Metilmetakrilatın Farklı Çözücü Ortamlarında Seryum (ıv) İle Kimyasal Ve Elektrokimyasal Polimerizasyonu

Abstract

Bu çalışmada Seryum(IV) Sülfat ve Seryum(IV) Amonyum Nitrat ile Elektrokimyasal ve kimyasal yolla başlatılmış metil metakrilat polimerizasyonu sulu sülfürik asit ortamında gerçekleştirilmiştir. Seryum,monomer ve asit konsantrasyon etkisi,sıcaklık,02,ışık etkilerinin polimerizasyon verimi üzerindeki farklılıkları incelenmiştir. Sentez edilen polimerlerin karakterizasyonu spektrofotometrik ve gravimetrik yolla yapılmıştır.(FT-IR,UV,GPC ve viskozimetrik) Ayrıca polimerizasyon asetonitril, DMF ve farklı DMF-su karışımlarında da yapılmış, polimerizasyon sırasındaki değişimler spektroskopik yolla (UV, Voltamogram) incelenmiştir. Kimyasal ve elektrokimyasal yolla elde edilen polimerlerin molekül ağırlığı farkına bakılmıştır.Bu sonuçlara göre kimyasal yolla elde edilen polimerin molekül ağırlığı yüksekken,elektrokimyasal yolla elde edilen ise daha düşük molekül ağırlıklı ve çok farklı zincir uzunluklarına sahip olduğu saptanmıştır.Bunun nedeni Ce(III)'ün Ce(IV)'e rejenerasyonu ile ortamda artan Ce(IV) konsantrasyonudur. Çünkü bu koşullarda sonlanma daha çabuk gerçekleşir. Polimerizasyon verimi üzerine karıştırma ve oksijen etkisine de bakılmış ve bunların verimi düşürdüğü gözlenmiştir. Literatüre göre daha düşük monomer konsantrasyonu ile daha yüksek polimerizasyon verimi elde edilmiştir.

