Ethene/4-methyl-1-pentene copolymers by metallocene based catalysts: an insight in 13C NMR assignment

This paper presents a 13C NMR investigation of a series of ethylene/4-methyl-1-pentene copolymers, prepared with the metallocene precatalyst, rac-CH2(3-tBuInd)2ZrCl2 and having a 4-methyl-l-pentene content spanning from 1 to 35% by moles. The high stereoregularity and the complete regioregularity obtained with this catalyst greatly simplify the spectra, as the spectral multiplicity can be ascribed to differences in comonomer sequences only. Previously assigned chemical shifts have been checked and novel sequence assignments have been obtained. The sequence assignment is based on: (i) comparison of the copolymer chemical shifts with those of the spectrum of the 4-methyl-1-pentene homopolymer obtained with the same catalyst, (ii) DEPT experiment on one significant sample, (iii) comparison of spectral intensities of samples of different compositions, (iv) correlation between integrated peak areas of signals of sequences linked by stoichiometric relationships, (v) comparison with selected signals of the spectrum of a sample of ethylene/4-methyl-1 -pentene copolymer obtained with a different catalyst. Correct assignment of S ??, S??, and S ?? methylene carbons allows for a precise measurement of the entire set of methylene peak areas present in the copolymers studied in this paper, and consequently, a correct determination of dyad and triad copolymer composition and of comonomer content.