Rate Determination of Azide Click Reactions onto Alkyne Polymer Brush Scaffolds: A Comparison of Conventional and Catalyst-Free Cycloadditions for Tunable Surface Modification

The postpolymerization functionalization of poly(N-hydroxysuccinimide 4-vinylbenzoate) brushes with reactive alkynes that differ in relative rates of activity of alkyne–azide cycloaddition reactions is described. The alkyne-derived polymer brushes undergo “click”-type cycloadditions with azido-containing compounds by two mechanisms: a strain-promoted alkyne–azide cycloaddition (SPAAC) with dibenzocyclooctyne (DIBO) and azadibenzocyclooctyne (ADIBO) or a copper-catalyzed alkyne–azide cycloaddition (CuAAC) to a propargyl group (PPG). Using a pseudo-first-order limited rate equation, rate constants for DIBO, ADIBO, and PPG-derivatized polymer brushes functionalized with an azide-functionalized dye were calculated as 7.7 × 10–4, 4.4 × 10–3, and 2.0 × 10–2 s–1, respectively. The SPAAC click reactions of the surface bound layers were determined to be slower than the equivalent reactions in solution, but the relative ratio of the reaction rates for the DIBO and ADIBO SPAAC reactions was consistent between solution and the polymer layer. The rate of functionalization was not influenced by the diffusion of azide into the polymer scaffold as long as the concentration of azide in solution was sufficiently high. The PPG functionalization by CuAAC had an extremely fast rate, which was comparable to other surface click reaction rates. Preliminary studies of dilute solution azide functionalization indicate that the diffusion-limited regime of brush functionalization impacts a 50 nm polymer brush layer and decreases the pseudo-first-order rate by a constant diffusion-limited factor of 0.233.