Why a Polymer Has Different Properties from Its Monomer

Ethene is a gas at room temperature. Polyethene, made entirely from ethene molecules, is a solid that can be moulded into bottles, containers, pipes, and plastic film. How can the same atoms produce materials with such different properties?

The answer is that **polymerisation changes the molecular structure, and processing determines how those polymer molecules arrange themselves in the finished material.** A small molecule and a very long molecular chain do not interact or move in the same way. Those differences become noticeable in the bulk properties of the material, such as its melting behaviour, flexibility, strength, and density.

## Why polymerisation changes the material

### Start with ethene

An ethene molecule, \(\ce{C2H4}\), contains two carbon atoms joined by a double bond. Each carbon is also bonded to two hydrogen atoms.

At room temperature, ethene is a gas. Its molecules are small, and the attractions between neighbouring ethene molecules are relatively weak London dispersion forces. They can move freely and separate easily.

[IMAGE: A clear molecular illustration of several separate ethene molecules, each showing two carbon atoms joined by a double bond and four hydrogen atoms. Show the molecules spaced apart in a gas, with no bonds connecting neighbouring molecules.]

Now consider what happens when many ethene molecules undergo addition polymerisation.

The carbon-carbon double bond in each ethene molecule opens, allowing the molecules to join through new carbon-carbon single bonds. This produces a long chain with the repeating unit \(\ce{-CH2-CH2-}\).

The overall reaction is:

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

Here, \(n\) represents the large number of ethene molecules that join to form a polymer chain. The product is polyethene, also called polyethylene.

[IMAGE: An annotated chemical reaction illustration showing several ethene molecules on the left, their carbon-carbon double bonds opening during addition polymerisation, and a long polyethene chain on the right. Clearly distinguish the original double bonds from the new single bonds in the backbone. Label the repeating unit and the polymer chain.]

Notice something important: the polymer contains the same types of atoms as the monomer. Polymerisation does not introduce a new element. Instead, it changes how the atoms are connected.

That change in connectivity produces a very different molecular structure.

### From small molecules to long chains

Think about the difference between a handful of loose shoelaces and a tangled collection of very long shoelaces. The material is still made from the same basic strands, but their length and entanglement change how the collection behaves.

Polymer chains behave in a related way. Their long backbones can bend, twist, become entangled, and interact with neighbouring chains over many points.

The analogy has a limit: polymer chains are molecules with specific covalent bonds and three-dimensional motion, not ordinary shoelaces. However, it helps explain why the behaviour of a collection of long chains differs from that of separate, small molecules.

Three changes are particularly important:

- **Molecular size:** A polymer chain contains many repeating units and has a much greater molar mass than its monomer.
- **Intermolecular attractions:** A long chain can make contact with neighbouring chains along much of its length, producing many simultaneous intermolecular attractions.
- **Chain entanglement:** Long chains can become physically entangled. This makes it harder for them to slide past one another, particularly when the material is cool.

In polyethene, the chains are non-polar, so London dispersion forces are the main intermolecular attractions. These attractions are individually weak, but their combined effect across long chains is significant.

The polymer does not become solid because its carbon-carbon covalent bonds between repeating units are weak or strong in some unusual way. It becomes a useful solid material because its molecular size, intermolecular interactions, and chain arrangement produce different behaviour from that of ethene gas.

[IMAGE: A comparison of separate, widely spaced ethene molecules and several long polyethene chains packed together. Show the polymer chains bending and overlapping, with subtle indications of intermolecular attractions between neighbouring chains. Do not draw covalent bonds between separate polymer chains.]

01A historical experiment: the discovery of polyethene

The difference between ethene and polyethene became particularly interesting when chemists first discovered how to produce solid polyethene.

In 1933, researchers Eric Fawcett and Reginald Gibson at Imperial Chemical Industries (ICI) were investigating whether very high pressures could cause substances to react in unexpected ways.

They weren’t initially trying to manufacture plastic.

What did the apparatus look like?

Ethene gas was supplied from a gas cylinder through steel pipes and pressure-control equipment. It entered a thick-walled steel reaction vessel containing ethene and benzaldehyde.

The vessel sat inside a temperature-controlled bath. A pressure gauge monitored the pressure, while the bath maintained the reaction temperature.

The mixture was heated to approximately \(170\\,^\circ\mathrm{C}\), under a pressure close to 2,000 atmospheres.

What should have happened?

The researchers were investigating a reaction between ethene and benzaldehyde. They expected evidence of a product formed from those substances.

Instead, they found a small amount of white, waxy solid coating part of the reaction tube.

