Article pubs.acs.org/Macromolecules
Short-Chain Branched Polar-Functionalized Linear Polyethylene via “Tandem Catalysis” Zhongbao Jian and Stefan Mecking* †
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany S Supporting Information *
ABSTRACT: Cationic PdII complex 1 chelated by an N-fixed phosphine sultam has been synthesized and structurally characterized. Exposure of 1 to ethylene resulted in the formation of short-chain olefins (1-butene: 2-butene: 1hexene: 1-octene = 86:7:6:1) with a high catalytic activity of 105 molE molPd−1 h−1. By combination of 1 and one of the well-known phosphinesulfonato PdII catalyst precursors 2−5, linear polyethylenes containing methyl, ethyl, and n-butyl branches of up to 100 per 1000 C were generated from the polymerization of ethylene alone in a “tandem catalysis” onepot approach. In further exploitation of this concept, linear polyethylenes with both various short-chain branches and a choice of different polar functional groups incorporated into the main chain were obtained for the first time from the copolymerization of ethylene and polar vinyl monomers (methyl acrylate, N-isopropylacrylamide, methyl vinyl sulfone, acrylonitrile, ethyl vinyl ether, vinyl acetate, and allyl bromide). All these apolar and polar branches are incorporated into the linear polyethylene backbones to varying degrees, while the type of initiating and terminating chain ends of the resulting polyethylenes depends significantly on the nature of polar vinyl monomer.
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INTRODUCTION Polyethylene (PE) is the highest volume of all synthetic plastics produced, and it is used ubiquitously in a variety of different applications. These different requirements can be met by adjusting the mechanical properties and processing characteristics of polyethylenes via their microstructures. For instance, an introduction of short-chain branches (SCB) into the backbone of linear polyethylene alters the crystallinity and in turn the density, rigidity, hardness, and permeability.1 As an extension of these well-established principles, an incorporation of polar functional groups into the backbone of inherently nonpolar polyethylene even in small amounts can also significantly enhance such properties as adhesion and compatibility with other materials.2 Thus, polyethylene with a combination of both short-chain branches and polar moieties in the backbone appears attractive. Using Ziegler-type or metallocene-type catalysts, short-chain branched polyethylenes are produced industrially by copolymerization of ethylene with nonpolar α-olefins such as 1butene, 1-hexene, and 1-octene.3 An alternative route to generate such materials by polymerization of ethylene alone has been termed “tandem catalysis”, employing a dimerization (or oligomerization) catalyst that generates the comonomers in situ in combination with a copolymerization catalyst.4 The high oxophilicity of the traditional early transition-metal catalysts employed, however, appears prohibitive for additional incorporation of polar vinyl monomers. © XXXX American Chemical Society
Over the past two decades, less oxophilic late-transitionmetal catalysts, especially palladium(II) catalysts, have been developed for the insertion polymerization of ethylene with a range of polar vinyl monomers to generate functionalized polyethylenes.5 The seminal cationic α-diimine PdII catalysts form highly branched amorphous, liquid ethylene-acrylate copolymers with ester groups at the end of branches as a result of extensive “chain-walking” (Chart 1A).6 Corresponding dinuclear α-diimine PdII catalysts were recently reported to produce branched copolymers containing ester groups both at the end of branches and also on the main chain (Chart 1B).7 Neutral phosphinesulfonato PdII catalysts represent another prototypical motif, yielding linear functionalized polyethylenes with only very low degrees of methyl branching and incorporation of polar vinyl monomer into both the main chain and the chain ends (Chart 1C).8 These neutral PdII catalysts are compatible with a broad scope of polar vinyl monomers including even such difficult candidates as acrylonitrile, vinyl acetate, vinyl ether or acrylic acid.9 These properties render the neutral PdII catalyst attractive candidates for a “tandem catalysis” concept to both short chain branched and polar functionalized polyethylenes. Received: March 22, 2016 Revised: May 2, 2016
