The solid-phase polycondensation (ester interchange) reaction leading to the formation of polyethylene terephthalate has been studied. Two types of material were polymerized: the monomer, dihydroxyethyl terephthalate (DHET), and polymeric samples. In the former case reaction only occurs at temperatures close to the melting point and the reaction rate is critically dependent on the temperature in this temperature region; the nature of the dependence is related to increase in mobility of molecules within the crystal lattice, and the presence of impurities used as catalysts broadens the region over which reaction may occur. Polymer reacts over a wider temperature range (e.g. perceptible reaction takes place at 150°C with polymer melting at ca. 260°C) and the activation energy for the reaction is approximately the same as that obtained for melt polymerization. From the kinetic data it is deduced that polymerization occurs only within a limited depth from the surface of each polymer particle. The velocity coefficients for the reaction are much greater than the velocity coefficients for melt polymerization extrapolated to the temperatures used for the solid phase studies. Suggestions are made about ways in which these increased rate constants could arise. The molecular weight distribution of polymer prepared in the solid phase is thought to be abnormal, both from kinetic and other evidence. This abnormality appears to arise from a spatial heterogeneity and differential thermal analysis confirms that the polymer is inhomogeneous. © 1969.