This was unexpected. Chemical analysis showed that the solid had an empirical formula of \(\ce{CH2}\), a molecular mass of approximately \(3700\ \mathrm{g\\,mol^{-1}}\) or greater, and no oxygen.

These results supported a different explanation: the solid was polyethene, formed from ethene molecules joining together.

The Science Museum Group preserves parts of the original apparatus and documents the discovery in its collection record.

Why did the researchers need to repeat the experiment?

The first result was difficult to reproduce. Some later attempts failed, and other reactions decomposed dangerously.

The pressure measurements also created a problem. A falling pressure could indicate that ethene was being consumed during polymerisation, but it could also indicate that gas was escaping through a leak.

The researchers needed a better-controlled investigation.

In 1935, a different team, including Michael Perrin and John Paton, revisited the work using improved equipment in a safer laboratory. They investigated ethene without benzaldehyde.

During a successful experiment, the pressure decreased steadily, and the researchers recovered 8.5 g of white polyethene powder.

Later investigations established that trace oxygen in the ethene had acted as an initiator for polymerisation. The original results had depended partly on this uncontrolled impurity.

This was an important experimental lesson: an observation can be genuine even when the original explanation or experimental conditions are incomplete.

The historical account is discussed in detail by polymer chemists Trossarelli and Brunella in their review, Polyethylene: Discovery and Growth.

The detailed historical reaction conditions are additional context, rather than an experiment you need to reproduce for the HSC. The important chemical result is that ethene molecules became covalently connected into long polymer chains.

02The same monomer can produce different polymer materials

So far, we’ve compared ethene with polyethene.

But there’s another misconception worth avoiding: knowing the monomer does not tell you everything about the finished polymer’s properties.

Consider low-density polyethene (LDPE) and high-density polyethene (HDPE).

Both are produced from ethene. Both have the same repeating unit:

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

Yet their physical properties differ.

Why?

The difference lies partly in the arrangement and branching of their polymer chains.

Branching changes how chains pack

LDPE contains more branching along its polymer chains. These branches make it harder for neighbouring chains to pack closely together.

HDPE has a more linear chain structure. Its chains can pack more closely and form more extensive ordered regions.

These ordered regions are called crystalline regions.

Crystallinity describes the proportion of a polymer’s structure that is arranged in a regular, ordered pattern. A polymer can contain both crystalline and amorphous regions.

The differences affect the properties of the finished materials.

Historically, the original high-pressure method of producing polyethene produced highly branched chains. Later catalytic methods allowed chemists to produce more linear chains, giving materials with higher density and greater rigidity.

The important point is that the monomer remained ethene.

The polymerisation method influenced the resulting chain structure, which influenced the bulk properties.

This is why two plastics can have the same repeating unit but behave differently.

03Processing also changes the finished material

There’s one more layer to consider.

Even after polymerisation has produced the polymer chains, the material still needs to be processed into a useful shape.

For polyethene, this commonly involves heating the polymer until it softens or melts, shaping it, and then cooling it.

The polymer chains do not necessarily return to exactly the same arrangement every time this happens.

Processing can influence:

• How closely polymer chains pack together.
• How much of the material becomes crystalline.
• Whether chains become aligned in a particular direction.
• How easily neighbouring chains can move past one another.

These changes can affect the final material without changing the repeating unit.

Cooling rate affects crystallinity

Imagine two samples of the same polyethene grade.

One is cooled slowly. The other is cooled rapidly.

Predict which sample is more likely to develop greater crystallinity.

Generally, the slowly cooled sample has more time for polymer chains to arrange themselves into ordered regions.

The rapidly cooled sample has less time for this arrangement to develop, so it may retain more disordered regions.

This is a general tendency, not a universal rule. The result also depends on the polymer’s molecular structure, the cooling conditions, sample thickness, and additives.

For a typical polyethene sample, greater crystallinity generally increases density and stiffness.

Stretching can change strength without changing the monomer

Processing can also influence the direction in which polymer chains are arranged.

When a polymer is stretched during manufacture, some of its chains become more aligned with the direction of stretching.

This is called molecular orientation.

When chains are aligned, the material can become stronger along the direction of alignment.

This occurs because the covalent bonds along the polymer backbone can carry substantial loads when the chains are arranged in the direction of the applied force.

However, the material may not have the same strength in every direction. Its properties can become anisotropic, meaning they depend on the direction in which the material is tested.

The covalent bonds have not become stronger simply because the material was stretched. The chains have been rearranged.

For a more detailed explanation of this effect, see How Chain Alignment Changes Polymer Fibre Tensile Strength.

Three different levels of explanation

When explaining polymer properties, keep these three levels separate.

