Letter pubs.acs.org/macroletters
Synthesis of “Necklace” Polymers by Chain-Walking Polymerization Guobin Sun, Jens Hentschel, and Zhibin Guan* Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697-2025, United States S Supporting Information *
ABSTRACT: We report an efficient catalytic synthesis of “necklace” polymers, polyethylene (PE) denpols, utilizing the chain-walking polymerization (CWP). The approach is based on the design of a linear multivalent chain-walking catalyst that can initiate hyperbranched polymerization of ethylene for in situ formation of multiple dendritic PEs covalently tethered to the linear polymer backbone. The simplicity and efficiency of this approach makes it promising for facile preparation of large soluble nanostructures for various potential applications.
Scheme 1. Catalytic Synthesis of “Necklace” PE by CWP
D
endronized polymer (denpol) represents an interesting type of polymer topology that has multiple dendrons covalently attached to a linear polymer backbone.1 When dendrimers instead of dendrons are used, a special type of denpol, referred to as a “necklace polymer”, is formed.1 Synthesis of denpols can be achieved by a graft-to, graft-from, or macromonomer approach.2 Combining these approaches with other polymerization methods,3,4 the structures and properties of denpols can be tailored to meet various criteria.5 Through varying the size and grafting density of the tethered dendrons/ dendrimers, various forms of denpols, ranging from relatively flexible random coil to cylindrical and even globular molecular shapes can be systematically controlled.4 This represents a promising approach toward direct synthesis of shape-persistent macromolecules that mimic not only the size and shape but also the function of biological objects, such as viruses and cells. However, the synthesis of dendrons or dendrimers, by either divergent or convergent strategy, is normally tedious, which limits denpol synthesis and further exploration of their applications. To the best of our knowledge, no catalytic polymerization approach for direct synthesis of denpols has been reported.1,6 Herein, we report an efficient catalytic synthesis of a “necklace” polymer, a polyethylene (PE) denpol, using the chain-walking polymerization (CWP). CWP has been reported7,8 to directly polymerize ethylene into dendritic polyethylenes (PEs) using the Brookhart Pd(II) α-diimine catalyst.9,10 Benefiting from good functionality tolerance of the catalyst, functional dendritic PEs can also be synthesized in one-pot by chain-walking copolymerization of ethylene and polar olefins.11−13 In addition, we developed a cascade CWP for the synthesis of large dendrimer-ondendrimer type organic nanoparticles (NPs).6 In the current study, we apply this catalytic dendritic polymerization to the efficient synthesis of PE denpols. For this purpose, a linear polymer covalently tethered with multiple single-site CWP catalysts is constructed, which subsequently initiates parallel CWP to grow multiple dendritic PEs grafted from the linear polymer backbone, resulting in a “necklace” polymer topology (Scheme 1). © 2012 American Chemical Society
Specifically, we first synthesized a linear norbornene copolymer A carrying multiple silyl ether functionalities via ring-opening metathesis polymerization (ROMP; Scheme 1). Removal of silyl protection groups followed by coupling with butenoate functional groups resulted in polymer C via polymer B, which upon treatment with the CWP catalyst precursor, Pd2, in the presence of NaB(Ar′)4 (Ar′ = 3,5-bis(trifluoromethyl)phenyl), led to covalent attachment of multiple active CWP catalysts onto the polymer backbone to form a linear multivalent chain-walking catalyst (LMVCWC), the polymer D. Finally, exposure of D to low pressure (0.1 atm) of ethylene commences CWP from multiple catalytic sites in parallel, resulting in the formation of PE denpols having a necklace-like topology with multiple dendritic PEs tethered onto the linear polymer backbone. Furthermore, chain-walking copolymerization of ethylene and polar olefins is expected to achieve PE denpols with a broad range of functionalities for various applications.14,15 In the first step of our synthesis, ROMP of substituted norbornenes (NB) was chosen to prepare the linear polymer backbone using the N-heterocyclic carbene-coordinated Grubbs Received: February 9, 2012 Accepted: April 9, 2012 Published: April 18, 2012 585
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ruthenium catalyst [(H2IMes)(3-Br-Py)2(Cl)2RuCHPh].16,17 This catalyst is known for its ability to tolerate a wide range of functionalities and for initiating ROMP of NB and its derivatives to yield polymers with high molecular weight and narrow polydispersity.16,17 For our purpose, TIPS-NB and tBuNB were copolymerized to form functional copolymer PNB (Scheme 2).18
Table 1. SEC Characterization (Mn and PDI) of Various Linear Polymer Precursors (A−C) and the LMVCWC (D)
a
molecular weighta
PNB
A
B
C
D
Mn (× 103 g/mol)a PDIa
286.0 1.12
331.0 1.14
314.7 1.11
332.7 1.12
268.9 1.37
SEC with RI detector in THF (1 mL/min).
