Surface-initiated polymerizations on initiator anchored substrates: Synthesis and characterization of nanometer thick functional polymer films

We describe the surface-initiated ring-opening polymerization (ROP) of lactide from poly(2-hydroxyethyl methacrylate) (PHEMA) brushes anchored to Au substrates. The resulting comb polymers have a "bottle brush" architecture. During hydrolytic degradation of PLA in pH 7.4 buffer at 55°C, large, highly symmetric domains (∼50-100 mum) unexpectedly formed. The purpose of the research described in this chapter was to devise a model that describes their formation. Control experiments during degradation study link high lactide polymerization temperature to the formation of the defects. A likely mechanism is the scission of Au-S bonds at high temperatures, causing defects that swell when placed in the buffer solution.; We demonstrated enhanced control over polymer brushes 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 drop 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. PHEMA 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.; We report the remarkably rapid synthesis of polymer brushes under mild conditions (50°C) using surface-initiated polymerization. The use of the highly active atom transfer radical polymerization catalyst Cu(1)1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane allows synthesis of 100 ran thick poly(tert-butyl acrylate) brushes from initiator-modified Au surfaces in just 5 minutes. Using the same catalyst, polymerization of hydroxyethyl methacrylate and methyl methacrylate yielded 100 nm thick films in 10 and 60 minutes, respectively. Such polymerization rates are an order of magnitude greater than those for traditional free-radical polymerizations initiated from surfaces. It is important to note that though these rapid polymerizations from surfaces are not "living", they retain some features of controlled radical polymerizations such as the ability to form block copolymer brushes. Such rapid polymerization from a surface will be very important in potential applications of polymer brushes as skin layers in separation membranes and as substrate coatings for probe immobilization in gene and protein chips.