Free Radical Polymerisation Questions and Answers

Q: A small amount of hydroquinone is added to methyl methacrylate monomer in order to

Hydroquinone is added to inhibit polymerization.Methyl methacrylate polymerises by free radicals, and traces of heat, light or oxygen can start it during storage. Hydroquinone and its monomethyl ether react with the radicals and stop chains from growing, so the monomer stays liquid in the drum. Before use it is removed by washing with alkali or distillation, or simply swamped with extra initiator.It does not improve the thermal or hydrolytic stability of the polymer, and molecular weight is regulated by chain transfer agents such as mercaptans, not by inhibitors.

Q: The initiator system which can be used for free radical polymerization at 5 °C is

The initiator for 5 °C is \(\mathrm{FeSO_{4}}\) + t-butyl hydroperoxide.Thermal initiators need heat to break their weak bonds at a useful rate: AIBN, benzoyl peroxide and potassium persulfate are typically used at about 50-90 °C, and at 5 °C they would give almost no radicals.A redox pair produces radicals by electron transfer instead: Fe2+ reduces the hydroperoxide to an alkoxy radical even at low temperature. This is the basis of the redox recipes used for 'cold' emulsion polymerization of styrene-butadiene rubber at about 5 °C.

Q: Norrish-Smith effect or Trommsdorff effect during free radical bulk polymerization at high conversion is characterized by ___.

The effect is an increase in rate of polymerization with time due to increase in viscosity.At high conversion in bulk free radical polymerization the medium becomes very viscous. Termination needs two large growing chains to meet, and their diffusion slows dramatically, so the termination rate constant falls. Small monomer molecules can still reach the radical ends, so propagation continues.With fewer terminations the radical concentration builds up, and the rate of polymerization and the molecular weight rise sharply: autoacceleration, the gel effect. This can cause a dangerous temperature runaway.The options with a decreasing rate describe the opposite behaviour, and those attributing the effect to falling viscosity have the cause backwards.

Q: In case of radical polymerization, the kinetic chain length is given by expression :\(\nu = \dfrac{k_{p}[M][I]^{-1/2}}{2k_{t}f^{1/2}(k_{d}/k_{t})^{1/2}}\)increase in the initiator concentration eventually leads to :

Increasing the initiator concentration leads to a low molecular weight polymer.The expression shows the kinetic chain length proportional to [I]−1/2: the more initiator, the shorter the average chain. More initiator produces more radicals at once, and since termination is bimolecular between radicals, chains meet and terminate sooner, so each chain adds fewer monomer units.The rate of polymerization rises as [I]1/2, so the reaction does depend on the initiator (option C is wrong) and does speed up, but option D describes the rate rather than what the chain-length expression shows; the question is about the polymer formed. A higher molecular weight (option A) is the opposite of what the inverse square-root dependence gives.

Q: The expression for the rate of chain initiation in addition polymerization is :

The rate of initiation is 2fkd[I].In free-radical polymerisation the initiator I decomposes with rate constant kd into two radicals, and only a fraction f (the initiator efficiency) of them start chains; the rest are lost to side reactions or cage recombination. So\[ R_{i} = 2fk_{d}[\mathrm{I}] \]kd[I] is only the rate of initiator decomposition, fkd[I] forgets that each molecule gives two radicals, and kd[I]/2 halves where it should double.

Q: How many initiator fragments are present in a polymer chain formed by coupling termination in addition polymerization ?

The chain contains 2 initiator fragments.In free-radical addition polymerization each growing chain starts from one initiator radical, such as a benzoyloxy or phenyl fragment from benzoyl peroxide, so each living chain carries one initiator fragment at its starting end. In termination by coupling (combination), two growing chains meet and their radical ends join into one dead chain, which therefore carries an initiator fragment at each end.One fragment per chain is the result of termination by disproportionation, where a hydrogen atom is transferred and the two chains stay separate. Three fragments would need a branching event, and zero would mean the chain never started from an initiator.

A small amount of hydroquinone is added to methyl methacrylate monomer in order to

A.improve its thermal stability
B.improve its hydrolytic stability
C.inhibit polymerization
D.regulate molecular weight

Exam Year(s)

GATE XE - 2012,

Answer: C

Hydroquinone is added to inhibit polymerization.

Methyl methacrylate polymerises by free radicals, and traces of heat, light or oxygen can start it during storage. Hydroquinone and its monomethyl ether react with the radicals and stop chains from growing, so the monomer stays liquid in the drum. Before use it is removed by washing with alkali or distillation, or simply swamped with extra initiator.

