Ring-opening metathesis polymerization of 18-e Cobalt(I)-containing norbornene and application as heterogeneous macromolecular catalyst in atom transfer radical polymerization.

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Ring-Opening Metathesis Polymerization of 18-e Cobalt(I)-Containing Norbornene and Application as Heterogeneous Macromolecular Catalyst in Atom Transfer Radical Polymerization Yi Yan, Jiuyang Zhang, Perry Wilbon, Yali Qiao, Chuanbing Tang*

In the last decades, metallopolymers have received great attention due to their various applications in the fields of materials and chemistry. In this article, a neutral 18-electron exo-substituted η4-cyclopentadiene CpCo(I) unit-containing polymer is prepared in a controlled/“living” fashion by combining facile click chemistry and ring-opening metathesis polymerization (ROMP). This Co(I)-containing polymer is further used as a heterogeneous macromolecular catalyst for atom transfer radical polymerization (ATRP) of methyl methacrylate and styrene.

1. Introduction Metallopolymers have received wide attention in the past decades due to their applications in the fields of materials and chemistry.[1–5] One of facile strategies to synthesize metallopolymers starts from functional metallocenes.[6–9] Both main-chain and side-chain ferrocene-containing polymers have found numerous applications in a variety of areas.[9–15] As an isoelectronic species to ferrocene, cationic cobaltocenium has recently become a focus of attention.[15–19] We have recently developed a series of sidechain cobaltocenium-containing polymers.[20–24] However,

Dr. Y. Yan, J. Zhang, P. Wilbon, Dr. Y. Qiao, Prof. C. Tang Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, USA E-mail: [email protected]

as the starting point for subsequent functionalization and polymerization, the synthesis of mono-substituted cobaltocenium is tedious and with low yield. Cyclopentadienyl-cobalt-1,3-cyclopentadiene (η4-cyclopentadiene CpCo(I) unit), another isoelectronic 18-electron species to ferrocene and cobaltocenium, can be facilely synthesized. It has some interesting properties due to its relative open structure and neutral state.[25–27] Similarly, Chadha and Ragogna[27] reported the synthesis of a series of neutral cyclopentadienylcobalt-cyclobutadiene-functionalized acrylate and methacrylate monomers, which can be polymerized through free radical polymerization.[27] Compared with the challenge in functionalization of cobaltocenium, cyclopentadienyl-cobalt-1,3-cyclopentadiene with a similar structure to cyclopentadienyl-cobalt-cyclobutadiene can be readily modified through facile nucleophilic addition of un-substituted cobaltocenium.[28–30] Therefore, synthesis of side-chain Co(I) unit-containing polymers

Macromol. Rapid Commun. 2014, DOI: 10.1002/marc.201400365

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DOI: 10.1002/marc.201400365

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could start from cyclopentadienyl-cobalt-1,3-cyclopentadiene-functionalized monomers. In parallel, Wang and co-workers[31,32] reported that the exo-substituted η4cyclopentadiene CpCo(I) can be used as a homogenous molecular catalyst in Co(I)-mediated atom transfer radical polymerization (ATRP). Building on the recent progress, we are particularly motivated to use side-chain Co(I) polymers as a heterogeneous catalyst to prepare polymers via controlled radical polymerization.[31,32] Here, we report the synthesis of Co(I)-containing polymers by ring-opening metathesis polymerization (ROMP), and their use as a heterogeneous polymeric catalyst for ATRP of methyl methacrylate (MMA) and styrene.

