Olefin Polymerization Catalyzed by Double‐Decker Dipalladium Complexes: Low Branched Poly(α‐Olefin)s by Selective Insertion of the Monomer Molecule

Abstract

Dipalladium complexes of a cyclic bis(diimine) ligand with a double-decker structure catalyze polymerization of ethylene and α-olefins and copolymerization of ethylene with 1-hexene. The polymerization of 1-hexene yields a polymer that is mainly composed of the hexamethylene unit formed by 2,1-insertion of the monomer into the palladium-carbon bond, followed by chain-walking (6,1-insertion). The polymerization of 4-methyl-1-pentene proceeds by 2,1-insertion with a selectivity of 92-97 %, and affords the polymer with methyl and 2-methylhexyl branches. 2,1-Insertion occurs selectively in all of the polymerization reactions of α-olefins catalyzed by the dipalladium complexes. Ethylene polymerization with the catalyst at 100 °C lasts over 24 h, whereas the monopalladium-diimine catalyst loses its activity within 8 h at 60 °C. Polyethylene obtained by the dipalladium catalyst is less-branched and has a higher molecular weight compared to that of the monopalladium catalyst under the same conditions. Copolymerization of ethylene with 1-hexene affords solid products with melting points and molecular weights that vary depending on the polymerization time, suggesting formation of a block and/or gradient copolymer.

Recent Progress in Ethylene Polymerization Catalyzed by Ni and Pd Catalysts

“…Dinucleating α‐diimine ligands are also common in recently reported bimetallic catalysts used for ethylene polymerization. [27c], , Takano et al designed the two binuclear Pd complexes 30a and 30b (Figure ) with double‐decker structures . In ethylene polymerization at 60 and 100 °C, binuclear complex 30b , with the help of NaBARF, showed higher activities than the corresponding mononuclear catalyst in each case.…”

“…Structures of binuclear complexes 30a , 30b , 31a – d , 32a – e , 33a , 33b , and 34a – c . [27c], , , , …”

“…Usually, PE with higher molecular weight can be generated by using bimetallic catalysts, due to the efficient blocking of the axial sites of the metal centers by the ligands, and to the metals suppressing β‐H elimination and/or chain transfer to some extent, whereas mononuclear catalysts experience only limited steric hindrance from ligands , ,. [27c], , , , For example, results (Table ) from Chen's group[27c] show that the molecular weight of the resulting polymer increase with increasing bulkiness at the ortho ‐aryl positions of catalysts and that higher molecular weight can be achieved by using bimetallic complexes 32a – e (Figure ) than with their mononuclear analogues 32f – h (Figure ). Likewise, the phenomena are similar in the case of complexes 33a and 33b (Figure ) and their mononuclear analogues .…”

Recent advances in thermally robust, late transition metal‐catalyzed olefin polymerization

“…This stability was evidenced by increasing turnover number (TON) and consistent molecular weight ( M n ) values as a function of time (Fig. ) . Finally, the bimetallic Ni‐based catalyst 17 , which is bridged on a single side by a naphthalene group, takes advantage of the cooperativity effect and is able to achieve a high activity (2.34 × 10 6 g (mol Ni) −1 h −1 ) at temperatures as high as 80 °C.…”

“…TON and M n data for catalyst 16 (squares) at 100 °C . Reprinted with permission from Chem Eur J 21 :16 209 (2015).…”