THERMOLYTIC BEHAVIOR OF POLYDIENYLLITHIUM AND POLYSTYRYLLITHIUM

UV absorption spectra of thermolyzed polybutadienyl- and polyisoprenyl -lithium reveal a chromophore group previously not recognized for such systems; its absorption band at 271 nm has been assigned to a structu re with three conjugated double bonds. A two-step mechanism for the fo rmation of this trienic structure is proposed: an intermolecular metal lation of associated living ends is followed by lithium hydride (LIH) elimination. Along thermolysis the presence of a dienic structure was also recognized, the latter arising from intramolecular elimination of LiH. The trienyllithium structure is also considered to be an effecti ve species for the observed molecular weight distribution (MWD) variat ions. The observed different extent of high molecular weight (HMW) for polyisoprenyl- and polybutadienyl-lithium is explained on the basis o f a different stability of the intermediates present along the propose d reaction mechanism. The thermolytic behavior of polystyryllithium do es not provide any significant change in MWD: the disappearance of the living chain ends, UV detected, is due to an intramolecular LiH elimi nation which obeys first-order kinetics. The influence of temperature and of the tetrahydrofuran (THF) level on kinetic rate constants was i nvestigated. (C) 1996 John Wiley & Sons, Inc.