Rate Of Chain Growth Research Articles

The effect of electrostatic and electrohydrodynamic forces on the chaining of carbon nanofibres in liquid epoxy

The formation of chains of aligned carbon nanofibres (CNFs) in polymers is a subject of great interest in the field of multifunctional nanocomposites. The mechanism of CNF chain assembly and growth in a low viscosity epoxy is investigated by developing a finite element model of a chain attached to an electrode. The model examines the combined effects of electrostatic and electrohydrodynamic forces on chain morphology. The electrohydrodynamic forces are modelled using the theory of ac electro-osmosis. The predictions of the model are supported by experimental results. The experiments were conducted on a CNF/epoxy/amine mixture by applying an ac field at frequencies ranging from 100 to 100 000 Hz. The predictions of the model qualitatively capture the variations of chain morphology and growth rate as functions of ac frequency. Higher frequencies promote a more uniform and denser network of chains. The rate of growth of chains is highest at an intermediate frequency. A uniform network of chains was observed at frequencies of 1 kHz and greater in the experiments. The rate of growth of chains was maximized at a frequency of 1 kHz for a liquid viscosity of 0.03 Pa s.

Chemistry and Mechanism of Alkene Polymerization Reactions with Metallocene Catalysts

AbstractThe structures of oligomers formed in ethylene polymerization reactions catalyzed by metallocene complexes are described. Ethylene was homopolymerized and copolymerized with four 1‐alkenes—propene, hex‐1‐ene, hept‐1‐ene, and 3,3‐dimethylbut‐1‐ene—using as catalysts (n‐Bu‐Cp)2ZrCl2 and (Me‐Cp)2ZrCl2 complexes activated with MAO, both in the absence and in the presence of H2. Oligomers generated in these reactions are the low molecular weight “tails” of the molecular weight distributions of the respective homopolymers and copolymers. GC analysis affords structural identification of each polymer molecule, albeit a very short one, individually. Analysis of the co‐oligomer structures provides explicit confirmation of the standard mechanisms and kinetics of chain growth and chain transfer reactions. The kinetic features include primary and secondary insertion reactions of 1‐alkene molecules into Cp2ZrC and Cp2ZrH bonds, chain transfer reactions to AlMe3, independence of chain growth rate constants on the size of polymer chains attached to active centers, etc. GC analysis also yields detailed information on an unusual feature of metallocene catalysis, namely the formation of saturated macromolecules in ethylene polymerization reactions performed in the absence of H2.magnified image

Academic Life: The Whole Package

When I received the unexpected but welcome invitation to write a Reflections article, I wondered which aspects of my life in science are sufficiently distinctive to make the article worth reading. One aspect may be my survival as a department chair for nearly 25 years while struggling to contribute as well in teaching and research, in other words, embracing the “whole package” of academic life: teaching, research, and service. Another part may be my experience as coauthor of a biochemistry textbook. To deal with both, I will begin with my introduction to biochemistry.

A DFT study of the chain growth probability in Fischer–Tropsch synthesis

The chain growth probability ( α value) is one of the most significant parameters in Fischer–Tropsch (FT) synthesis. To gain insight into the chain growth probability, we systematically studied the hydrogenation and C C coupling reactions with different chain lengths on the stepped Co(0001) surface using density functional theory calculations. Our findings elucidate the relationship between the barriers of these elementary reactions and the chain length. Moreover, we derived a general expression of the chain growth probability and investigated the behavior of the α value observed experimentally. The high methane yield results from the lower chain growth rate for C 1 + C 1 coupling compared with the other coupling reactions. After C 1, the deviation of product distribution in FT synthesis from the Anderson–Schulz–Flory distribution is due to the chain length-dependent paraffin/olefin ratio.

