Nitroxide-mediated radical polymerization of styrene in miniemulsion: model studies of alkoxyamine-initiated systems

A mathematical model has been developed to describe the behavior of the nitroxide-mediated miniemulsion polymerization (NMMP) of styrene initiated by alkoxyamine initiators. The model includes mechanisms describing reactions in the aqueous and organic phases, particle nucleation, the entry and exit of oligomeric radicals, and the partitioning of nitroxide and styrene between the aqueous and organic phases. The influence of nitroxide partitioning on the polymerization kinetics was examined by modeling systems initiated by the alkoxyamines BST and hydroxyl-BST; BST and hydroxyl-BST are benzoylstyryl radicals terminated by the nitroxides TEMPO and 4-hydroxyl-TEMPO, respectively.Predicted monomer conversions, number average molecular weights and polydispersities were in agreement with experimentally measured values. Simulations and mathematical analysis showed that the rate of styrene NMMP is not strongly influenced by the partitioning properties of TEMPO and 4-hydroxyl-TEMPO because of the complex interaction between reaction equilibrium, phase equilibrium, termination and thermal initiation. However, in the absence of styrene thermal initiation, nitroxide partitioning was found to have a significant influence on the polymerization kinetics. The model was also used to make quantitative estimates of: the population of active and dormant polymer radicals derived from both alkoxyamine initiators and thermal initiation; the population of dead polymer chains; and the number molecular weight distributions of living and dead polymer chains.     

Nitroxide-mediated radical polymerization in nanoreactors: Factors influencing compartmentalization effects on bimolecular termination

Modeling and simulations of nitroxide-mediated radical polymerization (NMP) in dispersed systems have been performed to elucidate what factors dictate the magnitude of the segregation effect on bimolecular termination between propagating radicals generated from alkoxyamine activation. The reduction in termination rate due to segregation cannot be predicted merely based on the average number of propagating radicals per particle . This is because the magnitude of the segregation effect is also governed by the distribution of propagating radicals between particles, which is influenced by both the termination (k_t) and the deactivation (k_deact) rate coefficients. The results have implications with regards to improvement of livingness (end-functionality) in NMP by exploitation of particle size, and are expected to apply (qualitatively) to other controlled/living systems based on the persistent radical effect (e.g. atom transfer radical polymerization).     

Nitroxide-mediated polymerization to form symmetrical ABA triblock copolymers from a bidirectional alkoxyamine initiator

Bidirectional alkoxyamine 2 was synthesized and used as the initiator in the polymerization of styrene (S), n-butyl acrylate (nBA), t-butyl acrylate (tBA), isoprene (I), and dimethylacrylamide (DMA). A variety of symmetrical ABA triblock copolymers were prepared, ranging in size from 5 to 59 kDa. Kinetics studies and gel permeation chromatography (GPC) confirmed the “living” nature of these polymerizations. Trifluoroacetic acid was used to convert the PtBA blocks of these polymers into poly(acrylic acid) (PAA) blocks, forming ABA amphiphilic triblock copolymers. AFM images of PAA-b-PnBA-b-PAA and PAA-b-PS-b-PAA triblock copolymers ionized by the addition of 2,2′-(ethylenedioxy)bis(ethylamine) show evidence of self-assembly.     

Solvent effect on TEMPO-mediated living free radical dispersion polymerization of styrene

Living free radical dispersion polymerization of styrene in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 2,2-azobisisobutyronitrile (AIBN) was conducted in various glycol media having different solubility parameters. Genuine heterogeneous polymerization nature was observed from the initial stage of the reaction. The solubility parameter of the media was found to significantly influence the living characteristics of the polymerization including the polymerization kinetics and the molecular weight evolution at a fixed concentration of TEMPO. A grafting of stabilizer molecules onto monomer and a possible partitioning of TEMPO between the continuous medium and particle phase were postulated for the deviation of the experimentally obtained molecular weight from the calculated value. For a poor medium for styrene, such as diethylene glycol, the living nature was achieved by increasing the amount of TEMPO. The polystyrene (PS) microsphere was well obtained in all tested glycol media and the average size was increased with enhanced affinity of the media to styrene and with decreasing concentration of TEMPO.     

