Compartmentalization and nitroxide partitioning in NMP in dispersed systems have been investigated by modeling and simulations. Compartmentalization comprises the segregation effect on termination and the confined space effect on deactivation. Under certain conditions, it is possible to obtain an improvement in both control and livingness. The particle size threshold for compartmentalization, decreases with any system change that leads to a decrease in the number of propagating radicals and/or nitroxides per particle, and vice versa. There is direct competition between the confined space effect on deactivation and nitroxide exit–the more water‐soluble the nitroxide, the weaker the confined space effect. Nitroxide partitioning leads to an increase in polymerization rate and loss in control/livingness.
Summary: Simulations based on the kinetics and mechanism of nitroxide‐mediated free radical polymerization (NMP) have been carried out in order to understand the hitherto largely unexplained effects (or lack thereof) of nitroxide partitioning in aqueous miniemulsion NMP. The focus has been on the miniemulsion NMP of styrene mediated by TEMPO and 4‐hydroxy‐TEMPO, two nitroxides with very similar activation‐deactivation equilibria, but very different organic phase‐aqueous phase partition coefficients. The general conclusion is that the organic phase propagating radical and nitroxide concentrations are unaffected by the partition coefficient in the stationary state, but the rate of polymerization and the extent of bimolecular termination increase with increasing nitroxide water solubility in the pre‐stationary state region. Specific NMP systems are, therefore, affected differently by nitroxide partitioning depending on whether polymerization predominantly occurs in the stationary state or not, which in turn is governed mainly by the activation‐deactivation equilibrium constant and the rate of thermal initiation.
Simulated organic‐phase propagating radical concentrations in the presence of thermal initiation for TEMPO‐mediated miniemulsion free radical polymerization of styrene for different nitroxide partitioning coefficients at 125 °C. 相似文献
Summary: 2,2,6,6‐Tetramethylpiperidinyl‐1‐oxy (TEMPO)‐mediated radical polymerization of styrene in aqueous miniemulsion at 125 °C using sodium dodecylbenzenesulfonate and poly(vinyl alcohol), respectively, as colloidal stabilizers has been investigated. The particle size had a dramatic effect on the polymerization process. Decreasing particle size led to a markedly higher polymerization rate, but less control and a lower degree of livingness. For particles with diameters greater than approximately 170 nm, the polymerization behavior was essentially the same as in the corresponding bulk system. By varying the particle size within an appropriate range, it is possible to tune the polymerization such that the polymerization rate is increased while still maintaining reasonable control and livingness.
Summary: The controlled/living radical polymerizations of methyl acrylate in 50% v/v of an ionic liquid initiated by the alkoxyamine generated in situ from 4‐oxo‐2,2,6,6‐tetramethyl‐1‐piperidinyl‐N‐oxyl (4‐oxo‐TEMPO) and 2,2′‐azoisobutyronitrile (AIBN) at 140–155 °C are reported. The number‐average molecular weights increased linearly with conversion, and polydispersity indices are approximately 1.4 in the best case. The rates of polymerization were greater than in anisole, and similar to the rate of spontaneous polymerization in the ionic liquid.
(filled symbols) and (open symbols) vs. conversion for the MA polymerization in the presence of [4‐oxo‐TEMPO]/[AIBN] (2.8:1) in 50% v/v anisole with 0.03 M AIBN (squares) and 50% v/v [hmim][PF6] with 0.03 M AIBN (circles), and 0.06 M AIBN (triangles). 相似文献
Summary: The nitroxide‐mediated controlled/living free radical copolymerization of styrene and divinylbenzene using a polystyrene‐TEMPO macroinitiator in aqueous miniemulsion and in bulk have been investigated. The crosslink densities were estimated based on the content of pendant vinyl groups as determined by 1H NMR. Considerably lower crosslink densities were revealed in the miniemulsion than in the corresponding bulk system. The rate of polymerization in the miniemulsion increased with decreasing particle size, and was significantly higher than in bulk.
Crosslink density for the TEMPO‐mediated free radical copolymerization of S(1) and DVB(2) (f = 0.99, f = 0.01) at 125 °C in bulk (□) and in miniemulsions with dn = 585 nm (○) and 53.3 nm (•). 相似文献
Summary: Compartmentalization in atom transfer radical polymerization (ATRP) in dispersed systems at low conversion (<10%) has been investigated by means of a modified Smith–Ewart equation focusing on the system n‐butyl acrylate/CuBr/4,4′‐dinonyl‐2,2′‐dipyridyl at 110 °C. Compartmentalization of both propagating radicals and deactivator was accounted for in the simulations. As the particle diameter (d) decreases below 70 nm, the polymerization rate (Rp) at 10% conversion increases relative to the corresponding bulk system, goes through a maximum at 60 nm, and thereafter decreases dramatically as d decreases further. This behavior is caused by the separate effects of compartmentalization (segregation and confined space effects) on bimolecular termination and deactivation. The very low Rp for small particles (d < 30 nm) is due to the pseudo first‐order deactivation rate coefficient being proportional to d−3.
