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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Synthesis of Self-Healing Polymers by Scandium-Catalyzed Copolymerization of Ethylene and Anisylpropylenes Haobing Wang,†,∥ Yang Yang,†,∥ Masayoshi Nishiura,†,‡ Yuji Higaki,§,# Atsushi Takahara,§ and Zhaomin Hou*,†,‡ †
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Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡ Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: Self-healing materials are of fundamental interest and practical importance. Herein we report the synthesis of a new class of self-healing materials, formed by the copolymerization of ethylene and anisyl-substituted propylenes using a sterically demanding halfsandwich scandium catalyst. The copolymerization proceeded in a controlled fashion, affording unique multi-block copolymers composed of relatively long alternating ethylene-alt-anisylpropylene sequences and short ethylene−ethylene units. By controlling the molecular weight and varying the anisyl substituents, a series of copolymers that show a wide range of glass-transition temperatures (Tg) and mechanical properties have been obtained. The copolymers with Tg below room temperature showed high elastic modulus, high toughness, and remarkable self-healability, being able to autonomously self-heal upon mechanical damage not only in a dry environment but also in water and aqueous acid and alkaline solutions, while those with Tg around or above room temperature exhibited excellent shape-memory property. The unique mechanical properties may be ascribed to the phase separation of the crystalline ethylene−ethylene nanodomains from the ethylene-alt-anisylpropylene matrix.
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INTRODUCTION Materials capable of self-healing upon damage have received tremendous attention over the past decades.1−3 Most of the self-healing materials reported to date have mainly relied on sophisticated designs and incorporation of chemical mechanisms into polymer networks, such as irreversible or reversible covalent bond formation,4−7 hydrogen bonding,7−12 metal− ligand interaction,13,14 ionic interaction,15 and donor−acceptor π−π stacking interaction.16 To achieve significant repair, the input of an external energy or stimulus such as heat,4,5,16 light,14 pressure,8 or solvent15 is usually required, although a few polymer materials were reported to show autonomous selfhealability with low toughness.6,9−13,15,17 The introduction of phase-separated morphologies may also facilitate damage repair,3,10,17 but self-healing materials based on such physical mechanisms remained scarce. In consideration of practical applications, it is highly important to develop tough, autonomous self-healing materials that can be produced in commercially relevant quantities and function in highly variable, real-world situations, including acidic and basic aqueous environments. Ethylene is the most widely used olefin monomer in the chemical industry. The copolymerization of ethylene with © XXXX American Chemical Society
polar functional olefins has received much recent attention, as such copolymerization can, in principle, serve as an atomefficient and practical route for the synthesis of functionalized polyolefins that may show improved beneficial properties.18−21 However, the copolymerization of ethylene and polar functional olefins often suffers from a trade-off between copolymer molecular weight and polar-monomer incorporation because of the distinct difference in reactivity between these two types of olefin monomers.21−36 Despite extensive studies and recent advances in this area, it is still difficult to synthesize functionalized polyolefins having both high molecular weight and high polar-monomer content in a controllable fashion. This has severely limited the exploration of well-defined, functionalized polyolefins with potential for a wide range of practical applications. An ethylene-based autonomous selfhealing polymer has remained unknown to date.37 We have recently found that heteroatoms (such as oxygen and sulfur) can serve as an efficient promoter for the copolymerization of functional α-olefins with ethylene in the presence of a rare-earth metal catalyst via a heteroatomReceived: December 13, 2018
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DOI: 10.1021/jacs.8b13316 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society Table 1. Scandium-Catalyzed Copolymerization of Ethylene (E) and Anisylpropylene (AP)a
run
[Sc]
[AP]/ [Sc]b
1g 2g 3 4 5 6
1 2 2 2 2 2
200/1 200/1 500/1 1000/1 2000/1 5000/1
time 10 15 5 15
min min min min 6h 24 h
