Hydrogen (H)-complex formation of maleic anhydride–acrylic acid (methyl acrylate) monomer system

Abstract

The results of the study of complex formation through H-bond between the –CO group of maleic anhydride (MA, electron-donor monomer) and the carboxyl group of acrylic acid (AA, electron-acceptor monomer) and the charge transfer complex between the double bond of MA and vinyl acetate (VA, electron donor monomer) using ultraviolet-visible spectroscopy (Uv-vis) and nuclear magnetic spectroscopy (¹H NMR) methods are discussed. Maleic anhydride–methyl acrylate (MAc) monomer pair as a model system is also studied. The formation of two types of complexes in the MA–AA monomer system with further determination of their K_H and K_C values were studied using ultraviolet-visible spectroscopy (UV-vis) and ¹H NMR spectroscopy and the following well known equations of Benesi–Hildebrand, Ketelaar and Hanna–Aushbaugh, respectively. The formation of H-complex in the MA–AA system is confirmed by the analysis of electronic spectra of free monomers and their various mixtures (MA≫AA) in dioxane at 25 °C. The equilibrium constant of the MA…AA charge transfer complex (CTC) formation was determined by ¹H NMR in p-dioxane and acetone at 25 °C.

Introduction

Hydrogen bonding is one of the important non-covalent interactions in nature. H bonding is achieved through the complexation of two or more molecules containing proton donor and proton acceptor groups [1]. The bonding energies of normal H-bonding are 10 and 50 kJ/mol. These directional interactions can be expressed as –X....H–Y– (X,YN,O, ....) stable and dynamic molecular complexes can be prepared by simple molecular self-assembly processes using nucleic acids, proteins and polysaccharides having hydrogen bonding groups, which participate in the formation of supramolecular structures and the induction of function. However, for design of synthetic polymers, hydrogen bonding has not been considered to be very useful. If the varieties of hydrogen bonding moieties are strategically introduced into synthetic polymeric and organic materials, new polymeric materials exhibiting a variety of functions can be obtained [2]. Thermodynamic peculiarities of H-complex formation in the monomer–monomer and monomer-solvent systems were described by Kabanov et al. [3], [4]. H-bond is formed due to both electrostatic [5] and donor–acceptor interactions in H-complexes [4]. Despite that considerable contribution of an electrostatic interaction to the energy of the H-bond the role of charge transfers (donor–acceptor interaction), which is reasonable for the change of electron state and reactivity of individual components of the monomer H-complex system, is highly essential. Thus, change of the electron structure of 2- and 4-vinylpyridines, dimers of unsaturated carbonic acids and amides with formation of H-complexes was confirmed by IR and NMR-spectroscopy [4]. In these systems, anomaly high shifts and broad band of H-bond in IR spectra and its high chemical shifts of the proton in NMR spectra were observed. It is demonstrated that the formation of H-complexes of (metha)acrylic acids with some N-containing monomers (amides of unsaturated carbonic acids, N-vinylcarbazole, vinylpyrrolidone, etc.) is accompanied by an essential change of copolymerization constants and parameters of Q and e mainly due to the change of the polarization sign of monomer double bonds. It was proposed that the unsaturated carbonic acids and amides in the dimer forms with the resonance structures should be characterized by higher energy of stabilization of the double bond when compared with monomer forms of these systems [4]. This phenomenon was studied in detail by Kerber and Hlaman [6]. It was shown that dimers dissociated in a solvent, capable of H-bond formation, were substituted with H-complexes of acids with the solvent. Equilibrium constants (46l l mol⁻¹) for organic acid–solvent complexes were determined by cryoscopy and FTIR-spectroscopy methods [5]. It was also shown that the equilibrium constant of dimerization of acrylic acid in the inert solvents was 360 l mol⁻¹. As evidenced from this value, the equilibrium shifts to dimer formation where it takes place in the acrylic acid–non-polar solvent system.

The determination of self-association and inter-association equilibrium constants of H-bond formation has been discussed in detail by Coleman et al. [7], [9] According to the authors H-bonds are dynamic, continually breaking and reforming under the influence of thermal motion, and there exists at any instant of time a distribution of species consisting of ‘free’ (non-hydrogen bonded) monomers, hydrogen bonded dimers and hydrogen bonded multimers (‘chain-like’ trimers, tetramers, etc.). Naturally, this distribution is affected by changes in the temperature and concentration. The IR spectral bands at 3630, 3530 and 3350 cm⁻¹ have been assigned to non-hydrogen bonded OH groups, hydrogen bonded OH dimers and hydrogen bonded multimers, respectively. This spectral information forms the basis for the determination of equilibrium constants and enthalpy of hydrogen bond formation that describe the self-association of low molar mass molecules [7]. The effect of H-complex (–CO…HO–) in radical alternating copolymerization of MA and fumaric acid (FA) with acrylic acid (AA) was observed by El'Said et al. [10]. They showed that copolymerization of these monomer pairs proceeds through formation of MA…AA and FA…AA H-complexes (–CO…HO–) and it is possible to direct the process away from the formation of alternating copolymers to the formation of random copolymers with different composition by using naphthalene as an electron donor substance forming a donor–acceptor complex with double bond of acceptor MA. Some interesting works concerning studies of specific interactions in polymer blends using FTIR spectroscopy have been published. In these articles, it is possible to calculate the concentration of hydrogen bonds and to determine quantitatively the fraction of ‘free’ and bonded (associated) carbonyl and hydroxyl or amine groups in polymer mixtures. This phenomena explains why the number of intermolecular hydrogen bonds formed between donor and acceptor groups in the miscible polymer blend is much smaller than that in the analogous mixture of low molecular compounds [11], [12], [13], [14], [15] Recently, we have reported that the H-complex formed in the maleic anhydride–acrylic acid system plays an important role in the radical terpolymerization of donor–acceptor monomers such as maleic anhydride, acrylic acid, vinyl acetate [16]. The objective of the present work is to investigate the complex formation through the H-bond between the –CO group of maleic anhydride (MA, electron-donor monomer) and carboxyl group of the acrylic acid (AA, electron-acceptor monomer) and the charge transfer complex between the double bond of MA and vinyl acetate (VA, electron donor monomer) using UV-vis and ¹H NMR spectroscopic methods. MA-methyl acrylate (MAc) monomer pair as a model system is also studied. The formation of two types of complexes in the MA–AA monomer system with further determination of their K_H and K_C values was studied using ultraviolet-visible spectroscopy (UV-vis) and ¹H NMR spectroscopy and the well known equations of Benesi–Hildebrand, Ketelaar and Hanna–Aushbaugh [17], [18], [19], respectively.