Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
pubs.acs.org/Macromolecules
Interaction between Two Active Sites of the Same Catalyst for Macromonomer Enchained Olefin Polymerization Thilina Gunasekara,†,§ Jungsuk Kim,‡ Silei Xiong,‡ Andrew Preston,†,§ D. Keith Steelman,† Grigori A. Medvedev,‡ W. Nicholas Delgass,‡ James M. Caruthers,*,‡ and Mahdi M. Abu-Omar*,§ †
Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States Charles D. Davidson School of Chemical Engineering, Forney Hall of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States § Department of Chemistry and Biochemistry, Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States ‡
S Supporting Information *
ABSTRACT: A zirconium amine bis(phenolate) catalyst that is capable of simultaneously producing both oligomers and polymers of 1-hexene was investigated. It was found that the polymer produced has more branching than the commonly encountered poly(1-hexene), suggesting an oligomer macromonomer enchainment. The polymer weight fraction and molecular weight are weakly dependent on conversion and are not affected by increasing the concentration of free oligomer macromonomers in solution. After considering a number of possible mechanisms, the most plausible supported by spectroscopic and kinetic data is a second-order reaction between an oligomer forming site and a polymer forming site, resulting in the transfer of oligomer macromonomers into the growing polymer chain. The ratio between oligomer and polymer products can be precisely tuned by varying the precatalyst activation conditions.
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to produce simultaneously oligomers and polymers of αolefin.4,9 Such a bimodal molecular weight distribution of products could perhaps be explained if the precatalyst is activated by a nonstoichiometric activator such as methylaluminoxane (MAO) as it can potentially give rise to more than one type of active site or introduce additional chain transfer pathways.4 However, the same behavior is exhibited by a precatalyst activated by a stoichiometric activator, which is inconceivable based on our current knowledge of olefin polymerization. The catalyst system detailed herein (Figure 1), and reported initially by Kol and co-workers,9 has a fair oligomerization activity of 60 g mmol cat.−1 h−1, where it also simultaneously produces a 300 000 g/mol polymer. In this study, we report a detailed kinetic analysis of 1-hexene oligomerization and polymerization for the Zr-NEt2/B(C6F5)3 catalyst system, with the objective of determining a plausible reaction mechanism that can explain all of the multiresponse data that we collected. Two straightforward mechanisms were considered: oligo-/polymerization by two types of active sites independent of each other (Mechanism 1) and oligomerization by one site and oligomer macromonomer insertion by another site (Mechanism 2). Other possibilities, such as cationic and
INTRODUCTION There is a continuing interest in post-metallocene transition metal catalyzed α-olefin polymerization. These single-site catalysts have led to the synthesis of polymers with tailored architecture and properties. However, tuning a catalyst system from producing polymers to oligomers (or vice versa) has been a challenging task, in part due to lack of understanding the mechanism and kinetics of the responsible active sites.1 Subtle changes in ligand framework, cocatalyst, and reaction conditions can have a substantial and unforeseen influence on the molecular weight of the product. In other instances, changes on the catalyst can result in a bimodal distribution of products.2,3 When such behaviors are observed, the reasons and the chemical structures of the active species are often not well understood.4 In selective ethylene oligomerization, such as in the Chevron Phillips system, it is quite common to obtain both oligomer and polyethylene concurrently through different mechanisms, most commonly through a metallacyclic pathway and a Cossee− Arlman mechanism, respectively.5,6 The ability to switch the activity of the catalyst system from ethylene polymerization to selective oligomerization3,7 or nonselective oligomerization8 has also been reported in the literature. However, unlike for ethylene, the increased steric bulk of α-olefins makes the metallacyclic mechanism less favorable for α-olefin oligomerization, where it is intriguing that certain catalyst systems are able © XXXX American Chemical Society
Received: June 25, 2017 Revised: October 24, 2017
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DOI: 10.1021/acs.macromol.7b01341 Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
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Article
EXPERIMENTAL PROCEDURES
The ligand and precatalyst were prepared following literature procedures.9,17,18 Detailed procedures of the synthesis and product analyses and related spectra are provided in the Supporting Information. The kinetic analysis employs multiresponse data that include (1) time-dependent monomer consumption, (2) evolution of the molecular weight distribution (MWD), (3) active site counts, and (4) end-group concentrations. Detailed procedures are reported in the Supporting Information.
