Microspheres from light–a sustainable materials platform

Polymeric microspheres (0.1–100 μm) are widely used due to their high surface-to-volume ratio, tunable properties, and facile surface functionalization, with applications spanning diagnostics, drug delivery, coatings, and electronics. Conventional particle syntheses in dispersed media commonly rely on thermally initiated radical polymerizations using initiators, surfactants, stabilizers, and other additives, which can contaminate the final product and require complex process control. Precipitation polymerization offers additive-free routes but is typically radical-based with low oxygen tolerance. This work addresses the need for milder, sustainable, initiator- and surfactant-free methods by developing a purely light-driven, step-growth precipitation polymerization that forms microspheres at ambient temperature via Diels–Alder cycloadditions of photo-generated ortho-quinodimethanes with bismaleimide dienophiles, including synthesis using natural sunlight.

Prior photopolymerization in dispersed systems has focused on radical processes such as photoinitiated PISA and PET-RAFT, enabling complex morphologies under mild conditions but generally requiring initiators, emulsifiers, or stabilizers. Adaptations from thermal to photo routes commonly use emulsion, dispersion, or miniemulsion techniques, still necessitating process additives. Precipitation polymerization is an additive-free approach but is typically radical-driven (low oxygen tolerance) and less explored mechanistically for heterogeneously induced particle formation. Light-mediated step-growth alternatives (e.g., thiol-ene polymerizations) have been reported, including miniemulsion routes to poly(thioether) latex nanoparticles and thiol–isocyanate ligations yielding uniform 2–8 μm particles. Building on wavelength-gated photochemistry generating ortho-quinodimethanes from methylisophthalaldehydes that undergo Diels–Alder reactions with maleimides, the present study leverages this non-radical, light-driven chemistry to enable precipitation polymerization without additives.

Reaction concept: Photoenolization of methylisophthalaldehyde (MIA) generates a reactive ortho-quinodimethane (o-QDM) that undergoes Diels–Alder cycloadditions with maleimide moieties to form benzo[j]isoindole-5-carbaldehyde adducts. Using AA/BB monomers (AA: photoactive MIA derivatives; BB: bismaleimides), an A8B8 step-growth network forms under UV (365–385 nm) or sunlight; growing chains precipitate to form particles. Monomers: AA1 = 4-methoxy-2,5-dimethylisophthalaldehyde (synthesized from 2,5-dimethyl-2-furan-dione and base-mediated dehydration); BB1 = commercially available 2,4-toluene bismaleimide. Additional monomers for backbone variation: AA2 = 4-(2-(2-methoxyethoxy)ethoxy)-2,5-dimethylisophthalaldehyde; AA3 = 4,4'-(1,4-phenylenebis(methylene))-bis(oxy)bis(2,5-dimethylisophthalaldehyde) (multi-o-QDM crosslinker); AA4 = 4,6-dimethoxy-2,5-dimethylisophthalaldehyde; BB2 = 1,1'-Methylenedi-2-((4-phenyl)bisaminomaleic). Typical particle synthesis (lab LED): Prepare AA and BB stock solutions in acetonitrile (ACN) at 5 mmol L−1. Filter 1 mL of each through 2.5 μm PTFE syringe filters and combine in an optical cell (V = 2 mL). Degas by N2 purging for 5 min under irradiation with a 3 W LED (λ = 360–390 nm, 4 cm distance) while rotating on a bottle/tube roller at 10 rpm. The clear solution gradually turns turbid over 4 h. Isolate particles by centrifugation (15,000 rpm, 5 min), discard supernatant, wash pellet with THF twice, redisperse in ACN, and characterize by SEM. A 10 W LED can be used to influence particle size and dispersity. Larger scale: 10 mL of each solution in a 20 mL crimp vial; centrifuge at 5,000 rpm for 5 min; typical yield ~68.9%. Sunlight synthesis: Identical conditions without LED; place bottle roller outdoors for 4–8 h. Initial clear, homogeneous solution becomes turbid; isolate and wash as above. Aging/stability tests: Store particles in ACN, THF, or CHCl3 at ambient temperature; assess after months by SEM. Thermal stability: store dry powder at 150 °C for 1 month; redisperse and analyze. Solvent/thermal stress: disperse in 1,2,4-trichlorobenzene (TCB) at 25 °C or 150 °C for up to 1 month; monitor morphology and size by SEM over time. Thermal analyses: DSC for crystallinity/phase transitions; TGA to 800 °C for decomposition profile. Surface functionalization: Use residual surface maleimide groups for post-modification. NITEC: UV-induced cycloadditions with tetrazoles (Tz1 phenyl; Tz2 methoxyphenyl) to produce fluorescent pyrazoles; isolate by centrifugation/washing; measure emission. Thiol–ene: mix particles with PEG-SH (Mn ~2000 g mol−1) at ambient temperature; isolate; assess water dispersibility and size changes by SEM.

