Polymerization, morphology, and crystal structure of poly(p-benzamide)

Isothermal polymerization of acetaminobenzoic acid at various temperatures in bulk and solution was monitored by acetic acid evolution. The polymerization took place initially in a homogeneous phase, but it soon changed to heterogeneous as the polymer precipitated from the melt or solution. The reaction course consisted of a short induction period, followed by a fast second-order reaction and a third slow, also second-order, reaction. The different polymerization rates in the second and third stage were due to polymer precipitation. The degree of polymerization of the resulting polymer was generally low, usually less than 5. The polymer was highly crystalline and prone to decarboxylation at temperatures above 300 degreesC. Sample structure and morphology were characterized by various lamellar formations similar to those seen previously in many aromatic polyesters. Epitaxial solution polymerization/crystallization on mica resulted in three different morphologies, the most unique being shish kebab-type structures. Polybenzamide [poly(p-aminobenzoate)] is polymorphic; the most frequently seen hk0 electron diffraction patterns were similar to those from phase I of aromatic polyesters. Phase II of polybenzamide was seen only rarely. Simulation, using Cerius(2), suggested equal agreement with [001], [100], and fiber electron diffraction (ED) patterns and X-ray powder patterns for two different orthorhombic unit cells (a = 7.71, b = 5.35, c = 12.94 Angstrom, rho = 1.55 g/cm(3)); both have a parallel packing of adjacent molecules, but in one, successive phenyl rings along a given chain are parallel, while in the other they are twisted at +/- 30 degrees. For both unit cells, neighboring molecules were displaced by +/-0.14 Angstrom along c to obtain better agreement with the observed patterns. The [010] patterns could distinguish between the two cells, but were not obtainable.