630 Rai and Sherigara Transition Met. Chem., 20, 630-633 (1995) Aqueous polymerization of acrylonitrile initiated by manganese(Ill) pyrophosphate-thiocyanate redox system: a kinetic study Sheshappa K. Rai S.D.M. College, Ujire 574 240, India Bailure S. Sherigara** Department of Postgraduate Studies and Research in Chemistry, Mangalore University, Mangalagangotri 574199, India Summary The kinetics of aqueous polymerization of acrylonitrile monomer (M) initiated by the Mnm-KNCS redox system have been studied under deaerated conditions in the temperature range 26-40~ at constant ionic strength. The overall rates of polymerization and the disappear- ance of Mn nl were determined. The polymerization was initiated by the free radicals arising from the Mn m- thiocyanate redox reaction. The rate of polymerization was investigated at various concentrations of monomer and initiator. The effects of varying [Mnm], [NCS-], pH, total [pzo4-], added [MnIl], metal ions, C104-, C1- and SO 2- were examined. Dependence of the rate of polymer- ization on temperature was studied and activation par- ameters were computed from an Arrhenius plot. A suitable kinetic scheme consistent with the observed results is proposed and discussed. Introduction Redox polymerization of vinyl monomers initiated by transition metal ions in their higher valency states in aqueous medium can provide valuable information re- garding the mechanistic details of the individual steps. A qualitative survey of manganese(Ill) reactions shows that some of them proceed via free radical mechanisms which may be capable of initiating radical-based polymer- ization. Thus, managanese(III), in the presence of various inorganic and organic reducing agents, has been used as an effective initiator for the polymerization of vinyl mono- mers ~-1~ The advantages of redox polymerization as compared to thermal initiation include the very short induction period and a relatively low energy of activation. This enables the polymerization to be carried out at a low temperature, thereby decreasing the possibility of side reactions and making control of the polymerization reac- tion easier. The kinetics of oxidation of thiocyanate by manga- nese(Ill) in pyrophosphate medium have recently been investigated by this group ~11). In the present paper we report a study on the polymerization of acrylonitrile initiated by the manganese(III)-thiocyanateredox system in aqueous medium. The anomalous kinetic features~a2) associated with acrylonitrile polymerization in aqueous solution and the importance of polyacrylonitrile in the fibre industry led us to the choice of acrylonitrile for this investigation. * Authorto whomall correspondence shouldbe directed. * Presentaddress:DepartmentofIndustrialChemistry, Kuvempu Uni- versity,Jnana Sahyadri, 577451,Shimoga District,Karnataka,India. Experimental Preparation of materials Acrylonitrile was washed with 5% NaOH followed by dilute orthophosphoric acid and finally with distilled water. It was dried over CaC12 and distilled under reduced pressure in a N 2 atmosphere. The middle fraction was collected and stored at 5 ~ A solution of Mn m pyrophosphate was prepared by the method of Belcher and West ~3). It was standardized by iodometry, the value was checked by titrating against standard ferrous ammonium sulphate solution using bar- ium diphenylamine sulphonate as internal indicator. KNCS solution was standardized argentometri- cally"4). All other materials were of analytical grade and were used without further purification. The water used for preparing all solutions was obtained by redistilling de- mineralized water from alkaline KMnO,. Kinetic measurements Reactions were carried out under N 2 in Pyrex glass vessels. In a typical experiment, a mixture of known amounts of monomer, KNCS, Mn z +, P20 4-, H3PO 4 (for maintaining pH), NaC104 (for ionic strength adjustment) and water (to keep the total volume constant) was ther- mally equilibrated at the desired temperature. A wash- bottle containing an aqueous solution of acrylonitrile of the same concentration as in the reaction vessel was interposed between the N 2 train and the reaction vessel to avoid loss of monomer during deaeration. O2-free N 2 was bubbled through the solution for 30min and the Mn ul pyrophosphate oxidant was then added. The reaction vessel was sealed with a rubber gasket. The reaction was arrested by adding a known amount of standard ferrous ammonium sulphate or by cooling to 0 ~ and blowing in air. The rate of polymerization (Rp) was determined gravimetrically. The rate of Mn nl disappearance (-Rm) was followed iodometrically by determining the concen- tration of Mn m in the reaction mixture before and after polymerization. Molecular weight determinations The molecular weights, My, of the purified polyacrylonit- rile samples were determined by viscometry. A solution of the polymer (0.1%) in dimethyl formamide was filtered through a glass filter and placed in an Ubbelohde-type suspended-level dilution viscometer. The intrinsic viscos- ity, It/], for the solution was determined and the ~t v value was evaluated by using the Mark-Houwink equation 0340-4285 9 1995Chapman &Hall