Radical polymerization as an indicator reaction for determination of organic compounds

Polymerization of methyl methacrylate (MMA) and 4-vinylpyridine (VP) has been carried out in an aqueous solution in the presence of the initiating system persulfate-tetramethylethylenediamine. The reaction rate has been monitored by measuring the light absorbance of the suspension of the resulting polymer. The effect of 26 model organic compounds on the polymerization rate has been studied. It has been shown that the VP polymerization is inhibited by a smaller number compounds (9 compounds) than the MMA polymerization (22 compounds), which indicates that the former reaction has better selectivity, whereas the determination of model compounds using the MMA polymerization reaction is more sensitive. This is explained by the lower chain growth rate constant for VP vs. MMA and different stationary concentrations of radicals in the systems. The use of these indicator polymerization reactions makes it possible to distinguish some closely related compounds, e.g., 1,4-benzoquinone and 9,10-anthraquinone (MMA reaction) or dinitrophenol and 4-nitrophenol or phenol (VP reaction). Determination of ascorbic acid in a pharmaceutical formulation has been carried out.

Chain recurrence, growth rates and ergodic limits

AbstractAverages of functionals along trajectories are studied by evaluating the averages along chains. This yields results for the possible limits and, in particular, for ergodic limits. Applications to Lyapunov exponents and to concepts of rotation numbers of linear Hamiltonian flows and of general linear flows are given.

Synthesis and structure of poly(ε-caprolactone)/synthetic montmorillonite nano-intercalates

Bulk polymerisations of ε-caprolactone (CL) were conducted at 170 °C in the presence of catalytic traces of water and 10, 30 and 50 wt% of hydrated synthetic montmorillonite SOMASIF ME100 (M) without additional catalysts. In both cases a low molecular poly(ε-caprolactone) (PCL) was produced ( M w = 5360–22,432). As revealed by 1H NMR and GPC analyses the montmorillonite present in the system induced both significantly higher lactone hydrolysis and polymer chain growth rates. Wide-angle X-ray scattering (WAXS) technique was applied to investigate the changes in clay stratification after swelling with monomer and after the polymerisation. The scattering peaks from clay (0 0 1) periodicity recorded for the silicate in dispersions and in the composites are shifted towards lower angles, which indicate an intercalation of CL as well as PCL in the galleries. Narrow intensity distribution of Bragg peaks recorded for both CL/M mixtures and resulting nano-intercalates testifies that the well ordered, layered structure of montmorillonite is retained in the silicate swollen with ε-caprolactone and after the polymerisation. The results suggest that PCL chains are flatly arranged onto each side of silicate platelet and they create pseudo-bilayers inside the silicate’s gallery.

Linear Chain Termination in the Kinetics of Postpolymerization of Dimethacrylates

The kinetics of postpolymerization (after turning off UV irradiation) of various dimethacrylates differing in their nature and molecular weight was studied over a wide range of temperatures. For each temperature, a series of kinetic curves, varying in the initial conversion during a dark period, was obtained. The proposed kinetic model is based on the following assumptions. The process in the interphase layer at the liquid monomer-solid polymer boundary has the most significant contribution to the kinetics of postpolymerization. Chain termination in the interphase layer occurs by the unimolecular reaction, is controlled by the chain growth rate, and presents the act of “self-burying” of an active radical in a conformational trap. A wide spectrum of characteristic times is inherent in unimolecular chain termination, and the relaxation function is described by the Kohlraush' stretched exponential law. The rate law obtained agrees well with experimental data. This fact made it possible to estimate the rate constants (kt) and the activation energies of chain termination and to establish the scale dependence of kt on the molar concentration [M0] of the monomer in a block. It is suggested that both the stretched exponential law and the scale kt-[M0] dependence are due to a wide spectrum of characteristic times of relaxation exhibiting the properties of a fractal set.

Zirconocene-catalyzed propene polymerization: a quenched-flow kinetic study.

