MChem Master of Chemistry Polymer Chemistry 2 — Questions and Answers
Question 1: Which of the following is an example of a condensation (step-growth) polymer?
- Polyethylene
- Polystyrene
- Nylon-6,6 (Correct answer)
- Polyisoprene
Correct answer: Nylon-6,6
Nylon-6,6 is formed by condensation of hexamethylenediamine and adipic acid with elimination of water, a classic step-growth mechanism.
Question 2: What does the Carothers equation predict in step-growth polymerization?
- The rate constant of chain initiation
- The number-average degree of polymerization as a function of monomer conversion (Correct answer)
- The glass transition temperature from monomer structure
- The dispersity as a function of initiator concentration
Correct answer: The number-average degree of polymerization as a function of monomer conversion
The Carothers equation, DP̄n = 1/(1-p), relates the number-average degree of polymerization to the fractional monomer conversion p, showing high conversion is needed for high molecular weight.
Question 3: In atom transfer radical polymerization (ATRP), what component serves as the persistent radical?
- The carbon-centered propagating radical
- The Cu(II) complex (oxidized metal halide) (Correct answer)
- The alkyl halide initiator
- The ligand coordinated to copper
Correct answer: The Cu(II) complex (oxidized metal halide)
In ATRP, the Cu(II)/L complex is the persistent (stable) radical that caps the propagating chain, maintaining low active radical concentration via the persistent radical effect.
Question 4: Which analytical method directly measures the absolute molecular weight of a polymer without calibration standards?
- GPC with refractive index detection
- Viscometry using the Mark-Houwink equation
- Multi-angle light scattering (MALS) (Correct answer)
- Matrix-assisted laser desorption ionization (MALDI) for all molecular weights
Correct answer: Multi-angle light scattering (MALS)
Multi-angle light scattering (MALS) measures the absolute weight-average molecular weight directly from the angular dependence of scattered light intensity without requiring calibration standards.
Question 5: What structural feature distinguishes an isotactic polymer from an atactic polymer?
- Isotactic polymers have no stereocenters along the backbone
- Isotactic polymers have all pendant groups on the same side of the backbone (Correct answer)
- Isotactic polymers have alternating R and S stereocenters
- Isotactic polymers are cross-linked while atactic polymers are linear
Correct answer: Isotactic polymers have all pendant groups on the same side of the backbone
In an isotactic polymer, all repeat units have the same relative configuration (all pendant groups on the same side of the extended backbone), allowing efficient chain packing and crystallization.
Question 6: The Mark-Houwink-Sakurada equation [η] = KM^α relates intrinsic viscosity to molecular weight. What does the exponent α indicate about polymer-solvent interactions?
- α = 0 indicates theta solvent conditions
- α values between 0.5 and 0.8 indicate a good solvent where chains are expanded (Correct answer)
- α = 1 always indicates rod-like rigid polymers
- α < 0 indicates poor solvent conditions where chains collapse
Correct answer: α values between 0.5 and 0.8 indicate a good solvent where chains are expanded
In a good solvent, polymer-solvent interactions favor chain expansion, giving α values of 0.5–0.8 (random coil); α = 0.5 corresponds exactly to theta (ideal) solvent conditions.
Question 7: What is the primary advantage of reversible addition-fragmentation chain-transfer (RAFT) polymerization over conventional free-radical polymerization?
- It requires no initiator, making it purely thermally driven
- It provides controlled molecular weights and narrow dispersity by reversible chain-transfer equilibria (Correct answer)
- It completely eliminates oxygen sensitivity
- It can only polymerize acrylates, giving excellent selectivity
Correct answer: It provides controlled molecular weights and narrow dispersity by reversible chain-transfer equilibria
RAFT polymerization uses a thiocarbonylthio chain-transfer agent to reversibly cap growing chains, maintaining pseudo-living character with predictable molecular weights and Đ typically below 1.3.
MChem Master of Chemistry Polymer Chemistry 2
Which of the following is an example of a condensation (step-growth) polymer?