Megadalton Macromolecules Made-to-Order in Minutes: A Highly Active Nanosphere Catalyst for Preparing High-Molecular Weight Polymers
Tang Tang,
§Sherilyn J. Lu,
§Guillermo Ahumada, and Christopher W. Bielawski *
Cite This:Macromolecules2022, 55, 9943−9950 Read Online
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sı Supporting InformationABSTRACT: A rhodium(I)-based polymer, [Rh2(cyclooctatetraene)- Cl2]n, was used as a heterogeneous catalyst to prepare stereoregular poly(phenylacetylene) under ambient conditions and within short periods of time. Ultrahigh-molecular weight polymers were obtained when silica and an amine were added to the reaction mixture. The additives promoted catalytic activity as the silica functioned as a support, while the amine induced the depolymerization of the polymer catalyst into its monomeric components. Kinetic analyses in conjunction with molecular weight measurements revealed that the polymerization reaction proceeded in a chain-growth fashion. The
poly(phenylacetylene)s were analyzed using a series of tensile tests and found to be mechanically robust (σ= 20.1 MPa andE= 2.0 GPa). Features intrinsic to the catalyst facilitated the realization of a novel “injection polymerization” method in which passing monomer through catalyst-loaded cartridges rapidly and conveniently afforded high-molecular weight polymer fibers.
C
onjugated polymers, such as poly(acetylene), poly- (alkylthiophene), and poly(p-phenylene vinylene), have been commonly employed in a wide range of electrical, optical, and liquid crystal-based applications.1−5 Among the con- jugated polymers, poly(phenylacetylene) (PPA) has received significant attention due to its high solubility in common organic solvents, excellent processability, and semiconductor characteristics.6,7 The stereochemistry and conformation adopted by PPA also affect its chemical and physical properties.5,8−10Although a range of catalysts for polymerizing phenyl- acetylene (PA) have been developed,11−16those that are based on Rh are advantageous in that they enable the synthesis of polymers in a stereoselective manner17−21 as well as in high molecular weight.13,19,22−25
Ultrahigh-molecular weight (UHMW) PPAs (>106 Da) are particularly interesting due to their pronounced chain entanglements, which can lead to marked increases in durability and other mechanical properties.
UHMW polymers also feature relatively few end-groups, which effectively reduce structural defects that can be detrimental to the physical properties exhibited by such materials.
A common Rh catalyst that is used to polymerize phenylacetylene is the chloro(1,5-cyclooctadiene)rhodium(I) dimer ([Rh(COD)Cl]2; COD = 1,5-cyclooctadiene).26Tang et al. demonstrated that [Rh(COD)Cl]2could be used to obtain stereoregular PPAs under ambient conditions, although the molecular weights of the polymers were relatively low as weight-average molecular weight (Mw) values of up to 46.2 kDa were reported.17 Tabata subsequently showed that
stereoregular PPAs in ultrahigh molecular weights (Mwup to 4443 kDa) were produced when norbornadiene (NBD) was used in lieu of COD as a supportive ligand in the Rh complex (i.e., [Rh(NBD)Cl]2), although long reaction times (20 h) and inert (N2 atmosphere) conditions were required.13 Masuda reported that the time needed to prepare stereoregular PPAs in high molecular weight (Mw up to 766.4 kDa) could be significantly reduced (1 h) by treating an activated monomer, Ph2CCPh(Li), with a mixture of [(tfb)RhCl]2 (tfb = tetrafluorobenzobarrelene) and Ph3P, although an inert atmosphere still appeared to be necessary.19
A key feature of the aforementioned catalysts is the chelating diene-based ligand, which saturates the metal centers and bestows stability.27We hypothesized that the use of a ditopic analogue, such as cyclooctatetraene (COT), should afford an air-stable Rh(I)-based polymer. While such a polymer is known,28it has not been explored as a polymerization catalyst to the best of our knowledge. Polymer catalysts, like heterogeneous catalysts, often exhibit high stabilities and can be isolated for reuse under some conditions.29They can also be depolymerized into their monomeric species,13,30 which
Received: June 21, 2022 Revised: October 20, 2022 Published: November 3, 2022
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should increase accessibility to their catalytically active centers and promote activity.
