Normal Atom Transfer Radical Polymerization Research Articles

Efficient synthesis of the undiluted vegetable juice-compatible hydrophilic quantum dot-labeled fluorescent molecularly imprinted polymer microspheres via facile one-pot surface-initiated ARGET ATRP

The development of fluorescent molecularly imprinted polymers (MIPs) applicable for herbicide optosensing in the undiluted vegetable juices is highly important for food safety control, but still remains a challenging task. Herein, we report on a facile and efficient new strategy for the synthesis of hydrophilic quantum dot (QD)-labeled fluorescent MIP microspheres capable of directly and selectively optosensing a widely used herbicide (i.e., 2,4-dichlorophenoxyacetic acid, 2,4-D) in the undiluted vegetable juices. Such fluorescent MIP particles were readily prepared through one-step grafting of a red CdTe QD-labeled 2,4-D-MIP layer with hydrophilic polyethylene glycol brushes onto the preformed uniform “living” polymer microspheres (with surface-bound alkyl halide groups) via one-pot surface-initiated activator regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP) (SI-ARGET ATRP). They proved to be a highly useful optosensor with high 2,4-D selectivity and sensitivity (the limit of detection ≤ 0.14 μM) in the undiluted Chinese cabbage and cucumber juices. They also showed excellent photostability and reusability and could be directly utilized for 2,4-D detection with high recoveries (98.2 %-102.4 %) and accuracy (RSD≤3.8 %) in the undiluted Chinese cabbage and cucumber juices at three spiking levels of both 2,4-D and its mixtures with several analogues. The use of SI-ARGET ATRP significantly reduced the applied copper catalyst concentration (about 5 μM), which greatly alleviated the quenching effect of the copper catalyst on the fluorescence of the QD-labeled MIP (usually observed for those prepared via normal ATRPs that typically use several hundreds of μM of copper catalysts), thus making this facile new strategy highly efficient for developing advanced fluorescent MIPs that hold much promise in various bioanalytical and diagnostic applications.

Revisiting to N‐alkylation of multi‐dentate amines ligand for ATRP

AbstractThe ligands role in normal atom transfer radical polymerization (ATRP) is not only for controlling the system but also regulate the rate of polymerization depending the solubility in the reaction media. Solubility of the catalyst/ligand complexes in organic media is of particular importance to attain homogeneous polymerization conditions. The linear multi‐dentate amine ligands with a long aliphatic chain on the nitrogen atoms provide solubility of its metal complexes in organic solvents. However, the increasing length of the alkyl substituents induces steric effects and can alter the redox potential of the metal center. Herein, to understand the effect of alkyl length, we report the synthesis of tri‐, tetra‐dentate linear amine ligands, diethylenetriamine, triethylenetetramine having n‐alkyl halides containing odd carbon chain lengths (C3H7‐, C5H9‐), respectively. Well‐defined polymers were obtained in ATRP of S and MMA using synthesized ligands. Apparently, increasing the alkyl chain length was sufficient to provide homogeneous ATRP medium, which was not achieved with methyl‐substituted ligands (e.g., pentamethyldiethylenetriamine, PMDETA and hexamethyltriethylenetetramine, HMTETA). In combining ATRP results for odd (synthesized) and even carbon number alkylated ligands (literature), linear decreasing trend was observed on apparent rate constant (kpapp) by the increasing the alkyl chain length.

