Donor–acceptor Stenhouse adduct functionalised polymer microspheres

Polymers that carry donor–acceptor Stenhouse adducts (DASAs) are a very relevant class of light-responsive materials. Capable of undergoing reversible, photoinduced isomerisations under irradiation with visible light, DASAs allow for on-demand property changes to be performed in a non-invasive fashion. Applications include photothermal actuation, wavelength-selective biocatalysis, molecular capture and lithography. Typically, such functional materials incorporate DASAs either as dopants or as pendent functional groups on linear polymer chains. By contrast, the covalent incorporation of DASAs into crosslinked polymer networks is under-explored. Herein, we report DASA-functionalised crosslinked styrene–divinylbenzene-based polymer microspheres and investigate their light-induced property changes. This presents the opportunity to expand DASA-material applications into microflow assays, polymer-supported reactions and separation science. Poly(divinylbenzene-co-4-vinylbenzyl chloride-co-styrene) microspheres were prepared by precipitation polymerisation and functionalised via post-polymerisation chemical modification reactions with 3rd generation trifluoromethyl-pyrazolone DASAs to varying extents. The DASA content was verified via19F solid-state NMR (ssNMR), and DASA switching timescales were probed by integrated sphere UV-Vis spectroscopy. Irradiation of DASA functionalised microspheres led to significant changes in their properties, notably improving their swelling in organic and aqueous environments, dispersibility in water and increasing mean particle size. This work sets the stage for future developments of light-responsive polymer supports in solid-phase extraction or phase transfer catalysis.


SI Experimental Procedures
FT-IR spectra were recorded on a Shimadzu IRAffinity-1 Spectrophotometer instrument.
High-resolution mass spectrometry (HRMS) was obtained from a Thermo-Fisher Scientific Exactive Plus Orbi-Trap Mass Spectrometer using electrospray ionisation Experimental S1. Synthesis of N-phenylpropane-1,3-diamine (1) Cu(I)Cl (0.13 g, 1.3 mmol), potassium hydroxide (1.38 g, 24.6 mmol), iodobenzene (1.40 mL, 12.5 mmol) and propane-1,3-diamine (3.10 mL, 37.1 mmol) were added to a glass sample vial equipped with a magnetic stirrer bar. The reaction mixture was left stirring for 8 h at a temperature of 0 °C before being diluted with DI water (50 mL) and extracted with dichloromethane (DCM; 2 x 50 mL). The organic layers were collected and dried over magnesium sulfate, filtered and concentrated under reduced pressure. The crude product was filtered through a silica gel column (DCM/methanol 2:1 with 5 % triethylamine as eluent) to give a yellow oil as the final product (1.24 g, 69 %).

Figure S4. 19 F ssNMR spectrum of D3.
To determine the functionalisation degree of the microspheres from the normalised integral per mg / a.u. the following calculations were performed: Due to the variance in fluorine T 1 relaxation and experimental repetitions for the DFB external standard (T 1 = 473 s) and the DMs (T 1 = 790 ms), raw integral values were corrected with respect to the number of repetitions and then normalised to get an integral per mg value. The fluorine content per mole of DFB external standard was calculated: Where is the number of fluorine atoms in the molecule and N A is Avogadro's The fluorine content represented by 1 normalised integral unit could then be calculated: Where is the moles of DFB used in the NMR experiment and is the normalised integral value. Since integral values between standard and samples were normalised according 1 mg of substance, is equal to or 4.583 x 10 -6 mol.

/ )
Where N(F) IU represents the number of fluorine atoms equal to one normalised integral unit, 218.2 g mol -1 is the molar mass of DFB.
Knowing the fluorine content represented by 1 normalised integral unit, the functionalisation degree of the microspheres could be calculated (example below for D3, 50 mol% VBC in initial monomer feed, 25 mol% aniline functionalisation).
Experimental fluorine content as observed by ssNMR spectroscopy at -61.7 ppm for D3: ( ) 3 = × ( ) = 1.098 × 10 18 The theoretical maximum fluorine content per mg microsphere (D3) was calculated based off the available aniline groups, i.e. the degree of aniline functionalised determined from elemental analysis.
( ) 3( ℎ ) = ( ( ℎ ) ) 3 = ( 1 × 10 -3 244.28 ) 3 × 0.25 = 1.849 × 10 18 M w(sphere) is the weighted average molecular weight of the microsphere, according to the initial monomer feed, conversion to chlorine groups to aniline groups (determined from elemental analysis) and assuming all aniline groups react to form DASA groups: Where M i is the molecular weight of a monomer species and is its mole fraction.
f M is the maximum functionalisation degree (in this case 25 mol%). N A is Avogadro's constant.
Multiplying by a factor of 3 was done to represent the 3 fluorines of the CF 3 on the DASA units.
The conversion of the aniline units to the DASA is then represented by: The overall degree of DASA functionalisation for the microspheres is finally determined by considering the aniline conversion with respect to the initial aniline functionalisation degree:

Raw UV-Vis data from integrated sphere spectrophotometer
Raw data processing: The raw data was processed using Origin software package. The raw data was smoothed using the Savitzky-Golay method. To account for baseline drifts due to the heterogeneous dispersion, absorbance values of spectra were corrected at 700 nm before being normalised.