Herein, we report the synthesis of a heterogeneous catalyst, [Rh2(cyclooctatetraene)Cl2]n, and show that it facilitates rapid access to ultrahigh-molecular weight PPA in a stereoselective manner (Scheme 1). Although the organometallic polymer was
found to be catalytically active, it was discovered that its intrinsic activity can be increased through exposure to an amine. The amine effectively competes with the diene for coordination sites on the Rh metal center in a manner that ultimately facilitates depolymerization and promotes catalysis.
Supporting the catalyst on silica was also found to be beneficial as its inclusion in the reaction mixture afforded PPA that was relatively high in molecular weight. Monitoring the reaction kinetics via 1H NMR spectroscopy and size exclusion chromatography (SEC) revealed that the polymerization occurred in a chain-growth fashion. Overall, the polymerization reactions are straightforward to perform, scalable, and merely require exposure of the monomer (i.e., phenylacetylene) to the catalyst under mild conditions. The practicality of the method was demonstrated through the development of an “injection polymerization” system wherein monomer was passed through syringe filters loaded with the catalyst to afford high-molecular weight polymer fibers within minutes.
To access the polymer complex, [Rh2(cyclooctatetraene)- Cl2]n, a solution of di-μ-chlorotetraethylene dirhodium(I) in CH2Cl2 ([Rh]0 = 0.02 M) was treated with stoichiometric COT (Figure 1). The reaction produced an insoluble yellow- orange solid, which was subsequently collected via filtration in quantitative yield. Field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron micros- copy (HR-TEM) revealed that the solid consisted of uniform microspheres with diameters that ranged from 300 to 500 nm.
The constituent elements (C, Cl, and Rh) were evenly distributed and well dispersed in the microspheres as determined by elemental mapping. Mass signals with m/z
ratios of 623.4, 761.1, and 865.9 were recorded upon analysis of the material using MALDI-TOF mass spectrometry and assigned to fragments of [Rh2(COT)Cl2]n.
Unfortunately, due to its insolubility, a fine structure of the complex could not be elucidated using standard spectroscopic techniques. As such, a small-molecule analogue, the [Rh- (COT)Cl]2dimer, was prepared by mixing excess COT with di-μ-chlorotetraethylene dirhodium(I) precursor in CH2Cl2 and then crystallized by slow evaporation of a saturated solution of the complex in a mixture of CH2Cl2and hexane. X- ray diffraction analysis revealed that the two Rh atoms were coordinated to COT ligands and connected via chloride bridges. The terminal alkenes were uncoordinated and thus capable of binding to additional Rh centers. Indeed, subjecting [Rh(COT)Cl]2to high vacuum resulted in the formation of a polymeric material that gave similar spectroscopic signals as [Rh2(COT)Cl2]n, presumably through the evaporative loss of COT.28Collectively, these results indicated that the structure of [Rh2(COT)Cl2]nfeatures a repeat unit that is structurally similar to the crystal structure obtained from its constituent dimer.
With the [Rh2(COT)Cl2]n complex in hand, a series of preliminary polymerizations were performed; the key results are summarized in Table 1. In an initial reaction (run 1), [Rh2(COT)Cl2]n was added to a solution of PA in CH2Cl2
([PA]0/[Rh]0 = 50; [PA]0 = 0.5 M) and stirred at room temperature. After 20 h, the mixture was poured into excess methanol, which caused orange solids to precipitate; the solids were collected via filtration, dried, and then analyzed.
Inspection of the1H NMR spectrum recorded for the product was consistent with stereoregular PPA that featured a repeating cis−trans geometry (Figure 2a).31,32 The absolute number average molecular weight (Mn) was determined by SEC to be 4.5 kDa with a polydispersity index (Đ) value of 2.4. Based on the strong correlation between intrinsic viscosity and molecular weight in the corresponding Mark−Houwink plots (Figure 2b−d) and that between the radius of gyration and molecular weight (R2 = 0.99), it can be deduced that the polymer obtained is uniform and linear over a wide range of molecular weights.
