Self-Condensing Polycondensations

The finishing reactors used for PET and other equilibrium-limited polymerizations pose a classic scaleup problem. Small amounts of the condensation product are removed using devolatilizers (rotating-disk reactors) that create surface area mechanically. They scale as, S2/3. 

The removal of condensation byproducts becomes increasingly difficult upon scaleup. Some commercial PET processes use CSTRs for the early stages of the reaction where most of the byproduct ethylene glycol is removed. They use only the top, visible surface of the liquid for mass transfer and rely on jacket heating to supply the latent heat of vaporization. The surface area scales as and limits the production rate in some processes because the previous limit, a downstream finishing reactor, has been 

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Aldol Reaction (Condensation) [24] Traditionally, it is the acid- or base-catalyzed condensation of one carbonyl compound with the enolate/enol of another, which may or may not be the same, to generate a P-hydroxy carbonyl compound— an aldol. The method is composed of self-condensation, polycondensation, generation of regioisomeric enols/enolates, and dehydration of the aldol followed by Michael addition, q.v. The development of methods for the preparation and use of preformed enolates or enol derivatives that dictate specific caibon-caibon bond formation have revolutionized the coupling of carbonyl compounds (Fig. 6.6) ... 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

This reaction is reported to be much more difficult to perform than it appears. According to Rajan et al. 72) the mechanism is not typical of polycondensations, since appreciable amounts of high polymer are present at low conversions and unreacted monomer is found at high conversions. Numerous other synthetic routes have been described. According to Cleary 71) the only other process which has been unequivocally demonstrated to give the required polymer is the self condensation of alkali metal salts of 4-halothiophenols, as described by Lenz et al. 73). 

FIGURE 4.19 (a) Synthesis of polyimides by polycondensation, (b) Self-condensation of isocyanate of trimellitic... 

Hyperbranched polymers are synthesized in a one-step method, often from AB monomers but also by combining A +B (x>3) monomers or variations of those. Polymerization methods have been applied that involve polycondensation, polyaddition, and ring-opening or self-condensing vinyl polymerization. Even though the one-pot synthetic approach leads to imperfectly branched structures because of uncontrolled growth, it is more suitable for the preparation on a larger scale and thus for commercial use. Nowadays, different... 

Metal salts, like lithium chloride, significantly enhance reactions of carboxylic acids with amines promoted by triphenyl phosphite. This allows direct polycondensation of dicarboxylic acids with diamines and self-condensation of p-aminobenzoic acid. The presence of a solvent markedly enhances the reaction with the best results being obtained in A methylpyrrolidone. High molecular weight polyamides form. Mixed solvents, like pyridine and -methylpyrrolidone, can be used to form polyisophthalamides. This combination of solvents, however, yields only low molecular weight polyterephthalamides. On the other hand, when the reaction is carried out in the presence of polymeric matrices of poly(ethylene oxide) or poly(4-vinylpyridine), high molecular weight polyterephthalamides form . ... 

Hyperbranched polymers can be prepared by a variety of techniques, including the polycondensation of AB monomers as originally described by Flory [113], the reaction of A2 + B3 monomers, and self-condensing vinyl polymerization [139-141]. The first report [142] of using click chemistry in the synthesis of hyperbranched materials appeared at about the same time as the initial report for dendrimers prepared using CuAAC however, but much fewer examples have been reported that describe hyperbranched materials involving click chemistry. Nevertheless, these polymers represent an important class of materials, and both CuAAC [142-147] and thiol-ene [148] chemistry have found their way into the hyperbranched hterature. 

The functional groups occurring in the reactions (17-l)-(17-4) may be attached to AB as well as to AA or BB molecules, that is, in self-condensation or foreign polycondensations. The reaction, (17-5), is confined to selfcondensations. 

Polyesters contain the ester group —COO— in the main chain. Many methods are suitable for their synthesis self-condensation of a,o>-hydroxy acids, ring-opening polymerization of lactones, the polycondensation of dicarboxylic acids with diols, transesterification, the polycondensation of diacyl chlorides with diols, polymerization of O-carboxy anhydrides of a- and jS-hydroxycarboxylic acids, and the copolymerization of acid anhydrides with cyclic ethers. The last reaction is commercially used in the curing of epoxides with anhydrides. 

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