- A purely light-driven, initiator/surfactant/additive-free step-growth precipitation polymerization yields monodisperse microspheres (0.4–2.4 μm) at ambient temperature, including synthesis under natural sunlight. - Sunlight synthesis (AA1/BB1 in ACN) produces narrow particles: Dn ≈ 0.76 μm, dispersity D ≈ 1.12 after ~4 h; comparable results under a 3 W 365 nm LED (Dn ≈ 0.79 μm, D ≈ 1.15). A 10 W LED yields larger particles with very low dispersity (Dn ≈ 1.60 μm, D ≈ 1.03). - Particle stability at ambient conditions over months in various solvents (ACN, THF, CHCl3) shows minimal size/dispersity changes (examples: Dn ≈ 1.02 μm, D ≈ 1.03; 0.93 μm, D ≈ 0.95; 0.91 μm, D ≈ 1.12). - Thermal/solvent robustness: Dry particles stored at 150 °C for 1 month remain stable (Dn ≈ 1.56 μm, D ≈ 1.05). In TCB, particles are stable at ambient temperature for 1 month (initial Dn ≈ 1.55 μm, D ≈ 1.08; after 1 month Dn ≈ 1.53 μm, D ≈ 1.09). At 150 °C in TCB, surfaces become coarse, and coalescence occurs over time; sizes increased over 2 weeks (e.g., to ≈1.71 μm), with more pronounced coalescence by 1 month. - Thermal analyses: DSC shows no crystallinity or phase transitions; TGA to 800 °C shows ~2% mass loss between 25–210 °C (attributed to water), onset of steady decomposition at ~360 °C, and final mass loss ~47.7% (residual carbon ~54%). - Clean, additive-free surfaces enable efficient post-functionalization: NITEC with tetrazoles produces strongly fluorescent particles (pyrazole adducts; emission maxima ~450 nm for Tz1 and ~330 nm for Tz2) without altering size/dispersity. Thiol–ene with PEG-SH imparts water dispersibility; SEM indicates a modest size increase (~100 nm). - Polymer backbone tuning via monomer choice controls particle size: AA2 (higher oligomer precipitation Mw ~10,000 g mol−1 vs ~6,000 g mol−1 for others) yields larger particles; AA3 (multi-o-QDM crosslinker) can produce smaller particles down to ~0.44 μm; AA4 behaves similarly to AA1. BB2 generally gives larger particles with broader distributions than BB1. These trends correlate oligomer critical precipitation molecular weight with final particle size.

The study demonstrates that light-induced, non-radical step-growth polymerization can be coupled with precipitation to form microspheres under exceptionally mild, sustainable conditions, addressing limitations of conventional radical-based, additive-reliant dispersed photopolymerizations. The ability to use sunlight or low-power LEDs, without initiators, surfactants, or heating, affords monodisperse particles with clean surfaces that are immediately available for post-functionalization, enabling straightforward modulation of properties such as fluorescence and hydrophilicity. Stability studies confirm robustness at ambient and elevated temperatures and in harsh solvents, indicating suitability for practical handling and potential applications (e.g., chromatographic media, diagnostic platforms). Structure–reactivity relationships are initiated by showing how monomer structure (AA/BB selection) and oligomer precipitation thresholds govern particle size and dispersity, offering levers to tailor microsphere dimensions and surface reactivity. Collectively, the results validate the feasibility and advantages of a sustainable, sunlight-enabled microsphere synthesis platform and suggest broad relevance for fields requiring clean, functional particle surfaces.

A simple, purely light-driven AA/BB step-growth precipitation polymerization produces polymeric microspheres from 440 nm to 2.4 μm under ambient conditions, including using natural sunlight, without initiators, surfactants, additives, or heating. The particles are stable in dry and solvated states, withstand elevated temperatures, and feature clean, reactive surfaces enabling rapid, mild functionalization (e.g., NITEC fluorescence labeling, thiol–ene PEGylation for aqueous dispersibility). Initial structure–reactivity insights show that monomer choice and backbone design govern critical precipitation molecular weight and particle size. Future research should deepen mechanistic understanding of backbone influences on nucleation and growth, systematically optimize particle size/dispersity via AA/BB ratios, solvent systems, and light intensity, and explore functional backbones for targeted properties such as degradability, chemiluminescence, conductivity, and biomedical compatibility.

- The influence of polymer backbone structure, monomer stoichiometry, solvent composition, and light intensity on nucleation/growth and particle size/dispersity was only preliminarily explored; comprehensive parametric studies are beyond the present scope. - High-temperature stability in harsh solvents (e.g., TCB at 150 °C) shows surface coarsening and particle coalescence over extended times, indicating limits under extreme conditions. - The approach currently relies on near-UV (365–385 nm) irradiation; while sunlight is effective, translation to strictly visible-light activation for all monomer sets was not established here. - Oxygen removal (N2 purge) was employed in laboratory procedures, suggesting potential sensitivity to dissolved oxygen that merits quantification.