Simulated propagating radical concentration ([P•]) as a function of particle diameter (d) at 10% conversion for ATRP of n‐butyl acrylate ([nBA]0 = 7.1 M , [PBr]0 = [CuBr/dNbpy]0 = 35.5 mM ) in a dispersed system at 110 °C. The dotted line indicates the simulated [P•] in bulk at 10% conversion. 相似文献
Summary : For the controlled/living radical polymerization (CLRP) in which the active period during the chain formation is extremely small, ϕA < 1, such as the cases of usual SFRP and ATRP, the polymerization rate can be made larger by increasing the average number of monomeric units added during a single active period, . The -value is inversely proportional to the trapping agent concentration [X], and the polymerization rate is controlled by [X]. For small particles, even with a single trapping agent, [X] in the particle could be larger than that in corresponding bulk polymerization, and the polymerization rate decreases with D, where Dp is the particle diameter. On the other hand, for CLRPs whose ϕA-value is not very much smaller than unity, say ϕA>0.01, such as some of RAFT polymerization systems, the polymerization rate can be made larger by increasing the kinetic chain length for a given initiation frequency. For such reaction systems, the polymerization rate can be enhanced significantly by employing the emulsified polymerization systems. 相似文献
It is demonstrated by experiment and simulation that the commercially available thioketone 4,4‐bis(dimethylamino)thiobenzophenone is capable of controlling AIBN‐initiated bulk butyl acrylate polymerization at 80 °C. On the basis of molecular weight data and from monomer conversion versus time curves, the associated rate parameters are estimated. The addition rate coefficient, kad, for the reaction of a propagating chain with the thioketone is close to 106 L · mol−1 · s−1 and the fragmentation rate coefficient, kfrag, is around 10−2 s−1 giving rise to large equilibrium constants in the order of 108 L · mol−1. Furthermore, cross‐ and self‐termination of the dormant radical species are identified to be operational.
Nitroxide‐mediated polymerizations of styrene in microemulsion have been carried out at 125 °C using the cationic surfactant tetradecyltrimethylammonium bromide and the nitroxides 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO) and N‐tert‐butyl‐N‐[1‐diethylphosphono‐(2,2‐dimethylpropyl)] nitroxide (SG1). TEMPO‐mediated polymerizations were extremely slow, with large particles (dn = 39–129 nm) and broad molecular weight distributions (MWDs). The origin of the broad MWDs was likely significant alkoxyamine decomposition and differing diffusion rates of monomer and low MW alkoxyamines (and nitroxide) between monomer‐swollen micelles and polymer particles. SG1‐mediated polymerizations proceeded at higher rates, resulting in nanoparticles (dn = 21–37 nm) and lower than for TEMPO.
Summary: Nitroxide‐mediated dispersion polymerization of styrene in supercritical carbon dioxide has been performed successfully at 110 °C using a new polymeric so‐called inistab species, which fulfils the dual functions of an initiator and a colloidal stabilizer. The inistab species comprised a poly(dimethylsiloxane) block and a polystyrene block end‐capped with the nitroxide N‐tert‐N‐butyl‐N‐[1‐diethylphosphono‐(2,2‐dimethylpropyl)] nitroxide (SG1). The dispersion polymerization resulted in sub‐micron sized polymer particles and polymers of narrow polydispersity.
TEM micrograph of PS particles prepared in the dispersion polymerization of S in scCO2 in the presence of PDMS(\overline M _{\rm n} = 6 500)‐b‐PS(\overline M _{\rm n} = 4 500)‐SG1 at 110 °C. 相似文献
A mechanistic model is developed for high‐temperature (138 °C) styrene semibatch thermally and conventionally initiated FRP, as well as NMP with a two‐component initiating system (tert‐butyl peroxyacetate, 4‐hydroxy‐TEMPO). The model, using kinetic coefficients from literature, provides a good representation of the FRP experimental results. Implementation of a gel effect correlation to represent the change in the diffusion‐controlled termination rate coefficient with conversion improves the fit to the thermally initiated system, but is not required to represent the production of low molecular weight material ( Dalton) by conventionally initiated FRP or NMP. The low initiator efficiency found in NMP is well explained by a reaction network involving combination of free nitroxide with methyl radicals formed from initiator decomposition.
Controlled/living radical polymerization systems in which the active period is extremely small, ϕA ≪ 1, such as the cases of stable free radical mediated polymerization (or nitroxide mediated polymerization) and atom transfer radical polymerization, are considered theoretically. The polymerization rate, Rp, for such systems increases by lowering the trapping agent concentration [X]. When the polymerization is conducted inside small particles, Rp decreases with D below the diameter Dp,SMC at which a single molecule concentration (SMC) is equal to [X]bulk. On the other hand, when the average number of trapping agents in a particle is smaller than about 10, the fluctuation of nX among particles is significant, which leads to a larger Rp than in the cases where all particles contain the same nX. Because of the effects of SMC and fluctuation, Rp may show an acceleration window, Dp,SMC < Dp < Dp,Fluct where Rp is slightly larger than that in bulk.
