Comparative kinetics of the induced radical autocondensation of polyflavonoid tannins. II. Flavonoid units effects

Comparative kinetics of the radical autocondensation induced by SiO₂ on a series of polyflavonoid tannins, namely, pine, pecan, mimosa, quebracho, gambier, sumach, and on the catechin monomer as a model compound were carried out by electron spin resonance. The induced radical autocondensation appeared to be independently catalyzed by the known base mechanism, as well as SiO₂ and Lewis acid attack directly at the heterocycle oxygen. The reaction occurs in two definite steps: the first, the radical anion formation, the second, the condensation proper with other flavonoid units of the reactive sites formed. The rate determining step depends on both the main flavonoid unit structure of each tannin and particularly, on the level of colloidal state of the tannin solution and the number-average degree of polymerization (DP_n), with the latter two parameters being the main determining ones for the second reaction step and the first two for the first reaction step. It is, however, the combination of the three parameters that determines the total observable effect for each of the flavonoid tannins. The SiO₂ attack at the heterocycle ether oxygen is of such an intensity that the A-rings, phenoxide radicals, which drive the reaction, surge very rapidly to such a higher proportion than the B-rings phenoxide radicals that the B-rings also start to surge later by shifting to the left of the ^*B ← A^* equilibrium. There are also indications that ionic mechanisms might be more important for the second step of the reaction. Different radical-anion species and the relative movements of the relevant equilibria involved can be clearly identified from the spectra peaks. The initial, maximum intensity of the peaks has been shown to be the parameter defining the first step of the reaction, while the radical decay rate has been shown to refer to the second step of the reaction. Hydrolyzable tannins have been shown not to undergo neither any silica-induced radical surge nor autocondensation as predictable from their structures. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 243–265, 1997     

Comparative kinetics of the induced radical autocondensation of polyflavonoid tannins. III. Micellar reactions vs. cellulose surface catalysis

Cellulose surface catalysis of the induced autocondensation of tannins has been found to occur also for the radical mechanism of the reaction. Equally effective in accelerating pyran ring opening of polyflavonoids by radical mechanisms appears to be the presence of any substance leading to micellar structures in the tannin solution. Thus, soaps and synthetic and natural polymeric colloids have been found to accelerate the reaction. The micellar effect is distinct and different from the surface catalysis of the process induced by cellulose. Amorphous and crystalline cellulose appear to differ somewhat as regards the extent of catalytic activation. As for ionic mechanisms, also for catalyzed radical mechanisms, pro-cyanidins invert their favorite reaction of cleavage of the interflavonoid bond to favor, instead, pyran ring opening. This inversion of the favorite reaction is induced both by the presence of cellulose and by the level of the micellar state of the reaction. © 1996 John Wiley & Sons, Inc.     

Polycondensation and autocondensation networks in polyflavonoid tannins. I. Final networks

A thermomechanical analysis (TMA) study, directly on a bonded joint during the process of adhesive hardening, on the sinergy and interference between polycondensation and autocondensation on procyanidin, and profisetinidin–prorobinetinidin-type polyflavonoid tannins network formation and hardening, confirmed that also at the higher curing temperatures characteristic of the hardening of tannin-based wood adhesives, hardening by polycondensation can be coupled with simultaneous hardening of tannins by autocondensation. Some coreactants appear to depress the tannin autocondensation while still leaving a small contribution of this reaction to the formation of the final crosslinked network. Other coreactants instead appear to enhance formation of the final network by sinergy between the 2 condensation mechanisms, while still others do not show any interference between the 2 types of reaction. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1083–1091, 1998     

Ionic polycondensation effects on the radical autocondensation of polyflavonoid tannins: An ESR study

An electron spin resonance (ESR) study of the presence or lack of interference by ionic hardening mechanisms and ionic coreactants on the polyflavonoid tannin radical autocondensation reaction indicated that in certain cases hardening by ionic coreactants can be coupled with the simultaneous hardening of the tannin by radical autocondensation. Some coreactants tend to depress the tannin radical autocondensation while still leaving a small contribution of this reaction to the formation of the final crosslinked network. Other coreactants instead appear to enhance formation of the final network by synergy between ionic and radical mechanisms, while still others do not show any interference between the two types of reaction. Mechanisms describing the interaction between the two reactions are proposed and discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2623–2633, 1997     

An ESR study of the silica-induced autocondensation of polyflavonoid tannins

Radical mechanisms of silica-induced polyflavonoid tannin autocondensation and hardening through alkaline pyran ring cleavage are shown to be important, especially at different levels according to the reaction conditions and type of tannin. For prodelphinidin tannins, radical mechanisms are shown to be more important than are ionic ones even under mild alkaline reaction conditions. For procyanidin tannins, radical mechanisms appear to be of little or no importance under mild alkaline reaction conditions, but acquire more importance at more alkaline pHs, a result confirmed also with a (+)-catechin monomer as a model compound. Under the latter conditions, the radical mechanism of pyran ring cleavage inverts the relative importance of the interflavonoid bond cleavage and pyran ring opening for procyanidins. Relative intensities of phenoxide radical and radical anion ESR signals indicate that pyran ring opening and autocondensation are rapid under alkaline conditions. © 1996 John Wiley & Sons, Inc.     

Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates. II. Cellulose effect and application

Polyflavonoid tannin autocondensation was found to be facilitated by the reaction occurring on cellulose and lignocellulosic substrates. Although the mechanism of polyflavonoid autocondensation induced by cellulose differs from that induced by the action of Lewis acids, the subsequent reaction of autocondensation appears to be similar. The determining step of tannin rearrangement pathways under alkaline conditions in cellulose-induced highertemperature autocondensation is the favoring of the heterocycle pyran ring opening over the normal interflavonoid bond cleavage and catechinic acid rearrangement. This is caused by relevant bond weakening and easier cleavage induced by strong attractive forces between flavonoid and cellulosic substrate. Applied bonding of cellulosic substrates by polyflavonoid tannin autocondensation reaction appear to be feasible and to occur as predicted from theoretical predictions. © 1995 John Wiley & Sons, Inc.     

Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates. I. 13C-NMR, 29Si-NMR,X-ray,and polarimetry studies and mechanism

Polyflavonoid tannins have been found to autocondense and harden when in presence of small amounts of SiO₂ at high pH. Small amounts of boric acid and AICI₃ were found to have the same effect. The mechanism of tannin autocondensation to hardening was found to depend on the Lewis acid behaviour of the additives used. Such mechanism involves Lewis acid acceptance of electrons from the ether oxygen of the flavonoid unit pyran ring with subsequent facilitation of base-induced heterocycle opening. The reactive C2 site created by heterocycle opening proceeds to autocondense with the reactive A-ring of a flavonoid unit on another chain denying to the flavonoid the possibility of intramolecular rearrangements to catechinic acid and phlobatannin which occur in model compounds. Si has been shown to pass through a coordination state of 5, by CP-MAS ²⁹Si-NMR. The portion of Si, which has not been able to complete the reaction due to premature hardening, remains attached to the flavonoids, in this coordination state, in the hardened network. The Si portion, which has completed the reaction and caused the hardening, reverts instead to SiO₂ and is detached from the flavonoid. The SiO₂, H₃BO₃, and AICI₃ mechanism shows that their presence also reverses the relative facility of cleavage between interflavonoid bond and heterocycle opening in mainly procyanidin and profisetinidin tannins. © 1994 John Wiley & Sons, Inc.     

Antioxidant characteristics of hydrolysable and polyflavonoid tannins: An ESR kinetics study

As part of an investigation of the role of tannin as antioxidants, the radical formation and radical decay reactions of some polyflavonoid and hydrolysable tannins has been followed by electron spin resonance (ESR) techniques. Comparative kinetics were determined for both light-induced radicals and by radical transfer from a less stable chemical species for the tannin alone and when the tannin is in a methanol solution. The five parameters which appear to have a bearing on the very complex pattern of the rates of tannin radical formation and radical decay were found to be (1) the extent of the colloidal state of the tannin in solution, (2) the stereochemical structure at the interflavonoid units linkage, (3) the ease of heterocyclic pyran ring opening, (4) the relative numbers of A- and B-rings hydroxy groups, and (5) solvation effects when the tannin is in solution. It is the combination of these five factors that appears to determine the behavior as an antioxidant of a particular tannin under a set of application conditions. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 475–482, 1997     

Study on the structure of mangrove polyflavonoid tannins with MALDI-TOF mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry is a suitable method for examining polyflavonoid tannin oligomers as it is capable to determine aspects of their oligomeric structure and characteristics, which are otherwise too difficult to determine by other techniques. It has been possible to determine by MALDI-TOF for Rhizophora apiculata mangrove polyflvonoid tannins that: (i) procyanidins oligomers formed by catechin/epicatechin, epigallocatechin, and epicatechin gallate monomers are present in great proportions; (ii) oligomers, up to nonamers, in which the repeating unit at 528–529 Da is a catechin gallate dimer that has lost both the gallic acid residues and an hydroxy group which are the predominant species; (iii) oligomers of the two types covalently linked to each other also occur. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microstructure elucidation of polyflavonoid tannins by MALDI‐TOF‐CID

