Preparation of porous polyacrylamide hydrogels by frontal polymerization

Polyacrylamide hydrogels with defined porous structure were synthesized through frontal polymerization (FP) in the presence of NaHCO₃ as a foaming agent. Pore properties were analyzed using scanning electron microscopy and mercury intrusion porosimetry. The as‐prepared hydrogels displayed a small cell diameter of ca 2 µm. The dissolved foaming agent dispersing at the level of molecules and the polymerization front propagating step by step should be responsible for the small uniform cell structure. The pore volume varied from 0.63 to 3.65 cm³ g^?1 and the bulk density changed from 0.48 to 0.28 g cm^?3 for a foaming agent content from 0 to 18%. The hydrogels synthesized by FP exhibited higher swelling rate and swelling ratio with respect to conventional batch polymerization. The highest swelling ratio and rate were obtained at a foaming agent concentration of 12% based on monomer. Copyright © 2007 Society of Chemical Industry     

Numerical modeling and experimental study of the frontal polymerization of the diglycidyl ether of bisphenol A/diethylenetriamine epoxy system

Frontal polymerization is a process in which a spatially localized reaction zone propagates through a monomer by converting it into a polymer. In particular, the heat produced during the curing process is exploited to promote the reaction of the monomer lying next to the propagating front, making this latter able to self‐sustain. This approach represents an alternative solution to traditional polymerization methods and can be successfully applied to the preparation of many polymeric materials. In this study, frontal polymerization was numerically modeled to better understand it and to provide the basis for processing simulation. A finite‐difference method was used to solve the thermal problem coupled with the equation describing the cure evolution for a reactor with a cylindrical geometry. The implicit backward time–centered space method was used. First, a one‐dimensional model, able to describe the process in an adiabatic tube, was developed. The front ignition was simulated as if it were a hot surface warming one end of the reactor to trigger reactant polymerization. The model was able to predict the formation of a reactive front advancing in the unreacted zone with a constant speed. The influence of the chemical and physical properties of the resin on process evolution was also investigated. By applying the alternate direction implicit method, a more detailed two‐dimensional model able to describe a three‐dimensional problem for a cylindrical reactor was also developed. With this model, it was possible to study the influence of boundary conditions on process evolution, considering a convective heat exchange with the environment through the reactor walls. Diglycidyl ether of bisphenol A, cured with diethylenetriamine (DETA), was used as the model system. Differential scanning calorimetry was used to produce a phenomenological model able to describe the cure process and to determine the physical properties of the resin. The validity of the approach was confirmed experimentally using a small cylindrical reactor. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1756–1766, 2005     

苯乙烯热引发本体聚合的动力学研究 1 转化率与时间的关系

基于三段聚合模型 (TSPM) ,研究了 10 0— 2 0 0℃苯乙烯 (St)热引发本体聚合。结果表明 ,TSPM同样适用于描述 St热引发本体聚合。用文献中发表的大量实验数据进行 TSPM标绘表明 ,由低转化阶段向凝胶效应阶段转变时的临界转化率 x1与聚合温度无关 ,为一定值 ,大致等于 0 .5。同时 ,聚合各阶段的表观速率常数可用阿累尼乌斯 (Arrhenius)方程关联。用得到的表观速率常数计算方程和 x1,对转化率与时间关系进行了计算 ,并与文献中发表的大量实验数据作了比较。结果表明 ,二者较为吻合     

Bulk polymerization of vinyl chloride with half-titanocene/MAO catalyst

Bulk polymerization of vinyl chloride (VC) with Cp^∗Ti(OPh)₃/MAO catalyst was investigated. The bulk polymerization of VC with Cp^∗Ti(OPh)₃/MAO catalyst proceeded to give poly(vinyl chloride) (PVC) with high molecular weight in good yields. The M_n of the polymer increased in direct proportion to polymer yields and the line passed through the origin. The M_w/M_n of the polymer decreased with an increase of polymer yield. The GPC elution curves were unimodal and the whole curves shifted clearly to the higher molecular weight as a function of reaction time. This indicates that the control of molecular weight can be achieved in the polymerization of VC with Cp^∗Ti(OPh)₃/MAO catalyst even in bulk. The structure of PVC obtained from the bulk polymerization of VC with Cp^∗Ti(OPh)₃/MAO catalyst consists of a regular structure. The thermal stability of the polymer obtained with Cp^∗Ti(OPh)/MAO catalyst was higher than that of PVC obtained from radical polymerization and depended on the molecular weight of the polymer. In contrast to that, the initial decomposition temperature of the polymer obtained from a radical polymerization did not depend on the molecular weight. We presumed that the decomposition of the polymer obtained with Cp^∗Ti(OPh)₃/MAO catalyst initiated at the chain end.     

