A new and highly active vanadium–magnesium catalyst (VMC) modification is used to obtain data on the effect of the content of hydrogen and hexene-1 on catalyst activity, along with the structure of synthesized polymers. It is established that the formation of polyethylene with a broad bimodal molecular mass distribution (MMD) on VMC is due to that these catalysts have two groups of active sites with different reactivity in the reaction of polymer chain transfer with hydrogen. It is also found that the presence of hexene-1 during copolymerization additionally broadens the molecular mass distribution of the copolymer, due to its molecular mass falling predominantly in the low-molecular-weight region. As a result, active VMC sites that produce the high-molecular-weight polymer virtually do not participate in the chain transfer reaction with hexene-1. At the same time, these sites are more reactive in the reaction of hexene-1 insertion, raising the content of butyl branches in the high-molecular-weight fraction of copolymers. The revealed kinetic features of highly active VMCs show they are promising candidates for the synthesis of tube- and film-grade polyethylene using a single-reactor scheme, instead of the two reactors now employed in the production of bimodal polyethylene on traditional titanium–magnesium catalysts.
Mikenas et al. (Mon,) studied this question.