Ring-Closing Metathesis and Ring-Opening Metathesis

Oct 20, 2018 - effectively resulted in terminal dienes with high yields (Table. 1). ... successful when the number of carbon atoms is three or five, w...
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Ring-Closing Metathesis and Ring-Opening Metathesis Polymerization toward Main-Chain Ferrocene-Containing Polymers Ye Sha,† Yudi Zhang,‡ Tianyu Zhu,† Shaobo Tan,† Yujin Cha,† Stephen L. Craig,*,‡ and Chuanbing Tang*,† †

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States Department of Chemistry, Duke University, Durham, North Carolina 27708, United States

‡

Macromolecules Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/07/18. For personal use only.

S Supporting Information *

ABSTRACT: We report the preparation of cyclic ferrocenyl olefins with various substituents and different ring sizes by ring-closing metathesis (RCM). These ferrocene-containing monomers were subject to ring-opening metathesis polymerization (ROMP), leading to main-chain ferrocene-containing homopolymers, random copolymers, and block copolymers. Depending on the substituents, ferrocenyl homopolymers are semicrystalline or amorphous with good solubility. A semicrystalline polymer was used in the crystallization-driven self-assembly (CDSA) of block copolymers to generate platelet nanostructures.

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INTRODUCTION Ferrocene-containing polymers, in which organic polymer frameworks and metal centers are combined together, are among the most explored hybrid metallo-materials that play useful roles in optical, electronic, magnetic, biomedical, and catalytic applications.1−11 Through a range of versatile polymerization strategies and postmodification methodologies, various structural topologies of ferrocene-containing polymers are now accessible.3,12,13 Among them, main-chain ferrocenecontaining polymers that have the metal as an integral part of the polymer backbone are usually more synthetically challenging.4 Main-chain polyferrocenes can be prepared through step growth polymerization such as polycondensation, intermolecular coupling, and cross-metathesis, all of which involve the reaction of difunctional monomers.4,14−19 Such techniques generally result in relatively low molecular weight (typically 65%. When the ester bonds were changed to ether linkages, cyclic monomer (compound 15) was not obtained after 10 h. However, after reaction for 24 h, we observed the conversion of 4 into a new cyclic dimer 16, as evidenced by its mass spectrum (Figure 1b). Similar extended reactions of RCM have B

DOI: 10.1021/acs.macromol.8b02064 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Results of RCM for Preparation of Cyclic Ferrocenyl Monomers

a

The stereochemistry of each isomer was determined from coupling constants of alkene protons. The molar ratio of the isomers was determined from 1H NMR.

ring closing irrespective of substituent length, as shown in Figure 2b. The formation of dominant cis-isomer conformation of cyclic olefins (monomers 12−14, Table 1) indicates that a functionality away from the RCM reaction site is capable to control over the stereochemistry of the products. Ether-based substituents, however, are less aligned with respect to the rotation of Cp rings, reducing the possibility for two olefin end groups to meet, as shown in Figure 2c.43 Finally, when the ester bonds are not directly linked to the Cp ring but with a

of the differential reactivity. All ferrocenyl ester-linked olefins undergo RCM effectively irrespective of chain length, as we explored. Being directly linked to the cyclopentadienyl (Cp) rings, the ester bonds result in a rigid conjugated coplanar structure (Figure 2b). Cp ring rotation allows the ester substituents to align, which in turn brings the terminal enes in greater proximity to each other. The conformation-constrained in-plane rotation of substituents with Cp rings increases the probability for the ene ends to collide,43 resulting in effective C

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Figure 1. (a) Proposed mechanism of head-to-tail backbiting to form cyclic ferrocenyl dimer. (b) EI (positive mode) mass spectrum of compound 16.

