Radiation-induced heterogeneous polymerization of acrylamide in acetone and acetone–water mixtures

The effects of temperature, dose rate, and monomer concentration on the heterogeneous polymerization of acrylamide in acetone–water mixtures have been studied. Heterogeneous polymerization takes place in mixtures containing less than 60 vol-% water. The polymerization is steady in acetone and nonsteady in acetone an nonsteady in mixtures containing 10–50 vol-% water. The average rate of polymerization is highest in mixtures with about 20 vol-% water. Polymer molecular weight increases with the increasing water content in range 0–10 vol-% and does not change in the range of 30–70 vol-% water. For the polymerization in acetone and an acetone–water 60/40 mixture the activation energies are 2.3 and ?1.8 kcal/mole, the dose rate exponents of rate are 0.78 and 0.52, and the monomer concentration exponents of rate are 0.5 and 1.6, respectively. The polymer molecular weight increases with decreasing dose rate, decreasing temperature, and increasing monomer concentration. These results are discussed in connection with the mechanism of heterogeneous polymerization and the solvent effect.     

A novel scheme of heterogeneous polymerization of ethylene

The heterogeneous polymerization of ethylene initiated by radiation in tert-butyl alcohol was studied. The polymerization was carried out in a 100-ml reactor at 25–100°C and pressures of 200–300 kg/cm² in the presence of 50 ml of tert-butyl alcohol containing 7 wt-% water. The amounts of polymerized monomer, the average molecular weight of polymer formed, and the molecular weight distribution of polymer were measured at various stages of reaction and at various temperatures. The molecular weight distribution was found to be very much dependent on the reaction time and temperature. For the polymer formed at 50–60°C in the very early stages of reaction, the molecular weight distribution is unimodal, and in the intermediate stage a shoulder appears at a molecular weight higher than the first peak which increases as the polymerization proceeds; eventually a bimodal curve is formed. The bimodal distribution curves were analyzed to determine the fractions and average molecular weights of the each peak. On the basis of these data for the molecular weight distribution and kinetic behavior, a new scheme for the heterogeneous polymerization is proposed which indicates that the polymerization proceeds via propagating radicals in two different physical states, namely, loose and rigid states.     

Solid-state polymerization of long-chain vinyl compounds. V. Two-step postpolymerization of octadecyl acrylate by temperature increase and by stepwise γ-ray irradiation

The mechanism of two-dimensional polymerization of octadecyl acrylate in lamellar crystal was investigated in two-step postpolymerizations by temperature increase and stepwiseγ-ray irradiation in lieu of the usual one-step reaction. Two-step postpolymerizations by these procedures are interpreted satisfactorily by the cone model, which assumes that the polymerization probability of the monomer molecules in a single layer is distributed conically around the initiation point. It was found that the propagating radicals were living, even in the saturated stage, and the effect of the polymer chains already formed on the propagating and terminating reactions was evident. Furthermore, molecular weight distributions of the resultant poly(octadecyl acrylate) measured by gel permeation chromatography (GPC) were broad. The values of M_w /M _n for the two-step post polymerizations were 4.71–7.03, whereas those for one-step reactions were 3.26–5.54.     

Atom transfer radical polymerization from nanoparticles: a tool for the preparation of well-defined hybrid nanostructures and for understanding the chemistry of controlled/"living" radical polymerizations from surfaces

