Polyethylene vs Polypropylene: How Methyl Groups Change Structure

Put a flexible polyethylene film pouch beside a polypropylene food container. The film bends easily, while the container holds its shape and can tolerate temperatures that would soften many common polyethylene products.

Yet polypropylene has an extra carbon-containing group sticking out from every second carbon in its main chain. Wouldn’t that extra bulk make its chains harder to pack together?

It does change how the chains fit together. But it doesn’t prevent them from forming ordered structures. The important difference is the methyl side group in polypropylene, and how regularly those groups are arranged.

The short answer: polyethylene has hydrogen atoms attached to its carbon backbone, while polypropylene has a methyl group (\(\ce{-CH3}\)) attached to every second backbone carbon. This changes chain movement, molecular packing, crystallinity, and ultimately the properties of the plastic.

01The structural difference: hydrogen versus methyl

Start with the repeating structures. These tell us exactly what has changed.

Look closely at the structures.

In polyethylene, each carbon in the repeating backbone section is a \(\ce{CH2}\) group. There are no carbon-containing side groups.

In polypropylene, the backbone alternates between \(\ce{CH2}\) and \(\ce{CH(CH3)}\). The \(\ce{CH3}\) group is attached to the backbone but does not form part of the continuous main chain.

This is an important distinction.

Although a polypropylene repeat unit contains three carbon atoms, only two belong to the main backbone. The third carbon belongs to the methyl side group.

Both polymers retain the two-carbon backbone pattern. The difference is what is attached to that backbone.

Why does the methyl group matter?

A methyl group is larger than a hydrogen atom. It occupies more space around the polymer chain.

This creates a steric effect. Steric effects occur when the size and position of atoms or groups influence how easily nearby parts of a molecule can move or approach one another.

Imagine placing two long zippers beside each other.

One zipper has no bulky attachments. The other has small tabs sticking out at regular intervals. Those tabs make some arrangements harder to achieve because they take up space.

The methyl groups in polypropylene have a similar effect. They influence:

• How freely sections of the carbon backbone can rotate.
• Which three-dimensional chain conformations are favourable.
• How closely neighbouring polymer chains can approach.
• How easily the chains form ordered crystalline regions.

There is one limitation to this analogy. Real polymer chains are flexible, three-dimensional molecules, not rigid zippers. Their carbon-carbon single bonds can rotate, so the chains continually adopt different conformations.

The methyl group changes which arrangements are favourable. It does not lock the entire chain into one position.

This is why saying that polypropylene cannot pack closely because it has bulky side groups is incorrect.

02Why chain packing is more complicated than it first appears

Polymer chains do not all arrange themselves in the same way.

Some sections form ordered regions called crystalline regions. Other sections remain disordered and are called amorphous regions.

A polymer containing both types of regions is described as semicrystalline.

The amount of crystalline material influences several physical properties:

• Greater crystallinity generally increases stiffness.
• Greater crystallinity often increases density within the same polymer family.
• Crystalline regions restrict the movement of polymer chains.
• The arrangement and stability of crystalline regions influence melting behaviour.

The important point is that the presence of a side group does not automatically determine the amount of crystallinity.

We need to consider the actual arrangement of the polymer chains.

Polyethylene: why branching changes its properties

Polyethylene provides an excellent example of why the repeat unit alone does not tell the whole story.

Two important commercial forms are low-density polyethylene (LDPE) and high-density polyethylene (HDPE).

Both have exactly the same basic repeat unit:

\[
\ce{[-CH2-CH2-]_{n}}
\]

So why do they have different properties?

The answer is their chain architecture.

HDPE (high-density polyethylene) has relatively linear chains with fewer branches.

This allows neighbouring chains to approach one another and form more ordered regions. HDPE generally has greater crystallinity and density than LDPE.

It is commonly used for rigid containers, bottles, and pipes.

LDPE (low-density polyethylene) has more branching along its carbon chains.

These branches interfere with close packing. As a result, LDPE generally has lower crystallinity and density, making it more flexible.

It is commonly used for plastic films, flexible packaging, and squeeze bottles.

Notice the important distinction:

• The methyl group in polypropylene is part of every repeat unit.
• The branches in LDPE vary in length and occur at different positions along the chain.
• HDPE has relatively few branches and can therefore pack more closely.

Do not confuse the regular methyl substituents in polypropylene with the irregular branching found in LDPE.

Polypropylene: tacticity changes how the chains pack

There is another structural detail that makes polypropylene particularly interesting.

The methyl groups do not have to point in random directions.

Their relative arrangement along the polymer chain is called tacticity.

There are three important forms.

Isotactic polypropylene

The methyl groups have the same relative stereochemical arrangement along the chain.

