Competitive Equilibria in Atom Transfer Radical Polymerization

With the recent development of new initiation techniques in atom transfer radical polymerization (ATRP) that allow catalysts to be employed at unprecedented low concentrations (∼10 ppm), a thorough understanding of competitive equilibria that can affect catalyst performance is becoming increasingly important. Such mechanistic considerations are discussed herein, including i) factors affecting the position of the ATRP equilibrium; ii) dissociation of the ATRP catalyst at high dilution and loss of deactivator due to halide dissociation; iii) conditional stability constants as related to competitive monomer, solvent, and reducing agent complexation as well as ligand selection with respect to protonation in acidic media; and iv) competitive equilibria involving electron transfer reactions, including the radical oxidation to carbocations or reduction to carbanions, radical coordination to the metal catalyst, and disproportionation of the Cu^I-based ATRP activator.     

Recent Developments in Atom Transfer Radical Polymerization (ATRP): Methods to Reduce Metal Catalyst Concentrations

Atom transfer radical polymerization (ATRP) was initially developed in the mid‐1990s, and with continued refinement and use has led to significant discoveries in new materials. However, metal contamination of the polymer product is an issue that has proven detrimental to widespread industrial application of ATRP. The laboratories of K. Matyjaszewski have made significant progress towards removing this impediment, leading the development of “activators regenerated by electron transfer” ATRP (ARGET ATRP) and electrochemically mediated ATRP (eATRP) technologies. These variants of ATRP allow polymers to be produced with great molecular weight and functionality control but at significantly reduced catalyst concentrations, typically at parts per million levels. This Concept examines these polymerizations in terms of their mechanism and outcomes, and is aimed at giving the reader an overview of recent developments in the field of ATRP.     

Visible light-driven acridone catalysis for atom transfer radical polymerization

Acridone as a new kind of visible light photocatalyst has been developed to catalyze metal free atom transfer radical polymerization (ATRP). The photocatalyst possess low excited state potential as can undergo an oxidative quenching pathway to initiate ATRP of vinyl monomers. Kinetic study and light on/off reaction demonstrate the “living”/controlled nature of the polymerization by light. Block copolymers can be achieved by using PMMA as macroinitiator to reinitiate polymerization of other vinyl monomers, which shows highly preserved Br chain-end functionality in the synthesized polymers. Moreover, the polymerization can be conducted under air atmosphere as most photocatalysts need anaerobic condition, which may give inspiration of further application of this kind of photocatalyst.     

A new class of ruthenium complexes containing Schiff base ligands as promising catalysts for atom transfer radical polymerization and ring opening metathesis polymerization

A novel series of Schiff base ruthenium complexes that are active catalysts in the field of atom transfer radical polymerization (ATRP), have been prepared. Moreover, when activated with trimethylsilyldiazomethane (TMSD), these species exhibit good catalytic activity in the ring opening metathesis polymerization (ROMP) of norbornene and cyclooctene. The activity for both the ROMP and ATRP reaction is dependent on the steric bulk and electron donating ability of the Schiff base ligand. The control over polymerization in ATRP was verified for the two substrates that exhibit the highest activity, namely MMA and styrene. The results show that the optimal ATRP equilibrium leading to a controlled polymerization, can be established by adjusting the steric and electronic properties of the Schiff base ligand.     

Electrochemically mediated atom transfer radical polymerization with dithiocarbamates as alkyl pseudohalides

Electrochemically mediated atom transfer radical polymerizations (ATRPs) provide well‐defined polymers with designed dispersity as well as under external temporal and spatial control. In this study, 1‐cyano‐1‐methylethyl diethyldithiocarbamate, typically used as chain‐transfer agent (CTA) in reversible addition–fragmentation chain transfer (RAFT) polymerization, was electrochemically activated by the ATRP catalyst Cu^I/2,2′‐bipyridine (bpy) to control the polymerization of methyl methacrylate. Mechanistic study showed that this polymerization was mainly controlled by the ATRP equilibrium. The effect of applied potential, catalyst counterion, catalyst concentration, and targeted degree of polymerization were investigated. The chain‐end functionality was preserved as demonstrated by chain extension of poly(methyl methacrylate) with n‐butyl methacrylate and styrene. This electrochemical ATRP procedure confirms that RAFT CTAs can be activated by an electrochemical stimulus. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 376–381     

