Renzo M. Paulus

Self-healing metallopolymers based on cadmium bis(terpyridine) complex containing polymer networks

The utilization of metal–ligand interactions within polymers generates materials which are of interest for several applications, including self-healing materials. In this work we use methacrylate copolymers containing terpyridine moieties in the side chain for the formation of self-healing metallopolymer networks. The materials were synthesized using the reversible addition–fragmentation chain transfer (RAFT) polymerization technique and subsequent crosslinking by the addition of a metal salt, here cadmium(II) salts, with different counter-ions. The influence of the counter-ions on the self-healing process within these structures was analyzed. The research resulted in a new polymeric material featuring a high (intrinsic) healing efficiency at relatively low temperatures (<75 °C).

Thermoresponsive Poly(2‐oxazine)s

The monomers 2-methyl-2-oxazine (MeOZI), 2-ethyl-2-oxazine (EtOZI), and 2-n-propyl-2-oxazine (nPropOZI) were synthesized and polymerized via the living cationic ring-opening polymerization (CROP) under microwave-assisted conditions. pEtOZI and pnPropOZI were found to be thermoresponsive, exhibiting LCST behavior in water and their cloud point temperatures (T(CP)) are lower than for poly(2-oxazoline)s with similar side chains. However, comparison of poly(2-oxazine) and poly(2-oxazoline)s isomers reveals that poly(2-oxazine)s are more water soluble, indicating that the side chain has a stronger impact on polymer solubility than the main chain. In conclusion, variations of both the side chains and the main chains of the poly(cyclic imino ether)s resulted in a series of distinct homopolymers with tunable T(CP).

Cationic poly(2-oxazoline) hydrogels for reversible DNA binding

A new 2-oxazoline monomer with a Boc-protected amino group in the side chain (BocOx) was synthesized. Homopolymerization as well as copolymerization with 2-ethyl-2-oxazoline (EtOx) revealed a pseudo first order kinetic. A series of homopolymers was synthesized, deprotected and characterized regarding their structure and thermal properties. The copolymerization with EtOx yielded a series of water soluble polymers with varying amino contents. After deprotection it was shown by the ethidium bromide assay that these polymers were able to form complexes with DNA. Treatment with epichlorohydrin leads to the formation of hydrogels. The swelling properties of the gels were investigated and it could be demonstrated that also the polymeric scaffolds were able to immobilize DNA from aqueous solution. Furthermore, the release of the DNA was accomplished using heparin.

High temperature initiator-free RAFT polymerization of methyl methacrylate in a microwave reactor

The reversible addition–fragmentation chain transfer (RAFT) polymerization of methyl methacrylate (MMA) was investigated under microwave irradiation. At first, a comparison was made between microwave and thermal heating for the RAFT polymerization of MMA with azobis(isobutyronitrile) (AIBN) as initiator and 2-cyano-2-butyldithiobenzoate (CBDB) as RAFT agent, revealing comparable polymerization kinetics indicating the absence of non-thermal microwave effects. Second, the CBDB-mediated RAFT polymerization of MMA was investigated at high temperatures (120°C, 150°C, and 180°C, respectively) in the absence of a radical initiator, showing a linear increase of the molar masses with conversion. The polydispersity indices remained below 1.5 up to 25% MMA conversion at 120°C and 150°C, indicating a controlled polymerization. This control over the polymerization was confirmed by the ability to control the molar masses by the concentration of RAFT agent.

Self-assembly of chiral block and gradient copolymers

Chiral micelles have a high potential for targeted drug delivery or chiral separation applications. In this contribution the self-assembly of chiral amphiphilic copolymers into chiral structures was investigated. Gradient copolymers could be obtained by statistically copolymerizing the hydrophilic 2-ethyl-2-oxazoline (EtOx) with the hydrophobic chiral R-2-butyl-4-ethyl-2-oxazoline (R-BuEtOx) or racemic RS-BuEtOx monomers. Self-assembly of the gradient enantiopure copolymers was studied by both cryogenic transmission electron spectroscopy (cryo-TEM) and dynamic light scattering (DLS) revealing the formation of spherical micelles in aqueous solution. Additionally, amphiphilic block copolymers were synthesized in a 1-pot-2-step manner. The type of self-assembled structure could be controlled by varying the hydrophobic to hydrophilic ratio within the block copolymer from spherical and cylindrical micelles to sheets and vesicles. When the enantiopure block was replaced by the corresponding racemic block, only spherical micelles could be observed, while the chiral block copolymers with similar hydrophobic content revealed cylindrical micelles.

