|242. Yakimansky, A.V.; Müller, A.H.E.: Quantum-Chemical Study of the Effect of Lewis Acids on the Chain-End Structure and Tacticity of Poly(N,N-Dimethylacrylamide) with Lithium Counterion in THF , Macromolecules 39, 4228-4234 (2006)|
Quantum-chemical DFT calculations of the structures and relative stabilities of poly(N,N-dimethylacrylamide) anionic living chain ends with lithium counterion and their aggregates in THF, both in the absence and in the presence of the Lewis acid (LA) triethylaluminium (Et3Al) were performed. LA-coordinated, solvated unimers are the most stable species in the presence of Et3Al, whereas unimers and dimers co-exist in the absence of LA. In all cases coordination of Li+ to the penultimate and/or antepenultimate amide unit of the chain end is energetically unfavorable. In the presence of LA, the triads with ultimate r-dyads are systematically more favorable than those with ultimate m-dyads. This correlates with the observed formation of heterotactic polymers in the presence of Et3Al. It was shown that the polymerization kinetics is determined by the competition for complexation with Et3Al between solvent, monomer, and carbonyl groups of predominantly heterotactic polymer chains (in the presence of Et3Al). Complex formation between LA and heterotactic in-chain triads is much less favorable than those between LA and either THF or monomer molecules. Therefore, the fraction of monomer activated by the LA complexation increases with increasing conversion, leading to an increase of the observed polymerization rate.