Origin of “Multisite-like” Ethylene Polymerization Behavior of the Single-Site Nonsymmetrical Bis(imino)pyridine Iron(II) Complex in the Presence of Modified Methylaluminoxane

Abstract

A detailed study of the effect of reaction temperature, time, and cocatalyst composition on the ethylene polymerization performance of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)­ethyl]-6-(1-mesityliminoethyl)­pyridyliron dichloride (1) is reported. In the presence of modified methylaluminoxane (MMAO), 1 behaves like a highly active, “multisite-like” ethylene polymerization catalyst, with the resulting polyethylenes having time-dependent bimodal-like molecular-weight distributions and featuring saturated (n-propyl- and i-butyl-terminated) chain ends. To readily distinguish between bimodal and bimodal-like molecular-weight distributions, we have proposed the use of the dN f/(d log M) – log M representation further to the mainstream dW f/(d log M) – log M one. The consensus mechanism of chain transfer and chain-end formation in the presence of MMAO has been proposed, which explains the composition and amount of terminal alkyl groups in the polymer, and the apparent “multisite-like” nature of the iron catalyst. A comparison between the catalytic behaviors of the “multisite-like” 1/MMAO catalyst system and the truly multisite catalyst system based on Brookhart’s symmetrical bis­(imino)­pyridine iron catalyst 2 is given.

Cycloheptyl‐fused NNO‐ligands as electronically modifiable supports for M(II) (M = Co, Fe) chloride precatalysts; probing performance in ethylene oligo‐/polymerization

“…Indeed, a mechanism of this type has recently been proposed by Bryliakov et al to account for a similar iso-butyl/n-propyl chain-end combination which occurs at high concentrations of MMAO using bis(imino)pyridine-iron catalysts. 25 Related linear polyethylenes are expected for the other samples prepared in this study and indeed the melting temperatures of between 127.1 -128.9 o C support this (entries 1-5, Table 7). …”

“…13a Table 7); expansion shows the aliphatic region In addition, peaks at δ 39.72, 28.52, 27.87 and 23.09 can be ascribed to an iso-butyl end-group (peaks 1 -4 in Figure 7), while the signals at δ 32.44, 23.13 and 14.46 to an n-propyl endgroup (peaks I -III in Figure 7). 25 Interestingly, no evidence of unsaturated chain ends could be detected hence precluding chain termination via β-H elimination. Given the presence of an iso-butyl end-group, it would seem probable that these catalyst systems undergo termination by chain transfer to aluminum and in particular to Al(i-Bu)3 and its derivatives (e.g., i-Bu2AlMe) present in MMAO.…”

Fusing Carbocycles of Inequivalent Ring Size to a Bis(imino)pyridine-Iron Ethylene Polymerization Catalyst: Distinctive Effects on Activity, PE Molecular Weight, and Dispersity

“…Through systematic variation of the steric and electronic properties of the ligand frame and in particular to the N-aryl groups, catalysts capable of generating highly sought-after materials such as α-olefins, linear PE waxes, and high-density polyethylene (HDPE) are all accessible [2,6,10]. Moreover, such targeted ligand manipulation has seen remarkable improvements to the temperature stability of the catalyst itself [5][6][7][8][9][11][12][13][14], a limitation often levelled against the first-generation catalysts [4,6]. Elsewhere, other types of neutral N,N,N-ligand skeleton such as N-[(pyridin-2-yl)-methylene]-8-amino-quinolines [15], 2-benzimidazolyl-6-imino-pyridines [16][17][18], 2,8-bis(imino)quinolines [19], and 2-imino-1,10-phenanthrolines [20,21] have witnessed some important developments [6][7][8].…”

Exceptionally high molecular weight linear polyethylene by using N,N,N′‐Co catalysts appended with a N′‐2,6‐bis{di(4‐fluorophenyl)methyl}‐4‐nitrophenyl group

“…The Co-N pyridine bond length [2.068(2) Å Co1, 2.036(3) Å Co3] is shorter than the exterior Co-N imino distances [Co(1)-N(1) 2.143(3) Å Co1, 2.141(3) Å Co3; Co(1)-N(3) 2.207(2) Å Co1, 2.193(3) Å Co3], an observation that is common in structurally related comparators and can be accounted for by the superior binding properties of the pyridine and by the constraints imparted by the N,N,N-ligand. [8][9][10][11][12][13][14][15]38,42,43,[48][49][50][51][52] Some disparity in the Co-N imino distances is also evident with the Co(1)-N(3) distance longer than in Co (1) is considered to play a crucial role in the polymerization performance. [43] There are no intermolecular contacts of note.…”

“…As the temperature was raised, the catalytic activity steadily reduced reaching a minimal value of 2.85 × 10 6 g (PE) mol −1 (Co) h −1 at 60°C, which is likely attributable to partial deactivation of the active species and lower solubility of ethylene at elevated temperature. [4][5][6][7][8][9][10][11][12][13][14][15]38,[48][49][50]54,55] Nevertheless, this activity at 60°C was relatively high when compared with related cobalt catalysts at a similar operating temperature, [10,13,30,[32][33][34][35][36][37][38] highlighting the appreciable thermal stability of this catalyst. In terms of the polymer molecular weight, it was evident that increasing the temperature gave polyethylene of lower molecular weight ( Figure S1), which is in line with more rapid chain transfer/chain termination at the higher temperature compared to chain propagation.…”