Surface‐initiated atom transfer radical polymerization (ATRP) of styrene from silica nanoparticles under UV irradiation

Diethyldithiocarbamyl‐modified silica nanoparticles were prepared and used as macroinitiator for the surface‐initiated atom transfer radical polymerization (SI‐ATRP) of styrene under UV irradiation. Well‐defined polymer chains were grown from the nanoparticle surfaces to yield particles composed of a silica core and a well‐defined, densely grafted outer PS layer with a mass ratio of styrene to silica, or percentage grafting, of 276.3% after an UV irradiation time of 5 h. Copyright © 2004 Society of Chemical Industry     

Nitroxide-mediated radical polymerization of styrene: Experimental evidence of chain transfer to monomer

Chain transfer to monomer during nitroxide-mediated radical polymerization of styrene has been investigated for 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (SG1) mediated polymerizations at 125 and 110 °C, respectively. A novel technique employing a fluorescence-labelled polystyrene-TEMPO macroinitiator enabled separate detection of the total chain distribution and the distribution of chains containing the original macroinitiator, thus directly confirming the presence of chains not containing macroinitiator. Chain transfer to monomer results in a low molecular weight tail, which can be very much pronounced, in particular in the number distributions. Quantitative analysis of the total number of chains in both the TEMPO and the SG1 systems, correcting for the contribution of thermal initiation of styrene, yielded chain transfer to monomer constants in agreement with the literature.     

Grafting of water‐soluble sulfonated monomers onto functionalized fumed silica nanoparticles via surface‐initiated redox polymerization in aqueous medium

Sulfonated polymer/fumed silica hybrid nanoparticles were prepared via surface‐initiated free radical polymerization of 2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid (PAMPS‐g‐FSN), styrene sulfonic acid sodium salt (PSSA‐g‐FSN) and vinyl sulfonic acid sodium salt (PVSA‐g‐FSN) from the surface of aminopropyl‐functionalized fumed silica nanoparticles (AFSNs) dispersed in aqueous medium. Cerium(IV) ammonium nitrate/nitric acid and sodium dodecyl sulfate were used as redox initiator and stabilizer respectively. AFSNs were prepared by covalently attaching 3‐aminopropyltriethoxysilane onto the surface of fumed silica nanoparticles. Sulfonated monomers (AMPS, SSA or VSA) were then grafted onto the AFSNs ultrasonically dispersed in water via redox initiation at 40 °C. Structure, thermal properties, particle size and morphology of the AFSNs and PAMPS‐g‐FSN, PSSA‐g‐FSN and PVSA‐g‐FSN hybrid nanoparticles were characterized by Fourier transform infrared spectroscopy, TGA, SEM, transmission electron microscopy (TEM) and dynamic light scattering (DLS). The results indicated that the sulfonated monomers were successfully grafted onto the fumed silica nanoparticles. Grafting amounts of the sulfonated polymers onto the fumed silica nanoparticle surface were estimated from TGA thermograms to be 59%, 13% and 29% for the PAMPS, PSSA and PVSA, respectively. From SEM, TEM and DLS analysis, polymer‐grafted fumed silica nanoparticles with an average diameter smaller than 70 nm and a (semi‐) spherical shape were observed. A significant bimodal particle size distribution was observed only for the PAMPS‐g‐FSN with average diameters of 39.6 nm (84.1% per number) and 106 nm (15.9% per number). The hydrophilic sulfonated polymer/grafted fumed silica obtained from the redox graft polymerization gave a stable colloidal dispersion in acidic aqueous medium. Copyright © 2012 Society of Chemical Industry     

