Elsevier

Applied Catalysis A: General

Volume 332, Issue 2, 20 November 2007, Pages 192-199
Applied Catalysis A: General

A novel immobilized cobalt(II)/copper(II) bimetallic catalyst for atom transfer radical polymerization (ATRP) of methyl methacrylate

https://doi.org/10.1016/j.apcata.2007.07.040Get rights and content

Abstract

A new immobilized catalytic system, cross-linked poly(acrylic acid) (PAA) resin immobilized cobalt(II)/copper(II) bimetallic catalyst, was successfully employed for atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). Well-defined poly(methyl methacrylate) (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 < 1 ppm, Cu residue ∼4 ppm) was achieved. Both main catalytic activity and good controllability over polymerization were retained by the recycled catalyst after reused several times. The catalyst was characterized by SEM, EDX, TGA and ESR techniques. The polymerization reaction mechanism was presented.

Graphical abstract

Cross-linked poly(acrylic acid) (PAA) resin immobilized Co(II)/Cu(II) bimetallic catalyst could well control over the polymerization of methyl methacrylate (MMA). After polymerization, the catalyst could be easily removed, resulting in the concentration of transition-metal residues in polymer product lower than 5 ppm. Both catalytic activity and controllability were retained by the recycled catalyst.

Introduction

Atom transfer radical polymerization (ATRP), discovered by Matyjaszewski and co-worker [1] and Sawamoto and co-workers [2], has become one of the most useful controlled/“living” radical polymerization techniques in polymer science. Numerous well-defined (co)polymers with desired molecular weight and low polydispersity index (PDI < 1.5) [3], [4], (co)polymers with complex architectures [5], [6], [7], [8], [9], [10], functional polymer or hybrid materials [11], [12], [13], [14], [15] and so on have been prepared by ATRP technique in laboratory. ATRP is based on the reversible activation/deactivation equilibrium between the active and dormant species catalyzed by transition-metal/ligand complex [1], [3]. Generally, in ATRP, organic halide is used as initiator, and transition-metal/ligand complex in its lower oxidation state is used as catalyst. The key point in ATRP is the catalytic system. So far, several catalytic systems based on copper [1], iron [16], [17], [18], [19], nickel [20], [21], [22], cobalt [23], [24], [25], [26], ruthenium [2], [27], molybdenum [28], [29], rhenium [30], rhodium [31], [32], osmium [33], and titanium [34] have been used in ATRP.

However, for traditional ATRP, the catalyst residue in the yielding polymer is difficult to be removed after polymerization. Not only does the colored and/or toxic transition-metal residue seriously contaminate the polymer, leading to the product rather hazardous [35], but also the valuable catalyst becomes waste after being used once and causes environmental problems. This is the main defect that hinders ATRP to become an applicable technique to produce polymers at low cost on large scale. So, it is urgent to develop new catalytic systems that do not contaminate the polymer products. One approach is to develop sufficiently active catalysts that can provide good control over polymerization at very low concentrations. Another is to design catalysts that can be removed and reused conveniently. Recently, Matyjaszewski and co-workers [36], [37] have developed a high active copper-based ATRP catalyst and ARGET (activators regenerated by electron transfer) ATRP catalysts. The polymerization could be well controlled with ppm quantities of these catalysts, so the catalyst residues in polymer products were very low. However, the synthesis of the special ligands and the reducing agents was difficult and costly. Alternatively, immobilizing ATRP catalyst on solid support is another good solution, because immobilized catalyst can be easily separated and reused. Therefore, immobilized and recyclable catalytic systems have been and continue to be developed [38].

As we know, metal complex catalysts have been immobilized on supports for many years in an effort to aid catalyst recovery [39]. Up to date, a few immobilized ATRP catalysts based on copper(I) [40], [41], [42], [43] and nickel(II) [22], [44], [45] have been reported. However, these immobilized catalysts had poor control over polymerization [46]. To improve the control over polymerization, a given amount soluble transition-metal/ligand as deactivator [22], [47], [48] or free ligand [44] was added. Although the control over polymerization could be improved to some extent, the added or produced soluble transition-metal/ligand complex was still difficult to be removed, which also inevitably caused high content of catalyst residue in the final polymer product. Additionally, these reported immobilized ATRP catalysts were prepared by reacting α-functionalized ligands to surface bound functional groups [38], which was complicated and uneconomical. Moreover, for the immobilized ATRP catalysts based on copper(I), regeneration steps were needed when they were reused [41], [48]. In fact, these reported immobilized ATRP catalysts are still impracticable for industry-scale reactions.

To overcome the limitations of the reported immobilized mono-metallic ATRP catalysts, we developed an efficient immobilized bimetallic ATRP catalyst by immobilizing both activator Co(II) and deactivator Cu(II) onto cross-linked poly(acrylic acid) (PAA) resin through ion exchange. All polymerization experiments were carried out at 80 °C in N,N-dimethylformamide (DMF), using p-toluenesulfonyl chloride (p-TsCl) as initiator (Scheme 1) except for chain extension reaction. The catalyst was characterized by Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Thermogravimetric Analysis (TGA) and Electron Spin Resonance (ESR) techniques.

Section snippets

Chemicals

PAA resin (cross-linked poly(acrylic acid) resin, average particle size 2 μm, 9.5 mmol/g –COONa+ groups) was purchased from Shanghai Huazhen Science Technology & Trading Co. Soluble PAAS (sodium polyacrylate, average Mn = 1200) was purchased from Changzhou Chunjiang Chemical Co. Other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. Methyl methacrylate (MMA) was washed with 5%NaOH solution, dried over anhydrous Na2SO4, and vacuum distilled from CaH2. N,N-dimethylformamide (DMF)

Characterization of PAA/Co(II)/Cu(II)

The immobilized catalyst PAA/Co(II)/Cu(II) was prepared by ion exchange of Na+ on PAA resin with Co2+ and Cu2+ in aqueous solution of CoCl2 and CuCl2 (Scheme 2). To determine the quantities of Co(II) and Cu(II) on PAA resin, UV–vis spectra were recorded during ion exchange (Fig. 1). UV–vis spectrum analysis showed that after 12 h since PAA resin was added, the absorbency of CoCl2 (λ = 511 nm) and CuCl2 (λ = 810 nm) in water decreased from 0.188 and 0.499 to 0.007 and 0.013, respectively, indicating

Conclusion

In this paper, ATRP of MMA using PAA resin immobilized Co(II)/Cu(II) bimetallic catalyst, p-TsCl as initiator, without any additional ligand or soluble transition-metal/ligand complex, was successfully carried out. PAA/Co(II)/Cu(II) could be conveniently prepared through ion exchange. Using PAA/Co(II)/Cu(II), the polymerization of MMA was well controlled and PMMA with desired molecular weight and low polydispersity (PDI = 1.27) was obtained. Importantly, the immobilized catalyst could be easily

Acknowledgements

The authors greatly appreciate the financial support of the National Natural Science Foundation of China (Nos. 50673057 and 20504020).

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