共查询到20条相似文献,搜索用时 0 毫秒
1.
Polymerization of hexamethylcyclotrisiloxane (D3 ) in CH2Cl2 at 30°C, initiated by triflic acid (TfOH) was studied in the presence of siloxanediols (HD2OH or HDxOH) which polycondense giving water, or in the presence of water alone for comparison. D represents a siloxane unit OSiMe2. In the second case, the relative amounts of cyclic oligomers (D6, D9) and of linear high polymer (HP) vary strongly with the molar ratio r =[H2O]/[TfOH]. When r increases from 1 to 100 (in homogeneous phase), D9 /D6 increases from 0.2 to 2 and D6 /HP decreases from 1 to 0.3. The large decrease of D6 amount and the increase in D9/D6 are attributed to the suppression of their formation through oxonium ions and to their exclusive formation, for r>100, by cyclization of silanol-esters, which is more rapid for HD9OTf than for HD6OTf. For polymerization of D3 in the presence of HD2 OH, there is a fast and limited ring-opening of D3 (about 10% conversion over 6 min) at the beginning, without formation of cyclics (D6, D9 , etc). Then reaction with D3 stops. Polymerization of HD2OH takes place simultaneously, at the same rate as in the absence of D3, with slow formation of high polymer. At the end of the polycondensation (after, eg, 4 h) D3 polymerization starts again, giving D6 , D9, etc, and HP. The inhibition period for D 3 is attributed to the complexation of triflic acid hydrates by silanol groups. These activated silanol groups do not react with D3. A comparison of D3 polymerization with the addition of water, HD2OH or HD15OH leads to the conclusion that when r is not too large, propagation involves silylester end-groups, but that for large r ratios (100 or higher), it probably occurs mainly on silanol groups reacting with an activated monomer. © 1999 Society of Chemical Industry 相似文献
2.
The radiation-induced bulk polymerization of liquid hexamethylcyclotrisiloxane has been achieved. Appreciable yields of polymer from octamethylcyclotetrasiloxane liquid have also been obtained. These results were achieved on “ultra-dry” monomers and are interpreted in terms of an ionic polymerization mechanism. 相似文献
3.
Summary A bifunctional living poly(dimethylsiloxane), poly(DMS), was synthesized through the anionic ring-opening polymerization of hexamethylcyclotrisiloxane(D3) (3) with an initiator system consisting of dilithium salt(2) of bis (p-hydroxydimethylsilyl)phenyl ether(1) and N-methylpyrrolidine in THF solution. The subsequent end-capping reaction of the bifunctional living poly(DMS) with dimethylvinylchlorosilane(4) provided the uniform size poly(DMS) having vinylsilane group at the both chain ends(5). 相似文献
4.
6.
7.
J. D. B. Smith 《应用聚合物科学杂志》1992,45(1):1-15
Using a model reaction system consisting of cumene hydroperoxide, N,N-dimethyl-p-to-luidine and O-benzoic sulfimide in toluene solvent (without reactive acrylic monomer), attempts have been made to understand the initiation mechanism operating during anaerobic polymerization. In these studies, the instrumental techniques of visible spectroccopy (VS), high performance liquid chromatography (HPLC) and infrared spectroscopy (IR) were employed. Valuable kinetic data have been obtained using these three instrumental techniques. HPLC studies have shown conclusively that the N,N-dimethyl-p-toluidine (N,N-DMpT) component is significantly depleted during the initiation step of the anaerobic process. By way of contrast, the benzoic sulfimide (BS) concentration was found to be essentially unchanged during initiation. Visible spectral changes with the model anaerobic solutions indicate that the initiation mechanism may involve radical-ionic species derived from N,N-DMpT rather than free radical reactive intermediates. Increasing the acidity of the reaction solution, with addition of acetic acid, appears to accelerate the initiation rate under anaerobic conditions. 相似文献
8.
