Quantum-Chemical Study of the Effect of Triethylaluminum on the Chain-End Structure, Reactivity, and Microtacticity of Poly(N,N-dimethylacrylamide) with Lithium Counterion in Nonpolar Solvents
journal contributionposted on 10.08.2010, 00:00 by Alexander V. Yakimansky, Axel H. E. Müller
Reactions of chain growth in the anionic polymerization of N,N-dimethylacrylamide (DMAAm) with lithium counterion in the presence of triethylaluminum (Et3Al) were studied using quantum-chemical DFT methods. The active sites were simulated by chain fragments consisting of two DMAAm units, the lithium counterion being coordinated to carbonyl oxygen atoms of both ultimate and penultimate units. For all local minima and transition states zero-point vibration energies and free energies, ΔG, at 200 K were calculated. It was shown that in the absence of Et3Al iso-addition of the monomer to the model chain end is the most favorable, while in the presence of two Et3Al molecules syndio- and iso-additions of DMAAm are equally probable. In the presence of one Et3Al molecule, an unambiguous prediction of the prevailing stereomode of DMAAm addition is impossible because different stereomodes are preferred at the BHLYP/SVP (syndio-addition via 1LA-DAS mechanism) and BHLYP/aug-cc-pVTZ//BHLYP/SVP (iso-addition via 1LA-AM mechanism) levels. Thus, on the basis of the performed DFT calculations, it is possible to predict the possibility to influence the microtacticity of poly(N,N-dimethylacrylamide), varying the [Al]/[Li] ratio: the polymer should be predominantly isotactic at [Al]/[Li] = 0 and atactic at [Al]/[Li] ≥ 2.