Leonie Barner

Reversible addition-fragmentation chain transfer polymerization initiated with γ-radiation at ambient temperature: an overview

Using gamma-radiation as initiation source at ambient temperatures (i.e. T approximate to 20 degreesC) for reversible addition-fragmentation chain transfer (RAFT) polymerizations allows for the generation of narrowly distributed polymeric material with living characteristics. It is shown that the living characteristics effected by RAFT agent mediated bulk polymerizations using gamma-irradiation are associated with a RAFT mechanism rather than with reversible termination processes. Furthermore, gamma-radiation as initiation source for an appropriate RAFT agent/monomer system allows for effective radical storage and the generation of long-lived reaction intermediates at ambient temperatures. The current overview further demonstrates how the RAFT process together with gamma-radiation as source of initiation can be employed to graft various monomers onto polypropylene surfaces in a controlled manner

Mild and modular surface modification of cellulose via hetero Diels-Alder (HDA) cycloaddition.

A combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and hetero Diels-Alder (HDA) cycloaddition was used to effect, under mild (T ≈ 20 °C), fast, and modular conditions, the grafting of poly(isobornyl acrylate) (M(n) = 9800 g mol(-1), PDI = 1.19) onto a solid cellulose substrate. The active hydroxyl groups expressed on the cellulose fibers were converted to tosylate leaving groups, which were subsequently substituted by a highly reactive cyclopentadienyl functionality (Cp). By employing the reactive Cp-functionality as a diene, thiocarbonyl thio-capped poly(isobornyl acrylate) synthesized via RAFT polymerization (mediated by benzyl pyridine-2-yldithioformiate (BPDF)) was attached to the surface under ambient conditions by an HDA cycloaddition (reaction time: 15 h). The surface-modified cellulose samples were analyzed in-depth by X-ray photoelectron spectroscopy, scanning electron microscopy, elemental analysis, Fourier transform infrared (FT-IR) spectroscopy as well as Fourier transform infrared microscopy employing a focal plane array detector for imaging purposes. The analytical results provide strong evidence that the reaction of suitable dienophiles with Cp-functional cellulose proceeds under mild reaction conditions (T ≈ 20 °C) in an efficient fashion. In particular, the visualization of individual modified cellulose fibers via high-resolution FT-IR microscopy corroborates the homogeneous distribution of the polymer film on the cellulose fibers.

Complex Molecular Architecture Polymers via RAFT

The synthesis of star and hydrogel polymers mediated by reversible addition-fragmentation chain-transfer (RAFT) polymerization was reported. The RAFT technique generates polymers with RAFT end groups, opening the possibility to restart the polymerization process. RAFT polymerization was therefore a very versatile route for the synthesis of star, comb, or block copolymers with good control over molecular weight.

Photo-induced macromolecular functionalization of cellulose via nitroxide spin trapping.

Cellulose is the most abundant organic raw material on the planet. Due to its renewability and biodegradability it is currently attracting much interest for the production of biofuels or platform chemicals. In addition, recent applications in the field of materials science have appeared, arising from the low density and the excellent thermal and mechanical properties of cellulose, particularly in the production of composites. However, there are still some major drawbacks to using cellulose including its water-absorbing nature and its poor compatibility with other materials, for example, synthetic polymers. To combat these problems, grafting synthetic polymers onto cellulose is the most straightforward method to alter its surface properties and thus to control the wettability, adhesion, or hydrophobicity of the biopolymer. Although grafting from methods such as surface-initiated nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP),reversible addition–fragmentation transfer polymerization (RAFT), or ring-opening polymerization (ROP)are considered as the most efficient approaches, particularly in terms of grafting density, efficient grafting to methods have recently produced very good results. Cellulose has been successfully modified with preformed polymers by both hetero-Diels–Alderand 1,3-dipolar nitrile imine-ene cycloadditions. In the latter case, light was used as the grafting trigger. Importantly, employing light offers temporal and spatial control of the reaction. In the present contribution, we introduce a very facile grafting to protocol based on the generation of radicals at the surface of cellulose by mild UV irradiation (λmax ∼ 311 nm) of an immobilized photoinitiator, followed by radical trapping with a nitroxide-functionalized polymer (see Scheme 1). Previously, nitroxide radical coupling was employed to efficiently link polymer strands, however via a copper-catalyzed mechanism using ATRP-made polymers to generate reactive radicals. A rather similar philosophy based on spin capturing was reported, employing nitrones that after a first radical reaction generate a nitroxide able to undergo a second radical coupling.

Surface Grafting via the Reversible Addition–Fragmentation Chain-Transfer (RAFT) Process: From Polypropylene Beads to Core–Shell Microspheres

Leonie Barner studied chemistry at the Universities of Kassel and Gottingen ( Germany). She joined Michael Bubacks group and received her Ph. D. in 1998. From 1998 to 2001 she held a senior research position at Sartorius ( Gottingen), developing microfiltration membranes for biotechnology applications. She then joined the Centre for Advanced Macromolecular Design, where she currently holds a senior research associate position. Her prime research interests are controlled/ living radical polymerization methods and the development of novel polymeric surfaces for biotechnology and combinatorial chemistry applications.

