Abstract:
Polymerisation of methyl methacrylate (MMA) and styrene in supercritical carbon dioxide (CO2) was carried out in a high pressure reactor. The investigation was focused primarily on the effect of experimental variables including reaction temperature and pressure, type and concentration of initiator, and stabiliser/fluorosurfactant, on the physical form, average molecular weight and polydispersity of the polymer products, together with reaction yield.
In optimised conditions, use of 2,2-azobrsisobutyronitrile (AIBN) as initiator in the presence of poly(dimethylsiloxane) monomethacrylate (PDMS-mma) stabiliser, gave high molecular weight poly(methyl methacrylate) (PMMA) in high yield, in the form of free-flowing powder with discrete spherical particle morphology. The polymerisation of MMA was also investigated in supercritical CO2 using the low temperature initiator Bis(4- tert - butylcyclohexyl) peroxydicarbonate (Perkadox 16). In contrast with the results obtained for polymerisation of MMA in supercritical CO2, polymerisation of styrene in that medium gave small yields of low molecular weight polymer.
Dispersion polymerisation of styrene in ethanol/2-methoxyethanol medium was carried out using poly(vinylpyrrolidone) (PVP) stabiliser. Polystyrene was formed in high yield with narrow particle size and molecular weight distributions.
Polystyrene formed in the ethanol/2-methoxyethanol medium was used to investigate the synthesis of polymers with core-shell morphology whereby the outer shell is composed of poly(vinyl acetate) polymer surrounding hydrophobic polystyrene polymer microspheres. The resulting product did not show the desired morphology as determined by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The polymerisation of MMA in n-heptane was investigated using poly(dimethylsiloxane)-vinyl terminated stabiliser. PMMA was obtained in high yield with broad molecular weight distribution. Extension of that study to polymerisation of MMA in pressurised liquefied petroleum gas (LPG) and liquid isobutane gave high yields of PMMA as free-flowing powder, even in the absence of a stabiliser.
The effect of temperature, and types and concentrations of initiator, stabilisers/fluorosurfactants on the resulting polymer properties was investigated for polymerization in the hydrocarbon solvents, both as liquids under pressure and as supercritical fluids. Use of supercritical isobutane as the reaction medium gave PMMA which adhered to the walls of the vessel as a hard, glassy solid. Polymerisation of styrene gave higher yields for reactions in supercritical isobutane than for polymerisations in liquid isobutane, and the SEM revealed discrete spherical particles formed in supercritical isobutane.