Design and Synthesis of Molecularly Imprinted Polymers for the Selective Extraction of Polychlorinated Biphenyls

Polychlorinated biphenyls (PCB)s are simple aromatic compounds which lack the appropriate functionality to form strong non-covalent interactions with a functional monomer (FM). This is a significant and as yet unaddressed challenge for the synthesis of molecularly imprinted polymers (MIP)s. The development of PCB selective MIPs presented herein was conducted by a fragment imprinting approach utilising the weak π-π stacking interactions between the chlorinated benzene templates and the aromatic FMs. Imprinting of these templates relied on optimising π-π stacking interaction energy, the strength of solvophobic effects, implementing a low temperature photochemical initiated polymerisation and adding a greater proportion of template to the pre-polymerisation mixture than a typical MIP protocol. 2, 4, 6-Trimethylstyrene (TMS) was predicted by our Effective Fragment Potential (EFP) computational approach as the FM capable of forming the most favourable π-π stacking interactions with the chlorinated benzene templates. The accuracy of this prediction was demonstrated by the higher specific binding (SB) afforded by 1, 2, 3, 4, 5-pentachlorobenzene (PenCB) imprinted MIPs synthesised using TMS as the FM (SB = 4.5 mM g^-1) compared to MIPs imprinted using 4-vinylpyridine (4VP), 2, 3, 4, 5, 6-pentaflurostyrene (PFS) or styrene (STY) as FMs (SB = 2.0 mM g^-1, 3.1 mM g^-1, 3.4 mM g^-1 respectively). Analysis of the IF of 1, 2, 3-trichlorobenzene (TriCB) imprinted MIPs by an extreme vertices mixture design predicted the optimum proportions of template: FM: cross-linking monomer added to the pre-polymerisation mixtures were 30: 12: 58 respectively. This was confirmed by the highest imprinting factor (IF) of 3.7 which was achieved for a TriCB imprinted MIP using PFS as the FM. The lower IF of TriCB MIPs synthesised using TMS as the FM was determined, by molecular dynamics simulation, to be caused by frequent interactions of MeOH with the π-orbital of TMS which disrupted the π-π stacking interactions between TMS and TriCB in the pre-polymerisation cluster. The success of the fragment imprinting technique was revealed by an average increase in extraction of PCBs which was 1.4 times higher by PenCB imprinted MIPs and 5.2 times higher by TriCB imprinted MIPs compared to the equivalent non-imprinted polymers.