Comparison of chemical and enzymatic emulsion polymerization of styrene

For the reasonable comparison of chemical and enzymatic polymerization, manganese(III) acetate and manganese peroxidase (MnP)‐catalyzed synthesis of polystyrenes were chosen and studied with respect to the yield and the polydispersity of polymer particles. MnP allowed the production of polystyrene in a higher yield and higher polydispersity (82.34% and 2.61) than manganese(III) acetate (77.90% and 1.75). This is believed to be due to a gradual supply of initiator radicals via a catalytic cycle in MnP‐catalyzed polymerization, whereas radicals are generated all at once in Mn(III)‐mediated polymerization. NMR spectroscopy revealed that atactic polystyrene was produced by emulsion polymerization using Mn(III) or MnP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nitroxide‐mediated polymerization of styrene initiated from the surface of montmorillonite clay platelets

Polymer‐grafted montmorillonite (MMT) hybrid composites which possess a hard backbone of MMT and a soft shell of brush‐like polystyrene (PSt) were prepared via “grafting from” strategy based on nitroxide‐mediated radical polymerization (NMRP) using 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) as mediator. Three steps were used to graft PSt chains to the surface of MMT: anchoring of methacrylatoethyl trimethyl ammonium chloride (DMC) onto the surface of MMT by ion exchange reaction first. And then, the surface alkoxyamine initiator was produced in a one‐step process by reacting simultaneously TEMPO, BPO, and DMC in the presence of MMT. Next, PSt chains with controlled molecular weights and polydispersities were grown from the alkoxyamine functionalized MMT surface. The prepared PSt‐g‐MMT hybrid particles have been extensively characterized by FTIR, XPS, XRD, TGA, TEM, respectively. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of binary mixed homopolymer brushes by combining atom transfer radical polymerization and nitroxide-mediated radical polymerization

This communication describes a novel strategy to synthesize binary mixed homopolymer brushes from mixed self-assembled monolayers (SAMs) on silica substrates by combining atom transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMRP). Mixed SAMs terminated by ATRP and NMRP initiators were prepared by coadsorption of two corresponding organotrichlorosilanes from toluene solutions. Mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes were synthesized by ATRP of MMA at 80 °C followed by NMRP of styrene at 115 °C. Corresponding ‘free’ initiators were added into the solutions to control the polymerizations. We have found that the brush thickness increases with molecular weight in a nearly linear fashion. For a series of binary brushes consisting of PMMA of molecular weight of 26,200 and PS of various molecular weights, we have observed a transition in water contact angles with increasing PS molecular weight after CH₂Cl₂ treatment. Moreover, binary mixed polymer brushes with comparable molecular weights for two grafted polymers undergo reorganization in response to environmental changes, exhibiting different wettabilities.     

Fractal dimension of fumed silica: Comparison of light scattering and electron microscope methods

Due to the enormous increase in nanopowder production, it becomes necessary to find and develop adapted characterization techniques. In the case of nanostructured agglomerates, the structure of these particles has a direct impact on flowing, and handling, but also on end-use final product properties. In this work, a fractal approach is used to characterize the agglomerate structure using two different, commercially available and widely used, methods: static light scattering (SLS) and image analysis of scanning electron microscope (SEM) photographs of the aggregates. Fumed silica aggregates are used for this comparison. The results by image analysis show that fumed silica aggregates have a two-level structure, made of compact aggregates of open aggregates of nanoparticles. This structure is not detected by SLS. For such a structure, SLS seems to be less accurate than image analysis method, although it could be an interesting technique in more simple cases, since it is a much less time-consuming technique.     

Combination of fumed silica with carbon black for simultaneously improving the thermal stability,flame retardancy and mechanical properties of polyethylene

The combination of fumed silica (SiO₂) with carbon black (CB) for improving thermal stability, flame retardancy and mechanical properties of linear low density polyethylene (LLDPE) was investigated. The temperature at the maximum weight loss rate of LLDPE was dramatically increased by 92 °C, and the peak value of heat release rate measured by cone calorimeter was significantly reduced by 80.7%. The improved thermal stability and flame retardancy of LLDPE were partially attributed to the formation of a percolated network structure in LLDPE matrix, and partially to the accelerated oxidation crosslinking reaction of LLDPE or other radicals by CB and SiO₂. More importantly, although both SiO₂ and CB were used without any pre-treatment, ternary LLDPE nanocomposites showed much higher mechanical properties compared to those of neat LLDPE. This was ascribed to good dispersions of two kinds of nanoparticles and strong matrix-nanoparticle interfacial interactions with LLDPE matrix.     

