Alkoxyamine Concentrations Research Articles

New Class of Alkoxyamines for Efficient Controlled Homopolymerization of Methacrylates.

Despite significant efforts, the design of alkoxyamines for polymerization of methacrylic monomers in a well-controlled fashion with good retention of the active chain ends remains a challenge. Herein, the facile synthesis of several alkoxyamines, which are capable of achieving this long sought-after goal, is reported. Controlled homopolymerization of methyl methacrylate is achieved as determined by a linear increase in molecular weight with conversion and first-order rate plots for various alkoxyamine concentrations. The versatility of the alkoxyamines is further exemplified by the ability to control the homopolymerization of styrene and by synthesis of a block copolymer of a second methacrylate in an efficient chain extension process.

Nitroxide-Mediated Polymerization at Elevated Temperatures.

A new alkoxyamine based on a highly thermally stable nitroxide is used for the controlled polymerization of styrene and butyl acrylate at temperatures up to 200 °C. High monomer conversions are reached in a few minutes with a linear increase in polymer chain-length with conversion, a final dispersity (Đ) of ∼1.2, and successful chain-extension of the resulting material. The alkoxyamine concentration was altered to target various chain lengths, with autopolymerization dictating the polymerization rate of styrene regardless of alkoxyamine concentration. Controlled polymerization of methacrylate monomers and acrylic acid was successful with the addition of styrene. The new material opens the possibility to increase the range of specialty products made for applications in coatings, inks, overprint varnishes, and adhesives.

Improving the control of styrene polymerization at 60 °C using a dialkylated α-hydrogenated nitroxide

A new dialkylated α-hydrogenated linear nitroxide and the corresponding 1-phenylethyl alkoxyamine were synthesized in two and three steps, respectively. The alkoxyamine was involved in the polymerization of styrene at 60 °C, and the in situ concentration of nitroxide was monitored by electron spin resonance spectroscopy. The enhanced characteristics of these new alkylated alkoxyamine and nitroxide (k = 1.5 × 10−4 s−1 and k = 5.7 × 104 L mol−1 s−1) yielded a monomer consumption one order of magnitude higher than styrene thermal polymerization. This resulted in well-defined polystyrenes up to 70,000 g mol−1 and the observation of a control occurring through the establishment of the radical persistent effect, that is, ln([M]0/[M]) = t2/3. Experimentally determined kinetic constants were involved in PREDICI modelings to investigate the influence of temperature and initial alkoxyamine concentration on the kinetics as well as on the livingness and the controlled character of the polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Nitroxide-Mediated Living/Controlled Radical Polymerization of N,N-Diethylacrylamide

Well-defined poly(N,N-diethylacrylamide)s were synthesized for the first time by nitroxide-mediated polymerization. The polymerizations were first performed in toluene solution, using BlocBuilder, a tertiary alkoxyamine based on the nitroxide SG1 (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide), as an initiator without initial free nitroxide. High monomer conversions were reached, up to completion and the number-average molar masses increased linearly with conversion. However the polydispersity indices (PDI) remained quite high, despite a constant diminution with increasing conversion until a value of 1.9 at high conversion. This could be corrected by the addition of free SG1 at the onset of the reaction, leading to PDI of 1.2 at 93% conversion in the best case. Various alkoxyamine concentrations were used to target different molar masses. The presence of SG1 functionality at the ω-end of the polymer chains was evidenced by size-exclusion chromatography coupled to electospray ionizatio...

Kinetics of nitroxide-controlled radical polymerization during the non-stationary state

The activation–deactivation equilibrium of nitroxide-controlled radical polymerization of styrene at 123°C was investigated. For this purpose the reaction solution was examined time dependently during the initial phase of the polymerization by using an SEC column combination providing a very good separation of the low-molecular weight species. By time-dependent measurement of the alkoxyamine concentration the activation rate of the alkoxyamines PhEt-TIPNO (N-tert-butyl-N-(2-methyl-1-phenyl-propyl)-O-(1-phenyl-ethyl)-hydroxylamine) kact=3.2×10−3s−1 and PhEt-BIPNO (N-tert-butyl-N-(1-isopropyl-2-methyl-propyl)-O-(1-phenyl-ethyl)-hydroxylamine) kact=6.4×10−3s−1 can be determined directly.Considering the Persistent Radical Effect theory, the measurement of the free nitroxide concentration allows to determine the pseudo-equilibrium constant of dissociation/combination between dormant and active species for polystyryl-TIPNO and polystyryl-BIPNO, K=7.5×10−9mol/L and 1.08×10−8mol/L, respectively.

Nitroxide‐mediated living free‐radical polymerization of styrene: A systematic study of the variation of the alkoxyamine concentration

AbstractThe effect of the variation of the alkoxyamine concentration on the conversion and polydispersity of the nitroxide‐mediated living free‐radical polymerization of styrene is discussed. Four different alkoxyamines (1–4) have been used for these studies. For an alkoxyamine with a small equilibrium rate constant (K), such as styryl–TEMPO 2, the conversion is governed by the autopolymerization of styrene. For efficient alkoxyamines 1, 3, and 4, the conversion at high alkoxyamine concentrations is higher than the conversion obtained by autopolymerization. At high alkoxyamine concentrations, the conversions vary to a small extent for all the alkoxyamines studied. As long as the conversion remains high, the polydispersity index is small. In addition, simulations of polymerizations with a program for modeling nonlinear dynamics are discussed. Polymerizations with efficient alkoxyamines at high alkoxyamine concentrations are well described by the kinetic scheme applied. K for alkoxyamines 1 and 4 has been estimated with the simulations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3342–3351, 2004

