Nitroxide-mediated photo-controlled/living radical polymerization of ethyl acrylate

The nitroxide-mediated photo-controlled/living radical polymerization of ethyl acrylate was attained using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator, and (4-tert-butylphenyl)diphenylsulfonium triflate as the photo-acid generator. The photopolymerization was performed in acetonitrile at room temperature by irradiation with a high-pressure mercury lamp. The molecular weight distribution of the resulting polymer decreased as the monomer concentration decreased. It was confirmed that the polymerization was controlled on the basis of the linear correlations for the first-order time-conversion plots and the plots of the molecular weight vs. the reciprocal of the initial concentration of the initiator, although the conversion–molecular weight plots did not show a completely linear correlation. The block copolymerization with methyl methacrylate accompanied by no deactivation of the growing polymer chain end supported the livingness of the polymerization.     

Nitroxide-mediated photo-living radical polymerization of methyl methacrylate in solution

The photoradical polymerization of methyl methacrylate (MMA) was performed in an acetonitrile solution at room temperature using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator, and (4-tert-butylphenyl)diphenylsulfonium triflate as the photo-acid generator. This solution polymerization showed a non-steady-state during the very early stage followed by a steady-state. The polymerization produced oligomers with several thousand molecular weights at a very low conversion under the non-steady-state. It was confirmed that the polymerization proceeded in accordance with a living mechanism under the steady-state based on the linear correlations for both the first-order time-conversion plots and the conversion–molecular weight plots. The molecular weight distributions of the polymers obtained in the steady-state were approximately 1.8. The block copolymerization with isopropyl methacrylate ( ⁱ PMA) demonstrated that the growing polymer chain ends of the MMA prepolymer were stabilized even at a high conversion and efficiently initiated the ⁱ PMA polymerization.     

Nitroxide-mediated photo-living radical polymerization of methyl methacrylate using (4-tert-butylphenyl)diphenyl-sulfonium triflate as a photo-acid generator

The photo-living radical polymerization of methyl methacrylate (MMA) was performed at room temperature using (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, and (4-tert-butylphenyl)diphenylsulfonium triflate ( ^t BuS) as the photo-acid generator. The livingness of the polymerization was confirmed on the basis of linear increases in the ln([MMA]₀/[MMA]_t) vs. time and in the molecular weight vs. the conversion. The molecular weight distributions of the resulting polymers were around 1.45. The polymerization rate was dependent both on the ^t BuS/MTEMPO and MTEMPO/r-AMDV molar ratios. Furthermore, it was found that the polymerization had a photo-latency because the polymerization was retarded by the interruption of the irradiation; however, it was accelerated again by further irradiation without deactivation of the growing polymer chain ends.     

Nitroxide-mediated photo-living radical polymerization of vinyl acetate

The photoradical polymerization of vinyl acetate was performed using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate (BAI). The MTEMPO/BAI system using 2,2’-azobis(isobutyronitrile) or 2,2’-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator did not succeed in controlling the molecular weight and produced polymers that showed a bimodal gel permeation chromatography with the broad molecular weight distribution. On the other hand, the polymerization using 1-(cyano-1-methylethoxy)-4-methoxy-2,2,6,6-tetramethylpiperidine and BAI proceeded by the living mechanism based on linear increases in the first order time–conversion and conversion–molecular weight plots. The molecular weight distribution also increased with the increasing conversion due to cloudiness of the solution as the polymerization proceeded. It was found that the polymerization had a photolatency because the propagation stopped by interruption of the irradiation and was restarted by further irradiation.     

Single electron transfer‐living radical polymerization of methyl methacrylate catalyzed by ytterbium powder

Zerovalent ytterbium (Yb) powder is firstly used as a catalyst in single electron transfer‐living radical polymerization of methyl methacrylate initiated by carbon tetrachloride in N, N‐dimethylformamide (DMF) and dimethyl sulfoxide, respectively. Polymerization proceeds in a “living”/controlled way as evidenced by kinetic studies and chain extension results, producing well‐defined polymers with controlled degree of polymerization and narrow molecular weight distribution. The apparent activation energy of polymerization in DMF is accounted to be 36.2 kJ/mol, and the energy of equilibrium state is calculated to be 13.9 kJ/mol. An increase in the concentration of Yb(0) yields a higher monomer conversion. It is observed that polymerization rate experiments a rapid increase in the presence of more polar solvent water, and increasing in the content of H₂O results in an increase in the apparent rate constant of polymerization, and a decrease in the molecular weight distribution. The reaction rate and molecular weight increase along with the decrease of DMF content. The effect of Yb(0) powder content, different ligands and concentration of initiator on the polymerization is also investigated. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

