Branched polyolefin microstructures resulting from so‐called “chain walking” are a fascinating feature of late transition metal catalysts; however, to date it has not been demonstrated how desirable branched polyolefin microstructures can be generated thereby. We demonstrate how highly branched polyethylenes with methyl branches (220 Me/1000 C) exclusively and very high molecular weights (ca. 106 g mol−1), reaching the branch density and microstructure of commercial ethylene–propylene elastomers, can be generated from ethylene alone. At the same time, polar groups on the main chain can be generated by in‐chain incorporation of methyl acrylate. Key to this strategy is a novel rigid environment in an α‐diimine PdII catalyst with a steric constraint that allows for excessive chain walking and branching, but restricts branch formation to methyl branches, hinders chain transfer to afford a living polymerization, and inverts the regioselectivity of acrylate insertion to a 1,2‐mode.

A series of novel constrained-geometry-configuration (CGC) rare-earth metal complexes (RCH 2 −Py)Ln-(CH 2 SiMe 3 ) 2 (THF) n (Py = pyridyl; Ln = Y, n = 1, R = C 5 Me 4 (Cp′) (1); Ln = Y, n = 1, R = C 9 H 6 (Ind) (2); Ln = Y, n = 1, R = C 13 H 8 (Flu) (3a); Ln = Lu, n = 1, R = C 13 H 8 (Flu) (3b); Ln = Sc, n = 0, R = C 13 H 8 (Flu) (3c)) have been synthesized by treating rare-earth metal trisalkyls with PyCH 2 −Cp′, PyCH 2 − Ind, and PyCH 2 −Flu compounds, respectively, and fully characterized by NMR and X-ray diffraction analyses. Complexes 1, 2, and 3a−b are monomeric THF solvates while the scandium complex 3c is solvent-free, in which all the CGC ligands adopt a η 5 /κ 1 bonding mode via coordination of five carbon atoms from the cyclopentadienyl fragment and the pyridyl nitrogen atom with the central metals. Upon activation with Al i Bu 3 and [Ph 3 C][B(C 6 F 5 ) 4 ], these complexes showed different performances toward styrene polymerization. The Cp′CH 2 −Py and IndCH 2 −Py ligated yttrium complexes 1 and 2 showed very low activity to afford syndiotactic enriched polystyrene. Strikingly, the bulky FluCH 2 −Py supported complexes 3a−c displayed outstanding activities up to 1.56 × 10 7 g/(mol Ln ·h) and perfect syndioselectivity (rrrr > 99%), giving high molecular weight sPS; in particular, thus excellent performance was independent of the central metal type for the first time. In addition, the relationship of the ligand structure with the catalytic performances toward specific selective polymerization of styrene was reasonably revealed by comparison with the other Flu-based rare-earth metal catalysts reported previously by us. This might open a new pathway for designing catalysts for specifically selective polymerizations. ■ INTRODUCTIONSyndiotactic polystyrene (sPS), discovered first by Ishihara at Idemitsu in 1986 by using unlinked half-sandwich titanium catalysts, 1 is a promising engineering plastic due to its high melting point (T m = 270°C), rapid crystallization ability, high tensile modulus, low dielectric constant, and excellent resistance to heat and chemicals. 2 Since this breakthrough, a large number of titanium analogues generally formulated as [LTiX 3 ], where L is an alkyl or alkoxyl-and amino-substituted cyclopentadienyl (Cp), indenyl (Ind), or fluorenyl (Flu) moiety, while X is halogen or alkoxyl group, have been extensively investigated, which exhibited obvious improvements in both catalytic activity and syndioselectivity for styrene polymerization. 3 In contrast, rare-earth metal catalysts have been noted for their inertness or low specific selectivity toward styrene polymerization. 4 In 2004, great successes were achieved by using two type of catalytic systems based on the unlinked half-sandwich cationic scandium alkyl species 5 and neutral lanthanidocene bearing Flu−CMe 2 −Cp ligand, 6 both displayed good activity to provide perfect sPS. Later on some derivatives supported by the substituted Cp′, hetero(B, N, P)−Cp′, oxygen-linked Cp′ and Ind′ ligands have also been reported as efficient c...