Cyclic Macromolecular Chains Visualized by Cryo-EM and AFM Reveal a Ring Expansion Polymerization Mechanism in a Classical Synthetic Pathway to Polyphosphazenes

We observed individual macromolecular chains with distinct ring topology when studying cryogenic electron microscopy (Cryo-EM) images of a high molar mass polyorganophosphazene in its vitrified aqueous solution. The presence of monocyclic chains was confirmed by visualizing samples of the same macromolecule in its absorbed form by using atomic force microscopy (AFM). The polymer, poly[di(carboxylatophenoxy)phosphazene], PCPP, is synthesized via a two-stage process, which includes a ring-opening polymerization (ROP) and a subsequent macromolecular substitution transformation. The visualization of macrocycles, which are 20-60 nm in diameter, provides direct proof of a ring extension polymerization (REP) mechanism occurring in the chain-growth process commonly employed for the synthesis of high molar mass polyphosphazenes. Asymmetric flow field flow fractionation (AF4) analysis shows the presence of a faster moving fraction, which can be potentially attributed to macrocycles.