Nitroxide-mediated Living Radical Polymerization Research Articles

Photo Nitroxide-Mediated Living Radical Polymerization of Hindered Amine-Supported Methacrylate

With the aim of obtaining giant polymer vesicles supporting a hindered amine that is converted into a redox catalyst on the vesicle shells, the living nature of the photo nitroxide-mediated living radical polymerization (photo NMP) of a monomer containing a hindered amine and the formation of the vesicles consisting of an amphiphilic diblock copolymer by the polymerization-induced self-assembly were investigated. The photo NMP of 2,2,6,6-tetramethyl-4-piperidyl methacrylate (TPMA) was performed in methanol using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, 2,2’-azobis[2-(2-imidazolin-2-yl)propane] (V-61) as the initiator, and (4-tert-butylphenyl)diphenylsulfonium triflate as the accelerator by UV irradiation at room temperature. The first-order time-conversion plots had an induction period in which the MTEMPO molecules were captured by the initiator radicals and the monomer radicals generated by the initiation. It was confirmed that the polymerization proceeded by a living mechanism based on linear correlations of the molecular weight of the poly(TPMA) (PTPMA) versus the monomer conversion and the reciprocal of the initial concentration of V-61. Based on the livingness of the polymerization, the photo NMP-induced self-assembly for the block copolymerization of methyl methacrylate (MMA) using the PTPMA end-capped with MTEMPO was carried out in methanol to produce microsized giant spherical vesicles consisting of the amphiphilic PTPMA-block-poly(MMA) diblock copolymer. A differential scanning calorimetry study demonstrated that the vesicles had a single bilayer structure.

C60-containing polymers for electron beam lithography

New C60-containing polymers were synthesized for electron beam lithography. The homo and random copolymers of p-chloromethylstyrene and p-tert-butoxystyrene were produced using a nitroxide-mediated living radical polymerization technique, and then the C60-containing polymers were obtained by the reaction of C60 with the nitroxide-terminated polymers and the subsequent transformation of the tert-butoxy groups to hydroxy groups. The highly sensitive lithographic performance of the resulting electron beam resists led to the fabrication of 50-nm-feature size patterning using the C60-containing copolymers of p-chloromethylstyrene and p-hydroxystyrene.

Effects of Dendron Generation and Salt Concentration on Phase Structures of Dendritic–Linear Block Copolymers with a Semirigid Dendron Containing PEG Tails

We prepared a series of dendritic–linear block copolymers (DLBCPs) bearing a semirigid Percec-type dendron with ionophilic poly(ethylene glycol) (PEG) tails and a polystyrene (PS) linear polymer by nitroxide-mediated living radical polymerization (NMRP). As the DLBCPs are connected by an ester linkage, through hydrolysis the molecular weights of the DLBCPs were precisely characterized by gel permeation chromatography and MALDI-TOF MS. Differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy were used to investigate the phase behaviors of the DLBCPs. Results show that the PEG tails of the semirigid dendron display a cold crystallization peak and a melting peak during the second heating process, while for the neat DLBCPs, the crystallization of the PEG tails is completely inhibited, and only the glass transition temperature (Tg) of the PS block is observed. However, Tg of the dendron block can be observed by complexing the DLBCPs with LiCF3SO3, suggesting that mi...

Synthesis of Functional Syndiotactic Polystyrene Copolymers with Graft and Block Structures by Nitroxide-Mediated Living Radical Polymerization

In this study, synthesis of controlled graft copolymers onto syndiotactic polystyrene using nitroxide-mediated living radical polymerization and 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO) iniferter is reported. Syndiotactic polystyrene has been synthesized using half-sandwich titanocene/ methylaluminoxane catalyst in homogenous conditions. Then, α-phenyl chloroacetylation of syndiotactic polystyrene was accomplished in a heterogeneous process using carbon disulfide as dispersing medium and α-phenyl chloroacetyl chloride and anhydrous aluminum chloride as acetylating agent and catalyst, respectively. Then, 1-hydroxy 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO-OH) was synthesized by the reduction of TEMPO with sodium ascorbate. This functional nitroxyl compound was coupled with arylated syndiotactic polystyrene. The graft copolymerizations of styrene and methyl styrene were initiated by arylated syndiotactic polystyrene carrying TEMPO groups as a macroinitiator. The product of polymerization was then heated in the presence of styrene and methyl styrene to form graft copolymer. The structure of obtained copolymer was investigated by 1H-NMR, and Fourier transform—infrared spectroscopy, and thermal behavior of copolymers was performed by differential scanning calorimetry analyses.