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DOI: 10.1021/acs.macromol.6b00581 Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
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Chart 1. Functional Polyethylenes with Different Microstructures
Article
RESULTS AND DISCUSSION
Synthesis of Ligand and PdII Complex. Although the less sterically encumbered O-atom of the sulfonamide moiety of II (Chart 2) chelates more easily to the PdII center in the catalyst precursor,11b the N-atom of the sulfonamide moiety may also compete for coordination to the PdII center of the active species as suggested by theoretical studies.11a To exclude this additional parameter, we fixed the N-atom in the form of a cationic phosphine sultam PdII complex (Chart 2, III). N-phenylbenzenesulfonamide was prepared, which underwent ortho metalation, as well as N-metalation, with 2 equiv of nBuLi to form dilithiosulfonamide that further reacted with benzophenone to generate the carbinol (Scheme 1). This carbinol underwent thermal cyclodehydration at 230 °C to yield the desired sultam.13 The resulting sultam was again metalated, and then reacted with (dianisyl)chloro phosphine to afford the desired phosphine sultam. Similar to reported procedures for acyclic phosphine sulfonamides,11a treatment of the phosphine sultam with 1 equiv of [(COD)PdMeCl] at room temperature for 30 min, followed by reaction with AgSbF6 in the presence of dmso at room temperature for another 10 min selectively generated the cationic phosphine sultam PdII complex 1 in a high yield (Scheme 1). Indicative 1H NMR resonances at δ = 0.62 and 2.70 ppm arise (cf. Supporting Information, Figure S4) from the protons of the Pd−CH3 group and a coordinated dmso molecule (compared to δ = 2.55 ppm for free dmso), respectively. The solid-state structure of 1 was determined by X-ray diffraction analysis (Figure 1). The phosphine sultam chelates to the PdII center in a κ2-P,O bidentate fashion, resulting in a distorted square planar coordination geometry. The dmso molecule binds to the PdII center in a κ1-O mode (Pd1−O5: 2.129(3) Å). As anticipated, the N-atom is remote to the PdII center and blocked from coordination. The Pd1−O4 bond (2.277(2) Å) in 1 is longer than in the analogous neutral dianisyl phosphinesulfonato PdII complex (2.165(3) Å)14 and a cationic phosphine−diethyl phosphonate PdII complex (2.164(2) Å),12a and is much longer than that in the phosphine−phosphine monoxide PdII complex (2.087(4) Å).12b The Pd1−P1 bond (2.217(9) Å) is slightly shorter than in the aforementioned three PdII complexes (2.235(6) Å, 2.232(1) Å and 2.232(2) Å, respectively).12,14 That is, complex 1 has a slightly stronger Pd−P bond but a significantly weaker Pd−O bond compared to other known neutral and cationic PdII complexes that are precursors to polymerization catalysts. Ethylene Oligomerization. Under pressure reactor conditions, exposure of 1 to ethylene at 80 °C for 30 min resulted in a high catalytic activity of 105 molE molPd−1 h−1 as
A comprehensive mechanistic understanding has revealed that an important structural feature of PdII catalysts containing phosphine sulfonates (Chart 2, structure I), which renders Chart 2. PdII Catalyst Precursors with Different Phosphine Ligands