两种气相法白炭黑水分测定方法的比较

使用卡尔·菲休库仑法和重量法分别测定了两种气相法白炭黑微粉中的水分质量分数,为白炭黑水分测定提供了技术途径,并对两种测定方法进行了比较。研究结果表明卡尔·菲休库仑法与重量法皆可用于固体物质的水分测定,且库仑法精度较高,更适用于微量水的测定,而重量法适用于较高水分质量分数的测定。     

气相法白炭黑表面处理的新方法

在辅助处理剂的存在下，六甲基二硅氮烷（HMDZ）可以很好地对气相法白炭黑进行表面处理。处理后的气相法白炭黑适合于制作双组分室温硫化（RTV-2）硅橡胶；讨论了气相法白炭黑与HMDZ的配比，反应温度，反应时间等对RTV-2硅橡胶性能的影响。最佳反应条件为：100份气相法白炭黑用17份HMDZ处理。反应温度120℃，反应时间60min以上，采用处理后的白炭黑填充的RTV硅橡胶具有优良的机械性能和操作性能。     

Ability of nitrones of various structures to control the radical polymerization of styrene mediated by in situ formed nitroxides

The ability of several nitrones to control the radical polymerization of styrene at 110 °C has been investigated by high-throughput experimentation. The nitrone/free radical initiator pair dictates the structure of the nitroxide and the alkoxyamine formed in situ, which determines the position of the equilibrium between the active and the dormant species operating in the nitroxide-mediated polymerization. For the styrene polymerization to be controlled, the nitrone must be reacted with 2,2′-azo-bis-isobutyronitrile (AIBN) at 85 °C, prior to addition of styrene and polymerization at 110 °C. The effect of the nitrone structure on the kinetics of the styrene polymerization has been emphasized. Amongst all the nitrones tested, those of the C-phenyl-N-tert-butylnitrone (PBN) type are the most efficient in terms of polymerization rate, control of molecular weight and polydispersity. Electrophilic substitution of the phenyl group of PBN by either an electrodonor or an electroacceptor group has only a minor effect on the polymerization kinetics. Importantly, the polymerization rate is not governed by the thermal polymerization of styrene but by the alkoxyamine formed in situ during the pre-reaction step. The initiation efficiency is, however, very low, consistent with a limited conversion of the nitrone into nitroxide and alkoxyamine.     

A comparative study on effects of two kinds of polymerization methods on grafting of polymer onto silica surface

In this article we present the result of a comparative study of two kinds of polymerization methods—solution polymerization (sol. poly.) and dispersion polymerization (dis. poly.) for grafting polymer onto silica. As a model for the grafting polymerization reaction, styrene was chosen as the monomer and azo diisobutyronitrile (AIBN) as the initiator. The study aims at supplying theoretical reference for better selecting polymerization method to graft polymer on the silica particle surface. First, monolayers of 3‐methacryloylpropyl trimethoxysilane were chemically bonded onto the surfaces of micrometer‐sized silica gel particles, and so double bonds were immobilized onto the silica surface. Second, the copolymerizations between the immobilized double bonds and the monomer styrene were carried out, homopolymerizations of styrene followed, and finally polystyrene was grafted to the silica surfaces. Two kinds of polymerization methods, sol. poly. and dis. poly., were adopted respectively, and the effects of polymerization methods on grafting process were examined mainly. At the same time, the effects of different polymerization conditions on the grafting degree were researched. It was found that in the dis. poly. system the grafting degree is obviously higher than that in the sol. poly. system under the same polymerization conditions, and the grafting degree can go up to 47%, i.e. 47g/100g. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5808–5817, 2006     

Effect of fumed silica properties on the thermal insulation performance of fibrous compact

The particle properties play an important role in developing advanced thermal insulation board made by fumed silica. In this study, the effect of hydrophilic or hydrophobic on the thermal conductivity and fracture strength of the resultant insulation board was investigated. A higher thermal conductivity and higher strength were obtained in using hydrophilic particles. Very low thermal conductivity, below 0.02 W/m K, was achieved in using hydrophobic particles. The difference of thermal conductivity was mainly dominated by solid part. The decrease of thermal conductivity and increase of fracture strength were related to the bonding condition between particles in the boards.     