Micelle formation of polystyrene-block-poly(4-tert-butoxystyrene) in hexane

A new type of hexane-soluble polymeric surfactant based on poly(4-tert-butoxystyrene) (PtBSt) was prepared by the nitroxide-mediated living radical polymerization, and their self-assemblies in hexane were explored. Polystyrene-block-PtBSt diblock copolymers with six different PtBSt block lengths were obtained by the sequential living radical polymerization of styrene followed by 4-tert-butoxystyrene using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator; Mn(PtBSt block) = 13,500, 21,700, 26,600, 47,500, 91,300, and 108,000 at the constant length of the PSt block (Mn = 12,900). Dynamic light-scattering studies demonstrated that the copolymers self-assembled into monodispersed spherical micelles in hexane. The hydrodynamic diameter of the micelles increased with an increase in the PtBSt block length. The micellar size also increased as the copolymer concentration increased. However, the size decreased as a result of the increasing temperature due to a decrease in the aggregation number. The 1H NMR analysis confirmed that the copolymers formed micelles with PSt cores.

Synthesis and characterization of an exfoliated modified syndiotactic polystyrene/Mg–Al-layered double-hydroxide nanocomposite

This paper describes the synthesis and characterization of a controlled graft and block copolymerization onto syndiotactic polystyrene (sPS) using nitroxide-mediated living radical polymerization and the preparation of its polymer/layered double-hydroxide (LDH) nanocomposite. The graft and block copolymerization of styrene and p-methylstyrene were initiated with arylated sPS carrying 2,2,6,6-tetramethyl-1-piperidinyloxy groups as a macroinitiator. The structures of obtained graft and block copolymers were determined by 1H nuclear magnetic resonance and Fourier transform infrared spectroscopy. The MgAl(Cl)-LDH precursor was prepared with microwave irradiation using a mixed solution of MgCl2 and AlCl3, which was slowly pumped into decarbonated water. Thereafter, the surfactant-modified MgAl(SDBS) was obtained by the anion exchange reaction of MgAl(Cl)-LDH with sodium dodecyl benzene sulfate (SDBS). Finally, the sPS-g-(PS-b-PMS)/MgAl-LDH nanocomposite was prepared by a solution intercalation method. The exfoliated structure of the nanocomposite was probed by X-ray diffraction and transmission electron microscopy (TEM). Compared with pure sPS-g-(PS-b-PMS), the nanocomposite shows a much higher decomposition temperature and higher glass transition temperature. This paper describes the synthesis and characterization of exfoliated modified syndiotactic polystyrene/Mg–Al-layered double-hydroxide nanocomposite. Synthesized modified syndiotactic polystyrene were characterized by means of Fourier transform infrared (FT-IR) and 1H nuclear magnetic resonance spectroscopy. The exfoliated sPS-g-(PS-b-PMS)/MgAl-LDH nanocomposite was characterized by means of FT-IR, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction and transmission electron microscopy.

Unimolecular micelle derived from hyperbranched polyethylenimine with well-defined hybrid shell of poly(ethylene oxide) and polystyrene: A versatile nanocapsule

The synthesis and properties of a macromolecular nanocapsule derived from hyperbranched polyethylenimine (HPEI) with well-defined hybrid shell of poly(ethylene oxide) monomethyl ether (mPEO) and polystyrene (PS) are described. HPEI is treated in sequence with 4-glycidol-2,2,6,6-tetrametyl-piperidin-1-oxyl, succinic anhydride, mPEO, leading to a HPEI derivative compatible with nitroxide-mediated living radical polymerization of styrene, thus a macromolecular nanocapsule, [email protected]/PS, is available with a well-defined and tunable hybrid shell of PEO and PS. Within certain PEO/PS ratio, the nanocapsule is soluble in a number of organic solvents as well as in water. The nanocapsule exists as three layer onion-like particle ([email protected]@PEO) in water, whereas in chloroform it exists as a hybrid shell particle ([email protected]/PS), and the particles generally exist in the form of unimolecular micelle. In a biphasic water/chloroform mixture, the nanocapsule can transfer anionic, water-soluble guest from an aqueous phase to the chloroform phase; while when dissolved in water, the nanocapsule can efficiently capture both ionic and apolar solutes. Release of the guest can occur under the stimulus of pH or the switch of medium. This is the first example of a unimolecular micelle that can simultaneously deliver both polar and apolar guests. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 681–691, 2010

