Thermal Reorganization and Molecular Weight Control of Dynamic Covalent Polymers Containing Alkoxyamines in Their Main Chains

Dynamic covalent polymers incorporating thermally reversible alkoxyamine units in the main chain were synthesized, and their thermal structural reorganization behavior was systematically investigated. The model reaction using alkoxyamine derivatives clearly revealed that the exchange reaction between the derivatives occurred upon heating over 60 °C, and the degree of exchange strongly depended on the reaction temperature, time, and concentration. Polycondensation from alkoxyamine-based diol and adipoyl chloride at room temperature successfully afforded the corresponding poly(alkoxyamine ester) without any decomposition of the alkoxyamine units. Their molecular weight can be readily estimated by conventional gel permeation chromatography (GPC). In contrast, their thermal reorganization behavior was completely different from common polymers; for instance, the peak in the GPC profiles of the fractionated polymer with a narrow molecular weight distribution broadened to 1.6−2.0 upon heating. Furthermore, the mixture of poly(alkoxyamine)s with different molecular weights reorganized to form the poly(alkoxyamine) with an average molecular weight that showed as a unimodal peak in the GPC profile. However, NMR spectroscopy measured after heating did not reveal any change in the primary chemical structure of the repeating unit. The thermal reorganization in the presence of excess N−O stable free radical resulted in the scission of the main chain due to the chain transfer reaction.