The effects of non‐ideal initiator decomposition, i.e., decomposition into two primary radicals of different reactivity toward the monomer, and of primary radical termination, on the kinetics of steady‐state free‐radical polymerization are considered. Analytical expressions for the exponent n in the power‐law dependence of polymerization rate on initiation rate are derived for these two situations. Theory predicts that n should be below the classical value of 1/2. In the case of non‐ideal initiator decomposition, n decreases with the size of the dimensionless parameter α ≡ (ktz /kdz) √rinkt, where ktz is the termination rate coefficient for the reaction of a non‐propagating primary radical with a macroradical, kdz is the first‐order decomposition rate coefficient of non‐propagating (passive) radicals, rin is initiation rate, and kt is the termination rate coefficient of two active radicals. In the case of primary radical termination, n decreases with the size of the dimensionless parameter β ≡ kt,srin1/2/kp,sMrt,l1/2, where kt,s is the termination rate coefficients for the reaction of a primary (“short”) radical with a macroradical, kt,l is the termination rate coefficients of two large radicals, kp,s is the propagation rate coefficient of primary radicals and M is monomer concentration. As kt is deduced from coupled parameters such as kt /kp, the dependence of kp on chain length is also briefly discussed. This dependence is particularly pronounced at small chain lengths. Moreover, effects of chain transfer to monomer on n are discussed. 相似文献
Calculations of polymerization kinetics and molecular weight development in the dithiolactone‐mediated polymerization of styrene at 60 °C, using 2,2′‐azobisisobutyronitrile (AIBN) as initiator and γ‐phenyl‐γ‐butirodithiolactone (DTL1) as controller, are presented. The calculations were based on a polymerization mechanism based on the persistent radical effect, considering reverse addition only, implemented in the PREDICI® commercial software. Kinetic rate constants for the reverse addition step were estimated. The equilibrium constant (K = kadd/k‐add) fell into the range of 105–106 L · mol?1. Fairly good agreement between model calculations and experimental data was obtained.
Summary: The influence of the Laplace pressure in polymer emulsion particles during aqueous heterogeneous free radical polymerization on the polymerization kinetics has been investigated. Calculations were carried out based on experimentally reported pressure dependences of propagation and termination rate coefficients. The results suggest that in most cases the effects are not likely to be significant, although under conditions of very small particles (diameter <20 nm) and high interfacial tensions effects of the order of a few percent on propagation (increase in rate) and termination (decrease in rate) were predicted.
(kp/kt0.5)/(kp/kt0.5)0 as a function of particle radius as a result of the Laplace pressure. 相似文献
Batch and semibatch styrene polymerizations are carried out using a heterogeneous ATRP catalyst system that provides excellent molecular‐weight control. The observed initiator efficiency is lower for semibatch operation due to the high initiator concentrations required to make a low‐MW polymer. Experiments verified that the insoluble metal complex does not participate in the polymerization and that Cu(I) solubility is an order of magnitude higher than that of Cu(II). A mechanistic model, using kinetic coefficients from literature and the solubility data from this study, provides a good representation of the experimental results.
A new MC simulation method is proposed for the controlled/living radical polymerization in a dispersed medium, assuming an ideal miniemulsion system. This tool is used to consider the effects of particle size on the polymerization rates and the molecular weight distributions. For NMP, the polymerization kinetics are basically governed by two conflicting factors, (i) the confined space effect that promotes the coupling reaction between a radical and a trapping agent and (ii) the isolation effect of radicals into different particles that suppresses the overall frequency of bimolecular termination. For RAFT polymerization, a significant rate enhancement by reducing the particle size could be observed only for the systems with fast fragmentation of adduct radicals.
Summary: The debate on the mechanism of dithiobenzoate‐mediated RAFT polymerization may be resolved by including the reaction between a propagating radical and the star‐shaped combination product from irreversible termination into the kinetic scheme. By this step, a highly reactive propagating radical and a not overly stable three‐arm star species are transformed into the resonance‐stabilized RAFT intermediate radical and a very stable polymer molecule. The time evolution of concentrations is discussed for the main‐equilibrium range of CDB‐mediated methyl acrylate polymerization.
Illustration of the novel understanding of the RAFT mechanism in dithiobenzoate‐mediated RAFT polymerization. 相似文献
Electron spin resonance (ESR) spectroscopy can contribute to understanding both the kinetics and mechanism of radical polymerizations. A series of oligo/poly(meth)acrylates were prepared by atom transfer radical polymerization (ATRP) and purified to provide well defined radical precursors. Model radicals, with given chain lengths, were generated by reaction of the terminal halogens with an organotin compound and the radicals were observed by ESR spectroscopy. This combination of ESR with ATRPs ability to prepare well defined radical precursors provided significant new information on the properties of radicals in radical polymerizations. ESR spectra of the model radicals generated from tert-butyl methacrylate precursors, with various chain lengths, showed clear chain length dependent changes and a possibility of differentiating between the chain lengths of observed propagating radicals by ESR. The ESR spectrum of each dimeric, trimeric, tetrameric, and pentameric tert-butyl acrylate model radicals, observed at various temperatures, provided clear experimental evidence of a 1,5-hydrogen shift. 相似文献