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Development and characterisation of molecularly imprinted suspension polymers

A controlled photo-polymerisation procedure for the synthesis of spherical molecularly imprinted polymer (MIP) beads through the use of a suspension polymerisation methodology is described.
As molecular imprinting moves towards commercialisation, the need to produce packing materials with good flow properties is critical.
A comparative study of non-covalent MIPs synthesised by suspension and bulk polymerisation methods using 2-aminopyridine (2-apy) as a template and methacrylic acid (MAA) as functional monomer to produce acrylate polymers is reported. The influence of experimental parameters such as temperature, agitation speed and cross-linker i.e. trimethylolpropane trimethacrylate
(TRIM) and ethylene glycol dimethacrylate (EGDMA) was examined in order to assess their impact on binding performance, yield, morphology and particle size distribution in the different polymers. It was concluded that for improved polymer performance, longer polymerisation times and lower temperatures resulted in the best polymer affinity for suspension polymerisation.
A detailed investigation of polymers produced by varying pre-polymerisation composition was performed using binding isotherm analysis. The optimum isotherm for use for MIP analysis was found to be the Langmuir-Freundlich isotherm with additional information subsequently obtained by plotting affinity distribution spectra. A greater number of binding sites with similar binding energies were found in the suspension MIPs in comparison to NIPs. A GC-MS methodology was utilised to characterise the performance of polymers synthesised under varying conditions in the absence of solvent. A direct comparison was found between imprinting performance assessed by a GC-MS methodology and solution phase analysis experiments.
To extend the scope of this study, MIPs were prepared for chlorpheniramine racemate (CP), dchlorpheniramine (d-CP), brompheniramine racemate (BP) and d-brompheniramine (d-BP) as the template molecules respectively employing a novel non-stabilised aqueous suspension polymerisation procedure. A cross-selectivity rebinding study was carried out on the polymers using chiral HPLC analysis. Higher uptake capacities and IF values were found for the BP polymers over the CP polymers. It is speculated the higher binding of BP polymers over CP is related to differences in polarisability between Br and Cl and steric constraints.
The d-CP MIP prepared at 9500 rpm was packed as a HPLC stationary phase and the enantioselectivity and chromatographic behaviour of CP and BP was studied using an aqueous mobile phase. The highest enantioselectivity factor (1.19) and resolution (0.88) was obtained for
CP over BP. Interestingly, BP was more retained on the d-CP MIP column than CP as a result of deprotonation correlating with equilibrium binding observations. The effect of column temperature on the d-CP MIP column was also studied for the separation of a mixture of d-CP and d-BP isomers and concluded that with increase in column temperature improved retention performance was observed with sharper peaks produced and an increase in resolution from 0.61 to
0.86. The d-CP imprinted column was also capable of separating pheniramine racemate (PHEN), CP and BP enantiomers using an aqueous mobile phase. The highest retention factor,
enantioselectivity and resolution were observed for BP over the PHEN and CP enantiomers. This was explained due to the fact that all compounds possess similar pKa values and in addition the
hydrophobicity is in the order of BP, CP and PHEN. The chromatographic behaviour and separation of CP, d-CP, BP and d-BP isomers were also studied on a d-BP imprinted column using an aqueous mobile phase. The retention, selectivity and resolution of BP and d-BP on the d-BP imprinted column were greater in comparison to CP and d-CP. The results showed that the d-CP and d-BP imprinted columns each gave the best enantioseparation for its own template.
Physical characterisation of the polymers was carried out using nitrogen sorption porosimetry, particle size distribution, solvent swell studies and SEM. All samples exhibited low specific surface areas, pore volumes and pore diameters indicating non-porous materials. Trends were observed between the polymer properties characterised by the above techniques and the synthesis conditions.