The “Living” Feature of the ATRP Macroinitiators in Different Catalytic Systems

Atom transfer radical polymerization (ATRP) has achieved widespread use in living polymerization. However, until now there has been little report that macroinitiators initiate polymerization in different catalytic systems. The preparation of bromine-terminated polymethyl methacrylate (PMMA-Br) and chlorine-terminated PMMA (PMMA-Cl) were carried out via reverse atom transfer radical polymerization (RATRP). The PMMA with halogen termination and narrow polydispersity (Mn = 12,000–15,000 g/mol, Mw/Mn = 1.1–1.2) were used as macroinitiators. The block copolymer of polymethyl methacrylate and polyacrylonitrile (PMMA-b-PAN) was prepared in different catalytic systems through normal ATRP. The analyses of the 1H NMR showed that the PMMA prepared by RATRP were end-functionalized by halogen atoms, demonstrated the activities of the PMMA macroinitiators. The molecular weight and polydispersity index (PDI) of the polymers were analyzed using gel permeation chromatography (GPC). The results indicated that the block polymers that the molecular weight of the block copolymer after chain extension has increased significantly and the molecular weight distribution is narrow (Mn = 17,000–25,000 g/mol, Mw/Mn = 1.1–1.3). The kinetics of these polymerization processes were studied as a function of monomers to the macroinitiator molar ratio. It was found that the polymerizations in different catalytic systems coincidence first-order kinetics with respect to monomers. In this paper, we reported that the chain extension in different catalytic systems is practicable. The PMMA-Cl and PMMA-Br were obtained in initiating systems of AIBN/FeCl3·6H2O/triphenylphosphine (PPh3) and AIBN/CuBr2/PMDETA, respectively. As the macroinitiators, the PMMA-Cl and PMMA-Br initiate the polymerization in FeCl2/PPh3 and CuBr/PMDETA catalytic systems, respectively. The molecular weight and PDI of the polymers were analyzed using GPC.

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.

Atom Transfer Radical Polymerization: Billion Times More Active Catalysts and New Initiation Systems.

Approaching 25 years since its invention, atom transfer radical polymerization (ATRP) is established as a powerful technique to prepare precisely defined polymeric materials. This perspective focuses on the relation between structure and activity of ATRP catalysts, and the consequent choice of the initiating system, which are paramount aspects to well-controlled polymerizations. The ATRP mechanism is discussed, including the effect of kinetic and thermodynamic parameters and side reactions affecting the catalyst. The coordination chemistry and activity of copper complexes used in ATRP are reviewed in chronological order, while emphasizing the structure-activity correlation. ATRP-initiating systems are described, from normal ATRP to low ppm Cu systems. Most recent advancements regarding dispersed media and oxygen-tolerant techniques are presented, as well as future opportunities that arise from progressively more active catalysts and deeper mechanistic understanding.

A Molecular Weight Distribution Polydispersity Equation for the ATRP System: Quantifying the Effect of Radical Termination

Polydispersity quantifies the breadth of polymer molecular weight distribution, making it an important and frequently quoted chain microstructural property for characterization. An explicit expression of such an important variable is desirable for ease of calculation, correlation with experiment data, and/or parameter estimation. A review of published literatures shows that great efforts have been put forth by many researchers to derive these equations for various polymerization mechanisms. In atom transfer radical polymerization (ATRP), polydispersity depends on three factors: monomer conversion, number of monomer addition per activation/deactivation cycle, and amount of dead chains. The existing expressions available in the literature only account for, at most, two of these three factors, with the contribution from dead chains commonly neglected. This assumption results in polydispersity monotonically decreasing with conversion, which is often not observed in experiments. In this work, a new equation for polydispersity, which accounts for contributions of all the three aforementioned factors, is proposed. The validity of assumptions involved in the derivation is evaluated by comparing the polydispersity profiles to those simulated by the method of moments. In addition, this new equation is used to correlate several experiment data sets for verification, namely from ATRP of 2-hydroxyethyl methacrylate, methyl methacrylate, and N-isopropylacrylamide, showing better agreement than the existing equation. Although the equation derived here is strictly applicable to homogeneous (bulk and solution) normal ATRP, with further effort it may be extended to other types of ATRP as well as NMP and RAFT systems.

Atom transfer radical polymerization of dimethyl(methacryloyloxymethyl) phosphonate

Atom transfer radical polymerization (ATRP) of dimethyl(methacryloyloxymethyl) phosphonate (DMMAMP, also referred to as MAPC1) with a copper/2,2′-bipyridine based catalytic system in DMSO resulted in a homopolymer with narrow molecular weight distribution. Initiators for continuous activator regeneration (ICAR) ATRP of this monomer was also successful when using tris[(2-pyridyl)methyl]amine as ligand and azobisisobutyronitrile as supplementary initiator for the regeneration of the CuI activator. Normal ATRP provided better control compared to ICAR ATRP. Differential scanning calorimetry measurements of the resulting poly(DMMAMP) revealed a glass transition temperature (Tg=63°C), lower than that of poly(methyl methacrylate). Chain extension of the poly(dimethyl(methacryloyloxymethyl) phosphonate) macroinitiator with styrene was successfully achieved with a high fraction of block copolymer.

