Polycondensation exchange reactions

The effects described by Eqs. 4.1 to 4.4 apply to both stepwise addition and condensation polymerizations. In polycondensation, two further factors must be considered The condensation equilibrium and the exchange reactions. The condensation equilibrium limits the conversion and hence the average molecular weight. As in the case of esterification of monofunctional compounds, the corresponding polycondensations are to be treated as equilibrium reactions, governed by the law of mass action. For example, in the case of polyesterification, if... 

The second factor that additionally effects polycondensations are exchange reactions which can occur between free end groups and junction points in the chain, for example, between OH end groups and ester groups of a polyester (transesterification) ... 

In 1958, Eastman Kodak introduced a more hydrophobic polyester fiber under the trade name Kodel. The raw material for this polyester is dimethyl terephthalate. Reduction leads to 1,4-cyclohexylene glycol, which is used with dimethyl terephthalate in the polycondensation (ester exchange) reaction. 

In reality, allophanate and biuret formation is reversible at temperatures above 130°C [252]. Formation of isocyanurates causes a reorganization of the CLD akin to what happens in reversible polycondensations because of exchange reactions. 

The reversible alternating polycondensation with FSSEs in both monomers (see Section 3.1.5), disregarding exchange reactions, is a convenient case study for discussing problems of modeling this kind of systems. It can be described by the rate laws of Eqs. (132) and (133). 

In a study on the example of model reactions of polycondensation of aroyl-bis-4-hydroxybenzoyl chlorides with aliphatic and aromatic dipoles, it was shown in [17] that the competing reactions of alcoholysis and phenolysis do not affect (in conditions identical to polycondensation) the internal aromatic ester bonds in the mesogenic fragment. In contrast to them, the tominal ester bonds in the fragment und go alcoholysis, but this does not result in perturbation of the regularity of the structure of the polyether. The interchain exchange reaction could be the cause of perturbation of the regularity of the structure of the chain, but NMR spectroscopy showed that the perturbations of the structure of the terephthaloyl-bis-4-hydroxybenzoate triad did not take place [17]. 

The preparation of polycondensates has been gradually abandoned and today ion-exchangers produced mainly by polymerisation reactions are used. Polymeric ion-exchangers possess higher chemical stability than polycondensates also it is easier to make ion-exchangers with the necessary outer shape, porosity and chemical composition from polymer resins. 

The polycondensation with formaldehyde according to the following reaction scheme was one of the first procedures for making cation-exchange membranes ... 

Clair It al (1996) reported major losses of concentrate due to the plumbing of their RO plant and observed precipitation in most samples. It was further reported that polycondensation reactions in the concentrate occurred which modified the acid-base characteristics. Phenolic compounds appeard to be lost with the permeate, but most likely, aliphatic carbon is lost. Clair it al (1996) considered the application of a cation exchange resin as a possible fractionation step. Sun it al (1995) suggested the use of a Na resin to avoid pH variation which leads to increased loss of organics in the permeate and enhances precipitation. 

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