yield (g)c
AP conv (%)
0.20 0.70 0.91 1.61 3.05 8.35
67 91 95 85 84 92
activity (g mol-Sc−1 h−1 atm−1) 1.4 1.1 6.4 5.1 3.5
− × 105 × 106 × 105 × 104 × 104
Mn (×103 g mol−1)d
Mw/ Mnd
AP/Ee
Tg (°C)f
Tm (°C)f
5 41 (P1) 90 (P2) 173 (P3) 344 (P4) 552 (P5)
1.65 1.68 1.58 1.94 1.70 1.98
100/0h 39/61 39/61 41/59 45/55 46/54
60 −6 −4 4 5 6
150 124 123 127 125 125
Conditions: [Sc] (0.01 mmol), [Ph3C][B(C6F5)4] (B) (0.01 mmol), ethylene (1 atm), 150 mL toluene, 20 °C, unless otherwise noted. bFeed ratio (in moles) of o-anisylpropylene (AP) and a scandium complex. cGrams of the polymer product. dDetermined by gel permeation chromatography (GPC) in o-dichlorobenzene at 140 °C against polystyrene standard. Mn = number-average molecular weight, Mw = weightaverage molecular weight. eMolar ratio of o-anisylpropylene (AP) and ethylene (E) in the copolymer, determined by 1H nuclear magnetic resonance (NMR) analysis. fDetermined by differential scanning calorimetry (DSC). g[Sc] = [B] = 0.02 mmol, 50 mL toluene. hSyndiotactic homopolymer of AP. a
assisted olefin polymerization (HOP) mechanism.36 Our previous studies have also demonstrated that the ether group in an anisole moiety can show an efficient interaction with a rare-earth metal catalyst to promote otherwise difficult C−H activation and transformation reactions.38−43 These findings have promoted us to examine the copolymerization of ethylene and anisyl-substituted propylenes by rare-earth catalysts. Herein we report the copolymerization of ethylene and anisylpropylenes by a sterically demanding half-sandwich scandium catalyst, which has led to the formation of the copolymers with both high molecular-weight and high polarmonomer content in a controllable fashion. By controlling the molecular weight and varying the anisyl substituents, we have successfully synthesized a brand-new class of well-defined, functionalized polyolefins ranging from soft viscoelastic materials to tough elastomers and rigid plastics. The elastomer copolymers showed high elastic modulus, high toughness and remarkable self-healing property, which autonomously selfhealed upon mechanical damage not only in a dry environment but also in water and aqueous acid and alkaline solutions without the need for any external energy or stimulus. The plastic copolymers demonstrated excellent shape-memory property, approaching to 100% shape fixation and shape recovery. It has been revealed that the formation of a multiblock copolymer structure composed of relatively long alternating ethylene−anisylpropylene sequences and short ethylene−ethylene segments is critically important to show the mechanical strength and self-healability. This is probably due to phase separation to form crystalline nanodomains of the ethylene−ethylene segments that may serve as the physical cross-linking points in a flexible alternating ethylene− anisylpropylene matrix.
2)44 with [Ph3C][B(C6F5)4] as a cocatalyst (Table 1). Under 1 atm of E and the AP monomer/catalyst feed ratio [AP]/[Sc] = 200/1 in toluene at room temperature, the C5H5-ligated complex 1 did not give a copolymer product, but afforded the AP homopolymer with high syndiotacticity (Table 1, run 1). This is probably due to the high activity of 1 for the homopolymerization of AP (see Table S1). By contrast, when the sterically demanding C5Me4SiMe3-ligated complex 2 was used in place of 1, the E−AP copolymer product (P1) was formed exclusively (Table 1, run 2), although 2 was not effective for the homopolymerization of AP under the similar conditions due to steric hindrance (see Table S1). As the [AP]/[2] feed ratio was raised from 200/1 to 500/1, 1000/1, 2000/1, and 5000/1 under 1 atm of E, the molecular weight (Mn) of the resulting copolymers showed a pronounced increase from 41 × 103 (P1) to 90 × 103 (P2), 173 × 103 (P3), 344 × 103 (P4), and 552 × 103 g mol−1 (P5), respectively, while the incorporation ratio of the AP monomer in the copolymers gradually increased from 39 mol% to 46 mol % (Table 1, runs 2−6). In contrast with the efficient AP incorporation, the copolymerization of ethylene with the methoxy-free allylbenzene under the same conditions gave a copolymer with only 2.5 mol% content of allylbenzene (Figure S60). These results suggest that the methoxy group played a highly important role in promoting the E−AP copolymerization. The 13C{1H} NMR analyses in C2D2Cl4 revealed that the E−AP copolymer products possess multi-block microstructures that mainly consist of the unique alternating E-alt-AP sequences (67−76%) and a smaller amount of the (E−E)n (n ≥ 1) segments (19−33%), while the AP−AP sequences were almost negligible (
