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.
Abstractω‐Alkenyltrimethylsilanes of different alkenyl moieties, i.e., 3‐butenyltrimethylsilane, 5‐hexenyltrimethylsilane, and 7‐octenyltrimethylsilane, are used as model compounds to study the alkenyl length effect in copolymerization of ethylene with steric‐hindered tri‐substituted silane‐functionalized α‐olefins over MgCl2/TiCl4 catalysts. The experimental results reveal that 3‐butenyltrimethylsilane tops the three α‐olefins in incorporation rate into PE while 7‐octenyltrimethylsilane is slightly better than 5‐hexenyltrimethylsilane. The coordination‐insertion events for different ω‐alkenyltrimethylsilanes are investigated by DFT simulation. The results suggest that the three ω‐alkenyltrimethylsilanes encounter similar energy barriers during insertion, with similar repulsive interactions between the bulky trimethylsilane substituent and growing PE chain found in the energy decomposition of transition state configuration. However, complexation abilities at the Ti active site for the three ω‐alkenyltrimethylsilanes follow the order of 3‐butenyltrimethylsilane > 5‐hexenyltrimethylsilane > 7‐octenyltrimethylsilane, in line with their molecular compactness, which are deemed to be where the alkenyl length effect originates in the ω‐alkenyltrimethylsilane/ethylene copolymerization.
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