Debromination of ATRP-made Wang soluble polymer supports

摘要

This article describes two convenient methods for removing the u-terminal bromine atom of well-defined soluble polymer supports prepared by atom transfer radical polymerization (ATRP). The targeted soluble supports are linear polystyrene chains that contain an acid-labile p-alkoxybenzyl ester linker (i.e. Wang linker) at their a-chain end. These polymers are synthesized by ATRP using a fluorenylmethoxycarbonyl (Fmoc)-protected amino functional ATRP initiator, namely 3-(Fmoc-amino) propyl 2-bromoisobutyrate. After polymerization and before Wang functionalization, the bromine-atom of the ATRP-made soluble supports was removed. Two different debromination approaches were considered. The first one consists in reducing the terminal alkyl bromide in the presence of a trialkyltin hydride. This method can be applied directly in the ATRP medium at the end of the polymerization or can be performed on a purified polymer sample. The latter conditions were found to be more suitable. It was also observed that the use of tributyltin hydride in the absence of additional radical initiator led to the best results. Indeed, well-defined polymer supports with controlled chain-length, molecular weight distribution and fully dehalogenated chain-ends were obtained. The second dehalogenation approach consisted in removing the terminal bromide by nucleophilic substitution with sodium azide. Afterwards, the formed terminal azide group was reacted with 1-pentyne by copper-catalyzed azide-alkyne 1, 3-dipolar Huisgen cycloaddition. This method was also found to be valid for preparing bromine-free polystyrene supports. After omega-chain-end debromination, Fmoc-deprotection was performed on the a-chain-end and the resulting amine function was reacted with 4-(hydroxymethyl) phenoxyacetic acid. Further esterification of the Wang linker is also possible. (C) 2015 Elsevier Ltd. All rights reserved.