A.V.Topchiev Institute of Petrochemical Synthesis
Russian Academy of Sciences, Moscow, Russia
Introduction. Advance of the methods of solvation effects calculation with semiempirical procedures as background of quantum-chemical calculation stage allows to investigate large chemical systems with rather reliable results for a quite reasonable computational time. In the present paper solvation effects calculations in the framework of continuum solvent model in realization [1] wasere performed by utilizing improved version based on semiempirical NDDO quantum-chemical background [2] to consider one of the interesting problems of radical polymerization ó "non-classical" polymerization of N-vinylpyrrolidone (VP). AM1 [3] and PM3 [4] methods were as used for quantum-chemical stage.
VP molecule was shown by physical-chemical measurements to be a very polar, to coordinate about fifty water molecules in aqueous solutions and to form hydrogen bonds between oxygen atom and solvent molecules in aqueous and alcohol solutions [5] (review). The specific feature of VP radical polymerization in aqueous and alcohol solutions is anomalous (having an extremum) polymerization rate dependence on monomer concentration [5]. In general opinion, this effect is caused by hydrogen bonding between water (alcohol) molecules and amide group of VP leading to decrease of negative charge of vinyl terminal carbon atom and, as a result, to diminishment of electrostatic repulsion in transition state and of activation energy, Ea , of propagation reactions in solution inby comparison with process in bulk [5]. The above conclusion did not seem convincing for some reasons. The study wais intended to establish specific hydrogen bonding and electrostatic solvation influence upon the activation energy of elementary propagation reaction in VP polymerization in water solutions. The first results of this investigation have been published recently [6].
Results and Discussion. Associations of VP molecule (M) and its radical (R) (within UHF) with one and two water molecules as well as with monomer itself have been calculated (with geometry optimization) in solvent with several ε values, simulating VP aqueous solutions of various concentrations. The application of the methods to systemsms under consideration was discussed and proper reliability was demonstrated. Potential energy curves of reactions (1) -(3)
[M+R ] (1), H2O...[M+R]...H2O (2), 2H2O...[M+R]...2H2O (3)
(3) were calculated in solvent. Each of reactions was considered as two-stage process: diffusion of reactants into solvent cellage (and reversebackward movement) and chemical reaction of encounter pare in solvent cellage. The observed propagation constant rate, kp , was expressed as (under chemical equilibrium condition): kp = Kd * kcell , where Kd is constant of diffusive equilibrium and kcell is chemical constant rate in solvent cellage. Different possible structures of hydrated reactants, as well as associated in cell ( M...R , M...H2O...H2O...R , H2O...M...H2O...H2O...R...H2O ) and solvated transition states were examined. From the data obtained it was established that despite the decrease of vinyl group terminal carbon atom negative charge Ea differed negligibly in reactions set (1)- (3). There was redistribution of atom charges in transition state to reduce Coulomb repulsion so that in reactional centers atom charges were nearly equal as well as full two-center Coulomb terms in interactions under consideration. It was shown that transition state structures wereas not influenced by H-bonding and electrostatic solvation. On the basis of these data it might be concluded that entropy factors in transition states (1)-(3) were similar.
Analysis of the results showed evidence that at VP polymerization in aqueous solutions propagation constant rate kcell in solvent cellage is almost independent on monomer concentration. The present result confirms that the energy of π-electron interaction is the fundamental measure of reactivity in the case of radical addition reactions with similar reactional centers. Obviously, the nature of the anomalous effect is closely related to dynamic and conformational characteristics of macroradicals and to transport diffusive processes in both propagation and termination steps. Detailed analysis of experimental data on VP polymerization in different solvents indicates that dynamic characteristics of solutions may play the main role in kinetic dependence on concentration.
References
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5. Kirsh, Yu.E. Polymer Science, 35, 271 (1993).
6. Timofeeva, L.M., Kabanova, E.Yu.,Martynenko, A.I., and Topchiev, D.A.
Chemical Physics Reports, 13, 1827 (1994).