Lesson Explainer: Polymerization Chemistry

In this explainer, we will learn how to describe the formation of different types of polymers and list common examples and uses.

A “polymer” is a type of chemical. A polymer is a chemical that can be made by joining together lots of individual units. We group chemicals that share this property together because they have lots of characteristics in common. The word is made up of poly- meaning “many” and “-mer” meaning “parts.”

The units that can be made into polymers are called monomers.

We can imagine that this is one monomer:

And that these are three separate monomers:

And this is a polymer made from three monomers:

Definition: Polymer

A chemical that can be formed from similar chemical units (monomers) that join together.

There is no strict minimum for the number of monomers you need to call a chemical a polymer, but, generally, we expect more than about 10 monomers in a polymer. There is no upper limit though, so polymers can be thousands of units long.

Sometimes, we use the word monomer to refer to a unit of the polymer, and sometimes we use it to refer to the unit the polymer was made from. When monomers combine, they will change chemically, but they will usually look very similar before and after the reaction.

Definition: Monomer

A monomer is a chemical unit that can combine with itself or similar units to form a chain or a chemical unit that is a part of a chain.

There are many examples of polymers: cellulose, nylon, starch, polystyrene, and so on. Some polymers occur naturally, and some polymers are entirely synthetic.

Broadly speaking, there are two types of polymers.

There are polymers that can form when the internal bonds of monomers change and the monomers bond together.

These are called addition polymers.

Definition: Addition Polymer

An addition polymer is a polymer that can form from monomers without the formation of by-products.

Also, there are polymers that form when bits of the monomers break off and the remainders bond together.

These are called condensation polymers.

Definition: Condensation Polymer

A condensation polymer is a polymer that can form from monomers with the production of a small-molecule by-product.

An addition or condensation polymerization reaction will always produce a substance with a bigger molecular mass than the monomers. For an addition polymerization, the molecular mass of the product will simply be the sum of the molecular masses of the monomers (if we ignore chemicals that trigger the polymerization and might be attached to the ends).

The names of polymers are often written like this: poly(monomername).

So, the polymer made from styrene is called poly(styrene). Sometimes, the parentheses are not written, and that is okay.

Some polymers have a number of different names: official names, common names, and commercial names.

The following are some of the names for poly(ethylene terephthalate).

Significant addition polymers include

  • poly(styrene) (PS),
  • poly(propene) (PP),
  • poly(ethene).

To make an addition polymer, we need a monomer with an unsaturated bond (we can make polymers by opening and joining rings together, but we will not be looking at that here).

For example, we can use a carbon–carbon double bond:

CC

If we have this type of bond in a chemical, we might be able to use that chemical to form a polymer by “opening” the carbon–carbon double bond to form 2 carbon–carbon single bonds:

CCCC

In this reaction, we see a 𝜋 bond break (2pz2pz) and two 𝜎 bonds form.

Let’s have a detailed look at the simplest addition polymer, poly(ethene), which is known commonly as polythene.

This is the monomer, ethene:

CHCHHH

In the presence of a catalyst and enough heat and pressure, ethene can be made to polymerize.

Two ethene molecules would react to leave carbons with only three bonds at both ends, which is not stable:

+CHHCHHCHHCHHCHHCHHCHCHHH

However, with the right catalysts and other chemicals, stable polymers can be produced:

We can represent the chain of monomers using circles to help us visualize it:

So, what goes on the ends of the chain? That is beyond the scope of this explainer, but it depends on how the polymerization is carried out. Generally speaking, what is on the end of a long polymer chain does not have much of an effect on how the polymer behaves.

Here are some commonly used addition polymers, with their monomers and common uses.

Example 1: Identifying the Possible Monomer That Can be Used to Make a Polymer given Their Structures

Consider the following polymer:

nCCHClHH

Which of the following halogenated hydrocarbons can be used to make it?

Answer

The structure of the polymer that has been given has a repeat unit composed of two carbons atoms, three hydrogen atoms, and one chlorine atom. The pattern will repeat n times, forming the whole polymer molecule (we can generally ignore what goes on the ends).

There are many ways to make this type of polymer, but the most efficient and straightforward will involve the polymerization of a monomer that contains only those atoms that end up in the polymer, in the right ratio.