This regularity allows neighbouring chains to adopt ordered three-dimensional arrangements. Isotactic polypropylene can therefore form substantial crystalline regions.

It is the most common form used in many commercial polypropylene products.

Syndiotactic polypropylene

The methyl groups alternate their relative stereochemical arrangement.

Although the groups alternate, the pattern is still regular. Syndiotactic polypropylene can also form crystalline structures.

This is a useful distinction: alternating does not mean random.

Atactic polypropylene

The methyl groups have a random arrangement.

This makes it much harder for neighbouring chains to form consistent, ordered structures. Atactic polypropylene is generally largely amorphous.

Tacticity is not the same as crystallinity

These terms are related, but they describe different things.

• Tacticity describes the regularity of the side-group arrangement along an individual polymer chain.
• Crystallinity describes how much of the polymer material is arranged into ordered regions.

Regular tacticity can encourage crystallisation, but it does not mean that every section of the polymer becomes crystalline.

Even isotactic polypropylene contains amorphous regions.

Processing conditions, cooling rates, and molecular structure all influence the final arrangement.

03How does the side group affect the properties of PE and PP?

Now we can connect the molecular structures to the properties of the finished plastics.

Why can polypropylene have a higher melting temperature?

Here is a result that might initially seem backwards.

You might predict that polyethylene should have a higher melting temperature because its smaller hydrogen substituents allow its chains to approach one another more easily.

However, common isotactic polypropylene grades often have higher melting temperatures than common polyethylene grades.

Why?

The methyl groups in isotactic polypropylene have a regular arrangement. This allows the chains to form stable, ordered crystalline structures despite the additional steric bulk.

These crystalline regions must be disrupted before the polymer can melt.

The regular molecular arrangement therefore contributes to polypropylene’s relatively high melting temperature.

Polyethylene behaves differently depending on its grade. HDPE can form closely packed crystalline regions, while the branching in LDPE makes this more difficult.

The melting behaviour depends on the overall polymer structure, not simply the size of its side groups.

Is polypropylene more polar than polyethylene?

No significant change in polarity occurs simply because a hydrogen substituent is replaced by a methyl group.

Both polymers contain only carbon and hydrogen. They are largely non-polar, and London dispersion forces are their main intermolecular attractions.

The main structural difference we are examining is therefore the effect of the methyl group on chain geometry, movement, and packing.

Avoid explaining polypropylene’s higher melting temperature by claiming that its methyl groups create strong dipole-dipole attractions. That is not the correct explanation.

A useful warning about density

Density and crystallinity are related, but they are not interchangeable.

Within the same polyethylene family, increasing crystallinity generally increases density because the chains occupy a smaller volume.

However, comparing the densities of two different polymers is more complicated.

Polyethylene and polypropylene have different repeat-unit compositions and different crystal structures. Their densities depend on both the mass of the repeating material and the volume occupied by the resulting structure.

A polypropylene sample can therefore have a lower density than an HDPE sample without necessarily having a lower degree of crystallinity.

This distinction becomes important when interpreting experimental results.

04Worked examples

05Questions and solutions

Question 1

Consider the following two polymer repeat units:

• Polymer X: \(\ce{[-CH2-CH2-]_{n}}\)
• Polymer Y: \(\ce{[-CH2-CH(CH3)-]_{n}}\)

(a) Identify each polymer.

(b) State the original monomer used to produce each polymer.

(c) Identify the side group in polymer Y.

(d) How many backbone carbon atoms are present in each repeat unit?

Solution 1

Polymer X is polyethylene, and polymer Y is polypropylene.

The structures can be identified by examining the groups attached to their carbon backbones.

For polymer X:

\[
\ce{[-CH2-CH2-]_{n}}
\]

The monomer is ethene:

\[
\ce{CH2=CH2}
\]

For polymer Y:

\[
\ce{[-CH2-CH(CH3)-]_{n}}
\]

The monomer is propene:

\[
\ce{CH2=CH-CH3}
\]

The side group in polymer Y is a methyl group, \(\ce{-CH3}\).

Both repeat units contain two carbon atoms in their main backbone. The third carbon in the polypropylene repeat unit belongs to the methyl side group.

This is a common examination mistake: counting every carbon atom in a repeat unit as part of the backbone.

Question 2

A manufacturer produces two polyethylene grades using ethene.

Both samples have the same basic repeat unit and similar molecular mass distributions. Assume they contain no additives that significantly affect their physical properties.

• Sample A has relatively linear polymer chains with very few branches.
• Sample B has numerous irregular branches of different lengths.

(a) Identify which sample is more likely to have the higher crystallinity.

(b) Predict which sample is likely to have the higher density.

