Copolymerization of ω-Alkenyltrimethylsilane/Propylene with Heterogeneous Ziegler-Natta Catalyst: How Dose Alkenyl Length Affect Comonomer Incorporation?

Three ω-alkenyltrimethylsilanes of different alkenyl moieties, i.e., 3-butenyltrimethylsilane, 5-hexenyltrimethylsilane, and 7-octenyltrimethylsilane, are copolymerized with propylene over a heterogeneous Ziegler-Natta catalyst. The experimental results reveal that, at odds with what the molecular volumes will foretell, 5-hexenyltrimethylsilane top the three ω-alkenyltrimethylsilanes in incorporation rate into PP while 3-butenyltrimethylsilane becomes the most sluggish of the three. This comonomer incorporation rate order is in line with that of ω-alkenylmethyldichlorosilanes in copolymerization with propylene-synthesizing long-chain-branched PP (LCB-PP), pointing to a peculiar alkenyl length effect on comonomer incorporation rate for these comonomers. DFT simulation is then applied to seek energetic basis in coordination-insertion for such an effect. It is revealed that complexation abilities of the three ω-alkenyltrimethylsilanes decrease in the following order: 3-butenyltrimethylsilane > 5-hexenyltrimethylsilane > 7-octenyltrimethylsilane, in line with their molecular sizes. However, the insertion energy barriers increase in the order of: 5-hexenyltrimethylsilane < 7-octenyltrimethylsilane < 3-butenyltrimethylsilane. The repulsive interaction between the bulky trimethylsilane functionality of ω-alkenyltrimethylsilanes and growing PP chain is found to contribute significantly to the insertion energy barrier, which grows disproportionally large with 3-butenyltrimethylsilane. The current discovery will be conducive to understanding the more complex ω-alkenylmethyldichlorosilane/propylene copolymerization that synthesizes the industrially important LCB-PP.