CuI-catalyzed aerobic oxidation of tertiary amines as chain initiation for radical polymerization of methyl methacrylate. Syndiotactic-rich alpha-amino PMMA via mono-centered initiation

Abstract

CuI-catalyzed aerobic oxidation of tertiary amines as the substrate involves active radicals as the oxidized intermediates, in which the reduced intermediates, CuI complexes, play a role of oxygen carriers to activate O2 into the proton/electron/hydrogen-atom acceptor, while tertiary amines serve as the corresponding donor and afford alpha aminoalkyl carbon-centered radicals. CuI-catalyzed oxidation of various tertiary amines using aerobic O2 diffused from air as the ultimate oxidant and air-stable CuII salts as the catalyst precursor was adopted as the chain initiation for radical polymerization of methyl methacrylate (MMA) in dimethylsulfoxide at 90°C. The polymerization kinetics was monitored by gas chromatography, while the molecular weight and stereotacticity of PMMA were determined by gel-permeation chromatography and nuclear magnetic resonance spectroscopy. Triethylamine behaved as an efficient substrate while CuII/2,2′-bipyridine complexes displayed a higher catalytic activity. The polymerization proceeded in a conventional fashion at a comparable rate even using several micromol of CuII/2,2′-bipyridine complexes per liter, leading to syndiotactic-rich PMMA (syndiotacticity >55%) with an alpha-amino terminal moiety as the residue from the tertiary amine substrate, which was confirmed by UV-induced benzophenone-initiated radical polymerization using the so-obtained PMMA as the macro-sensitizer. The catalyst-performance analysis suggested that each molecule of CuII/2,2′-bipyridine complexes could generate 3×102 propagating PMMA radicals in 3h under optimal conditions.