formation
does create a by-product, like in the lower example. When no by-product forms, we call
that an addition polymer because the monomers can simply be added together to create
the polymer. An addition polymer typically forms
when the internal bonds of the monomer can rearrange to form new bonds to create the
polymer. For example, a double bond in a
polymer might shift to form a single bond in the polymer plus a single bond to the
next monomer in the chain.
When a by-product does form in the
creation of a polymer, we call that polymer a condensation polymer. Water is a common by-product of
these reactions. It should be noted that since the
by-product is not a part of the polymer that forms, that polymer has a lower mass
than the combined mass of the starting monomers. However, since the formation of an
addition polymer only involves rearranging bonds, the resulting polymer has the same
mass as the combined mass of its starting polymers. Before we dive deeper into these
processes, let’s take a closer look at how we name and draw polymers.
The compound drawn here is called
tetrafluoroethene. It can exist as a standalone
molecule. It can also exist as the monomer
that makes up a polymer. The polymer is drawn like this with
a monomer unit in brackets or sometimes parentheses, followed by the letter 𝑛. This letter 𝑛 represents the
number of repeated units in the polymer chain, which is often a large number. To form this particular polymer out
of this monomer, the double bond between the carbon atoms breaks to form the new
bond to the next monomer unit in the chain. The drawing of the polymer shows
that new bond that continues the chain. It also shows that the double bond
has been replaced by a single bond connecting the two carbon atoms. We can also see the bond that
connects this monomer unit to the previous monomer unit in the chain.
To name this polymer, we simply
combine the prefix poly- with the name of the monomer. In this case, that makes
poly(tetrafluoroethene). As we’ve written it here, the name
of the monomer is in parentheses, although you may see it written without these
parentheses as well. Of course, in chemistry, a compound
will often have multiple names. Ethene and ethylene are two
different names for the same compound, so we can also call this polymer
poly(tetrafluoroethylene). This name can be a mouthful, so
it’s frequently shortened to a four-letter abbreviation PTFE. We can also use the more precise
IUPAC name for this compound, poly(1,1,2,2-tetrafluoroethylene). The 1,1,2,2 refers to the fact that
there are two fluorine atoms on the first carbon and two more fluorine atoms on the
second carbon.
When a compound is used in a
commercial product, it’s usually given a catchier, easier-to-remember name. The commercial name for
poly(tetrafluoroethene) is Teflon, the nonstick coating that appears on many pots
and pans. Let’s take a closer look at how
this polymer is formed. We start with a monomer. For the formation of an addition
polymer, we need an unsaturated bond. As we will see in a moment, the
extra electrons in this bond are key to the formation of the polymer. If we get multiple monomers
together, no reaction will occur spontaneously. However, if we add a catalyst,
heat, and pressure, changes start to emerge.
The double bond between the
monomer’s two carbon atoms represents four shared electrons. When we add a catalyst, heat, and
pressure in this scenario, two of the electrons remain as a single bond between the
two carbon atoms. Meanwhile, each of the carbon atoms
takes back one of the electrons to form a new bond somewhere else. This same shift happens in other
monomers as they get added to the chain. These electrons end up being shared
between carbon atoms of adjacent monomers. Those electrons form a single bond
that connects the monomers together in the chain.
This process gets repeated over and
over again, adding even more links to the chain. At some point, the polymer will
have to end. But what happens when that carbon
atom can no longer form a new bond with the next monomer in the chain? The precise answer is beyond the
scope of this video, but as a simplified answer, we can say that there is some form
of chemical cap that bonds to the carbon atom, adding stability to the end of the
polymer.
For now, let’s just focus on the
middle of the chain. It’s worth noting that as these new
bonds form, the atoms involved don’t stay in one place; they twist and shift. So, this chain of four connected
carbon atoms with eight attached fluorine atoms might more commonly be drawn like
this. And since we typically show just
one monomer unit of the polymer repeated, we can condense it down even further to
look like this. This final form with the monomer
unit in brackets followed by the letter 𝑛 is how we typically notate a polymer.
A variety of monomers can combine
to form addition polymers with a wide range of everyday uses. The monomer ethene can lose its
double bond to form a polymer called poly(ethene). Poly(ethene) is a plastic used to
make bags, bottles, and gloves. Another addition polymer is
poly(propene). Poly(propene) can be used to make
food containers, packaging, and toys as well as a synthetic fabric. The technical name for this third
polymer is poly(chloroethene). However, it’s more commonly
referred to as polyvinyl chloride or an abbreviation PVC. PVC is used to make pipes, window
frames, and cable insulation. The last polymer on the list,
poly(tetrafluoroethene), is also known as Teflon. As we mentioned before, Teflon is
used to make the nonstick coatings on many pots and pans.
Now, let’s take a look at the
formation of the other kind of polymer, a condensation polymer. As a reminder, the primary
difference between an addition polymer and a condensation polymer is that when we
make a condensation polymer, a by-product will form. In this example, we will take a
look at the combination of a dicarboxylic acid, or a molecule with two carboxyl
groups, and a diol, or a molecule with two hydroxy groups. When these two molecules combine in
this way, an oxygen atom and a hydrogen atom from the dicarboxylic acid and a
hydrogen atom from the diol can combine to form a water molecule as a
by-product.
