A sample of polystyrene preprared by bulk polymerization at 60°C using radioacti

Question

A sample of polystyrene preprared by bulk polymerization at 60°C using radioactive 14C-AIBN as initiator was found to have a number average molar mass (Mn) of 1000 kg/mol and a radioactivity of 6 x 103 counts/(s*g). Given that the 14C-AIBN has a radioactivity of 6x109 counts/(s*mol), determine the mode of termination that operated during preparation of the polystyrene sample. (Note: bulk polymerization means that no solvent is used, and the reaction is carried out in pure liquid monomer. Use the density of styrene, 0.88 g/mL, when solving this problem).

Explanation / Answer

given data

Mn= 1000 kg/mol = 106g/mol

Radioactivity of the polymer = 6 x 103 counts s-1g-1x 106g/mol = 6 x 109 counts s-1mol-1

Radioactivity of AIBN = 6 x 109 counts s-1mol-1

Since radioactivity per moles is equal, there is only 1 AIBN molecule in each polymer molecule. Therefore, termination is by coupling (i.e. AIBN splits into 2 radicals during initiation, and recombine during coupling). If the termination was disproportionation, radioactivity of the polymer would be halved since 1 molecule of radical would be present for each polymer molecule.)

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