A Highly Active Iron-Based Catalyst System for the AGET ATRP of Styrene

Development of Environmentally Friendly Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is one of the most successful techniques for the preparation of well-defined polymers with controllable molecular weights, narrow molecular weight distributions, specific macromolecular architectures, and precisely designed functionalities. ATRP usually involves transition-metal complex as catalyst. As the most commonly used copper complex catalyst is usually biologically toxic and environmentally unsafe, considerable interest has been focused on iron complex, enzyme, and metal-free catalysts owing to their low toxicity, inexpensive cost, commercial availability and environmental friendliness. This review aims to provide a comprehensive understanding of iron catalyst used in normal, reverse, AGET, ICAR, GAMA, and SARA ATRP, enzyme as well as metal-free catalyst mediated ATRP in the point of view of catalytic activity, initiation efficiency, and polymerization controllability. The principle of ATRP and the development of iron ligand are briefly discussed. The recent development of enzyme-mediated ATRP, the latest research progress on metal-free ATRP, and the application of metal-free ATRP in interdisciplinary areas are highlighted in sections. The prospects and challenges of these three ATRP techniques are also described in the review.

Effects of various Cu(0), Fe(0), and proanthocyanidin reducing agents on Fe(iii)-catalysed ATRP for the synthesis of PMMA block copolymers and their self-assembly behaviours

Well-defined PMMA, PMMA-b-PBzMA and PMMA-b-PBMA polymers were obtained via green Fe-ATRP with the aid of proanthocyanidins. Interestingly, microphase separation was observed in PMMA-b-PBMA polymer with upper critical ordering temperature behaviour.

Investigating the Effect of Silica Aerogel Nanoparticles on the Kinetics of AGET ATRP of Methyl Methacrylate

Hexamethyldisilazane-modified silica aerogel nanoparticles were used for in situ polymerization of methyl methacrylate by activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) to synthesize tailor-made PMMA nanocomposites. Appropriate dispersion of silica aerogel nanoparticles in the monomer solution and improvement in interfacial interaction between the PMMA matrix and nanoparticles are two main reasons for application of HMDS-modified silica aerogel nanoparticles. Nitrogen adsorption/desorption isotherm was employed to examine surface area and structural characteristics of the HMDS-modified silica aerogel nanoparticles. Evaluation of size distribution and morphological studies were also performed by SEM and TEM. Conversion and molecular weight determinations were carried out using GC and SEC, respectively. Addition of 3 wt% HMDS-modified silica aerogel nanoparticles leads to decrement of conversion from 85 to 64%. Molecular weight of PMMA chains also decreases from 13,912 to 10,810 g⋅mol−1 by addition of only 3 wt% HMDS-modified silica aerogel nanoparticles; however, polydispersity index values increases from 1.18 to 1.51. Linear increase of ln(M0/M) with time for all the samples shows that polymerization proceeds in a living manner. In addition, suitable agreement between theoretical and experimental molecular weight in combination with low PDI values can appropriately demonstrate the living nature of the polymerization. TGA results indicate that by increasing HMDS-modified silica aerogel nanoparticles content, slight improvements in thermal stability of the nanocomposites were obtained. DSC results show a decrease in Tg from 86.9 to 80.1°C by addition of 3 wt% HMDS-modified silica aerogel nanoparticles.

Synthesis and Characterization of Polystyrene/Mesoporous Diatomite Composites via Activators Generated by Electron Transfer for Atom Transfer Radical Polymerization

Diatomite platelets were employed to synthesize different polystyrene/diatomite composites. Mesoporous diatomite platelets were used for in situ polymerization of styrene by activators generated by electron transfer for atom transfer radical polymerization to synthesize tailor-made polystyrene nanocomposites. FTIR spectroscopy and thermogravimetric analysis were employed for evaluating some inherent properties of the pristine mesoporous diatomite platelets. Nitrogen adsorption/desorption isotherm is applied to examine surface area and structural characteristics of the diatomite platelets. Evaluation of pore size distribution and morphological studies were also performed by scanning and transmission electron microscopy. Conversion and molecular weight determinations were carried out using gas and size exclusion chromatography, respectively. Addition of 3 wt% pristine mesoporous diatomite leads to increase of conversion from 78 to 95%. Molecular weight of polystyrene chains increases from 15800 to 20000 g·mol−1 by addition of 3 wt% pristine mesoporous diatomite; however, polydispersity index values increases from 1.14 to 1.38. Increasing thermal stability of the nanocomposites is demonstrated by thermogravimetric analysis. Differential scanning calorimetry shows an increase in glass transition temperature from 93.8 to 97.4°C by adding 3 wt% of mesoporous diatomite platelets.

