Condensation reaction model

Grote R F and Hynes J T 1980 The stable states picture of chemical reactions. II. Rate constants for condensed and gas phase reaction models J. Chem. Phys. 73 2715-32... 

Early studies of the condensation reaction on the monomer level did not give the full picture of this process and only in the 1980s was polycondensation of siloxanols studied by using oligomeric model compounds (76,77). These studies revealed that in the presence of strong protic acids three processes must be considered linear condensation (eq. 14), cyclization (eq. 15), and disproportionation (eq. 16). 

Model Networks. Constmction of model networks allows development of quantitative stmcture property relationships and provide the abiUty to test the accuracy of the theories of mbber elasticity (251—254). By definition, model networks have controlled molecular weight between cross-links, controlled cross-link functionahty, and controlled molecular weight distribution of cross-linked chains. Sihcones cross-linked by either condensation or addition reactions are ideally suited for these studies because all of the above parameters can be controlled. A typical condensation-cure model network consists of an a, CO-polydimethylsiloxanediol, tetraethoxysilane (or alkyltrimethoxysilane), and a tin-cure catalyst (255). A typical addition-cure model is composed of a, ffl-vinylpolydimethylsiloxane, tetrakis(dimethylsiloxy)silane, and a platinum-cure catalyst (256—258). 

Alkaline co-condensation to yield commercial resins and the products of reaction obtained thereof [93,94] as well as the kinetics of the co-condensation of mono methylol phenols and urea [104,105] have also been reported [17]. Model reactions in order to prove an urea-phenol-formaldehyde co-condensation (reaction of urea with methylolphenols) are described by Tomita and Hse [98,102, 106] and by Pizzi et al. [93,104] (Fig. 1). 

Model studies directed toward the synthesis of Ecteinascidin 743 employed an elegant Pictet-Spengler cyclization of phenethylamine 54 and the 1,2-dicarbonyl compound 55 to assemble the spiro tetrahydroisoquinoline 56 in a stereospecific fashion. " The silica-catalyzed condensation reaction provided 56 in excellent yield. 

The mechanisms for model condensation reactions of para-hydroxymethyl-substituted phenol (and therefore para-quinone methide) with reactive ortho positions are described in Fig. 7.29. The phenolate derivatives react with para-quinone... 

The results in Table IV suggest that the condensation reactions cannot be described adequately by the ion-induced dipole model. In this regard the results agree with conventional studies which have frequently found a higher power inverse dependence of the cross-section on the field strength E for condensation reactions than for hydrogen transfer reactions. 

Model systems indicate that aldehydes may also be produced by the action of polyphenoloxidases on amino acids in the presence of catechin, all of which are present in coffee beans at some stage between green and roasted. For example, valine yields isobutanal, leucine yields isopentanal, and isoleucine yields 2-methyl-butanal.14 Some of these aldehydes probably undergo condensation reactions in the acidic medium of the roasted bean when moisture is present.15 Some dienals in green coffee beans have recently been identified as (E,E)-2,4- and (E,Z)-2,4-nonadienal and (E,E)-2,4- and (E,Z)-2,4-decadienal.18... 

Thermochemical investigations of molybdate solutions have been carried out and reaction heats were measured (108,109). As the interpretation of calorimetric data depends heavily on the correct reaction model, progress in determining reliable enthalpy and entropy changes for condensation reactions have been hampered. However, since there is little doubt that [M07O24]6 is the first polyanion which forms on acidification, the enthalpy and entropy changes obtained for its formation should be meaningful. The values for Eq. (30) are AH° =... 

Darzens reaction of (-)-8-phenylmethyl a-chloroacetate (and a-bromoacetate) with various ketones (Scheme 2) yields ctT-glycidic esters (28) with high geometric and diastereofacial selectivity which can be explained in terms of both open-chain or non-chelated antiperiplanar transition state models for the initial aldol-type reaction the ketone approaches the Si-f ce of the Z-enolate such that the phenyl ring of the chiral auxiliary and the enolate portion are face-to-face. Aza-Darzens condensation reaction of iV-benzylideneaniline has also been studied. Kinetically controlled base-promoted lithiation of 3,3-diphenylpropiomesitylene results in Z enolate ratios in the range 94 6 (lithium diisopropylamide) to 50 50 (BuLi), depending on the choice of solvent and temperature. ... 

The threshold concentration of monomer that must be exceeded for any observable polymer formation in a self-assembling system. In the context of Oosawa s condensation-equilibrium model for protein polymerization, the cooperativity of nucleation and the intrinsic thermodynamic instability of nuclei contribute to the sudden onset of polymer formation as the monomer concentration reaches and exceeds the critical concentration. Condensation-equilibrium processes that exhibit critical concentration behavior in vitro include F-actin formation from G-actin, microtubule self-assembly from tubulin, and fibril formation from amyloid P protein. Critical concentration behavior will also occur in indefinite isodesmic polymerization reactions that involve a stable template. One example is the elongation of microtubules from centrosomes, basal bodies, or axonemes. 

LaMer, Monomer Addition Growth Model. Most of the recent publications (13,18,37,43-45) concerning the Stober silica precipitation describe a first-order hydrolysis of TEOS as the rate-limiting process in the silica particle precipitation. The second reaction step, the condensation reaction, was found to be faster by at least a... 

In this review, we will briefly discuss selected literature on hydrolysis and condensation reactions at silicon which are relevant to organofunctional silanes. Some studies will involve simpler (lower functionality) models some may involve related chemical reaction classes illustrating a mechanistic point. We will then discuss selected references focused on silanes of the types used as coupling agents and crosslinkers. Finally, some reactions at the interface will be mentioned. 

Position 5 is also the most reactive in the nitration of guaiacyl propane units containing a free phenolic hydroxyl group and in analogous model compounds (7). By contrast, acid-catalyzed condensation reactions appear to proceed more rapidly at position 6 (15) and in this respect resemble the protodedeuteration reactions. 

Condensation of TEOS could be controlled by the reaction rate and/or the diffusion of water, while copolymerization could be controlled solely by the diffusion rate of PDMS. Proposed structural models of ormosils based on the reaction mechanisms before gelation are shown in Figure 14. The TEOS/PDMS ratio of the ormosils was 1/0.082. Immediately after mixing, the self-condensation of TEOS(I) was predominant over copolymerization between PDMS and TEOS. As the reaction time increased, copolymerization between PDMS and TEOS(II) was promoted. At this time, the PDMS chains were broken into shorter chains and/or cyclic D4C tetramers. As copolymerization and condensation reactions of TEOS proceeded, the solution gelled (III). After gelation, syneresis (IV) occurred and nonbridging PDMS chains and cyclic D4C tetramers were released from the gel. 

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