To enhance catalytic activity, it was reasoned that the addition of an amine may ligate to the polymer catalyst and facilitate depolymerization into relatively more active mono- meric analogues.13,30,33 Indeed, adding triethylamine (TEA) ([Rh]:[TEA] = 1:5) (run 2) improved performance, resulted in high conversion of monomer (96%) over a short period of time (1 min), and also afforded a relatively high-molecular weight polymer (Mn= 114 kDa;Đ= 2.8). In addition, as metal species are often stabilized by silica, which can lead to better dispersions or enhanced catalytic activities,34−37 SiO2 was explored as a support for the polymer catalyst. Adding modest quantities of silica (SiO2:catalyst = 20:1 w/w) (run 3) produced a relatively large polymer (Mn = 282 kDa; Đ = 2.0), while the conversion remained high (91%). The initiation efficiency improved with increasing quantities of TEA, while an opposite trend was observed when larger amounts of SiO2
were used (seeTable S2). Regardless, the addition of amine and silica38was critical to achieve high catalytic activity and a high-molecular weight polymer product.
Subsequent efforts were directed toward refining the polymerization conditions. Decreasing the temperature (0
°C) produced polymers with higher molecular weights (Mn= 441 kDa;Đ = 2.0) and in high conversions (86%). As such, Scheme 1. Polymerization of Phenylacetylene Using Various
Rh Catalysts
Macromolecules pubs.acs.org/Macromolecules Article
further optimizations were carried out at 0°C. Increasing the monomer-to-catalyst feed ratio resulted in an increase in molecular weight of up to 797 kDa but was accompanied with a decrease in conversion. Setting the initial [PA]-to-[Rh] ratio to 150 was determined to be optimal for accessing high- molecular weight polymers. The ratio of Rh to amine was also varied. When the feeds of these two components were equimolar ([Rh]:[TEA] = 1:1), ultrahigh-molecular weight polymers (1027 kDa) were produced, albeit in low conversion (31%). When excess amine ([Rh]:[TEA] = 1:10) was used, monomer conversion increased (91%), although polymer molecular weight was relatively reduced (617 kDa). The initial monomer concentration was also varied. When a relatively high initial concentration was used ([PA]0 = 1.0 M), the polymer obtained from the reaction featured an ultrahigh molecular weight. Under optimal conditions ([PA]0/[Rh]0 = 150; [Rh]:[TEA] = 1:2; [PA]0= 1.0 M), 96% of the monomer
converted to a polymer with a molecular weight of 1293 kDa (Đ = 1.5) in less than 40 min (run 12).39
To gain additional insight into the polymerization mechanism, the kinetics of the reaction were monitored by periodically removing aliquots from an optimized set of starting materials ([PA]0 = 1.0 M; [PA]0/[Rh]0 = 150;
[Rh]:[TEA] = 1:2). Conversions were monitored via1H NMR spectroscopy in CDCl3, and the molecular weights of the polymers that were produced over time were measured via SEC (Figure 3a). Polymer molecular weight was measured to be high (Mn= 1282 kDa;Đ= 1.5), even during the initial stage of the reaction (conversion ≈ 56%). The molecular weight remained relatively constant (Mn≈1300 kDa;Đ≈1.5) as the reaction proceeded (Figure 3b) and ultimately reached a conversion of 96% after 40 min. Collectively, these data indicated that the polymerization proceeded in a chain-growth fashion.
Figure 1.(a) Synthesis of [Rh(COT)Cl]2and [Rh2(COT)Cl2]n. (b) SEM (left) and TEM (right) data that were recorded for [Rh2(COT)Cl2]nat different magnifications (indicated). (c) HRTEM-EDS elemental maps that were collected for [Rh2(COT)Cl2]n. The elements analyzed are indicated. (d) ORTEP diagram of the [Rh(COT)Cl]2dimer with thermal displacement ellipsoids drawn at the 50% probability level.
The role of the amine in the polymerization mechanism was also probed. It was reasoned that the additive facilitates dissociation of the polynuclear complex, [Rh2(COT)Cl2]n, into its monomeric components which, in turn, could accelerate the polymerization reaction (Figure 3c).30,33,40The hypothesis was probed with a series of dynamic light scattering measurements.
