Preparation of t-butyl acrylate, methyl acrylate, and styrene block copolymers by atom transfer radical polymerization: precursors to amphiphilic and hydrophilic block copolymers and conversion to complex nanostructured materials

Atom transfer radical polymerization conditions with copper(I) bromide/pentamethyldiethylenetriamine (CuBr/PMDETA) as the catalyst system were employed for the polymerization of tert-butyl acrylate, methyl acrylate, and styrene to generate well-defined homopolymers, diblock copolymers, and triblock copolymers. Temperature studies indicated that the polymerizations occurred smoothly in bulk at 50°C. The kinetics of tert-butyl acrylate polymerization under these conditions are reported. Well-defined poly-(tert-butyl acrylate) (PtBA; polydispersity index = 1.14) and poly(methyl acrylate) (PMA; polydispersity index = 1.03) homopolymers were synthesized and then used as macroinitiators for the preparation of PtBA-b-PMA and PMA-b-PtBA diblock copolymers in bulk at 50°C or in toluene at 60 or 90°C. In toluene, the amount of CuBr/PMDETA relative to the macroinitiator was important; at least 1 equiv of CuBr/PMDETA was required for complete initiation. Typical block lengths were composed of 100-150 repeat units per segment. A triblock copolymer, composed of PtBA-b-PMA-b-PS (PS = polystyrene), was also synthesized with a well-defined composition and a narrow molecular weight dispersity. The tert-butyl esters of PtBA-b-PMA and PtBA-b-PMA-b-PS were selectively cleaved to form the amphiphilic block copolymers PAA-b-PMA [PAA = poly(acrylic acid)] and PAA-b-PMA-b-PS, respectively, via reaction with anhydrous trifluoroacetic acid in dichloromethane at room temperature for 3 h. Characterization data are reported from analyses by gel permeation chromatography; infrared, 1H NMR, and 13C NMR spectroscopies; differential scanning calorimetry; and matrix-assisted, laser desorption/ionization time-of-flight mass spectrometry. The assembly of the amphiphilic triblock copolymer PAA90-b-PMA80-b-PS98 within an aqueous solution, followed by conversion into stable complex nanostructures via crosslinking reactions between the hydrophilic PAA chains comprising the peripheral layers, produced mixtures of spherical and cylindrical topologies. The visualization and size determination of the resulting nanostructures were performed by atomic force microscopy, which revealed very interesting segregation phenomena.