Epoxides

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

A Bifunctional β‑Diiminate Zinc Catalyst with CO2/Epoxides Copolymerization and RAFT Polymerization Capacities for Versatile Block Copolymers Construction Yao-Yao Zhang, Guan-Wen Yang, and Guang-Peng Wu* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Construction of block copolymers is a practical method for modifying the properties of CO2-based polycarbonates (CO2-PCs) in order to meet specific needs. Herein, we report a well-defined single-site β-diiminate zinc complex 1 equipped with the capacities of coordination copolymerization of CO2/epoxide and reversible addition−fragmentation chain transfer (RAFT) polymerization of vinyl monomers. Complex 1 is specifically designed to possess a 3-(benzylthiocarbonothioylthio)propionate (BSTP) initiating group, which enables the controlled ring-opening copolymerization of epoxides and CO2, leaving a polycarbonate with BSTP functional group at the end of the chain. The end-capped BSTP allows direct chain extension via living RAFT polymerization, thus providing a robust method to construct various CO2-based block copolymers in one pot via a tandem catalysis strategy. The structure of 1 is established by single-crystal X-ray diffraction as well as 1H and 13C NMR. By utilizing 1, a wide range of CO2-based block copolymers including poly(cyclohexene carbonate)-block-poly(2-(dimethylamino)ethyl methacrylate) (PCHC-b-PDMAEMA), poly(cyclohexene carbonate)-block-polystyrene (PCHC-b-PS), and poly(cyclohexene carbonate)-block-poly(N-isopropylacrylamide) (PCHC-bPNIPAM) with controlled molecular weight and compositions are prepared. These block copolymers are fully characterized by DSC, 1H NMR, and 13C NMR spectroscopy. Especially, a thermoresponsive PCHC-b-PNIPAM copolymer is for the first time synthesized as a functional nanomaterial. The complex and catalytic process provided here offers a straightforward catalytic access to CO2-based block copolymers.

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properties for materials14−17 and particularly by the promise to manufacture high-value-added materials stemming from their attractive phase separation behavior in the formation of nanomaterials. For example, the amphiphilic CO2-based block copolymers could self-assemble into micelles with diverse morphologies and dimensions in water, exhibiting diverse nanostructure and therefore attractive applications in drug loading/releasing.18−20 By incorporating PPC blocks, Wiesner and Coates reported a well-defined triblock copolymer that could be used to construct ordered complex nanoreactors for advanced catalysis.21 Very recently, we have found that polystyrene-block-poly(propylene carbonate) (PS-b-PPC) could be well-assembled to sub-10 nm domains for the nextgeneration lithography.22,23 By carefully optimizing the stiff and soft domains, CO2-based block copolymers with impressive self-healing capability could also be constructed in our recent study.24 As shown in Scheme 1, three main approaches have been employed for the preparation of CO2-based block copolymers.25 The most direct strategy is via sequential addition of

INTRODUCTION Aliphatic polycarbonates (PCs) produced via alternating copolymerization of CO2 and epoxides have intrigued intensive research attention because of their scientific merit and application potency for mitigating the negative environmental consequences of energy crisis.1 For nearly half a century,2 intensive effort has been directed toward the development of strategies to improve the chemical transformation efficiency as well as the related material performance.3−6 Regardless of such advances, current attempts at the industrialization of this transformation are limited to propylene oxide and its derivatives to produce poly(propylene carbonate) (PPC) polyols.7 This limited scope of the application for CO2-PCs is attributed to their unsatisfactory thermophysical properties and lack of functionalities,8 thus preventing their use in high-valueadded and functional materials. In this context, the search of the approaches to obtain novel materials for new applications is encountering its most rapid development.9−12 Among various strategies for modifying the properties of manufactured plastics in order to meet specific needs, the preparation of block copolymers (BCPs) by incorporating different macromolecular blocks is emerging as a useful method.13 The motivation for construction BCPs comes in part from the improvement in physical and mechanical © XXXX American Chemical Society