(c) Explain why the two samples can have different physical properties despite having the same repeat unit.

(d) A student claims that sample B must be polypropylene because it contains side groups. Evaluate this statement.

Solution 2

Sample A is expected to have greater crystallinity and a higher density than sample B.

The difference comes from the arrangement of the polymer chains, rather than the chemical composition of the basic repeat unit.

(a) Sample A has more linear chains. These chains can approach one another more closely and form ordered crystalline regions.

Sample B has more irregular branches. These interfere with close packing and make it harder for the chains to form ordered regions.

Therefore, sample A is expected to have greater crystallinity.

(b) Sample A is expected to have the higher density.

Its more linear chains can pack more closely, reducing the volume occupied by a given mass of polymer.

(c) Both samples have the same repeat unit:

\[
\ce{[-CH2-CH2-]_{n}}
\]

However, they have different chain architectures.

The greater branching in sample B interferes with chain packing. This generally produces lower crystallinity, lower density, and greater flexibility.

Sample A is more likely to be associated with HDPE, while sample B is more likely to be associated with LDPE.

(d) The student’s statement is incorrect.

The presence of side branches does not automatically identify polypropylene.

Polypropylene contains a methyl group attached to every second backbone carbon as part of its regular repeat-unit structure.

LDPE instead contains irregular carbon-chain branches along its polyethylene backbone.

The distinction is between a regular methyl substituent and irregular branching within the polymer architecture.

Question 3

Two polypropylene samples are prepared using the same monomer, with similar molecular mass distributions and additive content.

Their methyl groups have different arrangements:

• Sample P is isotactic polypropylene.
• Sample Q is atactic polypropylene.

(a) Which sample is more likely to form substantial crystalline regions?

(b) Which sample is expected to be more amorphous?

(c) Explain how the methyl-group arrangement affects the physical properties of the polymers.

(d) A student argues that syndiotactic polypropylene cannot crystallise because its methyl groups alternate sides. Explain why this reasoning is incorrect.

Solution 3

Sample P, the isotactic polypropylene, is more likely to form substantial crystalline regions. Sample Q is generally more amorphous.

The difference comes from the regularity of the methyl-group arrangement.

(a) Sample P is isotactic. Its methyl groups have the same relative stereochemical arrangement along the chain.

This regularity allows neighbouring chains to adopt ordered arrangements and form crystalline regions.

(b) Sample Q is atactic.

Its methyl groups are arranged randomly, making consistent chain packing difficult.

It therefore tends to have a much larger amorphous fraction.

(c) The regular arrangement in isotactic polypropylene allows the formation of crystalline regions.

These regions restrict chain movement and generally increase stiffness and resistance to deformation.

The random arrangement in atactic polypropylene makes ordered packing more difficult. Its chains have greater freedom of movement, so it is generally softer.

The two polymers have the same basic repeat-unit composition. Their different properties arise from their different stereochemical arrangements.

(d) The student’s reasoning is incorrect because alternating does not mean random.

Syndiotactic polypropylene has an alternating pattern of methyl-group configurations. This is still a regular arrangement.

Both isotactic and syndiotactic polypropylene can form crystalline regions, although their crystal structures and resulting properties differ.

Question 4

Two polymer samples, X and Y, are cut into identical blocks.

Each block has a volume of \(2.00\ \mathrm{cm^3}\).

Their masses are:

• Sample X: \(1.80\ \mathrm{g}\)
• Sample Y: \(1.92\ \mathrm{g}\)

Both samples are placed in separate containers of fresh water.

Assume:

• The density of water is \(1.00\ \mathrm{g\,cm^{-3}}\).
• Both blocks are completely solid, with no trapped air.
• The blocks have identical shapes and volumes.

(a) Calculate the density of each sample.

(b) Determine which sample floats higher in the water.

(c) Can the floating positions establish which sample has greater crystallinity? Explain.

Solution 4

Sample X has the lower density and floats slightly higher than sample Y. However, the floating positions cannot establish which polymer has greater crystallinity.

(a) Use the density equation:

\[
\rho=\frac{m}{V}
\]

For sample X:

\[
\begin{aligned}
\rho_X&=\frac{1.80}{2.00}\\
&=0.900\ \mathrm{g\,cm^{-3}}
\end{aligned}
\]

For sample Y:

\[
\begin{aligned}
\rho_Y&=\frac{1.92}{2.00}\\
&=0.960\ \mathrm{g\,cm^{-3}}
\end{aligned}
\]

(b) Sample X floats higher.

Both samples have densities below that of water, so both float.

According to Archimedes’ principle, a floating object displaces a mass of water equal to its own mass.

Because the blocks have equal volumes, the less dense block has less mass and needs to displace less water to float.

