Synthesis and in-depth characterization of reactive, uniform, crosslinked microparticles based on free radical copolymerization of 4-vinylbenzyl azide

Abstract

The introduction of functional groups into microparticles is commonly accomplished through, at times, low-yielding post-synthesis modification. In this detailed study, the introduction of azide functionality into uniform, crosslinked, macroporous microparticles through direct copolymerization of styrene, divinylbenzene (DVB), and 4-vinylbenzyl azide (VBA) in varying ratios inside swollen polystyrene seed (template) particles is investigated. Formulations contained up to 40 wt% of VBA in the monomer mixture. Resulting microspheres were characterised by SEM, porosimetry, FT-IR spectroscopy, and CHN elemental analysis. Uniform spherical particles with diameters ranging from 7.3 to 10.8 μm with diameter dispersities typically below 1.01 and with tuneable azide loadings from 0.11 to 1.17 mmol g⁻¹ were obtained. Interestingly, severe effects of VBA addition on porosity, surface smoothness, and particle shape were observed. Specific surface areas and cumulative pore volumes increased with the amount of DVB in feed, decreased with increasing VBA feed ratio, and increased drastically for the use of azide-functional template particles with measured cumulative pore volumes reaching up to 0.60 cm³ g⁻¹. With increasing VBA feed, formation of smaller, secondary particles was observed and attributed to an incomplete swelling of VBA into seed particles, which is discussed as a main reason for lower-than-expected azide contents in product particles. For high VBA feed ratios (>25 wt%), dented, hollow, or hollow collapsed azide-functional particles were found, presumably due to immiscibility of the growing azide-functional copolymer with the polystyrene seeds. Finally, successful click-modification is demonstrated with phenylacetylene and an alkyne-functional Rhodamine B dye allowing for mapping of functionalization density through confocal fluorescence microscopy.