ε-Caprolactam polymerization in presence of polyhedral oligomeric silsesquioxanes (POSS)

Polyhedral oligomeric silsesquioxanes (POSS) have been covalently linked to polyamide 6 (PA6) chains with the aim of synthesizing hybrid organic/inorganic polymer materials. The synthesis has been achieved by in situ polymerization of ε-caprolactam (CL) in presence of increasing amounts of POSS molecules, using two polymerization mechanisms (hydrolytic and anionic). The latter method has been carried out by three different approaches, in order to get PA6 samples characterized by various morphologies and content of structural defects: (i) quasi-adiabatic bulk polymerization; (ii) isothermal bulk polymerization; (iii) quasi-isothermal suspension polymerization. The products obtained have been characterized in term of structure, morphology, thermal properties and molecular mass.     

Bistability of propagating front with spin‐mode in a frontal polymerization of trimethylopropane triacrylate

We have studied a nonlinear dynamic behaviour in a frontal polymerization process. Frontal polymerization is a mode of converting a monomer into a polymer via a localized reaction zone that propagates as a front. Interest in studying the dynamic behaviour of such propagating fronts is growing. Many studies focus on the spin‐mode front propagation characterized by a nonplanar front having one or more high temperature regions which move in a helical path along the axis of the reaction vessel. In this study, the bistable character of the spin‐mode induced by reaction vessel diameter has been analyzed. First, a reactor with a discontinuously varying diameter for preliminary tests, and second, a conical reactor with a continuously varying diameter have been used. We have obtained evidence of bistability of the spin‐mode front in the frontal polymerization of trimethylopropane triacrylate under certain reaction conditions. Copyright © 2003 Society of Chemical Industry     

Synthesis and characterization of amphiphilic block copolymers of methyl methacrylate with poly(ethylene oxide) macroinitiators formed by atom transfer radical polymerization

Amphiphilic ABA triblock copolymers of poly(ethylene oxide) (PEO) with methyl methacrylate (MMA) were prepared by atom transfer radical polymerization in bulk and in various solvents with a difunctional PEO macroinitiator and a Cu(I)X/N,N,N′,N″,N″‐pentamethyldiethylenetriamine catalyst system at 85°C where X=Cl or Br. The polymerization proceeded via controlled/living process, and the molecular weights of the obtained block copolymers increased linearly with monomer conversion. In the process, the polydispersity decreased and finally reached a value of less than 1.3. The polymerization followed first‐order kinetics with respect to monomer concentration, and increases in the ethylene oxide repeating units or chain length in the macroinitiator decreased the rate of polymerization. The rate of polymerization of MMA with the PEO chloro macroinitiator and CuCl proceeded at approximately half the rate of bromo analogs. A faster rate of polymerization and controlled molecular weights with lower polydispersities were observed in bulk polymerization compared with polar and nonpolar solvent systems. In the bulk polymerization, the number‐average molecular weight by gel permeation chromatography (M_n,GPC) values were very close to the theoretical line, whereas lower than the theoretical line were observed in solution polymerizations. The macroinitiator and their block copolymers were characterized by Fourier transform infrared spectroscopy, ¹H‐NMR, matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry, thermogravimetry (TG)/differential thermal analysis (DTA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). TG/DTA studies of the homo and block copolymers showed two‐step and multistep decomposition patterns. The DSC thermograms exhibited two glass‐transition temperatures at ?17.7 and 92°C for the PEO and poly(methyl methacrylate) (PMMA) blocks, respectively, which indicated that microphase separation between the PEO and PMMA domains. SEM studies indicated a fine dispersion of PEO in the PMMA matrix. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 989–1000, 2005     

