Hydroxylamine-based inhibitors of auto-initiated styrene polymerization Chiara Baldassarri PhD University of York Chemistry September 2014 Abstract 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. 2 Table of Contents Abstract .................................................................................................................... 2 List of Figures ........................................................................................................... 8 Acknowledgements ................................................................................................ 16 Declaration ............................................................................................................. 17 1 Introduction .................................................................................................... 18 1.1 Styrene .............................................................................................................. 18 1.2 Mechanism of radical polymerisation .............................................................. 18 1.3 Auto-initiated styrene polymerisation ............................................................. 20 1.4 Inhibitors of polymerisation ............................................................................. 22 1.4.1 Addition reaction ...................................................................................... 22 1.4.2 Hydrogen abstraction................................................................................ 24 1.4.3 Redox reaction .......................................................................................... 26 1.5 Inhibitors vs retarders ...................................................................................... 27 1.6 Synergistic effect .............................................................................................. 28 1.6.1 Restrictions about the use of nitrophenol compounds ............................ 30 1.7 Aim of the project ............................................................................................. 32 2 N,N-Dibenzylhydroxylamine as an inhibitor of styrene polymerization ............. 33 2.1 Introduction ...................................................................................................... 33 2.1.1 Background ............................................................................................... 33 2.1.2 Aim of the chapter .................................................................................... 39 2.2 Inhibition efficiency of N,N-dibenzylhydroxylamine in the thermally initiated styrene polymerisation ............................................................................................... 40 2.2.1 Dilatometry ............................................................................................... 40 2.2.2 Results ....................................................................................................... 41 2.3 N,N-Dibenzylnitroxide as inhibitor ................................................................... 44 2.3.1 Monitoring N,N-dibenzylnitroxide and oxygen evolution by EPR ............ 44 2.3.2 Oxidation of N,N-dibenzylhydroxylamine by oxygen ............................... 52 3 2.3.3 N,N-Dibenzylnitroxide as a radical trap .................................................... 56 2.4 Conclusions ....................................................................................................... 65 3 2,5-Di-tert-butyl-hydroquinone as an inhibitor of the auto-initiated styrene polymerisation ....................................................................................................... 67 3.1 Introduction ...................................................................................................... 67 3.1.1 Background ............................................................................................... 67 3.1.2 Aim of the chapter .................................................................................... 73 3.2 Investigation of the antioxidant properties of 2,5-DTBHQ .............................. 74 3.3 Investigation of the inhibition properties of 2,5-DTBBQ ................................. 76 3.4 Reactivity of 2,5-di-tert-butyl-1,4-benzoquinone towards alkyl radicals ........ 77 3.4.1 Reactivity of 2,6-di-tert-butyl-1,4-benzoquinone ..................................... 81 3.5 Investigation of the reactivity of 2,5-di-tert-butyl-hydroquinone ................... 84 3.6 Conclusions ....................................................................................................... 90 4 2,5-Di-tert-butyl-1,4-benzoquinone and N,N-dibenzylhydroxylamine: reaction in a non-polymerizable solvent ................................................................................... 91 4.1 Introduction ...................................................................................................... 91 4.1.1 Background ............................................................................................... 91 4.1.2 Aim of the chapter .................................................................................... 94 4.2 Investigation of charge transfer complexes ..................................................... 94 4.3 Investigation of N,N-dibenzylnitroxide formation by EPR spectroscopy ......... 96 4.4 Investigation of intermediates and products by NMR spectroscopy............... 97 4.4.1 Reaction order and rate constant ............................................................. 97 4.4.2 Substituents effect .................................................................................. 105 4.5 Reaction mechanism ...................................................................................... 109 4.6 Conclusions ..................................................................................................... 111 4.7 ................................................................................................................................ 91 5 Products analysis of inhibition of the styrene polymerisation by DBHA/2,5- DTBBQ mixture ..................................................................................................... 112 4 5.1 Introduction .................................................................................................... 112 5.1.1 Background ............................................................................................. 112 5.1.2 Aim of the chapter .................................................................................. 115 5.2 Investigation on a milligram-scale .................................................................. 115 5.2.1 Analysis of the styrene polymerisation inhibited by DBHA/2,5-DTBBQ . 115 5.2.2 Products analysis in a non-polymerizable solvent .................................. 126 5.3 Investigation of the inhibition products in reactions carried out on a large scale 128 5.3.1 DBHA/2,5-DTBBQ mixture ...................................................................... 128 5.3.2 Product analysis in the presence of either DBHA or N,N- benzylidenebenzylamine-N-oxide ......................................................................... 148 5.3.3 Investigation of fluorine containing compounds .................................... 150 5.4 Conclusions ..................................................................................................... 161 6 A new inhibitor composition for the auto-initiated styrene polymerisation .... 162 6.1 Aim of the chapter .......................................................................................... 162 6.2 Inhibition efficiency of the DBHA/2,5-DTBBQ