Alkyl Effects on the Chain Initiation Efficiency of Olefin Polymerization by Cationic Half-Sandwich Scandium Catalysts: A DFT Study

The effect of alkyls on the chain
initiation efficiency of ethylene,
propene, 1-hexene, styrene, butadiene, and isoprene polymerizations
catalyzed by the half-sandwich cationic rare-earth-metal alkyl complexes
[(η5-C5Me5)­ScR]+ (R = CH2SiMe3, 1; R = o-CH2C6H4NMe2, 2; R = η3-C3H5, 3) has been studied by using a DFT approach. It has
been found that 2 with the largest sterically demanding
aminobenzyl group results in the lowest initiation efficiency and
thus longest induction period among the three catalysts investigated.
In contrast, 1 with CH2SiMe3 displays
the best chain initiation ability, and 3 with η3-allyl gives moderate chain initiation activity, mainly due
to the most stable resulting coordination complex. Species 1 and 3 have better regioselectivity in the chain initiation
of styrene polymerization than species 2. In addition,
species 1′ ([(η5-C5Me5)­Sc­(CH2SiMe3)­THF]+) with a THF ligand has better chain initiation efficiency in styrene
and isoprene polymerizations than species 2 but is reasonably
worse than the analogue 1 without a THF ligand.