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

Three ?-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 ?-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 ?-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 ?-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 ?-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 ?-alkenylmethyldichlorosilane/propylene copolymerization that synthesizes the industrially important LCB-PP.

Automated summary

The copolymerization of three different α-alkenyltrimethylsilanes with propylene over a Ziegler-Natta catalyst was studied. Experimental results showed that the incorporation rate of 5-hexenyltrimethylsilane was higher than that of the other two, contrary to the predicted molecular volumes. The order of comonomer incorporation rate was in line with that of α-alkenylmethyldichlorosilanes in copolymerization with long-chain-branched PP. DFT simulation revealed that the complexation abilities and insertion energy barriers varied among the three α-alkenyltrimethylsilanes, with the bulky trimethylsilane functionality playing a significant role.