Received: March 17, 2018 Revised: April 20, 2018

A

DOI: 10.1021/acs.macromol.8b00576 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Methods Reported for Synthesis of CO2-Based Block Copolymers: (a) Sequential Monomer Addition, (b) Terpolymerization of Cyclohexene Oxide (CHO), CO2, and Cyclic Anhydride with Pre-Rate-Determining Selectivity, and (c) Immortal Copolymerization Using Macro-ChainTransfer Reagent

functional macro-CTA with coordination capacity (such as polyamides), the polymerization could not proceed even extending reaction time because of the tender tolerance of metal catalysts, indicating the great limitation for this methodology.31 Therefore, a more effective strategy that gives CO2-based block copolymer materials with high quality is still desired and remains an outstanding challenge in material science. Considering that the RAFT polymerization is one of the most useful and powerful tools in modern polymer synthesis for their compatibility with most vinyl monomers,44 we are setting out to use this technique to develop a procedurally simple method for constructing functional CO2-based block copolymers. Herein, we report the design and synthesis of a well-defined bifunctional catalytic β-diiminate zinc ((BDI)Zn− BSTP) complex (1, Scheme 2), which integrates the capacities

different epoxides during the CO2/epoxides coupling reaction (Scheme 1a).10,26−28 Via the regio- and enantioselective copolymerization of propylene oxide with CO2, Nozaki’s group reported the first synthesis of stereogradient PPC with higher thermal decomposition temperature than the typical PPCs.29 The second facile synthesis of CO2-based block copolymers is pioneered by Coates and co-workers as shown in Scheme 1b,30 where a third monomer was introduced to copolymerize with epoxides at a much faster rate than CO2. This strategy is particularly suitable for cyclic anhydride monomers, since a few monomers are more reactive for copolymerization with epoxides than CO2.31−35 By replacing cyclic anhydrides with lactones, Williams and Rieger prepared polyester/polycarbonate block and/or gradient copolymers by switching on/off CO2 during the polymerization, respectively.36−38 The main limitation of these two strategies is the well compatibility between the polyester and polycarbonate blocks, which has led to a situation where the materials are hard to microphase separate to be functional materials. Postfunctionalization of these BCPs holds promise in these materials, but the manufacturing process usually suffers from complementary chemical modifications.24 The most commonly used approach for the preparation of CO2-based block copolymers involves a macromolecule with a terminal initiating site that can be used as a chain-transfer agent (CTA) to incorporate epoxides/CO2 (Scheme 1c).39 Lee and co-workers prepared a series of di-/ triblock copolymers which sequentially open a door to diverse materials.40 Analogously, Feng’s group reported a novel noncoordinating anionic system for synthesizing polyolefinblock-poly(cyclohexene carbonate) materials by the use of lithium alkoxide ion pairs.41 Most recently, Xie and Wang utilized a reversible addition−fragmentation chain transfer (RAFT) agent as a bifunctional chain transfer agent and prepared various PPC-based block copolymers by using a porphyrin aluminum chloride/ammonium salt binary catalyst system.42 In similar circumstances, Nakano et al. utilized atom transfer radical polymerization (ATRP) agent as CTA to construct CO2-based block copolymers in a Co−salen catalytic system.43 Although elegant, this method requires a large amount of CTA to promote chain shuttling, thus leading to compromised reactivity and small molecular weight for the reaction. The methodology based on macro-CTA provides a powerful toolbox for the synthesis of CO2-based block copolymers; however, the low blocking efficiency and compromised activity are usually observed during the reaction as demonstrated in our latest study.31 Furthermore, upon

Scheme 2. (a) Synthetic Pathway for (BDI)Zn−BSTP 1 and (b) the Tandem Strategy for CO2-Based Block Copolymers

of controlled metal-coordinated CO2/epoxide copolymerization and the living RAFT polymerization, enabling a robust method to construct various CO2-based block copolymers in one pot via tandem catalysis strategy. The single-site βdiiminate zinc complex 1 is specifically designed to possess a 3(benzylthiocarbonothioylthio)propionate (BSTP) initiating group,45 which enables the controlled ring-opening copolymerization of epoxides and CO2, leaving a polycarbonate with functional BSTP group at the end of the chain. Subsequently, the end-capped BSTP allows direct chain extension via living RAFT polymerization, affording well-defined block copolymers in a single pot. Our catalytic process circumvents the aforementioned limitations and offers a straightforward catalytic access to the formation of CO2-based block copolymers. By using this strategy, a wide range of CO2-based block copolymers with controlled molecular weight, narrow polydispersity (PDI), and different functionalities are constructed efficiently in one pot. For example, poly(cyclohexene carbonate)-block-poly(N-isopropylacrylamide) (PCHC-b-PNIPAM) with a thermoresponsive capability was, for the first time, synthesized as a functional nanomaterial.