Dispersing the Rh(I)-polymer in CH2Cl2 (0.5 mg mL−1)
revealed that the nanoparticles possessed an average radius of 353 nm. However, adding TEA ([Rh]0:[TEA]0= 1:10) caused the average radius to decrease to 336 and 271 nm after 10 and 20 min, respectively. The use of larger relative quantities of TEA ([Rh]0:[TEA]0 = 1:20 and [Rh]0:[TEA]0 = 1:100) reduced the nanoparticle size even further to 276 and 125 nm, respectively, within 10 min. These results may explain why Table 1. Polymerization of PA in Aira
run [PA]0/[Rh]0 [Rh]: [TEA] temp (°C) time convb(%) yieldc(%) Mnd(kDa) Đd Mne(kDa) Đe IVe(dL/g)
1f 50 1: 0 r.t. 20 h 47 18 5 3.4
2f 50 1: 5 r.t. 1 min 96 88 114 2.8 91 3.0 0.60
3 50 1: 5 r.t. 1 min 92 80 282 2.0 221 2.3 0.82
4 50 1: 5 0 1 min 86 79 441 2.0 339 2.6 1.10
5 100 1: 5 0 20 min 95 93 570 2.0 428 2.4 1.27
6 150 1: 5 0 40 min 72 68 797 2.4 645 2.6 1.61
7 200 1: 5 0 60 min 32 28 725 2.3 611 2.5 1.48
8 150 1: 1 0 40 min 31 30 1027 2.1 1012 2.4 1.95
9 150 1: 2 0 40 min 45 43 848 1.9 806 2.2 1.73
10 150 1: 10 0 40 min 91 86 617 2.6 574 2.4 1.41
11g 150 1: 1 0 40 min 42 39 1121 1.9 1081 2.1 1.97
12g 150 1: 2 0 40 min 96 90 1293 1.5 1267 2.3 2.35
aGeneral reaction conditions: CH2Cl2 (total volume = 3 mL); dark; [PA]0 = 0.5 M; SiO2:catalyst = 20:1 w/w. r.t. = room temperature.
bDetermined by1H NMR spectroscopy in CDCl3.cIsolated yield (%) assuming 100% conversion.dDetermined using SEC in THF and reported as their absolute values.eDetermined using SEC in THF using a universal calibration curve with polystyrene as a standard.fNo silica was used.g[PA]0
= 1.0 M.
Figure 2.(a)1H NMR spectrum of a PPA that was obtained by introducing PA to [Rh2(COT)Cl2]n. (b) Plot of refractive index (red) and log molecular weight (blue) versus retention volume. (c) Mark−Houwink plot. (d) Log−log plot of radius of gyration (rg) versus molecular weight.
The molecular weight and polydispersity data for the PPA described in (b) to (d) was determined by SEC:Mn= 1232 kDa andĐ= 2.0.
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adding increasing quantities of TEA to the polymerization reactions resulted in higher conversions of monomer to polymer over the same periods of time (c.f.Table 1, runs 8−
10).
The high molecular weights of the PPAs facilitated the formation of large-area films via solution casting and enabled the assessment of the intrinsic mechanical properties of the
polymers with a universal testing machine. As shown inFigure 4, a tensile strength of 20.1 MPa and Young’s modulus of 2.0 GPa were measured for a high-molecular weight PPA (562.1 kDa). From the tensile stress−strain curve, it can be deduced that the PPA is rigid and brittle, presumably due to its high stereoregularity,41 as the elongation at break did not exceed 2%. Compared to ortho-substituted PPAs, where it has been Figure 3.(a) Size exclusion chromatograms recorded for aliquots taken during various times (indicated) over the course of polymerization of phenylacetylene. Conditions: [PA]0= 1.0 M; 0°C; CH2Cl2; silica was used as a substrate. Conditions: SiO2:Rh(I)-polymer catalyst = 20:1 (w/w);
[Rh]:[TEA] = 1:2. Conversion was determined by1H NMR spectroscopy in CDCl3. (b) Plot ofMnandĐversus monomer conversion. (c) (Top) Depiction of how [Rh2(COT)Cl2]nmay dissociate when exposed to TEA and (bottom) summary of dynamic light scattering data.
