SUMMARY: Elastomeric poly(propylene) with low percentages of isotactic pentads and high molecular weight was synthesized using unbridged "oscillating" metallocenes. The polymer sample with the highest [mmmm] content (22%) shows a small amount of crystallinity, which is highly influenced by the thermal history, and differs significantly in stress-strain and dynamic mechanical properties from all other samples. Polymers with lower isotacticity index, although lacking crystallinity in thermal analysis, differ in their viscoelastic behaviour from those of atactic poly(propylene). In dynamic mechanical analysis the c relaxation at low temperature shows high sensitivity to low isotactic contents.
Bis(2-N,N-dimethylamino-indenyJ) zirconium dichloride, (2-(CH 3)2 N-C 9 H 6)2 ZrC1 2 • and dimethylsilyl-bridged bis(2-N,N-dimethylamino-indenyl) zirconium dichloride. (CH3)2Si(2-(CH3)2N-C9Hs)2ZrCI2' were prepared by reaction of the corresponding ligand lithium salts with ZrCl" in toluene. Diffractometric structure determinations reveal C 2-symmetrlc complex geometries for both complexes. An increased electron density at the Zr center of the dimethylamino-substituted complexes is indicated by reduction potentials which are 0.3-0.4 V morr negative than those of their unsubstituted analogs. When activated with methyl aluminoxane in toluene solution. (CH3)2Si(2-(CH3)2N-C9Hs)2ZrCI2 catalyzes the polymerization of propene to polymers with a microstructure comparable with that of polymers produced with other Mel Si-bridged bis(indenyJ)ZrCl 2 complexes. but with a substantially increased fraction of i-propyl end groups derived from alkyl exchange between Zr-polymer and AI-Me species.
Up to 94% yields of isotactic pentads are achieved in the polymerization of propene with the ansa‐zirconocene catalysts 1. It emerges that bulky substituents are decisive for the stereoselectivity, while α‐substituents are decisive for the regioselectivity. Moreover, α‐substituents favor the synthesis of long chains (Mw up to 9190). R1 tBu, iPr; R2 H. Me; μ‐X Me2CCMe2, Me2Si.
67Isotactic polypropene was prepared using rac-dimethyl-silylbis(2-methyl-4-t-butyl-cyclopentadienyl)zirconiumdichloride (1) and methyla!umoxane (MAO) at temperatures between 0 and 80°C and with Al/Zr ratios of 300 and 1500. The polymers, prepared at temperatures below 30°C had melting points of 162°C and above. The chain ends were determined by means of distortionless enhancement by polarization transfer (DEPT)-NMR spectroscopy. The pentad intensities were corrected according to the chemical shifts of the end groups. The intensity distributions are shown to arise from enantiomorphic site control of the chiral catalyst species.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.