Interchain transesterification reactions

The potential for rapid randomizing processes in the copolyesters at elevated temperatures has been demonstrated conclusively by heating a mixture of the two homopolymers of PHBA and PHNA at 450 °C at a pressure of around several hundred psi [40]. Within a few seconds a viscous melt was observed to extrude from the cracks in the mold. Analysis of this material showed a structure consistent with the random 50/50 copolymer of HBA/HNA (see Figs. 18 and 19). We estimate that at this very high temperature the rate of interchain transesterification reactions corresponds to 1000 ester interchanges/chain/10 s. 

Only after heating for 15 min does the crystallinity disappear after quenching (see Fig. 27c). This experiment suggests that diffusion processes in the melt are very slow and that there is a strong memory which persists well into the nematic state. In fact it is not clear that the primary mechanism acting to eliminate the memory is diffusion or interchain transesterification or possibly both. At this point it is safe to argue that both diffusion and interchain transesterification reactions are operative over the 15 min time span at 35 °C above Tcn. 

SCHEME 25.4 (a) Inlrachaiii and (b) interchain transesterification reactions occurring in LA polymerization (P, P = polymer chain M = metal L = ligand). 

Abstract Recent progress on elucidating the role of interchain transesterification reactions in the copolyesters is described. New data are presented supporting the chemical nature of the ordering process. An unexpected degradation reaction in the p-hydroxybenzoic acid/2,6-hydroxynaphthoic acid copolyester system is discussed. 

The above conclusion on the role of reactivity ratios on microstructure assumes the absence of rapid transesterification reactions between chains. Since such processes might also tend to randomize the microstructure, it seemed important to isolate the role of interchain transesterification. A unique experiment was designed in which a 13c labeled carbonyl in acetoxy benzoic acid monomer (B ) was reacted with the dimer of HBA-HNA. At 99% enrichment, the only resonances in the carbonyl region of the spectrum will arise from the enriched benzoic acid carbonyl. In the absence of any interchain transesterification the microstructure of the polymer would consist only of B -B dyads (see scheme 1). 

On the other hand presence of B -N dyad in the polymer could only arise from a transesterification reaction where the B was inserted between the -BN-. As shown in Figure 3, NMR displays a small but distinct peak (ca.14%) at the resonance position corresponding to B -N diad. Since the polymerization was run at the same temperature of 245°C forl70 min and to the same degree of polymerization (M n 2252) as the earlier experiment on the reactivity ratios, one can conclude that the role of interchain transesterification is relatively small and that the monomers have approximately equivalent reactivity ratios. The reaction of B with the HBA-HNA dimer was also examined at 225° and 285°C to determine the possibility of a temperature effect The times of reaction were 20 hrs and one hour, respectively,... 

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