Solid Phase Synthesis of Polymacromer and Copolymacromer Brushes

Abstract

We report a novel solid phase method for the sequential coupling of heterobifunctional macromonomers to form a new class of polymeric materials that we refer to as polymacromers. Starting from an azide functional substrate, α-azido, ω-protected-alkyne macromonomers are added sequentially by thermally initiated click reactions to form polymacromer brushes wherein macromonomers are linked via triazole groups. After each addition step, the terminal alkyne group can be deprotected to allow addition of the next macromonomer. The use of highly chemoselective click chemistry for the coupling reactions allows virtually any macromonomer to be employed, regardless of its chemical nature. The method is illustrated by formation of two different homopolymacromer brushes: one made by sequentially coupling four polystyrene macromonomers and another by sequentially coupling eight poly(tert-butyl acrylate) macromonomers. Two different types of copolymacromer brushes are also prepared: one by alternating sequential coupling of polystyrene macromonomers with poly(methyl methacrylate) macromonomers and another by alternating sequential coupling of polystyrene macromonomers with poly(tert-butyl acrylate) macromonomers. The polymacromer brushes are characterized by ellipsometry, contact angle analysis, X-ray photoelectron spectroscopy, and X-ray reflectivity measurements. In all cases the coupling reactions are found to be highly efficient. Analysis of thickness data for the poly(tert-butyl acrylate) homopolymacromer brushes indicates that a conversion within the range of 97.5–100% is achieved for each cycle. This solid phase synthesis method is extremely versatile because it can be used to prepare complex sequenced block copolymers from almost any polymers and because the brushes formed can be made to span the entire region of brush behavior between the “grafting from” and grafting to” limits by varying the macromonomer molecular weight.