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245	1	0	_aThe formation of polyethylene terephthalate by ester interchange. II. The kinetics of reversible melt-polycondensation
490	0		_vDie Makromolekulare Chemie: MacroMolecular Chemistry and Physics, 38(1), p.123-137, 1960
520	3		_aA kinetic investigation of uncatalyzed reversible polycondensation in polyethylene terephthalate has been made at different temperatures and average DP's. Since immediate and complete removal of ethylene glycol from the melt is impossible, the experiments were carried out in a closed system while the reverse reaction, i. e. glycolysis of polyethylene terephthalate, was fully taken into account. Free ethylene glycol, end-groups and monomer were determined quantitatively as a function of reaction time. The results have been evaluated in terms of the second-order rate constants k and k? for polycondensation and glycolysis, respectively. Both rate constants show about the same dependence on temperature, corresponding with activation energy of ?? 23 kcal./mole. Further, k increases with the extent of polycondensation, whereas k? remains constant. Thus, all ethylene diester groups along polymer chains are equally reactive in glycolysis, but all end-groups are not equally reactive in polycondensation. Finally, the monomer contents of polycondensates exceed the theoretical contents given by the FLORY-SCHULZ distribution function. Like in our previous equilibrium study, the increase of k as well as the deviation of the monomer contents can be described in terms of a ratio k0/k1 = 1.8, where k0 is the rate constant for polycondensation between polymer molecules and k1 that for polycondensation between monomer and polymer molecules. This deviation of monomer reactivity is not due to structural energy effects, but appears to be related to a lower activation entropy for reaction of monomer. The hypothesis is that small and mobile monomer molecules loose more rotational and translational entropy upon entering into the transition-state than the long and rigid polymer molecules do. May be, such an explanation generally applies to the reactivities of small molecules in melt-polycondensation.
700	1	2	_aChalla, G.
856	4	0	_uhttps://drive.google.com/file/d/1dDPJrcCzssIYBOVJ7uKMvIH0DNUtSJO4/view?usp=drivesdk _zPara ver el documento ingresa a Google con tu cuenta: @cicy.edu.mx
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