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RESULTS AND DISCUSSION The synthesis of well-defined block copolymers requires that a controlled/“living” catalyst system be employed. In addition, the nontoxic catalysts with high activity appear more favorable for the environmental perspective. Since the pioneering work by Coates research group,46−48 the BDI−Zn complexes have attracted enormous attention given their unprecedented catalytic activity for the coordination polymerization of epoxide/CO2 with controlled catalytic behavior.49−51 In particular, the related polymers possess high fidelity of the end-capped initiating groups, such as acetate and alkoxide B

DOI: 10.1021/acs.macromol.8b00576 Macromolecules XXXX, XXX, XXX−XXX

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BDI ligand. The BSTP nucleophilic anion is nearly perpendicular to the coordination plane. Each metal center adopts a slightly distorted tetrahedral geometry, where the corresponding Zn(1)−N(1) (1.987(6) Å) and Zn(1)−N(2) (1.996(5) Å) as well as Zn(1)−O(1) (1.945(5) Å) and Zn(1)− O(2) (1.954(5) Å) bonds differ slightly in length within their standard deviation. The Zn··Zn separation of the dimer is about 4.07 Å, indicating a loosely bound dimer and high activity potency for epoxides/CO2 copolymerization.47 To verify the feasibility of our proposal, the copolymerization of CHO and CO2 was initially carried out (entry 1, Table 1). The reaction was performed in toluene at [CHO]/[1] = 10/1 (mole ratio), 50 °C, and 3.0 MPa of CO2. Both the 1H NMR and MALDI-TOF analyses of the corresponding PCHC oligomer confirm that the BSTP initiating group attaches at the end of the polymer chain (Figure 2). As shown in the 1H NMR spectrum (Figure 2a), the restricted motion of the polymer backbone causes the broadened peaks at Hb (3.64 ppm) and Hc (2.71 ppm), indicating the direct linkage of the BSTP to the end of the rigid PCHC oligomer. Whereas, the signals at 3.60 ppm (He) and 4.60 ppm (Hf) reveal the hydroxyl group on the other chain end.41 Moreover, the molar mass of the polymers can be determined by integration of the corresponding peaks. The high fidelity of the BSTP endcapped PCHC was further substantiated by the MALDI-TOF analysis (Figure 2b), where one main series signal was assigned to [C11H11O2S3 + (C7H12O3)n + C6H11O + H]+ = [270.99 + (142.06)n + 99.08 + 1.00], in spite of some noise signals that were observed on the baseline. Increasing the ratio of [CHO]/ [1] to 200 (entry 2), the quantitative conversion of the epoxide with PCHC with >99% polycarbonate linkages was observed in 3 h, indicating the high reactivity of the catalyst system. In addition, the scope of epoxide could broaden to 4-vinylcyclohexene oxide (VCHO) that contains a pendant double bond group (entry 3), thus holding promise for further postfunctionalization and application potency.50 The gel permeation chromatography (GPC) traces of all the resulted PCHC are monomodal with narrow polydispersity, making a promise for the subsequent RAFT polymerization for the construction of well-defined block copolymers. Previously, we have reported the preparation of CO2-based amphiphilic triblock copolymers by sequential copolymerization of different epoxides with CO2.24,31 These amphiphilic block copolymers exhibit attractive properties for self-healing and biomedical applications. However, their manufacturing process requires complementary chemical transformations for introducing the functional groups or/and suffers from low blocking efficiency. In this study, the BSTP end-capped polycarbonate equipped with the ability to directly introduce hydrophilic block through RAFT polymerization of vinyl monomers (entries 4−8). First, we are interested in PNIPAM since it is widely studied as a typical hydrophilic polymer with a lower critical solution temperature (LCST) close to human body temperature,52 showing great potential in drug loading/ releasing. Further, we have demonstrated that it is impossible to introduce the PNIPAM block using chain transfer polymerization because of the strong coordination interaction between amide groups to the metal center.31 Here, we would like to highlight the use of this strategy to address the abovementioned challenges that observed in chain transfer polymerization. In a representative procedure, after the full conversion of epoxides and release of the CO2, the N-isopropylacrylamide (NIPAM) monomer as well as 2,2′-azobis(2-methyl-

