Science Record

In the polyesterification reaction between ethylene glycol and 1,2-propylene glycol with dibasic acid, their electrons are transferred as follows:

HO > CH > CH. >O cee ! : = (x : fe HO —> CH(CHs)—»>CH: ——> 06—CG=0. (5)

Because the methyl group can form hyperconjugation with the lone pair of electrons of oxygen atom, the oxygen atom in the 1,2-propylene glycol thus carries less negative charge than that in the ethylene glycol, and therefore the reaction in the former proceeds more easily than that in the latter. This explains why the first phenomenon occurs. For the second phenomenon we may explain in a similar manner as for the first. When the length of the chain along the two hydroxyl groups of the glycol is increased, the effect of the electronic transference becomes small in formula (5), the reaction in eq. (4) proceeds easily, and therefore, the velocity constants are increased as the length of the chain becomes longer. Of course, the velocity constant will tend toward a definite value as the length of the chain becomes sufficiently long so that the interaction between the two hydroxyl groups can be neglected.

In conclusion, from the large amount of experimental data which we have obtained by our kinetic studies on the polyesterification reactions among the thirty five pairs of dibasic acids and glycols, we have confirmed once again the newly suggested mechanism of hydrogen ion catalysis in polyesterification reaction, and furthermore, we have verified that the fundamental assumption in the kinetics of the polycondensation reaction, ie., that all similar functional groups have the same reactivity and hence the same velocity constant no matter whatever the chain-length of the molecules may be, is essentially correct. We have also made a detailed analysis of the relationships between the velocity constants of polyesterification reactions and the molecular structures of the dibasic acids and the glycols, and explained those phenomena theoretically.

REFERENCES

[1] Tang, Au-chin, Yao, Kuo-sui, Lin, Te-hou, Li, Yu-wei and L. Shin-wei 1957 Science Record, New Ser. 1, 243.

[2] Tang, Au-chin, Yao, Kuo-sui and Co-workers 1958 Acta Chimica Sinica, 24, (6). .

[3] Flory, P. J. 1939 J. Am. Chem. Soc. 61, 3334.