It does not improve the thermal or hydrolytic stability of the polymer, and molecular weight is regulated by chain transfer agents such as mercaptans, not by inhibitors.

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The initiator system which can be used for free radical polymerization at 5 °C is

A.\(\mathrm{FeSO_{4}}\) + t-butyl hydroperoxide
B.Azobisisobutyronitrile
C.Potassium persulfate
D.Benzoyl peroxide

Exam Year(s)

GATE XE - 2022,

Answer: A

The initiator for 5 °C is \(\mathrm{FeSO_{4}}\) + t-butyl hydroperoxide.

Thermal initiators need heat to break their weak bonds at a useful rate: AIBN, benzoyl peroxide and potassium persulfate are typically used at about 50-90 °C, and at 5 °C they would give almost no radicals.

A redox pair produces radicals by electron transfer instead: Fe²⁺ reduces the hydroperoxide to an alkoxy radical even at low temperature. This is the basis of the redox recipes used for 'cold' emulsion polymerization of styrene-butadiene rubber at about 5 °C.

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Norrish-Smith effect or Trommsdorff effect during free radical bulk polymerization at high conversion is characterized by ___.

A.increase in rate of polymerization with time due to increase in viscosity
B.decrease in rate of polymerization with time due to increase in viscosity
C.decrease in rate of polymerization with time due to reduction in viscosity
D.increase in rate of polymerization with time due to reduction in viscosity

Exam Year(s)

GATE XE - 2026,

Answer: A

The effect is an increase in rate of polymerization with time due to increase in viscosity.

At high conversion in bulk free radical polymerization the medium becomes very viscous. Termination needs two large growing chains to meet, and their diffusion slows dramatically, so the termination rate constant falls. Small monomer molecules can still reach the radical ends, so propagation continues.

With fewer terminations the radical concentration builds up, and the rate of polymerization and the molecular weight rise sharply: autoacceleration, the gel effect. This can cause a dangerous temperature runaway.

The options with a decreasing rate describe the opposite behaviour, and those attributing the effect to falling viscosity have the cause backwards.

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In case of radical polymerization, the kinetic chain length is given by expression :

\(\nu = \dfrac{k_{p}[M][I]^{-1/2}}{2k_{t}f^{1/2}(k_{d}/k_{t})^{1/2}}\)

increase in the initiator concentration eventually leads to :

A.Higher molecular weight polymer
B.Low molecular weight polymer
C.Rate of reaction and thus the extent polymerization does not depend on the concentration of initiator
D.Fast polymerization reaction

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

Increasing the initiator concentration leads to a low molecular weight polymer.

The expression shows the kinetic chain length proportional to [I]^−1/2: the more initiator, the shorter the average chain. More initiator produces more radicals at once, and since termination is bimolecular between radicals, chains meet and terminate sooner, so each chain adds fewer monomer units.

The rate of polymerization rises as [I]^1/2, so the reaction does depend on the initiator (option C is wrong) and does speed up, but option D describes the rate rather than what the chain-length expression shows; the question is about the polymer formed. A higher molecular weight (option A) is the opposite of what the inverse square-root dependence gives.

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The expression for the rate of chain initiation in addition polymerization is :

A.\(fk_{d}[\mathrm{I}]\)
B.\(k_{d}[\mathrm{I}]/2\)
C.\(2fk_{d}[\mathrm{I}]\)
D.\(k_{d}[\mathrm{I}]\)

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

The rate of initiation is 2fk_d[I].

In free-radical polymerisation the initiator I decomposes with rate constant k_d into two radicals, and only a fraction f (the initiator efficiency) of them start chains; the rest are lost to side reactions or cage recombination. So

\[ R_{i} = 2fk_{d}[\mathrm{I}] \]

k_d[I] is only the rate of initiator decomposition, fk_d[I] forgets that each molecule gives two radicals, and k_d[I]/2 halves where it should double.

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How many initiator fragments are present in a polymer chain formed by coupling termination in addition polymerization ?

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

The chain contains 2 initiator fragments.

In free-radical addition polymerization each growing chain starts from one initiator radical, such as a benzoyloxy or phenyl fragment from benzoyl peroxide, so each living chain carries one initiator fragment at its starting end. In termination by coupling (combination), two growing chains meet and their radical ends join into one dead chain, which therefore carries an initiator fragment at each end.

One fragment per chain is the result of termination by disproportionation, where a hydrogen atom is transferred and the two chains stay separate. Three fragments would need a branching event, and zero would mean the chain never started from an initiator.

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