groups. Inspired by this, we postulated that a single-site BDI− Zn complex might combine CO2/epoxide alternating copolymerization and the living RAFT polymerization to provide various CO2-based block copolymers in one-pot tandem catalysis reaction. Our rationale is provided in Scheme 2, the reaction of the β-diimine ligands with ZnEt2 affords the (BDI)Zn−Et complexes, which can be efficiently converted to the zinc propionate analogues (BDI)Zn−BSTP 1 in the presence of 3-(benzylthiocarbonothioylthio)propionic acid (BSTPA). Thus, a single-component system equipped with capacities of coordination copolymerization and RAFT polymerization was delivered. Under 1, chain propagation is initiated by the coordination/activation of epoxide by BSTP to provide a Zn−alkoxide intermediate that undergoes an insertion reaction with CO2 to afford a Zn−carbonate. Successive alternating incorporation of epoxide and CO2 produces linear polycarbonates end-capped with the BSTP group, from where the trithiocarbonate could be activated for RAFT polymerization of vinyl monomers, thus giving the welldefined block copolymers. Catalyst 1 was isolated as a yellow solid and was recrystallized from chloroform with a total 75.9% yield (Scheme 2a). As shown in Figure S1, the 1H NMR measurement in chloroform-d shows characteristic signals (Ha−Hd) corresponding to the BSTP initiating group which is coordinated onto the zinc metal present in 1. Two sets of shifts from 1 were detected (such as Ha at 4.65 and 4.56 ppm, Hb at 3.17 and 3.13 ppm, Hc at 2.21 and 1.92 ppm, Hg at 2.57− 2.52 and 2.34−2.24 ppm, Hh at 1.20 and 1.02 ppm), demonstrating that monomeric species and reversible carboxylate bridged dimeric species both appeared in the system.46 This can be also clearly discerned through 13C NMR, further confirming the concomitant system (Figure S2). The result conforms to the previous studies of Coates and Rieger that the complexes with smaller steric hindrance are likely to adopt a dimeric formation.48,51 The X-ray crystal structure of 1 is provided in Figure 1. The complex 1 is a dimer in solid state with four-coordinate zinc center containing carboxylate bridging while the zinc center is almost in plane with the

Figure 1. ORTEP drawing of 1 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Zn(1)−N(1) 1.987(6), Zn(1)−N(2) 1.996(5), Zn(1)−O(1) 1.945(5), Zn(1)− O(2) 1.954(5), O(1)−Zn(1)−O(2) 113.6(2), O(1)−Zn(1)−N(1) 106.6(2), O(1)−Zn(1)−N(2) 117.9(2), O(2)−Zn(1)−N(1) 117.6(2), O(2)−Zn(1)−N(2) 103.3(2), N(1)−Zn(1)−N(2) 97.2(2). C

DOI: 10.1021/acs.macromol.8b00576 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Tandem Strategy for Block Copolymer Catalyzed by 1a entry

epoxide

[epoxide]/[1] (molar ratio)

1 2 3 4 5 6 7

CHO CHO VCHO CHO CHO CHO CHO

10 200 400 200 200 200 200

vinyl monomer

NIPAM NIPAM DMAEMA styrene

[VM]/[1] (molar ratio)

100/1 200/1 100/1 400/1

t (h)

12 24 24 12

convb (%)

Mn(cal)b (kg/mol)

Mn(GPC)c (kg/mol)

PDIc

Tgd (°C)

95.3 90.0 95.0 96.8

1.7 28.6 66.8 39.4 48.9 38.2 68.9

2.1 31.1 72.7 42.5 51.0 41.3 68.6

1.19 1.11 1.17 1.10 1.09 1.03 1.20

112 110 121/140 140 24/102 103

a

Experimental procedure: in a glovebox, 0.1 mmol of 1 and a certain amount of epoxide were dissolved with 1 mL of toluene in a homemade 50 mL autoclave. The autoclave was pressurized to 3.0 MPa of CO2. The reaction was carried out at 50 °C for 3 h to make sure the full conversion of the epoxide. After the release of CO2, the vinyl monomer (VM) as well as AIBN was directly added into the autoclave, then the temperature was increased to 65 °C. After a certain time, a small amount of the polymerization mixture was removed from the autoclave for 1H NMR analysis. bThe conversion of the vinyl monomer and the Mn(cal) were obtained from 1H NMR spectroscopy. cThe Mn(GPC) and PDI of the final product were determined by GPC against polystyrene standard using THF as eluent. dThe Tg values were obtained from DSC.

