Mechanistic Insights into Water-Mediated Tandem Catalysis of Metal

Jan 14, 2016 - Ironically, during the history of CO2/epoxide coupling development, water was generally considered primarily as an aversion reagent. Th...
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Mechanistic Insights into Water-Mediated Tandem Catalysis of Metal-Coordinating CO2/Epoxide Copolymerization and Organocatalytic Ring-Opening Polymerization: One-Pot, Two Steps, and Three Catalysis Cycles for Triblock Copolymers Synthesis Guang-Peng Wu*,† and Donald J. Darensbourg*,‡ †

MOE Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: The addition of water as a chain transfer reagent during the copolymerization reaction of epoxides and carbon dioxide has been shown as a promising method for producing CO2-based polycarbonate polyols. These polyols can serve as drop-in replacements for petroleum derived polyols for polyurethane production or designer block copolymers. Ironically, during the history of CO2/epoxide coupling development, water was generally considered primarily as an aversion reagent. That is, in its presence, low catalytic activity and high polydispersity was normally observed. Recently, we reported a water-mediated tandem metal-coordination CO2/epoxide copolymerization and organobase catalyzed ring-opening polymerization (ROP) approach for the one-pot synthesis of an ABA CO2-based triblock copolymers. As in previous studies, water was deemed as the chain transfer reagent in this tandem strategy for producing CO2-based polyols. Herein is presented a mechanistic study aimed at determining the intimate role water plays during the metal-catalyzed CO2/epoxide copolymerization process. In this regard, it was observed that under the commonly employed (salen)Co(trifluoroacetate)/onium salt binary catalyst system, water was not the true chain-transfer reagent, but instead reacted initially with the epoxides to afford the corresponding diols which serves as the chain-transfer reagent. The further studies the resultant afforded α,ω-dihydroxyl endcapped polycarbonates were utilized in direct chain extension via ROP of the water-soluble cyclic phosphate monomer, 2methoxy-2-oxo-1,3,2-dioxaphospholene employing an organocatalyst. These triblock copolymers displayed narrow PDI and were found to provide nanostructure materials which should be of use in biomedical applications.

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distinct homopolymer from the end-capped −OH group, since block copolymers are able to self-assemble into microphase separated domains.5 Regardless of such practical applications for polycarbonate polyols, the general requirement of fossilderived hydroxyl or acid containing precursors as the chaintransfer reagents inevitably leads to high production costs and environmental pressure for the scalable process.4b,6,7 In contrast, using water instead of hydroxyl- or acid-containing compounds as chain-transfer reagents represents an exciting way to produce CO2-based polyols with the compelling potential to reduce the production costs for their large scale preparation. On the basis of this idea, Williams and co-workers reported the preparation of poly(cyclohexene carbonate) (PCHC) diols by introducing water as the chain-transfer reagent during the copolymerization of cyclohexene oxide (CHO) and CO2 under magnesium and zinc catalysts.8 After purification, these PCHC diols could be used subsequently as macroinitiators for ring-

INTRODUCTION The synthesis of polymeric materials based on main chains that incorporate renewable resources, thereby replacing totally, or in part, petroleum-based plastics, is of considerable current interest.1 Concomitantly, the use of carbon dioxide as a carbon replacement for the preparation of useful organic chemicals can serve to contribute to a sustainable chemical industry. In this regard, the catalytic transformation of CO2 into polymers by way of alternating copolymerization with epoxides provides a platform for the development of a library of materials with tunable chemical and physical properties.2 Indeed, with indepth mechanistic understanding of the copolymerization process, along with optimal catalyst design, efforts at industrialization of CO2/epoxide derived copolymers are currently underway by several companies throughout the world.3 During this growth in commercial activity, an important driving force is the production of polycarbonate polyols which can undergo condensation reactions with diisocyanates to produce one of the most widely used polymers in modern society, polyurethanes.4 In addition, these CO2-based polyols can be used for constructing nanomaterials by incorporating © XXXX American Chemical Society