Structurally well-defined polymer--nanoparticle hybrids were prepared by modifying the surface of silica nanoparticles with initiators for atom transfer radical polymerization and by using these initiator-modified nanoparticles as macroinitiators. Well-defined polymer chains were grown from the nanoparticle surfaces to yield individual particles composed of a silica core and a well-defined, densely grafted outer polystyrene or poly(methyl methacrylate) layer. In both cases, linear kinetic plots, linear plots of molecular weight (M(n)) versus conversion, increases in hydrodynamic diameter with increasing conversion, and narrow molecular weight distributions (M(w)/M(n)) for the grafted polymer samples were observed. Polymerizations of styrene from smaller (75-nm-diameter) silica nanoparticles exhibited good molecular weight control, while polymerizations of methyl methacrylate (MMA) from the same nanoparticles exhibited good molecular weight control only when a small amount of free initiator was added to the polymerization solution. The difference in polymerization behavior for styrene and MMA was ascribed to the facts that styrene undergoes thermal self-initiation while MMA does not and that termination processes involving freely diffusing chains are faster than those involving surface-bound chains. The polymerizations of both styrene and MMA from larger (300-nm-diameter) silica nanoparticles did not exhibit molecular weight control. This lack of control was ascribed to the very high initial monomer-to-initiator ratio in these polymerizations. Molecular weight control was induced by the addition of a small amount of free initiator to the polymerization but was not induced when 5--15 mol % of deactivator (Cu(II) complex) was added.     

γ-Radiation-initiated post-polymerization of methacrylic acid in the solid state

The solid-state postpolymerization of slowly crystallized methacrylic acid was studied at 0°C with ⁶⁰Co γ-radiation as the initiator. The yield, molecular weight, molecular weight distribution, and stereosequencing of the polymer product were determined as a function of polymerization time. The narrow molecular weight distribution and the linear dependence of molecular weight on polymer yield were attributed to a polymerization mechanism characterized by both independent chain propagation and essentially no termination step. The overall polymerization rate was substantially faster than that reported previously for shock-crystallized monomer, a result which was attributed to termination by the occlusion of propagating radicals at defects in the shock-crystallized monomer. Although largely atactic, the polymer synthesized in the solid state contained a secondary kind of stereosequencing; the meso triad probability was highest at the end of the chain, where propagation had initiated and decreased continuously with chain growth. The gradient in stereosequencing along the chains was attributed to defects that were introduced into the monomer crystals by the growing polymer chains.     

单电子转移活性自由基聚合制备支化聚丙烯酸甲酯的研究

以丙烯酸2-(2-溴异丁酰氧基)乙酯(BIEA)为引发剂单体(inimer),丙烯酸甲酯(MA)为单体,Cu0/CuBr2和N,N,N',N″,N″-五甲基二亚乙基三胺(PMDETA)为催化体系,二甲亚砜(DMSO)为溶剂,在常温(25℃)下通过单电子转移活性自由基聚合(SET-LRP)合成支化聚丙烯酸甲酯.聚合反应过程中,采用气相色谱(GC)、核磁共振(1H-NMR)和三检测体积排除色谱(TD-SEC)等测试手段跟踪分析和表征支化聚合物的结构.研究结果表明,采用SET-LRP方法,铜粉作为催化剂,常温下聚合反应就能快速进行,130 min之内MA的转化率已达99%以上,制备出高分子量支化聚合物.随着反应的不断进行,聚合物支化程度不断提高,相比较同分子量下的线型聚合物其黏度不断下降,Mark-Houwink特征常数α最小可达0.290.此外,低分子量聚合物组分随着反应不断减少,在高单体转化率下,聚合体系中以高支化度的聚丙烯酸甲酯为主.     

Effect of dose rate on the radiation-induced polymerization of ethylene in tert-butyl alcohol

The dependence of the dose rate on the rate of radiation-induced polymerization of ethylene in tert-butyl alcohol containing 5 vol-% water was studied. The reaction was carried out by use of a reactor with a capacity of 100 ml under the following conditions: pressure, 200 kg/cm²; temperature, 24 ± 3°C; dose rate, 3.7 × 10⁴?1.6 × 10⁵ rad/hr; amount of medium, 70 ml. The dose rate exponents for rate of the polymerization, the molecular weight, and the number of polymer chain were found to be about 0.8, ?0.1, and 0.9, respectively. These results were well explained with kinetic results (obtained by a novel analytical method) for the polymerization which contain both first-order and second-order terminations for the concentrations of propagating radical. The individual values of the rate constants in each elementary reaction were also obtained.     

