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

The terminal microstructure of the hydrogenated α,ω-diisopropenyloligo(propylene) prepared by thermal degradation of isotactic poly(propylene) at 370°C was determined by means of ¹³C 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)₂CH (i-Pr) formed by hydrogenation, the mole fractions of triads (m meso, r racemo) F_mm = 0.64, F_mr = 0.09, 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(δ_tζ_t) + F(δ_eζ_e) = 0.74 (m); F(δ_tζ_e) + F(δ_eζ_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_rr + 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 β-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.