Bonding continuum and materials (polymers)

IB Chemistry Higher Level· S2.3· 12 min read

1. The Bonding Continuum Model★★☆☆☆HL only⏱ 3 min

The introductory model of discrete, mutually exclusive bonding types is a simplification. In reality, all bonding exists on a continuous spectrum, with most real compounds exhibiting mixed character that does not fit neatly into a single category. Fajans' rules quantify this by describing how high charge density cations can polarize anions to introduce partial covalent character to otherwise ionic compounds.

📘 Definition

Bonding Continuum

A qualitative and quantitative scale that describes bonding as a continuous spectrum from 100% ionic, through partially polar covalent, to 100% non-polar covalent and metallic, with no sharp dividing lines between categories.

📐 Worked Example

Classify the bonding character of tin(IV) chloride, SnCl₄, given its electronegativity difference ΔEN = 1.3, and melting point of -33°C

1
First recall that the old discrete rule classifies ΔEN > 1.8 as ionic, 0.5-1.8 as polar covalent, but the continuum accounts for additional factors like cation polarizing power.
2
Sn⁴⁺ has very high charge density, so it strongly polarizes Cl⁻ anions, introducing significant covalent character that overrides the ionic classification.
3
The low melting point confirms no extended ionic lattice exists, so SnCl₄ sits far on the covalent side of the continuum, despite its relatively high electronegativity difference.
4
Final classification: predominantly covalent, with negligible ionic behaviour.

Exam tip:

Always reference Fajans' rules when justifying a compound's position on the bonding continuum for 2+ mark questions to get full marks.

2. Polymer Chain Structure and Primary Bonding★★★☆☆HL only⏱ 3 min

Addition and condensation polymers are built from covalently bonded monomer units, forming long linear chains with a strong covalent backbone. The length of these chains is defined by the degree of polymerization, which directly impacts bulk material properties.

📘 Definition

Degree of Polymerization

$n_{D P}$

The total number of repeating monomer units covalently linked in a single polymer chain.

M_{\text{polymer}} = n_{DP} \times M_{\text{monomer}}

📐 Worked Example

Calculate the approximate molar mass of a polyethylene chain with a degree of polymerization of 2500, given ethene monomer molar mass is 28.0 g mol⁻¹.

1
Identify the given values: $n_{D P} = 2500$ , $M_{monomer} = 28.0$ g mol⁻¹.
2
Substitute into the polymer molar mass formula: $M_{polymer} = 2500 \times 28.0$ g mol⁻¹.
3
Compute the final result: 70 000 g mol⁻¹, or 70 kg mol⁻¹.

✓ Quick check

Test your understanding of basic polymer structure

Which of the following is an addition polymer?
- Nylon 6,6
- Polyethylene terephthalate (PET)
- Polytetrafluoroethene (PTFE)
- Urea-formaldehyde resin
Reveal answer
Polytetrafluoroethene (PTFE) —
PTFE forms from addition polymerization of tetrafluoroethene monomers, with no small elimination product during synthesis.

3. Intermolecular Forces and Polymer Bulk Properties★★★☆☆HL only⏱ 3 min

The physical properties of non-crosslinked thermoplastics are almost entirely determined by the strength of intermolecular forces between adjacent polymer chains, not the strong covalent bonds in the polymer backbone. Polar side groups on the chain can drastically increase intermolecular attraction, raising melting point and tensile strength.

📐 Worked Example

Explain why polyvinyl chloride (PVC, -CH₂-CHCl- repeating unit) has a much higher melting point than polyethylene (PE, -CH₂-CH₂- repeating unit) of the same degree of polymerization.

1
Compare the side groups on each polymer backbone: PE has only non-polar H atoms, PVC has electronegative Cl atoms on every second carbon.
2
The C-Cl bond is polar, so permanent dipole-dipole interactions form between adjacent PVC chains, while PE only has weak London dispersion forces.
3
More energy is required to overcome the stronger intermolecular forces in PVC, leading to a significantly higher melting temperature.

4. Modifying Polymer Properties: Cross-Linking and Plasticizers★★★★☆HL only⏱ 3 min

Polymer properties can be tuned for specific industrial uses via two common modification strategies: cross-linking and plasticizer addition. These two interventions have opposite effects on chain mobility and bulk material behaviour.

📐 Worked Example

Predict and explain how adding 10% by mass of a plasticizer to rigid PVC (used for water pipes) changes its flexibility and melting point.

1
Recall the function of a plasticizer: small, non-volatile molecules insert between adjacent PVC polymer chains.
2
The plasticizer molecules separate the chains, reducing the strength of dipole-dipole interactions between them.
3
Less force is required to make chains slide past each other, so flexibility increases, and the melting point / glass transition temperature decreases. This is how flexible PVC products like shower curtains are manufactured.

5. Common Pitfalls

Wrong move:

Stating that a compound is '100% ionic' or '100% covalent'

Why:

Virtually all real bonds have some mixed character, no substance sits exactly at the extreme end of the bonding continuum.

Correct move:

Describe the dominant bonding character, and note any partial covalent/ionic contributions from Fajans' rules.

Wrong move:

Claiming polymer melting points depend on breaking strong covalent backbone bonds

Why:

Thermoplastics melt when intermolecular forces between chains are overcome, the covalent backbone bonds remain fully intact.

Correct move:

Explicitly state that only intermolecular forces are disrupted during melting of non-crosslinked thermoplastics.

Wrong move:

Confusing cross-linking effect with plasticizer effect

Why:

Cross-linking increases rigidity and melting point, while plasticizers decrease both, they have exactly opposite effects.

Correct move:

Memorize that cross-links are covalent connections between chains, plasticizers are spacers that reduce chain interaction.

Wrong move:

Classifying all polymers as covalent network solids

Why:

Most thermoplastics are made of separate long polymer chains held by intermolecular forces, not a single continuous covalent network.

Correct move:

Reserve 'covalent network' classification only for fully crosslinked thermoset polymers or giant covalent substances like diamond.

Wrong move:

Forgetting to reference electronegativity difference when justifying position on the bonding continuum

Why:

IB exam markers require explicit use of electronegativity or Fajans' rules to support continuum claims, not arbitrary classification.

Correct move:

Always cite ΔEN or cation/anion charge density to justify your placement of a compound on the bonding spectrum.

6. Quick Reference Cheatsheet

Factor	Effect on polymer melting point	Effect on tensile strength
Increasing degree of polymerization	Increase	Increase
Polar side groups on chain	Significant increase	Significant increase
Cross-linking between chains	Large increase (no melting for high cross-link)	Large increase
Added plasticizer	Decrease	Decrease

What's Next

You now have a full framework to map bonding character across the entire spectrum, and connect polymer structure to real-world material properties. This knowledge will be critical for upcoming topics on nanotechnology, environmental impact of polymer waste, and organic reaction pathways for monomer synthesis. You will also use the bonding continuum model to explain unexpected properties of ionic compounds like aluminium oxide in the acids and bases unit. Mastering these links will help you score maximum marks on the Paper 2 long answer questions that combine bonding, structure and material applications.