Alkyne-functional polymers through sonogashira coupling to poly(4-bromostyrene)

Alkyne-functional polymers and block copolymers were synthesized by postpolymerization modification of poly(4-bromostyrene) (PBrS) via palladium-catalyzed coupling with terminal alkynes. Several coupling methods, most notably bis(benzonitrile)palladium dichloride/tris-tert-butylphosphine catalyst systems at room temperature, were examined on well-defined polymers and copolymers of 4-bromostyrene prepared by nitroxide-mediated polymerization (NMP) with TEMPO. High conversions to alkyne-functional repeat units were obtained with low molecular weight homopolymers of PBrS (8-15 kg/mol) to afford alkyne-rich random copolymers of poly[(4-phenylethynyl)styrene]-co-poly(4- bromostyrene) and poly[(4-hexynyl)styrene]-co-poly(4-bromostyrene). Coupling reactions run on higher molecular weight polymers (70 kg/mol) or multiblock copolymers also gave high conversions to poly[(4-phenylethynyl)styrene], while coupling reactions with 1-hexyne suffered from side reactions. ¹H NMR analysis of the coupled products suggests that up to 80% of terminal TEMPO groups are lost under standard coupling conditions. The polymers were purified from residual catalyst by treatment with tetrakis(hydroxymethyl)phosphonium chloride.