Effect of ligand on the synthesis of star polymers by resorcinarene‐based ATRP initiators

The effect of the steric hindrance on the initiating properties of two multifunctional resorcinarene‐based initiators in atom transfer radical polymerization (ATRP) was studied by using Cu(I)‐complexes of three multidentate amine ligands in the polymerization of tert‐butyl acrylate and methyl methacrylate. These ligands are less sterically hindered and have higher activities in the catalysis of ATRP of (meth)acrylates than 2,2′‐bipyridine. The polymerizations were faster and more controlled than with the 2,2′‐bipyridyl catalyst, but the tendency for bimolecular coupling increased. Even though the initiator was octafunctional, the resulting star polymers had only four arms. This indicates that the steric hindrance arising from the conformations of the initiators determines the structure of the polymer, but the ligand noticeably affects the controllability of the polymerization © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3349–3358, 2005     

Effect of the synthetic method and support porosity on the structure and performance of silica‐supported CuBr/pyridylmethanimine atom transfer radical polymerization catalysts. II. Polymerization of methyl methacrylate

A systematic study of the effect of the synthesis method and catalyst structure on the atom transfer radical polymerization (ATRP) performance of copper(I) bromide/pyridylmethanimine complexes supported on silica is described. Four different synthetic routes, including multistep‐grafting (M1), two‐step‐grafting (M2), one‐pot (M3), and preassembled‐complex (M4) methods, have been evaluated on three different silica supports (mesoporous SBA15 with 48‐ and 100‐Å pores and nonporous Cab‐O‐Sil EH5). The resulting solids have been used for ATRP of methyl methacrylate. The catalysts allow for moderate to poor control of the polymerization, with polydispersity indices (PDIs) ranging from 1.46 to greater than 2. The materials made with the preassembled‐complex (M4) and one‐pot (M3) approaches are generally more effective than those prepared with a grafting method (M1 and M2) on porous silica, whereas all the methods provide similarly performing catalysts on the nonporous support. Nonporous Cab‐O‐Sil EH5 is the most effective support because of its small particle size, lack of porosity, and relative compatibility in the reaction media. All the catalysts leach copper into solutions in small amounts. In addition, the catalysts can be effectively recycled, with improved controlled character in recycle runs (PDI ～ 1.2). Control experiments have shown that this improved performance of the used catalysts is likely due to the presence of a soluble Cu(II) complex in the materials that effectively deactivates the growing polymer chain, leading to narrow PDIs and controlled molecular weights. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1384–1399, 2004     

Cobalt(II) octanoate and cobalt(II) perfluorooctanoate catalyzed atom transfer radical polymerization of styrene in toluene and fluorous media—A versatile route to catalyst recycling and oligomer formation

Cobalt(II) perfluorooctanoate‐catalyzed atom transfer radical polymerization (ATRP) and reverse ATRP were developed to prepare oligostyrenes (M_n < 2500) with low polydispersities M_w/M_n < 1.5. Fluorous biphase catalysis was applied for effective recycling of catalyst and fluorous solvent. The homogeneous polymerization reaction was performed at 90 °C in toluene/cyclohexane/perfluorodecalin mixture (1:1:1) and fluorine‐free solvents. Temperature‐induced phase separation of this fluorous solvent mixture occurred at room temperature and proved to be the key for the very effective separation of the cobalt(II) perfluorooctanoate from the oligostyrene and fluorine‐free solvents. Both the fluorine‐tagged cobalt catalysts and the fluorous media were recycled and reused up to three times without encountering catalyst activity losses. The roles of cobalt catalysts, fluorous media, and monomer/initiator ratio were examined with respect to the polymerization kinetics. Fluorine‐containing and fluorine‐free cobalt(II) octanoate catalyzed controlled styrene oligomerization according to the ATRP mechanism. The molar mass control range was limited in fluorous biphase catalysis most likely because of precipitation of high molar mass polystyrenes in the fluorous reaction medium. To the best of our knowledge, this is the first time temperature‐induced phase separation of fluorous and fluorine‐free solvents has been successfully applied to polymerization processing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3804–3813, 2005     

