Control of the Density of Polymer Brushes Prepared by Surface-Initiated Atom Transfer Radical Polymerization

Growth of polymer brushes from solid substrates is an attractive method for generating robust surfaces with controlled properties and functionality. This work demonstrates enhanced control over such films through variation of the areal density of the immobilized initiators used for their growth. Reaction of mercaptoundecanol monolayers on Au with both an acyl bromide initiator and a structurally similar acyl bromide diluent yields monolayers whose composition reflects the ratio of the acyl bromides in solution. Similarly, derivatization of SiO₂ with an initiator and a diluent monochlorosilane also affords control over initiator density. The thickness of polymer films grown from these modified substrates drops dramatically when the fractional coverage of the surface by initiator decreases below 10% of a monolayer because the area per polymer chain increases. However, reduced termination at low initiator coverage results in substantial increases in initiator efficiency as measured by film growth rates normalized by the fractional coverage of the surface by initiator. Variation of chain density also affords control over film swelling. Poly(2-hydroxyethyl methacrylate) films prepared with 0.1% initiator densities swell 20-fold more in water than films grown from monolayers containing only initiators. Such control should prove valuable in the use of brushes for immobilization of active, accessible biomacromolecules such as single-stranded DNA or antibodies.