New efficient boron-containing initiators of methyl methacrylate radical polymerization

An efficient method for methyl methacrylate radical polymerization by tri-n-propyl-, triisopropyl-, and triisobutylborane ammonia complexes, including the addition of a boron-containing initiating agent into the monomer in air, was developed. An advantage of this method is that the reaction occurs at room temperature, requires no peroxide components, and leads to polymers with enhanced thermal stability.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2120–2125, October, 2004.     

Control of particle size in dispersion polymerization of methyl methacrylate

Poly(methyl methacrylate) (PMMA) particles ranging in diameter from 2 to 10 μm were prepared by dispersion polymerization. The effects of various polymerization parameters on the size and monodispersity were systematically investigated. The particle size was found to increase with increasing polymerization temperature, concentration and decomposition rate of the initiator, and solvency of the dispersion medium. It also increased with increasing concentration and molecular weight of the polymeric stabilizer, poly(vinyl pyrrolidone) (PVP). As the monomer concentration was increased from 5 to 20 wt %, a minimum was found in the particle size at a monomer concentration of 10 wt %. A costabilizer was found to be necessary for preparing monodisperse particles at stabilizer concentrations below 2 wt %. A recycling experiment showed that the consumption of PVP was quite small in each cycle and the residual materials in this system could be reused readily. © 1993 John Wiley & Sons, Inc.     

Nitroxide-mediated radical polymerization of acrylamide in water solution

This study shows that reasonably controlled nitroxide-mediated polymerization of acrylamide is achieved in pure water solution at 120 °C and high concentration (40 wt%), using a combination of a conventional hydrosoluble radical initiator (Vazo56) and a β-phosphonylated nitroxide, SG1. Moreover, some chain extensions can be performed from a polyacrylamide macroinitiator. Under these experimental conditions, we have demonstrated the conservation of the acrylamide structure without hydrolysis side reactions. The physico-chemical characterizations of polymers obtained from this method demonstrates that the controlled growing chain arises as a star-like shape from the hydrophobic core composed of SG1-functionalized polyacrylamide in the presence of a slight excess of SG1.     

Giant vesicles prepared by nitroxide-mediated photo-controlled/living radical polymerization-induced self-assembly

Giant vesicles with several-micrometer diameters were prepared by self-assembly induced by the nitroxide-mediated photo-controlled/living radical polymerization. The random block copolymerization of methyl methacrylate (MMA) and methacrylic acid (MAA) were performed using poly(methacrylic acid) (PMAA) as the prepolymer in an aqueous methanol solution to produce a PMAA-block-poly(MMA-random-MAA) random block copolymer (PMAA-b-P(MMA-r-MAA)). PMAA₁₉₅-b-P(MMA_0.817-r-MAA_0.183)₂₂₄ formed spherical vesicles with a 4.74 μm diameter and 0.108 μm wall thickness. A differential scanning calorimetry analysis demonstrated that the vesicles had a bilayer structure consisting of a hydrophilic PMAA surface and hydrophobic P(MMA-r-MAA) interface. The wet vesicles before air-drying were flexible and easily transformed by stress, whereas the dry vesicles were fragile and cracked. The vesicles in the solution were dissociated into much smaller vesicles by increasing the temperature. They were also transformed by a further temperature increase into hollow fibers and finally into membranes retaining the bilayer structure.     