The kinetics of propene polymerization catalyzed by ansa-metallocenes were studied using quenched-flow techniques. Two catalyst systems were investigated, (SBI)ZrMe2/Al(i)Bu3/[Ph3C][CN[B(C6F5)3]2] (1:100:1) at 25.0 degrees C and (SBI)ZrCl(2)/methylalumoxane at 40.0 degrees C (Al:Zr = 2400:1) (SBI = rac-Me(2)Si(1-Indenyl)2). The aims of the study were to address fundamental mechanistic aspects of metallocene-catalyzed alkene polymerizations, catalyst initiation, the quantitative correlation between catalyst structure and the rate of chain propagation, and the nature of dormant states. One of the most important but largely unknown factors in metallocene catalysis is the distribution of the catalyst between dormant states and species actively involved in polymer chain growth. Measurements of polymer yield Y versus reaction time t for propene concentrations [M] = 0.15-0.59 mol L(-1) and zirconocene concentrations in the range [Zr] = (2.38-9.52) x 10(-5) mol L(-1) for the borate system showed first-order dependence on [M] and [Zr]. Up to t approximately 1 s, the half-life of catalyst initiation is comparable to the half-life of chain growth; that is, this phase is governed by non-steady-state kinetics. We propose a rate law which takes account of this and accurately describes the initial rates. Curve fitting of Y(t) data provides an apparent chain growth rate constant k(p)(app) on the order of 10(3) L mol(-1) s(-1). By contrast, the evolution with time of the number-average polymer molecular weight, which is independent of the concentration of catalyst involved, leads to a k(p) which is an order of magnitude larger, (17.2 +/- 1.4) x 10(3) L mol(-1) s(-1). The ratio k(p)(app)/k(p) = 0.08 indicates that under the given conditions only about 8% of the total catalyst is actively engaged in chain growth at any one time. The system (SBI)ZrCl(2)/methylalumoxane is significantly less active, k(p)(app) = 48.4 +/- 2.7 and k(p) = (6 +/- 2) x 10(2) L mol(-1) s(-1), while, surprisingly, the mole fraction of active species is essentially identical, 8%. Evidently, the energetics of the chain growth sequence are strongly modulated by the nature of the counteranion. Increasing the counteranion/zirconium ratio from 1:1 to 20:1 has no influence on catalyst activity. These findings are consistent with a model of closely associated ion pairs throughout the chain growth sequence. For the borate system, propagation is approximately 6000 times faster than initiation, while for the MAO catalyst, k(p)/k(i) approximately 800. Polymers obtained at 25 degrees C show 0.1-0.2 mol % 2,1-regioerrors, and end-group analysis identifies 2,1-misinsertions as the main cause for chain termination (66%), as compared to 34% for the vinylidene end groups. The results suggest that 2,1-regioerrors are a major contributor to the formation of dormant species, even at short reaction times.

Living and Block Copolymerization of Ethylene and α-Olefins Using Palladium(II)−α-Diimine Catalysts

Living polymerizations of ethylene with palladium(II) diimine complexes coupled with use of a functionalized initiator and/or cleavage of the palladium−polymer bond with various reagents provides a protocol for synthesis of mono- and di-end-functionalized, branched, amorphous polyethylenes. The functional initiator used is the chelate complex (Ar = 2,6-(iPr)2C6H3) (3). The alkyl chain is cleaved by insertion of alkyl acrylates or methyl vinyl ketone, followed by cleavage with Et3SiH to generate alkyl ester or methyl ketone end groups, respectively. Insertion of 5-hexen-1-ol, followed by chain running and β-elimination, results in formation of aldehyde end groups. Conditions for living polymerization of propylene, 1-hexene, and 1-octadecene have also been established. Rates of first monomer insertion and subsequent chain growth are shown to be a sensitive function of the palladium complex used for initiation and the nature and concentration of auxiliary nitrile ligands. Block copolymers of ethylene and 1-o...

The ultrasensitivity of living polymers.

Synthetic and biological living polymers are self-assembling chains whose chain length distributions (CLDs) are dynamic. We show these dynamics are ultrasensitive: Even a small perturbation (e.g., temperature jump) nonlinearly distorts the CLD, eliminating or massively augmenting short chains. The origin is fast relaxation of mass variables (mean chain length, monomer concentration) which perturbs CLD shape variables before these can relax via slow chain growth rate fluctuations. Viscosity relaxation predictions agree with experiments on the best-studied synthetic system, alpha-methylstyrene.

