Authors:
Lois Alexander, Kevin Dhaliwal, John Simpson, and Mark Bradley
Summary:
Small mono-disperse particles are valuable materials in many areas of research such as DNA sequencing, cellular analysis, biochemical reaction multiplexing as well as carriers for in vitro cellular sensors. They are typically prepared by dispersion polymerization, a widely used approach usually carried out via a simple one-pot procedure using a mixture of surfactant, initiator and the required monomers. It is an attractive method of polymerization due to the procedural simplicity, the ability to form mono-dispersed particles and the typically excellent yields and conversions obtained. However, although the method is widely applied the exact mechanism is complicated and not fully understood but it is believed that, following initiation, so-called nuclei are generated which grow by homo- and hetero-coagulation to form primary particles. Swelling of these primary particles with monomer and short-chain oligomers then permits the growth of polymer spheres which eventually reach a size that allows them to precipitate from the reaction media. Many factors are capable of altering the size and quality of the particles generated, with solvent often playing a crucial role due to its control over both monomer and polymer solubility.
During investigations, mono-dispersed beads were prepared for cellular uptake analysis using ethanol as the major solvent with the addition of n-butanol, 1,2-dimethoxyethane, isobutanol, cyclohexanone and formamide all giving uniform microspheres. However, an unexpected effect was observed following the addition of 5% dioxane in ethanol to the polymerization mixture with the generation, in 37% yield, of unusual particles with a narrowly dispersed micron-sized doughnut -like morphology (3.2 ± 0.48 µm, see ESI for details). SEM analysis showed their doughnut-like structure, which contained a 1 µm central hole. These particles maintained their morphology after re-solvation in all solvents evaluated, as well as showing the expected incorporation of aminomethylstyrene (amine loading: 9.6 µmol g−1). Doughnut formation was also observed with co-solvents of diethylketone, isopropyl alcohol, tetrahydrofuran or toluene in ethanol, but in these cases irregular rough shaped doughnuts or only small proportions were produced.
Source:
Chemical Communications; 3507-3509, 06/12/08