Ion exchange enables MALDI-TOF mass spectrometry analysis of sequence-controlled polymers with a positively charged backbone

Abstract

Transforming negative ions of cationic polymers with a positively charged backbone into Tf₂N⁻((SO₂CF₃)₂N⁻) or FSI⁻((SO₂F)₂N⁻) through an ion exchange reaction makes it possible to achieve matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry analysis. In this study, a series of sequence-defined polymers with positively charged backbones were synthesized via alternately performing the Menshutkin reaction and Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction to verify the feasibility of the method. The electron cloud density of the cation determines the appearance of the fragmentation pattern in the mass spectra. This method is suitable for positively charged polymers with molecular weights of up to 4000 Da containing 9 cations. The sequence of the cationic polymer can be elucidated through the molecular-ion peak and regular fragment peaks, making it a valuable method for sequence, component and end-group analysis. Such advantages make it an easy route to achieve coding–decoding processes with sequence-defined cationic polymers.