宽分子量分布高聚合度PVC的合成和性能

针对高聚合度聚氯乙烯加工性能较差的缺点，采用２段聚合温度法合成了宽分子量分布的ＰＶＣ树脂，并对树脂的热稳定性、加工性能和力学性能进行了研究。     

Study on spatial–temporal kinetics of photo‐initiated frontal polymerization in stacked reaction cells

The spatial–temporal kinetics for photo‐initiated frontal polymerization(PFP) of isobornyl acrylate with 2,4,6‐trimethylbenzoyldiphenyl phosphine oxide (TPO) as photobleaching initiator was studied experimentally in stacked reaction cells. FTIR and NMR spectroscopy were employed to measure the polymerization conversion, which is dependent on the exposure time, sample depth, light intensity and photo‐initiator concentration. The experimental results are consistent with the theoretical model prediction and show that prolonged irradiation time, higher light intensity and lower photo‐initiator concentration are favorable in enhancing the advance of the polymerization front. The depth‐resolved GPC analysis shows that the average molecular weight of the PFP product dramatically increases with sample depth, while the molecular weight polydispersity reduces steadily with increase in sample depth. Copyright © 2006 Society of Chemical Industry     

甲基丙烯酸甲酯本体聚合中物性参数的测定及关联

甲基丙烯酸甲酯(MMA)本体聚合过程中比热容、黏度和导热系数等物性参数是影响聚合动力学和体系传热的重要因素.研究了聚合转化率和温度对MMA本体聚合体系密度、比热容、黏度和导热系数的变化,发现随着转化率增大,体系比热容减小,密度、黏度和导热系数增大,并存在黏度发生突变的临界转化率;随着体系温度增大,密度和导热系数减小,比热容和黏度突变对应的临界转化率增大.建立了能描述各物性参数随转化率(聚合物浓度)和温度变化的数学关联式,计算值和实验值吻合较好,建立的各关联式能较好地预测MMA本体聚合中的物性的变化,可为聚合配方及传热的设计提供基础.     

An Epoxy Adhesive Crosslinked through Radical-Induced Cationic Frontal Polymerization

UV-initiated cationic frontal polymerization is exploited as a solvent-free, extremely fast, and low-temperature technique to obtain epoxy-based adhesives. Epoxy formulations are prepared by blending commercial resins at different weight ratios and adding photo and thermal initiators at different percentages. In addition, the influence of other critical parameters, including the joint thickness, the nature of the adherends, and the temperature, is studied. As the reaction front is thermally sustained, the boundary conditions play a key role during the curing process and heat dissipation through the adherends in particular. The thermal properties of the epoxy formulation are studied through differential scanning calorimetry analysis, and the joint strengths are investigated by carrying out single lap off-set shear tests under compression. The results demonstrate the feasibility of obtaining joints by means of the radical induced cationic frontal polymerization of the epoxy adhesives, which exhibit comparable epoxy group conversion and mechanical performances to the ones cured by traditional energy-intensive techniques.     

Preparation of functionally gradient materials via frontal polymerization

Ascending frontal polymerization in a body with a moving boundary was accomplished experimentally. This process was shown to be a steady‐state process within a certain range of the parameters. Temperature profiles of the front were recorded. This new method gives an excellent opportunity to prepare functionally gradient materials because composition of a monomer feedstream can be varied in a programmable manner. Polymer samples with hyperbolic gradients of optical dye concentration were manufactured at ambient pressure and temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2398–2404, 2000     

低分子量聚N-乙烯基甲酰胺-co-乙烯胺的合成

以N-乙烯基甲酰胺为单体,N,N-二甲基甲酰胺和乙酸乙酯为混合溶剂,偶氮二异丁腈为引发剂,十二硫醇为链转移剂,通过沉淀聚合法制备了低分子量的聚N-乙烯基甲酰胺。详细研究了单体质量分数、混合溶剂配比、引发剂用量、链转移剂用量、反应温度及反应时间对聚合反应的影响。在最佳聚合条件下,聚合物收率可达93.1%、聚合物的数均分子量(Mn)为2 975.2,PDI=2.64,且聚合残液可循环利用。然后将所得聚合物在酸性条件下水解,制备了不同胺化度的聚N-乙烯基甲酰胺-co-乙烯胺,产品的数均分子量为1 000~1 400,PDI为1.34~1.40。     

