A novel solid state photocatalyst for living radical polymerization under UV irradiation

This study presents the development of a novel solid state photocatalyst for the photoinduced controlled radical polymerization of methacrylates under mild UV irradiation (λmax ≈ 365 nm) in the absence of conventional photoinitiators, metal-catalysts or dye sensitizers. The photocatalyst design was based on our previous finding that organic amines can act in a synergistic photochemical reaction with thiocarbonylthio compounds to afford well controlled polymethacrylates under UV irradiation. Therefore, in the current contribution an amine-rich polymer was covalently grafted onto a solid substrate, thus creating a heterogeneous catalyst that would allow for facile removal, recovery and recyclability when employed for such photopolymerization reactions. Importantly, the polymethacrylates synthesized using the solid state photocatalyst (ssPC) show similarly excellent chemical and structural integrity as those catalysed by free amines. Moreover, the ssPC could be readily recovered and re-used, with multiple cycles of polymerization showing minimal effect on the integrity of the catalyst. Finally, the ssPC was employed in various photo-“click” reactions, permitting high yielding conjugations under photochemical control.


Synthesis of 3-(2-bromoisobutyramido)propyl(triethoxy)-silane (BIBAPTES). 3-(2-
Bromoisobutyramido)propyl(triethoxy)-silane (BIBAPTES) was synthesized according to literature 2 . The 3-aminopropyltriethoxysilane (10 g, 45.2 mmol) and triethylamine (10.1 g, 100 mmol) were placed in a round-bottom flask containing 100 mL of anhydrous THF. 2bromoisobutyryl bromide (16.1 g, 70 mmol, 1.55 eqv.) was dissolved in 30 mL of anhydrous THF and the solution was added dropwise to the mixture of APTES and TEA placed in an ice bath over 60 min under vigorous stirring. The solution was allowed to warm to room temperature, and stirred overnight. The precipitate of triethylamine bromide (TEA· HBr) was removed by filtration. The solution was then concentrated in vacuo (1 mbar) and product was re-dissolved in 200 mL of DCM. The organic solution was neutralized by washing with 0.
Synthesis of propargyl 1-pyrenebutyrate (PPy). 1-Pyrenebutyric acid (2.88 g, 10 mmol, 1.0 equiv) was dissolved in chloroform (50 mL) in a round bottomed flask under nitrogen and cooled to 0 °C. Oxalyl chloride (1.03 mL, 12 mmol, 1.2 equiv) was then added slowly into the flask. Two drops of dimethyl formamide (DMF) were added as a catalyst for the reaction. The reaction mixture was stirred at room temperature for 20 h with constant stirring. The solvent and any excess oxalyl chloride was removed by rotary evaporation to obtain the acid chloride. The acid chloride was directly used in the next step without further purification. The acid chloride (1.0 equiv.) was dissolved in chloroform (20 mL) and added slowly to a solution of propargyl alcohol (0.87 mL, 15 mmol, 1.5 equiv) dissolved in chloroform (50 mL) and under nitrogen. The mixture was then stirred for 3 h. The chloroform and excess propargyl alcohol were removed in vacuo. The

Synthesis of α-azido PEG.
The mono-azido-functionalized PEG (PEG-azide) was synthesized via a procedure reported in the literature 4 . PEG monomethyl ether (M n = 1 kDa, 2 g, 2 mmol, 1 equiv) was initially dried via azeotropic distillation with toluene (20 mL). Subsequently, triethylamine (2.4 mmol, 1.2 equiv) and dichloromethane (20 mL) were added and the mixture was cooled to 0 o C before methanesulfonyl chloride (3 mmol, 1.5 equiv) was added dropwise. The reaction was kept at 0 o C for 30 min and then at room temperature for 12 h. The reaction was filtered and the filtrate was concentrated in vacuo. The residue was dissolved in DMF (20 mL), and NaN 3 (20 mmol, 10 equiv) was added. The reaction mixture was heated to 65 o C for 12 h, cooled to room temperature and concentrated in vacuo. The residue was dissolved in water (50 mL) and then washed with dichloromethane (25 mL × 2). The organic extracts were collected, washed with water (50 mL) and saturated NaCl (50 mL × 2), dried (MgSO 4 ), filtered, and concentrated in vacuo to afford the desired PEG-

Kinetic study of poly(methyl methacrylate) for Figure 4
In this experiment, a 25 mL flask was charged with monomer (MMA: 1.78 mL, 16.7 mmol), TTC-1 (58 mg, 0.167 mmol), solid state photocatalyst (Si wafer 0.5 cm 2 ) and DMSO (50 vol % w.r.t monomer), [MMA]:[TTC] = 100:1. After polymerization, the MMA conversion was estimated from 1 H NMR by integrating the peaks corresponding to methyl group at δ H = 3.5-3.7 ppm, (s, 3H, -COOCH 3 ) and the protons corresponding to the unsaturated methacrylate double bond (δ H = 5.3-6.0 ppm, m, 3H, CH 2 =C(CH 3 )-). These peaks account for all protons derived from the monomer species, from which the percentage of remaining unreacted monomer can be calculated. The theoretical molecular weight was calculated using the equation:    Figure S5. Plot of the log z-average diameter vs. log molecular weight for polystyrene in toluene.

Local concentration of ssPC
For example, a ssPC has the PDMAEMA coating (Area = 1 cm 2 , thickness = 40 nm), and the amount of TA on the ssPC can be estimated using the equation below:  Table 1 In this study, a 25 mL flask was charged with monomer, TTC, ssPC and solvent (50 vol % w.r.t monomer), [M]:[TTC] = 100:1. This was de-gassed via two freeze-pumpthaw cycles before the UV light source was switched on. The reaction was performed under argon positive pressure. Samples were taken after 6 h and immediately diluted with either CDCl 3 or THF, for NMR and GPC analysis, respectively. Figure S6. GPC traces of polymethacrylayes obtained from PRP using ssPCs (a-h correspond to Entry 1-8 in Table 1).        Figure S11. 1 H NMR spectrum of crude PMMA-b-PBMA using ssPC.

Preparation of pseudo-triblock copolymer.
General procedure for PMA chain extension was followed. After 16 hours a 1: 2 (v/v) mixture of degassed MA (100 equiv.) used for diblock formation and DMSO was added to the reaction mixture via degassed syringe. Samples were taken and conversions were measured using 1 H NMR analysis. After another 24 hours, a 1: 4 (v/v) mixture of degassed MA (100 equiv.) and DMSO was added to the reaction mixture via degassed syringe for triblock preparation. MA conversion was measured using 1 H NMR and the resultant (pseudo)triblock copolymer was characterized by GPC measurement.   Figure S13. a) GPC evolution of the prepared PEG-b-PCL block copolymer via 'CuAAC' reaction using ssPC. b) 1 H NMR spectra of (i) alkyne-functionalized PCL (PCL-alkyne), (ii) azide-functionalized PEG (PEG-azide) and (iii) the resultant PEG-b-PCL block copolymer.