Synthesis of Lewis Acidic, Aromatic Aminotroponiminate Zinc Complexes for the Ring-Opening Polymerization of Cyclic Esters.

Three novel aminotroponiminate (ATI) zinc complexes I-III (I = [(Ph2)ATI]Zn-N(SiMe3)2, II = [(C6H3-2,6-C2H5/Ph)ATI]Zn-N(SiMe3)2, and III = [(C6H3-2,6-CH(CH3)2/Ph)ATI]Zn-N(SiMe3)2) were synthesized and tested in the ring-opening polymerization of the lactones ß- rac-butyrolactone (BBL) and rac-lactide (LA). The ligands, with two of them literature unknown, were readily obtained via a three-step synthesis from tropolone. Forming a five-membered metallacycle with zinc, the complexes were further structurally examined via single-crystal X-ray analysis and compared with that of the established, 6-ringed ß-diiminate (BDI) complex IV ([CH(CMeNPh)2]Zn-N(SiMe3)2). The influence of the varying metallacycle ring size on the polymerization was evaluated. In situ IR measurements indicate a higher catalytic activity of the novel ATI complexes I-III for BBL compared with the BDI system IV. The activity and degree of control were further improved by an in situ generated alkoxy initiating group generated after the addition of 2-propanol. An enhanced initiator efficiency allowed the synthesis of polymers with controlled molecular weights and narrow polydispersities. Furthermore, II and III exhibited a high activity in the ring-opening polymerization of rac-LA. Hereby, reaction time and initiator efficiency could also be optimized at a higher temperature or by the addition of 2-propanol.

Next Generation Multiresponsive Nanocarriers for Targeted Drug Delivery to Cancer Cells.

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.

Gated Channels and Selectivity Tuning of CO2 over N2 Sorption by Post-Synthetic Modification of a UiO-66-Type Metal-Organic Framework.

The highly porous and stable metal-organic framework (MOF) UiO-66 was altered using post-synthetic modifications (PSMs). Prefunctionalization allowed the introduction of carbon double bonds into the framework through a four-step synthesis from 2-bromo-1,4-benzenedicarboxylic acid; the organic linker 2-allyl-1,4-benzenedicarboxylic acid was obtained. The corresponding functionalized MOF (UiO-66-allyl) served as a platform for further PSMs. From UiO-66-allyl, epoxy, dibromide, thioether, diamine, and amino alcohol functionalities were synthesized. The abilities of these compounds to adsorb CO2 and N2 were compared, which revealed the structure-selectivity correlations. All synthesized MOFs showed profound thermal stability together with an increased ability for selective CO2 uptake and molecular gate functionalities at low temperatures.

In Situ Generated ABA Block Copolymers from CO2, Cyclohexene Oxide, and Poly(dimethylsiloxane)s.

Chain-transfer polymerization reactions with siloxanes, CO2, and cyclohexene oxide have been conducted, utilizing two ß-diiminate (BDI) zinc-based catalysts, BDICF3(1)-ZnEt and BDICF3(2)-ZnEt ((BDICF3(1))H = [CH(CCF3NC6H4-2,6-C2H5)2] and (BDICF3(2))H = [CH(CCF3NC6H4-2,6-CH(CH3)2)2]). The correlation between equivalents of siloxane and the corresponding molecular masses and glass transition temperatures is exhibited. Furthermore, the in situ preparation of ABA block copolymers from carbon dioxide, cyclohexene oxide, and α,ω-bis(hydroxymethyl)poly(dimethylsiloxane)s is presented. This reaction was found to strongly relate to a robust Lewis acid catalyst like the outlined complexes. The polymer properties can be tuned by varying the amount of chain-transfer agent or changing the catalyst. The resulting polymer structures and incorporation of siloxanes were revealed by 29Si NMR spectroscopy, 1H NMR spectroscopy, ESI-MS, GPC, and DSC.

Mechanistic Investigations of the Stereoselective Rare Earth Metal-Mediated Ring-Opening Polymerization of ß-Butyrolactone.

Poly(3-hydroxybutyrate) (PHB) is produced by numerous bacteria as carbon and energy reserve storage material. Whereas nature only produces PHB in its strictly isotactic (R) form, homogeneous catalysis, when starting from racemic (rac) ß-butyrolactone (BL) as monomer, can in fact produce a wide variety of tacticities. The variation of the metal center and the surrounding ligand structure enable activity as well as tacticity tuning. However, no homogeneous catalyst exists to date that is easy to modify, highly active, and able to produce PHB with high isotacticities from rac-ß-BL. Therefore, in this work, the reaction kinetics of various 2-methoxyethylamino-bis(phenolate) lanthanide (Ln=Sm, Tb, Y, Lu) catalysts are examined in detail. The order in monomer and catalyst are determined to elucidate the reaction mechanism and the results are correlated with DFT calculations of the catalytic cycle. Furthermore, the enthalpies and entropies of the rate-determining steps are determined through temperature-dependent in situ IR measurements. Experimental and computational results converge in one specific mechanism for the ring-opening polymerization of BL and even allow us to rationalize the preference for syndiotactic PHB.

Functionalization of metal-organic frameworks through the postsynthetic transformation of olefin side groups.

For the first time, the adaptability of the C=C double bond as a versatile precursor for the postsynthetic modification (PSM) of microporous materials was extensively investigated and evaluated. Therefore, an olefin-tagged 4,4'-bipyridine linker was synthesized and successfully introduced as pillar linker within a 9,10-triptycenedicarboxylate (TDC) zinc paddle-wheel metal-organic framework (MOF) through microwave-assisted synthesis. Different reactions, predominately used in organic chemistry, were tested, leading to the development of new postsynthetic reactions for the functionalization of solid materials. The postsynthetic oxidation of the olefin side groups applying osmium tetroxide (OsO4) as a catalyst led to the formation of a microporous material with free vicinal diol functionalities. The epoxidation with dimethyldioxirane (DMDO) enabled the synthesis of epoxy-functionalized MOFs. In addition to that, reaction procedures for a postsynthetic hydroboration with borane dimethyl sulfide as well as a photoinduced thiol-ene click reaction with ethyl mercaptan were developed. For all of these PSMs, yields of more than 90% were obtained, entirely maintaining the crystallinity of the MOFs. Since the direct introduction of the corresponding groups by means of pre-synthetic approaches is hardly possible, these new PSMs are useful tools for the functionalization of porous solids towards applications such as selective adsorption, separation, and catalysis.