Addition polymerisation differs from condensation polymerisation in that it
Answer: A
The monomer adds across a double bond and the repeat unit has the same composition as the monomer. Condensation eliminates water or a similar small molecule at each step. That by-product must be removed to drive the reaction.
Please log in or register to report this question.
Match each polymer with the mechanism by which it is made.
List I
A. Polyethylene
B. Nylon 66
C. Polyester
D. Polystyrene
List II
1. condensation between an acid and an alcohol
2. addition across a double bond
3. addition of a vinyl monomer
4. condensation with loss of water
Answer: B
Correct answer: A-2, B-4, C-1, D-3
Two addition polymers and two condensation polymers. Polyethylene and polystyrene grow by addition across the monomer’s double bond, with nothing eliminated and the repeat unit having the same composition as the monomer. Nylon 66 forms by condensation between a diamine and a diacid with loss of water, and polyester by condensation between a diacid and a diol - and in both cases the small molecule must be removed to drive the chain to high molecular weight.
Please log in or register to report this question.
Assertion (A): Condensation polymerisation requires very accurate stoichiometry between the two monomers.
Reason (R): An excess of one monomer caps the growing chains and limits the molecular weight.
Answer: C
Both are true and the reason gives the mechanism: with a step growth reaction the molecular weight rises only in the last few per cent of conversion, and any imbalance stops it short. A monofunctional impurity has the same effect.
Please log in or register to report this question.
Consider the following statements about step growth polymerisation.
1. High molecular weight is reached only at very high conversion.
2. Monomer disappears early while the chains are still short.
3. The molecular weight rises steeply from the first few per cent of reaction.
Answer: D
The first two are correct and are the characteristic signature of step growth. The third describes chain growth polymerisation, where full length chains form from the outset. The two mechanisms behave quite differently with conversion.
Please log in or register to report this question.
The four industrial polymerisation techniques are bulk, solution, suspension and
Answer: A
The last three are processing operations carried out on a polymer that already exists. The four techniques differ in how the heat of reaction is removed and how the product is recovered. Each gives the polymer in a different physical form.
Please log in or register to report this question.
Match each polymerisation technique with its characteristic feature.
List I
A. Bulk
B. Solution
C. Suspension
D. Emulsion
List II
1. monomer droplets in water giving beads
2. monomer alone, with difficult heat removal
3. a surfactant gives a latex of very fine particles
4. a solvent controls viscosity but must be removed
Answer: B
Correct answer: A-2, B-4, C-1, D-3
Four techniques distinguished by what surrounds the monomer. Bulk polymerisation uses monomer alone, so the product is pure but the heat of reaction is very hard to remove from a thickening mass. Solution adds a solvent that controls the viscosity and carries the heat away, at the cost of removing it afterwards. Suspension disperses monomer droplets in water, each acting as a small bulk reactor and giving beads. Emulsion uses a surfactant to give a latex of very fine particles, with excellent heat transfer and high rate together.
Please log in or register to report this question.
Assertion (A): Bulk polymerisation is difficult to control at industrial scale.
Reason (R): The reaction is strongly exothermic and the viscosity rises sharply as it proceeds.
Answer: C
Both are true and the reason gives the difficulty: the rising viscosity impedes both mixing and heat transfer just as the heat release is greatest, which can lead to a runaway called the gel effect. Cooling capacity is what limits the batch size.
Please log in or register to report this question.
Emulsion polymerisation is distinctive in that it
Answer: D
Because each particle contains at most one growing radical at a time, termination is suppressed and both rate and chain length can be high. Every other technique forces a trade-off between the two. The latex may be used directly as a coating.
Please log in or register to report this question.
State whether the following is true or false.
A thermoplastic can be melted and reshaped repeatedly.
Answer: A
Its chains are held together only by physical forces, which soften on heating. A thermoset has covalent crosslinks that decompose rather than soften, so it cannot be remoulded. Recycling therefore works only for the thermoplastics.
Please log in or register to report this question.
A thermoset differs from a thermoplastic in having
Answer: B
The crosslinks make the whole article a single molecule, so it cannot flow at any temperature. Bakelite and epoxy are familiar examples, and neither can be melted and reused. Curing is irreversible once it has taken place.
Please log in or register to report this question.
