No abstract

A variety of new dinuclear zinc catalysts was developed and tested for the copolymerization of cyclohexene oxide and carbon dioxide. Electron-withdrawing groups thereby led to unprecedented activities with turnover frequencies up to 155,000 h(-1). These are by far the highest polymerization rates ever reported for the copolymerization of cyclohexene oxide and CO2.

The present study is one of the first examples for rare earth metal-mediated group transfer polymerization (REM-GTP) with non-metallocene catalyst systems. 2-Methoxyethylaminobis(phenolate)yttrium trimethylsilylmethyl complexes were synthesized and showed moderate to high activities in the rare earth metal-mediated group transfer polymerizations of 2-vinylpyridine, 2-isopropenyl-2-oxazoline, diethyl vinylphosphonate, diisopropyl vinylphosphonate, and N,N-dimethylacrylamide as well as in the ring-opening polymerization of β-butyrolactone. Reaction orders in catalyst and monomer were determined for the REM-GTP of 2vinylpyridine. The mechanistic studies revealed that the catalyst systems follow a living monometallic group transfer polymerization mechanism allowing a precise molecular-weight control of the homopolymers and the block copolymers with very narrow molecular weight distributions. Temperaturedependent reaction kinetics were conducted and allowed conclusions about the influence of the bulky substituents around the metal center on the polymerization activity. Additional polymerization experiments concerning the combination of REM-GTP and ROP to obtain block copolymers were performed.

The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co-polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co-polymerization experiments with propylene oxide (PO) and CO2 revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO2 co-polymerization was not stable enough to deactivate the catalyst due to steric repulsion.

Herein, we present a fundamental study of isostructural 2-methoxyethylamino-bis(phenolate)-lanthanide complexes [(ONOO)M(X)(THF)] (M = Lu, Y; R = Bu, CMePh, X = CHTMS, collidine; THF = tetrahydrofuran; TMS = trimethylsilyl) for rare-earth metal-mediated group-transfer polymerization (GTP). This analysis includes the differentiation of electron-donating and nondonating vinyl monomers and two metal centers with regard to the ionic radius (yttrium and lutetium). In addition, highly nucleophilic alkyl initiators are compared with electron-donating heteroaromatic initiators. Our examinations include the impact of these parameters on the activity, initiator efficiency, and tacticity of the obtained polymers. Density functional theory calculations and proposed catalyst structure determinations via X-ray analysis support these investigations. This facilitates the selection of the best metal and initiator combination to address efficient and stereospecific polymerization of a broad range of Michael monomers. [(ONOO)Lu(X)(THF)] shows the highest activity of 2220 h (normalized turnover frequency) for the polymerization of 2-vinylpyridine due to the higher Lewis-acidity of lutetium. Through C(sp)-H bond activation, catalysts with higher initiator efficiency in N,N'-dimethylacrylamide (DMAA) and diethylvinylphosphonate polymerization were synthesized. Remarkably, [(ONOO)Y(collidine)(THF)] was capable of stereospecifically polymerizing DMAA to highly isotactic poly(DMAA) (P = 0.94). Overall, the kinetics studies reveal a living-type GTP mechanism for all of the tested catalysts, enabling precise molecular-weight predeterminations with narrow molecular weight distributions (Đ ≤ 1.06).

2-Methoxyethylamino-bis(phenolate)-yttrium complexes were employed in the catalytic precision polymerization of 2-vinylpyridine (2VP). The C 1 -symmetric catalyst systems are able to isospecifically polymerize prochiral 2-vinylpyridine with moderate to high activities. Tacticities ranging from atactic to isotactic can be achieved (P m = 0.54-0.74). Mechanistic studies through 13 C NMR microstructure analysis of the resulting isotactic P2VP show an enantiomorphic site control mechanism.

A series of β-diiminato zinc(II) complexes bearing two electronwithdrawing trifluoromethyl groups in the pentane ligand backbone were successfully isolated ((). The solid-state structures illustrate differences in the ligation of the Zn atom compared to a literature known BDI-ZnEt complex. All catalysts show good activities in the ring-opening polymerization of the cyclic ester (rac)-β-butyrolactone (BL), whereas only BDI CF 3 -II-ZnEt and BDI CF 3 -III-ZnEt are active initiators for the polymerization of (rac)-lactide (LA).

C-H bond activation of 2-methoxyethylamino-bis(phenolate)-yttrium catalysts allowed the synthesis of BAB block copolymers comprised of 2-vinylpyridine (2VP; monomer A) and diethylvinylphosphonate (DEVP; monomer B) as the A and B blocks, respectively, by rare-earth-metal-mediated group-transfer polymerization (REM-GTP). The inherent multi-stimuli-responsive character and drug-loading and -release capabilities were observed to be dependent on the chain length and monomer ratios. Cytotoxicity assays revealed the biocompatibility and nontoxic nature of the obtained micelles toward ovarian cancer (HeLa) cells. The BAB block copolymers effectively encapsulated, transported, and released doxorubicin (DOX) within HeLa cells. REM-GTP enables access to previously unattainable vinylphosphonate copolymer structures, and thereby unlocks their full potential as nanocarriers for stimuli-responsive drug delivery in HeLa cells. The self-evident consequence is the application of these new micelles as potent drug-delivery vehicles with reduced side effects in future cancer therapies.