On the stereoisomerization in thermal degradation of isotactic poly(propylene)

The terminal microstructure of the hydrogenated alpha,omega-diisopropenyloligo(propylene) prepared by thermal degradation of isotactic poly(propylene) at 370 degrees C was determined by means of C-13 NMR spectroscopy, and therefrom the stereoisomerization reaction of the reacting polymer segment is discussed in relation to polymer dynamics. With respect to the asterisked carbons of the propyl end group CH3CH2*CH2- (n-Pr) and of the isopropyl end group (*CH3)(2)CH- (i-Pr) formed by hydrogenation, the mole fractions of triads (m meso, r raceme) F-mm = 0.64, F-mr = 0.09, F-rm = 0.15 and F-rr = 0.12 of n-Pr-*CH2 differ from the microtacticity of the main chain of telechelic oligomers, and the mole fraction of i-Pr-*CH3 is as follows: F(delta(t) zeta(t) + F(delta(e) zeta(e)) = 0.74 (m); F(delta(t) zeta(e)) + F(delta(e) zeta(t) = 0.26(r). These results clearly show that the stereoisomerization occurs only at the asymmetric carbon near the end group of the telechelic oligomer. The value of the mole fraction of i-Pr*CH3 is consistent with F-m = 0.73 (F-mm + F-mr and F-r = 0.27 (F(r)r) + F-rm) of n-Pr-*CH2 at the outer position of the main chain rather than with F-m = 0.79 (F-mm + F-rm) and F-r = 0.21 (F-rr + F-mr) at the inner position. These results could be consistently traced by a simulation based on a reaction model including stepwise intramolecular hydrogen abstraction of the tertiary on-chain macroradical near the center of the main chain before the successive beta-scission. This stepwise back-biting is estimated to occur once (20%) and twice (80%) via a quasi six-membered cyclic structure in a transition state, and occurs in a very limited part of the molecular chain.