Solid Phase Synthesis of Polymacromer and Copolymacromer Brushes

Adjusting the surface areal density of click-reactive azide groups by kinetic control of the azide substitution reaction on bromine-functional SAMs

Azide-alkyne click chemistry has emerged as an important and versatile means for tethering a wide variety of guest molecules to virtually any substrate. In many of these applications, it is important to exercise control over the areal density of surface functional groups to achieve a desired areal density of the tethered guest molecule of interest. We demonstrate herein that the areal density of surface azide groups on flat germanium surfaces and nanoparticle substrates (silica and iron oxide) can be controlled kinetically by appropriately timed quenching of the S(N)2 substitution reaction of bromo-alkane-silane monolayers induced by the addition of sodium azide. The kinetics of the azide substitution reaction on monolayers formed on flat Ge substrates, determined by attenuated total reflection infrared spectroscopy (ATR-IR), are found to be identical to those for monolayers formed on both silica and iron oxide nanoparticles, the latter determined by transmission infrared spectroscopy. To validate the method, the percentages of surface bromine groups converted to azide groups after various reaction times were measured by quenching the S(N)2 reaction followed by analysis with ATR-IR (for Ge) and thermogravimetric analysis (after a subsequent click reaction with an alkyne-terminal polymer) for the nanoparticle substrates. The conversions found after quenching agree well with those expected from the standard kinetic curves. The latter result suggests that the kinetic method for the control of azide group areal density is a versatile means for functionalizing substrates with a prescribed areal density of azide groups for subsequent click reactions, and that the method is universal for any substrate, flat or nanoparticle, that can be modified with bromo-alkane-silane monolayers. Regardless of the surface geometry, we find that the azide substitution reaction is complete within 2-3 h, in sharp contrast to previous reports that indicate times of 48-60 h required for completion of the reaction.

Mixed silane monolayers for controlling the surface areal density of click-reactive alkyne groups: a method to assess preferential surface adsorption on flat substrates and a method to verify compositional homogeneity on nanoparticles

SAMs formed from mixtures of alkyne-silanes and alkane-silanes are used to control the areal density of click-reactive alkyne groups on the surface of flat germanium substrates, silicon wafers, and silica nanoparticles. Two new analytical tools are described for characterization of the mixed SAMs: a thermogravimetric analysis (TGA) technique for quantifying the compositional homogeneity of the mixed monolayers formed on nanoparticles, and an infrared spectroscopy (IR) technique to detect preferential surface adsorption. The TGA technique involves measurement of the change in weight when azide-terminated polymers react with surface alkyne groups on silica nanoparticles via a click reaction, while the IR technique is based on the use of attenuated total reflectance infrared spectroscopy (ATR-IR) to monitor click reactions between azide compounds with infrared "labels" and alkyne-functional mixed SAMs deposited on germanium ATR plates. Upon application of the new characterization techniques, we are able to prove that the mixed silane monolayers show neither phase separation nor preferential surface adsorption on any of the three substrates studied. When reacted with azide terminal polymers, the areal density at saturation, σ(sat) is found to scale with molecular weight according to σ(sat) ≈ N(-0.57). We conclude that mixed monolayers of alkyne-silanes and alkane-silanes are an effective means of controlling the surface areal density of click-reactive alkyne groups on both flat and nanoparticle substrates.