Rhodanine-based Knoevenagel reaction and ring-opening polymerization for efficiently constructing multicyclic polymers

Cyclic polymers have a number of unique physical properties compared with those of their linear counterparts. However, the methods for the synthesis of cyclic polymers are very limited, and some multicyclic polymers are still not accessible now. Here, we found that the five˗membered cyclic structure and electron withdrawing groups make methylene in rhodanine highly active to aldehyde via highly efficient Knoevenagel reaction. Also, rhodanine can act as an initiator for anionic ring-opening polymerization of thiirane to produce cyclic polythioethers. Therefore, rhodanine can serve as both an initiator for ring-opening polymerization and a monomer in Knoevenagel polymerization. Via rhodanine-based Knoevenagel reaction, we can easily incorporate rhodanine moieties in the backbone, side chain, branched chain, etc, and correspondingly could produce cyclic structures in the backbone, side chain, branched chain, etc, via rhodanine˗based anionic ring-opening polymerization. This rhodanine chemistry would provide easy access to a wide variety of complex multicyclic polymers.


General Reagent Information
All reagents were used as received unless otherwise stated. Glycidyl propargyl ether (90%), PEG˗OH (Mn = 2000 g/mol) and 4˗dimethylaminopyridine (98%) were purchased from Aladdin. Rhodanine (99%), rhodanine˗3˗acetic acid (98%), 2˗(phenoxymethyl) oxirane (98%), 5˗formyl˗2˗thiopheneboronic acid (98%), tetrakis(triphenylphosphine)palladium (99%), 2,4,6˗Trimethylbenzaldehyde (98%) and pentaethylene glycol (97%) were purchased from Energy Chemical. was used as a matrix. Imaging of the cyclic brush-like polymer and multicyclic molecular brush were accomplished using Bruker atomic force microscope system in ambient air. Thermogravimetric analysis was measured on a TA Q5000IR instrument with a heating rate of 10 °C/min from room temperature to 700 °C. Differential scanning calorimetry thermograms were measured on a TA Q2000 differential scanning calorimeter instrument in aluminum pans with a heating or cooling rate of 10 °C/min under a flowing nitrogen atmosphere from ˗40 °C to 120 °C. All Tg values were obtained from the second scan after removing the thermal history. PL spectra were obtained from Hitachi F˗7000 fluorescence spectrophotometer. The absolute fluorescence quantum yields of copolymer 3 solid and multicyclic copolymer 4 solid were measured by were used to image the cyclic structures.

Model reaction of rhodanine-aldehyde condensation
A typical procedure is as follow: 1 (0.2 mmol), 2 (0.2 mmol) and TEA (0.1 mmol) were dissolved in 0.5 mL DMSO˗d6 and transferred into a NMR tube. Then the tube was sealed and immersed in an oil bath at 70 °C. After the certain reaction time, the conversions were analyzed by NMR measurement.

Ring˗opening polymerization of thiiranes using rhodanine and derivatives as initiators
A typical procedure is as follow: rhodanine (33 mg, 0.25 mmol), POMT (830 mg, 5 mmol) and tetrabutylammonium chloride (41 mg, 0.15 mmol) were dissolved in NMP to obtain 2.5 mL solution and transferred into a transparent glass tube. After two freezepump-thaw cycles, the tube was sealed and immersed in an oil bath at 75 °C. After 24 h reaction, the solution was precipitated into methanol several times and the product as yellow viscous solid was obtained after dried in vacuum.
After the reaction had been carried out at room temperature for 12 h, the mixture was precipitated into diethyl ether three times. Then the crude product was dissolved in methanol and further purified by dialysis (cut off 3500 Da MWCO). The methanol solvent was changed about 8 h later, and it was changed for at least five times at 8 h periods. The resulting cyclic brush˗like polymer was obtained as milk-white solid after concentration.

Synthesis of multicyclic polymers with cyclic units in the backbone.
Synthesis of the precursor polymer: 1d (292 mg, 0.5 mmol), terephthalaldehyde 3f (67 mg, 0.5 mmol) and trimethylamine (25 mg, 0.25 mmol) were dissolved in DMF to obtain 2.5 mL solution and immersed in an oil bath at 70 °C. After 3 h reaction, the solution precipitated into methanol several times and the product precursor polymer as light brown solid was obtained after dried in vacuum.
The precursor polymer (72.2 mg, 0.2 mmol heterocycle), POMT (664 mg, 4 mmol) and TBACl (56 mg, 0.2 mmol) were dissolved in NMP to obtain 2 mL solution and transferred into a transparent glass tube. After two freeze˗pump˗thaw cycles, the tube was sealed and immersed in an oil bath at 75 °C. After 18 h reaction, the solution was precipitated into methanol several times and the product as light brown solid was obtained after dried in vacuum.

Synthesis of red/near˗infrared AIE multicyclic polymer with pendant cyclic units.
2 Then this copolymer 1 and 40 fold excess AIBN were dissolved in 50 mL THF to remove RAFT end groups. After three freeze˗pump˗thaw cycles, the solution was sealed and immersed in an oil bath at 80 °C. After 16 h reaction, the solution was precipitated into diethyl ether several times and the copolymer 2 as white powder was obtained after dried in vacuum.
Copolymer 3 (57 mg, 0.03 mmol RDA units), POMT (150 mg, 0.9 mmol) and tetrabutylammonium chloride (13 mg, 0.045 mmol) were dissolved in NMP to obtain 0.75 mL solution and transferred into a transparent glass tube. After two freeze-pumpthaw cycles, the tube was sealed and immersed in an oil bath at 75 °C. After 10 h reaction, the solution was precipitated into methanol several times and the