Each of the options contains carbon and chlorine, but B does not contain any hydrogen at all, so we can eliminate that right away.

To form a polymer from a monomer, the monomer must have the potential to form extra single bonds (for instance, by having double or triple bonds that can “open up”), or parts of the monomer need to be lost so that the next monomer can attach to the chain. E and A are fully saturated, so we can eliminate them.

The remaining potential monomers are C and D. C has two chlorines rather than one, so the polymer that would be produced from it would not match the polymer that is given.

If we polymerized D, the carbon–carbon double bond would break, and single bonds would form between the ends of the monomers, producing a chain with the right ratio of carbon, chlorine, and hydrogen.

The answer is D.

Example 2: Determining the Monomer from the Acronym of the Polymer

Fill in the blank: PVC polymer is produced from the addition polymerization of .

Answer

PVC is the acronym for poly(vinyl chloride). Poly(vinyl chloride) is the polymer made from the monomer vinyl chloride. Vinyl is a term for an ethenyl group:

Vinyl chloride consists of a vinyl group attached to a chlorine atom:

The IUPAC name for this chemical is chloroethene.

Therefore, the answer is chloroethene.

Significant condensation polymers include

  • DNA,
  • starch,
  • cellulose,
  • polyesters, like poly(ethylene terephthalate) (PET),
  • polyamides, like nylon.

Condensation polymers tend to be more biodegradable than addition polymers, particularly if the small molecules that are eliminated when they form are in the surroundings.

The most common type of condensation polymerization involves the production of water as a by-product.

For example, the reaction of two different monomers produces poly(ethylene terephthalate), otherwise known as PET. This plastic is used to make disposable plastic water bottles. In one possible reaction, a diol (a chemical with two alcohol groups) reacts with a dicarboxylic acid (a chemical with two carboxylic acid groups).

This is the condensation reaction that produces poly(ethylene terephthalate) and water:

nnnn++COOHCOOHCOOHCOOCCHHHHOHOCHCHHOHHHH2O

We can simplify this, just showing the bits that are involved in the reaction:

nnnn++H2OCOOHOHOHCOOHOHOCOCOOH

When we look closer, we can see that this polymer is made using ester linkages, so poly(ethylene terephthalate) is known as a polyester. This is the ester group that links monomers together:

OCO

We can simplify our picture further and see that the two monomers alternate along the chain:

This type of polymer is called a copolymer.

Definition: Copolymer

A copolymer is a polymer that can be made from two or more different monomers.

Monomers do not necessarily alternate.

It is possible to make poly(ethylene terephthalate) from just one monomer, but it is more expensive to do it that way.

Example 3: Identifying the Correct Structure for a Generic Condensation Polymer

What is the correct structure of the condensation polymer formed between the following two monomers?

AABB

Answer

Here, there are two types of monomers: a rectangle with A groups on both ends and an oval with B groups on both ends.

A condensation polymer is a type of polymer that is produced by a polymerization reaction with small-molecule by-products. This means that we must lose something from these monomers when we form the polymer.

In practice, the small-molecule by-product will be made of pieces from the two monomers (e.g., H and HO forming HO2 or H and CHO3 forming CHOH3). Therefore, we expect something to come off both monomers when the polymer forms.

The A and B groups can be detached, forming a by-product AB. This leaves a polymer composed of alternating rectangles and ovals.

This corresponds with C.

n

Here are some commonly used condensation polymers, with their monomers and common uses,

Polymer Type
Example
Common Monomers
Small-Molecule By-Product
Uses
Polyester
Poly(ethylene terephthalate)
A diol + a dicarboxylic acid
Water
Packaging
Polyamide
Nylons
A diamine + a dicarboxylic acid
Water
Clothing
Polypeptide
Proteins
Amino acids
Water
Supporting biological processes
Polynucleotide
DNA
Nucleic acids
Nonwater substance
Storing genetic data

Let’s summarize what we have discussed regarding polymerization.

Key Points

  • Polymers are chemicals that can be made from repeating units called monomers.
  • Addition polymers are formed from monomers without any by-product.
  • Condensation polymers are formed from monomers with the loss of a small-molecule by-product, like water (HO2).
  • Copolymers are polymers that can be made using two or more different monomers.

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