在甲基丙烯酸甲酯(MMA)单体中,以偶氮二异丁腈(AIBN)与I2反应原位合成α-碘代异丁腈(IIBN),进一步作为原子转移自由基聚合引发剂分别与CuCl、FeCl2.4H2O等催化剂,MA5-DETA、PMDETA等络合剂相匹配引发MMA的ATRP,同时又作为衰减链转移聚合(DT)的可逆链转移剂同步进行DT聚合。实验结果表明:所得聚合物分子量可控,分子量分布(PDI)很窄(1.05~1.25),引发剂效率较高。动力学的研究结果显示:分子量随转化率而增长;由Ln[M]0/[M]t对时间作图,呈现良好的线形关系,表明聚合过程中增长自由基浓度是一个恒定值,证明由IIBN引发的聚合是典型的活性自由基聚合;这种方法克服了极不稳定的在α-位带有吸电子基团的碘化物的购运和储存的困难。不失是一种简单,快捷,方便的方法。 相似文献
9.
This work reports on using microwave energy to increase the rate of solid-state polymerization for PET and nylon 66. Theoretical analysis and experimental evidence show that the increase in polymerization rate is not due to an increase in the bulk temperature. Instead, the effect is consistent with directed heating of the condensate leading to enhanced diffusion rates. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1203–1212, 1998 相似文献
10.
Tetrahydrofuran was polymerized using the heteropolyacid H3PW12O40 as the initiator and ethylene oxide as the promoter, which effectively increased the rate and conversion of the polymerization. Water and butylene glycol were used to control the molecular weight of the product in the range of 1000–3000. The polymer was found to be polyether glycol containing 10–22 mol % oxyethylene moieties with hydroxyl groups at both chain ends. The melting point was ∼ 10°C lower compared to polytetramethylene ether glycol having the same molecular weight. The concentration of active species remained unchanged in the main period of the polymerization, indicating the absence of chain termination. The values of the chain propagation rate constant of tetrahydrofuran polymerization at 0 and 20°C were found to be 3.78 × 10−3 and 1.98 × 10−2 L mol−1 s−1, respectively, which are close to the rate constant of chain propagation of tetrahydrofuran on ionic active species. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2303–2308, 1999 相似文献
11.
Potassium iodate–initiated polymerization of aniline was carried out in an acidified aqueous medium in the presence and in the absence of sodium thiosulfate salt. The nature of the polyaniline (PANI) produced depended on the aniline/potassium iodate (A/PI) mole ratio. Green emeraldine salt (ES) and blue pernigraniline salt (PS) were produced at A/PI mole ratios greater than 2 and less than 2, respectively. The sodium thiosulfate salt played a significant role in the purification and properties of PANI. The optimum aniline‐to‐oxidant mole ratio was found to be 2 : 1. Spectral, thermal, and electrical characteristics and viscosity of the materials were studied and compared with the results. A reaction scheme has been proposed to elucidate the role of iodine(V). Sodium thiosulfate makes it easier for iodine to be separated from a system and increases the conductivity of the products. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1626–1631, 2007 相似文献
12.
Acrylamide was polymerized under different voltages with ammonium persulfate as the initiator at room temperature. The polymerization reactions were performed in two different ways. In one case, the reaction was performed in a classical, three‐electrode electrochemistry cell, and in the other, the cell was placed in an ultrasonic bath, and ultrasound was applied during the reaction. Both reactions were performed at room temperature. The conversion was rapid; even in the normal electrochemical case, the composite rate constant was comparable to the case with free‐radical polymerization of acrylamide at 50–70°C. When ultrasound was applied, the conversion values increased more. The heterogeneity index and molecular weights of the polymers produced with and without ultrasound were measured with size exclusion chromatography. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 83–89, 2002; DOI 10.1002/app.10233 相似文献
13.
Makoto Takeishi Norikazu Iwasaki Atsushi Sone Yuichi Chisaka Kenichi Sekiya Rikiya Sato 《Polymer International》1993,30(1):73-79
Methyl methacrylate (MMA) was polymerized by thiophenol without oxidants. Hydroquinone had no effect on the polymerization, indicating that the polymerization proceeded via a non-radical process. Since other monomers with an x,β-unsaturated carbonyl group polymerized similarly, the initiation and propagation were explained by the Michael addition. Radical scavengers such as 1,1-diphenyl-2-picrylhydrazyl(DPPH) and galvinoxyl enhanced the polymerization because of an accompanying radical polymerization initiated by radical species formed by the reaction of them with thiophenol. Aluminium acetylacetonate, which has no effect on radical polymerization, accelerated the thiophenol-initiated polymerization supporting the postulated mechanism. 相似文献
14.
