Ethylene polymerization initiated by tertiary diamine/n-butyllithium complexes: an interpretation from density...

Area of Science:

• Organometallic Chemistry
• Polymer Chemistry
• Computational Chemistry

Background:

• Ethylene polymerization is crucial for producing plastics.
• Anionic coordination polymerization mechanisms are less understood than cationic ones.
• Tertiary diamine/n-butyllithium complexes are potential catalysts for anionic polymerization.

Purpose of the Study:

• To elucidate the mechanism of ethylene insertion into tertiary diamine/n-butyllithium complexes.
• To compare the energy profiles of anionic and cationic ethylene polymerization.
• To explain the lower activity observed in anionic ethylene polymerization.

Main Methods:

• Density Functional Theory (DFT) calculations using the BLYP/DNP level of theory.
• Investigation of the transition state structures and energy barriers for ethylene insertion.
• Comparative analysis of reaction energy profiles.

Main Results:

• A weak van der Waals complex is formed between ethylene and the catalyst before insertion.
• Ethylene insertion occurs via a four-membered-ring transition state.
• Insertion barriers (6.9–11.0 kcal/mol) are comparable to cationic polymerization, but anionic polymerization exhibits lower activity.
• Transition states in anionic polymerization are higher in energy than reactants, unlike cationic polymerization.

Conclusions:

• The higher energy transition states in anionic polymerization require additional energy input, contributing to lower activity.
• Differences in energy profiles, not just insertion barriers, explain the disparity in polymerization activities.
• This study provides insights into the mechanistic differences between anionic and cationic ethylene polymerization.