Artjom Döring

Responsive hydrogels – structurally and dimensionally optimized smart frameworks for applications in catalysis, micro-system technology and material science

Although the technological and scientific importance of functional polymers has been well established over the last few decades, the most recent focus that has attracted much attention has been on stimuli-responsive polymers. This group of materials is of particular interest due to its ability to respond to internal and/or external chemico-physical stimuli, which is often manifested as large macroscopic responses. Aside from scientific challenges of designing stimuli-responsive polymers, the main technological interest lies in their numerous applications ranging from catalysis through microsystem technology and chemomechanical actuators to sensors that have been extensively explored. Since the phase transition phenomenon of hydrogels is theoretically well understood advanced materials based on the predictions can be prepared. Since the volume phase transition of hydrogels is a diffusion-limited process the size of the synthesized hydrogels is an important factor. Consistent downscaling of the gel size will result in fast smart gels with sufficient response times. In order to apply smart gels in microsystems and sensors, new preparation techniques for hydrogels have to be developed. For the up-coming nanotechnology, nano-sized gels as actuating materials would be of great interest.

Lactide polymerisation with air-stable and highly active zinc complexes with guanidine-pyridine hybrid ligands.

The synthesis of zinc complexes of guanidine-pyridine hybrid ligands [Zn(DMEGpy)Cl(2)] (C1), [Zn(TMGpy)Cl(2)] (C2), [Zn(DMEGqu)Cl(2)] (C3), [Zn(TMGqu)Cl(2)] (C4), [Zn(DMEGpy)(CH(3)COO)(2)] (C5), [Zn(TMGpy)(CH(3)COO)(2)] (C6), [Zn(DMEGqu)(CH(3)COO)(2)] (C7), [Zn(TMGqu)(CH(3)COO)(2)] (C8), [Zn(DMEGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C9) and [Zn(TMGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of D,L-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (M(w)) up to 176,000 g mol(-1) could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure-reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline-guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.

Mechanism of the Living Lactide Polymerization Mediated by Robust Zinc Guanidine Complexes

Zinc bis(chelate) guanidine complexes promote living lactide polymerization at elevated temperatures. By means of kinetic and spectroscopic analyses the mechanism has been elucidated for these special initiators that make use of neutral N-donor ligands. The neutral guanidine function initiates the polymerization by a nucleophilic ring-opening attack on the lactide molecule. DFT calculations on the first ring-opening step show that the guanidine is able to act as a nucleophile. Three transition states were located for ligand rearrangement, nucleophilic attack, and ring-opening. The second ring-opening step was modeled as a representation for the chain growth because here, the lactate alcoholate opens the second lactide molecule via two transition states (nucleophilic attack and ring-opening). Additionally, the resulting reaction profile proceeds overall exothermically, which is the driving force for the reaction. The experimental and calculated data are in good agreement and the presented mechanism explains why the polymerization proceeds without co-initiators.

A Comprehensive Study of Copper Guanidine Quinoline Complexes: Predicting the Activity of Catalysts in ATRP with DFT†

Copper complexes of the hybrid guanidine ligands 1,3-dimethyl-N-(quinolin-8-yl)-imidazolidin-2-imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)-guanidine (TMGqu) have been studied comprehensively with regard to their structural and electrochemical properties and their activity in atom transfer radical polymerization (ATRP). A simple analysis of the molecular structures of the complexes gives no indication about their activity in ATRP; however, with the help of DFT and NBO analysis the influence of particular coordinating donors on the electrochemical properties could be fully elucidated. With an adequate DFT methodology and newly applied theoretical isodesmic reactions it was possible to predict the relative position of the redox potentials of copper complexes containing DMEGqu and TMGqu ligands. In addition, predictions could be made as to whether the complexes of DMEGqu or TMGqu are more active in ATRP. Four new Cu(I) complexes were tested in standard ATRP reactions and kinetically investigated both in bulk and in solution. It could be proven that complexes featuring DMEGqu possess a lower redox potential and are more active in ATRP, although the tetramethylguanidine moiety represents the stronger donor.

Neue Bisguanidin-Kupfer-Komplexe und ihre Anwendung in der ATRP/ New Bisguanidine-Copper Complexes and their Application in ATRP

The ligands TMG2e [bis(N,N,N´,N´-tetramethylguanidino)ethane] and DMEG2e [N1,N2-bis(1,3-dimethylimidazolin-2-ylidene)ethane-1,2-diamine] were used in the complexation of copper cations to give the new complexes [Cu(TMG2e)2][Cu2I4], [Cu(TMG2e)Cl2] and [Cu(DMEG2e)2]-[CuCl2]. Single-crystal structure determination shows that the complexes [Cu(TMG2e)Cl2] and [Cu(DMEG2e)2][CuCl2] both crystallise in the monoclinic space group C2/c, the complex [Cu(TMG2e)2][Cu2I4] in the orthorhombic space group Pbca. The copper atoms in all complex cations reside in a coordination environment between tetrahedral and square-planar geometry. The application of copper complexes with TMG2e and DMEG2e as ligands in atom transfer radical polymerisation (ATRP) was investigated with styrene as monomer. The polymerisation process with both ligand systems shows even at low temperature unexpected high conversions and molecular weight distributions that are evidence of a well controlled ATRP. These first results in the application of guanidine ligands in ATRP show that these ligands have high potential, but that further process optimisations and ligand tuning are necessary to develop highly active catalysts for ATRP. Graphical Abstract Neue Bisguanidin-Kupfer-Komplexe und ihre Anwendung in der ATRP/ New Bisguanidine-Copper Complexes and their Application in ATRP

CCDC 819814: Experimental Crystal Structure Determination

Related Article: J.Borner, I.dos S.Vieira, M.D.Jones, A.Doring, D.Kuckling, U.Florke, S.Herres-Pawlis|2011|Eur.J.Inorg.Chem.||4441|doi:10.1002/ejic.201100540