Suppressing Pseudomonas aeruginosa adhesion via non-fouling polymer brushes

In the current study, well-defined polymer brushes are shown as an effective surface modification to resist biofilm formation from opportunistic pathogens. Poly[oligo(ethylene glycol)methyl ether methacrylate] (poly(MeOEGMA)) and poly[N-(2-hydroxypropyl)methacrylamide] (poly(HPMA)) brushes were grown by surface initiated atom transfer radical polymerization (SI-ATRP) and subsequently characterized by Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and dynamic water contact angle measurements. Their remarkable resistance to protein fouling after long term contact with biological media was evidenced by surface plasmon resonance spectroscopy. Challenging these brushes with an environmental strain of Pseudomonas aeruginosa in mineral media as well as a casein–soja–pepton–agar (CASO) medium resulted in no biofilm formation, while a decrease of the biofilm formation by 70% (poly(HPMA)) and 90% (poly(MeOEGMA)) was observed when the medium was rich in nutrients and proteins (fetal bovine serum). In contrast to the antibiotic sensitive strains, biofilm formation was observed using an antibiotic multi-resistant P. aeruginosa strain on both brushes. Protein fouling was fully prevented on both types of brushes, which might challenge the proposed mechanism of biofilm formation mediated by a pre-formed conditioning film of proteins. The resistance to biofilm formation and the possibility to precisely control their growth and functionalities makes these brushes ((poly(HPMA) and (poly(MeOEGMA)) promising candidates for surface modification of various biomaterials as well as platforms for basic studies into the mechanisms of bacteria fouling.

Facile Access to Hydroxy-Functional Core–Shell Microspheres via Grafting of Ethylene Oxide by Anionic Ring-Opening Polymerization

We present a facile access route to hydroxy-functional narrow disperse microspheres of well-defined grafting density (GD). Ethylene oxide has been grafted from highly crosslinked poly(divinyl benzene) microspheres by anionic ring-opening polymerization using sec-butyllithium as activator together with the phosphazene base t-BuP(4) . Initially, core microspheres have been prepared by precipitation polymerization utilizing divinyl benzene (DVB, 80 wt.-%). The grafting of poly(ethylene oxide) (PEO) from the surface resulted in the formation of functional core-shell microspheres with hydroxy-terminal end groups. The number average particle diameter of the grafted microspheres was 3.6 µm and the particle weight increased by 5.7%. The microspheres were characterized by SEM, FT-IR spectroscopy, elemental analysis, and fluorescence microscopy. The surface GD (determined via two methods) was 1.65 ± 0.06 and 2.09 ± 0.08 chains · nm(-2) , respectively.

Modular design of glyco-microspheres via mild pericyclic reactions and their quantitative analysis

The facile and efficient functionalization of porous poly(glycidyl methacrylate) (pGMA) microspheres via hetero Diels–Alder (HDA) chemistry with poly(3-O-acryloyl-1,2:5,6-di-O-isopropylidene-α-D-glucofuranoside) (pAIpGlc) prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization employing electron deficient thiocarbonylthio compounds (benzyl pyridin-2-yldithioformate (BPDF)) is described in detail. The efficiency of the employed ‘grafting to’ approach is qualitatively and quantitatively analyzed. Initially the microspheres are functionalized with a highly reactive diene – cyclopentadiene (Cp) – in one step with sodium cyclopentadienide, and subsequently reacted with a protected glycopolymer (number-average molecular weight, Mn = 4200 g mol−1; polydispersity index, PDI = 1.2) that carries a thiocarbonyl moiety functioning as a dienophile. The functionalization of the microspheres is achieved under mild conditions (T = 50 °C) with trifluoroacetic acid (TFA) as a readily removable catalyst. Deprotection of the grafted pAIpGlc to poly(3-O-acryloyl-α,β-D-glucopyranoside) (pAGlc) can be performed after functionalization in one pot with formic acid at ambient temperature. The obtained loading capacity is 2.63 × 1019 chains per g and the grafting density is close to 0.16 chains per nm2. Quantitative analysis of the grafting densities is achieved via elemental analysis; the pore size distribution before functionalization was analyzed by inverse size exclusion chromatography (iSEC). Further employed characterization techniques include scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and high resolution attenuated total reflectance (ATR) FT-IR microscopy supporting the successful modification of the microspheres.

A Facile Route to Boronic Acid Functional Polymeric Microspheres via Epoxide Ring Opening

Boronic acid-functionalized microspheres are prepared for the first time via mild epoxide ring opening based on porous cross-linked polymeric microspheres (diameter ≈ 10 μm, porosity ≈ 1000 Å). Quantitative chemical analysis by XPS and EA evidences that there is a greater functionalization with boronic acid when employing a sequential synthetic method [1.7 atom% boron (XPS); 1.12 wt% nitrogen (EA)] versus a one-pot synthetic method [0.2 atom% boron (XPS); 0.60 wt% nitrogen (EA)] yielding grafting densities ranging from approximately 2.5 molecules of boronic acid per nm(2) to 1 molecule of boronic acid per nm(2), respectively. Furthermore, the boronic acid-functionalized microspheres are conjugated with a novel fluorescent glucose molecule demonstrating a homogeneous spatial distribution of boronic acid.

Synthesis and On‐Demand Gelation of Multifunctional Poly(ethylene glycol)‐Based Polymers

The synthesis of a novel photoreactive poly(ethylene glycol) (PEG)-based polymer with caged carbonyl groups is reported. We further demonstrate its use for the on-demand fabrication of hydrogels. For rapid gelation, a hydrazide-functionalized PEG is used as the second component for the hydrogel preparation. The photoreactive PEG-based polymer is designed for controlled cleavage of the protecting groups upon exposure to UV light releases free aldehyde moieties, which readily react with hydrazide groups in situ. This hydrogel system may find applications in controlled release drug delivery applications, when combined with in situ gelation. Furthermore, the possibility of forming gels specifically upon UV irradiation gives an opportunity for 3D fabrication of degradable scaffolds.