Preparation Of Poly(Ethylene Glycol)-based Nitroxyls Block Copolymers And Their Catalytic Performances

In the selective catalytic oxidation of alcohols,2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)is particularly attractive as a highly efficient catalyst.However,TEMPO is expensive and its molecular weight is too small to recycle.The immobilization of TEMPO can effectively solve the above problems.In this study,the amphiphilic nitroxide-containing block copolymers were synthesized by grafting poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl acrylamide)(PTMA)containing nitroxides on polyethylene glycol(PEG)with good solubility and phase transfer properties.The catalytic performances of the block copolymers in the selective catalytic oxidation of alcohols were investigated and the effects of the block copolymer structure on the catalytic performance were studied.The block copolymers containing nitroxyl radicals were prepared via activstors regenerated by electron transfer atom transfer radical polymerization(ARGET ATRP).Firstly,2-bromoisobutylyl bromide was immobilized to PEG to obtain macromolecular initiator.Then,the block copolymer PEG-PTMPM was prepared by ARGRT ATRP of 2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl acrylamide(TMPM).Finally,3-chloroperoxybenzoic acid(m-CPBA)was used to oxidize the amino groups in PTMPM to nitroxyl radicals to obtain the block copolymer containing nitroxides.The nitroxide-containing block copolymers were characterized by fourier transform infrared spectroscopy(FT-IR),ultraviolet-visible spectrophotometry(UV-Vis),gel permeation chromatography(GPC)and transmission electron microscopy(TEM).The obtained block copolymer(PEG-PTMA)had the advantages of controllable structure and narrow molecular weight distribution.The molecular weight of the methyl-terminated polyethylene glycol-supported diblock copolymer(MPEG-PTMA)was from 8700 to 17000,and the molecular weight of the polyethylene glycol-supported triblock copolymer(PTMA-PEG-PTMA)was from 11200 to 39,000.The dispersion index was around 1.5.The prepared PEG-PTMA was used as catalyst in the selective catalytic oxidation of alcohols with sodium hypochlorite(NaClO)and molecular oxygen(O2)as oxidants.The effect of PEG-PTMA structure on the catalytic performance was studied.With NaClO as oxidant,the block copolymer loaded to MPEG2000 and PEG2000 showed excellent catalytic performances,even better than that of free TEMPO.Moreover,the catalyst had good catalytic effect on other alcohols.When the molecular oxygen was used as oxidant,the catalytic performances of the catalyst were studied with N,N-dimethylformamide(DMF)and acetonitrile/water as solvent respectively.With DMF as solvent,the catalytic performance of MPEG5000-PTMA was good,while other block copolymers were not satisfactory.With the mixture of acetonitrile/water as solvent,the catalytic effect of block copolymer loaded to MPEG2000 was better than TEMPO.The catalyst recovery could be achieved by precipitation and filtration and the cyclic catalytic performances were also good.