Condensation kinetics

In this section we turn to a consideration of the experimental side of condensation kinetics. The kind of ab links which have been most extensively studied are ester and amide groups, although numerous additional systems could also be cited. In many of these the carbonyl group is present and is believed to play an important role in stabilizing the actual chemical transition state involved in the reactions. The situation can be represented by the following schematic reaction ... 

The flux boundary condition accounting for vaporization and condensation kinetics at membrane-vapor interfaces is... 

Investigation of template poly condensation kinetics has only been studied within a very narrow scope. Polymerization of dimethyl tartrate with hexamethylene diamine was found to be enhanced by using as a template poly(vinyl pyrrolidone), poly(2-vinyl pyridine), or polysaccharides and poly(vinyl alcohol), poly(4-vinyl pyridine). In this case, the template can be treated as a catalyst. No information exists on the influence of the template on the order of reaction. The increase in molecular weight of the polymerization product by the template can be induced by a shift of equilibrium or by an increase in the reaction rate. A similar increase in the reaction rate was observed when poly(4-vi-nyl pyridine) was used in the synthesis of poly(terephtalamides) activated by triphenyl phosphite.The authors suggested that a high molecular weight template was involved in the increase of the local concentration of the substrate (terephthalic acid) by adsorption and activation via N-phosphonium salt of poly(4- vinyl pyridine). 

The fact that silanol persistence can be favored by equilibrium conditions rather than control of condensation kinetics by steric or electronic factors is usually not considered. The phase separation which results from highly condensed systems continuously removes material from deposition solutions, depleting soluble silane species. While condensed silanols or siloxanes are typically not regarded as participating in a reversible reaction with water or alcohol, they do indeed participate in an equilibrium reaction. Iler [16] has shown that even hydrated amorphous silicon dioxide has an equilibrium solubility in methanol, which implies the formation of soluble low molecular... 

Work by Sigwalt, Bischoff and Cypryk22 have used this inter-and intramolecular catalysis to explain the condensation kinetics and cyclic formation processes in siloxane condensations. The kinetics show a very complex dependence on siloxane chain length, complicated by equilibria involving acid, silanol and water. They do indicate that the dominating reaction in the process is condensation and that chain disproportionation and chain scrambling are negligible The kinetics of condensation are influenced by the involvement of triflic acid in several equilibria, i.e. the formation of triflate esters, shown in equation 3, the possible involvement of triflic acid in the reaction of these esters with silanol, shown... 

Domine and Gogos (88-90) considered a very long, very wide, and thin mold being fed by a constant temperature mixture of AA, BB molecules. Both types are bifunctional and the feed has a molecular weight Mq. The polymerization, assumed to be reversible, proceeds by the reaction of A-ends with B-ends, and follows idealized step polymerization (condensation) kinetics without the generation of a small molecule (91). Specifically, we have... 

Hydrolysis and Condensation Kinetics of Dimeric Sol-Gel Species by Si NMR Spectroscopy... 

High-resolution Si NMR spectroscopy was used to study the hydrolysis and condensation kinetics of monomeric and dimeric species in the silicate sol-gel system. Peak assignments for the kinetics experiments were determined by comparing add-catalyzed reaction solutions prepared with limited amounts of water with the synthetically prepared dimeric, trimeric, and tetrameric species. Si NMR peaks were assigned for 5 of the 10 possible dimeric species. The temporal evolution of hydrolysis and condensation products has been compared with a kinetic model developed in our laboratory, and rate constants have been determined. The results indicate that the water-producing condensation of dimeric species is approximately 5 times slower than the water-producing condensation of the monomeric species. The alcohol-producing condensation of dimeric species is comparable with that of monomeric species. 

Analysis of Condensation Kinetics. An earlier publication (8) explained how, under certain limiting conditions, the observed rate constants for water- and alcohol-producing condensation reactions in the TMOS sol-gel reactions can be calculated. The limiting conditions are (1) hydrolysis occurs on a time scale short compared with condensation and (2) the concentration of Si-O-Si bonds formed by condensation reactions is small compared with the initial concentration of the methoxy functional group. If these conditions... 

Tachibana S., Nagahara H., and Ozawa K. (2001) Condensation kinetics of metallic iron and its application to condensation in the solar nebula. Lunar Planet. Sci. XXXII, 1767. Lunar and Planetary Institute, Houston (CD-ROM). 

Ethanol purchased from Verbiese (France) (purity > 99.77 %) is mixed with double distilled and deionized water (resistivity 18 MO/cm) for the preparation of aqueous solutions of concentration 0.6, 1, 2, 3, 4.5, 9 and 17 mol%. The solutions are placed in an insulated glass bulb connected to a vacuum line. The vapour-liquid equilibrium conditions are first obtained at 295 K before a contact between the gas phase in equilibrium above the liquid and the pre-cooled sample holder of a cryostage is established. The deposition takes place at 10 Torr and 88 K in 3-5 min. Using thermodynamic modelling and a condensation kinetic model (relation below), one derives the concentration XEton) of EtOH in the deposited solids as ... 

For silica gels a number of parameters have been demonstrated to have a large effect on the evolution of porosity and subsequently on the resulting silica materials [1]. Almost dense, micro- or mesoporous silica materials can be obtained depending on the experimental conditions in which hydrolysis and condensation reactions of silicon alkoxides are carried out. This is not the case for transition metal alkoxides which are very sensitive to hydrolysis. They do not cillow the adaptation of sol-to-gel transition in order to obtain controlled porous textures. Some years ago special attention was paid to the utilization of amphiphilic systems as reactive media to control hydrolysis and condensation kinetics with transition metal alkoxides [37]. In a more recent work Ayral et al. 

Preparation of Materials with Special Functions. The porosity not only defines the adsorption parameters of a material, but also its mechanical stability. On the basis of this idea and the hydrolysis and condensation kinetics and its effect on microstructure, an abrasive powder was developed (12-13) with an abrasion-controlling mechanism for human skin. The investigation of the synthesis parameter shows a direct connection between the composition and the mechanical properties of the granular material (Figure 6). As expected from the previous experiments, HCl had to be used to provide reproducible CH3 concentrations and well-defined, reproducible material properties. 

There is a wide range of polymers that are polar in their compositional makeup. These macromolecules can be produced in numerous ways, including chain growth polymerization and step growth polymerization. Chain growth polymerization involves a rapid macromolecule formation that suddenly stops. Step growth polymerization exhibits condensation kinetics. It includes not only small molecule removal, but also reactions in which there is no small molecule removal. Analytical pyrolysis can be a useful qualitative as well as quantitative tool in the characterization of these macromolecules. 

Average molecular weight development can be measured directly through GPC or SEC, as we mentioned earlier. These measurements have their own problems, but can be very useful when properly tested and interpreted. They provide an excellent basis for predicting PF performance. They can also give an overview of PF condensation kinetics and even some information about polymer shapes. However, they do not provide detailed information on the chemical structure of the polymer. Such information is required to propose reasonable mechanisms. - C-... 

Carruthers, C., Francoeur, K., and Teitelbaum, H. Studies of condensation kinetics behind incident shock waves. Shock Tubes and Waves (H. Gronig, ed.), VCH, pp.327-333 (1988). 

Reaction/condensation kinetics can be controlled with their functionalization of particle properties in the nanoscale... 

---