The formation of polyethylene terephthalate by ester interchange. I. The polycondensation equilibrium

The equilibrium conditions pertaining to reversible melt-polycondensation in the polyethylene terephthalate system have been investigated at different temperatures and average DP's. Free ethylene glycol, end-groups and monomer were determined quantitatively. The equilibrium constant K depends slightly on temperature but increases markedly with increasing extent of polycondensation, thus showing that polyethylene terephthalate does not obey the principle of equal reactivity. In addition, it appears that the monomer contents of various equilibrium products definitely exceed the theoretical contents given by the FLORY-SCHULZ distribution function. By assuming as a first approximation, that only monomer does not fulfil the principle of equal reactivity, it is possible to describe both the abnormal monomer contents and the increase of K in terms of a ratio K0/K1 = 1.8, where K0 denotes the equilibrium constant for polymer molecules and K1 that for monomer with polymer molecule. A detailed interpretation of the abnormal monomer reactivity will be given in a next paper dealing with the kinetics of reversible polycondensation.