Enhanced spin capturing polymerization: Numerical investigation of mechanism

Simulation on the kinetic scheme of enhanced spin capturing polymerizations (ESCP) were performed to reveal the influence of different parameters, e.g., initiation rate, spin capturing and side reactions on the properties (the monomer conversion, degree of polymerization, polydispersity index, and the fraction of “living” chains) of polymer obtained. The kinetic scheme of ESCP was solved numerically as well as the method of statistical moments of molecular weight distribution was applied to get some numerical expressions for analysis. The simulations showed that fast initiation rate can lead to decrease of living fraction and broadening of molecular weight distribution. Whereas high rate of spin capturing makes the polymer chains shorter but increases living character. And side reactions always decrease livingness of the polymer obtained. Finally in the case of absence of side reaction we were able to formulate criteria for formation of polymer with target molecular weight and quota of living chains via ESCP process. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2546–2556     

Pulsed laser polymerization of styrene in compartmentalized systems

Pulsed laser polymerizations of styrene were performed in microemulsion droplets and in latex particles. From the molecular weight distribution of the resultant polymer either the propagation rate coefficient in the droplets or the monomer concentration in the particles could be determined. Furthermore for microemulsion droplets with a low initiator concentration the rate coefficient for transfer to monomer could be determined. Because in very small particles the termination rate can be very high, instantaneous termination may occur, leading to a shift of the low molecular weight inflection point as the best measure of the propagation rate coefficient to the maximum in the molecular weight distribution. forming pulsed laser polymerization experiments directly in emulsion systems gives specific information on the special features associated with doing polymerizations in heterogeneous and compartmentalized systems.     

Stereospecific polymerization of benzyl vinyl ether by BF3·OEt2

Polymerization of benzyl vinyl ether was carried out by BF₃·OEt₂, and the effects of polymerization conditions on the stereoregularity of the polymer were studied by NMR analysis. The polymerization at ?78°C in toluene gave a highly isotactic polymer. The isotacticity of the polymer was independent of the catalyst concentration but increased with a decrease in the initial monomer concentration and decreased slightly on raising the reaction temperature. When the polymerizations were carried out in toluene—nitroethane mixtures, a gradual decrease in the isotacticity and a rapid decrease in the molecular weight of the polymer were observed with increasing nitroethane in the solvent. The molecular weight of the polymer was almost constant, regardless of the catalyst concentration, and increased with increasing initial monomer concentration and decreasing polymerization temperature. When the polymerization was performed in toluene at ?78°C with a small amount of water or benzyl alcohol, a linear relationship was found between the reciprocal DP of the polymer and water or benzylalcohol concentration. The mechanisms of the initiation reaction and the stereoregulation in the polymerization were also discussed.     

Role of chain transfer in heterogeneous polymerization of ethylene

The role of chain transfer was studied for the radiation-induced polymerization of ethylene in precipitating media, namely n-butyl alcohol, tert-butyl alcohol and their mixtures. The affinities of those solvents for polyethylene are similar, but the chain-transfer coefficient of n-butyl alcohol is larger than that of tert-butyl alcohol. The polymerizations were carried out in a reactor of 100 ml under a pressure of 300 kg/cm², at 60°C, dose rate of 3.07 × 10⁴–1.75 × 10⁵ rad/hr in the presence of 50 ml of solvents. The polymerization in tert-butyl alcohol shows the kinetic behavior characteristic of a heterogeneous polymerization, such as rate acceleration, high dose rate dependence of polymerization rate, and low dose rate dependence of polymer molecular weight, whereas the polymerization in n-butyl alcohol does not exhibit such behavior and gives polymer having a molecular weight much lower than that of polymer obtained in tert-butyl alcohol. The polymer formed in tert-butyl alcohol exhibits a bimodal molecular weight distribution measured by gel permeation chromatography. In mixed tert-butyl alcohol and n-butyl alcohol solvent, with increasing fraction of n-butyl alcohol, the two peaks not only shift to lower molecular weight but the higher molecular weight peak becomes relatively small. Eventually, the polymer formed in n-butyl alcohol exhibits a unimodal distribution. Those results are well explained on the basis of the proposed scheme for heterogeneous polymerization.     

