Semiaromatic polyamides are high-performance thermoplastics with a huge potential but also a high price. In this study, the condensation kinetics of aliphatic diamines with aromatic diacids, diesters, and polyesters were investigated, to find ways of producing semiaromatic polyamides at lower costs. The condensation of hexamethylene diamine with phthalic acids and their dimethylesters are discussed in this Part I. The amidation of phthalic acids turned out to proceed like the amidation of adipic acid leading to PA66, only more slowly. The amidation of dimethylphthalates proved to be difficult. It is accompanied by a side reaction of amino alkylation which prevents the growth of long polyamide chains. This side reaction was investigated kinetically in detail, using model compounds that were unable to polymerize. The aim was to provide a basis for attempts to amidate polyesters, in particular, PET, which will be reported in Part II.

Polyphthalamides are distinguished by outstanding properties. Only their price keeps them from replacing PA66 in many applications, in particular; in automobiles. This article deals with a simple, economically attractive process of upcycling postconsumer polyethylene terephthalate (PET) waste to polyphthalamides. Basic studies on the amidation of acids and esters were described in Part I. The amidation of esters is accompanied by a side reaction of amine alkylation, which, in the case of polyesters, leads to branching and crosslinking. But the crosslinking can be avoided in mixtures with excessive diamine. Therefore, a heterogeneous amidation process was developed in which solid PET granules are simultaneously dissolved in and amidated by a liquid diamine. As long as the PET diamine system is heterogeneous, it reacts under amino excess conditions. Later, when all PET is dissolved, it comes back to stoichiometric conditions again, so long-chained polyamides can be obtained. In terms of molar mass and viscosity, this amidation of solid PET proceeds very similarly to the amidation of terephthalic acid (TPA), which is so far the common route towards polyphthalamides. PET waste could replace TPA.