The structure of cationic polypropylene obtained both from natural-abundance 13C monomer and from monomers selectively enriched with 13C in positions 1, 2, and 3 was studied through 13C NMR spectroscopy. The presence of methyl, ethyl, n-propyl, n-butyl, isopropyl, gem-dimethyl, isobutyl, and terf-butyl groups was ascertained in samples obtained under different conditions. The content of the branched groups increased with the oligomerization temperature and the monomer conversion. The study showed that appreciable amounts of 1,3 enchainment of the monomer are ruled out and the irregular tail-to-head addition is of small importance. The formation of the linear groups is accounted for by 1,2-hydride and -methide shifts, while some branched groups can arise through protonic initiation followed by classical cationic rearrangements. Other unusual rearrangement mechanisms must operate if experimental results are to be explained. These possibilities are discussed in light of current views on carbenium ion chemistry.
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