Sample X therefore has a smaller fraction of its volume submerged.

(c) No. Floating behaviour provides information about density, not a direct measurement of crystallinity.

Density can be influenced by the chemical composition of the repeat unit, the arrangement of the chains, and the degree of crystallinity.

If X and Y are different polymer types, their densities cannot be used by themselves to rank their crystallinity.

A more direct comparison would require a suitable technique for measuring the ordered and disordered regions, such as differential scanning calorimetry or X-ray diffraction.

Question 5

A materials laboratory investigates three polymer samples.

The results are shown below. All values are illustrative.

Assume that P and R have identical repeat-unit chemistry, tacticity, molecular mass distribution, and additive content. Their cooling histories are the only processing difference.

A student examines the results and makes two claims:

Claim 1: Sample Q must have the greatest crystallinity because it has the highest density.

Claim 2: Sample R must have a different polymer structure from sample P because its melting peak has shifted to a lower temperature.

Evaluate both claims.

In your response:

(a) Explain why density alone cannot establish the relative crystallinity of P and Q.

(b) Explain how P can have a higher melting temperature than Q despite having a lower density.

(c) Explain why P and R can have different measured properties despite having the same repeat unit and tacticity.

(d) Identify a more suitable way to compare their degrees of crystallinity. Explain why comparing the melting peak temperatures alone would be insufficient.

Solution 5

Neither claim is justified. The results can be explained by differences in polymer chemistry and physical structure without requiring any change to the polypropylene repeat unit.

This question requires separating three different ideas:

• Chemical structure: which atoms and groups make up the repeat unit.
• Chain arrangement: how the polymer chains are positioned and organised.
• Crystallinity: how much of the polymer is arranged into ordered regions.

(a) Sample Q having the highest density does not prove that it has the greatest crystallinity.

Density depends on both the mass of the polymer material and the volume it occupies.

P and Q have different repeat units and different crystal structures. Therefore, their densities cannot be compared as though crystallinity were the only variable.

Q may have greater crystallinity than P, but the density measurements alone cannot establish this.

The tempting approach is to assume that the densest polymer must have the greatest crystallinity. This can be a useful general trend when comparing samples of the same polymer under controlled conditions, but it is not a reliable rule across different polymer chemistries.

(b) P can have a higher melting temperature because isotactic polypropylene can form stable crystalline structures.

Its methyl groups have a regular stereochemical arrangement, allowing the chains to adopt ordered conformations despite the additional steric bulk.

HDPE also forms crystalline regions, but its crystal structure and chain arrangement are different.

Consequently, the melting temperatures do not need to follow the same order as the densities.

The lower density of P does not contradict its higher melting temperature.

(c) P and R have the same chemical structure, but their cooling histories are different.

During slow cooling, polymer chains have more time to rearrange and form ordered crystalline regions.

During rapid cooling, the chains have less time to organise themselves. This can reduce the amount of crystallisation and produce smaller or less perfect crystalline structures.

The lower density of R is consistent with less effective crystalline packing.

Its broader, lower-temperature melting endotherm is also consistent with a distribution of less perfect crystalline regions.

However, the melting peak temperature alone does not tell us the exact percentage of crystalline material.

Most importantly, the methyl side groups have not disappeared or changed their stereochemical arrangement.

The difference is in the physical organisation of the polymer chains, not their chemical repeat units.

(d) A more suitable approach is to measure the melting enthalpy using differential scanning calorimetry (DSC) and compare it with an appropriate reference value for the same polymer in a fully crystalline state.

This allows the degree of crystallinity to be estimated.

X-ray diffraction can also be used to distinguish ordered crystalline regions from disordered amorphous regions.

When comparing P and R, the reference must be appropriate for polypropylene. When comparing P and Q, each polymer needs its own suitable reference because their crystal structures and chemical compositions differ.

A melting peak tells us about the temperature at which crystalline regions melt. The area of the melting endotherm provides information about the energy absorbed during melting and is more useful for estimating the amount of crystalline material.

The central lesson: a change in measured physical properties does not necessarily mean the polymer’s chemical structure has changed. Molecular structure, chain arrangement, and processing history can all influence the final material.

06Where to take this next

When comparing polymers, always identify whether the difference is in the repeat unit, the arrangement of side groups, or the packing of the finished chains.

Polypropylene and polyethylene mainly demonstrate how a methyl substituent changes chain geometry and crystallisation.

PVC provides a useful contrast. Its chlorine side group introduces polar carbon-chlorine bonds, changing the intermolecular attractions as well as the polymer’s structure.

Continue with PVC vs Polyethylene: Monomers, Repeat Units and Uses to see how changing a side group can produce a different set of chemical and physical effects.