Engineering Nitroxide Functional Surfaces Using Bioinspired Adhesion.

We pioneer a versatile surface modification strategy based on mussel-inspired oxidative catecholamine polymerization for the design of nitroxide-containing thin polymer films. A 3,4-dihydroxy-l-phenylalanine (l-DOPA) monomer equipped with a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-derived oxidation-labile hydroxylamine functional group is employed as a universal coating agent to generate polymer scaffolds with persistent radical character. Various types of materials including silicon, titanium, ceramic alumina, and inert poly(tetrafluoroethylene) (PTFE) were successfully coated with poly(DOPA-TEMPO) thin films in a one-step dip-coating procedure under aerobic, slightly alkaline (pH 8.5) conditions. Steadily growing polymer films (∼1.1 nm h-1) were monitored by ellipsometry, and their thicknesses were critically compared with those obtained from atomic force microscopic cross-sectional profiles. The heterogeneous composition of surface-adherent nitroxide scaffolds examined by X-ray photoelectron spectroscopy was correlated to that examined by in-solution polymer analysis via high-resolution electrospray ionization mass spectrometry, revealing oligomeric structures with up to six repeating units, mainly composed of covalently linked dihydroxyindole along the polymer backbone. Critically, the reversible redox-active character of the nitroxide-containing polymer scaffolds was investigated by cyclic voltammetric measurements, revealing a convenient and facile access route to electrochemically active nitroxide polymer coatings with potential application in electronic devices such as organic radical batteries.

Wavelength-Dependent Photochemical Stability of Photoinitiator-Derived Macromolecular Chain Termini.

Herein, we report the unique-and first time-wavelength-dependent investigation with strictly monochromatic light of 305-405 nm wavelength into the stability of photoinitiator-derived chain termini of poly(methyl methacrylate) using a tunable laser system fused with pulsed-laser irradiation and size exclusion chromatography hyphenated to high-resolution electrospray mass spectrometry (PLI-SEC-ESI-MS). We assess several substitution patterns of methyl groups on the common benzoyl-type radical fragment. Critically, methyl substitution in the 2- and 6-positions of the benzoyl moiety, i.e., in both ortho-positions, resulted in stable chain ends up to approximately 350 nm. The stability can be attributed to a blue-shift of the n-π* transitions (relevant for the end group reactivity) as predicted by earlier density functional theory (DFT) calculations on model species. In sharp contrast, our experiments show a far reduced stability of the end groups commencing from 400 nm onwards, when the dual ortho-methyl substitution in the benzoyl fragment is missing. Thus, we demonstrate that the substitution pattern on the phenyl ring of the benzoyl group dictates the chain end stability as a function of wavelength in excellent agreement with the quantum chemical predictions. Our study thus provides critical insights into selecting suitable photoinitiation systems for specific wavelength regimes.

Imaging Single-Chain Nanoparticle Folding via High-Resolution Mass Spectrometry.

Herein, we introduce the first approach to map single-chain nanoparticle (SCNP) folding via high-resolution electrospray ionization mass spectrometry (ESI MS) coupled with size exclusion chromatography. For the first time, the successful collapse of polymeric chains into SCNPs is imaged by characteristic mass changes, providing detailed mechanistic information regarding the folding mechanism. As SCNP system we employed methyl methacrylate (MMA) statistically copolymerized with glycidyl methacrylate (GMA), resulting in p(MMA-stat-GMA), subsequently collapsed by using B(C6F5)3 as catalyst. Both the precursor polymer and the SCNPs can be well ionized via ESI MS, and the strong covalent cross-links are stable during ionization. Our high-resolution mass spectrometric approach can unambiguously differentiate between two mechanistic modes of chain collapse for every chain constituting the SCNP sample.

Just Add Salt: A Mass Spectrometric Analysis Method for Imaging Anion-Exchanged Poly(Ionic Liquid)s.

We report the first mass spectrometric analysis of poly(ionic liquid)s (PILs) containing weakly coordinating anions introduced by a fast, simple, and quantitative postmodification method on the example of the hydrophilic, well-defined poly(vinylbenzylpyridinium chloride) p([VBPy]Cl) species, analyzed with an in-source collision induced dissociation-Orbitrap mass spectrometry (MS) protocol. Using the MS approach allows for the precise structural elucidation of ion-exchanged p([VBPy]Cl) utilizing AgX (X = NO3- , CF3 CO2- , BF4- ) salts. The anion exchange is shown to be quantitative - without observing residual chlorinated PIL - on rapid time scales, using only filtration as a standard procedure during sample preparation. In addition, the influence of weakly coordinating anions on the ionization behavior of PILs is studied in detail.

Universal mass spectrometric analysis of poly(ionic liquid)s.

We introduce a universal high resolution mass spectrometric method for the analysis of poly(ionic liquid)s (PILs), which belong to the most challenging polyelectrolytes from an analytical perspective, by fusing high resolution collision-induced dissociation (CID)-Orbitrap mass spectrometry (MS) with supercharging agents as well as quadrupole time-of-flight (QToF) MS. The study includes a wide array of hydrophilic halide-containing PILs, which were analyzed in negative mode. The influence of the core structures (based on imidazolium, triazolium, ammonium, phosphonium and pyridinium moieties), and variable styrene-, acrylate- and vinyl-type IL polymers on the ionization behavior is mapped in detail. Variable end group functionalities were introduced via functional chain transfer agents (CTA) in reversible addition-fragmentation chain transfer (RAFT) polymerization to study their behavior during the MS analysis. Furthermore, the demanding class of vinylimidazolium halide IL polymers was investigated. The current contribution thus introduces a new analytical technology platform for an entire polymer class.