Hydroxylamine-based inhibitors of auto-initiated styrene polymerization

The object of this thesis was to investigate the inhibition mechanism of N,N-dibenzylhydroxylamine (DBHA) and 2,5-di-tert-butyl-1,4-benzoquinone (2,5-DTBBQ) mixture towards auto-initiated styrene polymerisation. This non-toxic composition represents a valid alternative to the quite efficient, but harmful 2,4-di-nitro-6-sec-butyl phenol. A dilatometry study revealed that DBHA/2,5-DTBBQ mixture shows synergism, therefore in order to decipher its mechanism of inhibition, the inhibitors were first investigated individually and then together. DBHA is a good inhibitor only in oxygenated systems. The main mechanism of inhibition of DBHA is the quenching of peroxyl radicals at the end of the propagating chains by hydrogen abstraction. In the presence of oxygen N,N-dibenzylnitroxide also contributes to the inhibition to some extent. During the inhibition of styrene with DBHA/2,5-DTBBQ, 2,5-DTBBQ is reduced to 2,5-di-tert-butyl-hydroquinone (2,5-DTBHQ). Dilatometry study revealed that the 2,5-DTBHQ/2,5-DTBBQ mixture shows a limited retardation towards the styrene polymerisation. The ability of these compounds to stop propagation radicals by addition reactions was ruled out, since no addition products were detected. Product analysis of the inhibition of styrene polymerization in the presence of DBHA and 2,5-DTBBQ allowed the detection of a few compounds, which were tested by dilatometry either singularly or as a mixture. This approach provides a way to rule out several molecules as responsible for the synergism of DBHA-2,5-DTBBQ. N,N-Dibenzylnitroxide is formed during the inhibition, and a combination of dilatometry and EPR analyses revealed a correlation between its concentration in the mixture and inhibition time. N,N-Dibenzylnitroxide is likely to be involved in the inhibition but not in the retardation. UV-Vis study confirmed that DBHA and 2,5-DTBBQ form a charge transfer complex, and its role in the inhibition and retardation cannot be excluded.