Perfectly Isotactic Polypropylene upon In Situ Activation of Ultrarigid meso Hafnocenes

For more than 40 years, the synthesis of C₂ -symmetric indenyl-based racemic metallocenes for the isoselective polymerization of propylene relied on a tedious separation of the produced rac and meso isomers. Status quo, latter are considered wasteful as they produce atactic polypropylene (aPP) rather than isotactic polypropylene (iPP) if activated with methylaluminoxane (MAO). Unexpectedly, the in situ activation of meso hafnocene I yielded perfectly isotactic polypropylene. We verified an isomerization of the meso compound to the corresponding racemic one upon triisobutylaluminum (TIBA) addition via nuclear magnetic resonance (NMR) spectroscopy and established an easy and convenient polymerization protocol, enabling productivities comparable to pure rac-I if applied to pure meso-I or a mixture of both isomers. With this established isomerization protocol, the potential yield of iPP was enhanced by more than 400 %. This protocol was also shown to be applicable to other meso hafnocenes and some initial mechanistic insights were received.

Synthesis of a Triphenylphosphinimide-Substituted Silirane as a "Masked" Acyclic Silylene

Phosphinimides are long known as useful ligands in transition metal chemistry, but examples of these in low-valent silicon chemistry are rather rare. Hence, in this work, we report on the implementation of a triphenylphosphinimide moiety as a ligand of a novel silylene that is trapped as a silirane with cyclohexene. By performing activation reactions with B(p-Tol)₃, HSiEt₃, N₂O, and NH₃, we demonstrate that the silirane exhibits a silylene-like behavior, making it a "masked" silylene. Furthermore, we treated the silirane with ethylene, propylene, and trans-butene, which led to an olefin exchange. In the case of ethylene and propylene, an additional insertion of the olefin into the silicon-silicon bonds of the respective siliranes could be achieved. As the insertion of trans-butene was not feasible, we surmise that the scope of this reactivity is restricted by the steric demand of the olefin.

Synthesis, characterisation and functionalisation of BAB-type dual-responsive nanocarriers for targeted drug delivery: evolution of nanoparticles based on 2-vinylpyridine and diethyl vinylphosphonate

The emerging field of nanomedicine gives new opportunities in the treatment of cancer. Aspects such as dosage, bioavailability or the application to the patient can be drastically improved. Previously our group reported an efficient route towards cross-linked nanospheres based on ABB' block copolymers made from 2-vinylpyridine (2VP), diethyl vinylphosphonate (DEVP) and diallyl vinylphosphonate (DAlVP). Followed by thiol-ene click chemistry stable nanoparticles were formed. Herein, this promising concept was extended to copolymers with the analogous B'BABB' architecture. In this context the new yttrium complex 5 was investigated in the rare-earth metal-mediated group transfer polymerisation (REM-GTP) and used for the generation of copolymers with different monomer feeds (2VP: 100-300 equiv.; DEVP: 200-300 equiv.; DAlVP: 6-20 equiv.) to explore the influence of the copolymer compositon on the nanoparticle properties. After successful cross-linking with various cross-linking agents, all nanoparticles were characterised via DLS and TEM. These size measurements revealed defined, almost spherical particles (d DLS = 17-52 nm; d TEM = 17-43 nm) and were mainly affected by the 2VP content and the cross-linking density. Zeta potential measurements resulted in values in the range from -6 mV to -22 mV and revealed an influence of the cross-linking agent on the surface charge. Studies on the release behaviour exhibited the fastest release at pH = 4.5. Temperature-wise best results were achieved at 42 °C. Furthermore, we aimed for the conjugation of folic acid as a model compound for a potential application in active drug targeting. The consecutive couplings of cysteamine and dithiol 6 enabled the formation of an amine-modified precursor which was reacted with a folic acid derivative. Zeta potential measurements and analysis by NMR spectroscopy corroborated a successful conjugation while DLS and TEM (d DLS = 44 nm; d TEM = 38 nm) indicated defined nanoparticles.