Hyperbranched macromolecules through donor‐acceptor type copolymerization of allyl–vinylene bifunctional monomers

AbstractPresent review is an attempt to generalize and systematize the results accumulated in synthesis of cyclolinear and hyperbranched reactive macromolecules via radical alternating copolymerization of various bifunctional monomers containing donor and acceptor type double bonds. Synthesis of hyperbranched reactive macromolecules was carried out using complex‐radical cyclocopolymerization of donor‐acceptor type bifunctional monomers such as monoallyl ester of maleic acid (MAM), allyl acrylate (AA), allyl methacrylate (AM), allyl trans‐cinnamate (AC), methylallylmaleate (MeAM), methylallylfumarate (MAF) and allyl‐α‐(N‐maleimido)acetate (AMI), and maleic anhydride (MA) and styrene (St) as typical acceptor and donor comonomers, respectively. The kinetic parameters of these reactions, constants of cyclization, complex‐formation and copolymerization, as well as the ratios of chain growth rates for the participation of monomeric charge transfer complexes and free monomers, were all determined. It was demonstrated that in the studied systems, copolymerizations predominantly proceed according to alternating mechanism with formation of macromolecules having cyclolinear structure in the steady‐state and hyperbranched structure in the high conversion conditions. It was shown that formation of linear and hyperbranched macromolecules containing allyl or vinylene groups in the side chain occurs selectively carry out and depends on the nature of used comonomer. General schemes and proposed mechanism of hyperbranching and crosslinking reactions were also described. Some useful properties of synthesized reactive copolymers were discussed.© 2002 Society of Chemical Industry.

Mechanistic aspects of the copolymerization reaction of carbon dioxide and epoxides, using a chiral salen chromium chloride catalyst.

The air-stable, chiral (salen)Cr(III)Cl complex (3), where H(2)salen = N,N'-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a M(n) value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 degrees C and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by (13)C NMR spectroscopy. Whereas the mechanism for the (salen)Cr(III)-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst]. This was demonstrated herein by way of in situ measurements at 80 degrees C and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)Cr(III)-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation.

Diffusion-limited aggregation of magnetic particles under a field

Nanosize magnetic particles in dispersions should be strongly stabilized in order to avoid flocculation. Even with the use of steric surfactants or ionic peptization of particle surface, potential minima due to magnetic interaction may result in temperature-dependent transient polymerization. To this extent, it seems that the angular degrees of freedom are paramount to stability against aggregation. The last conclusion can be drawn from experiments in which an aligning magnetic field is applied to a magnetic fluid. Aggregation in such conditions have been measured using light-scattering techniques. Commercial ferrofluid was diluted in water to about 5×10 −3 particle volume fraction, where it becomes semi-transparent. With an applied field of 1.6×10 5 A/m the time-dependent low angle scattered intensity has been measured. Data analysis shows the occurrence of two successive power-law aggregation regimes. For the long-term growth, contrary to the traditional diffusion-limited aggregation of isotropic particles, the chain growth rate increases with time. At low concentrations this process may go on for hours and no evidence of an equilibrium or saturation was seen in this particular set of experiments.

Aqueous Homo- and Copolymerization of Ethylene by Neutral Nickel(II) Complexes

Aqueous coordination polymerization of ethylene by different neutral nickel(II) complexes [(X∧O)NiR(L)] (X = P: type 1; X = N: type 2) was investigated. With catalyst precursors of type 1, productivities and molecular weights were reduced by comparison to conventional polymerization in organic solvents. This effect results to a large extent from a lowering of the chain growth rate due to a low concentration of ethylene at the catalytically active centers in the aqueous polymerization. The catalysts are stable in water for hours. Stable latices of low-molecular-weight linear polyethylene were obtained with a hydrophilic complex of type 1 in an emulsion-type polymerization (Mw ca. 3 × 103 g mol-1, Mw/Mn 2−3; solids content of polymer dispersions up to 10%). With complexes of type 2, productivities in aqueous ethylene polymerization were also reduced by comparison to polymerization in organic solvents. However, moderately branched semicrystalline polymers of high molecular weight are accessible (Mn > 104 g mol-1). Employing norbornene as a comonomer, high-molecular-weight amorphous copolymers were obtained in water as a reaction medium. Ethylene solubility in water−acetone mixtures was estimated experimentally.