When the atoms making up the water
molecule break away from the carbon atom of the dicarboxylic acid and the oxygen
atom of the diol, a new bond is formed between those two atoms connecting the
previously separate monomers. A similar process can repeat to
connect the end of the diol molecule to the beginning of the next dicarboxylic acid
molecule. Again, a water molecule forms,
creating a new bond to add the next monomer onto the chain. When we link together these
monomers in this way, the polymer that emerges is called poly(ethylene
terephthalate). Since the molecules in this example
are a bit complicated, we don’t have to worry for now about how to precisely name
them.
Right here, we can see the new bond
that has formed between the end of the dicarboxylic acid and the beginning of the
diol. Poly(ethylene terephthalate) is an
example of a copolymer or a polymer that can be made from two or more different
monomers. In this example, instead of having
a single monomer to make up the repeated unit, two different monomers combine to
make up the repeated unit of the polymer. If we wanted to write this process
as a chemical equation, we could say that 𝑛 molecules of this dicarboxylic acid and
𝑛 molecules of this diol combine to make a poly(ethylene terephthalate) molecule
with 𝑛 repeated subunits and two 𝑛 minus one molecules of water.
As we discussed before, during this
process, a water molecule is created in between these two monomers, and another
molecule is created when we link the diol to the beginning of the next dicarboxylic
acid. At some point, the chain will need
to end with a hydroxy group instead of producing another water molecule. Two water molecules per repeated
unit minus one for the cap at the end explains the two 𝑛 minus one water molecules
that we see written in this equation. Water is a common by-product of the
formation of a condensation polymer, although other by-products are possible.
Let’s review some examples of
condensation polymers. The polymer we just looked at,
poly(ethylene terephthalate), is an example of a polyester. Polyesters are commonly made from
the combination of a diol and a dicarboxylic acid with a by-product of water. It’s used in packaging and
fabrics. You may be familiar with the fabric
nylon. There are many different types of
nylon, each of which falls under the category polyamide.
A diamine plus a dicarboxylic acid
is one way to make a nylon, although a variety of combinations exist. The by-product is water. Nylons are primarily used in
clothing. The proteins that carry out the
functions of living cells and organisms are polymers that are made up of amino acid
monomers. We sometimes refer to proteins as
polypeptides. Once again, the by-product is
water. Biological functions such as
breathing, digesting, and fighting illness are all carried out on the smallest scale
by proteins.
Another biological polymer is DNA,
which we can say is a polynucleotide. DNA is a polymer chain of nucleic
acid monomers. As nucleic acids get added to the
chain, the chemical pyrophosphate is released as a by-product. The genetic information that gets
passed down from organism to organism is stored as a polymer of nucleic acids. Now that we’ve learned about
polymers, let’s do a practice problem to review.
Consider the following polymer. Which of the following halogenated
hydrocarbons can be used to make it?
This is a question about
polymers. Specifically, it’s about making
polymers. Which of the five answer choices is
the repeated subunit or monomer that makes up the polymer? A first question we can ask is, is
this polymer an addition polymer or a condensation polymer? The key difference between an
addition polymer and a condensation polymer is that when we make an addition
polymer, no by-product forms. But when we make a condensation
polymer, a by-product will form. Often, that by-product is
water.
If we look at the answer choices,
several key features stick out. First, there is a lack of a hydroxy
group in any of the answer choices. No hydroxy group means no water
molecule can form as a by-product, suggesting that it is not a condensation
polymer. And while another by-product is
technically possible, the presence of double bonds in several of the answer choices
suggests that it is an addition polymer.
As we will see in a moment, the
extra electrons of the double bond allow for the formation of new bonds that make a
polymer without a by-product. To put it simply, addition polymers
are made of monomers with unsaturated bonds. Usually, that means a double bond,
although it could mean a triple bond. So, we can eliminate choice (A) and
choice (E) from consideration as they contain only saturated single bonds. Let’s erase choice (E) to give
ourselves more room to work with. The remaining answer choices each
have the same central structure, two carbon atoms with a double bond in between
them. This structure is a common
structure for the monomer of an addition polymer, as the 𝜋 bond from the double
bond can break, and the electrons can be used to form a new bond to the next monomer
in the chain.
The pattern can continue to link
more and more monomers together. The end result is a continuous
chain of carbon atoms with single bonds between them. But what about the atoms that are
attached to the carbon chain? These atoms are different in each
of the three answer choices remaining. Well, the attached atoms remain
unchanged throughout this process. While the carbon atoms change the
way they bond with one another, the attached atoms simply remain attached as the
central structure changes. So, if our ending polymer has a
repeated unit with three hydrogen atoms and one chlorine atom attached to the carbon
chain, that means our starting polymer also had three hydrogen atoms and one
chlorine atom attached to its carbon atoms.
The structure shown in choice (D)
is the correct answer. It has the double bond necessary to
form the spine of the addition polymer, as well as the correct attached atoms. So, which of the following
halogenated hydrocarbons can be used to make this polymer? That’s choice (D).
Now that we’ve learned about
polymerization, let’s review the key points of the video. Polymers are molecules made of many
repeating subunits called monomers. One type of polymer, addition
polymers, are formed without a by-product. Usually, the bonds of the monomer
can rearrange to form the polymer. Examples of addition polymers
include poly(ethene), PVC, and Teflon. The other type of polymer,
condensation polymers, are produced alongside a by-product. Often, that by-product is
water. Examples of condensation polymers
include nylons, proteins, and DNA.
