Polymerisation of Aromatic Monomers

It has been reported that styrene can be polymerised Iqr HCl and HBr -but that the balance between polymerisation and simple additions is a delicate one which can be easily swung to either side by a change of solvent. The original work of Pepper and Sommerfield showed that uncontrollatJe factors tended to marr the reproducibility of these systems and unfortunately this stu was not pursued °°  

Tsuda found that dry HCl in fairly high craicentrations in methylene chloride gave low yields of polystyrene, DP = 1,000 at -78 °C and about 500 at -21 °C. 

This author however did not search for 1-phenylethyl chloride among the products. 

In sulphur dioxide a high concentration of HBr produced some polystyrene but HCl failed to polymerise this monomer Finally, Giusti and Andruzzi showed that in 1,2-dichloroethane styrene is not polymerised by HI, even wiien high acid concentrations are used. The only product of this reaction was the l henylethyl iodide, presumably at room temperature. 

One can conclude that the 1-phenylethyl halide is by far the preferred product of these interactions, but that suitable conditions of polarity and temperature can create the conditions required for some polymerisation to take place. Pocket et al. have demonstrated that 1-phenylethyl chloride exchanges chlorine atoms with HCl in nitromethane, i.e., that carbenium ion pairs can be generated in tiiis system. It seems likely that the low yields of polymers observed in specific conditions as described above resulted from the solvation of the halide by excess acid, a contingency which restricted active species to a short period. From the few scattered results published it is nevertheless difficult to derive a sound interpretation of such apparent anomalies as the fact that the strongest of all hydrogen halides, HI, failed to induce polymerisation. 

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These experiments and similar ones with other monomers, to be described elsewhere, show that the explanation of the allegedly cationic polymerisations of aromatic monomers in terms of ions must be revised drastically, and that the interpretation of the rate-constants reported must be treated with the greatest circumspection. 

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