“One-Pot” Aminolysis/Thiol–Maleimide End-Group Functionalization of RAFT Polymers: Identifying and Preventing Michael Addition Side Reactions

We show that many of the nucleophiles (catalysts, reducing agents, amines, thiols) present during "one-pot" aminolysis/thiol-maleimide end-group functionalization of RAFT polymers can promote side reactions that substantially reduce polymer end-group functionalization efficiencies. The nucleophilic catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene and the reducing agent tributylphosphine were shown to initiate anionic polymerization of N-methylmaleimide (NMM) in both polar and nonpolar solvents whereas hexylamine-initiated polymerization of NMM occurred only in high-polarity solvents. Furthermore, triethylamine-catalyzed Michael reactions of the representative thiol ethyl 2-mercaptopropionate (E2MP) and NMM in polar solvents resulted in anionic maleimide polymerization when [NMM]0 > [E2MP]0. Base-catalyzed enolate formation on the α-carbon of thiol-maleimide adducts was also shown as an alternative initiation pathway for maleimide polymerization in polar solvents. Ultimately, optimal "one-pot" reaction conditions were identified allowing for up to 99% maleimide end-group functionalization of dithiobenzoate-terminated poly(N,N-dimethylacrylamide). Much of the work described herein can also be used to ensure near-quantitative conversion of small molecule thiol-maleimide reactions while preventing previously unforeseen side reactions.