them capable of insertion chain growth, is the presence of one hard sulfonate oxygen (O) and one soft phosphine (P) σ-donor ligand.10 On the basis of this P/O framework, very recently cationic PdII catalysts chelated by phosphine sulfonamides (Chart 2, structure II) have been reported,11,12 which catalyze the dimerization of ethylene to predominantly generate 1butene. We now report on the generation of both short chain branched and polar functionalized polyethylene (Chart 1D) from ethylene and a polar vinyl monomer as the sole feedstocks via a “tandem catalysis” approach employing a combination of a neutral and a cationic PdII catalyst (Chart 2, structures I and III). Scheme 1. Synthesis of Phosphine Sultam and PdII Complex 1
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DOI: 10.1021/acs.macromol.6b00581 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
two metal sites toward ethylene must be well-matched, and the catalysts must not interfere with each other chemically.4f In view of these issues, the combination of catalyst precursors 1 and 2 (Chart 3) appears reasonable. Because of their overall Chart 3. Phosphinesulfonato PdII Catalyst Precursors 2−5 Applied in Tandem Catalysis
similar structure, no specific unfavorable interactions are anticipated between active species formed from 1 and 2. The catalytic activity and lifetime of 1 toward ethylene compare to those of 2 as monitored by mass flow (Table 1, entries 1 and 2).9p Additionally, 2 is well-known to afford linear polyethylene, and to copolymerize ethylene and 1-hexene.9p,15 By a one-pot polymerization procedure with ethylene as the only monomer starting with catalyst precursor 1 and subsequently adding 2, 3, or 4, respectively (cf. Supporting Information and the Experimental Section), linear polyethylenes containing methyl (Me), ethyl (Et), and n-butyl (Bu) branches were produced. Under otherwise identical conditions, with an increasing amount of 1, the Et and Bu branches increase as anticipated, resulting in a decrease of the melting point (Tm). However, polymer molecular weights decrease as well (Table 1, entries 5−8). By combination of 1 with a precursor to a more active polymerization catalyst 3,9p the degree of branching in the resulting linear polyethylene decrease as expected (Table 1, entries 6 vs 9). Conversely, by
Figure 1. ORTEP plot of 1 drawn with 50% probability ellipsoids. Hydrogen atoms and the SbF6 counterion are omitted for clarity. Selected bond lengths [Å]: Pd1−O4 2.277(2), Pd1−P1 2.217(9), Pd1−C1 2.022(4), Pd1−O5 2.129(3).
monitored by mass flow (Table 1, entry 1). This is approximately twice as high compared to cationic phosphine sulfonamide PdII catalysts reported previously,11 thus the fixation of the N-atom in 1 obviously enhances catalytic activity. Both 1H NMR spectroscopy and GC-MS analyses of the resulting products revealed the formation of 1-butene as the major product, along with 2-butene, and small amounts of 1hexene and 1-octene (typical ratio: 86:7:6:1). The formation of 1-olefin products predominately is attractive for employing this catalyst in tandem catalysis of ethylene to generate branched polyethylene. Short-Chain Branched Linear Polyethylene. For the approach of tandem catalysis to be viable, the reactivity of the
Table 1. Short-Chain Branched Linear Polyethylene Produced by One-Pot Tandem Catalysisa entry 1 2h 3i 4j 5 6 7 8 9 10 11 12k 13k 14k 15l 16n
step 1 oligo
step 2 (co)poly
1 (3.5) − − − 1 (5) 1 (10) 1 (20) 1 (40) 1 (10) 1 (10) 1 (40) 1 (40) 1 (40) 1 (40) − −
− 2 (3.5) 3 (2) 4 (3.5) 2 (20) 2 (20) 2 (20) 2 (20) 3 (20) 4 (20) 4 (20) 2 (3.5) 2 (8) 2 (20) 2 (20) 2 (20)
pE [bar] (1/2) 10/− −/5 −/5 −/10 10/5 10/5 10/5 10/5 10/5 10/5 10/5 5 5 5 − 5
t [min] (1/2) 30/− −/30 −/30 −/30 30/60 30/60 30/60 30/60 30/40 30/60 30/60 60 60 60 60 20
yield [g]
act.b
f
g
4.6 4.9 3.7 1.0 13.4 11.5 7.4 4.7 19.0 3.5 1.1 1.5 3.4 6.6 0.25 3.2
263 280 370 57 67 58 37 24 143 18 6 43 43 33 1.3m 48
Me brsc /1000C
Et brsc /1000C
Bu brsc /1000C
− 3
