Bulk polymerisation is difficult to control chiefly because
Answer: C
With no diluent to absorb it, the exotherm raises the temperature and the viscosity together and the reaction can run away. Its compensating merit is a product of the highest purity, needing no solvent or surfactant removal.
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The degree of polymerisation of a chain is the number of
Answer: D
Correct answer: repeat units it contains
The degree of polymerisation is
\[ DP = \frac{M_n}{M_0} \]the number of monomer residues in the chain, obtained by dividing the chain’s molecular mass by the mass of one repeat unit. It is the direct measure of chain length, and almost every property that distinguishes a polymer from its monomer - strength, viscosity, melting behaviour - depends on it.
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The number average molecular mass of a polymer weights each chain by
Answer: A
Correct answer: the number of chains of that mass
The number average counts chains, not mass:
\[ M_n = \frac{\sum N_iM_i}{\sum N_i} \]so every chain contributes equally regardless of its size. It is what colligative methods such as osmometry and end group analysis measure, since those count molecules. The weight average, by contrast, weights each chain by its own mass and is therefore always the larger.
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The weight average molecular mass exceeds the number average because it gives more weight to the
Answer: B
Correct answer: longer chains
The weight average is
\[ M_w = \frac{\sum N_iM_i^2}{\sum N_iM_i} \]with the mass appearing squared in the numerator, so a long chain counts far more heavily than a short one. Since it emphasises exactly the chains that dominate melt viscosity and mechanical strength, it is the more useful average for processing and property prediction, while the number average is the one thermodynamic methods measure.
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The polydispersity index of a polymer is the ratio of
Answer: C
Correct answer: weight average to number average molecular mass
The polydispersity index is
\[ PDI = \frac{M_w}{M_n} \]and it measures how broad the distribution of chain lengths is. It cannot be less than one, and a value of exactly one would mean every chain identical - achieved only by some living polymerisations and by natural proteins. Commercial polymers run from about 1.5 to well over 10.
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The glass transition temperature is the temperature at which an amorphous polymer changes from
Answer: D
Correct answer: a rigid glass to a rubbery state
Below the glass transition the chain segments cannot move past one another and the material is hard and brittle; above it they gain enough thermal energy to move cooperatively and the material becomes rubbery. It is not melting - nothing crystalline is involved, and the change occurs over a range rather than at a point. Whether a plastic is rigid or flexible at room temperature is simply a question of which side of its \( T_g \) that temperature falls.
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A plasticiser lowers a polymer's glass transition temperature by
Answer: A
A small molecule inserted between the chains reduces their mutual attraction, which is how rigid poly(vinyl chloride) becomes flexible sheeting. Its gradual loss by migration is why such articles stiffen and crack with age.
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Crystallinity in a polymer raises its
Answer: B
Closely packed ordered regions resist both deformation and solvent penetration, and they scatter light, so a crystalline polymer is translucent rather than clear. No polymer is wholly crystalline; amorphous regions always remain.
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A thermoplastic differs from a thermosetting polymer in that a thermoplastic
Answer: C
Correct answer: softens on heating and can be remoulded
A thermoplastic consists of separate linear or branched chains held together only by secondary forces, so heating overcomes those forces and the material flows; cooling restores them. The cycle can be repeated, which is what makes injection moulding and recycling possible. A thermoset is one covalently bonded network, and heating it degrades rather than softens it.
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A thermosetting resin cannot be remoulded because its chains are joined by
Answer: D
Correct answer: covalent cross-links
Curing creates covalent bonds between the chains, so the whole moulding becomes a single three-dimensional molecule. There is nothing to melt: the covalent network cannot be loosened by heat, and raising the temperature far enough breaks the bonds irreversibly and chars the material. This is why a thermoset moulding cannot be reprocessed and why thermoset waste is so much harder to recycle than thermoplastic.
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