C to form some interchain byproducts. 1H NMR data of C shows it contains ∼3.0 mol % of the 3-butenoate functionality. After complexation of C with Pd-2 in the presence of NaB(Ar′)4, 1H NMR indicates all the olefinic moieties on polymer C disappeared, confirming that all the 3-butenoate functionalities were reacted with the active Pd(II) to tether these Pd(II) catalysts. Assuming one 3-butenoate group covalently attaches one Pd(II) catalyst, the distance between two neighboring Pd(II) catalysts would be ∼24 nm for a fully stretched backbone, each polymer chain would contain ∼33 Pd(II) catalytic sites, and ideally, each Pd(II) catalytic site initiates one CWP to form one tethered dendritic PE with maximum of 300 KD number-average molecular weight (Mn).6 Due to sensitivity, the LMVCWC (D) was used directly for subsequent chain-walking polymerization of ethylene without further purification. Exposure to ethylene at 0.1 atm, the LMVCWC D initiates CWP at multiple catalytic sites in parallel, leading to the formation of the PE denpol. According to the mechanism of CWP, ethylene should first coordinate to the palladium(II) center to open the ester chelate. Subsequent migratory insertion of ethylene will commence the growth of a dendritic PE covalently attached to the PNB backbone. Under such polymerization conditions, the CWP is not living and chain transfer reaction10 will occur after each dendritic PE grows to its maximal size, governed by the polymerization kinetics. After chain transfer, the cleaved Pd catalyst can initiate new CWP in the solution to form individual dendritic PE unattached to the PNB backbone. Therefore, the polymerization product is a mixture of PE denpols and single dendritic PEs. Nevertheless, the large difference in molecular weight between the single dendritic PE and the PE denpol allows for easy purification through size exclusion chromatography (SEC) to obtain the pure denpol polymer, DPE-1 (Figure 1, Table 2, and SI, Figure
Scheme 2. Synthesis of the Polynorbornene-Based LMVCWC
Whereas TIPS-NB will provide a functional handle for attaching the CWP catalysts, tBu-NB is introduced to enhance the solubility of the polymer. The PNB backbone is fully reduced to saturation to achieve polymer A in order to avoid undesired complexation of backbone double bonds to the CWP Pd catalyst. Subsequent functionalization steps led to the linear polymers B, C, and D, respectively (Scheme 2). The reason to utilize 3-butenoate instead of acrylate along the polymer backbone in the synthesis of polymer C is similar to the one explained in the previously reported work:6 the separation of the double bond from the ester carbonyl functionality by a methylene group significantly reduces the reactivity of the vinyl group toward radical or nucleophilic addition. It avoids the troublesome gelation problem and eases the overall synthesis. The LMVCWC D carries multiple activated CWP catalysts, which will initiate simultaneous CWP of ethylene to form the “necklace” PE. The details of synthesis and characterization of the polymers can be found in the Supporting Information. Following the route illustrated in Scheme 2, the initial copolymer (PNB with TIPS-NB to tBu-NB molar ratio of 5:95) was synthesized, which was sequentially converted to HPNB (A), HPNB-OH (B), HPNB-BU (C), and finally, LMVCWC (D). As measured by size exclusion chromatography (SEC), all of the linear polymers are of high molecular weight with relatively narrow polydispersities (Table 1 and Supporting Information, Figure S5). Polydispersity of D is slightly higher than those of A−C, and the possible reason might be that, during the reaction of C to D, a tethered Pd(II) might continue to react with the 3-butenoate functionality on another chain of
Figure 1. Left: SEC chromatographs of the PE denpol DPE-1 before (DPE-1 crude) and after purification (DPE-1 purified) by gel-filtration column, in comparison with the single dendritic PE. Right: Dynamic light scattering (DLS) of purified DPE-1 (number-average value).