Controlled radical polymerization of n-hexadecyl methacrylate mediated by tris(2,2′-bipyridine)iron(III) complexes

Poly(n-hexadecyl methacrylate) (PHMA) with narrow molecular weight distribution has been synthesized by atom transfer radical polymerization (ATRP) and reverse ATRP of n-hexadecyl methacrylate (HMA) at 80 °C in N, N-dimethylformamide (DMF) using the CBr4/tris(2,2′-bipyridine)iron(III) complex initiation system in the presence of 2,2′-azobisisobutyronitrile (AIBN). From the kinetic studies and molecular weight data, it reveals the controlled nature of polymerization. The effect of various reaction parameters on number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) have been investigated. For the reverse ATRP, the catalyst tris(2,2′-bipyridine)iron(III)complex, [Fe(bpy)3]3+, played an important role in polymerization rate and Mw/Mn. The resulting PHMA that obtained by reverse ATRP shows the best control of molecular weights and its distribution as compared to normal ATRP system. PHMA has been characterized by different techniques such as GPC, XRD and NMR spectroscopy.

Synthesis of functional poly(styrene)-block-(methyl methacrylate/methacrylic acid) by homogeneous reverse atom transfer radical polymerization: Spherical nanoparticles, thermal behavior, self-aggregation, and morphological properties

This study investigates the use of homogeneous reverse atom transfer radical polymerization for the synthesis of polystyrene (PS) initiated by conventional radical peroxide with copper bromide in the lower oxidation state and a 2,2′-bypyridine complex as the catalyst. In a second stage, an amphiphilic block copolymer containing methyl methacrylate (MMA) was synthesized via normal atom transfer radical polymerization in two steps, followed by partial hydrolysis of the methyl ester linkage of the MMA block under acidic conditions. The block copolymer PS699-b-P(MMA232/MAA58) obtained had a narrow molecular weight dispersity (Ð < 1.3). The structure of the precursor, PS-b-PMMA, and resultant polymer, was characterized and verified by FTIR and 1H-NMR spectroscopy as well as size exclusion chromatography. The self-aggregation of PS699-b-P(MMA232/MAA58) in organic solvents was monitored by UV spectroscopy, whereas the morphology and size of the formed microaggregates were investigated by transmission electron microscopy and dynamic light scattering. The results indicate that this copolymer formed regular spherical reverse micelles with a core–shell structure. The atomic force micrographs of PS699-b-P(MMA232/MAA58) showed a rough surface morphology owing to microphase separation of the block copolymer. In addition, thermal characterization was performed by differential scanning calorimetry and thermogravimetric analysis. The glass transition temperature of PS699-b-P(MMA232/MAA58) decreased significantly (65°C), when compared to PS and PMMA, suggesting that an enhanced movement of the polymer chains resulted by the segregation of the hydrolyzed P(MMA232/MAA58) block. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Kinetic study of in situ normal and AGET atom transfer radical copolymerization of n–butyl acrylate and styrene: Effect of nanoclay loading and catalyst concentration

AbstractThe effect of clay nanolayers and catalyst concentration on the kinetics of atom transfer radical copolymerization of styrene and butyl acrylate initiated by activators generated by electron transfer (AGET initiation system) or an alkyl halide (normal initiation system) was studied. Monomer conversion was studied by attenuated total reflection–Fourier transform infrared spectroscopy, and also proton nuclear magnetic resonance (1H NMR) spectroscopy was utilized to evaluate the heterogeneity in the composition of poly(styrene‐co‐butyl acrylate) chains. A decrease in the copolymerization rate of styrene and butyl acrylate in the presence of clay platelets was observed since clay layers confine the accessibility of monomer and growing radical chains. Considering the linear first‐order kinetics of the polymerization, successful AGET and normal atom transfer radical polymerization (ATRP) in the presence of clay nanolayers were carried out. Consequently, poly(styrene‐co‐butyl acrylate) chains with narrow molecular weight distribution and low polydispersity indices (1.13–1.15) were obtained. The linearity of ln([M]0/[M]) versus time and molecular weight distribution against conversion plots indicates that the proportion of propagating radicals is almost constant during the polymerization, which is the result of insignificant contribution of termination and transfer reactions. Controlled synthesis of poly(styrene‐co‐butyl acrylate)/clay is implemented with the diminishing catalyst concentration of copper(I) bromide/N,N,N′,N′′,N′′‐pentamethyl diethylene triamine without affecting the copolymerization rate of normal ATRP. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 789–799, 2012