Synthesis and reactions of self-crosslinkable epoxy- and anhydride-containing macromolecules

Synthesis of ternary copolymers of some unsaturated electron-donor epoxide monomers (UEM) such as 4-methyl-4,5-epoxypentene-1 (MEP), vinyl- (VGE), allyl- (AGE) and p-isopropenyl-phenyl (PhGE) glycidyl ethers, maleic anhydride (MA) and styrene (St) was carried out in methylethyl ketone (MEK) in the presence of benzoyl peroxide as an initiator at 50–80 °C. Constants of charge-transfer complex (CTC) formation (Kc) for MEP…MA (Kc = 0.10 L mol−1), VGE…MA (Kc = 0.17 L mol−1), AGE…MA (Kc = 0.09 L mol−1), PhGE…MA (Kc = 0.14 L mol−1) and St…MA (Kc = 0.29 L mol−1) donor–acceptor complexes in deuterated acetone at 37 °C were determined by 1H NMR spectroscopy. Constants of copolymerization for monomer and complexed monomer pairs were determined by the Kelen–Tudos and Seiner–Litt methods. Kinetic parameters together with ratios of chain growth rates for the participation of monomer CTC and free monomers were obtained for all monomer systems studied. The results showed that terpolymerization was carried out through a primary ‘complex’ mechanism in the state close to binary copolymerization of UEM…MA and MA…St complexes. The synthesized terpolymers with free anhydride and epoxide reactive groups easily underwent crosslinking by thermotreatment and by UV-irradiation, as confirmed by DTA and FTIR analyses. © 2001 Society of Chemical Industry

Short Stopping of Runaway Methyl Methacrylate Polymerizations

Polymerization reactions are generally risky reactions for two reasons. On the one hand, they are strongly exothermic reactions; and on the other hand, the viscosity of the reaction masses can strongly increase during the course of the reaction. Especially in homogeneous systems, the increase of viscosity causes a sharp decrease of the heat transfer coefficient. Moreover, there are several polymerization systems which show strongly autocatalytic behavior at higher polymer volume fractions. In the case of loss of agitation or cooling, it is necessary to bring the reaction back to safe operating conditions. The addition of a so-called stopping agent is one of the possibilities to prevent a reaction system from runaway under such circumstances. These stoppers are radical-trapping agents which can react with free radicals by formation of a terminated product with respect to the polymerization process. With respect to the free radical, inhibition and chain growth are the two reactions which compete with each other. According to the reaction rate ratio of these two reactions, these agents are classified into retarders and true inhibitors. In the case of an inhibitor the rate of inhibition is much faster than the rate of chain growth, in the case of a retarder both reaction rates are of nearly the same order of magnitude. Surveys of the literature with respect to the inhibition of polymerization reactions are given in [1,3]. The present paper deals with the inhibition of radical solution and suspension polymerizations of methyl methacrylate using 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl in an adiabatic reaction calorimeter.

Complex‐radical cyclocopolymerization of allyl a‐(N‐maleimido)acetate with styrene and maleic anhydride

