Single Chain Nanoparticles in Catalysis

Over the last six decades folded polymer chains—so-called Single Chain Nanoparticles (SCNPs)—have evolved from the mere concept of intramolecularly crosslinked polymer chains to tailored nanoreactors, underpinned by a plethora of techniques and chemistries to tailor and analyze their morphology and function. These monomolecular polymer entities hold critical promise in a wide range of applications. Herein, we highlight the exciting progress that has been made in the field of catalytically active SCNPs in recent years

Wavelength-gated photoreversible polymerization and topology control

We exploit the wavelength dependence of [2 + 2] photocycloadditions and-reversions of styrylpyrene to exert unprecedented control over the photoreversible polymerization and topology of telechelic building blocks. Blue light (λmax = 460 nm) initiates a catalyst-free polymerization yielding high molar mass polymers (Mn = 60 000 g mol-1), which are stable at wavelengths exceeding 430 nm, yet highly responsive to shorter wavelengths. UVB irradiation (λmax = 330 nm) induces a rapid depolymerization affording linear oligomers, whereas violet light (λmax = 410 nm) generates cyclic entities. Thus, different colors of light allow switching between a depolymerization that either proceeds through cyclic or linear topologies. The light-controlled topology formation was evidenced by correlation of mass spectrometry (MS) with size exclusion chromatography (SEC) and ion mobility data. Critically, the color-guided topology control was also possible with ambient laboratory light affording cyclic oligomers, while sunlight activated the linear depolymerization pathway. These findings suggest that light not only induces polymerization and depolymerization but that its color can control the topological outcomes.</p

From precision photochemistry to advanced microsphere design

This thesis explores polymer chemistry in three distinct fields of real-world applications. First, the design of polymers with reversible remote control over their degree of polymerization and three-dimensional shape purely by application of light constitutes a critical first step towards truly recyclable materials. Second, investigations of polymer coated surfaces, one of the largest fields of application for functional materials, to characterize light driven reactions on solid-liquid interfaces gave critical insight for enabling the synthesis of advanced functional interfaces. Third, the development of well-defined nanopatterned materials for the analysis of polymeric materials afforded nature-inspired particle shapes for advanced chromatographic applications

The long and the short of polymer grafting

We demonstrate that grafting a distribution of polymer chains onto an interface critically affects the shape of the distribution, with shorter chains being preferentially attached. This distortion effect is herein quantified for the first time, exploiting a quartz crystal microbalance – underpinned by single-molecule force spectroscopy – on the example of grafted poly(methyl methacrylate) (PMMA) chain distributions of different molar mass. ‘Grafting-to’ of different ratios of number average molecular weight of PMMA distributions unambiguously establishes the preferred surface grafting of shorter polymers, which can be correlated to their smaller radius of gyration. Our findings allow to establish a preferential grafting factor, k, which allows to predict the molar mass distribution of polymers on the surfaces compared to the initial distribution in solution. Our findings not only have serious consequences for functional polymer interface design, yet also for the commonly employed methods of grafting density estimation

N-Heterocyclic olefins as initiators for the polymerization of (meth)acrylic monomers: A combined experimental and theoretical approach

The zwitterionic organopolymerization of four different acrylic monomers (N,N-dimethylacrylamide, methyl acrylate, methyl methacrylate and tert-butyl methacrylate) based on neutral initiators, so-called N-heterocyclic olefins (NHOs), is presented. The scope and underlying (deactivation-)mechanisms were studied in a combined experimental and computational effort. From a range of differently structured NHOs it was shown that imidazole-derivatives, in contrast to imidazoline- and benzimidazole-derivatives, readily polymerize the selected monomers. While the additive-free reactions proceed with a relatively low degree of control to yield a largely atactic material, for the acrylamide the addition of LiCl as a μ-type ligand has been shown to result in a rapid and quantitative monomer consumption. The thus generated poly(N,N-dimethyl acrylamide) was found to be highly isotactic (>90% isotactic dyads) with high molecular weight (Mn= 250000-650000 g mol-1, DM= 1.3-1.6). Complementing DFT calculations considered the zwitterionic chain growth with respect to competing side reactions, namely spirocycles and enamine formation. It was found that NHOs with an unsaturated backbone better support the zwitterionic chain growth, with the spirocycles acting as dormant species that slow down but do not quench the polymerization process. Contrasting this, enamine formation irreversibly terminates the polymerization and is found to be energetically favored. This pathway can be blocked by the introduction of substituents on the exocyclic carbon of the NHO, resulting in structures like 2-isopropylidene-1,3,4,5-tetramethylimidazoline (4) which consequently deliver the most controlled polymerizations. Finally, a good correlation of the initiation energy barrier with the buried volume (%VBur) and the Parr electrophilicity index is described, allowing for a quick and reliable screening of potential monomers based on these two readily accessible parameters