The finished material’s properties depend on the combined effects of molecular structure and processing.

For example, a polymer’s flexibility may depend on the ability of its chains to move past one another. Its stiffness may be influenced by crystallinity and chain arrangement. Its tensile strength may depend on chain alignment and how effectively forces are transferred between chains.

One final distinction: processing does not always mean a purely physical change. Some manufacturing processes also involve chemical reactions, such as cross-linking, which covalently connect polymer chains. This produces another change in molecular structure.

If you want to explore that difference, see Thermoplastics vs Thermosets: Structure and Properties.

04Questions and solutions

Question 1

Ethene is a gas at room temperature, whereas polyethene is a solid.

Both contain the same types of atoms and have the same repeating-unit formula, \(\ce{C2H4}\).

Explain why their physical properties are different.

Solution 1

Polyethene is solid at room temperature because its molecules consist of long covalently bonded chains, rather than separate, small ethene molecules.

Ethene has a low molar mass, and the weak London dispersion forces between its molecules allow them to move freely and separate easily.

In polyethene, each long chain has a much greater molar mass and can interact with neighbouring chains over many points. The combined intermolecular attractions and physical entanglement restrict chain movement.

Consequently, polyethene behaves as a solid material at room temperature, whereas ethene is a gas.

The important distinction is that polymerisation changes the molecular connectivity and size. The individual dispersion forces have not become covalent bonds.

Question 2

Two polyethene samples have the following properties:

Both samples were produced using ethene as the monomer.

(a) Identify which sample is more likely to be LDPE and which is more likely to be HDPE.

(b) Explain how differences in polymer chain structure account for the measured properties.

(c) A student claims that sample Y must contain stronger covalent bonds because it has a higher melting interval. Evaluate this claim.

Solution 2

(a) Sample X is most likely LDPE, while sample Y is most likely HDPE.

(b) Sample X is likely to contain more branched polymer chains. These branches prevent neighbouring chains from packing closely, reducing crystallinity and density.

Its less regular packing also contributes to its greater flexibility.

Sample Y is likely to contain more linear chains. These chains pack more closely, producing more extensive crystalline regions. This increases density and generally increases stiffness and melting temperature.

The measurements are consistent with these structural differences.

(c) The claim is incorrect.

A higher melting interval does not demonstrate that the covalent bonds along the polymer backbone are stronger.

Melting involves overcoming the interactions that hold neighbouring polymer chains in their arrangement, rather than breaking the covalent bonds along each chain.

The greater crystallinity of sample Y means that more of its chains are arranged in ordered regions. This contributes to its higher melting interval.

The student has confused stronger bulk cohesion and more ordered packing with stronger covalent bonds.

Question 3

A manufacturer produces two flat sheets using the same grade of linear polyethene.

Both sheets have identical polymer chain lengths, branching characteristics, and additives. Neither undergoes stretching during manufacture.

Sheet A is cooled slowly, while sheet B is rapidly cooled.

Assume that neither sheet undergoes chemical degradation and that all other manufacturing conditions are comparable.

(a) Predict which sheet is likely to have the greater degree of crystallinity.

(b) Predict which sheet is likely to have the greater density.

(c) Explain why the two sheets can have different physical properties despite having the same monomer and polymer chain structure.

Solution 3

(a) Sheet A is generally expected to have the greater degree of crystallinity.

Slow cooling gives the polymer chains more time to arrange themselves into ordered regions before their movement becomes restricted.

Rapid cooling can limit the development of these regions.

(b) Sheet A is generally expected to have the greater density.

Its greater crystallinity allows more efficient packing of polymer chains, reducing the amount of space occupied by less ordered regions.

(c) The sheets can have different physical properties because processing influences the arrangement of polymer chains within the finished material.

Although both sheets have the same repeating unit and chain architecture, their different cooling histories can produce different degrees of crystallinity.

This can affect density, stiffness, and melting behaviour.

The key distinction is that molecular structure describes the polymer chains themselves, whereas processing influences how those chains are arranged in the finished material.

These predictions assume comparable conditions. Cooling rate alone cannot be used to calculate an exact density or tensile strength without further experimental data.

Question 4

During an investigation into the polymerisation of ethene, a researcher observes that the pressure inside a reaction vessel decreases over time.

The researcher immediately concludes that ethene must be undergoing polymerisation.

Evaluate this conclusion.

Explain why the pressure measurement alone is insufficient and identify further evidence that would support the formation of a polymer.

Solution 4

The conclusion is not justified by the pressure measurement alone.

A decrease in pressure could indicate that ethene is being consumed during polymerisation. However, it could also result from gas escaping through a leak.

Temperature changes could also affect the measured pressure.

