Radical polymerization of butadiene mediated by molecular iodine: A comprehensive kinetic study on solution copolymerization with acrylonitrile

Solution homoand copolymerization of butadiene (Bu) and acrylonitrile (AN) in 1,4-dioxane (Dox) was carried out in the presence of molecular iodine (I2) and 4,4 '-azobis (4-cyanovaleric acid) (ACVA), both acting as an in situ generator of carboxyl-functionalized alkyl iodide as a chain transfer agent (CTA), to synthesize alpha-carboxyl co-iodine heterotelechelic polymer architectures. Effect of various variables including solvent concentration and type, ACVA to I-2 molar ratio and reaction temperature (60 and 70 degrees C) on the overall conversion versus time data was investigated. It was found that before complete consumption by initiator-derived radicals during the induction period, molecular I-2 inhibits the reaction. Polymerization period was then started in the presence of an in situ generated CTA. From conversion versus time data, rate constant of initiator decomposition (k(d)), induction time (t(ind)) and overall average rate constant of (k) over bar (2)(p)/(k) over bar (t) were estimated and compared with those reported for conventional free-radical homoand copolymerization of Bu and AN. A good agreement between the theoretical and experimental variations in the conversion versus time was also observed, indicating accuracy of the kinetic constants calculated in the present work. Comparison of the theoretical and experimental induction times revealed that molecular I-2 reacts with Bu, resulting in butadiene diiodide species (I - Bu - I) which can act along with carboxyl-functionalized alkyl iodide as CTAs. Formation of I - Bu - I was also confirmed by H-1 NMR analysis. Also, fraction of I-2 consumed by Bu (i.e. [I - Bu - I]/[I-2](0)) was estimated from kinetic data at hand. [I - Bu - I]/[I-2](0) value as high as 20.8% was observed in the case of Bu homopolymerization and decreased by increasing the AN content in the initial feed. In the case of AN homopolymerization, no consumption of I-2 by AN was observed.