A theoretical approach to zwitterionic derivatives of 4-vinylpyridine R. Salcedo, J. Cardoso, O. Manero and V. M. Monroy Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de M#xico, A. Postal 70-360, 04510 M~xico DF, Mexico and J. L. V. Escobar and M. F. Rubio-Arroyo Instituto de Quimica, Universidad Nacional Autonoma de M#xico, Circuito Exterior, 04510 M&xico DF, Mexico (Received 15 August 1988; revised 7 November 1988; accepted 2 January 1989) 4-Vinylpyridineand a quaternized derivativewere calculated by the modifiedneglect of differentialoverlap (MNDO) method to obtain values related to their geometry, energetic magnitudes and dipole moments for the closed-shell molecules and the corresponding free radicals. A discussion of the chemical behaviour of all structures is presented and some evidence for a possible free-radical pathway for polymerization is analysed. (Keywords: 4-vinylpyridine; MNDO method; zwitterion; quaternization; free radicals; polymerization) INTRODUCTION Monosubstituted pyridines are organic bases with known nucleophilic properties and, most importantly, they have a strong tendency to polymerize ~-3. Zwitterionic derivatives of 4-vinylpyridine may produce polymers with real charges, with interesting properties that include an increase in ionic strength in aqueous solution4. In addition, a number of molecular characteristics make them particularly interesting as surfactants or detergents. Pyridinic derivatives often have a complicated chemical structure, with active sites where polymerization may take place. It is therefore important to predict theoretically the more feasible pathway of the polymer- ization reaction, if specific characteristics of conformation and stability of the final products (macromolecules) are sought. Prediction of possible reaction mechanisms is valuable in experimental characterization as well. Vinylpyridine isomers have been studied in relation to their base strength in both gas phase and solution4, with further attention given to their chemical characteristics 5. As Barone et al. 5 pointed out, the conformational characteristics of conjugated molecules are determined by a balance between steric and conjugative effects (double bonds conjugated to aromatic rings), and this kind of analysis represents one of the most difficult tasks for semiempirical methods. Moreover, the zwitterionic derivatives obtained after quaternization of the vinyl- pyridine group are larger molecules where straight- forward computations by more reliable ab initio methods are extremely expensive. In the present investigation, attention is given to a carboxylic derivative of 4-vinylpyridine obtained after quaternization, by using the locally modified version of the MNDO method. These calculations included total optimization of the molecular structures of the closed-shell molecules and of the free radicals produced in the polymerization process. The program was computed on a 7800 Burroughs computer using standard parameters 6. CLOSED-SHELL MOLECULAR CALCULATIONS In this section we address results of the calculations corresponding to 4-vinylpyridine and the carboxylic derivative obtained after the quaternization reaction: H H \ I C=C H +lO~C~ 2 N CHz-- CH2 4 - vinylpyridine + /~- loctone LX~ H H \ I C= C\ N, I CH 2 I carboxybetaine /c% O_ 0 The relevant geometrical parameters and net charges for 4-vinylpyridine and the quaternized molecule are shown in Figures 1-3. Calculations in Figure 3 considered a planar position of the vinyl group with respect to the aromatic ring. This equilibrium structure corresponds to the minimum energy or maximum stability of the compound molecule 5. These results show that the negative charge expected on the carboxylic group is split over the two oxygen atoms, but the positive charge on the nitrogen atom is not localized, due to the aromaticity of the ring in the quaternized derivative. These results also show that the distribution of the nitrogen charge over the ring causes the reactivity of this atom to be lower than expected. As a consequence, enhanced reactivity is induced on the carbon and hydrogen atoms of the ring. In addition, the first carbon atom of the vinyl group shows a small negative value in both cases but it is slightly larger in the quaternized molecule. Another interesting aspect related to the chemical properties of the quaternized molecule lies in the possible sites for nucleophilic and electrophilic reactions. The two 0032-3861/89/0917474)4503.00 © 1989 Butterworth & Co. (Publishers) Ltd. POLYMER, 1989, Vol 30, September 1747