Chemistry-only: an alkene becomes an addition polymer
| English | 中文 | Pinyin |
|---|---|---|
| addition polymerisation/əˈdɪʃn ˌpɒlɪməraɪˈzeɪʃn/ | 加成聚合 | jiā chéng jù hé |
| monomer/ˈmɒnəʊmə/ | 单体 | dān tǐ |
What would explain this observation?
- Ethene is a small molecule with C=C. Poly(ethene) contains a long chain with single carbon–carbon bonds: joining changes the bonding arrangement while retaining the atoms of the ethene units.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- In addition polymerisation 加成聚合 many small monomer 单体 molecules join to form a very large polymer molecule. For ethene, C=C becomes a single bond within each two-carbon unit, and each carbon also bonds along the new chain. Poly(ethene) has repeating unit –CH₂–CH₂–. No other molecule is formed in this addition reaction, so the repeating unit contains the same two carbon and four hydrogen atoms as one ethene monomer.
- monomer: A small molecule that can join with others to form a polymer; addition polymerisation: Joining many unsaturated monomers without forming another small molecule in the stated reaction.
Which feature belongs in the poly(ethene) repeating unit?
Draw the two-carbon backbone horizontally, keep both hydrogens on each carbon and show a single bond extending out of each side of the brackets. Put n outside the brackets to represent many repeats. A continuation bond joins the neighbouring unit; it is not an extra hydrogen or a separate monomer still containing C=C. Carbon has four bonds when the continuation bonds are counted. The bracketed unit represents the repeating interior of the chain, not its complete end-group structure.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Draw the two-carbon backbone horizontally, keep both hydrogens on each carbon and show a single bond extending out of each side of the brackets. Put n outside the brackets to represent many repeats. A continuation bond joins the neighbouring unit; it is not an extra hydrogen or a separate monomer still containing C=C. Carbon has four bonds when the continuation bonds are counted. The bracketed unit represents the repeating interior of the chain, not its complete end-group structure.
- Use a molecular kit or atom-labelled cards to build several ethene molecules. Replace each C=C by a single link, then join their carbon backbones without removing any atoms. Translate that model into a short chain segment and a bracketed repeating unit. Reverse the reasoning for a supplied unit: identify the adjacent backbone carbons that came from the double bond, restore C=C and remove the continuation bonds when drawing the monomer.
Which two habits make the investigation or model in this case more defensible?
Use a molecular kit or atom-labelled cards to build several ethene molecules. Replace each C=C by a single link, then join their carbon backbones without removing any atoms. Translate that model into a short chain segment and a bracketed repeating unit. Reverse the reasoning for a supplied unit: identify the adjacent backbone carbons that came from the double bond, restore C=C and remove the continuation bonds when drawing the monomer.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a model chain interior contains 25 ethene-derived repeating units. Each contributes two carbon atoms, so the interior contains 25×2=50 carbon atoms and 25×4=100 hydrogen atoms. These counts concern the stated repeating units; they do not specify the actual initiator-derived end groups or claim every polymer molecule has exactly that chain length.
Thirty ethene-derived repeating units contain two carbon atoms each. Count their carbon atoms. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Thirty ethene-derived repeating units contain two carbon atoms each. Count their carbon atoms.
The result is 60 atoms. Known: a model chain interior contains 25 ethene-derived repeating units. Each contributes two carbon atoms, so the interior contains 25×2=50 carbon atoms and 25×4=100 hydrogen atoms. These counts concern the stated repeating units; they do not specify the actual initiator-derived end groups or claim every polymer molecule has exactly that chain length.
Check the conclusion and its limits
- The polymer does not retain a double bond in every repeating unit, and addition polymerisation does not release water. Do not draw disconnected brackets around a complete alkane molecule: the chain must continue through single bonds. This separate Chemistry objective applies to both tiers. Detailed industrial mechanisms and named initiators are not required by this section.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Addition polymerisation releases water from every ethene monomer. This claim is false: The polymer does not retain a double bond in every repeating unit, and addition polymerisation does not release water. Do not draw disconnected brackets around a complete alkane molecule: the chain must continue through single bonds. This separate Chemistry objective applies to both tiers. Detailed industrial mechanisms and named initiators are not required by this section.
Chemistry-only: an alkene becomes an addition polymer: Draw the two-carbon backbone horizontally, keep both hydrogens on each carbon and show a single bond extending out of each side of the brackets. Put n outside the brackets to represent many repeats. A continuation bond joins the neighbouring unit; it is not an extra hydrogen or a separate monomer still containing C=C. Carbon has four bonds when the continuation bonds are counted. The bracketed unit represents the repeating interior of the chain, not its complete end-group structure.
Addition polymerisation releases water from every ethene monomer.
The polymer does not retain a double bond in every repeating unit, and addition polymerisation does not release water. Do not draw disconnected brackets around a complete alkane molecule: the chain must continue through single bonds. This separate Chemistry objective applies to both tiers. Detailed industrial mechanisms and named initiators are not required by this section.
A small molecule that can join with others to form a polymer: write the technical term.
monomer means A small molecule that can join with others to form a polymer.