Wednesday, October 24, 2012

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


Using a pseudo-first-order limited rate equation, rate constants were determined for 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). 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. 

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