Akrilonitrilin kimyasal ve elektrokimyasal yöntemle polimerizasyonu

Abstract

Seryum (IV) sülfat etilendiamin tetra asetik asit (EDTA) ve nitrilotriasetik asit (NTA) serbest radikal polimerizasyonlannda başlatıcı olarak kullanılabilmektedirler. Vinil kökü ihtiva eden monomerler yapılarındaki çift bağlar nedeniyle kolay polimerize olabilmektedirler. Akrilonitril de yapısmdaki vinil kökü ve -CsN nedeniyle kolay polimerize olabilen bir kimyasaldır. Bu çalışmada akrilonitril monomerinin kimyasal ve elektrokimyasal yöntemlerle polimerizasyonu çeşitli koşullarda incelenmiştir. Kimyasal polimerizasyon sırasmda NTA, Seryum (IV), akrilonitril, EDTA, sülfirik asit konsantrasyonu ile sıcaklık, zaman parametrelerinin verime olan etkileri tespit edilmeye çalışılmıştır. Elektrokimyasal polimerizasyonda Seryum (TV), sülfirik asit, akrilonitril monomerinin konsantrasyonu, uygulanan gerilim ve akınım etkisi karşılaştırmalı olarak belirlenmiştir. Sonuç olarak akrilonitril monomeri sülfirik asitli çözeltide ve nitrik asidi çözeltide çok düşük Seryum (IV) konsantrasyonu altında platin elektrotlar ile elektrokimyasal olarak yüksek verimle polimerize edilmiştir. Kimyasal polimerizasyon Seryum (TV) konsantrasyonunun elektrokimyasal koşullarda kullanıldığı değerler kullanıldığı hallerde gerçekleşmemiştir. Elektrokimyasal polimerizasyon çalışmalarında EDTA, NTA biç kullanılmadan ve çok düşük Seryum (TV) sülfat tuzu kullanılarak uygun akım verildiğinde ürün verimi yüksek olan sonuçlar elde edilmiştir.