S6). The small peak before the main peak of the purified DPE1 could be due to the interchain byproduct during the synthesis of D as described above. DPE-1 was characterized by SEC equipped with both refractive index (RI) and MALLS detector (Table 2). Due to its unique denpol structure, the Mn obtained by normal SEC with 586
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Table 2. Characterization of PE Denpols DPE-1 and -2 sample
Mn (× 106 Da)c
PDIc
Mn (× 106 Da)d
PDId
Rgd (nm)
Rhe (nm)
DPE-1a DPE-2b
1.49 1.33
1.54 1.39
4.39 4.55
1.10 1.10
41.2 38.7
28.2 24.0
a
CWP under 0.1 atm of ethylene at r.t. for 20 h. bCWP under 0.1 atm of ethylene at r.t., copolymerizing with tert-butyl-(2, 2-dimethyl-pent4-enyloxy)-diphenylsilane (0.22 M) for 20 h. cSize exclusion chromatography (SEC) with RI detector in THF (1 mL/min). d SEC coupled with multi-angle laser light scattering (MALLS) detector in THF (1 mL/min). eMeasured by dynamic light scattering (DLS, radius, number average) in THF with polymer concentration 1.0 mg/mL.
Figure 2. AFM images of individual dendritic PE (left) and DPE-1 (right), spin-coated from 0.01 mg/mL heptane solutions.
RI detector are significantly lower than the values obtained by MALLS, which provides an absolute Mn of 4.39 million Dalton. If each of the ∼33 Pd(II) catalytic sites on the PNB backbone leads to the formation of one tethered dendritic PE, the estimated Mn for the PE denpol should be ∼10 million Dalton assuming each single dendritic PE with a Mn of 300 KD.6 The discrepancy between the theoretical value and the Mn measured by MALLS could be attributed to the fact that a fraction of the Pd(II) catalysts might decompose or remain uninitiated during the polymerization. To further reveal the shape of the polymer, the radius of gyration (Rg) and hydrodynamic radius (Rh) of DPE-1 were measured by MALLS and dynamic light scattering (DLS), respectively. The shape factor, that is, ratio Rg/Rh, of 1.46 agrees with a linear rod-shape polymer architecture.19,20 This confirms that DPE-1 has been successfully achieved with a linear shape that is characteristic for denpol structures. For many applications it is desirable to have denpols carrying multiple functional groups. To demonstrate that the linear catalyst D is able to catalyze the synthesis of functional PE denpols, chain-walking copolymerization of ethylene and a model polar olefin, a tert-butyl-(2,2-dimethyl-pent-4-enyloxy)diphenylsilane (TBDPS) protected alcohol (T-OH), was tested with D (Scheme 3). The silyl-ether functionalized PE denpol,
separate nanoparticles. In contrast, under identical sample preparation conditions, DPE-1 shows a much more organized structure, with multiple nanoparticles aligned in tandem arrays, showing a bead-on-string topology (Figure 2, right). The AFM images unambiguously confirm the necklace-like denpol structure for DPE-1. The AFM image also indicates some chain entanglement of multiple denpol polymers to form larger networks. The size for the individual “beads” on the “necklace” chain is comparable to the size of single dendritic PE, except that there are also some larger beads distributed along the chain, which most likely result from aggregation (fusion) of neighboring dendritic PEs tethered on the chain. Multichain intertwined topology was also observed. Presumably, both bead fusion and chain intertwining result from the interplay between the unique denpol structure and AFM sample preparation process (see Supporting Information). The AFM sample was prepared by spin-casting a polymer solution on mica. As solvent evaporates during spin-casting, increasing denpol concentration led to chain entanglement, forming the intertwined morphology on the mica surface. The intertwined network topology prevents us from estimating the chain length and number of beads per chain. Furthermore, due to proximity of tethered dendritic PE beads and their soft nature (Tg ∼ −60 °C), as well as the sharp contrast of hydrophobicity/hydrophilicity between the PE beads and mica substrate, aggregation or fusion of adjacent individual PE domains can occur either intramolecularly or intermolecularly, resulting