Atom Transfer Radical Polymerization Catalyzed By Immobilized Cobalt Catalyst

Atom transfer radical polymerization (ATRP) is one versatile living polymerization technique to achieve polymer with well-controlled molecular weight and narrow molecular weight distribution and to realize polymer structure design. However, for the traditional ATRP, the difficulty to remove catalyst residue after polymerization limits its application in industry.In this paper, ATRP of methyl methacrylate(MMA)catalyzed by soluble cobalt carboxylate including cobalt acetate and soluble cobalt polyacrylate was investigated. The results indicated that cobalt carboxylate salt had high catalytic activity and good controllability over polymerization, and polymer with preset molecular weight and narrow molecular weight distribution was obtained.To solve the difficulty of catalyst removal and recycle for traditional ATRP, ATRP catalyzed by immobilized cobalt catalyst supported on cross-linked poly(acrylic acid) resin was presented. This catalyst had high catalytic activity using DMF as solvent. To improve the control over polymerization, Ion exchange resin immobilized Co(II) catalyst with a small amount of soluble CuCl2/Me6TREN catalyst was successfully applied to ATRP of MMA. Poly(methyl methacrylate) (PMMA) with predicted molecular weight and narrow molecular weight distribution (Mw/Mn=1.09~1.42) was obtained. The immobilized catalyst can be easily separated from the polymerization system by simple centrifugation after polymerization, resulting in the concentration of transition metal residues in polymer product was as low as 10ppm. Both main catalytic activity and good controllability over the polymerization were retained by the recycled catalyst without any regeneration process. Using m-xylene as solvent, ATRP MMA using ion exchange resin supported Co(II) without any additional deactivator, was successfully performed. Well-defined PMMA was achieved. The concentration of catalyst residue in the yielding polymer was so low that it could not be detected by ICP. When a given amount of deactivator CuCl2/Me6TREN was added, the control over polymerization was further improved. Whereas the quantity of Cu residue in the final polymers could still be controlled less than 5ppm after the supported catalyst was removed by simple centrifugation. Main catalytic activity and good controllability over polymerization were remained by the recycled catalyst. To remove and recycle the catalyst more conveniently, a novel immobilized catalytic system, cross-linked poly(acrylic acid) (PAA) resin immobilized cobalt(II)/copper(II) bimetallic catalyst, was successfully employed for ATRP. Well-defined PMMA with low polydispersity (PDI=1.27) was synthesized using PAA/Co(II)/Cu(II) catalyst without any additional ligand or soluble deactivator. As the immobilized catalyst could be effectively separated from the polymer solution by simple centrifugation after polymerization, colorless PMMA (Co residue<1ppm, Cu residue ~ 4ppm) was achieved. Both main catalytic activity and good controllability over polymerization were retained by the recycled catalyst after being reused several times.To understand the elementary reaction process, ATRP of MMA catalyzed by the supported Co(II) catalyst was studied by online ESR (electron spin resonance) spectroscopy. By adding a given amount of PMMA into the bulk ATRP system of MMA to enhance the viscosity, the radical signal was observed. The signal of MMA radical was as same as that initiated by AIBN. Chain extension reaction using PMMA-Cl as initiator catalyzed by supported Co(II) was also detected by online ESR and stronger radical signal was observed as well. The results indicated that the polymerization proceeded vs radical process. ESR spectroscopy on ATRP of MMA and EGDMA mediated by supported cobalt(II) catalyst also proved the radical nature during polymerization. By adding a small amount of deactivator CuCl2/Me6TREN, ESR analysis showed that concentration of Cu(II) decreased during polymerization, which indicated that CuCl2/Me6TREN indeed worked as deactivator. In this case, ESR signals of radical species obviously became weaker. The observation of the propagating radical by ESR strongly supported the radical mechanism of ATRP. In the case of ATRP of MMA with addition of TEMPO (2,2,6,6-Tetramethyl-1-piperidinyloxy), ESR signal of TEMPO sharply decreased, indicating that free radical was involved in polymerization. Also, free radicals were dected by ESR after PBN(N-Butyl-α-phenylnitrone) was added.All these strongly proved the radical mechanism for ATRP.In addition, ATRP of styrene, n-butyl acrylate and ethyl methacrylate catalyzed by immobilized cobalt catalyst was studied as well. The results elucidated that the catalyst had high catalytic activity and good controllability over polymerization of these monomers.