Free Radical Polymerisation Questions and Answers

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.

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.

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.

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.