Synthesis and Characterization of Cyclic Brush-Like Polymers by N-Heterocyclic Carbene-Mediated Zwitterionic Polymerization of N-Propargyl N-Carboxyanhydride and the Grafting-to Approach

Cyclic brush-like polymers were synthesized by tandem organo-mediated zwitterionic polymerization and a grafting-to approach. The cyclic polymer backbone, consisting of poly( N -propargylglycine) ( c -PNPG), was synthesized by an N -heterocyclic carbene (NHC)-mediated zwitterionic ring-opening polymerization of N -propargyl N -carboxyanhydride. The polymerization proceeds in a quasi-living manner, allowing access to c -PNPG of well-defined chain length. The cyclic architecture of the polymers was verified by size exclusion chromatography (SEC) and mass spectroscopy (MS), as well as scanning probe characterization. Poly(ethylene glycol) functionalized with azido end-groups was subsequently grafted onto the c -PNPG by the copper-mediated azide/alkyne cycloaddition reaction (CuAAC). The side chain grafting density was determined by 1 H NMR spectroscopy and SEC analysis. The grafting efficiency is low (<19%) when the cyclic backbone is comprised of a c -PNPG homopolymer. The efficiency can be significantly improved (up to 93%) by utilizing cyclic poly( N -propargylglycine)- ran -poly( N -butylglycine) random copolymers ( c -PNPG- r -PNBG). This has been attributed to the ease of access to the propargyl groups in c -PNPG and c -PNPG- ran -PNBG: the strong tendency of the former to aggregate in common organic solvents (including the CuAAC reaction medium) restricts access to the propargyl groups.