cyclic compound in this study has a ring of 11 atoms, which is in the medium-ring regime. Increasing the ring size would decrease the strain energies, thus facilitating RCM.51 Synthesis of Main-Chain Ferrocenyl Polymers by ROMP. After the synthesis of cyclic ferrocenyl monomers, ROMP was performed by using Grubbs II catalyst in dichloromethane at room temperature, and the concentration of monomers was in the range of 0.2−0.8 M, depending on their solubility. The feed ratio of monomer to catalyst was fixed at 500:1 for all monomers. Table 2 depicts the results of Table 2. Molecular Weight Information for Main-Chain Polymers from Various Cyclic Ferrocenyl Monomers via ROMP

Figure 2. (a) Ring strain of cyclic ferrocenyl olefins calculated by DFT; the calculation details are described in the Supporting Information. Compound labels are shown in the figure; illustration of cyclopentadienyl ring rotation and the possible chain ends of substituted ferrocenyl structure for (b) ester group, (c) ether group, and (d) methyl acetate. a

methylene spacer in between instead, the now non-coplanar bulky ester groups might hinder the terminal dienes from encountering each other, as shown in Figure 2d. Therefore, 19 and 20 cannot be obtained due to strong steric hindrance, resulting in the highest ring strain among all cyclic ferrocenyl compounds in Figure 2a. Another reason that 19 cannot be prepared via RCM is attributed to its electron-deficient bisacrylate strcuture, which is less favorably to react intramolecularly. When the substituent chain length becomes longer, the steric hindrance is less prominent. The resulting cyclic monomers 21−23 are all trans-dominated isomers with energy-favorable stereoselectivity. Apparently, the increase of substituent chain length favors the RCM reaction. The smallest

polymer

monomer

conv (%)

Mn (Da)

Đ

P12 P13 P14 P16 P17 P18 P21 P22 P23

12 13 14 16 17 18 21 22 23

80 0 70 0 84 89 94 84 91

−a − 36200 − 25700 26900 21800 17300 25000

− − 1.72 − 1.68 1.79 1.73 1.79 1.77

Polymer cannot be dissolved in THF.

ROMP. Monomers 13 and 16 cannot be polymerized under such conditions. The other seven monomers can be polymerized with conversions over 70% and molecular weight ∼25000 Da (Table 2). These results demonstrate that cyclic ferrocenyl monomers are polymerizable even with relatively low strain, which is attributed to an entropy-driven (ED) process, termed as ED-ROMP.53−55 Large hydrocarbon alkenes are usually easier for ROMP due to the larger entropy gained. The extent of ED-ROMP critically depends upon the olefin concentration, i.e., critical equilibrium monomer concentration, [M]c, below which only oligomers are D

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Figure 3. 1H NMR spectra (CDCl3 as solvent) of main-chain ferrocenyl-containing polymers prepared via ROMP.

formed.56,57 Monomers 13 and 16 cannot be polymerized might be due to their poor solubility. As their saturated concentration is only ∼0.2 M, it cannot reach a high critical monomer concentration to polymerize. 1 H NMR spectra in Figure 3 show peaks that are characteristic of main-chain ferrocenyl-containing polymers. The broad peak between 6.1 and 5.3 ppm denotes the olefin structures. The Cp peaks are regulated by the electronwithdrawing ester group to high chemical shift and electrondonating methoxyl groups to low chemical shift. UV−vis spectra show a peak at ∼440 nm originating from characteristic d−d transitions of ferrocene.58 P12 and P14, which have electron-withdrawing esters on the Cp ligands, show redshifted absorption around 450 nm, whereas P17, P18, P21, P22, and P23 with electron-donating groups show a blue-shift to 430 nm, as shown in Figure 4. The electrochemical properties of homopolymers were studied using cyclic voltammetry (CV). All voltammograms were recorded by using tetrabutylammonium hexafluorophosphate as an electrolyte, and the electrochemical potentials were determined versus standard calomel electrode (SCE), as shown in Figure 5. The electron-withdrawing effect of ester

Figure 5. Cyclic voltammogram of homopolymers in dichloromethane vs SCE at a scan rate of 100 mV/s.

