Direct polycondensation

The relative rates of reaction between the hydrolysis, condensation, and polycondensation directly affect the type and quality of sol-gel produced. For instance, in base-catalyzed sol-gel synthesis, the rate of the hydrolysis reaction is much slower than the polycondensation step, which tends to form a polymeric structure that is highly branched with relatively large particles. In contrast, in acid-catalyzed sol-gel synthesis, the rate of the hydrolysis reaction is much faster than the polycondensation step, which favors the formation of linear polymers. This also tends to form much smaller particles that the gel is composed of, which results in materials with much higher porosities and higher surface areas than is formed with sol-gels formed in base-catalyzed reactions. Therefore, acid-catalyzed synthesis tends to produce materials that have properties that are preferable for microfluidic applications. 

The relative rates of reaction between the hydrolysis, condensation, and polycondensation directly affect the type and quality of sol-gel produced. For instance, in base catalyzed sol-gel synthesis, the rate of the hydrolysis reaction is much slower than the polycondensation step, which tends to form a pol)meric structure that is highly branched with relatively large particles. In contrast, in... 

Starting with DMT, methanol is removed from the reaction starting with TA, water is removed. Catalysts ate used to transesterrfy DMT but not for direct esterification of TA. The second step is the polycondensation reaction which is driven by removing glycol. A polycondensation catalyst is used. 

Activating agents, such as trifluoroacetic anhydride 1,1 -carbonyldiimidazolc carbodiimides sulfonyl, tosyl, and picryl chlorides and a range of phosphorus derivatives can promote direct solution reactions between dicarboxylic acids and diols or diphenols in mild conditions. The activating agents are consumed during the reaction and, therefore, do not act as catalysts. These so-called direct polycondensation or activation polycondensation reactions proceed via the in situ transformation of one of the reactants, generally the carboxylic acid, into a more... 

This method involves the direct polycondensation of aromatic diamines with aromatic diacids in the presence of an aryl phosphite (triphenyl phosphite) and an organic base like pyridine.7,9 70 71 The addition of salts improves die solubility of the polymer and, with this, the maximum attainable molecular weight.71 The concentrations are, however, lower than by the dichloride method. 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

Somewhat more complicated is the Markov chain describing the products of polycondensation with participation of asymmetric monomers. Any of them, AjSaAj, comprises a tail-to-head oriented monomeric unit Sa. It has been demonstrated [55,56] that the description of molecules of polycondensation copolymers can be performed using the Markov chain whose transient states correspond to the oriented units. A transient state of this chain ij corresponds to a monomeric unit at the left and right edge of which the groups A, and A are positioned, respectively. A state ji corresponds here to the same unit but is oriented in the opposite direction. However, a drawback of this Markov chain worthy of mention is the excessive number of its states. 

Acceleration of the Template- and Metal-Directed Polycondensation of Nucleoside-5 -phosphoric Imidazolides by Acylation... 

We have reported that the reaction of 2,4,6-tri(chloro)borazine CI3B3N3H3 with three equivalents of tris(isopropylamino)borane B(NHPr )3 led to the formation of 2,4,6-tri[bis(isopropylamino)boryl(isopropyl)amino]borazine 11 [Fig. 5(a)] in quantitative yield.26,27 The subsequent polycondensation carried out in vacuo up to 150°C, led to the formation of the polymer 12 containing B3N3 rings linked either through direct B—N interring bonds (minor pathway) or three-atoms —N—B—N— bridges... 

As mentioned previously, the main drawbacks of the thermal route to poly-borylborazine are (1) the presence of both direct intercyclic bonds and three-atom bridges between the rings, and (2) a difficulty in controlling the polycondensation rate. One solution we investigated to address these drawbacks is a route based on the room temperature reaction of /i-chloroborazine with trialkylaminoborane.31 32 We used 2-methylamino-4,6-dichloroborazine instead of 2,4,6-trichloroborazine to prepare a two-point polymer (scheme 4), which is theoretically less cross-linked. 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

S.-i. Kawano, S.-i. Tamaru, N. Fujita, and S. Shinkai, Sol-gel polycondensation of tetraethyl orthosilicate (TEOS) in sugar-based porphyrin organogels Inorganic conversion of a sugar-directed porphyrinic fiber library through sol-gel transcription processes, Chem. Eur. J., 10 (2004) 343-351. 

Generally speaking, we should note that the synthetic potentials of the polycondensation approach for the preparation of protein-like copolymers are only now beginning to be explored. Nonetheless, even the first results show promising directions for future synthetic developments that can hardly be reached through the polymerization processes. 

A powerful and efficient method for the preparation of poly(ketone)s is the direct polycondensation of dicarboxylic acids with aromatic compounds or of aromatic carboxylic acids using phosphorus pentoxide/methanesulfonic acid (PPMA)16 or polyphosphoric acid (PPA)17 as the condensing agent and solvent. By applying both of these reagents to the synthesis of hexafluoroisopropylidene-unit-containing aromatic poly(ketone)s, various types of poly(ketone)s such as poly(ether ketone) (11), poly(ketone) (12), poly(sulfide ketone) (13), and poly-... 

Fluorine-containing aromatic poly(benzimidazole)s are synthesized by direct polycondensation of 2,2-bis(4-carboxyphenyl)-l, 1,1,3,3,3-hexafluoropropane (15) with 3,3 -diaminobenzidine tetrahydrochloride (23) (Scheme 13) and 1,2,4,5-benzenetetramine tetrahydrochloride (24) (Scheme 14) in PMMA or PPA.24... 

High-molecular-weight fluorine-containing aromatic poly(benzoxazole)s have not been obtained either by the direct solution polycondensation in PPA at 200°C or by the low-temperature solution polycondensation in DMAc at 0 to 5°C from 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoropropane and aromatic diacid derivatives because the fluorine-containing monomer has low nucleophilicity owing to the presence of the electron-withdrawing hexafluoroiso-propylidene unit. 

The esterification of TPA is catalyzed by protons and in standard industrial operations neither an additional esterification catalyst nor a polycondensation catalyst is added to the esterification reactor. Some new antimony-free polycondensation catalysts [125-128] also affect the speed of esterification significantly and it could be advantageous to add them directly into the slurry preparation vessel. Co-monomers, which should be randomly incorporated into the polymer chains, are usually fed into the slurry preparation vessel. How and when additives, catalysts, colorants and co-monomers are added influences the overall reaction rate and therefore affects the product quality. 

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