Atom transfer radical bulk polymerization of methyl methacrylate under microwave irradiation

Microwave irradiation (MI) was applied to the atom transfer radical bulk polymerization of methyl methacrylate. The influence of the amount of the refluxing solvent used for controlling the polymerization temperature, irradiation power, irradiation time, and initiator concentration on the conversion, molecular weight, and molecular weight distribution of the polymers was studied with a benzyl chloride/cuprous chloride/2,2′‐bipyridyl initiation system and compared with the corresponding conventional heating (CH) process. In comparison with CH, the results can be summarized as follows. The polymerization rate for reaching 70% conversion increased 2.6–5.1 times under an irradiation power of 270–630 W. The apparent increasing rate constant was much larger than that with CH and increased with the irradiation power. MI produced a higher polymerization rate and conversion even if the concentration of the initiation system was very low (initial monomer concentration/initial initiator concentration = 200:0.33 mol/mol) and the polydispersity index (DI) was narrower; however, CH yielded almost no polymers. MI promoted the activities of the catalyst and monomer, and its initiation efficiency was higher than that with CH and increased with the irradiation power. MI obviously played an important role in promoting the polymerization rate of atom transfer radical polymerization (ATRP). MI reduced the concentration of the initiation system and perhaps made ATRP controlled (cf. uncontrolled ATRP with CH); at the same time, it made the DI values of the polymers narrower. In comparison with the initiation efficiencies found with benzyl bromide and 2,2′‐azobisisobutyronitrile used as initiators, the initiation efficiency with p‐toluene sulfonyl chloride used as an initiator was higher; moreover, DI was much narrower (1.17), and the polymerization rate was greater. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1787–1793, 2003     

Polymerization of vinyl chloride at reduced monomer accessibility. IV. The effect of diffusion control on polymerization rate,molecular weight,and thermal stability

The effect of diffusion control on the polymerization of vinyl chloride has been studied by observing the rate as well as the molecular weight and the thermal stability of the polymer formed. The polymerizations were performed at 97% of saturation pressure in a water-suspended system at 55°C, using emulsion PVC latex as seed and a water-soluble initiator. The monomer was charged as vapor from a storage vessel kept at a lower temperature. Characterization included determination of molecular weight distribution by GPC and viscometry, and thermal dehydrochlorination. The gas–liquid contact was varied by changing the speed of agitation and the design of the stirrer. With a propeller the polymerization rate increased with the agitation up to ca. 1000 rpm, where after it became almost constant. Simultaneously, the molecular weight and the thermal stability increased. This indicates diffusion control, and thus decreased monomer concentration, at low agitation speeds, while the polymerization becomes reaction controlled at higher speeds. By a comparison with earlier data, obtained at different pressures and under reaction control, the actual monomer concentration could be calculated, which allowed an evaluation of the mass transfer constant. The possibilities to encounter problems with diffusion control in commercial polymerization of vinyl chloride is discussed. It is demonstrated that diffusion control is utilized in continuous polymerization of vinyl chloride, which explains the lower thermal stability of such materials.     

Investigation of redox initiators for free radical frontal polymerization

BACKGROUND: The reaction temperature for frontal polymerization (FP) initiated by redox initiators can be greatly decreased compared with FP initiated by peroxide initiator and disulfide initiator. We report the synthesis of poly(hydroxyethyl acrylate)s via free radical FP using benzoyl peroxide (BPO)/N,N‐dimethylaniline (DMA) and ammonium persulfate (APS)/N,N,N′,N′‐tetramethylethylenediamine (TMEDA) couples as redox initiators at ambient pressure. RESULTS: The results show that unlike the phenomenon of bubbles and ‘fingers’ when using BPO alone, a self‐sustaining and stable front can be obtained when the [DMA]/[BPO] ratio is higher than 1 (mol/mol). A slight increase of the DMA (or TMEDA) reductant concentration causes a marked decrease of front temperature to 53 °C (or 61 °C). CONCLUSION: We investigated the effects of the ratio of the oxidant to the reductant and the initiator and monomer concentrations on certain parameters of FP: formation of bubbles, front velocity and front temperature. This opens the way to the potential development of FP using more appropriate monomers with low boiling points. Copyright © 2009 Society of Chemical Industry     