First Nitroxide-Mediated Controlled Free-Radical Polymerization of Acrylic Acid

Controlled poly(acrylic acid) homopolymers were synthesized for the first time by direct nitroxide-mediated polymerization of acrylic acid. The polymerizations were performed in 1,4-dioxane solution at 120 °C, using an alkoxyamine initiator based on the N-tert-butyl-N-(1-diethyl phosphono-2,2-dimethyl propyl) nitroxide, SG1. The kinetics were controlled by the addition of free nitroxide at the beginning of the polymerization and the optimal amount was 9 mol % with respect to the initiator. In this case, whatever the initiator concentration, all polymerizations exhibited the same rate and conversion reached 85−90% within 5 h. Although the rate constant of propagation of acrylic acid is very large, its reactivity is moderated by a low activation−deactivation equilibrium constant between active macroradicals and SG1-capped dormant chains. Various alkoxyamine concentrations were investigated to target different molar masses. At high initiator concentrations, the number-average molar mass, Mn, increased linear...

Kinetic study of the nitroxide‐mediated controlled free‐radical polymerization of n‐butyl acrylate in aqueous miniemulsions

AbstractThe controlled free‐radical homopolymerization of n‐butyl acrylate was studied in aqueous miniemulsions at 112 and 125 °C with a low molar mass alkoxyamine unimolecular initiator and an acyclic β‐phosphonylated nitroxide mediator, N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl) nitroxide, also called SG1. The polymerizations led to stable latices with 20 wt % solids and were obtained with neither coagulation during synthesis nor destabilization over time. However, in contrast to latices obtained via classical free‐radical polymerization, the average particle size of the final latices was large, with broad particle size distributions. The initial [SG1]0/[alkoxyamine]0 molar ratio was shown to control the rate of polymerization. The fraction of SG1 released upon macroradical self‐termination was small with respect to the initial alkoxyamine concentration, indicating a very low fraction of dead chains. Average molar masses were controlled by the initial concentration of alkoxyamine and increased linearly with monomer conversion. The molar mass distribution was narrow, depending on the initial concentration of free nitroxide in the system. The initiator efficiency was lower than 1 at 112 °C but was very significantly improved when either a macroinitiator was used at 112 °C or the polymerization temperature was raised to 125 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4410–4420, 2002

Synthesis of Six-Arm Star Polymer by Nitroxide-Mediated “Living” Radical Polymerization

A dendritic multifunctional initiator with 6 TEMPO-based alkoxyamine moieties was prepared from 4-bromophenylethylbenzene in 7 steps and 6-arm star polymers were prepared by the radical polymerization of styrene (St) using the dendritic alkoxyamine as an initiator. The St polymerizations were carried out at 120°C using the dendritic alkoxyamine concentrations of 5.0, 12.8, and 18.8 mmol L -1 When the alkoxyamine concentration was 5.0 mmol L -1 , the polydispersity indexes of the resulting star polymers increased with conversion and that of the star polymer at 72% conversion was 1.59. On the other hand, when the alkoxyamine concentrations were 12.8 and 18.8 mmol L -1 , the polymerization was well controlled to give star polymers with a low polydispersity index even at high conversion. The side reactions disturbing the living fashion were discussed on the basis of the SEC elution curves of the star polymers obtained.

Determination of Alkoxyamine Concentrations in Nitroxyl-Mediated Styrene Polymerization Products

A fluorescence method is presented for measuring the concentration of TEMPO-terminated chains in polystyrene samples prepared by controlled free radical polymerization. Exchange of TEMPO with a naphthoyloxy derivative is used to fluorescence-label those alkoxyamine-terminated polymer chains that are capable of further styrene addition. Determination of the fluorophore concentration in the exchanged polymer provides an accurate assessment of the concentration of alkoxyamine-terminated chains within a given sample. Fractionation of samples by gel permeation chromatography and simultaneous analysis by differential refractive index and fluorescence provides a measure of the distribution of nitroxyl-terminated chain ends as a function of molecular weight. This method is demonstrated on a series of polystyrene samples synthesized under various polymerization conditions.

Synthesis of a New Stable β-Sulfinyl Nitroxide and the Corresponding Alkoxyamine for Living/Controlled Radical Polymerization of Styrene: Kinetic and ESR Studies

The syntheses of a new nitroxide bearing a sulfoxide group at the β-position (with respect to nitrogen) and the corresponding N-β-sulfinyl alkoxyamine are reported. Styrene was polymerized in bulk in the presence of this new alkoxyamine. The polymerization satisfies the usual criteria of a living/controlled radical polymerization, with a linear increase of molecular weight vs yield and a constant transient radical concentration with time. However, the polymerization rates were independent of alkoxyamine concentration but much higher than the thermal polymerization ones: Rp/Rth = 2.6 at 90 °C, 3.7 at 100 °C, and 3.5 at 110 °C. Kinetic and ESR studies showed that both transient and persistent radical concentrations do not follow the corresponding theoretical evolutions with time but reach stationary states. The different rate constants (kd and kc) and corresponding activation energies were estimated, showing that the sulfoxide group has a large effect mainly on the combination reaction, the value of which is unusually low for such system (kc ≈ 105 L mol-1 s-1).

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...