C-13 NMR STUDIES OF ETHYLENE-PROPYLENE COPOLYMERS PREPARED WITH HOMOGENEOUS METALLOCENE-BASED ZIEGLER-NATTA CATALYSTS

摘要

The recent discovery of homogeneous metallocene-based Ziegler-Natta catalysts has led to the synthesis of EP copolymers with new comonomer and stereochemical structures. In the present paper C-13 NMR investigation of EP copolymers at high ethylene content, prepared with highly isospecific, highly syndiospecific, and aspecific catalysts, is presented. The high stereoregularity of the iso- and syndiotactic samples and the complete regioregularity of all three samples greatly simplify the spectra and make it possible to check some of the chemical shift predictions and to perform other sequence assignments. The assignment of new sequences is based on (i) correlation of chemical shifts of homo- and copolymers performed with the same catalysts, (ii) comparison of spectral intensities for samples of different compositions, and (iii) comparison with chemical shift values predicted by calculations. The detailed information obtained on comonomer and tactic sequences opens the possibility of describing the chain growth mechanism for the various catalytic systems and of correlating chain structure and copolymer physical and mechanical properties. [References: 14]