MChem Master of Chemistry Polymer Chemistry 1 — Questions and Answers
Question 1: Which mechanism describes the polymerization of styrene initiated by a free radical?
- Step-growth polymerization
- Chain-growth (addition) polymerization (Correct answer)
- Ring-opening metathesis polymerization
- Cationic coordination polymerization
Correct answer: Chain-growth (addition) polymerization
Styrene undergoes chain-growth (addition) polymerization via a free-radical mechanism, where monomers add sequentially to a growing radical chain end.
Question 2: What is the degree of polymerization (DP) of a polymer sample with a number-average molecular weight (Mn) of 50,000 g/mol and a monomer molecular weight of 100 g/mol?
- 50
- 500 (Correct answer)
- 5,000
- 50,000
Correct answer: 500
DP = Mn / M0 = 50,000 / 100 = 500, giving the average number of repeat units per chain.
Question 3: Which of the following best defines the dispersity (Đ) of a polymer sample?
- Đ = Mn / Mw
- Đ = Mw / Mn (Correct answer)
- Đ = Mz / Mw
- Đ = Mn / Mz
Correct answer: Đ = Mw / Mn
Dispersity Đ = Mw/Mn (weight-average over number-average molecular weight), and a value of 1 indicates a perfectly monodisperse sample.
Question 4: In Ziegler-Natta polymerization of ethylene, what is the primary role of the titanium catalyst?
- It acts as a free-radical initiator
- It acts as a coordination site that controls monomer insertion stereospecifically (Correct answer)
- It provides a cationic active center for ionic polymerization
- It opens strained ring monomers via metathesis
Correct answer: It acts as a coordination site that controls monomer insertion stereospecifically
The titanium center in Ziegler-Natta catalysts coordinates the monomer and controls the stereochemistry of insertion, enabling isotactic or syndiotactic polymer production.
Question 5: Which technique is most commonly used to determine the molecular weight distribution of a polymer?
- Nuclear magnetic resonance (NMR) spectroscopy
- Gel permeation chromatography (GPC/SEC) (Correct answer)
- Differential scanning calorimetry (DSC)
- Thermogravimetric analysis (TGA)
Correct answer: Gel permeation chromatography (GPC/SEC)
Gel permeation chromatography (size exclusion chromatography) separates polymer chains by hydrodynamic volume, allowing determination of Mn, Mw, and dispersity.
Question 6: What is the glass transition temperature (Tg) of a polymer?
- The temperature at which the polymer crystallizes from the melt
- The temperature above which amorphous regions transition from a glassy to a rubbery state (Correct answer)
- The melting point of crystalline polymer domains
- The temperature at which thermal degradation begins
Correct answer: The temperature above which amorphous regions transition from a glassy to a rubbery state
The Tg marks the reversible transition of amorphous polymer segments from a rigid glassy state to a mobile, rubbery state as temperature increases.
Question 7: In living anionic polymerization of styrene using n-BuLi as initiator, why is the dispersity typically very low (Đ ≈ 1.05)?
- Termination reactions are very fast, limiting chain length
- There are no termination or chain-transfer reactions, so all chains grow at the same rate (Correct answer)
- The ionic mechanism inherently produces only oligomers
- Chain transfer to monomer dominates, creating uniform chain lengths
Correct answer: There are no termination or chain-transfer reactions, so all chains grow at the same rate
Living anionic polymerization lacks irreversible termination and chain-transfer steps, so all chains initiate simultaneously and grow at equal rates, producing narrow molecular weight distributions.
MChem Master of Chemistry Polymer Chemistry 1
Which mechanism describes the polymerization of styrene initiated by a free radical?