SULFONATION OF STYRENE - ALLYL GLYCIDYL ETHER COPOLYMERS

Sulfonated polymeric materials are widely applied in the development of high-performance proton-conducting membranes. In terms of sulphating agents, concentrated sulphuric- and chlorosulfonic acids, a mixture of methanesulfonic- and concentrated sulphuric acid, and acetyl sulphate are most commonly used. A high degree of sulfonation of membrane materials provides efficient proton transport and excellent current-voltage characteristics of fuel cells. In order to develop a new proton-conducting membrane, the sulfonation of copolymers of styrene and allyl glycidyl ether is carried out, the composition and structure were confirmed by elemental analysis, IR and NMR spectroscopy. Obtained copolymers represent powdery substances, having a cream to dark brown colour, and are characterised by good solubility in benzene and acetone. The degree of sulfonation varies from 12 to 98 mol. %. Additionally, a quantum chemical study of the sulfonation mechanism of styrene and allyl glycidyl ether copolymers is studied using Gaussian 09 software; MP2//B3LYP level of theory and 6-311++G(d,p) basis set and composite CBS-QB3 method. Studying the process of copolymers sulfonation and comparing the obtained results with the data of quantum chemical calculations is essential for the development of additional methods for obtaining effective proton-conducting membranes.

In terms of sulphating agents, the most commonly used are concentrated sulphuric and chlorosulfonic acids, a mixture of methanesulfonic and concentrated sulphuric acids, and acetyl sulphate. The degree of sample sulfonation ranges from 30 to 100%. At the same time, the chemical destruction and «crosslinking» are not observed, and the resulting membranes demonstrate sufficiently high proton conductivity [2].
The development of polystyrene-based proton-conducting membranes is promising due to its commercial availability and low cost [21]. The first industrial proton-conducting membranes [22], having a level of conductivity comparable to that of commercial perfluorinated membranes, were obtained from sulfonated polystyrene [22]. It should be noted the main disadvantage of polystyrene membranes is their short lifetime due to their low thermal-oxidative stability and mechanical strength.
Chemical modification of polystyrene materials by copolymerisation of styrene with functionalised vinyl monomers (for example, allyl glycidyl ether), as well as the formation of hybrid composite materials based on it, which include blocks of inorganic components, provides an opportunity to reduce these disadvantages and obtain proton conducting materials comparable to commercially-available perfluorinated membranes (Nafion, MF-4SK) [23].
It has been shown that the proton conductivity of polymer membranes is determined not only by the presence of functional groups that provide proton transfer, but also by the structure of the membrane itself [24]. In particular, macromolecules forming membrane structure should be capable of forming clusters to effectively adsorb water.
Noteworthy, hybrid composites microstructure is formed, among other things, at the stage of copolymer sulfonation. Therefore, in order to develop additional methods for obtaining proton-conducting membranes on this basis, an understanding of the mechanism of styrene and allyl glycidyl ether copolymer sulfonation is of great importance.
The aim of this work is to study the products of sulfonation of styrene and allyl glycidyl ether copolymer and to compare the obtained results with the data of quantum-chemical calculations.