2. Results and Discussion To introduce the Co(I) unit into a polymer, an acetylenecontaining η4-cyclopentadiene CpCo(I)[33] was converted to monomer through copper-catalyzed azide–alkyne cycloaddition (CuAAC),[34] as shown in Scheme 1. We prepared monomer 1 containing a norbornene group and a functional Co(I) unit. Due to the potential side reaction between azide group and norbornene double bond,[35] the monomer 1 could not be directly synthesized from the click reaction between norbornene azide and alkyne-containing η4-cyclopentadiene CpCo(I). We first carried out a click reaction between alkyne-containing η4cyclopentadiene CpCo(I) and 2-azido ethanol, followed by an esterification with norbornene carboxylic acid. Figure 1a shows the 1H NMR spectrum of monomer 1. The characteristic triazole proton can be found around 6.9 ppm. The multiple peaks at 6.1 ppm were assigned to the double bond protons from norbornene. Meanwhile, the peaks around 5.3, 4.8, and 2.9 ppm can be assigned to the Cp ring and the 1,3-cyclopentadiene ring from the Co(I) unit. The two triplet peaks at 4.0–4.5 ppm were assigned to the methylene protons. Furthermore, 13C NMR, high-resolution mass spectrum (Figures S3 and S4, Supporting Information), and elemental analysis of organic components

also confirmed the structure and purity of monomer 1. As shown in Figure S3 (Supporting Information), all peaks in 13C NMR were well assigned and matched well with the structure of monomer 1. The high-resolution mass spectrum peak (Figure S4, Supporting Information) at m/z of 421.1209 matched with the theoretical molecular mass of monomer 1 (421.1201). We then carried out room temperature ring-opening metathesis polymerization (ROMP)[36] of monomer 1 with the aid of 3rd generation Grubbs catalyst in dichloromethane (DCM) at room temperature (Scheme 1). As shown in Figure 1a, compared with the 1H NMR of monomer 1, the disappearance of norbornene double bond around 6.1 ppm and its upfield shift to 5.3 ppm (the broad shoulder on the left) demonstrated the successful polymerization.[14,37] The presence of triazole proton at 6.9 ppm and protons from the Cp ring and 1,3-cyclopentadiene ring (5.3, 4.8, and 2.9 ppm) indicated the integrity of η4-cyclopentadiene CpCo(I) unit in the resultant polymer. Meanwhile, UV−vis spectrum (Figure S5, Supporting Information) of the polymer 2 shows similar absorption to the monomer 1, which also demonstrated the integrity of the CpCo(I) unit. The polymer 2 was obtained as a light-orange powder (Figure 1b, inset) after precipitation from diethyl ether. According to 1H NMR conversion analysis, its molecular weight was estimated at 21 000 g mol−1. Furthermore, the elemental analysis results matched well with theoretical calculation, which confirmed no formation of Co(0) in this system. As shown in Figure 1c, the linear relationship between ln([M]0/[M]) and reaction time indicated a controlled/“living” characteristic of the ROMP process. The polymerization achieved 90% conversion within 9 min, highlighting the efficiency of ROMP. As shown in Figure 1d, the observed molecular weight (Mn) by gel permeation chromatography (GPC) also increased linearly with the monomer conversion, while the dispersity is below 1.2 during the polymerization (Figure 1d). The GPC result (Figure 1b) of final polymer 2 gives a unimodal peak with a dispersity of 1.05, indicating the good control of ROMP process.

Scheme 1. Synthesis of 18-e Co(I)-containing monomer and polymer.

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Ring-Opening Metathesis Polymerization of 18-e Cobalt(I)-Containing Norbornene . . .

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Figure 1. a) 1H NMR spectra of monomer 1 and polymer 2 in CDCl3; b) gel permeation chromatography (GPC) trace and optical image (inset) of polymer 2; c) semilogarithmic plot for ROMP process of monomer 1 with the aid of 3rd generation Grubbs catalyst; d) molecular weight (Mn, GPC), dispersity (GPC) versus monomer conversion (1H NMR) for the ROMP process of monomer 1.