High molar mass wood tannin extracts are complex mixtures that are distributed in both molar mass and chemical composition. Condensed tannins from quebracho and mimosa woods were analyzed and compared with cacao tannins using matrix‐assisted laser desorption/ionization‐time of flight (MALDI‐TOF) mass spectrometry. Although MALDI‐TOF MS reveals the oligomer structure of the tannins, this method cannot distinguish between isomers with isobaric masses, and therefore, ambiguous structural assignments were made in a number of cases. To determine the actual microstructures present, MALDI‐TOF collision‐induced dissociation (CID) experiments were conducted. MALDI‐TOF‐CID enables monomer sequence determination, and positive assignments of isobaric structures can be made. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Comparative rheological characteristics of industrial polyflavonoid tannin extracts

Commercial, industrially produced mimosa, quebracho, pine, and pecan polyflavonoid tannin extracts water solutions of different concentrations were examined by rheometry by measuring dynamic moduli both as a function of strain amplitude and at varying frequency. The water solutions of these materials have been found to behave in general mainly as viscous liquids at the concentrations that are generally used for their main industrial applications. Clear indications of viscoelastic response are also noticeable, among these the crossover of the elastic and viscous moduli curves at the lower concentrations of the range investigated, with some differences being noticeable between each tannin and the others—pine and quebracho tannin extracts showing the more marked viscoelastic behavior. Other than pH dependence (and related structural considerations), the parameters found to be of interest as regards the noticeable viscoelastic behavior of the tannin extracts were the existence in the solutions of labile microstructures that can be broken by applied shear. This is supported by the well‐known thixotropic behavior of concentrated, commercial polyflavonoid tannin extracts water solutions. Such microstructures appear to be due (1) to the known colloidal interactions of these materials already reported, or (b) to other types of secondary interactions between tannin oligomers and particularly between tannin and carbohydrate oligomers. The latter is supported by the dependence of this effect from both the average molecular masses of the tannin and of the carbohydrate oligomers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1634–1642, 2001     

Rheology of polyflavonoid tannin–formaldehyde reactions before and after gelling. II. Hardener influence and comparison of different tannins

Good correspondence of the gel‐time values obtained by two different methods, G′ = G″ and 1/η₀→0, was observed for different types of natural and modified tannin extracts. The pH presents the predominant effect on both the activation energies and the gel times observed, while the proportion of a paraformaldehyde hardener has a much lesser effect on these parameters. The rate constants of the different phases of the reaction of polycondensation with formaldehyde, both before and after the gel point, were obtained for the six commercial tannin extracts tested. The viscoelastic properties of the different tannins/formaldehyde gels were measured. The gel stiffness S, relaxation coefficient n, and relaxation time λ were determined and their dependence on the proportion of the formaldehyde hardener, on the temperature, and on the type of tannin was determined. The gel stiffness S appears to be influenced considerably by the proportion of the formaldehyde hardener. Its value decreased as the percentage of the hardener increased: This was due to early network immobilization and the resulting lower level of crosslinking resulting from it. The influence on S of the temperature is not very pronounced. The relaxation coefficient n appears to depend mainly on the reactivity of the tannin used: The faster the reactivity, the higher was the value of n. This appears to be valid exclusively in tannin extracts where the colloidal state is still present, while it is not valid in extracts where the colloidal state was eliminated by, for example, solvent extraction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 864–871, 2002     

Rheology of polyflavonoid tannin–Formaldehyde reactions before and after gelling. I. Methods

The gelling and hardening reactions of different, commercial polyflavonoid tannin extracts of the prorobitenedin/profisetinidin, procyanidin, and prodelphinidin types with formaldehyde were studied by parallel‐plate rheometry. To do this, methods to determine the rheological characteristics of tannin–formaldehyde polycondensation reactions both before and after the gel point were developed. The validity of some modifications of known methods was checked on the best‐known commercial tannin extract, namely, the reaction of the mimosa tannin extract with 5% paraformaldehyde, over a range of different temperatures. The gel point was determined by three different methods, namely, by the crossover point of viscous and elastic moduli, by the extrapolation to ∞ of the zero‐frequency viscosity, and by determining the point at which the value of tan δ is constant whatever frequency is used in the measurement. The results obtained were critically compared. The comparisons obtained were good and were reproducible when the first two methods were used, but not when the third one was used. This allowed the determination of the energy of activation at different stages in the tannin–aldehyde polycondensation reaction and the calculation of the degree of conversion of the tannin–aldehyde polycondensation before, at, and after the gel point. Mathematical expressions defining the degree of conversion for tannin–aldehyde reactions before, at, and after the gel point as a function of different moduli were developed and checked with the experimental data obtained. Methods and mathematical expressions for the determination of the rate constants of the different stages of the reaction were also developed and checked. After the gel point, the system was modeled using both a diffusion‐controlled model and a second‐order kinetic law. The applied results obtained indicated that, although a certain element of diffusion control is present, a second order, diffusion‐independent kinetic is more valid under the experimental conditions used. This indicates that the rheometry approach to a polycondensation network is limited mostly by the capability of the equipment to the region after the gel point, where diffusion control does not as yet play a predominant role. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 852–863, 2002     