The kinetics of the polymerization of methyl methacrylate initiated by t-butylmagnesium bromide in THF,toluene or mixed solvent

The kinetics of the solution polymerization of methyl methacrylate in THF, toluene and their mixtures were studied between 200 and 300 K using dilatometry (in the systems where it was valid), gravimetric determination and monitoring monomer and polymer concentrations by NMR spectrometry. The reaction followed zero order kinetics at 200 K, first order kinetics at 275 K and mixed order in between. At both the limits and intermediate range, the reaction followed an integrated rate equation consistent with terminationless propagation proceeding through a complex between monomer and the propagating species. Above 275 K, termination and side reactions were evident and the yields of high mol. wt polymer were small. Density-temperature calibrations for monomer in THF, toluene and mixtures were constructed for the range 190–283 K. However for polymerizations in toluene-rich mixtures, where very high mol. wt polymer forms, the contraction did not correlate linearly with conversion.     

N‐Bromosuccinimide as a thermal iniferter for radical polymerization

N‐Bromosuccinimide (NBS) was used as a thermal iniferter for the initiation of the bulk polymerizations of methyl methacrylate, methyl acrylate, and styrene. The polymerizations showed the characteristics of a living polymerization: both the yields and the molecular weights of the resultant polymers increased linearly as the reaction time increased. The molecular weight distributions of the polymers were 1.42–1.95 under the studied conditions. The resultant polymers could be used as macroiniferters to reinitiate the polymerization of the second monomer. The copolymers poly(methyl methacrylate)‐b‐polystyrene and polystyrene‐b‐poly(methyl methacrylate) were obtained and characterized. End‐group analysis of the resultant poly(methyl methacrylate), poly(methyl acrylate), and polystyrene confirmed that NBS behaved as a thermal iniferter. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2567–2573, 2005     

Modeling the reversible addition–fragmentation transfer polymerization process

A kinetic model has been developed for reversible addition–fragmentation transfer (RAFT) polymerization with the method of moments. The model predicts the monomer conversion, number‐average molecular weight, and polydispersity of the molecular weight distribution. It also provides detailed information about the development of various types of chain species during polymerization, including propagating radical chains, adduct radical chains, dormant chains, and three types of dead chains. The effects of the RAFT agent concentration and the rate constants of the initiator decomposition, radical addition, fragmentation, disproportionation, and recombination termination of propagating radicals and cross‐termination between propagating and adduct radicals on the kinetics and polymer chain properties are examined with the model. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1553–1566, 2003     

An Alkenyl Boronate as a Monomer for Radical Polymerizations: Boron as a Guide for Chain Growth and as a Replaceable Side Chain for Post‐Polymerization Transformation

The ability of isopropenyl boronate pinacol ester to serve as a monomer in radical polymerizations was established and exploited for the synthesis of polymers that are difficult to access using other polymerization techniques. Although the monomer exhibits an α‐methyl‐substituted unconjugated structure, which is usually unfavorable for radical propagation, both free and controlled radical polymerizations smoothly afford the corresponding polymers. A density‐functional‐theory‐based investigation revealed that the boron atom moderately stabilizes the radical species, which leads to the suppression of the degradative chain transfer to the α‐methyl groups, and thus guides the reaction towards the radical polymerization. The boronyl pendants, which are directly attached to the polymer backbone, can be replaced with ‐OH or ‐NH₂ to yield poly(α‐methyl vinyl amine) or poly(α‐methyl vinyl alcohol), which has been inaccessible by conventional synthetic methods.     

Polymerization of methyl methacrylate with ceric ion-methanol redox system

The polymerization of methyl methacrylate initiated by Ce⁴⁺ methanol redox system was studied in aqueous solution of nitric acid at 15°C. The polymerization was initiated by primary radicals formed from Ce⁴⁺/alcohol complex. Poly(methyl methacrylate) chains containing the alcohol residue were obtained. Variations in the temperatuare and concentration of the components of the redox system allowed the control of the rate of polymerization and molecular weight of the polymer. The concentration of the hydroxyl end groups in the poly(methyl methacrylate) of low molecular weight was determined by titration and by spectrometric method.     