Recent advances in controlled/living radical polymerization in emulsion and dispersion

Effective ways to conduct controlled/living radical polymerization (CRP) in emulsion systems are necessary for commercial latex production without significant modification of current industrial facilities. Conducting CRP in emulsion media is more complicated and more challenging than its application in homogeneous bulk. These challenges come from the intrinsic kinetics of emulsion polymerization. They include mass transport, slow chain growth mechanism, and exit of short radicals from polymeric particles. This review describes the recent developments of CRP in heterogeneous dispersion, including miniemulsion, microemulsion, dispersion, and especially emulsion. Various approaches for conducting emulsion CRP are detailed, including controlled seeded emulsion polymerization, nanoprecipitation, use of short oligomers as macroinitiators for in situ block copolymerization, and RAFT‐mediated self‐assembly. In addition many remaining challenges of the current methods barring wide spread industrial application of emulsion CRP are also suggested. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6983–7001, 2008     

Photo-living radical polymerization of methyl methacrylate by a nitroxide mediator

The novel photo-living radical polymerization was determined using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) and bis(alkylphenyl)iodonium hexafluorophosphate (BAI) as the photo-acid generator. The polymerization of methyl methacrylate was performed using azobisisobutylonitrile as an initiator in the presence of MTEMPO and BAI at room temperature by irradiation with a high-pressure mercury lamp to produce poly(methyl methacrylate) with a comparatively narrow molecular weight distribution (M _w/M _n?=?1.3–1.7). The polymerization proceeded by a living mechanism based on the fact that the first-order time-conversion plots linearly increased. A linear increase in the plots of the molecular weight versus the conversion also supported the living nature of the polymerization. It was found that MTEMPO had an interaction with the propagation chain end to control the molecular weight, while BAI weakened the interaction of MTEMPO with the propagation chain end to reduce the molecular weight distribution and polymerization time.     

Self-initiated atom transfer radical polymerization of methyl methacrylate in cyclohexanone

The self-initiated atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in cyclohexanone (CHO) in the presence of CuCl₂/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) is reported. The linear semilogarithmic plot of ln([M]₀/[M]) vs time, the linear increase of number-average molecular weight (M_n) with conversion, and rather narrow molecular weight distributions (MWDs) have been observed, which are in agreement of the characteristics of living/controlled polymerization. The NMR spectrum revealed the existence of terminal chlorine. The chain extension further proved the living characteristic. The polymerization can only be successful using CHO as the solvent, and is well controlled at the temperature as low as 50 °C. The effects of ligand, solvent, temperature and monomer to catalyst ratio are all discussed.     

Effects of chlorophosphines on the radical polymerization of methyl methacrylate

The effect of chlorophosphines (phosphorus trichloride, dichlorophenylphosphine, chlorodiphenylphosphine) on the radical polymerization of methyl methacrylate was investigated in benzene solution. The polymerization was carried out at 50°C by the standard solution method, α,α′-azobisisobutyronitrile being used as an initiator. These chlorophosphines accelerated the polymerization of methyl methacrylate but did not affect the rate of decomposition of α,α′-azobisisobutyronitrile. Ultraviolet and infrared spectral data suggested that the acceleration effect was due to the complex formation of methyl methacrylate with each chlorophosphine. From the result of a copolymerization with styrene, it was found that the reactivity of methyl methacrylate monomer increased in the presence of dichlorophenylphosphine.     

Effect of azoinitiators on nitroxide-mediated photo-living radical polymerization of methyl methacrylate

In order to clarify the initiator factor dominating the molecular weight distribution of the resulting polymer, the nitroxide-mediated photo-living radical polymerization of methyl methacrylate was performed using eight different kinds of azoinitiators: i.e., 2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 1,1′-azobis(cyclohexane-1-carbonitrile), racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile), meso-(2RS,2′SR)-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl 2,2′-azobis(2-methylpropionate), and 2,2′-azobis(N-butyl-2-methylpropionamide). The bulk polymerization was carried out at room temperature for 3 h using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate as the photo-acid generator. All the initiators provided a molecular weight distribution below 1.7 for the MTEMPO/initiator ratio of 2, although at the ratio of unity, about half of the initiators produced the molecular weight distribution around 2.3–3.4. The UV analysis revealed that the initiators having a higher ε value tended to more strictly control the molecular weight and provide a higher initiator efficiency. The half-lives of the initiators had little effect on the molecular weight control and initiator efficiency.     

Ruthenium carborane complexes in the controlled radical polymerization of methyl methacrylate

New initiators based on closo-and exo-nido-ruthenacarboranes with phosphine and diphosphine ligands were proposed as chain growth regulators. They allow conducting the controlled synthesis of poly(methyl methacrylate) under radical initiation conditions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 85–89, January, 2006.