Multifilament cellulose/chitin blend yarn spun from ionic liquids

Cellulose and chitin, both biopolymers, decompose before reaching their melting points. Therefore, processing these unmodified biopolymers into multifilament yarns is limited to solution chemistry. Especially the processing of chitin into fibers is rather limited to distinctive, often toxic or badly removable solvents often accompanied by chemical de-functionalization to chitosan (degree of acetylation, DA, +[OPr]-) and the obtained one-pot spinning dope is used to produce multifilament fibers by a continuous wet-spinning process. Both the rheology of the corresponding spinning dopes and the structural and physical properties of the obtained fibers have been determined for different biopolymer ratios. With respect to medical or hygienic application, the cellulose/chitin blend fiber show enhanced water retention capacity compared to pure cellulose fibers.</p

Quantifying solvent effects on polymer surface grafting

When grafting polymers onto surfaces, the reaction conditions critically influence the resulting interface properties, including the grafting density and molar mass distribution (MMD) on the surface. Herein, we show theoretically and experimentally that the application of poor solvents is beneficial for the "grafting-to" approach. We demonstrate the effect by grafting poly(methyl methacrylate) chains on silica nanoparticles in different solvents and compare the MMD of the polymer in solution before and after grafting via size exclusion chromatography (SEC). The shorter polymer chains are preferentially grafted onto the surface, leading to a distortion effect between the MMD in solution and on surfaces. The molecular weight distortion effect is significantly higher for ethyl acetate (good solvent quality, difference in Mw surface to solution 14%) than for N,N-dimethylacetamide (poor solvent quality, 6%). The difference in MMD on the surface to the solution significantly affects both the surface properties (e.g. the grafting densities) and their determination.</p

Wavelength-Gated Photoreversible Polymerization and Topology Control

We exploit the wavelength dependence of [2+2] photocycloadditions and -reversions of styrylpyrene to exert unprecedented control over the photoreversible polymerization and topology of telechelic building blocks. Blue light affords high molar mass polymers that are stable at wavelengths exceeding 430 nm yet highly responsive to shorter wavelengths. UVB irradiation induces a rapid depolymerization yielding linear oligomers, whereas violet light generates cyclic entities. Different colors of light thus allow switching between a depolymerization that either proceeds through cyclic or linear topologies. The light-controlled topology formation was evidenced by correlation of mass spectrometry (MS) with size exclusion chromatography (SEC) and ion mobility data.<br /

Wavelength-gated photoreversible polymerization and topology control

We exploit the wavelength dependence of [2 + 2] photocycloadditions and-reversions of styrylpyrene to exert unprecedented control over the photoreversible polymerization and topology of telechelic building blocks. Blue light (λmax = 460 nm) initiates a catalyst-free polymerization yielding high molar mass polymers (Mn = 60 000 g mol-1), which are stable at wavelengths exceeding 430 nm, yet highly responsive to shorter wavelengths. UVB irradiation (λmax = 330 nm) induces a rapid depolymerization affording linear oligomers, whereas violet light (λmax = 410 nm) generates cyclic entities. Thus, different colors of light allow switching between a depolymerization that either proceeds through cyclic or linear topologies. The light-controlled topology formation was evidenced by correlation of mass spectrometry (MS) with size exclusion chromatography (SEC) and ion mobility data. Critically, the color-guided topology control was also possible with ambient laboratory light affording cyclic oligomers, while sunlight activated the linear depolymerization pathway. These findings suggest that light not only induces polymerization and depolymerization but that its color can control the topological outcomes.</p