Dong-Sook Kwon

Thermal/oxidative degradation and stabilization of polyethylene glycol

Thermal degradation and stabilization of polyethylene glycol (PEG) with a molecular weight of 6000 have been investigated in order to develop PEG as a thermal energy storage material. Low molecular weight esters including formic esters were produced as the main products of the thermal degradation of PEG at 80°C in air. No degradation was observed for PEG aged in a vacuum. The mechanism of thermal degradation is found to be the random chain scission of the main chain. Thermal degradation could be suppressed by adding 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) (MBMTBP) as an antioxidant. In the stabilizing process, MBMTBP itself is transformed to dimers or trimers.

Thermal degradation of poly(ethyleneglycol)

The thermal degradation of poly(ethyleneglycol) (PEG) with an average molecular weight of 4000 has been investigated from the point of view of developing PEG as a phase change material in thermal energy storage. The PEG was maintained at 80 °C for 1000 h in air and in vacuo. It was severely degraded in air; the melting point and heat of fusion were reduced by as much as 13 °C and 32 kJ/kg, respectively. In vacuo, however, almost no degradation was observed. Both the fresh and degraded PEG samples were examined by thermal and spectroscopic methods. The results suggest that the thermal degradation of PEG in air follows a random chain scission oxidation mechanism. This was confirmed by adding an antioxidant which successfully suppressed the degradation.

Acid dissociation constants of phenols and reaction mechanism for the reactions of substituted phenyl benzoates with phenoxide anions in absolute ethanol

Abstract Acid dissociation constants of 10 substituted phenols have been measured by a kinetic method together with second-order rate constants for the reactions of aryl benzoates (X-C6H4CO-OC6H4-Y) with Z-substituted phenoxide (Z-C6H4O−) and EtO in absolute ethanol at 25.0±0.1°C. The kinetic results support a stepwise mechanism for the present acyl-transfer reaction.

Solvent effect on rate for anionic nucleophilic substitution reaction of 4-nitrophenyl acetate in aqueous acetonitrile

Abstract Second order rate constants have been measured for anionic nucleophilic substitution reactions of 4-nitrophenyl acetate in MeCN-H2O mixtures. The results show rate minima near 30–40 mole % MeCN, which are attributed to changes in solvent structure.

Evidence of a stepwise acyl-transfer reaction mechanism: Nonlinear Hammett plots for reactions of p-nitrophenyl substituted benzoates with hydroxide and p-chlorophenoxide

Abstract The Hammett plots obtained for the title reactions exhibit a break, i.e. ϱ acyl values decrease from 2.21∼2.44 to 1.45∼1.52 as the acyl substituent becomes a strong electron withdrawing group ( σ >0.6). Such a break in the Hammett plots is suggestive of a change in the reaction mechanism and strong evidence of a stepwise mechanism for the acyl-transfer reaction.

Transannular Diels-Alder reaction of a macrocyclic triene, (E,E,E)-13-trideca-2,8,10-trienolactone☆

Abstract ( E , E , E )-13-trideca-2,8,10-trienolactone 3 undergoes rapid transannular Diels-Alder reaction to produce the trans-anti-cis tricyclic lactone 4 as a single cycloadduct.

The effect of solvent on the α-effect: Abnormal nucleophilic reactivity of substituted acetophenone oximate anions in aqueous DMSO

Abstract The nucleophilic reactivity of substituted acetophenone oximate anions decreases with increasing base strengthening ability of the substituent, resulting in a positive Hammett ϱ value in H 2 O containing 20 mole % DMSO.