around 1.1 and 1.5 V versus Ag/AgCl electrode, which can be attributed to the oxidation of the center Co(I).[38,39] According to thermogravimetric analysis (TGA) (Figure S8, Supporting Information), polymer 2 is stable until 140 °C. At 500 °C, there is 14% residue left, which could be due to the formation of cobalt oxide species.[40] The η4-cyclopentadiene CpCo(I) complex was reported to be a catalyst for ATRP due to its reversible activation of halide initiator.[31,32] However, the molecular catalyst is soluble in either monomer or Table 1. Characteristics of PMMA and polystyrene synthesized by using polymer 2 as solvent, can only be used for homogenous polymerization, which results catalyst (representative data from Figure 2). in polymers contaminated with a color Entrya) Monomer Conversion Mnb Dispersityb) from the catalyst. Therefore, it would [%] [g mol−1] be more desirable if the side-chain η4cyclopentadiene CpCo(I)-containing 1 MMA 15 3200 1.75 polymer can be used as a heterogeneous 2 MMA 22 4400 1.84 macromolecular catalyst. The cata3 MMA 40 13 000 1.69 lytic property of polymer 2 was then 4 MMA 70 36 000 3.42 evaluated using methyl methacrylate (MMA) and styrene as monomers and 1 Styrene 14 4500 1.42 2-ethyl bromoisobutyrate (EBiB) as an 2 Styrene 24 6700 1.69 initiator in toluene (polymerization 3 Styrene 31 10 300 1.85 condition is shown in Table 1). Com4 Styrene 42 13 400 1.92 pared with monomer 1, polymer 2 is a) Conditions: solvent = toluene, no solvent for styrene, temperature = 80 °C, [monomer]: insoluble in MMA, styrene, and toluene [initiator]:[catalyst] = 200:1:1, the amount of catalyst is based on the number of Co(I) (see Figure 2e and Figure S9, Supporting unit; b)Determined by GPC. Information), which indicates the

The obtained polymer 2 shows good solubility in common organic solvents, such as DCM, chloroform, and tetrahydrofuran (THF). It is stable in air for several days and can be stored under N2 at room temperature longer than 30 d without noticeable change, as monitored by 1 H NMR (Figure S6, Supporting Information). As shown in Figure S9 (Supporting Information), the cyclic voltammetry of polymer 2 in DMF with TBAPF6 as a supporting electrolyte displays two irreversible oxidation peaks




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Figure 2. Semilogarithmic plots of polymerization of a) methyl methacrylate and b) styrene using polymer 2 as a heterogeneous catalyst; plots of molecular weight (Mn, GPC) versus monomer conversion (1H NMR): c) for MMA and d) for styrene; e) possible mechanism for the ATRP process by using polymer 2 as catalyst, and the corresponding optical images showing the homogeneous and heterogeneous process catalyzed by monomer 1 (left) and polymer 2 (right), respectively.

heterogeneous nature of polymerization process. As shown in Figure 2a,b, the linear semilogarithmic plots demonstrate that both the polymerization of MMA and styrene displayed “living” characteristic when polymer 2 was used as a catalyst. The polymerization rate was comparable to those molecular catalysts reported in the literature.[31,32] Meanwhile, the molecular weight (Mn) also increases linearly with monomer conversion, as shown in Figure 2c,d. We also attempted to use monomer 1 as a molecular catalyst for the ATRP process. While reasonable control was achieved for styrene polymerization, the polymerization of MMA was very slow, only producing

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30% conversion after 24 h (Table S1 and Figure S10, Supporting Information). However, as shown in Table 1, the dispersity of the resultant polymers is high (ca. 1.5–1.9, Figure 2c,d; Figures S11 and S12, Supporting Information) and increases with the increase of conversion. One of the possible reasons for this peculiar behavior may be related with diffusions of both the macromolecular catalyst and propagating polymer chains. The activation–deactivation process relies on diffusion of propagating radicals onto the heterogeneous catalyst. With the increase of molecular weight of propagating radicals, it would become more challenging for radicals to simultaneously and




Ring-Opening Metathesis Polymerization of 18-e Cobalt(I)-Containing Norbornene . . .