A 13C NMR study of polyflavonoid tannin adhesive intermediates. I. Noncolloidal performance determining rearrangements

Chemical and heat treatments introduce noticeable modifications on the structure of four different polyflavonoid tannin extracts during their modification for use as tannin-form aldehyde polycondensates. Such modifications are performance determining and account for the profound differences in behaviour of the four tannin extracts when used as tanninformaldehyde adhesives. Such modifications are directly followed by ¹³C NMR on the tannin extracts. Although some of the modifications observed by ¹³C NMR appear to correlate well with previous findings by model compounds studies, others could only be observed by ¹³C NMR of the modified tannin extracts themselves. Structural rearrangements to phlobatannins, depolymerization, recyclation, rearrangement of the type of interflavonoid monomer links, and autocondensation have been followed by ¹³C NMR and found to occur to different extents in the four tannins accounting for many of their differences in adhesive performance and characteristics. A novel reaction of autocondensation was observed for pecan nut pith tannins, which is different in applied characteristics from other polyflavonoid tannin adhesives. © 1994 John Wiley & Sons, Inc.     

Formation of monodisperse polyacrylamide particles by radiation‐induced dispersion polymerization. I. Synthesis and polymerization kinetics

Highly monodisperse polyacrylamide (PAM) microparticles were directly prepared by radiation‐induced dispersion polymerization at room temperature in an aqueous alcohol media using poly(N‐vinylpyrrolidone) (PVP) as a steric stabilizer. Monomer conversion was studied dilatometrically and polymer molecular weight was determined viscometrically. The gel effect was found evidently from the polymerization kinetics curves. The influence of the dose rate, monomer concentration, stabilizer content, medium polarity, polymerization temperature on the polymerization rate, and the molecular weight of the polymer was examined. The polymerization rate (Rp) can be represented by Rp ∝ D^0.15[M]^0.86[S]^0.47[A/W]^0.64 and the molecular weight of the polymer can be represented by M_w ∝ D^?0.19 [M]^1.71[S]^0.43[A/W]^0.14 at a definite experimental variation range. The overall activation energy for the rate of polymerization is 10.57 kJ/mol (20–35°C). Based on these experimental results, the polymerization mechanisms were discussed primarily. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2567–2573, 2002     

Microstructure and tacticity in radical copolymerization I.

The NMR spectra of styrene-methyl methacrylate copolymers were found to exhibit a multiplicity in the 2.0–3.6δ region characteristic for methoxy protons. The methoxy pattern is explained by taking into account the degree of shielding of methoxy protons, in relation to both number and configuration of the neighbouring styrene units along the chain. The relative areas of the methoxy peaks are compared with results of the statistical analysis of monomer sequence probabilities.     

Thermoplastic elastomers via radical polymerization. I

Thermoplastic elastomers were synthesized using a method for preparation of block copolymers introduced by Tobolsky and Rembaum. A hydroxy-terminated polyester or polyether was chain extended with tolylene diisocyanate and capped with cumene hydroperoxide. The high molecular weight peroxycarbamate was used as initiator for polymerization of styrene, methyl methacrylate, or acrylonitrile monomer. Their physical properties were investigated by modulus–temperature curves. The effect of composition on modulus was also examined.     

A Comparative Study Between the Adsorption Behavior of Activated Carbon Fiber and Modified Alginate I. Basic Dyes Adsorption

Activated carbon fibers and modified alginate were prepared by using a cheap precursor material. The adsorption behavior of these materials was studied toward basic dyes. It was found that the most effective factor in the adsorption process onto ACF is the high surface area followed by a little assist of the carboxylic groups on the surface. On the contrary, only the carboxylic groups created on the modified alginate (MA) will govern the adsorption behavior of MA. The equilibrium time for ACF in the adsorption of basic dyes was found to be 37 min, whereas in MA it was 70 min. The difference in the adsorption rate between ACF and MA was attributed to the difference in their structure features. Langmuir and Freundlich isotherm models were tested in the study to compare their applicability with the adsorption systems used. The result revealed that the adsorption of basics dyes onto ACF and MA fit very well with both Langmuir and Freundlich isotherms. The adsorption of basic dyes onto ACF follows the micropore-filling mechanism, whereas in MA, the adsorption is done via a physicochemical interaction between the dyes molecules and the carboxylic groups onto MA.