High resolution mass spectrometric access to nitroxide containing polymers†
We introduce a mass spectrometric access route to nitroxide containing polymers via high resolution electrospray ionization mass spectrometry (HR ESI MS), a polymer class that is – due to the presence of unpaired spins – highly challenging to analyze via NMR techniques. The nitroxide content within the polymer chain structure was varied between 11.3 and 29.1 mol% in a statistical copolymer consisting of styrene-stat-4-(chloromethyl)styrene (p(S-stat-CMS), 4800 ≥ Mn/g mol−1 ≥ 11 100), where 4-carboxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-carboxy-TEMPO) units were attached by post-polymerization modification. By carefully evaluating the isotopic pattern of the nitroxide containing polymers, we demonstrate that the persistent nitroxyl radical retains its structural integrity during the soft ionization process employing spray currents up to 4.3 keV and in-source collision induced dissociation energies up to 30 eV using chloride attachment technology in negative ion mode. Interestingly, high-molecular weight gas-phase aggregates are identified with increasing amounts of nitroxide side-chain functionalization. To further exemplify the power of the introduced mass spectrometric protocol, a well-defined styrene based polymer was synthesized via atom transfer radical polymerization (ATRP, Mn = 5600 g mol−1, Đ = 1.05) containing functional groups, i.e. a terminal cyanuric acid unit and Hamilton Wedge moiety as well as a free nitroxide in the penultimate position to the Hamilton wedge. The intact persistent nitroxide radical at the chain end was unambiguously identified by its isotopic pattern in a highly defined polymer structure.