Mathematical model for the bulk polymerization of styrene chemically initiated by sequential and total decomposition of the trifunctional initiator diethyl ketone triperoxide 下载免费PDF全文
Emilio Berkenwald Cecilia Spies Graciela Morales Diana Estenoz 《Polymer Engineering and Science》2015,55(1):145-155
This work experimentally and theoretically investigates the use of the symmetrical cyclic trifunctional initiator diethyl ketone triperoxide (DEKTP) in the bulk polymerization of styrene (St). The study focused on temperatures of 150 to 200°C, considering chemical initiation by both sequential and total decomposition reactions. The experimental work consisted of a series of isothermal batch polymerizations at higher temperatures, 150 and 200°C, with an initiator concentration of 0.01 mol/L. The mathematical model is based on a kinetic mechanism that includes thermal and chemical initiation (both sequential and total decomposition reactions), propagation, transfer to monomer, termination by combination and re‐initiation reactions. Experimental and theoretical results show that the decomposition mechanism of the initiator is modified by the reaction temperature and can be modeled as a set of two parallel reactions with different temperature dependences. The developed mathematical model simulates the bulk polymerization of St in the presence of DEKTP for a wide temperature range (120–200°C). It was found that due to these two decomposition mechanisms, the system may behave as a “dead‐end” polymerization system above a certain temperature, yielding low molecular weights and a limiting conversion value. Simulation results indicate the value of this temperature to be about 185°C. POLYM. ENG. SCI., 55:145–155, 2015. © 2014 Society of Plastics Engineers 相似文献
15.
The thermally induced solid-state polymerization of 66 nylon was investigated. It was found that the rate-controlling step in the process is chemical reaction. A mechanism of the form, rate = ktn was shown to hold. The reaction rate constant was found to be k = 1.53 × 1010 exp [ ? 12,960/RT]. Units of k are (hours)?0.51. Activation energy determined in this work compared closely to that determined for nylon 6 solid-state polymerization. 相似文献
16.
17.
A thermally and chemically heterogeneous medium is modeled by a set of cylinders pressed from a mixture of solid reagents
with coaxially aligned cylindrical rods made from an inert material. The change in velocity of combustion waves propagating
over one cylinder is studied by numerical methods under the assumption that there is no heat release from the cylinder surface.
The mean velocity of the combustion front in the specimen is shown first to decrease and then to increase with increasing
thermal conductivity of the inert rod. Spinning waves are obtained in the range of low velocities of the combustion front.
The laws of variations of the maximum temperature in the combustion front are determined. It is shown that the inert rod may
serve as a heat sink from the hot charge mixture, may be manifested as dilution by an inert component, enhance heat recuperation,
and increase the combustion-wave velocity. Introduction of the inert rod may either destabilize combustion-wave propagation
in the region of a stable plane front or stabilize the combustion wave in the range of parameters where the plane front is
unstable.
__________
Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 21–30, November–December, 2007. 相似文献
18.
Summary A novel initiator, i.e. trimethylsilylmethyllithium has been successfully used for the ring opening polymerization of cyclosiloxanes,
in toluene or in the bulk, in the presence of the cryptand [211] or DMSO. Suitable conditions have been found in which monomodal
distributions of molecular weights are observed for homopolymers of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane
(V4) as well as for copolymers of V4 with D3 or D4.
Laboratoire de Chimie Macromoléculaire associé au CNRS, URA 24 相似文献
19.
The results of initial studies showing the high activity of intercalation compounds of Lewis acids (C30SbCl5, C77FeCl3, C64AlBr3) as initiators of the cationic polymerization of cyclosiloxanes are presented. The influence of some reaction conditions on the yield and the molecular mass of the polymers formed is described. 相似文献
20.
Summary The kinetics of aqueous polymerization of methacrylamide initiated by potassium persulphate-malonic acid redox pair has been investigated at 50±1°C under nitrogen atmosphere. The initial rate of polymerization has been found to be square root dependence to the initial concentration of potassium persulphate and malonic acid. The order of reaction with respect to monomer has been found to be unity. The overall energy of activation has been found to be 66±1 KJ/mol. The effect of addition of manganous sulphate and sulphuric acid has been investigated. On the basis of experimental results a suitable mechanism has been proposed. 相似文献