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.
As modifications of the effective catalyst precursor
[dimethylsilanediylbis(benz[e]indenyl)]zirconium dichloride, the ansa-zirconocene complexes
Me2Si(cyclopenta[c]phenanthryl)2ZrCl2,
Me2Si(cyclopenta[l]phenanthryl)2ZrCl2,
Me2Si(2-methylcyclopenta[l]phenanthryl)2ZrCl2,
and
Me2Si(2-methyltetrahydrobenz[e]indenyl)2ZrCl2
have been synthesized. When activated by
methylaluminoxane (MAO), these complexes give highly active catalysts
for the polymerization of propene to polymers with high isotacticities and molecular
weights.
(2-Methylcyclopenta[l]phenanthryl)2ZrCl2 and (2-phenylcyclopenta[l]phenanthryl)2ZrCl2
were synthesized and used, after activation with MAO, as catalysts for the polymerization
of propene. The resulting polymers had low isotacticities (8−18% [mmmm]). Ligand rotation
barriers for the corresponding Zr−dibenzyl derivatives were determined by dynamic NMR
spectroscopy. Pathways and transition states for ligand rotation in this series of complexes
were studied by molecular-mechanics calculations and are discussed in relation to the
microstructure of the respective polymer products.
Chiral ansa-zirconocene complexes with a trimethylene link between the Si bridge atom and an a-position of each C s ring ligand were prepared; two representatives, with i-propyl and t-butyl groups as f3-substituents, were structurally characterized. The properties of these complexes with respect to MAO-activated propene polymerization were studied in comparison with those of their Me 2 Si-bridged analogs. The t-butyl-substituted spirosilane complex gives a polymer with relatively high content of 3, I-insertions. These increased regioirregularities appear to be associated with decreased coordination gap aperture and increased lateral extension angles of the spirosilane-bridged zirconocene complexes; this notion is supported by a molecular-mechanics analysis of alternative olefin-insertion transition states.
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.