Tailoring Mono-, Bi-, and Trimodal Molar Mass Distributions and All-Hydrocarbon Composites by Ethylene Polymerization on Bis(imino)pyridine Chromium(III) Supported on Ultrathin Gibbsite Single Crystal Nanoplatelets

Abstract

Multimodal molar mass distributions (MWD) of high-density polyethylene (HDPE) were tailored either by reactor cascade technology using a chain transfer agent or by multisite polymerization catalysis combining different single-site catalysts on the same support in a single reactor. Herein,2,6dimethylphenylimino)ethyl]pyridine chromium(III) (CrBIP) is supported on methylaluminoxane (MAO)-tethered ultrathin γ-Al(OH) 3 (gibbsite) single crystal nanoplatelets to produce reactor blends of HDPE wax and higher molar mass HDPE in a single reactor without adding either a second catalysts or a chain transfer agents. Lowering the MAO/gibbsite weight ratio enables unique switching from single-site to multisite nature of this catalyst system. In sharp contrast, ethylene polymerization on both homogeneous MAO/CrBIP and state-of-the-art heterogeneous CrBIP@MAO@SiO 2 catalysts exclusively produce HDPE wax (1000 g/mol) with narrow MWD (1.5). Both the MAO/gibbsite weight ratio of the CrBIP@MAO@gibbsite catalyst system and the polymerization time govern the HDPE wax/HDPE weight ratio. In addition, gibbsite single crystal nanoplatelets are calcinated at different temperatures prior to MAO tethering to establish the correlations between calcination temperature, MWDs, and catalyst activity. Upon calcination at 600 °C, highly active catalysts are obtained, but the gibbsite single crystal structure is destroyed, and the resulting catalyst fails to produce higher molar mass HDPE. Cosupporting CrBIP together with quinolylsilylcyclopentadienylchromium(III) (CrQCp), which produces ultrahigh molar mass HDPE (UHMWPE), on the same gibbsite support yielded CrQCp&CrBIP@MAO@gibbsite dual-site catalysts producing trimodal MWDs. Here the UHMWPE content is increased by increasing the CrQCp/CrBIP molar ratio. Moreover, the nanophase separation of UHMWPE during polymerization and melt-flow processing accounted for the formation of all-hydrocarbon nanocomposites self-reinforced by unentangled extended chain UHMWPE 1D nanostructures.

Disentangled UHMWPE@silica powders for potential use in power bed fusion based additive manufacturing

“…The DCSs show one melting isotherm, like is typical for in situ blends with single supports [49,[67][68][69]. The thermal properties of the polymer are typical for not (fully) entangled UHMWPE [61]. The pristine material had a first (standard) melting point of 142.1 • C and a high degree of crystallinity of 73 % [27,70].…”

“…Thus, the typical reactor blends of HDPE/UHMWPE were obtained with only one catalyst in form of 1 [9,16,[60][61][62]. The ratio of HDPE to UMWPE would be adjustable by the reaction time or TEA concentration, however, this was not pursued here [63].…”

Mathematical modeling of ethylene polymerization over advanced multisite catalysts: an artificial intelligence approach

“…Over the recent years, the multisite catalyst technology has been proposed by researchers to immobilize homogeneous active sites on different supports comprising MgCl 2 , silica, and graphene [7][8][9][10]. In this technology generally two or three different types of metallocenes and post-metallocene catalysts are immobilized on one or more preferred kinds of supports [8,[10][11][12][13][14]. As a consequence, polymers produced using multi component catalysts exhibit multi modal molecular weight distribution [7].…”

“…As a consequence, polymers produced using multi component catalysts exhibit multi modal molecular weight distribution [7]. A series of multisite catalysts were developed by Mülhaupt and Co-workers in which bimodal or trimodal polyethylene possessing precisely controlled microstructures are obtained [8,[10][11][12][13][14][15][16][17][18] One of their outstanding results, was the successful development of a polyethylene additive, that caused significant improvement in HDPE mechanical properties. The polymer stiffness, toughness and tensile strength were increased simultaneously, in terms of tensile strength (+ 392%), impact resistance (+ 197%) and Young's modulus (+ 365%) [8].…”

Tuning Polyethylene Molecular Weight Distributions Using Catalyst Support Composition

“…34 Recently, Mulhaupt and co-workers demonstrated that by varying the amount of methylaluminoxane (MAO) used to immobilize a chromium catalyst, distinct MWDs could be obtained. 35 A conceptually similar approach would be to create a support with a range of sites, so that one metallocene might behave in very different ways when immobilized on the surface. A classical support for immobilizing metallocenes is silica, often by using MAO as a cocatalyst.…”