リビングラジカル重合 １．重合機構と方法 １

Living radical polymerization (LRP) has been becoming an indispensable method for the synthesis of functional polymer materials, because this method possesses attractive features of both radical and living polymerizations. This four-series article describes the methods to carry out LRP and their uses for material synthesis. We shall mainly focused on nitroxide-mediated LRP (NMP), atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer polymerization (RAFT), organotellurium-, stibine-, and bismuthine-mediated LRP (TERP, SBRP, and BIRP, respectively), and cobalt-mediated LRP (CORP). This first article mainly focuses on methods and mechanisms of NMP and ATRP, in which the persistent radical effect plays an important role in the polymerization mechanism.

Preparation of molecularly imprinted polymers using nitroxide-mediated living radical polymerization

The use of molecularly imprinted polymers (MIPs) in chemical and bioanalytical applications has been gaining in interest in recent years. Compared to their biological receptor counterparts, MIPs are easy to prepare, have long shelf stability and can be used under a variety of harsh conditions. The majority of MIPs currently used are produced by traditional free radical polymerization. One drawback with the use of standard free radical initiators is that little control can be exerted over the chemical processes that form the final imprinted cavities. In this study we set out to investigate the application of controlled (living) free radical polymerization to the preparation of MIPs. This was exemplified by the synthesis of cholesterol-imprinted bulk polymers by nitroxide-mediated polymerization (NMP). A sacrificial covalent bond was employed to maintain imprinting fidelity at elevated temperature. Selective uptake of cholesterol from solutions in hexane was studied with imprinted polymers prepared under different conditions. The imprinted hydrolyzed MIP prepared by NMP displayed higher selective cholesterol binding than that prepared by a traditional radical polymerization.

Synthesis of Shell Cross-Linked Block Copolymer Micelles with Poly(p-styrenesulfonic acid) in the Micelle Core

Fine particles having poly(p-styrenesulfonic acid) (polySS) in the core and a polystyrene (polySt) shell were synthesized by cross-linking and successive chemical transformation of triblock copolymer micelles. A triblock copolymer, polySt-b-poly(St0.8-co-SBS0.2)-b-polySSPen, was synthesized by nitroxide-mediated living radical polymerization, where SBS and SSPen denote p-(1-methylsilacyclobutyl)styrene and neopentyl p-styrensulfonate, respectively. Block copolymer micelles carrying a polySSPen core and polySt shell with cross-linkable silacyclobutyl groups in the shell were prepared by dissolving the triblock copolymer in cyclohexane. Cross-linking of the shell by Pt-catalyzed ring-opening of silacyclobutane followed by thermolysis of the neopentyl esters in polySSPen in tetralin in the presence of p-toluenesulfonic acid gave the cross-linked micellar particles with polySS in the micelle core. The obtained particles showed unique environmentally responsive solubility due to the core−shell structure with a moderately cross-linked shell.

Nitroxide-Mediated Living Radical Polymerization of 2-Hydroxyethyl Acrylate and the Synthesis of Amphiphilic Block Copolymers

Nitroxide-mediated radical polymerizations of 2-hydroxyethyl acrylate (HEA) were performed in bulk, in organic solvent (N,N-dimethylformamide), and in aqueous media at 100−120 °C, using an alkoxyamine initiator based on N-tert-butyl-N-(1-diethyl phosphono-2,2-dimethylpropyl) nitroxide, SG1. By the addition of free nitroxide (6−12 mol% with respect to the initiator), the polymerizations were controlled, exhibiting a linear relationship of ln([M]0/[M]) vs time and Mn vs conversion up to 50−60% conversion. Poly(HEA) with Mn ≈ 90 000 and Mw/Mn < 1.3 can be obtained by bulk polymerization. Solution polymerizations of HEA in N,N-dimethylformamide and in water exhibit comparable kinetics to those of HEA bulk polymerizations at the same temperature. Chain transfer to polymer produced up to 1.7 mol% branches for poly(HEA) depending on the reaction conditions, with a trend of increasing long chain branching with conversion. Amphiphilic block copolymers of poly(BA-b-HEA) with different molecular weights and block co...

