Highly Isoselective Ring-Opening Polymerization of rac-ß-Butyrolactone: Access to Synthetic Poly(3-hydroxybutyrate) with Polyolefin-like Material Properties.

We report the highly isoselective ring-opening polymerization (ROP) of racemic ß-butyrolactone (ß-BL) using in situ-generated catalysts based on Y[N(SiHMe2)2]3(THF)2 and salan-type pro-ligands. The catalyst system produces isotactic poly(3-hydroxybutyrate) (PHB) with record productivity (TOF up to 32 000 h-1) and the highest isoselectivity (Pm up to 0.89) in ROP of ß-BL achieved to date. In contrast to bacterial PHB, the chemically synthesized PHB has beneficial material properties, such as increased melt processing window attributed to a lowered melting temperature (Tm ≈ 140 °C) and drastically reduced brittleness. The produced PHB showed an elongation at break of 392%, thus demonstrating promising polyolefin-like thermomechanical material properties.

Fiber Spinning of Ultrahigh Molecular Weight Isotactic Polypropylene: Melt Spinning and Melt Drawing.

Herein, this work reports fiber spinning of tailored isotactic polypropylene (iPP) by melt spinning and melt drawing, yielding an adjustable diameter of 40-400 µm. The crystallinity of all obtained fibers with a molecular weight between 330-1400 kg/mol is increased by thermal annealing and investigated via differential scanning calorimetry (DSC) as well as wide angle X-ray scattering (WAXS). The potential of ultrahigh molecular weight iPP (UHMW-iPP) fibers compared to fibers manufactured from industrially available iPP becomes evident when the mechanical performance is compared: fibers spun from UHMW-iPP (1400 kg/mol) enable a tensile strength of up to 400 MPa, whereas commercially available fibers (330 kg/mol) show a tensile strength of approximately 50 MPa. However, UHMW-iPP exhibits a short timeframe, in which extrusion is possible, thereafter extrusion rupture occurs, probably induced by an increased melt viscosity.

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.