Photocatalyzed iron-based ATRP of methyl methacrylate using 1,3-dimethyl-2-imidazolidinone as both solvent and ligand

A simple photocatalyzed Fe-based ATRP of MMA was conducted under UV irradiation using the “green” solvent DMI as both the solvent and ligand.

A facile, simple, and inexpensive ionic liquid, 1-alkyl-3-methylimidazole chloride, as ligand for the iron(iii)-mediated reverse atom transfer radical polymerization of methyl methacrylate

The facile, simple, and inexpensive ILs, 1-alkyl-3-methylimidazole chloride ([Rmim][Cl]), are explored for the first time as ligands for the reverse ATRP of methacrylates.

ICAR ATRP of Acrylonitrile under Ambient and High Pressure

It is well known that well-defined polyacrylonitrile (PAN) with high molecular weight (Mw > 106 g·mol-1) is an excellent precursor for high performance carbon fiber. In this work, a strategy for initiators for a continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) system for acrylonitrile (AN) was firstly established by using CuCl₂·2H₂O as the catalyst and 2,2'-azobis(2-methylpropionitrile) (AIBN) as the thermal initiator in the presence of ppm level catalyst under ambient and high pressure (5 kbar). The effect of catalyst concentration and polymerization temperature on the polymerization behaviors was investigated. It is important that PAN with ultrahigh viscosity and average molecular weight (Mη = 1,034,500 g·mol-1) could be synthesized within 2 h under high pressure.

Iron-Mediated Homogeneous ICAR ATRP of Methyl Methacrylate under ppm Level Organometallic Catalyst Iron(III) Acetylacetonate

Atom Transfer Radical Polymerization (ATRP) is an important polymerization process in polymer synthesis. However, a typical ATRP system has some drawbacks. For example, it needs a large amount of transition metal catalyst, and it is difficult or expensive to remove the metal catalyst residue in products. In order to reduce the amount of catalyst and considering good biocompatibility and low toxicity of the iron catalyst, in this work, we developed a homogeneous polymerization system of initiators for continuous activator regeneration ATRP (ICAR ATRP) with just a ppm level of iron catalyst. Herein, we used oil-soluble iron (III) acetylacetonate (Fe(acac)₃) as the organometallic catalyst, 1,1'-azobis (cyclohexanecarbonitrile) (ACHN) with longer half-life period as the thermal initiator, ethyl 2-bromophenylacetate (EBPA) as the initiator, triphenylphosphine (PPh₃) as the ligand, toluene as the solvent and methyl methacrylate (MMA) as the model monomer. The factors related with the polymerization system, such as concentration of Fe(acac)₃ and ACHN and polymerization kinetics, were investigated in detail at 90 °C. It was found that a polymer with an acceptable molecular weight distribution (Mw/Mn = 1.43 at 45.9% of monomer conversion) could be obtained even with 1 ppm of Fe(acac)₃, making it needless to remove the residual metal in the resultant polymers, which makes such an ICAR ATRP process much more industrially attractive. The "living" features of this polymerization system were further confirmed by chain-extension experiment.

Recent Progress on Transition Metal Catalyst Separation and Recycling in ATRP

Atom transfer radical polymerization (ATRP) is a versatile and robust tool to synthesize a wide spectrum of monomers with various designable structures. However, it usually needs large amounts of transition metal as the catalyst to mediate the equilibrium between the dormant and propagating species. Unfortunately, the catalyst residue may contaminate or color the resultant polymers, which limits its application, especially in biomedical and electronic materials. How to efficiently and economically remove or reduce the catalyst residue from its products is a challenging and encouraging task. Herein, recent advances in catalyst separation and recycling are highlighted with a focus on (1) highly active ppm level transition metal or metal free catalyzed ATRP; (2) post-purification method; (3) various soluble, insoluble, immobilized/soluble, and reversible supported catalyst systems; and (4) liquid-liquid biphasic catalyzed systems, especially thermo-regulated catalysis systems.

