Polyamidation in the Solid Phase

Fundamentals of Solid State Polymerization

“…In particular, regarding the acid-catalyzed polyamidation, the activity of catalysts follows the scheme of the nucleophilic acyl substitution. The proton of an acid (e.g., H 3 PO 4 , H 3 BO 3 , H 2 SO 4 ) becomes attached to the carbonyl oxygen, making the carbonyl group even more susceptible to nucleophilic attack by NH 2 [reaction (1.8)]; oxygen can now acquire the π electrons without having to accept a negative charge as is in the uncatalyzed substitution shown in (1.9) [67][68][69]. (1.9) Zinc chloride (ZnCl 2 ), phosphoric acid (H 3 PO 4 ), magnesium oxide (MgO), boric acid (H 3 BO 3 ), and others have been examined as catalysts during melt polycondensation of amino acids or diamines with dicarboxylic acids [68].…”

“…The dependence of the reaction temperature on the SSP rate constant is indicated by the values of the SSP activation energy (E a ), reported to be between 43.9 and 340.7 kJ mol −1 in the case of PAs and between 62.7 and 177.6 kJ mol −1 for polyesters, being in general higher than those for melt processes [79]. Optimized temperature ranges have been set for a variety of polymers: 20 to 160 • C below the final T m , and the most preferred temperatures are just below T m [67,68,82,[87][88][89][90][91]. Temperature-step processes are also not excluded, and in some cases they are preferred so as to avoid polymer grain agglomeration through gradual increases in the prepolymer softening temperature [87,[92][93][94][95].…”

“…An additional SSP parameter studied is the initial end-group concentration, which is assumed to affect significantly segmental mobility and diffusion of the reactive chain ends. The lower the concentration of end groups (the higher the value of initial number-average molecular weight, M n 0 ), the higher the number-average molecular weight, M n , at the end of the SSP reaction [44,67,85]. The high M n 0 value ensures a more effective confinement of the amorphous phase and therefore a high concentration and homogeneous distribution of reactive chain ends in the reaction zone according to the two-phase model.…”

“…Characteristic is the case of polyamides and polyesters, where the equilibrium constant for PAs is hundreds of times larger than that for polyesters, and much higher by-product concentration can thus be tolerated in the first case, having much less severe removal requirements [40]. For example, in polyamides, K eq varies between 100 and 750, depending on the water content in the reacting system [67,[106][107][108]; meanwhile, the relevant values for transesterification and esterification, used widely in PET melt condensation models, are 0.5 to 1 and 1.25, respectively. Correlating these K eq values with SSP data, it was found that in the case of PET SSP, a decrease in the particle diameter from 0.266 cm to 0.14 cm results in reducing the residence time by 56%, whereas the relative decrease in PA 66 SSP is only 3% [40].…”

“…The drawback here relates to the energy requirements for heating and drying the gas. The inert gases used most often in SSP processes are nitrogen, carbon dioxide, helium [118,119], superheated steam [67], and supercritical carbon dioxide [102,103,120,121]. In addition, three alternatives exist for the inert surroundings:…”