Download files
Abstract
There is a growing need to reduce the reliance on environmentally harmful organic solvents. Considerable efforts are devoted to the advancement of polymerisation processes in benign media such as CO2-expanded liquids. In the present thesis, the application of CO2-expanded media in polymerisations has been demonstrated in bulk and heterogeneous systems.
Nitroxide-mediated radical polymerisation (NMP) based on TEMPO has been performed for methyl methacrylate (MMA) under conditions of CO2-expanded monomer. The fraction of propagating radicals lost due to disproportionation with nitroxide, a well-known problem in NMP of methacrylates, decreased in the presence of CO2. However, the polymerisation ceased at relatively low conversions both with and without CO2, indicating that disproportionation between propagating radical and nitroxide is also a major problem in CO2-expanded MMA.
Dispersion polymerisation of styrene in CO2-expanded ethanol was performed at various pressures. Particle size distributions were narrow in CO2-expanded media at 10 and 20 vol% styrene loading. However, the addition of CO2 produced larger particles at a greatly reduced polymerisation rate compared with the CO2-free approach. Overall, dispersion polymerisation in a CO2-expanded medium likely suffers from dilution effects (volumetric expansion) and reduced monomer absorption in the particle phase.
A novel type of heterogeneous polymerisation was developed that relies on CO2-assisted particle formation exploiting the homogeneous expansion limit. This approach enables complete decoupling of the particle formation process and the polymerisation. The nanoparticles are generally smaller and more uniform in size than those obtained in the corresponding CO2-free approach (dispersion polymerisation). Polymerisation in CO2-induced emulsions has potential application to a range of polar and non-polar monomers and is anticipated to be applicable to "controlled/living" radical polymerisation techniques, which are difficult to implement as traditional dispersion polymerisations.