In this work, we computationally analyze the effect of the strain at the Cr(III) sites of a Cr(III)/SiO2 catalyst in the ethylene polymerization initiation considering two possible pathways: ethylene C–H activation and ethylene insertion into the Cr–O bond. We use the activation strain model (ASM), which dissects the activation energy of a reaction into strain and interaction energies, on a series of Cr(III) clusters with different sizes of the Si–O–Si rings. Our results indicate that more strained Cr(III) sites are more active than less strained ones and that the ethylene insertion into the Cr–O bond is more favorable than C–H bond activation for all sites. The ASM analysis reveals that the activation energies of both initiation mechanisms are dually controlled by interaction and strain energies. However, the comparison of the two mechanisms indicates that the preference for the ethylene insertion pathway is due to lower distortion of the ethylene fragment at the transition states, which is ultimately controlled by one intermolecular and one intramolecular interaction between the activated ethylene and the Cr(III) site fragments. © 2021 American Chemical Society