groups linked to the Cp ligands (P14) shows a positive shift of the oxidation potential compared to other homopolymers. P17, P18, and P21, with linker length less than or equal to six atoms, reveal two reversible waves characteristics, demonstrating interacting iron centers along the polymer backbone.28,31,59,60 When the linker length is more than six atoms, e.g., P22 and P23, the multiple wave properties are weakened to an indistinguishable level, indicating that these ferrocene moieties interact to a less extent. However, P14 with a linker length equal to six atoms only shows a single wave, which can be attributed to the shielded interaction between metal centers since the linker units are rigid, conjugated ester groups. No data was obtained for P12 due to its insoluble nature. Most earlier reported main-chain ferrocene polymers with only alkyl chains as the linker show poor solubility in common solvents.27,29 In contrast, except for the polymers prepared from monomer 12, all other polymers show good solubility (>200 mg/mL) in common organic solvents like THF and dichloromethane. This is mostly due to the presence of oxygen-containing substituents between ferrocene units as solubilizing groups. This solubility potentially enables the preparation of a variety of polymeric compositions. For

Figure 4. Normalized UV−vis spectra of main-chain ferrocenecontaining polymers in CHCl3. E

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Figure 6. (a) DSC traces of P12. (b) Powder XRD curve of P12.

example, copolymers can be obtained by copolymerization of cyclooctene with cyclic ferrocenyl monomers, in which the molecular weight can reach as high as 100000 Da (see Supporting Information, Table S1). Semicrystalline Ferrocenyl Polymers and Their Use for Crystallization-Driven Self-Assembly. During polymerization of monomer 12, polymer P12 precipitated from the solution. It is a semicrystalline polymer, as demonstrated by differential scanning calorimetry (DSC, Figure 6a) and X-ray diffraction (XRD, Figure 6b). DSC shows an endothermic peak at 169.2 °C with an enthalpy of 41.2 J/g. XRD shows diffraction peaks at 18.7°, with a lattice period of 4.7 Å. From the diffraction, the crystal size was determined to be 45 Å. In comparison with most well-known poly(ferrocenyldimethylsilane) polymers that have a crystallinity of ∼50%,61 P12 shows a higher melting point, smaller crystallite size, and a higher enthalpy of fusion. Optical microscopy also indicates a sperulitic morphology (Figure S1), similar to poly(ferrocenyldimethylsilane) polymers. Semicrystalline polymers have been used to induce crystallization-driven self-assembly (CDSA) in solution if they are an integral segment of a block copolymer. During the CDSA process, the crystalline block drives the selfassembly of the block copolymer to form a micellar core, while the solvophilic corona block stabilizes the formed aggregates.6,21,22,46,47 Although extensive CDSA studies have been well explored, the crystalline cores reported to date are limited to polyferrocenylsilanes,21 polycaprolactone,62 poly(lactic acid),63 polyethylene,64 poly(ethylene oxide),65 polythiophene,66 and polyacrylonitrile.67 Among them, polyferrocenylsilanes are the most well-established. We therefore attempted the synthesis of a block copolymer containing polymer P12, which can induce CDSA. As shown in Figure 7a, we chose poly(5-methoxycyclooctene) as an amorphous block. We prepared a pseudodiblock copolymer by consecutive ROMP of 5-methoxycyclooctene and ferrocenyl monomer 12. This copolymer is probably not a strict diblock copolymer, since chain transfer along the unhindered backbone would probably still occur. We studied CDSA using only one composition of this copolymer, which has molecular weight of 74000 Da with 172 ferrocenyl monomeric units. The copolymer was dispersed in THF, annealed at 110 °C for 1 h, and then cooled to room temperature slowly. The resulting micelles are expected to have a semicrystalline core of P12 and an amorphous corona of poly(5-methoxycyclooctene). Figure 7b shows an oval-shaped platelet morphology with a relatively uniform length of ∼600