Graft polymerization of vinyl monomers initiated by peroxycarbonate groups introduced onto silica surface by Michael addition

The introduction of peroxycarbonate groups onto a silica surface and the graft polymerization of vinyl monomers initiated by peroxycarbonate groups introduced onto a silica surface were investigated. The introduction of peroxycarbonate groups onto a silica surface was achieved by Michael addition of amino groups introduced onto the silica surface to t‐butylperoxy‐2‐methacryloyloxyethylcarbonate (HEPO). The amount of peroxycarbonate groups was determined to be 0.17 mmol/g. The graft polymerization of various vinyl monomers such as styrene (St), N‐vinyl‐2‐pyrrolidinone (NVPD), and 2‐hydroxyethyl methacrylate (HEMA) was initiated by peroxycarbonate groups introduced onto the silica surface to give the corresponding polymer‐grafted silicas. The percentage of poly(St)‐grafting reached about 120% after 5 h. This means that 1.20 g of poly(St) is grafted onto 1.0 g of silica. The surface of poly(St)‐grafted silica shows a hydrophobic nature, but the surfaces of poly(NVPD) and poly(HEMA)‐grafted silica show a hydrophilic nature. Furthermore, the poly(St)‐grafted silica was found to give a stable colloidal dispersion in a good solvent for the grafted polymer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1491–1497, 1999     

Atom transfer radical polymerization of styrene initiated by bromoacetylated syndiotactic polystyrene macroinitiator

Atom transfer radical polymerization of styrene was conducted with bromoacetylated syndiotactic polystyrene as macroinitiator and copper bromide combined with N,N,N′,N′,N′‐pentamethyldiethylenetriamine as catalyst. A two‐stage process has been developed to synthesize the macroinitiator. First, syndiotactic polystyrene (sPS) was functionalized in the side phenyl rings with acetyl groups using the Friedel–Crafts reaction; second, the acetyl groups were converted to bromoacetyl groups by an acid‐catalyzed halogenation reaction. The initiator was found to be active in the polymerization of styrene, leading to the production of graft chains with well‐defined structure. The molecular weight and molecular weight distribution of the graft chains were determined using gel permeation chromatography after cleaving from the sPS backbone using peroxide acid oxidation followed by hydrazine‐catalyzed hydrolysis. The results indicated that the polymerization process was characteristic of a ‘living’ nature. Copyright © 2007 Society of Chemical Industry     

苯乙烯的间歇-半连续RAFT细乳液聚合

进行了苯乙烯的间歇-半连续RAFT细乳液聚合,考察了半连续段的起点、单体滴加速率及最终胶乳固含量的影响。结果发现：从最终胶乳的稳定性考虑,半连续聚合的起点选择在间歇聚合的高转化率时期更好;若综合考虑胶乳的稳定性、分子量及其分布、固含量、乳化剂及共稳定剂在胶乳中的残留率等因素,半连续聚合的起点可适当提前,但必须在间歇聚合成核期结束后。过早容易引起乳液的失稳;过迟会延长反应时间,降低聚合物的制备效率,导致死聚物链含量升高。聚合体系的稳定性与胶乳的固含量密切相关,最终固含量不宜超过40％。采用间歇-半连续二段聚合工艺可以制得窄分子量分布(PDI＝～1.3),低乳化剂及共稳定剂残留量（～1.5%,质量）的高分子量聚合物(≈8×10⁴g•mol^-1)。