Judicious Reduction of Supported Ti Catalyst Enables Access to Disentangled Ultrahigh Molecular Weight Polyethylene Ravindra P. Gote, † Dipa Mandal, † Ketan Patel, †,⊥ Krishnaroop Chaudhuri, †,§ C. P. Vinod, ‡ Ashish K. Lele, † and Samir H. Chikkali* ,†,§ † Polymer Science and Engineering Division and ‡ Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India § Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110001, India * S Supporting Information ABSTRACT: Heterogeneous Ziegler-Natta and homogeneous metallocene catalysts are known to produce ultrahigh molecular weight polyethylene (UHMWPE) in the entangled state. On the other hand, only (two) homogeneous single-site catalysts are reported to yield disentangled UHMWPE (dis-UHMWPE). This disparity between the two types of catalysts and the two states of polyethylene can be bridged if a heterogeneous single-site catalyst that can yield dis-UHMWPE is made accessible. Here, one-pot two-step synthesis of a MgCl 2 supported [Ti(OEt) 4 ] derived catalyst 1 with a two-stage activation strategy is reported to produce dis-UHMWPE. Second activation of catalyst 1 was performed by adding excess modified methylaluminoxane (MMAO12), and XPS analysis indicated that the catalyst existed in only Ti(III) state at [Al]/[Ti] ratio of 600. Catalyst 1 after second activation with MMAO12 was found to be highly active in ethylene polymerization and produced dis-UHMWPE. Polymerization conditions were tailored to obtain molecular weight (M w ) as high as 13 million g/mol PE. To the best of our knowledge, this is the first time a heterogeneous catalyst (catalyst 1) that displays pseudosingle site nature is able to produce dis-UHMWPE. The thus-prepared nascent polyethylene revealed a melting temperature of 141-144 °C, which is a characteristic melting transition for a dis- UHMWPE. The disentangled state of the nascent PE and its M w and MWD were further authenticated by rheological investigations. Isothermal time sweep oscillatory experiments in linear viscoelastic limit revealed a rapid rise in elastic modulus followed by equilibration to plateau modulus, which are characteristic features of the disentangled state. Thus, a pseudo-single- site heterogeneous catalyst has been accessed, which upon second activation with excess MMAO12 led to the production of dis- UHMWPE. ■ INTRODUCTION Among the 300 odd million tons of polymers produced today, polyolefins constitute nearly half of the total volume and are most commonly manufactured by employing the supported Ziegler-Natta (ZN) catalyst. 1-6 The mode of action and fundamental principles of the Ziegler-Natta polymerization were deeply investigated in the early 1970s, and the monometallic mechanism proposed by Cossee-Arlman is unequivocally accepted by the scienti fic community. 7-9 Conventional ZN catalysts are known to be multisite in nature, as the alkylaluminum used for activating Ti(IV) precursor leads to titanium species with mixed oxidation states, which probably coexist in the catalytic system. 10 Among other reasons, the existence of Ti in various oxidation states is believed to be responsible for regularly observed broader molecular weight distribution (MWD) in ZN-catalyzed olefin polymerization reaction. 11-13 Therefore, detecting the distribution of oxidation states of a ZN catalyst and quantifying them has become indispensable, if we were to advance the seemingly matured olefin polymerization reaction. However, it is an impeding challenge to detect the distribution of oxidation states of titanium in a supported ZN catalyst and quantify them. This is especially inconvenient due to the extreme air and moisture sensitivity of ZN catalysts, and such studies have been rarely undertaken. About a decade ago, Somorjai and co-workers took up this challenge and investigated the oxidation states of a model ZN catalyst using high-resolution X-ray photoelectron spectroscopy (XPS). 14 Their investigation concluded that after reaction with triethylaluminum, the Ti species exists in Ti(IV), Ti(III), and Ti(II) states with different peak intensities. A consequence of simultaneous existence of multiple oxidation states is broadening of molecular weight distribution and a compromise on the properties offered by the polymer. Remarkably, their findings revealed that the predominant oxidation state of an activated titanium in the model catalyst was Ti 2+ , which contradicts the previous belief that the active species in a ZN system is generally a Ti 3+ species. 10 Although the above model system probably mimicked the actual catalytic system, access to a single site heterogeneous catalytic system in reality remains a formidable challenge. In contrast to the Received: March 19, 2018 Revised: May 19, 2018 Article Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b00590 Macromolecules XXXX, XXX, XXX-XXX