AbstractRadical copolymerizations of allyl a‐(N‐maleimido)acetate (AMI) with styrene (D, electron‐donor) or maleic anhydride (A, electron‐acceptor) were carried out in benzene and/or methyl ethyl ketone (MEK) at 50–70°C in the presence of 2,2′‐azoisobutyronitrile (AIBN) as initiator. The structure and properties of copolymers synthesized were derived from IR, chemical, DTA and TGA analyses. Side‐chain unsaturation of macromolecules was also proved by the crosslinking effect observed. Kinetic parameters of copolymerization such as complex‐formation (Kc), cyclization (kcyc), and copolymerization constants and ratios of chain growth rates for the participation of monomer charge transfer complexes (CTC) and free monomers for both systems were obtained: Kc = 0,20 ± 0,01 and 0,05 ± 0,005 L/mol in CH3COOH‐d4 at 35 ± 0,1°C (1H NMR method) for D…AMI and AMI… A complexes, respectively; kcyc · 105 = 1.71 and 0,66 L/(mol · s), r1 = 0,13 ± 0,01 and 0,037 ± 0,002, r2 = 0,048 ± 0,002 and 0,052 ± 0,002 (by Kelen‐Tüdőş method), k12/k21 = 0,20 and 0,62, k1c/k12 = 0,6 and 29,8, k2c/k21 = 4,8 and 9,1 for the D‐AMI and AMI‐A systems, respectively. The results show that alternating cyclocopolymerization reactions are realized which are carried out via a “mixed” mechanism in the D‐AMI system and via a “complex” mechanism in the AMI‐A system, with formation of cyclolinear macromolecules containing side‐chain unsaturated fragments of “allyl” (D‐AMI copolymer) and “imide” (AMI‐A copolymer) types. The synthesized copolymers easily undergo crosslinking by thermotreatment (105°C, 30 min) and/or by UV‐irradiation (25°C, 15 min), which was confirmed by DTA and TGA analyses.

A novel ribosome-associated protein is important for efficient translation in Escherichia coli

Previously, we showed that strains which have been deleted for the 21K gene (hereafter called yfjA), of the trmD operon, encoding a 21-kDa protein (21K protein) have an approximately fivefold-reduced growth rate in rich medium. Here we show that such mutants show an up to sevenfold reduced growth rate in minimal medium, a twofold-lower cell yield-to-carbon source concentration ratio, and a reduced polypeptide chain growth rate of beta-galactosidase. Suppressor mutations that increased the growth rate and translational efficiency of a delta yfjA mutant were localized to the 3' part of rpsM, encoding ribosomal protein S13. The 21K protein was shown to have affinity for free 30S ribosomal subunits but not for 70S ribosomes. Further, the 21K protein seems to contain a KH domain and a KOW motif, both suggested to be involved in binding of RNA. These findings suggest that the 21K protein is essential for a proper function of the ribosome and is involved in the maturation of the ribosomal 30S subunits or in translation initiation.

Study of Polycondensation of Tetraethoxysilane Catalyzed with Dibutylbis[1-oxo(dodecyl)oxy]stannane Using 1H NMR and 29Si NMR Spectrometry and Quasi Elastic Light Scattering

The polycondensation of tetraethoxysilane (TEOS) catalyzed with dibutylbis[1-oxo(dodecyl)oxy]stannane (dibutyltin dilaurate, DBTDL) has been investigated with the help of 1H and 29Si NMR spectrometry and quasi elastic light scattering (QELS). It has been found that in contrast to the acid catalyzed polycondensation the siloxane chain grows in linear and branched way, the proportion of cyclization reactions being slight. The rate of chain growth is relatively high, which is indicated by the impossibility to detect the presence of shorter oligomers in the reaction mixture by means of 29Si NMR. The found rate constant of polycondensation of TEOS catalyzed with DBTDL is k = 0.052 dm6 min-1 mol-1. From the determined condensation degree at the gelation point (0.20-0.35) and the ratio of consumed amount of ethoxysilane units to the consumed amount of monomer at the gelation point (0.49-0.69) it follows that the gel is predominantly formed by cross-linking of branched macromolecules, the reaction mixture containing no distinct proportions of polycyclic and highly condensed structural units. The density of network of the gel formed is relatively low, which was proved by 29Si CP/MAS NMR spectra of the solid gels. Investigation of QELS of the reaction mixture showed that the relatively large particles, microgels, were formed practically immediately after the reaction had started. The presence of such particles in the reaction mixture of acid catalyzed polycondensation of TEOS was not proved.