Exchange reactions, transesterification

Covalent polymers with reversible properties arising from dynamic covalent bonds such as disulfide exchange reaction [47 9], transesterification [50,51], transetherification [52], and boronate ester formation [53] were reported without respect to DCC. These studies should involve DCLs in... 

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) ... 

Transesterification reactions have been extensively used for the preparation of further alkoxides.158-165 Mixed alkoxides [M(OR)5. r(OR )j ] were obtained from the same alcoholysis reactions.166,167 Exchange reactions between [Nb(OEt) ] or [Nb(OPr )s] and organic acetates have also been exploited for the preparation of higher alkoxides. 

Transesterification [236] and the ester exchange reaction [237] were reported to be efficiently catalyzed (Eqs. 30 and 31). Either of the exchange reactions are sensitive to steric constraints of the substrates and to metal ion size. For example, transesterification is most applicable to primary alcohols. Increased catalytic activity in the presence of larger lanthanide centers is explained by enhanced coordinative unsaturation and increased basicity of the alkoxide complexes. Strong basicity of the lanthanide isopropoxides is considered to catalyze effectively the transhydrocyanation from acetone cyanohydrin to sev-... 

This particular reaction is called ester exchange, or transesterification—the conversion of one ester into another. Because it is a reversible reaction, the methanol byproduct needs to be removed from the reaction mixture as the polymerization proceeds in order to drive the reaction toward high polymer. 

In fact, we now have an equilibrium reaction. If R1 and R2 are different then the reaction is called ester exchange or transesterification and we should drive it in the direction we want by using a large excess of one of the two alcohols. If we carried out the reaction on one ester using an equivalent of the other alkoxide in that alcohol as solvent, the other ester would be formed in good yield. 

It can be speculated that the ester exchange reaction has already occurred but complete randomization of the monomers has not yet occurred. In that case, a blocky structure would be expected. However, as will be demonstrated later, the observed DSC traces are not consistent with the occurrence of a blocky structure. Thus, the conclusion is reached that the present data suggests that transesterification is not a dominant mechanism in determining the behavior of the present blends. 

Ester-exchange reactions in polyesters (and analogous amide reactions in nylons) that result in a new block copolymer. An example is the formation of a blend between polycarbonate and poly(butylene terephthalate) in which the compatibilizer is formed by a transesterification reaction at elevated temperatures (350 °C), but, when processed at lower temperatures, the systems are not compatible. 

The transesterification reactions in PBT/PC melt blends could be suppressed by using organo-phosphites and phosphonates which probably function by deactivating the titanium or antimony type polymerization catalyst residues present in PBT [Golovoy et al., 1989]. Even in the presence of phosphite stabilizers, PBT/PC blends showed dual phase behavior. However, a partial miscibility was evident since the T of PC phase was still reduced from the normal 150°C to about 140°C. This partial miscibility between PBT and PC which occurs even in the absence of an exchange reaction is responsible for the good compatibility and interfacial strength of the blend. 

The kinetics of transesterification reaction in PET/ PEN blends was studied by the model compounds ethylene dibenzoate (BEB) and ethylene dinaphthoate (NEN). The exchange reaction between BEB and NEN was followed by H NMR spectroscopy [24]. 

How may also result in mechanochemical degradation processes that generate reactive sites, viz., radicals, peroxides, acids, etc. Furthermore, transesterification and ester-amide exchange reactions are well documented. These reactions affect the phase equifibrium as well as the regularity of the chain structure, thus dispersion in the blend and its crystallinity. 

These exchange reactions of Fischer carbene complexes occur through tetrahedral intermediates, just like the transesterification of esters. Reaction of a Fischer carbene complex with an alcohol in the presence of base generates the same type of anionic intermediate that an ester does. This intermediate breaks down like an ester to form a new alkoxy-substituted carbene complex. Reactions of amines with Fischer carbene complexes occur similarly, but the high basicity of amines and the high electrophilicity of the carbene complexes alleviates the need for any additional base. 

Related exchange processes that could be alternatives to the transesterification reaction to produce similar DCLs of receptors under different reactions conditions are the exchange of thioesters and the exchange of amides. Thioesters have been used recently as the reversible bond in the formation of DCLs in water [39-41]. The exchange reaction is usually performed by mixing stoichiometric amounts of thiols and thioesters in aqueous solution (pH 7-9), without the need for any activation procedure. 

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