DISCUSSION OF THE RESULTS
Copolymers of styrene (St) and allyl glycidyl ether (AGE) are obtained using the suspension radical copolymerisation method according to [25]. Sulfonation of copolymers is carried out with concentrated sulphuric acid (ρ=1,825 g/cm 3 ) in a solution of benzene or toluene at a temperature varying from 60 to 90 °C for 2 hours. AGE-St sulfonated copolymers represent powdery substances coloured from cream to dark brown with good solubility in benzene and acetone. According to the elemental analysis, the degree of sulfonation ranges from 12 to 98 mol. % (see Table below).
Absorption IR bands of sulfonated copolymers are observed in the regions of 1260-1150 cm , which can be assigned to asymmetric and symmetric stretching vibrations of sulfogroups. The presence of oxirane cycle is confirmed by the presence of absorption bands in the following regions: 810 and 950 cm -1asymmetric and 1250 cm -1symmetric stretching vibrations of oxirane ring; 3040 cm -1vibrations of the methylene group in oxirane ring.
The reactive oxirane cycle of AGE and aromatic fragments of St represent the main active centres for sulfonation. The 13 C NMR spectroscopy data confirm the reaction course toward to both aromatic ring and epoxy group, indicating the formation of corresponding products.
Quantum chemical calculations are performed using the Gaussian 09 software [26]. Geometry optimization of structures is studied using B3LYP DFT functional [27] with 6-311++G(d,p) basis set [28] with zero-point vibrational energy correction. The refinement of the total energy is performed at Møller-Plesset MP2 level of theory [29] with the same basis set. At all stages of the calculations, toluene is used as a solvent and non-specific solvation is accounted for in the framework of IEF-PCM approximation [30]. For transition states, a descent along the internal reaction coordinate (IRC) is also performed [31] to prove the relationship between the transition state and initial reagents, as well as the reaction products. The relative energies presented in the text are given for the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) method taking into account zero-point vibrational energy (ZPVE) calculated at B3LYP/6-311++G(d,p).
In order to study the sulfonation mechanism of the obtained copolymers, model molecules ethylbenzene (EB) and 2-(propoxymethyl)oxirane (PO) are used as structural elements of the AGE-St copolymer. The choice of these structures is justified by the fact that in the case of an atactic polymer, consideration of the three-units-block is not enough due to possible conformational changes, while involving a larger number of units in the calculation significantly increases the calculation cost. It should be noted that for model structures, in contrast to the copolymer, steric effect reveals much lesser extent. This must be considered when switching to the real object of study, such as AGE-St copolymer.
As in the case of other aromatic hydrocarbons, sulfonation of polystyrene proceeds through the initial attack by a S-electrophile (A E ). However, the precise role of the S-electrophile in this process is still debatable. The nature of «true» electrophile in the reaction between aromatic hydrocarbons and sulfuric acid H 2 SO 4 depends on the concentration of the latter [18]. Thus, in diluted solutions, H 3 SO 4 + (or H 2 SO 4 and H 3 O + associate) acts as S-electrophile, while at concentrations above 80-85% it is H 2 S 2 O 7 (or H 2 SO 4 and SO 3 associate) that performs this function [32]. However, the true electrophile in the A E reaction of aromatic hydrocarbons is presented by SO 3 in free form or its associate.
In order to clarify the nature of the true electrophile and to study the sulfonation mechanism of obtained copolymers, model experiments are carried out. Energy profiles for generation of various S-electrophiles (H 3 SO 4 + , HSO 3 + , SO 3 и H 2 S 2 O 7 ) presented in Figure 1, energy values are obtained using precision composite CBS-QB3 method developed by Peterson et al. [33] taking into account the polarity of the solvent (toluene) in the framework of the IEF-PCM model.
As can be seen from Figure 1, due to strong hydrogen bonds, the association of two H 2 SO 4 molecules with the formation of dimer (H 2 SO 4 ) 2 leads to energy decrease of 15,8 kcal/mol. Further protonation of one of the molecules in (H 2 SO 4 ) 2 by the other with the formation of H 3 SO 4 + HSO 4 ion pair requires high energy input (93,7 kcal/mol).  fig. 2) turns out to be the true S-electrophile. This S-electrophile was used for further calculations.
In case of reaction between H 2 S 2 O 7 and ethylbenzene both para-and ortho-position attack of the phenyl ring is possible (see energy profile in Fig. 3). It is due to +I and +M-effects of ethyl substituent at phenyl ring. In both cases, the reaction proceeds through transition states shown in Fig. 4. These transition states (TS4 and TS2, respectively) lie 24,6 and 23,6 kcal/mol above the original non-reacting system (i1336 and i1351 cm -1 , respectively, B3LYP/6-311++G(d,p)). The descent along internal coordinate of the reaction shows these TSs bind pre-reaction complexes (pre-TS4 and pre-TS2) with the corresponding sulfonation products at para-(Et p C 6 H 4 SO 3 HH 2 SO 4 ) and ortho-positions (Et C 6 H 4 SO 3 HH 2 SO 4 ) of phenyl ring in ethylbenzene. As can be seen from Figure 3, the energy profiles of their formation are almost the same; ethyl substituent favours the substitution into orthoposition of the phenyl ring with very minor extent. Thus, sulfonation proceeds with equal probability at both ortho-and para-positions of phenyl ring in ethylbenzene.