QUESTION 149 (5 marks)
The diagram shows schematic methyl-group arrangements in three polypropene chains. Chain lengths and all other manufacturing conditions are the same.
(a) Identify X, Y and Z as isotactic, syndiotactic or atactic. [3 marks]
(b) Compare the expected ability of X and Z to pack regularly, and relate this to stiffness. [2 marks]
Practice marking scheme
Answer
X is isotactic, Y is syndiotactic (syntactic), and Z is atactic. X generally packs more regularly than Z, supporting ordered regions and greater stiffness.
Working
X places successive methyl groups on the same side; Y alternates them regularly; Z has an irregular arrangement. The regular arrangement in X helps neighbouring chains align and form ordered, closely packed regions. This increases collective interchain attractions and restricts chain movement relative to the irregular Z arrangement. The question does not require a strength ranking between isotactic and syndiotactic samples.
Marking criteria
- Identifies X as isotactic. [1 mark]
- Identifies Y as syndiotactic/syntactic. [1 mark]
- Identifies Z as atactic. [1 mark]
- Links X’s regular arrangement to more regular/ordered packing than Z. [1 mark]
- Links ordered packing to restricted chain motion and greater stiffness. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.