Received: December 21, 2015

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DOI: 10.1021/acs.macromol.5b02752 Macromolecules XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Scheme 1. depicts the widely accepted proposal for the mechanistic pathway involving water in the epoxide/CO2 copolymerization process.

opening polymerization (ROP) of lactide using a yttrium catalyst for the preparation of PCHC and polylactide (PLA) block copolymers. Recently, we reported an in situ watermediated tandem catalytic approach for one-pot synthesis of ABA CO2 triblock copolymers (Figure 1).9,10 In this strategy,

Scheme 1

Figure 1. Tandem strategy for synthesis CO2-based ABA triblock copolymer and the related catalysts system, (salen)CoTFA (1)/ PPNTFA = 1/1 (molar ratio), TFA = CF3COO−.

Prior to delineating our recent findings on the role of water in the CO2/epoxide coupling process, we wish to summarize the presumed involvement of water during this process catalyzed by the extensively studied aluminum, chromium, and cobalt porphyrin and salen catalyst systems.11 As indicated in Scheme 1, chain propagation was initiated by the ringopening of the coordinated (activated) epoxide by the nucleophilic anion (X) to afford a metal−alkoxide intermediate. This species then undergoes a successive alternating incorporation of CO2 and epoxide to provide an active metal center containing a growing anionic polymer chain. In the presence of adventitious water, the metal−alkoxide active center is hydrolyzed into a LnM−OH species and a neutral polymer chain with an end-capped −OH group. The released LnM−OH intermediate then serves as an active center in which the −OH nucleophile initiates the alternate incorporation of epoxide and CO2, thereby producing α,ω-dihydroxyl end-capped polycarbonates. This proposal provides a rational explanation of the commonly observed experimental observations; i.e., the two growing polymer chains, one with −OH and −X end groups and the other with two −OH end groups, lead to different molecular weight copolymers which are reflected as a bimodal distribution in the gel permeation chromatography (GPC) traces. Nonetheless, very little direct evidence has been presented to support this mechanism. In an effort to assess the intimate role of water during the CO2/epoxide coupling reaction, we initially performed an infrared spectroscopic study monitoring the process in the presence of the binary complex 1/PPNTFA catalyst system. In a typical experiment, complex 1 (57.3 mg, 80 μmol) and PPNTFA (52.0 mg, 80 μmol) were placed into a housedesigned 25 mL stainless steel reactor (see Supporting Information) under CO2, with the CO2 pressure subsequently increased to 0.3 MPa at ambient temperature. The coupling reaction was initiated upon injecting a solution mixture of propylene oxide (PO) (4.64 g, 80 mmol) and water (72 mg, 4 mmol) into the reactor. As the reaction proceeded, small aliquots of solution were withdrawn from the reactor and subjected to infrared analysis on a Nicolet 6700 FTIR spectrometer in a CaF2 solution cell of 0.01 mm path length. It is worthwhile noting here parenthetically that in contrast to other PPN salts, e.g., PPNCl, the complex 1/PPNTFA catalyst system has excellent solubility in neat epoxide, hence ensuring a homogeneous process at ambient temperature.

