A 13C NMR Study of End Groups and Stereoregularity Changes Induced by γ Irradiation of Molten Polypropylenes

Abstract

Samples of predominantly isotactic and syndiotactic polypropylenes have been irradiated with gamma rays to high doses in the molten state. Solution state 13C NMR analysis has identified propyl, vinylidene, and isopropyl end groups. The yield of the former shows that the extent of chain scission in isotactic polypropylene is strongly dose dependent with an initial G value of 11.6 reducing to 5.5 at doses greater than 500 kGy. Molecular weight measurements suggest that G(crosslink) is approximately independent of dose with a values of 3.5. The triad sequence contents of the polymers after 2 MGy irradation in the molten state, show that an equilibrium triad distribution has not been achieved, even though the rates of change of triad sequences are virtually zero by this dose. Initial rate kinetics suggests that this is mainly due to the preferential formation of syndiotactic sequences from isotactic ones.