Bis(benzylamine) monomers: One-pot preparation and application in dendrimer scaffolds for removing pyrene from aqueous environments

Biorenewable triblock copolymers consisting of l-lactide and ε-caprolactone for removing organic pollutants from water: a lifecycle neutral solution

Background

Current methods of removing organic pollutants from water are becoming ineffective as the world population increases. In this study, a series of biorenewable triblock copolymers with hydrophobic poly(ε-caprolactone) block and hydrophilic poly(l-lactide) blocks were synthesized and tested as agents to remove environmental pollutants from an aqueous solution. The percent of pollutant removed and equilibrium inclusion constants were calculated for the polymers. These values were compared to previously known removal agents for their effectiveness.

Results

Triblock copolymer samples removed over 70% of the polycyclic aromatic hydrocarbon (PAH) phenanthrene from an aqueous solution, with selectivity for the adsorption of phenanthrene over other PAHs tested. The inclusion constant was 7.4 × 10⁵ M⁻¹ and adsorption capacity was 5.8 × 10⁻⁷ mol phenanthrene/g polymer. Rose Bengal was used to further probe the nature of interactions between the copolymers and a small molecule guest. Solid samples of the block-poly(l-lactide)–block-poly(ε-caprolactone)–block-poly(l-lactide) (PLLA–PCL–PLLA) systems were found to rapidly remove over 90% of Rose Bengal from aqueous solution, resulting in a complete disappearance of the characteristic pink color. Solutions of the copolymers in dichloromethane also removed Rose Bengal from water with a similar level of efficiency. Large inclusion constant values were obtained, ranging from 1.0 × 10⁵ to 7.9 × 10⁵ M⁻¹, and the average adsorption capacity value of 6.2 × 10⁻⁷ mol/g polymer was determined. Aged polymer samples exhibited different adsorption characteristics and mechanistic theories for the removal of Rose Bengal were determined.

Conclusion

The triblock copolymer consisting of l-lactide and ε-caprolactone was effective in removing various organic pollutants in aqueous environments. It is a biorenewable material which leads to minimal waste production during its lifecycle. These polymers were in general more effective in removing organic pollutants than commercially available pollution removal systems.

Effect of dendrimeric composition on the removal of pyrene from water

Several closely related branched structures related to the well-established benzyl ether dendrimers were tested as hosts for the removal of pyrene from water. Fluorescence spectroscopy based complexation studies showed that the first generation benzyl ether dendrimers removed on average 94 % of pyrene from a saturated aqueous solution after 30 min. By contrast, branched systems containing modified benzyl ether or cyclohexane functional groups removed between 0 and 75 % of pyrene at the same time point while systems with benzyl amine resulted in a 77 % decrease in pyrene. This selectivity eroded over two days after which time all dendrimeric types removed pyrene to approximately the same extent. Inclusion constants of 10⁹–10¹¹ M⁻¹ were calculated for the branched systems studied.

Methyl 3,5-bis(cyclo-hexyl-meth-oxy)benzoate

In the title compound, C22H32O4, the atoms of the methyl ester group and the alk-oxy O atoms are all coplanar with the central aromatic ring, with an r.m.s. deviation of 0.008 Å. Bonds to the methyl-ene and cyclo-hexyl groups are also very close to this plane, so that the mol-ecule is essentially flat, apart from the cyclo-hexyl groups. The mean planes through the cyclo-hexyl groups are tilted by 30.08 (9) and 36.14 (7)° with respect to the central aromatic ring. In the crystal, pairs of mol-ecules linked by C-H⋯O hydrogen bonds form planar units which are stacked along the a axis, with an average inter-planar distance of 3.549 (2) Å. Stacking appears to be stabilized by further weak C-H⋯O hydrogen bonds.

Benzene-1,3,5-triyl tribenzoate

The title compound, C27H18O6, commonly known as phloroglucinol tribenzoate, is a standard unit for the family of benzyl ether dendrimers. The central phloroglucinol residue is close to planar, with out-of-plane distances for the three oxygen atoms of up to 0.095 (3) Å, while the three attached benzoate groups are approximately planar. One benzoate group is twisted [C-C-O-C torsion angle = 98.2 (3)°] from the central plane, with its carbonyl O atom 2.226 (4) Å above that plane, while the other two benzoate groups are twisted in the opposite direction [C-C-O-C torsion angles = 24.7 (2) and 54.8 (2)°], so that their carbonyl O atoms are on the other side of, and closer to the central plane, with distances from the plane of 1.743 (4) and 1.206 (4) Å. One benzoate group is disordered between two conformers, with occupancies of 86.9 (3) and 13.1 (3)%, related by a 143 (1)° rotation about the bond to the central benzene ring. The phenyl groups of the two conformers occupy the same space. The mol-ecule packs in the crystal with two of the three benzoate phenyl rings stacked parallel to symmetry-related counterparts, with perpendicular distances of 3.715 (5) and 3.791 (5) Å. The parallel rings are slipped away from each other, however, with centroid-centroid distances of 4.122 (2) and 4.363 (2) Å, respectively.