Model Studies of Nitroxide‐Mediated Styrene Miniemulsion Polymerization – Opportunities for Process Improvement

Modeling studies were performed to investigate how persulfate‐initiated nitroxide‐mediated styrene miniemulsion polymerizations are influenced by changes to the polymerization recipe. By manipulating the initial concentrations of potassium persulfate and nitroxide, and the aqueous phase volume, trends in the predicted polymerization time, number average molecular weight, polydispersity and degree of polymer livingness were identified that indicate operating conditions for improved process performance. Specifically, our model predicts the existence of experimental conditions that simultaneously minimize polymer polydispersity and maximize the livingness of the polymer. The mechanisms responsible for the predicted trends were identified from the predicted molecular weight distributions of the living and dead polymer chains.

Predicted number MWDs at 20% monomer conversion for styrene NMMP systems employing various levels of [KPS]_aq,0. Dormant KPS‐initiated polymer radicals.     

An experimental investigation on the evolution of the molecular weight distribution in styrene emulsion polymerization

Styrene ab initio emulsion polymerizations were conducted at 70°C in an automated reaction calorimeter. Two polymerizations were performed, one above and the other below the critical micelle concentration (CMC) of the surfactant, thus ensuring differing polymerization kinetics between the two: the system below the CMC gave large particles that were expected to follow pseudobulk kinetics, while that above the CMC gave small particles that were expected to follow zero-one kinetics. The evolutions of the molecular weight distributions (MWDs) were characterized by removing samples periodically during the course of the reactions and analyzing with gel permeation chromatography. Interpretation of the data used average molecular weights, the GPC MWDs, and the number MWDs, as functions of conversion. It was found that all of the number MWDs (plotted as ln (number of polymer chains) vs. molecular weight of polymer chains) were concave-up at low molecular weights and become nearly linear at molecular weights (≥3−4 × 10⁶); this linearity is expected from theory. The slope of the high molecular weight region was consistent with theory for the dominant mode for chain stoppage: termination and transfer for the pseudobulk system and (predominantly) chain transfer to monomer for the zero-one system. The most likely explanation for the concavity of the number MWDs is a heterogeneity of radicals: some surface anchored with sulfate end groups and others (with hydrogen end groups arising from transfer to monomer and/or reentry) being more mobile. Thus, two types of termination are proposed: slow reaction-diffusion for the less mobile surface anchored chains, and rapid short-long (center of mass) termination for the more mobile hydrogen-terminated chains. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 989–1006, 1997     

β-CD存在下MMA细乳液体系的RAFT聚合

近年来，活性自由基聚合已成为高分子合成领域中的一个热门课题．Rizzardo研究小组提出了一种新型活性自由基聚合反应，即RAFT(Reversible addition-fragmentation chain transfer)聚合．RAFT反应在传统的自由基聚合中加入了具有高链转移常数和特定结构的链转移剂——双硫酯类化合物．当链转移剂的浓度足够大时，链转移反应由不可逆变为可逆，聚合反应也随之发生质的变化，由不可控     

Modeling molecular weight distribution in emulsion polymerization reactions with transfer to polymer

A mathematical model was developed for the computation of the dynamic evolution of molecular weight distributions (MWDs) during nonlinear emulsion polymerization reactions. To allow the direct computation of the whole MWD, an adaptive orthogonal collocation technique was applied. The model was validated with experimental methyl methacrylate/butylacrylate (BuA) semicontinuous and vinyl acrylate (VA)/Veova10 continuous emulsion polymerization results. Both systems considered introduce significant chain‐transfer reactions to polymer chains as a result of the presence of BuA and VA, respectively. The model developed was able to represent quite properly the kinetics and MWD of polymer samples during emulsion polymerizations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3513–3528, 2001