The researcher therefore needs additional evidence.

First, the apparatus must be checked to establish whether the pressure decrease could be caused by leakage. Temperature must also be monitored so that changes in pressure can be interpreted correctly.

Second, any solid product should be isolated and analysed.

Evidence supporting polymer formation would include:

• A solid product with a high molar mass.
• An empirical formula consistent with the ethene repeating unit.
• Evidence that the product consists of many covalently connected repeating units rather than a small molecule formed from the original reactants.

In the historical discovery of polyethene, analysis of the recovered waxy solid helped establish that it was a polymer of ethene.

The important experimental principle is that a measurement can support a hypothesis without proving it. Alternative explanations must be considered before drawing a conclusion.

Question 5

Three polyethene samples are manufactured from the same linear polymer grade.

They have identical repeating units, average chain lengths, and additives. Assume that there are no voids or changes in chemical composition.

Their manufacturing histories are different:

• Sample A is stretched along its length before cooling.
• Sample B is stretched across its width using the same draw ratio and cooling conditions as sample A.
• Sample C is not stretched and is cooled slowly.

The following illustrative measurements are obtained:

Assume that the higher density of sample C is due to its greater crystallinity.

A designer makes the following claim:

“Sample C must be the strongest material because it has the highest density. It also has the strongest covalent bonds.”

Evaluate the claim.

Your answer must:

(a) Explain why samples A and B have different directional tensile strengths.

(b) Explain why sample C has a higher density but does not have the greatest tensile strength in either direction.

(c) Select the most appropriate sample for a strap loaded along its length.

(d) Select the most appropriate sample for a panel that must withstand tension in either of two perpendicular directions, assuming the designer wants to maximise the minimum tensile strength.

(e) Explain why none of these results demonstrates that the samples have different covalent bond strengths.

Solution 5

The designer’s claim is incorrect. Sample C has the highest density, but samples A and B have greater tensile strengths in their respective directions of molecular alignment.

(a) Samples A and B have different directional tensile strengths because their polymer chains are oriented in different directions.

Sample A was stretched along its length. This caused many of its polymer chains to become aligned with the length of the strip.

When tension is applied parallel to the length, the covalent backbones can carry the load effectively. This produces a tensile strength of 42 MPa.

However, when tension is applied perpendicular to the length, the material is less effective at carrying the load. Its tensile strength is only 18 MPa.

Sample B was stretched across its width. Its molecular orientation is therefore approximately perpendicular to that of sample A.

This reverses the directional behaviour:

• Parallel to its length: 18 MPa.
• Perpendicular to its length: 42 MPa.

The two samples demonstrate anisotropy: their physical properties depend on the direction of measurement.

(b) Sample C has the highest density because it has the greatest crystallinity, but greater crystallinity does not automatically produce the greatest tensile strength in every direction.

Slow cooling allowed more of its polymer chains to arrange into ordered regions.

This increased its density.

However, sample C was not stretched, so its chains do not have the strong preferred alignment present in samples A and B.

Its tensile strength is therefore more balanced between the two directions, at 30 MPa in each.

Samples A and B have lower densities but greater tensile strengths along their respective alignment directions.

This demonstrates why density alone cannot be used to rank the tensile strength of polymer materials.

(c) Sample A is the most appropriate choice for a strap loaded along its length.

Its tensile strength parallel to its length is 42 MPa, compared with 18 MPa for sample B and 30 MPa for sample C.

The chain alignment in sample A is suited to the direction of the applied load.

(d) Sample C is the most appropriate choice when the panel must withstand tension in either perpendicular direction and the minimum strength must be maximised.

Compare the lowest tensile strength of each sample:

Sample C has the greatest minimum tensile strength.

Although samples A and B each reach 42 MPa in one direction, both have a much weaker direction.

Sample C provides a more balanced response, making it the better choice when the loading direction could be either of the two perpendicular axes.

(e) The results do not demonstrate different covalent bond strengths because all three samples have the same polymer chemistry and chain structure.

Their differences arise from processing.

Stretching changes the orientation of the chains, while cooling affects how much crystalline structure develops.

Neither process, under the assumptions given, changes the chemical identity of the repeating unit or the strength of the covalent bonds along the backbone.

The tempting mistake is to treat the highest density as proof of the strongest material. However, tensile strength also depends on direction, chain alignment, and the way forces are transferred through the polymer structure.

This is why sample C can have the highest density while sample A is stronger along its length.

The next time you compare polymer materials, separate three things before explaining the data: what the polymer chains are made of, how those chains are arranged, and how the material was processed. That distinction is the key to explaining why the same monomer can produce very different finished materials.