Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Tailoring Mono‑, Bi‑, and Trimodal Molar Mass Distributions and AllHydrocarbon Composites by Ethylene Polymerization on Bis(imino)pyridine Chromium(III) Supported on Ultrathin Gibbsite Single Crystal Nanoplatelets Fan Zhong, Ralf Thomann, and Rolf Mülhaupt*
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Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry, Albert-Ludwigs-University Freiburg, Stefan-Meier-Strasse 31, Freiburg D-79104, Germany 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, 2,6-bis-[1-(2,6dimethylphenylimino)ethyl]pyridine chromium(III) (CrBIP) is supported on methylaluminoxane (MAO)-tethered ultrathin γAl(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@SiO2 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.
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INTRODUCTION Among the polymeric materials, the hydrocarbon polymers such as polyethylene and polypropylene, produced in highly cost-, energy-, and resource-efficient solvent-free catalytic polymerization process, are the clear leaders in both world plastics manufacturing and life cycle assessment.1,2 Because polyolefins possess hydrocarbon nature and oil-like high energy content, they can be readily recycled as materials or serve as a valuable source of hydrocarbon feedstocks and energy. Hence, polyolefins as hydrocarbon materials meet the demands of both sustainable development and green chemistry.3 Currently, more than 150 million tons of polyolefins account for over 50% worldwide polymeric materials with applications ranging from high-performance fibers to lightweight engineering plastics and transparent packaging materials.4−6 Key to the success of polyolefins is the facile tailoring of properties via catalyst technology, reaction engineering, and processing.7−9 Since the pioneering advances by Karl Ziegler and Giulio Natta in the 1950s, the catalyst development has been aimed at improving the control © XXXX American Chemical Society
of microstructures, molar masses, and molar mass distributions (MWD) of polyolefins.10−12 Using fluorinated bis(phenoxy-imine)Ti (FI) catalysts, Fujita et al. succeeded to enhance the precision of olefin polymerization producing narrow MWDs by living olefin polymerization.13 However, in view of shear thinning and melt strengthening during melt processing, it is highly desirable to broaden MWDs in a controlled fashion. Tailoring MWDs by blending together short and high molar mass HDPE during polymerization and by selectively branching ultrahigh molar mass polyethylene (UHMWPE) improves the balance between easy processing and superior mechanical properties in terms of stiffness/strength/toughness. In nanophase-separated HDPE wax/HDPE/UHMWPE reactor blends, which is also termed all-hydrocarbon composite or self-reinforcing polyethylene, the low molar mass PE enhances processability by lowering melt Received: January 15, 2019 Revised: February 21, 2019
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DOI: 10.1021/acs.macromol.9b00091 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
HDPE with controlled particle morphology without encountering reactor fouling.43 The group of Mülhaupt has developed robust multisite catalysts supported on meso- and nanoporous silica as well as 2D nanomaterials like functionalized graphene to tailor bi- and trimodal HDPE reactor blends in a single reactor.44−47 Owing to nanophase separation accounting for the absence of massive entanglement, much higher UHMWPE contents are tolerated in melt processing without requiring specialized expensive processing methods. When HDPE wax serving as lubricant is produced on a second catalytic site, up to 24 wt % UHMWPE is tolerated in injection molding typical for commodity polyolefins. Opposite to commodity HDPE, the flow-induced oriented UHMWPE crystallization affords ultrastrong extended chain UHMWPE 1D nanostructures resembling nanofibers which nucleate HDPE crystallization. Hence, this self-reinforcement simultaneously improves strength, stiffness, and toughness of the reactor blends which are far superior to commodity HDPE and approach ranges typical for glass fiber reinforced composites.48 Herein, we describe an alternative synthesis of mono- and bimodal HDPE on 2,6-bis-[1-(2,6-dimethylphenylimino)ethyl]pyridine chromium(III) complexes (CrBIP) catalyst supported on MAO-tethered ultrathin hexagonal gibbsite. This approach goes well beyond the use of micrometer-sized Al(OH)3 produced by the Bayer process and applied as the flame retardant for polyolefins.49−52 Hexagonal gibbsite nanoplatelets with an average thickness around 20 nm were obtained by hydrothermal crystallization.53−55 Special emphasis is placed upon elucidating how the MAO/gibbsite weight ratio, the calcination temperature, and polymerization parameters of CrBIP@MAO@gibbsite catalysts affect the composition of bimodal HDPE/HDPE wax reactor blends. Moreover, by blending dichloro-η5-[3,4,5-trimethyl-1-(8-quinolyl)-2-trimethylsilylcyclopentadienyl]chromium(III) (CrQCp), producing UHMWPE, together with CrBIP on the MAO@gibbsite support two-site CrQCp&CrBIP@MAO@ gibbsite are formed to produce trimodal HDPE/HDPE wax/ UHMWPE reactor blends which in the past were exclusively formed on three-site catalysts. The influence of CrQCp/CrBIP molar ratio on the UHMWPE content and the UHMWPE nanophase separation is examined to establish new synthetic routes toward fabricating self-reinforced all-hydrocarbon composites containing unentangled, extended-chain UHMWPE as reinforcing phases of all-hydrocarbon singlecomponent composites.
viscosity whereas the formation of nanophase-separated extended-chain UHMWPE accounts for highly effective HDPE matrix reinforcement.14,15 Because both the matrix and the reinforcing phase are made of polyethylene, allpolyethylene composites offer great promise as sustainable materials and single-component composites which do not require the addition of nanoparticles or other alien fillers and fibers. Generally, the melt-processable UHMWPE/HDPE blends are obtained either by melt blending or reactor blending.1 Owing to the massive chain entanglements typical for UHMWPE, it is paralleled by drastic viscosity buildup. The melt blending is restricted to