Iron-catalyzed atom transfer radical polymerization

This article reviews the preparation of polymers using iron-catalyzed atom transfer radical polymerization.

Fe(iii)-catalyzed grafting copolymerization of lignin with styrene and methyl methacrylate through AGET ATRP using triphenyl phosphine as a ligand

A novel, effective and environment friendly Fe(iii)-catalyzed AGET ATRP has been presented to carry out the grafting copolymerization of lignin with styrene and methyl methacrylate for the first time.

Facile iron(iii)-mediated ATRP of MMA with phosphorus-containing ligands in the absence of any additional initiators

Fe(iii)-mediated ATRP using phosphorus reagents was studied without any additional initiator and reducing agent. The polymerization was demonstrated as reverse ATRP, in which phosphorus reagents acted as both ligand and thermal radical initiator.

Fe(iii)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst

Iron catalyst separation and recycling was successfully achieved in a liquid/liquid biphasic TPSC-based ICAR ATRP system.

Magnetic nanomaterials with near-infrared pH-activatable fluorescence via iron-catalyzed AGET ATRP for tumor acidic microenvironment imaging

This work provides a fluorescent/magnetic iron oxide nanomaterials prototype to visualize the solid tumor in vivo by sensing the tumor acidic microenvironment, and a satisfactory tumor-to-normal tissue signal ratio (T/N ratio) and a prolonged time-window for 4T1 tumor visualization were observed in vivo.

Initiators for Continuous Activator Regeneration Atom Transfer Radical Polymerization of Methyl Methacrylate and Styrene with N-Heterocyclic Carbene as Ligands for Fe-Based Catalysts

Iron-based N-heterocyclic carbene (FeX₃(NHC)) complexes were used in initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP). A series of ICAR ATRPs of methyl methacrylate (MMA) and styrene (St) were carried out using FeX₃(IDipp) where IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene with X = Cl (I-Cl) or Br (I-Br) and FeX₃(HIDipp) (HIDipp =1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) with X = Cl (H-Cl) or Br (H-Br). The polymerizations showed good activity, resulting in polymers with controlled molecular weights and narrow molecular weight distribution (MWD) with low loading of catalysts (50 ppm). In particular, I-Br and H-Br generated polymers with narrower MWD. For example, ICAR ATRP of MMA with 50 ppm catalysts (H-Br) after 24 h at T = 60 °C in 50% v/v anisole resulted in PMMA with M_w/M_n = 1.20 at 65% monomer conversion. ICAR ATRP of St after 72 h resulted in PSt with M_w/M_n = 1.15 at 53% monomer conversion.

Iron-Based ICAR ATRP of Styrene with ppm Amounts of FeIIIBr3 and 1,1'-Azobis(cyclohexanecarbonitrile)

A successful ICAR (initiators for continuous activator regeneration) ATRP (atom transfer radical polymerization) of styrene was conducted with iron(III) bromide and 1,1'-azobis(cyclohexanecarbonitrile) (ACHN) as the thermal initiator. A polymerization, started with 50 ppm of FeBr₃ and 50 mol equivalents of ACHN in 33% (v/v) anisole at 90 °C, reached 70% conversion in 24 h and was well controlled, giving a polymer with a narrow molecular weight distribution (M_w/M_n = 1.15). The number average molecular weight (M_n) corresponded well to theoretical values, as conversion increased. The rate of polymerization was dependent on the amount of ACHN initially added to the reaction. A polymer with a relatively narrow molecular weight distribution, M_w/M_n = 1.29 at 65% of conversion, was obtained with 5 ppm of FeBr₃ and the appropriate amount of ACHN. This procedure therefore provides an efficient controlled polymerization in addition to creating a robust, cheap, and environmentally friendly catalytic system. Control of polymerization with ACHN was better than with tert-butyl peracetate as a thermal initiator or tin(II) 2-ethylhexanoate, Fe⁰, or Zn⁰ wire as reducing agents.