Maximizing polymer livingness in nitroxide-mediated miniemulsion polymerizations

Recent efforts to adapt nitroxide-mediated living radical polymerization chemistry to colloidal systems have shown considerable progress when miniemulsions are used. Miniemulsion SFRP in fact can yield higher conversions (>99%) and faster reaction rates than bulk SFRP. Much of the advantage of operating in miniemulsion arises from the inherent advantages of having a two phase system, in which the aqueous phase can be employed as a medium to introduce rate enhancing additives such as ascorbic acid to the reaction loci with a high degree of control. Additives such as ascorbic acid act by consuming excess nitroxide, and therefore directly influence the rate. Results on the effect of ascorbic addition on rate, molecular weight and livingness are presented. We have also begun to address the issue of high operating temperatures in TEMPO-mediated SFRP (∼120–135 °C) that require a pressurized reactor. We have run experiments using TEMPO at 100 °C with slowly decomposing initiator added to maintain an appropriate free nitroxide concentration. Reasonable reaction rates are obtained, albeit with higher polydispersities (∼1.6). Challenges and opportunities related to lower temperature operation are discussed.

Preparation of micelles with azobenzene at their coronas or cores from ?nonamphiphilic? diblock copolymers

Micelles with azobenzene at the coronas or the cores were prepared by the micellization of ‘nonamphiphilic’ diblock copolymers through hydrogen bond cross-linking. We used 4-(phenylazophenoxymethyl)styrene (AS) as the azobenzene. A poly(vinylphenol)-block-poly(AS-co-styrene) diblock copolymer (PVPh-b-P(AS-co-St)) was prepared by combination of the nitroxide-mediated living radical polymerization and the hydrolysis. The copolymer contained ca. 1 mol% of the azobenzene units in the P(AS-co-St) blocks on the basis of 1H NMR analysis. The PVPh-b-P(AS-co-St) copolymer showed no micellization in 1,4-dioxane, the nonselective solvent. Dynamic light scattering demonstrated that the copolymer formed micelles in the presence of 1,4-butanediamine (BDA) in this solvent. 1H NMR analysis revealed that the azobenzene moieties were located at the coronas of the micelles, because the signals of the aromatic protons originating from the azobenzene had no changes in the shape and the intensity by the micellization. UV analysis supported the presence of the azobenzene at the micellar coronas. The size of the PVPh-b-P(AS-co-St) micelles was independent of the copolymer concentration. On the other hand, the aggregation number of the micelles was dependent not only on the copolymer concentration but also on the kind of the diamine. A poly(AS-co-vinylphenol)-block-polystyrene diblock copolymer (P(AS-co-VPh)-b-PSt) formed the micelles with the azobenzene at the cores of the micelles by BDA. UV analysis demonstrated that the azobenzene at the micellar cores still had the potential to function as photorefractive switching.

New Sterically Hindered Nitroxides for the Living Free Radical Polymerization: X-ray Structure of an α-H-Bearing Nitroxide

The synthesis of three new sterically hindered Hawker−Braslau type alkoxyamines for the nitroxide-mediated living radical polymerization is described. Efficient polymerizations can be performed at 105 °C. With alkoxyamine 11 controlled polymerization of n-butyl acrylate and styrene is possible even at 90 °C. AA and AB diblock copolymers can be prepared in a controlled manner using alkoxyamine 11. Kinetic EPR experiments for the determination of the C−O bond activation energies of the new alkoxyamines are described. Furthermore, 1H NMR alkoxyamine decomposition studies are presented. These kinetic experiments are used for the discussion of the polymerization results. Furthermore, the X-ray structure of a Hawker-Braslau type nitroxide is presented. X-ray data and ESR data are used to explain the high stability of these nitroxides.

Some aspects of nitroxide-mediated living radical polymerization of N-( p-vinylbenzyl)phthalimide

The free radical polymerization of N-( p-vinylbenzyl)phthalimide (VBP) “initiated” with the adduct of 2-benzoyloxy-1-phenylethyl and TEMPO (BS-TEMPO) or TEMPO-terminated polystyrene (PS-TEMPO) in N,N-dimethylformamide (DMF) at 125 °C was found to proceed in a living fashion, providing low-polydispersity PVBP and block copolymers of the type PS- b-PVBA, where TEMPO is 2,2,6,6-tetramethylpiperidinyl-1-oxy. Unlike TEMPO-mediated styrene polymerization, the polymerization rate slightly but distinctly depended on the adduct concentration, which was interpretable as a pre-stationary behavior. The hydrolysis of those polymers gave poly( p-aminomethylstyrene) (PAMS) and PS- b-PAMS, and further treatment of the block copolymer with hydrogen chloride provided an amphiphilic block copolymer. The polymeric amphiphile was used as an emulsifier in emulsion polymerization to produce a positively charged polymeric microsphere.