Redox polymerizations are initiated by the transient free-radical species produced by the reaction taking place between an oxidizing agent called catalyst or initiator and a reducing agent called activator. Redox polymerizations have very short induction periods,they can be carried out at low temperatures since their activation energies are much lower than thermally initiated polymerizations and they provide production of high molecular weight polymers with high yield in a short time. In addition,polymers with functional end-groups can be obtained by redox polymerizations. The initiation step in the redox polymerization may be represented as follows: Oxidant + Reductant -> R» + Products R* + M -> R-M» Here,k is the rate constant of the redox reaction. In redox polymerizations,a proper choice of the oxidizing and reducing agents should be provided for an efficient initiation. The metal ions in their valance states such as Co(III), Cr(VI), Mn(III), Fe(II) and V(V) are commonly used oxidizing agents. Ce(IV) salts or Ce(IV)-reducing agent systems in aqueous acidic solution are also used in initiation of vinyl polymerization Aqueous solutions of eerie perchlorate, nitrate and sulfate in their respective acids initiate vinyl polymerization in the dark and at room temperature and also photochemically. The initial rate of polymerization decreases in the order of eerie perchlorate > eerie nitrate > eerie sulfate. The oxidation of the organic compound by eerie salts are found to follow different mechanisms depending on the type of acid media. In nitric acid, the higher the acid concentration the higher the rate. The average-molecular weights decrease as the acid concentration increases. The well known reaction between Ce(IV) salt and an organic reducing agent such as alcohol,glycol, aldehyde,ketone and carboxylic acid is a redox reaction. The most important feature of oxidation with the eerie ion is that it proceeds via a single-electron transfer with the formation of free radicals on the reducing agent. The mechanism of the redox initions may be represented as follows: Ce(IV) + R -> (Ce(IV)-R) (Ce(IV)-R) -> R- + Ce(III) + H+ complex In this work, aqueous polymerization of MMA initiated by electro lytically generated Ce(IV) redox system was studied. A few papers have been published on the electrochemical initiation of methyl methacrylate polymerization; the first workers in the field were probably Dinnen,Schwan and Wilson [11]. Initiation of methyl methacrylate polymerization could be successfully performed on the cathode only. The process was studied in sulfuric acid as the electrolyte, as well as on various cathodes (Hg, Pb, Sn, Pt, Bi, Fe, Al) and polymeric products were obtained. Parravano [12] studied the polymerization of methyl methacrylate during electrolysis on different cathodes and established the relationship between the rate of formation of poly(methyl methacrylate) and the cathode material; he found that the operative factor is the value of hydrogen overvoltage on the cathode. According to Parravano,the polymerization is initiated by free hydrogen radicals. Same results were confirmed by Kern and Quast [59]. In agreement with Wilson, it was assumed by Tsvetkov [17],who studied the polymerization of methyl methacrylate on the cathode,that the polymerization is produced by atomic hydrogen and takes place by a radical chain mechanism.lt was found that an increase in the current density and the temperature of the reaction medium results in an accelerated polymerization. The degree of polymerization of the product increases with decreasing cathodic current density, while temperature variations between 0 and 40°C have no significant effect. Stirring the reaction mixture during the electrolysis retards the process [21, s 499-501] and the polymerization is much less advanced, intensive stirring (700 rpm) suppresses the formation of the polymer, probably owing to the disturbance in the conditions favorable to the initiation and propagation of the polymer chain which occurs at the electrode surface. After the current has been disconnected,the polymerization spontaneously continues for a long time,until all of the monomer is practically spent. The yield of the polymer during the post-polymerization will depend on the duration of the preliminary electrolysis. The initiation mechanism of the polymeryzation of methylmethacrylate was subsequently studied in more detail. It was shown by Fedorova, Li Gun and Shelepin [18] that the polymerization of methylmethacrylate in an acid solution on a lead cathode is not initiated by atomic hydrogen, but it produced by the cathodic reduction of the peroxy compounds formed in the methylmethacrylate in contact with air. A similar effect is noted when hydrogen peroxide is added to an aqueous solution of methylmethacrylate. Methyl methacrylate, purified from traces of peroxide compounds by double distillation in a stream of nitrogen under reduced pressure, is not polymerized in aqueous sulphuric acid even on prolonged electrolysis. In certain cases the polymerization of methyl methacrylate may be initiated by electrochemical reduction of oxygen to hydrogen peroxide [45], which takes place on a mercury cathode in an acid medium,as follows: 02 + e" + H+ -> H02« H02. + e" + H+ -» H202 It was experimentally shown that oxygen has a two-fold effect on the polymerization of methyl methacrylate. On one hand, the process is inhibited by oxygen, which, on the other hand,takes part in its initiation, probably via H02* radicals. Studies of the capacitance of the electric double layer on a dropping mercury cathode in the presence of methyl methacrylate showed [18] that methyl methacrylate polymerization is initiated by the radicals formed as a result of the reversible electrochemical process: [CH2 = C - C = O H+]ads + e" ^=^ [CH2 - O - C = O ] II II CH3 OCH3 CH3 OCH3 The adsorbed hydrogen atoms do not participate in the initiation of the polymerization process. This is confirmed by the fact that polymer formation ceases altogether if a neutral salt such as Li2S04,which produces only an insignificant change in the pH, is added to the solution. It is easily seen that the less proton-donating the solution the higher the polymer yield and its molecular weight. In this study Ce(IV) salt was used for the polymerization of methyl methacrylate (MMA) under electrolytic conditions comparatively with chemical method in water. Electrc process allowed Ce(III) to be converted Ce(IV) during polymerization and had an advantage over the nonelectrolytic method for which polymerization did not occur under these conditions. The effect of Ce(IV) monomer,and sulfuric acid concentration, temperature, time, 02 and light on the polymerization yield was studied and compared with nonelectrolytic conditions. The role of Ce(IV) salt on the polymerization was followed potantiostatically during reaction period. The metal ion concentration was varied from 1.10-4 M to 1.1 0"2 M both in the chemical and electrochemical polymerization. The yield increases considerably up to the concentration of 5.1 0"^ M Ce(IV) of concentration and beyond this point the yield decreases in both chemical and electrochemical methods. Approximately 20% increase was observed in the case of electrochemical polymerization method,comparing with the chemical method (fig. 1). This is due to the regeneration of Ce(IV) from Ce(III) which is reduced by electrolytic conditions. Electrolytic and chemical polymerization gave almost the same yield at the same concentration of monomer which electrochemical method shows an increase in the yield about 20% compared to chemical method. Polymerization was carried out with Ce(IV) salt in H2SO4 solution at different temperatures. The molecular weights of some products and the results of GPC measurements were collected in Table 1. Table 1.Molecular weight of PMMA obtained by GPC and viscozimetric methods. Almost all polymerization reactions were performed in the presence of O2. In order to understand the effect of O2 and light under our experimental conditions the next experiments were carried out at N2 atmosphere and in the dark. The increase in conversion in N2 atmosphere shows the inhibition effect of O2. Electrochemical investigation of the polymerization reaction of MMA in ACN, in DMF and in a mixture of DMF and water was performed by cyclic voltammetry and spectrofotometric measurements. İn the mixture of DMF and water, the yield was almost 160%, because in this medium DMF behaved like a initiator. The results of FT-IR and viskozimetric measurements were performed. In this study, we studied the kinetics of Ce(IV) in different solvents (asetonitrile and the mixture of dimethylformamid and water). The rate constants were calculated by spectrofotometric measurements. Also this study was compared with literature (table 3.8). Results of all these detailed studies support our conclusion that the presence of cerium salt can initiate the MMA polymerization according to the following reaction: Ce(IV) +R -> R- + Ce(III) R* + MMA -> RMMA* -»polymerization R:may be H2O or monomer.