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equally access the catalyst. On the other hand, these radicals may preserve their “living” activities, thus still exhibiting minimal termination reactions. To further confirm the role of Co(I) unit in the catalysis, we carried out several control experiments. First, we evaluated the thermal stability of Co(I) unit during the catalysis. 1H NMR spectra (Figure S17, Supporting Information) show that there is no difference before and after heating Co(I) monomer 1 in d8-toluene at 80 °C for 5 h, the same condition used in the ATRP experiments. After heating, the solution was still clear, no precipitation was formed. Therefore, this molecule is stable at least within the time scale of catalysis in ATRP. Second, to exclude homogeneous components released from the polymer catalyst, we heated the polymer catalyst in toluene at 80 °C for 5 h (the same condition used in the ATRP experiments), filtered the solid, and then attempted the catalysis without the polymer catalyst in the above treated solvent. However, no polymerization was observed even with a longer time of reaction (18 h) (Figure S18A, Supporting Information). In addition, the polymer catalyst exhibited almost identical GPC traces before and after heating (Figure S18B, Supporting Information), indicating its good stability. Finally, to exclude the possibility of formation of Co(0) nanoparticles (NPs), we carried out a Hg poisoning test.[41] All the reaction conditions were as the same as the ATRP experiment, except at the middle of the reaction, 1000 equiv. of mercury was added. If there is some catalytic Co(0) NPs in the system, the addition of mercury would form amalgam and poison the catalyst, and thus slow down or stop the polymerization. However, as shown in Figure S19 (Supporting Information), the polymerization still kept going after the addition of mercury. Thus, we believe that there was no Co(0) NPs formed. All these control experiments suggested that there are no other species that could work as catalysts, except for the Co(I) unit during the ATRP process. Wang and co-workers[31,32] proposed that the halide and Co(I) unit undergoes a redox process involving atom transfer during activation and deactivation steps. To further confirm or disapprove the mechanism as shown in Figure 2e, we carried out a radical trap experiment that is designed to obtain the intermediate during the redox process.[42] We mixed monomer 1, initiator EBiB, and TEMPO together and kept the system at 60 °C for 5 h. After the reaction, we purified the product and characterized it by 1 H NMR. As shown in Figure S13 (Supporting Information), 1H NMR spectra show that protons from the η4CpCo(I)-η5Cp unit in monomer 1 disappear, with the appearance of new downfield peaks at 7.2–7.8 ppm, which most likely indicate the formation of oxidized η4Cp-Co(II) Br-η5Cp unit, as the integration areas perfectly match with each other. There is still one puzzling piece, we are


not clear: the integration of double bond protons from norbornene is off the theoretical value. Due to the heterogeneity of polymerization process using polymer 2 as the catalyst, the catalyst polymer 2 can be easily removed from the polymerization system by filtration. The obtained PMMA and polystyrene do not have any color from the catalyst and are much whiter than the light green polymer obtained with the use of monomer 1 as catalyst (Figure S15, Supporting Information). To further demonstrate the “living” nature of the polymerization catalyzed by polymer 2, a block copolymer PMMA-b-PSt was also successfully synthesized through ATRP by using polymer 2 as a heterogeneous macromolecular catalyst. There was a clean shift toward higher molecular weight (from 10 400 to 22 000 g mol−1) in GPC traces, while the dispersity increased from 1.36 to 1.80 (Figure S16, Supporting Information). The 1H NMR spectrum of the copolymer confirmed the formation of a block copolymer.

3. Conclusions In conclusion, we synthesized a neutral 18-electron exo-substituted η4-cyclopentadiene CpCo(I) complex functionalized with norbornene group through click chemistry. Controlled/“living” polymerization was carried out by ROMP with the aid of a 3rd generation Grubbs catalyst at room temperature. Furthermore, this polymer was used as a heterogeneous macromolecular catalyst for the Co(I)mediated ATRP of MMA and styrene, yielding polymers free of catalyst color.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: The support from the National Science Foundation (CHE-1151479) is acknowledged. Received: June 30, 2014; Revised: August Published online: ; DOI: 10.1002/marc.201400365

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