Figure 2. (a) 1H NMR spectrum and (b) MALDI-TOF spectrum of PCHC oligomers (chloroform-d, 600M, entry 1, Table 1). The peaks marked with purple circle on the NMR spectrum derive from the BDI ligand; the major signal series on the MALDI-TOF spectrum are assigned to C11H11O2S3(C7H12O3)nC6H11O·H+.

propionitrile) (AIBN) was directly added into the autoclave. Then the RAFT polymerization was carried out in one pot at 65 °C, and the chain extension grows from the end-capped BSTP, thus giving the well-defined block copolymers. The success of synthesis of PCHC-b-PNIPAM block copolymers was supported by GPC and NMR spectroscopy analyses (Table 1, entries 4 and 5). In all cases, controlled copolymerization of epoxides/CO2 was observed, as evidenced by the molecular weight (Mn) being predictable and corresponding closely to the values predicted on the basis of monomer conversion and the ratio of catalyst added (Figure 3). To further test the scope of our method, various vinyl monomers with different hydrophilicity and functional groups were shown to be suitable for polymerization under 1. The universality of our catalyst was evidenced by the well-prepared poly(cyclohexene carbonate)block-poly(2-(dimethylamino)ethyl methacrylate) (PCHC-bPDMAEMA) and typical poly(cyclohexene carbonate)-blockpolystyrene (PCHC-b-PS) with different molecular weight and compositions (entries 6 and 7). These block copolymers were also fully characterized by differential scanning calorimetry (DSC), 1H NMR, and 13C NMR spectra (see Supporting Information). With the well-defined amphiphilic diblock in hand, the selfassembly behavior and thermoresponse of PCHC-b-PNIPAM (entry 5) was examined. The dynamic light scattering (DLS) measurement indicates the decrease in hydrodynamic diameter

Figure 3. GPC analysis of PCHC and the corresponding PCHC-bPNIPAM block copolymers with different molecular weight: (a) PCHC-b-PNIPAM with 51.0 kg/mol (entry 5, Table 1), (b) PCHC-bPNIPAM with 42.5 kg/mol (entry 4, Table 1), and (c) PCHC with 31.1 kg/mol (entry 2, Table 1).

(Dh) of the micelles with increased temperature,53 corresponding to the hydration/dehydration changes of PNIPAM block D

DOI: 10.1021/acs.macromol.8b00576 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. (a) Temperature sequenced DLS measurement of a solution of PCHC-b-PNIPAM (0.15 mg mL−1). (b) Nanoparticle distribution of a solution of PCHC-b-PNIPAM at 25 °C (Dh = 131.9 nm) and 33 °C (Dh = 101.3 nm). (c) Temperature sequenced UV−vis measurement of the transmittance of the solution. (d) Four cycles of reversible change of the transmittance upon cooling/heating between 25 and 45 °C.

CO2-based block copolymers is the focus of our ongoing attention.

(Figure 4a). The shrinking of the PNIPAM shell can be clearly demonstrated by the Dh decreasing from 131.9 to 101.3 nm when the temperature gradually increases from 25 to 33 °C (Figure 4b). The transformation of transmittance with temperature is depicted in Figure 4c, where the transparent PCHC-b-PNIPAM water solution changes into a milky-like emulsion with the increase of the temperature. Four circles of reversible change of transmittance upon cooling/heating between 25 and 45 °C are shown in Figure 4d. The combination of the ready biodegradability of both the PCHC and PNIPAM blocks and the thermal-responsive nanomaterial shown here allows us to anticipate that these types of block copolymers should be useful in biomedical applications.

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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.8b00576. Experimental details and characterization of catalyst and polymers (PDF) Crystallographic data for complex 1 (CIF)

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CONCLUSION In summary, a well-defined single-site β-diiminate zinc catalyst integrating the capacities of coordination copolymerization and RAFT polymerization is designed, synthesized, and characterized. Given the high catalytic activity and great compatibility with functional vinyl monomers of the catalyst, the direct construction of versatile CO2-based block copolymers could be carried out in one reactor with controlled molecular weight and specific functionality (e.g., temperature-sensitive materials). Compared to the common methods applied, our strategy circumvents the complementary postmodification process and provides a more direct and effective methodology to construct functional CO2-based block copolymers. The well biocompatibility of the CO2-polycarbonate and the well-defined responsive self-assembly nanostructures make our strategy hold the promise of providing a potent platform for the biomedical applications, and the development of such advanced

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.-P.W.). ORCID

Guang-Peng Wu: 0000-0001-8935-964X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is acknowledged to National Natural Science Foundation of China (Grant 21674090), Qianjiang Talent-D Foundation (QJD1702025), the Fundamental Research Funds for the Central Universities (2018QNA4056), and the “Hundred Talents Program” of Zhejiang University from China. G.-P.W. greatly acknowledges the help from Prof. Xiao-Bing Lu at Dalian University of Technology and Prof. Donald J. Darensbourg at Texas A&M University. E

DOI: 10.1021/acs.macromol.8b00576 Macromolecules XXXX, XXX, XXX−XXX

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