Glycoconjugated Polymer. 4. Synthesis and Aggregation Property of Well-Defined End-Functionalized Polystyrene with β-Cyclodextrin

2,2,6,6-Tetramethylpiperidinyloxy (TEMPO)-substituted β-cyclodextrin (β-CyD), 1, was synthesized as the initiator for the nitroxide-mediated living radical polymerization. Styrene (St) was polymerized with 1 using a molar ratio of St and 1 in a feed ([St]/[1]) of 200 at 120 °C for 6 h. The size exclusion chromatography (SEC) trace of the product exhibited a symmetrical peak with a weight-average molecular weight (Mw,SEC) of 11 400 and a polydispersity (Mw/Mn) of 1.13. In the 1H NMR spectrum of the product, the signals due to polystyrene (PSt) appeared along with the characteristic signals due to the 1 unit. Thus, the product was assignable to the end-functionalized PSt with acetylated β-CyD, 2. The introduction of acetylated β-CyD into 2 was found to occur in a quantitative fashion, which was confirmed by thin-layer chromatography equipped with a flame ionization detector (TLC-FID). For the [St]/[1] of 600 at 120 °C, the St/1 system proceeded in a controlled manner to afford 2 with polymerization times of 1.5−9 h with yields of 5−40% and Mw,SEC's of 7500−26 000. On the other hand, the St/1 system for 15 and 20 h afforded polymers with 54 and 75% though an unexpected side reaction occurred. Polymer 2 with Mw,SEC's of 7500−26 000 was modified by deacetylation using sodium methoxide in dry THF into end-functionalized PSt with β-CyD, 3. Particle formation was observed during dynamic laser light scattering measurements of 3 in benzene, indicating that 3 was stably suspended in a good solvent for PSt by forming aggregates with β-CyD as a core. The average aggregation numbers (NA) of the aggregates were found to range from 42 to 125, which were determined by static laser light scattering measurement.

Synthesis of random and block copolymers of styrene and styrenesulfonic acid with low polydispersity using nitroxide-mediated living radical polymerization technique

Poly(styrene-ran-styrenesulfonic acid) and poly(styrene-block-styrenesulfonic acid) with low polydispersity were prepared using nitroxide-mediated living radical polymerization technique. Random or block copolymerization of styrene and neopentyl p-styrenesulfonate smoothly proceeded by AIBN/2,2,5,5-tetramethyl-4-diethylphosphono-3-azahexane-3-nitroxide initiating system. Transformation of the sulfonate ester to sulfonic acid was carried out by the reaction with trimethylsilyl iodide or by thermolysis at 150°C. Those polymers showed amphiphilic characters.

Synthesis of Polystyrene-block-poly(butyl acrylate) Copolymers Using Nitroxide-Mediated Living Radical Polymerization in Miniemulsion

Living free-radical butyl acrylate polymerization in miniemulsion was initiated by polystyrene bearing a nitroxyl end group to yield polystyrene-block-poly(butyl acrylate) block copolymers. Polystyrene macro-initiator was obtained using different initiating systems (potassium persulfate or benzoyl peroxide) in the presence of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) or the more water-soluble 4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl (OH-TEMPO). The nitroxide water-solubility has an important influence in determining molecular weight distribution and controlling the growth of the second block.

Nitroxide-Mediated Living Radical Polymerization: Determination of the Rate Coefficient for Alkoxyamine C−O Bond Homolysis by Quantitative ESR

The rate coefficient for alkoxyamine C−O bond homolysis has been determined over a range of temperatures for both 2-tert-butoxy-1-phenyl-1-(1-oxy-2,2,6,6-tetramethylpiperidinyl)ethane (1) and a polystyrene−TEMPO (approximately 75 units) adduct using quantitative ESR. In a thermostated solution of the alkoxyamine species in toluene the nitroxide concentration was monitored via ESR. Measurements were performed under atmospheric conditions to create pseudo-first-order kinetics with oxygen to serve as a scavenger for the carbon-centered radicals. The acquired kinetic data for alkoxyamine 1 were compared with data obtained via nitroxide-exchange experiments using HPLC to monitor the alkoxyamine concentrations. The data for the polystyrene alkoxyamine were compared with literature data obtained by Fukuda via his GPC method. The quantitative ESR measurements proved to be an easy and more important an accurate method to determine values for the rate coefficient for alkoxyamine C−O bond homolysis. The Arrhenius pa...