Highly Active Bipyridine-Based Ligands for Atom Transfer Radical Polymerization.

A series of 2,2'-bipyridines with 4,4'-substituents (R-bpy) were investigated for atom transfer radical polymerization (ATRP) of methyl acrylate (MA) and methyl methacrylate (MMA). Ligand substituents with a large range of Hammett parameters (R = Cl, H, Me, dinonyl (dN), MeO, and (Me)2N) were studied with cyclic voltammetry (CV), revealing that increasing the strength of electron donating groups (EDGs) resulted in more stable CuII complexes and larger ATRP equilibrium constants. Normal ATRP experiments confirmed the obtained CV data by showing the fastest rates of polymerization with R-bpy ligands containing EDGs ((Me)2N and MeO) and the slowest with electron withdrawing Cl. A 400-fold increase in the polymerization rate was observed with bpy ligands containing p-(Me)2N compared to H substituents. Linear increases in molecular weight with monomer conversion, and narrow molecular weight distributions were obtained with (Me)2N-bpy and MeO-bpy ligands. Low catalyst concentrations of 50 to 100 parts-per-million (ppm) were successfully employed with highly active R-bpy ligands (R = MeO and (Me)2N) and found to be effective in polymerizing MA and MMA, respectively, with narrow molecular weight distributions <1.3.

ATRP under Biologically Relevant Conditions: Grafting from a Protein.

Atom transfer radical polymerization (ATRP) methods were developed in water-based media, to grow polymers from proteins under biologically relevant conditions. These conditions gave good control over the resulting polymers, while still preserving the protein's native structure. Several reaction parameters, such as ligand structure, halide species, and initiation mode were optimized in water and PBS buffer to yield well-defined polymers grown from bovine serum albumin (BSA), functionalized with cleavable ATRP initiators (I). The CuCl complex with ligand 2,2'-bipyridyne (bpy) provides the best conditions for the polymerization of oligo(ethylene oxide) methacrylate (OEOMA) in water at 30 °C under normal ATRP conditions (I/CuCl/CuCl2/bpy = 1/1/9/22), while the CuBr/bpy complex gave better performance in PBS. Activators generated by electron transfer (AGET) ATRP gave well-controlled polymerization of OEOMA at 30 °C with the ligand tris(2-pyridylmethyl)amine (TPMA), (I/CuBr2/TPMA = 1/10/11). The AGET ATRP reactions required slow feeding of a very small amount of ascorbic acid into the aqueous reaction medium or buffer. The reaction conditions developed were used to create a smart, thermoresponsive, protein-polymer hybrid.

Reducing Copper Concentration in Polymers Prepared via Atom Transfer Radical Polymerization

AbstractThree different initiation systems for atom transfer radical polymerization (ATRP): normal ATRP, activators regenerated by electron transfer (ARGET) ATRP, and initiators for continuous activator regeneration (ICAR) ATRP were used to synthesize polystyrene with a number‐average molecular weight ≈20 000 g · mol−1. Each polymerization mixture was divided and subjected to one or more purification methods including passing through a column filled with neutral alumina, stirring with an ion exchange resin (ATRP Pure), or precipitation into hexanes or methanol. Inductively coupled mass spectrometry was used to analyze the concentration of copper in each sample. For the polystyrene prepared by normal ATRP, purification by a neutral alumina column was the most effective at removing copper (down to ≈2 parts per million by mass). For the ARGET and ICAR ATRP, purification by a neutral alumina column and stirring with 10 wt.‐% ATRP Pure gave comparable results (each ≈1 part per million by mass).magnified image