Tripodal imidazole containing ligands for copper catalyzed ATRP

Tripodal imidazole containing ligands, bis((2‐pyridyl)methyl)(1‐methylimidazole‐2‐yl)methyl)amine (BPIA) and bis(1‐methylimidazole‐2‐yl)methyl)((2‐pyridyl)methyl)amine (BIPA), were synthesized and used for copper catalyzed atom transfer radical polymerization (ATRP) of n‐butyl acrylate (nBA). The molecular weights of poly(n‐butyl acrylate) (PnBA) catalyzed by CuBr/BPIA and CuBr/BIPA complexes increased linearly with nBA conversions and they were close to theoretical values with low polydispersities. ATRP equilibrium rate constant (K_ATRP) measurements showed that bothCuBr/BPIA and CuBr/BIPA complexes had high K_ATRP values, similar to that of CuBr/tri(2‐pyridylmethyl)amine (TPMA), which is one of the ATRP most active ligands. Activators regenerated by electron transfer (ARGET) ATRP of nBA with CuBr₂/BPIA and CuBr₂/BIPA complexes were also conducted and polymerization reached high nBA conversions, resulting in PnBA with low polydispersities. This suggests that the copper complexes with BPIA and BIPA were sufficiently stable and active to conduct ATRP when catalyst concentration was low. ARGET ATRP to form high molecular weight PnBA with CuBr₂/BPIA and CuBr₂/BIPA complexes was also successful. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2015–2024, 2008     

无金属可见光诱导原子转移自由基聚合法制备聚丙烯酰胺@氧化石墨烯/纳米钯复合物修饰电极及其对乙醇的检测

在金电极表面,用无金属可见光诱导原子转移自由基聚合(MVL ATRP)的方法制备聚丙烯酰胺@氧化石墨烯/纳米钯复合物修饰电极(Au/PAM@GO/Pd)。采用电化学循环伏安法(CV)、交流阻抗法(EIS)、扫描电子显微镜(SEM)、能量色散X射线光谱法(EDS)对Au/PAM@GO/Pd电极进行表征,结果表明在金电极表面成功制备了复合物。利用Au/PAM@GO/Pd电极作为电化学传感器,该传感器能成功地检测溶液中的乙醇。在最佳条件下,利用差分脉冲伏安法(DPV)该传感器检测乙醇的线性范围为1.0×10-8~1.0 mol/L,检出限(S/N=3)为1.3×10-9 mol/L,线性相关系数为0.996。     

CuBr/PMDETA combined with triethanolamine as an economic and highly active catalyst for atom transfer radical polymerization

CuBr ligated with N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) is a common catalyst for atom transfer radical polymerization (ATRP). A catalyst/initiator ratio of 0.5–1 is generally required for most CuBr/PMDETA-catalyzed polymerizations, leading to high catalyst loading and high cost of post-polymerization purification. In this work, triethanolamine is found to drastically improve the catalytical performance of CuBr/PMDETA and the strong promotion effects of triethanolamine on ATRP of methyl acrylate (MA), methyl methacrylate (MMA) and styrene (St) have been investigated. In the presence of triethanolamine, the catalyst loading of CuBr/PMDETA is substantially reduced from a normal catalyst/initiator ratio of 1 to 0.01, 0.05 and 0.05 respectively in the polymerization of MA, MMA and St. As CuBr/PMDETA is one of the cheapest ATRP catalysts, the combination of CuBr/PMDETA with triethanolamine further markedly decreases the catalyst consumption and reduces the cost of post-purification for ATRP at large scales, and therefore is promising for potential industrial applications.     

