Boyd A. Laurent

Synthetic approaches for the preparation of cyclic polymers

Despite decades of studies devoted to the unique physical properties and potential applications of cyclic polymer topologies, their exploration has remained limited because of synthetic inefficiencies and acyclic impurities. Many recently developed synthetic techniques offer efficient routes to well-defined cyclic macromolecules to answer this need. This tutorial review aims to provide a concise overview of the most significant synthetic contributions in this field, and highlight the relative advantages and disadvantages of each approach.

Synthesis of cyclic amphiphilic homopolymers and their potential application as polymeric micelles

Although amphiphilic copolymers have been widely studied due to their ability to phase segregate in bulk and form micelle-like nanostructures in solution, previous research has focused primarily on block copolymers. Amphiphilic homopolymers, in which each monomer along the backbone contains an amphiphilic unit, have seen only limited exploration, while non-linear amphiphilic homopolymers remain largely unexplored. Building from methods established in our laboratories for the synthesis of cyclic polymers, bifurcated amphiphiles were attached via a highly efficient “click” coupling to access analogous sets of linear and cyclic amphiphilic homopolymers (the first reported example of cyclic amphiphilic homopolymers). These amphiphilic homopolymers showed solubility in a wide range of solvents with varying polarities and also have demonstrated the ability to encapsulate guests in incompatible solvents. For the mass range examined, the cyclic polymers showed only a negligible difference in guest encapsulation when compared to linear analogs.

Differentiation of Linear and Cyclic Polymer Architectures by MALDI Tandem Mass Spectrometry (MALDI-MS2)

Abstract[M + Ag]+ ions from cyclic and linear polystyrenes and polybutadienes, formed by matrix-assisted laser desorption ionization (MALDI), give rise to significantly different fragmentation patterns in tandem mass spectrometry (MS2) experiments. In both cases, fragmentation starts with homolytic cleavage at the weakest bond, usually a C–C bond, to generate two radicals. From linear structures, the separated radicals depolymerize extensively by monomer losses and backbiting rearrangements, leading to low-mass radical ions and much less abundant medium- and high-mass closed-shell fragments that contain one of the original end groups, along with internal fragments. With cyclic structures, depolymerization is less efficient, as it can readily be terminated by intramolecular H-atom transfer between the still interconnected radical sites (disproportionation). These differences in fragmentation reactivity result in substantially different fragment ion distributions in the MS2 spectra. Simple inspection of the relative intensities of low- versus high-mass fragments permits conclusive determination of the macromolecular architecture, while full spectral interpretation reveals the individual end groups of linear polymers or the identity of the linker used to form the cyclic polymer. FigureMacrocyclic and linear polystyrene and polydiene architectures are conclusively distinguished by the MS2 fragmentation patterns of Ag+-cationized oligomers.