Phosphorus ligands for iron(III)‐mediated ATRP of styrene via generation of activators by monomer addition

AbstractStyrene polymerization via generation of activators by monomer addition (GAMA) for atom transfer radical polymerization (ATRP) has been examined extensively with bulk FeX3 and FeX2 at 110 °C in conjunction with various phosphorus‐bearing ligands. It was found that GAMA possesses advantages over normal ATRP. Most importantly, narrower polydispersity index (PDI) values were observed from the styrene polymerizations with Fe(III) over those with Fe(II). Every instance of 2‐(diphenylphosphino)‐N,N′‐dimethyl‐[1,1′‐biphenyl]‐2‐amine and 2‐(diphenylphosphino) pyridine with the Fe(III) system were controlled excellently without addition of any radical initiator or reducing agent additives. Initiator type was found to exert a significant factor to influence on the controllability of polymerization. The initiation of 1‐phenylethyl chloride and methyl‐2‐chloropropionate gave rise to formation of polymers with narrow PDI (1.05–1.20), whereas those from 1‐phenylethyl bromide increased to 1.35. The GAMA of bulk styrene exhibited the best performance in terms of both rate and controllability compared with toluene and anisole. Both formation of block copolymer from the macroinitiator and efficient perturbation of polymerization with 2,2,6,6‐tetramethylpiperidine 1‐oxyl provided firm evidence to support the living and radical characteristics for the GAMA of styrene. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 144–151, 2010

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

AbstractSummary: 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 &gt; 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 &lt; 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.

Controlled Copolymerization of n-Butyl Acrylate with Nonpolar 1-Alkenes Using Activators Regenerated by Electron Transfer for Atom-Transfer Radical Polymerization

Controlled copolymerization of n-butyl acrylate with 1-octene was achieved by ARGET (activators regenerated by electron transfer) ATRP (atom transfer radical polymerization) in anisole. When a normal ATRP of n-butyl acrylate and 1-octene was conducted, the polymerization resulted in relatively low conversion (<50%), 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 were obtained because the low concentration of Cu(II) deactivator reduced the frequency of producing the nonreactive dormant species. When n-butyl acrylate and 1-octene were copolymerized with 10 ppm of CuCl2/TPMA vs monomer and 10 mol % of Sn(EH)2 vs initiator, a copolymer was prepared in higher yield and with low polydispersity (PDI < 1.4).

Synthesis of High Molecular Weight Poly(styrene-co-acrylonitrile) Copolymers with Controlled Architecture

High molecular weight styrene−acrylonitrile (SAN) copolymers were prepared under azeotropic conditions (60 mol % of styrene) by ARGET (activators regenerated by electron transfer) ATRP (atom transfer radical polymerization) at 80 °C in anisole. When a normal ATRP of styrene and acrylonitrile was conducted, the molecular weight of the resulting SAN copolymers was limited due to outer-sphere electron-transfer reactions. This was due to oxidation of polystyryl radicals to carbocations or reduction of polyacrylonitirile radicals to carbanions via reactions with Cu(II) and Cu(I) species, respectively. Since ARGET ATRP employs much lower concentrations of copper catalyst, the contributions of these side reactions are reduced, enabling formation of high molecular weight SAN copolymers (Mn ∼ 200 000) with low polydispersity (Mw/Mn < 1.3). Additionally, SAN copolymers with controlled chain architecture were prepared including block copolymers and starlike copolymers.

Recent mechanistic developments in atom transfer radical polymerization

Recent mechanistic innovations concerning catalyst development in atom transfer radical polymerization (ATRP) are described. The following topics will be discussed: structure–reactivity relationships of the polymerization catalyst, including correlating reaction parameters with catalyst, alkyl halide, and monomer structure; concurrent reactions that may occur during ATRP and will affect its efficiency, including oxidation/reduction of radicals to radical cations/anions, solvent, monomer, and radical coordination to the active catalyst, and side reactions particular to aqueous media. In addition, novel methods of fine tuning initiation, activation, and deactivation processes, including simultaneous reverse and normal initiation ATRP, activators generated by electron transfer ATRP, hybrid catalyst systems, and bimetallic ATRP will be presented.