Radical polymerization by a supramolecular catalyst: cyclodextrin with a RAFT reagent

Supramolecular catalysts have received a great deal of attention because they improve the selectivity and efficiency of reactions. Catalysts with host molecules exhibit specific reaction properties and recognize substrates via host–guest interactions. Here, we examined radical polymerization reactions with a chain transfer agent (CTA) that has α-cyclodextrin (α-CD) as a host molecule (α-CD-CTA). Prior to the polymerization of N,N-dimethylacrylamide (DMA), we investigated the complex formation of α-CD with DMA. Single X-ray analysis demonstrated that α-CD includes DMA inside its cavity. When DMA was polymerized in the presence of α-CD-CTA using 2,2'-azobis[2-(2-imidazolin-2-yl)propane dihydrochloride (VA-044) as an initiator in an aqueous solution, poly(DMA) was obtained in good yield and with narrow molecular weight distribution. In contrast, the polymerization of DMA without α-CD-CTA produced more widely distributed polymers. In the presence of 1,6-hexanediol (C6 diol) which works as a competitive molecule by being included in the α-CD cavity, the reaction yield was lower than that without C6 diol.

All chemicals except monomers and α-CD were used as supplied. Monomers were purified to remove inhibitors prior to use. Acrylamide (AAm) was recrystallized from water. Acrylic acid (AA) and N,N'-dimethylacrylamide (DMA) were purified by distillation at reduced pressure.

Measurements and procedures.
The 1 H NMR and 13 C NMR spectra were recorded at 600 MHz with a VNS 600 NMR spectrometer, 500 MHz with a JEOL JNM-ECA 500 NMR spectrometer and at 300 MHz with a Varian Mercury 300 NMR spectrometer. Positive-ion matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry experiments were performed on a BRUKER autoflex TM speed MALDI-TOF system calibrated by dihydoxybenzoic acid (DHBA) and peptide standard (Aldrich).
Gel permeation chromatography (GPC) was performed in 10 mmol LiBr in formamide (0.30 mL/min, 40 °C) using an TOSOH HLC-8320GPC EcoSEC ® equipped with a TOSOH TSK gel α-M column to determine molecular weights. M n and M n /M w of the obtained polymers were measured with respect to polystyrene sulfonate sodium salt (PSSNa) standards (American polymer standards Corp., Mentor, Ohio), and polyacrylamide (PAAm) standards (American polymer standards Corp., Mentor, Ohio).

Single crystal X-ray crystallographic measurements
Single crystal X-ray crystallographic measurements were performed using a Rigaku RAXIS-RAPID imaging plate diffractometer with MoKα radiation. Obtained data were calculated using the Crystal Structure 4.0.1. crystallographic software package except for refinement, which was performed using SHELXL-97. CCDC 1500212 and 1500213 contain the data for the structures. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.

Preparation of single crystals of α-CD-N,N'-dimethyl acrylamide complex
To 5 mL of an aqueous α-CD solution (75 mM) in a sample tube, N,N'-dimethyl acrylamide (5 mmol) was added and the mixture was shaken until a clear solution was obtained. The mixture was left standing for days and the obtained crystals were analyzed by X-ray diffraction.

Aqueous polymerization of acrylamide (AAm)
α-CD-CTA (0.01 mmol), acrylamide (144 mg, 2 mmol), VA-044 (0.13 mg, 4 μmol) were dissolved in acetate buffer (4 mL). After three successive freeze-and-thaw cycles, the Schlenk flasks were subsequently immersed in an oil bath at 45 °C, and the polymerization was allowed to proceed for 24 h before being quenched with by immersion in MeOH/dry ice bath. The resulting solution was purified by reprecipitated in MeOH three times and dried in vacuo. The resulting polymer was characterized by 1 H-NMR and GPC (respect to PAAm standard).

Aqueous polymerization of Acrylic acid (AA)
α-CD-CTA (0.01 mmol) and VA-044 (1.30 mg, 4 μmol) were dissolved in freshly distilled water (2 mL) under Ar atmosphere. After adding acrylic acid (140 μL, 2 mmol), the solution was degassed by two successive freeze-and-thaw cycles. The schlenk flasks were subsequently immersed in an oil bath at 45 °C, and the polymerization was allowed to proceed for 24 h before being quenched by immersion in an MeOH/dry ice bath. The resulting solution was freeze-dried to obtain the crude polymer. The crude polymer was dissolved in MeOH (8 mL), then reprecipitated in diethyl ether (80 mL) for three times and dried in vacuo. The resulting polymer was characterized by 1 H NMR S4 and GPC (respect to PSSNa standard).

Aquaous polymerization of N,N'-dimethylacrylamide (DMA)
The polymerization of DMA was carried out in an analogous manner to that of acrylic acid using DMA (205 μL, 2 mmol). After freeze-drying the reaction mixture, the crude polymer was dissolved in DCM (8 mL), then reprecipitated in diethyl ether (80 mL) three times and dried in vacuo. The resulting polymer was characterized by 1 H NMR and GPC (respect to PAAm standard).

Time-conversion measurement
The entire solution was diluted 4 times to slow down the polymerization rate

Preparation Preparation of 3-(benzylthiocarbonothioylthio)propanoic acid (CTA-COOH).
CTA-COOH was synthesized in a similar manner as described in [1]. To a stirred suspension of K 3 PO 4 (4.0 g, 18.8 mmol) in acetone (10 mL      β-CD-CTA was prepared in an analogous manner to that of α-CD-CTA using