Trans-esterification, melt

It is expected that if trans esterification occurs, the intrinsic viscosity of the samples will decrease and the rates of crystallization and ultimate degrees of crystallinity will also decrease. Therefore a series of studies on the 50/50 blend which was kept in the melt for 2, 3, 5, 7, and 10 minutes were performed. The crystallization was followed by DSC and IR. Simultaneous measurements of the intrinsic viscosity were made on a 40 60 mixture of 1,1,2,2,-tetrachloroethane phenol. The results are plotted in Figures 28, 29, and 30. 

Blends of polybutylene terephthalate and polyethylene terephthalate are believed to be compatible in the amorphous phase as judged from (a) the existence of a single glass-transition temperature intermediate between those of the pure components and (b) the observation that the crystallization kinetics of the blend may be understood on the basis of this intermediate Tg. While trans esterification occurs in the melt, it is possible to make Tg and crystallization kinetics measurements under conditions where it is not significant. When the melted blend crystallizes, crystals of each of the components form, as judged from x-ray diffraction, IR absorption, and DSC. There is no evidence for cocrystallization. There is a slight mutual melting point depression. 

A common method for polyester formation is via trans-esterification. The procedure described below uses a procedure that is modified from that described in Sorenson. In this case, aromatic di-acid 5 and di-ester 4 monomers are reacted together in the melt. During the polymerization, the molecular weight grows while volatile side-product is continuously removed from the reaction vessel by means of a vacuum pump. Scheme 2 describes the formation of a main-chain liquid crystalline poly(ester ether) containing... 

Polymer-bound Catalysts Various quaternary phosphonium salts are known to be excellent catalysts for trans-esterification and condensation polymerizations, e.g. melt polymerization of polycarbonates and polyesters. The ionomers from Exxpro elastomer can be used in those applications with added advantages such as solubility in the melt, ease of recovery and thermal stability. 

The product of the prepolymerization stage is continuously fed into the melt polymerization reactor, where the condensation reaction of the diol-ester to form the corresponding polyalkylene terephthalate (PAT) (either PET or PBT), shown in Figure 13.10. In this self-trans-esterification, the ester-diol is the source of both ester and alcohol functionalities. Since these PE products are semicrystalline, the reaction must be carried out above their melting point (r j) to prevent solidification of the... 

In the copolymers described in Sect. 3.4, the multiple components of the system are joined by chemical bonds and demixing, needed for complete phase separation of the components, is strongly hindered and may lead to partial or complete decoupling from crystallization. The resulting product is then a metastable micro- or nanophase-separated system with arrested, local equilibria. In some cases, however, it is possible to change the copolymer composition during the crystallization or melting by chemical reactions, such as trans-esterification or -amidation. In this case, the chemical and physical equilibrium must both be considered and a phase separation of the copolymer into either crystalline homopolymers or block copolymers is possible. 

Of special importance is in this case that the backbone of the copolymer can change chemically by trans-esterification (see Fig. 3.47). In fact, heating poly(ethylene terephthalate) and acetoxybenzoic acid in the melt is a common synthetic route to PETcoOB. The inclusion of oxybenzoate groups makes poly(ethylene terephthalate) increasingly stiffer (see Fig. 1.50). In POBcoON, oxynaphthoate adds an additional off-set of the large zig-zag of the benzoate. As a result of this chain stiffening, the PETcoOB with more than 30 mol-% oxybenzoate shows an anisotropic melt, as for a nematic main-chain liquid crystal (see Fig. 5.136) and the system POBcoON has a mesophase character over the whole concentration range. 

Zhang et al. [133] reported that PHB/PDLLA blends prepared by solvent casting were immiscible. However, greater miscibility was observed for 60% PLA and 40% PHB samples prepared by melt blending. The blend solution was magnetically stirred and evaporated at 190°C for 30 min. The miscibility was ascribed to possible small-scale trans-esterification. The rate of crystallization of PHB decreased with an increase in the PLA content in the blends, and the final blends showed improvement in mechanical properties compared to the neat polymers. 

Such distannoxanes have been found to be highly active and selective catalysts for (trans)esterification reactions of methyl esters or acids with a twofold excess of alcohol and 0.5 mol% of distannoxane, either solventless or in toluene.A similar 100% yield in esters could be observed with a stoichiometric amount of alcohol in fluorous solvent that permitted recycling of the catalyst without loss of its performances. " The proposed mechanism consists in the activation of the alcohol via Sn-X (Y) ligands exchange to Sn-OR ones (Scheme 21.13b), while the ester is coordinated through the ketonic o>ygen atom to an adjacent tin centre (Scheme 21.13c). A similar dinuclear activation was reported for the melt transesterification of a variety of polyesters and polyacetates in the presence of such distannoxane catalysts. 

Suitable trans-esterification catalysts are numerous and include a wide variety of bases and Lewis acids. Bases derived from alkaline and alkaline earth metals, such as, magnesium hydroxide and sodium stearate are known to cause transesterification of polyesters and polycarbonates in an extruder however, these basic catalysts can cause molecular weight loss during or post-extmsion, especially if the catalysts are not quenched downstream. Thus, there is a need for transesterification catalysts which are reactive enough to cause transesterfication in the extruder without the detrimental effect of causing molecular weight degradation or melt stability problems. 

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