High‐throughput experimentation in atom transfer radical polymerization: A general approach toward a directed design and understanding of optimal catalytic systems

High‐throughput experimentation (HTE) was successfully applied in atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) for the rapid screening and optimization of different reaction conditions. A library of 108 different reactions was designed for this purpose, which used four different initiators [ethyl 2‐bromoisobutyrate, methyl 2‐bromopropionate, (1‐bromoethyl)benzene, and p‐toluenesulfonyl chloride], five metal salts (CuBr, CuCl, CuSCN, FeBr₂, and FeCl₂), and nine ligands (2,2′‐bipyridine and its derivatives). The optimal reaction conditions for Cu(I) halide, CuSCN, and Fe(II) halide‐mediated ATRP systems with 2,2′‐bipyridine and its 4,4′‐dialkyl‐substituted derivatives as ligands were determined. Cu(I)‐mediated systems were better controlled than Fe(II)‐mediated ones under the examined conditions. A bipyridine‐type ligand with a critical length of the substituted alkyl group (i.e., 4,4′‐dihexyl 2,2′‐bipyridine) exhibited the best performance in Cu(I)‐mediated systems, and p‐toluenesulfonyl chloride and ethyl 2‐bromoisobutyrate could effectively initiate Cu(I)‐mediated ATRP of MMA, resulting in polymers with low polydispersities in most cases. Besides, Cu(I) halide‐mediated ATRP with 4,5′‐dimethyl 2,2′‐bipyridine as the ligand and p‐toluenesulfonyl chloride as the initiator proved to be better controlled than those with 4,4′‐dimethyl 2,2′‐bipyridine as the ligand, and polymers with much lower polydispersities were obtained in the former cases. This successful HTE example opens up a way to significantly accelerate the development of new catalytic systems for ATRP and to improve the understanding of structure–property relationships of the reaction systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1876–1885, 2004     

原子转移自由基聚合引发体系的最新研究进展

本文介绍了关于原子转移自由基聚合（ATRP）引发-活化-失活过程的最新研究情况,包括RATRP、SR&NI ATRP、AGET ATRP、ARGET ATRP以及ICAR ATRP等新型ATRP引发体系。这些新型ATRP体系逐渐克服了通常ATRP体系的局限性,尤其是后两种体系仅需微量（1～50 ppm）价态稳定的过渡金属络合物调控聚合,大大简化了ATRP方法的工业化生产程序;本文同时介绍了杂化及双金属ATRP催化体系,这两种新型催化体系具有较高的催化活性和对聚合反应的调控能力,而且通过简单地过滤或沉降就可从聚合产物当中脱除。     

原子转移自由基细乳液聚合*

本文从正向、反向、同时正向/反向、电子转移活化剂等不同原子转移自由基聚合（ATRP）细乳液引发体系的角度,综述了近年来国内外关于ATRP细乳液聚合的研究进展。在细乳液体系中进行正向ATRP,聚合可控性不理想,反向ATRP相对适合于细乳液体系,其缺点是表面活性剂用量较大。同时正向/反向引发体系的ATRP中催化剂用量大为减少,并且聚合具有良好的可控性;电子转移活化剂（AGET）ATRP是通过电子转移反应来还原过渡金属的氧化态,克服了同时正向/反向ATRP中需要引入自由基引发剂的缺点。     

原子转移自由基聚合反应及其进展

活性自由基聚合是目前高分子科学中最为活跃的研究领域之一,原子转移自由基聚合反应是实现活性聚合的一种颇为有效的途径,可实现众多单体的活性可控自由基聚合。此文综述有关原子转移自由基聚合反应及其最新进展。     

Copolymerization of (Meth)acrylates with Olefins Using Activators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization (ARGET ATRP)

Summary: Controlled copolymerization of polar (meth)acrylates with non-polar olefin monomers (1-octene, norbornene, vinylcyclohexane) was studied by ARGET (activators regenerated by electron transfer) ATRP (atom transfer radical polymerization). When a normal ATRP of n-butyl acrylate (nBA) and 1-octene was conducted, the polymerization resulted in relatively low conversion, limited control over the polymerization process and high polydispersity (PDI > 1.6). This was due to formation of a dormant species, by reaction of 1-octene radicals with Cu(II) deactivator, that could not be reactivated. However, in ARGET ATRP with 10 ppm amounts of Cu-based catalyst, higher yields and a better controlled copolymerization was obtained (PDI < 1.4), because the low concentration of Cu(II) deactivator reduced the formation of the non-reactive dormant species. The influence of the amount of Cu catalyst, ligand structure, initiators with different halogens, the reaction temperature, and monomer feed ratio were also investigated for ARGET ATRP. In copolymerization of (meth)acrylates with non-polar alkenes, the level of control and the total conversion in ARGET ATRP were higher than those for normal ATRP.     