Frontal Polymerization of Metal-Containing Monomers: A Topical Review

The peculiarities of the frontal polymerization of metal-containing monomers containing acrylamide (AAm) complexes of transition metal nitrates, M(AAm) _x (H₂O) _y (NO₃)₂ (M=Co, Ni, Fe, Cu, Zn, Cd, Ba), that are thermally initiated at 353–433K are considered. It was found that nitrogen oxides (e.g., NO₂, NO, N₂O), which were formed by partial oxidation of the nitrate groups, were able to initiate the polymerization reaction. The reaction front and a self-sustained thermal wave were determined by structural-chemical and energy factors. The application of a self-propagating frontal polymerization to synthesize metal-containing copolymers was demonstrated. The constants of reactivity of the monomers (r ₁=0.63, r ₂=0.70) confirmed the statistical character of the copolymers obtained. Frontal polymerization of the AAm complexes that were filled by silica or glass powder was studied. Adding fillers stabilized the front and resulted in spatially uniform products. By following the controlled thermolysis of the metallopolymer or the controlled change of frontal polymerization into the burning regime lead to the formation of metallopolymer nanocomposites. The products were metal (or metal oxides) nano-scale particles that were stabilized by the polymer matrix that is formed.     

Frontal polymerization of a cyanate ester

Thermal frontal polymerization is an exothermic process that uses a propagating wave to polymerize monomers via an external heat source, such as a soldering iron, to initiate front propagation. Herein, for the first time, the curing of a cyanate ester via thermal frontal polymerization is described with two different external heat sources. However, issues of bubbling due to vaporization of the amine catalyst generally resulted in incomplete frontal polymerization when a soldering iron was used as the external heat source. To counter this issue, dual‐strip polymerization systems were used, wherein the heat from the exothermic polymerization of a free‐radical system was used to initiate the frontal polymerization of a cyanate ester system with an amine catalyst. As a result, complete frontal polymerization occurred. Additionally, the effect of the width of the acrylate strip and its impact on the front temperature, initial velocity, and steady‐state velocity of the adjacent cyanate ester system were studied. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

热引发前线聚合法固化脂环族环氧树脂

采用热引发前线聚合（TFP） 方法固化了脂环族环氧树脂（221树脂）,研究了固化剂用量、预热温度和反应器倾斜角度等因素对聚合前线的推动速率和前线温度的影响,并利用FTIR、DSC、TG等手段对固化物结构和热性能进行了表征。研究结果表明,固化剂用量越大或预热温度越高,引发前线聚合反应所需的时间越短,前线推动速率V_f 越快,前线最高温度T_max 越高,达到最高前线温度所需时间越短;试管倾斜一定角度后,下行前线方向发生偏离,V_f有所下降;FTIR测试结果表明,采用TFP法得到的环氧固化物与采用传统热固化工艺得到的固化物有相似的红外吸收,DSC和TG测试结果表明前线聚合产物具有更高的玻璃化温度和更好的热稳定性。     

Modeling of diffusion‐controlled reactions in free radical solution and bulk polymerization: Model validation by DSC experiments

In this work, a detailed experimental study of diffusion‐controlled reactions in free radical polymerization by using differential scanning calorimetry (DSC) was carried out. The systems studied include the methyl methacrylate bulk polymerization as well as the solution and the bulk polymerization of vinyl acetate at a wide range of experimental conditions including initial initiator concentration, reaction temperature, and type and amount of solvent. The conversion data obtained by DSC was successfully simulated by using a mathematical model based on sound principles such as the free volume theory. The estimated parameters, by fitting predictions of this model to conversion data obtained by DSC were found to be in close agreement with the reported parameters in the literature, thus validating both the experimental and theoretical methods used in this work. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010