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RESULTS A series of experiments were performed using the Zr-NEt2 catalyst at various reaction conditions (Table 1). For every condition in Table 1, monomer consumption and MWD were obtained by analyzing the samples quenched with methanol-d4. In addition, for runs A-1 and B-2, the active site concentrations and end-group concentrations were measured. The 15 runs presented in Table 1 are divided into sets A through F, wherein for each set a particular condition is systematically varied. Run A-1 is chosen as the reference condition. Thus, set E (i.e., E-10, A-1, E-11, and E-12) illustrates the effect of the premix time between precatalyst and activator. Similarly, the effect of the premix temperature is seen in set F (i.e., A-1, F13, and F-15). Depending on the catalyst activation conditions, i.e., the premix time and premix temperature, the reaction may produce (1) only oligomers (molecular weight up to ca. 1 kg/mol), (2) only polymers (molecular weight of 22−220 kg/mol), or (3) both oligomers and polymers. In the discussion that follows, these regimes are referred to as case 1, 2, and 3, respectively. Table 1 shows that case 1 includes runs B-2 and E-10 (i.e., when only oligomers are observed), case 2 includes runs F-14 and F-15 (i.e., when only polymers are observed), and case 3 includes the rest of the data (i.e., when both oligomers and polymers are observed). The specific trends of oligomerization vs polymerization determined from the data in Table 1 are as follows: substoichiometric activator conditions, shorter premix times, and lower premix temperatures result in case 1 (oligomers only). Stoichiometric (and above stoichiometric) activator
Figure 1. 1-Hexene polymerization catalyzed by Zr-NEt2 when combined with the activator B(C6F5)3.
radical polymerization, have also been considered and dismissed where the details are given in the Supporting Information. The above possibilities are ruled out on the basis of inconsistency with the kinetic and structural features of the polymer; in addition, it was observed that the polymer has a branched architecture as compared to linear poly(1-hexene) and that the free vinylidene-terminated oligomers (macromonomers) do not reinsert. Consequently, it was necessary to postulate a more intricate kinetic model than the traditional Cossee-type mechanism, involving a second-order reaction between an oligomeric and a polymeric active site resulting in the insertion of oligomer into the growing polymer chain. The uniqueness of the mechanism proposed in this study is that the product of one catalytic cycle becomes the reactant for a second catalytic cycle via an associative interaction between two distinct active catalysts, where both sites originate from the same precatalyst, to produce, in this case, a branched olefin polymer.10,11 This discovery offers in principle a new pathway for the production of olefin copolymers using a single feedstock and a single precatalyst vs the traditional method of using a monomer and a comonomer, e.g., LLDPE from the copolymerization of ethylene and 1-hexene.12 Previous attempts to produce polymers with this type of architecture from a single monomer employing two different catalysts in tandem have been thwarted by challenges in matching rates of reactions and catalysts compatibility.13−16
Table 1. Results under Various Activation Conditions for Zr-NEt2 Catalyst ([Cat.] = Zr-NEt2, [Act.] = B(C6F5)3, and [M]0 = Initial 1-Hexene) run d
A-1 B-2d B-3e B-4d C-5d C-6d D-7d D-8d D-9d E-10d E-11d E-12e F-13d F-14d F-15e
[Cat.] (mM)
[Act.] (equiv)
[M]0 (M)
premix timea (h)
premix tempb (°C)
kobs (10−4) (s−1)
wtpolyc (%)
Mw (kg/mol) (PDI)
9.0 9.0 9.0 9.0 9.0 9.0 18.0 4.5 2.25 9.0 9.0 9.0 9.0 9.0 9.0
1.1 0.5 0.5 2.0 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1
4.5 4.5 4.5 4.5 2.25 0.9 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 0.45
1.0 1.0 3.0 1.0 1.0 1.0 1.0 1.0 1.0 0.0 5.0 8.0 8.0 8.0 8.0
25 25 45 25 25 25 25 25 25 25 25 25 45 65 65
1.68 0.94 0.10 1.48 1.47 1.66 5.75 0.76 0.34 1.82 1.25 1.17 0.07 0.03 0.02
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