Scaling-up the Synthesis of 1-Butyl-3-methylimidazolium Chloride under Microwave Irradiation

Ionic liquids are considered to be ‘green’ solvents on account of their non-volatility and non-flammability – which are results of their negligible vapour pressures – as well as reusability. On the basis of ecological concerns, ionic liquids seem to be an attractive alternative to conventional volatile organic solvents. In the present work, the reaction conditions for the synthesis of 1-butyl-3-methylimidazolium chloride were optimized on a small scale (~2 mL, 10 mmol) using a single-mode microwave system. The conditions obtained were subsequently transferred to various microwave reactors, both batch and continuous flow, as well as mono-mode and multi-mode, for the direct scale-up of the synthesis from 0.01 to 1.15 mol.

A strong cationic Brønsted acid, [H(OEt2)2][Al{OC(CF3)3}4], as an efficient initiator for the cationic ring-opening polymerization of 2-alkyl-2-oxazolines

In this contribution, the cationic ring-opening polymerization (CROP) and copolymerization of 2-ethyl-2-oxazoline and 2-tert-butyl-2-oxazoline using a strong cationic Bronsted acid, [H(OEt2)2][Al{OC(CF3)3}4], as an initiator are described. First, various poly(2-ethyl-2-oxazoline) (PEtOx) samples are prepared and the living/controlled character of the reaction is demonstrated. We could show that the microwave-assisted CROP of EtOx using this initiator system proceeds faster if compared to classical initiators such as methyl tosylate. These results were then extended to the CROP of poly(2-tert-butyl-2-oxazoline) (PtButOx) and to PEtOx/PtButOx random and block copolymers of different compositions. The resulting materials were characterized using spectroscopic (1H-NMR, FT-IR), chromatographic (SEC), and thermoanalytic techniques (DSC, TGA). Although samples containing more than 13 wt% PtButOx were insoluble in common organic solvents, thermogravimetric (TGA, DSC), spectroscopic (IR), scattering methods (wide-angle X-ray scattering, WAXS), and SEC in hexafluoro-iso-propanol (HFIP) hinted at the successful formation of block copolymers. In particular, WAXS revealed increasing crystallinity for samples containing higher weight fractions of PtButOx.

Simulation and prediction of the thermal sintering behavior for a silver nanoparticle ink based on experimental input

In order to develop a prediction model for resistivity evolution during isothermal sintering, a commercial silver nanoparticle ink was characterized for its metal content, particle size and behavior upon heating. Electrical properties, mass loss behavior, grain size development and material densification were studied for thermal sintering at 175 °C. The correlation between mass loss, height loss of the resulting sintered structures, grain size and electrical resistivity was investigated to gain further understanding of the silver nanoparticle sintering process. The results of thermal sintering were used to calibrate a discrete element sintering model that provides microstructural properties with which the resistivity development at 150 and 200 °C was successfully predicted. The model was validated by experimental data obtained at these temperatures. A variation of particle size and particle size distribution in the simulations furthermore illustrate their influence on final resistivity showing that using small particles with a broad distribution are preferable for reducing the final resistivity of the inkjet-printed pattern.

Determination of the relative ligand-binding strengths in heteroleptic IrIII complexes by ESI-Q-TOF tandem mass spectrometry

An electrospray ionization quadrupole time-of-flight mass spectrometer has been utilized to investigate the relative ligand-binding strengths in a series of heteroleptic-charged iridium(III) complexes of the general formula [(C^N)(2) Ir(III) (S-tpy)](PF(6) ) by using variable collision energies. Collision-induced dissociation experiments were performed in order to study the stability of the Ir(III) complexes that are, for instance, suitable phosphors in light-emitting electrochemical cells. The ratio of signal intensities belonging to the fragment and the undissociated complex depends on the collision energy applied for the tandem mass spectra (MS/MS) analysis. By defining the threshold collision energy and the point of complete complex dissociation, it is possible to estimate the relative complex stabilities depending on the nature of the coordinated ligands [i.e. type of cyclometalating ligand (C^N), substituents on the S-shaped terpyridine (S-tpy)]. The collision energy values differed as a function of the coordination sphere of the Ir(III) centers.

Cationic ring-opening polymerization of protected oxazolidine imines resulting in gradient copolymers of poly(2-oxazoline) and poly(urea)

Poly(urea)s are a polymer class widely used in industry. Their utilization in biomedical applications is already described, however, the use of controlled polymerization methods instead of polycondensation approaches would allow a better control over the degree of polymerization and the dispersity of the resulting polymers, improving their suitability for this particular field of application. Cationic ring-opening polymerization (CROP) as a chain growth polymerization enables those requirements and, additionally, allows the copolymerization with 2-oxazolines, which are generally known for their biocompatibility. In this report, a Boc protected oxazolidine imine monomer is synthesized and polymerized in a homopolymerization, as well as in a copolymerization with 2-ethyl-2-oxazoline (EtOx) via CROP. The synthesized polymers were analyzed regarding their chemical and physical properties, using NMR, GC, MALDI-MS, SEC, TGA and DSC. Copolymerization kinetics revealed the formation of quasi-block copolymers, able to self-assemble in aqueous solution as indicated by DLS.