Reaction rate constant, polyamides

It is therefore a valid approximation to look upon polyesterification, polyamidation, or polyurethane-forming reactions, and so on, as general reactions of the forms shown by Eqs. (5.13) to (5.15), respectively, in which the respective reaction rate constant does not depend on the sizes of the molecules to which the functional groups are attached ... 

Others have presented the kinetics of polyamidation differently. At high water concentrations (5-10 mol kg-1), a second-order reaction is given with an activation energy of approximately 86 kJ mol-1.5 612 28 At low water concentrations in die final stages of die polymerization, a mixed uncatalyzed second-order reaction and an acid-catalyzed third-order reaction are observed. The rate constant k in (3.13) can tiien be written as... 

Second-order rate constants for some acid-catalyzed polyesterifications and polyamidations obtained by the above method are shown in Table 5.3. The Arrhenius parameters A and E of the equation k = Aqxp —E/RT) are also tabulated for those reactions that have been studied kinetically at more than one temperature. 

In general, the reactions for polyamide formation are seldom catalyzed. The reactions to produce thermosets, such as melamine-urea or phenol-formaldehyde, require a basic or an acidic catalyst. Some polyurethane formation reactions are catalyzed by basic reactants. Equation 3.11 applies when the rate constant of the externally catalyzed reaction (k ) is much larger than that of the self-catalyzed... 

While deriving the dynamic batch-reactor models in Tables 7.3 and 7.4, the equal-reactivity-of-functional-groups assumption was made, so that kp is the rate constant for all forward linear polyamidation reactions and Ay is the rate constant for all reverse hydrolysis reactions involving amide links on linear chains. Let the rate constant for the forward cychzation reaction be Ay and the rate constant for hydrolysis of an amide link on cyclic oligomers be A . In practice all of these rate constants depend on the reactor temperature and... 

P-lactams. Monomers having a low rate constant of propagation are preferable for the synthesis of uniform polyamides. Drawbacks in the preparation of uniform polyamides from P-lactams are the very frequent occurrence of depolymerization and transamidation (at the acyl lactam chain end and on the polyamide chain), both causing broadening of molar mass distribution of the resultant polyamide. The latter reaction occurs more frequentiy at high conversion. Therefore, it is advisable to stop the polymerization at low to intermediate... 

Deleens C, Foy P and Marechal E (1977) Synthesis and characterization of copolycondensate sequences poly (amide-seg-ether) - III. Study of the polycondensation reaction of a ,a -dicarboxy polyamide-11 and a ,a -dihydroxy polyoxyethylene. Determination of the rate constants and the activation energy, Eur Polym J 13 353-360 (in French). 

Whenever possible the fourth point has been taken into consideration and radical concentrations have been corrected for losses due to recombination or other decay reactions of the (secondary) radicals under consideration. In the case of 6 polyamide fibers [5, 11, 17, 18] the radical decay is a second order recombination reaction with the rate constant depending on type, location, and mobility of the radicals (see Section III below). 

Ogata [94a] showed that the vEilue of the rate coefficient of polyamidation depended on the concentration of water when the reaction was carried out in a sealed tube in the presence of fixed amounts of water. He regarded the concentration of water as being constant under the conditions used, and the reaction then appeared to follow second-order kinetics, though the rate coefficient clearly must contain a term for the water concentration. 

The most frequently used is TGA at constant temperature or heating rate of = 5-20°C/min in vacuum, air or inert gas. " The degradation process has been described taking into account the reactions (i) initiation by random chain scission, (ii) depropagation, and (iii) chain termination." TGA is an integral part of synthesis and characterization of HTP and their blends. For example, developing new isomeric polythioetherimides (PTEI)," polyamides based on s-triazine," polyaryletherketones (PAEK)" or polyimides (PI)" " and their... 

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