in bigger beads separated by larger spacing. Due to these complications, whereas the AFM imaging provides direct evidence for the formation of necklace architecture, it is difficult to extract quantitative structural information to directly correlate with other quantitative solution characterization data such as 1H NMR, GPC, and DLS. The similar bead-on-string structures were observed for DPE-1 samples casted from a few different solvents (see SI, Figure S2). Finally, AFM image of DPE-2 also shows similar necklace-like topology, confirming its denpol structure (Figure S3). In summary, we have developed a new catalytic approach for facile synthesis of PE denpols using a linear multivalent chainwalking catalyst (LMVCWC). Simply exposing the LMVCWC to low pressure ethylene generates a necklace-like polymer via parallel chain-walking polymerization from the tethered catalytic sites. Similarly, chain-walking copolymerization of ethylene and polar olefins catalyzed by the LMVCWC yields functionalized PE denpols. The resulting PE and functional PE denpols were characterized by a number of techniques including 1H NMR, SEC, MALLS, DLS, and AFM imaging.
Scheme 3. Synthesis of the Functionalized PE Denpols through Chain-Walking Copolymerization
DPE-2, was successfully synthesized with high molecular weight and relatively narrow polydispersity (Table 2). Based on the Mn and 1H NMR data, it is estimated that in average each DPE-2 denpol carries ∼4900 TBDPS-protected alcohol groups. Upon deprotection of the TBDPS groups, the hydroxyl-functionalized polymer can be conveniently functionalized with many other functional moieties. Again, the shape factor Rg/Rh ratios of 1.61 agrees with a linear rod-shaped polymer architecture for DPE-2. Finally, in addition to these solution characterizations, atomic force microscopy (AFM) was used to directly image the polymer chains on substrate (Figure 2). Polymer samples prepared by casting dilute polymer solutions on mica substrates were imaged by AFM in tapping mode. The denpol structures were compared with single dendritic PE NPs made from free CWP catalysts.6 As shown in Figure 2 (left), the single dendritic PE are randomly distributed on the substrate as 587
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(13) Chen, G. H.; Huynh, D.; Felgner, P. L.; Guan, Z. B. J. Am. Chem. Soc. 2006, 128 (13), 4298−4302. (14) Zhu, B.; Han, Y.; Sun, M. H.; Bo, Z. S. Macromolecules 2007, 40 (13), 4494−4500. (15) Lau, K. N.; Chow, H. F.; Chan, M. C.; Wong, K. W. Angew. Chem., Int. Ed. 2008, 47 (36), 6912−6916. (16) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41 (21), 4035−4037. (17) Choi, T. L.; Grubbs, R. H. Angew. Chem., Int. Ed. 2003, 42 (15), 1743−1746. (18) Lee, L. B. W.; Register, R. A. Macromolecules 2005, 38 (4), 1216−1222. (19) Burchard, W. Adv. Polym. Sci. 1983, 48, 1−124. (20) Burchard, W. Adv. Polym. Sci. 1999, 143, 113−194.
These results unambiguously confirm the formation of necklace-shaped denpols. These denpols have excellent solubility in a wide range of organic solvents and can be further functionalized. The polymer architecture can be further tuned by varying the polymerization conditions such as ethylene pressure.7,9 The versatility of CWP and simplicity of the current approach provide facile preparation of other denpol nanostructures for various potential applications.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, including catalyst synthesis and characterizations, polymerization and polymer purification procedures, polymer characterizations by SEC coupled with MALLS, NMR spectra, DLS, and polymer morphology study by AFM. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0456719, CHE-0723497, and DMR-0703988) for financial support. The Alexander von Humboldt foundation is thanked for their financial support through a Feodor Lynen research fellowship for J.H. We also thank Dr. Phil Dennison, Dr. John Greaves, and Dr. Wytze van der Veer for valuable assistance with NMR, mass spectrum, and DLS measurement.
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REFERENCES
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