Fibers of poly(p‐phenylene terephthalamide) (PPTA) have a fibrillar morphology, the individual fibrils having a high proportion of extended chains passing through periodic defect layers. A pleat structure is superimposed. The fibers are fully crystalline (within the limits of determination) with a small fraction of randomly oriented crystalline material. The major distinction between PPTA and conventional fibers lies in the high level of extended chains passing through the defect layers of the former structure. These extended chains result in crystallographic register being maintained between adjacent ordered zones. Quantitatively, a measure of this order is obtained from a comparison of the correlation length, obtained from meridional x‐ray peak widths, and the defect spacing. In conventional fibers the defect spacing, i.e., long period, is longer than the correlation length (i.e., crystal size). In PPTA, the analog of the long period, the defect spacing (about 35 nm) is smaller than the correlation length, which is over 80 nm.

Poly(2,2-dialkyl-3-hydroxypropionic acid)s (usually prepared by living anionic polymerization of the corresponding 2,2-dialkyl-3-propiolactone) thermally degrade by two independent mechanisms depending on the state of the carboxylate group at the termination end of the polymer chain. When the carboxylate is deprotonated (X = Li+, Na+, K+, Cs+, R4N+, or R4P+) the polymer degrades predominantly by reverse polymerization. This paper concentrates on the reverse polymerization mechanism for polymer degradation. Reverse polymerization is dependent on the nucleophilicity of the terminal carboxylate group. This is demonstrated by the effect of the terminal counterion on the rate of degradation. Furthermore, reverse polymerization is shown to be a "living depolymerization". By living depolymerization we mean that all polymer chains degrade simultaneously from the living ends of the chains. Kinetics, change in MW distributions, and degradation of block copolymers are consistent with a living depolymerization.

The thermal degradation of poly(2,2-dialkyl-3-propiolactones) (I) has been studied. By comparing the thermal degradation of capped (methyl esterified) and uncapped (carboxylic acid terminated) polymer, it is demonstrated that above ~350 °C, I degrades by unzipping from carboxylic acid termini. Random scissions can generate chains with carboxylic acid termini. Capped I requires random scission generation of carboxylic acid termini before polymer unzipping can ensue. One random scission process elucidated requires the presence of a C-H group at the 2 position of one of the alkyl side groups in I and generates two distinguishable chains. One of the chains has a carboxylic acid terminus that can unzip. The second chain is still capped and must undergo further random scissions before mass loss can occur.

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