Nonconjugated alpha,omega-Diolefin/Propylene Copolymerization to Long Chain-Branched Polypropylene by Ziegler-Natta Catalyst: Overcoming Steric Hindrance by Introducing an Extra Electronic Pulling Effect

Synthesis of long chain-branched polypropylene (LCB-PP) by propylene copolymerization with nonconjugated alpha,omega-diolefin is a steric hindrance-prevailing reaction process which involves in copolymerization not only alpha,omega-diolefin itself (alpha-olefin copolymerization) but also polymeric olefin intermediate derived from the first alpha-olefin copolymerization (omega-olefin copolymerization). This reaction mishap reaches its extreme when ZieglerNatta catalysts based on MgCl2-supported TiCl4 (MgCl2/TiCl4 catalysts) are considered as catalyst, which produce active sites that are highly sensitive to olefin monomers steric bulkiness. A proposition is put forward that such a steric difficulty may be overcome by functionalization of alpha,omega-diolefin with Lewis base functionality, which would usher in an extra electronic pulling effect to help with olefin coordination to the active center in both the alpha,omega-diolefins monomeric alpha-olefin and the polymeric omega-olefin polymerization steps. Three model compounds, including di-n-hexyldiethoxysilane, di-5-hexenyldiethoxysilane, and di-5-hexenyldimethylsilane, were synthesized and used to attest to the validity of the hypothesis. The experimental results evidently proved that the Lewis base functionality enabled the functionalized alpha,omega-diolefin to establish dynamic electron-donating interactions with MgCl2/TiCl4 catalysts, making it far more effective in prompting LCB in copolymerization with propylene due to greatly enhanced polymerization reactivity of both its monomeric alpha-olefin and polymeric omega-olefin in their respective polymerization steps. The electronic promotion effect was found to be so robust that it could not be offset by reducing the initial alpha,omega-diolefin molecular steric hindrance. This approach is promising to solve the real issue of synthesizing LCB-PP by ZieglerNatta catalyst.