Polymers of acrylonitrile (vinyl cyanide), CH2 = CHCN, are extremely important in synthetic fibers, elastomers and plastics. High polymers are most often prepared by additive polymerization and consist simply of the union of several molecules with a redistribution of the valency bonds, but with no spliting off of water or hydrogen chloride. This feature is that which distinguishes "condensation " from " addition" polymerisation; in the former a simple compund is split off, in the latter nothing is split off. As its name implies, addition polymerisation consists of the addition of one molecule to another. Many unsaturated compounds, particularly ethylene and its derivatives, will polymerise under suitable conditions. The simplest case, in theory, is the polimerisation of ethylene to give polyethilene. The reaction - known as addition polimerisation - is purely and simply one of addition; it is brougth about by the application of high pressures and temperatures, usually in the presence of a catalyst. Mechanism of Addition Polymerisation There are three stages in the polymerisation of vinil compounds; initiation, propagation (growth) and termination. Initiation Some vinyl compounds, notably vinyl acetate and styrene, will polymerise slowly at room temperature, but most of them require the application of heat or light or a catalyst. In practice vinyl polimerisations of acrylic fibers are usually initiated by heat and the presence of catalyst together. Two kinds of catalyst are used; free radical and ionic. vu Free Radical Initiation : A free radical is a broken molecule usually very unstable and ready to combine to amore complete state. Some unstable compounds such as peroxides and diazonium compounds and persulphates are known to produce free radical intermediates when they decompose and these intermediates are powerful catalysts for vinyl polimerisation. Benzoyl peroxide is often used; at about 80°C it decomposes in solution into free phenyl and bezoate radicals : A A CeHs -C-O-O-C- QHs > 2 CöHs - C - O. » Ü 0 0 0 -» 2C6H5+C02 ( The dot at the side is used to indicate the free radical nature of the compound, sometimes it is placed on top. Its significance is that the elctron structure of the dotted atom is incomplete.) When the free radical comes in contact with a vinyl compound, it distributes the electronic structure of the carbon atoms which are at either end of the doble bond and we represent this by saying that the double bond opens. If R- is the free radical : R+CH2 = CHX > RCH2CHX The two have combined together to form another radical, which itself is capable of further growth; the reaction has been initiated. It is noteworthy that the catalyst (more properly called initiator) enters into the reaction and its parts, for example benzoate radicals, can often be detected in the final polymer. Ionic Initiation : The catalysts used for kind of initiation are Friedel - Craft type : aluminium chloride, boron trifluoride, titanium tetrachloride and stannic chloride. Somtimes they will initiate a polimerisation that a peroxide will not start, for example the polimerisation of isobutene and vice versa. The ionic catalysts are all strong electron acceptors. First the catalyst will dissociate into negative and positive and positive ions in an organic envirement : for example, boron trifluoride hydrate dissociates thus : BFyOHa c 3 HT+ tBF30H2]- The hydrogen ion attached itself to the vinyl compound at the double bond and leaves the carbon atom at the other side of the bond in a highly active state and ready to combine vin fT+CH2 = CHX > H3C-CH2+ Propagation In a condensation polymerisation the small polymers that are formed early in the reaction, such as the dimers and trimers, react with each other, so that after a very short time there is none of the original monomer left, even altough the average degree of polymerisation may still be very low. In vinyl polymerisation matters are very different and a partially polymerised mixture will consist of high molecular weight polymer and of unchanged monomer with virtually no constituents at intermediate stages of growth; some individual polymer molecules grow fo maturity whilst most of the monomers still remain intact. A given molecule of very high D.P. is formed by consecutive steps of a single chain process initiated by a free radical or an ion. This is very important; if it were otherwise it would be almost impossible to prepare the isotactic and other stereoreguiar polymers. Termination The monotonous "one and one and one" process goes a long way but is not endless. There comes a time when the free radical at the end of one chain meets that at the end of another, the two combine and that long molecule is finished: it cannot grow anymore. Alternatively, instead of joining together, the two active chain ends may disproportionate, one giving up ahydrogen atom to the other, so that both molecules are finished and can grow no more. Either way, whether by combination or by disproportionation, the growth of those particular molecules is stopped. Electrochemical Initiation The polimerisation of acrylonitrile on the anode failed to give satisfactory results. An insignificant amount of the polymeric product was obtained by electrolysis of acetate or propionate solutions in appropriate anhydrous acids in the presence of acrylonitrile. Better results are obtained when the polymerisation of acrylonitrile is initiated on the cathode. This is carried out with the aid of additives such as persulfate or hydrogen peroxide in the presence of a ferric salt. It is thougt that the polymerization of acrylonitrile may be initiated by free radicals formed by the following reactions : IX S2082- +e -> S20g" Fe3+ + e -> Fe2+ Fe2+H202 -? Fe3+ +0H + OH- The mixture of ferric ions with hydrogen peroxide is known as Fenton' s reagent and is widely used as a source of free hydroxyl radicals. The reduction was carried out on mercury, platinum and rotating graphite cathodes. Best results were obtained when the electrolysis of acrylonitrile solution in the presence of a ferric salt and hydrogene peroxide, at a ferric ion concentration 200 times higher than that of the peroxy compound. Conclusion Cerium (TV) sulphate, Cerium (TV) nitrate, Cerium (TV) perchlorate have been used for additive polimerization. These eerie salts also form very effective redox systems in the presence of organic reducing agent such as alcohols, aldehyds, carboxylic acids. Ceric salts have been used either alone or İn combination with reducing agents as initiators of vinyl polymerisation. In this work the polimerization of acrylonitrile initiated by the Cerium (TV) sulphate, EDTA, NTA is done. The effect of Cerium (IV) is given for the systems of electrogenerated Cerium (TV) in a seperated cell and chemical polimerization of monomer by using Cerium (IV) - EDTA. Polymerization were carried out in a cell divided in to two compartments by means of a sintered disc. Platin Gause electrodes of 4x10 cm and 5x4 cm area were used as cathode and anode respectively. The cell assembly was thermostatted and reaction miture containing required amounts of monomer and Ce(rV) in the aqueous solution of H2SO4 was stirred continuosly at cathode compartment white anolyte contains only H2SO4 solution. Electrolysis was carried out using a constant power DC source at a potential of 4 V (1=0.42 mA) Polymerization starts in water within a minute of electrolysis and whole precipation occurs after about 30 minutes electrolysis. But in the case of chemical polimerization by Ce(TV)-EDTA system it was noticed that the polimerization reaction starts within 15 minutes induction period. This might be due to the complexation of Ce(TV) with EDTA which can act as a slower source of free radicals. The precipitated polymer was filtered off and dried to constant weight at 60 °C. The rates of polymerization were obtained grawimetrically and coversation percentages were calculated using the following relationship : Wt of polymer Conversation % = - - xlOO Wt of monomer The polymerization of AN by CS salt was carried out under simiral conditions of with and without electrolysis. In these experiments, for comparison purposes, chemical polymerization by Ce(rV) and chemical polymerization by only EDTA gave no conversion, but chemical polymerization by Ce(TV)-EDTA system have given a 1.7% conversion with an induction period of « 30 mins. while electrochemical polymerization gave a 45% conversion with an induction period of «30 sees. Under the same conditions without Ce(IV), electrochemical polymerization gave no conversion. Fig. 1 shows a typical set of conversion curves at 50°C for various Ce(TV) concentrations (oxlO^xlO"2 M) for electrochemical polymerization in the presence of Ce(TV) (I) and chemical polymerization by Ce(IV)-EDTA system (IT). Fig. 1. The effect of Ce(TV) consentration on electrochemical and chemical polymerization yield At lower Ce(IV) concentrations, polymerization in the presence of electrolysis gave much higher conversion values, i.e. for 3x 10 M Ce(TV) about 45% and 1.7% conversion is obtained with and without electrolysis respectively. In the case of electroinitiated polymerization of AN, the polymer yield increases linearly and reaches a plateau and becomes constant. The yield of the chemical polymerization by Ce(TV)-EDTA system shows a very low conversion (7%) even at the highest Ce Ce(m).