Figure 4.(Left) Representative tensile stress−strain curve that was obtained for a film of PPA (Mn= 562.1 kDa,Đ= 2.4). (Right) Photographs of the films.
reported that the tensile strength ranged from 16 to 23 MPa and the Young’s modulus ranged from 0.7 to 1.1 GPa, the PPA films are relatively stiff but exhibit comparable strength.42
Since the polymerization of PA proceeded rapidly under ambient conditions and the catalyst can be supported on silica, a type of injection polymerization system was devised. A dispersion of the Rh(I)-polymer catalyst and SiO2in CH2Cl2
were packed into a PTFE filter. Next, a mixture of PA and TEA in CH2Cl2([PA]0= 1.0 M; [PA]:[TEA] = 1:10) was passed through the cartridge directly into methanol in a controlled manner using a syringe pump (flow rate = 0.2 mL min−1) (Figure 5). The process afforded orange polymer fibers, which were collected and dried.43 A cross-sectional analysis of the fibers using SEM indicated that they were hollow with a diameter of 150μm (c.f. inner diameter of needle = 495μm).
During the initial 10 min of the injection process, polymers with aMnof 135.3 kDa (Đ =2.2) were obtained in high yield
(90%) as determined by dividing the mass of the isolated polymer collected by the theoretical mass calculated from the quantity of monomer introduction over the same period. The yield subsequently began to decrease, although theMn of the polymer remained relatively high (ca. 200 kDa) as the experiment proceeded. Activity was exhausted after 2 h due to leaching (seeTable S5), and a catalyst turnover number of 205 mol PA/mol Rh was calculated. This system can be compared to contemporary flow polymerization techni- ques.44,45 While the latter is typically performed in a series of pressurized tubular reactors zoned for timed reagent delivery, mixing, and chemical reaction, the direct injection approach is relatively straightforward to perform and facilitates access to high-molecular weight polymer fibers in seconds (see Video S1).
In summary, an air-stable heterogeneous Rh(I)-polymer catalyst was synthesized and used to polymerize phenyl- Figure 5.(a) Preparation of a cartridge packed with [Rh2(COT)Cl2]n. A mixture of the polymer catalyst and silica was dispersed in CH2Cl2and then injected into a PTFE filter via a syringe. (b) Reaction vessel setup. The polymerization reaction was performed by passing a CH2Cl2solution of the monomer and amine through the cartridge containing [Rh2(COT)Cl2]ndirectly into excess methanol and produced orange fibers. The reaction was conducted in air and at room temperature. (c) SEM image of a PPA fiber at different magnifications (indicated).
Macromolecules pubs.acs.org/Macromolecules Article
acetylene. The catalyst afforded PPA in a stereoselective manner under mild conditions and within minutes. When used with silica as a support and TEA as a ligand, ultrahigh- molecular weight polymers were obtained. The metal-mediated polymerization reaction proceeded in a chain-growth fash- ion,46,47 and the addition of TEA facilitated catalyst depolymerization into monomer derivatives that displayed relatively high activities. Tensile tests showed that the PPA produced from this method is stiff but mechanically robust.
Finally, a new type of injection polymerization system was developed by packing the heterogeneous Rh(I) polymer catalyst into a PTFE filter and then used to obtain high- molecular weight fibers of PPA in short periods of time.
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ASSOCIATED CONTENT*sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.macromol.2c01284.
General information, synthetic procedures, SEC data, NMR spectra, single-crystal XRD data, DLS data, and mechanical test data (PDF)
Video of the direct injection approach (MP4)
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AUTHOR INFORMATION Corresponding AuthorChristopher W. Bielawski−Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea; Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea;
orcid.org/0000-0002-0520-1982; Email:bielawski@
unist.ac.kr Authors
Tang Tang−Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
Sherilyn J. Lu −Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
Guillermo Ahumada−Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea; orcid.org/0000-0002-1507- 0816
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.2c01284 Author Contributions
§T.T. and S.J.L. contributed equally to this work as co-first authors.
Notes
The authors declare no competing financial interest.
CCDC 1872663 contains the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre.
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ACKNOWLEDGMENTSWe are grateful to the IBS (IBS-019-D01) for its support.
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Macromolecules pubs.acs.org/Macromolecules Article
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