Gt. Viola et al., THERMOLYTIC BEHAVIOR OF POLYDIENYLLITHIUM AND POLYSTYRYLLITHIUM, Journal of polymer science. Part A, Polymer chemistry, 34(1), 1996, pp. 13-24
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.