Figure 7. (a) Synthetic scheme of ROMP for main-chain ferrocenecontaining block copolymer and 1H NMR (CDCl3 as solvent) spectrum of the block copolymer. (b) TEM micrograph of nanostructures by CDSA of this block copolymer in THF.

nm. The semicrystalline polyferrocene block is expected to pack in a folded structure due to its high energy of crystallization. Then the pseudo-block copolymer preferentially forms a thin-layer platelet morphology with the insoluble polyferrocene block capped by solvated corona layers of the soluble poly(methoxycyclooctene) block.68 The iron-rich core was verified by using energy-dispersive X-ray spectroscopy (EDX), as shown in Figure S3. The presence of well-defined diffraction patterns (Figure S2) indicates the crystalline nature F

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Macromolecules *E-mail: [email protected].

of polyferrocene core, from which the self-assembled platelet structure was demonstrated to be induced by crystallization. Using poly(ferrocenyldimethylsilane) as the core, Manners et al. reported a series of platelet structures, such as lenticular platelet,69−71 tapelike platelet,72 rectangular platelet,73 quasihexagonal platelet,74 and pointed-oval-shaped platelet.75 Both the average size and aspect ratio of our oval-shaped platelets are smaller than the corresponding oval-shaped platelets observed in the poly(ferrocenyldimethylsilane)-based nanostrutures.75

ORCID

Ye Sha: 0000-0003-3338-1228 Yudi Zhang: 0000-0003-0115-9008 Tianyu Zhu: 0000-0001-9115-6462 Shaobo Tan: 0000-0003-4205-8588 Stephen L. Craig: 0000-0002-8810-0369 Chuanbing Tang: 0000-0002-0242-8241 Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Looking forward, the structure−activity relationships observed here suggest strategies, challenges, and potential opportunities in the preparation of main-chain ferrocenyl polymers by ROMP. First, the synthetic accessibility of olefinic macrocycles by RCM is found to depend not only on ring size but also on the substituents linking the terminal alkenes to the Cp rings of the ferrocene. The observed interplay target ring size and Cp substitution patterns on the ring-closing metathesis of ferrocenyl terminal dienes provides a framework by which readily access linkers might be used to influence the structure of olefinic ferrocenyl macrocycles, including the balance between cyclic monomers and dimers and the cis/trans ratio of the alkene. The ability to prepare multiple cyclic ferrocenyl olefin monomers revealed further structure−activity relationships in the ROMP of these monomers into main-chain ferrocene-containing polymers. The resulting variety in backbone structure led to the discovery of a new semicrystalline main-chain ferrocene-containing polymer, prepared from the smallest 11-membered ring monomer. This polymer was then integrated into a main-chain ferrocene-containing block copolymer, which was explored for crystallization-driven selfassembly to prepare ferrocene-containing micellar nanostructures. The ability to use classical ROMP chemistry to fabricate such structures is appealing and might open the doorway to facile elaborations of such nanostructures, although the same level of particle structure is not yet available as can be achieved through existing poly(ferrocenyl) chemistry such as the anionic polymerization of ferrocenylsilanes. Further work will be necessary to reach the similar level of structural complexity, but the initial structure−activity relationships reported here provide a starting point for such investigations. In all, this work demonstrates that RCM can be utilized to access a variety of ferrocenyl monomers as a new entry into the ROMP of mainchain metallocene-containing polymers with rich compositions and potential applications.

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ACKNOWLEDGMENTS This work is partially supported by the National Institutes of Health (Grant R01AI120987 to C.T.) and by the US Army Research Laboratory and the Army Research Office (Grant W911NF-15-0143 to S.L.C).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02064. Synthesis and characterization of ferrocenyl dienes, cyclic monomers and polymers; DFT calculation, crystallization of polymers and crystallization-driven self-assembly, 1H NMR, 13C NMR, and mass spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected].

G

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DOI: 10.1021/acs.macromol.8b02064 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02064 Macromolecules XXXX, XXX, XXX−XXX