Electron transfer reactions relevant to atom transfer radical polymerization

The continuous development of more active and stable catalysts in atom transfer radical polymerization (ATRP) has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. Special attention is provided in this short review to such processes, including disproportionation, most pronounced in Cu-mediated ATRP, the reduction of radicals to carbanions or oxidation to carbocations, and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.     

Cu(II)‐Mediated Atom Transfer Radical Polymerization of Methyl Methacrylate via a Strategy of Thermo‐Regulated Phase‐Separable Catalysis in a Liquid/Liquid Biphasic System: Homogeneous Catalysis,Facile Heterogeneous Separation,and Recycling

A strategy of thermo‐regulated phase‐separable catalysis (TPSC) is applied to the Cu(II)‐mediated atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in a p‐xylene/PEG‐200 biphasic system. Initiators for continuous activator regeneration ATRP (ICAR ATRP) are used to establish the TPSC‐based ICAR ATRP system using water‐soluble TPMA as a ligand, EBPA as an initiator, CuBr₂ as a catalyst, and AIBN as a reducing agent. By heating to 70 °C, unlimited miscibility of both solvents is achieved and the polymerization can be carried out under homogeneous conditions; then on cooling to 25 °C, the mixture separates into two phases again. As a result, the catalyst complex remains in the PEG‐200 phase while the obtained polymers stay in the p‐xylene phase. The catalyst can therefore be removed from the resultant polymers by easily separating the two different layers and can be reused again. It is important that well‐defined PMMA with a controlled molecular weight and narrow molecular weight distribution could be obtained using this TPSC‐based ICAR ATRP system.

     

原子转移自由基法合成八臂星型聚苯乙烯

设计合成了星型聚合物引发剂四(2,2-二氯乙酸)季戊四醇酯(TDCAP), 并通过原子转移自由基聚合合成了八臂星型聚苯乙烯. 用FTIR, 1H NMR和GPC等手段对引发剂和星型高分子的结构进行了表征.     

A Facile Strategy for Catalyst Separation and Recycling Suitable for ATRP of Hydrophilic Monomers Using a Macroligand

How to simply and efficiently separate and recycle catalyst has still been a constraint for the wide application of atom transfer radical polymerization (ATRP), especially for the polymerization systems with hydrophilic monomers because the polar functional groups may coordinate with transition metal salts, resulting in abundant catalyst residual in the resultant water‐soluble polymers. In order to overcome this problem, a latent‐biphasic system is developed, which can be successfully used for ATRP catalyst separation and recycling in situ for various kinds of hydrophilic monomers for the first time, such as poly(ethylene glycol) monomethyl ether methacrylate (PEGMA), 2‐hydroxyethyl methacrylate (HEMA), 2‐(dimethylamino)ethyl methacrylate (DMAEMA), N,N‐dimethyl acrylamide (DMA), and N‐isopropylacrylamide (NIPAM). Herein, random copolymer of octadecyl acrylate (OA), MA‐Ln (2‐(bis(pyridin‐2‐ylmethyl)amino)ethyl acrylate), and POA‐ran‐P(MA‐Ln) is designed as the macroligand, and heptane/ethanol is selected as the biphasic solvent. Copper(II) bromide (CuBr₂) is employed as the catalyst, PEG‐bound 2‐bromo‐2‐methylpropanoate (PEG₃₅₀‐Br) as the water‐soluble ATRP initiator and 2,2′‐azobis(isobutyronitrile) (AIBN) as the azo‐initiator to establish an ICAR (initiators for continuous activator regeneration) ATRP system. Importantly, well‐defined water‐soluble polymers are obtained even though the recyclable catalyst is used for sixth times.