Comparison of Pyrylium and Thiopyrylium Photooxidants in Metal-Free Ring-Opening Metathesis Polymerization

Abstract

Materials and Methods. Dichloromethane (CH 2 Cl 2) and acetonitrile (CH 3 CN) were obtained from a solvent purification system. 1 H NMR spectra were recorded on Bruker AVance 300 MHz or 500 MHz spectrometers. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane using the residual protio-solvent as an internal standard (CDCl 3 , 1 H: 7.26 ppm; DMSO-d 6 , 1 H: 2.50 ppm and 13 C: 39.5 ppm). Gel permeation chromatography (GPC) was performed using a GPC setup consisting of: a Shimadzu pump, 3 in-line columns, and Wyatt light scattering and refractive index detectors with tetrahydrofuran (THF) as the mobile phase. Numberaverage molecular weights (M n) and weight-average molecular weights (M w) were calculated from light scattering and refractive index data. All polymerizations were carried out under an inert atmosphere of nitrogen in standard borosilicate 2-dram glass vials purchased from Fisher Scientific with magnetic stirring unless otherwise noted. Irradiation of photochemical reactions was done using a 2 W Miracle blue LED indoor gardening bulb. The output of the blue LED bulb was measured using an Ocean Optics USB2000 spectrometer. The (thio)pyrylium tetrafluoroborate salts were prepared according to literature procedure. 1,2 Norbornene was sublimed prior to use. All other reagents and solvents were obtained from commercial sources and used as received unless otherwise noted. Cyclic Voltammetry. Inside a glovebox under N 2 atmosphere, 1.0 mmol (67 mM) of photo-oxidant and 320 mg (0.200 M) LiClO 4 were dissolved in 15 mL of CH 3 CN from the solvent still in a 3-neck round bottom flask equipped with a stir bar. The flask was sealed with septa pierced with a glassy carbon working electrode (3 mm), a Pt basket counter electrode (Premier Lab Supply), and a 0.1 M Ag/AgNO 3 reference electrode (0.1 M AgNO 3 in dry CH 3 CN). The cell was removed from the glovebox and equipped with a N 2 balloon. The mixture was stirred prior to cyclic voltammetry measurements. Cyclic voltammograms (CVs) were obtained using a BASi Epsilon potentiostat. Ferrocene (0.15 mmol) was added as an internal standard after each voltammogram and a subsequent CV was taken.

Recent advances on visible light induced cationic polymerization

“…Further studies found that PC with strongly excited oxidability will lead to descent of monomer conversion rate and/or even polymerization inhibition. In comparison to thiopyrane‐based catalysts and pyrane‐based catalysts, it revealed that 2,4,6‐tris(4‐methoxyphenyl)thiopyran salt was the best PC for polymerization control among these three typical kinds of PCs 86 . Moreover, the conversion rate of monomer was higher in the air (87%) than in the nitrogen atmosphere (80%) 87 .…”

“…In comparison to thiopyrane-based catalysts and pyrane-based catalysts, it revealed that 2,4,6-tris(4-methoxyphenyl)thiopyran salt was the best PC for polymerization control among these three typical kinds of PCs. 86 Moreover, the conversion rate of monomer was higher in the air (87%) than in the nitrogen atmosphere (80%). 87 In recent work, it has been found that the counter ion of the catalyst affects the stereoscopic regularity of the polymer.…”

Radical chemistry in polymer science: an overview and recent advances

“…The reduced PC •− terminates the catalytic loop by reducing the propagating species to provide a polymer chain ( Scheme 12B ). Boydston and co-workers [ 91 ], systematically studied various pyrylium and thiopyrylium PCs ( Scheme 13 ). It is necessary for these PCs having a high excited-state redox potential to oxidize the enol ether initiators.…”