Long-chain branched polyethylene via coordinative tandem insertion and chain-transfer polymerization using rac-{EBTHI}ZrCl2/MAO/Al-alkenyl combinations: An experimental and theoretical study

In situ synthesis of topologically modified linear polyethylenes (PE) using single-site polymerization catalysis is a challenging task, but it can enable the production of valuable advanced polymer materials with tailored properties. Described herein is an investigation aimed at the efficient generation of long-chain branches (LCB) in linear PEs using Al-alkenyl species, namely, iBuAl(oct-7-en-1-yl)2 (Al-1), in combination with homogeneous rac-{EBTHI}ZrCl2 (Zr-1)/MAO or heterogeneous MAO on silica-supported-rac-{EBTHI}ZrCl2 (supp-Zr-1)/TIBAL catalytic systems. As corroborated by extensive rheological studies and 13C NMR spectroscopy, the Al-alkenyl reagent was found to be quite efficient in the formation of LCB, via a mechanistic pathway involving both insertion and transmetallation reactions. Formation of LCB has been rationalized by density functional theory (DFT) computations carried out on the putative [rac-{EBTHI}Zr-R]+ (R = Me, nPr, and pentyl) cationic species and including a solvent model. Of the three possible isomers of Al/Zr heterobimetallic complexes derived from the cationic species [rac-{EBTHI}Zr-R]+ and Al-1, only one was identified, on kinetic and thermodynamic grounds, as the key intermediate. The DFT study also unveiled that (i) insertion of ethylene into the Zr-alkyl bond of the growing PE chain is accompanied by a reversible decoordination of the Al-vinyl transfer agent (AVTA), (ii) the vinyl 1,2-coordination/insertion of the alkenyl moieties of Al-1 into the Zr-alkyl bond, resulting in the formation of branching, is in direct kinetic competition with the insertion of ethylene, and (iii) the recoordination of the AVTA after either insertion step is thermodynamically favored and mostly responsible for the transmetallation phenomenon.