water was added along with the propylene oxide (PO)/CO2 copolymerization using the binary catalyst system of complex 1 (salen)CoTFA and PPNYTFA (TFA = trifluoroacetate) to in situ generate α, ω-dihydroxyl end-capped poly(propylene carbonate)s (PPCs). These generated polycarbonate polyols could be directly used as macroinitiator for ROP of cyclic ester or continuing chain extension by introducing the second epoxide/CO2 coupling reaction, thus obtaining well-defined triblock copolymers in one-pot. In these results, water has proved to be a powerful reagent for producing PPC polyols with controlled molecular weight (Mw) and very narrow polydispersity (PDI). The coupled product of PO and CO2 was exclusively PPC with >99% carbonate linkages even in the presence of multiequivalents of water. This observation was in marked contrast to the copolymerization of epoxide and CO2 in the presence of water in previous studies, where the detrimental effects on the copolymerization including low catalytic activity, high PDI was normally observed.11 In fact, since 1964 when the epoxide/CO2 coupling reaction was first reported, extensive experimental studies have shown that water played a significant role in reaction process,12 however, a fundamental description for how the presence of water affects CO2/epoxide polymerization has not been fully understood.13 Because of our early efforts to synthesize CO2 based polyols in the presence of water for the tandem strategy of constructing CO2 block copolymers, we have become interested in the role water plays in this process. Herein, we communicate our initial studies designed to address the mechanistic details of their process. From these studies, it is apparent that water is not the true chain-transfer reagent during the epoxide/CO2 coupling reaction employing complex 1/PPNTFA as catalyst. It appears that water participates in the hydrolysis of the epoxide to produce the corresponding diol which serves as the true effective chain-transfer reagent which controls the molecular weight of the resulting polycarbonate polyols. This hydrolysis reaction involving the epoxide prior to the epoxide/CO2 catalyzed coupling reaction in the presence of complex 1/ PPNTFA was studied by infrared and NMR spectroscopies. Furthermore, we intend to describe the use of this predesigned tandem strategy to directly prepare amphiphilic CO2-based triblock copolymers, thereby adding to the arsenal of these extremely important new polymeric materials. B

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Figure 2. Partial stack plot of IR spectra and the resulting reaction profile at various time points during the PO/CO2 copolymerization in the presence of water under 1/PPNTFA, (PO/H2O/(salen)CoTFA/PPNTFA = 1000/50/1/1, molar ratio; CO2 0.3 MPa). Deconvolution was performed. The absorbance between 3100−1850 cm−1 is omitted for clarity.

Figure 3. Reaction profile and the resulting three-dimensional stack plot of IR spectra collect during the PO/CO2 copolymerization without intentional water added under 1/PPNTFA− (PO/1/PPNTFA = 1000/1/1, molar ratio; CO2 0.3 MPa). The absorbance at 1740 cm−1 is the absorbance of the carbonyl group from the resulting poly(propylene carbonate) copolymer.

these active species initiating the copolymerization process, or (ii) Water was involved in a rapid process suppressing the CO2/PO copolymerization reaction which did not take place until the water was consumed. To probe which postulated scenario is more reasonable, 1H NMR spectroscopy was employed to monitor the reaction between CO2/epoxide/H2O in the presence of catalyst system complex 1/PPNTFA. Figure 4A−C presents stacked plots of 1H NMR spectra displaying the proton signals of the reaction mixture at different times. The reaction conditions were the same as the IR experiments shown in Figure 2. From this NMR analysis, three main features are evident: (1) the characteristic proton resonance of water at 1.56 ppm essentially disappears after a 30 min reaction period, which is consistent with the IR experiment in Figure 2; (2) meanwhile a new set of resonances at δ 3.82, 3.57, 3.34, 2.05, 1.84, and 1.12 ppm with integral of 1/1/1/1/1/3 gradually increase; (3) little to no cyclic propylene carbonate (around 4.7 ppm) or poly(propylene carbonate) (around 5.0 ppm) products were produced during the induction period. Taking the IR and NMR measurements together, these observations clearly suggest that water is first consumed in some sort of a reaction before the CO2/PO copolymerization reaction takes place. Apparently, our initial assessment of the role of water as a chain-transfer reagent shown in Scheme 1 needs to be re-evaluated.9,10 That is, if water follows the pathway shown in Scheme 1 during the epoxide/CO2 copolymerization, the signals of polycarbonate in

Figure 2 displays the three-dimensional profile of the infrared absorbance vs time plots for the reaction. For clarity, only the infrared regions between 1700−1850 cm−1 (νCO2) and 3100− 3800 cm−1 (νH2O) are shown for the reaction proceeding between CO2/PO and water in the presence of complex 1/ PPNTFA, respectively. As is readily apparent from the reaction profile, the copolymerization reaction exhibited an induction period of nearly 0.5 h during which only a very weak νCO2 absorption at ∼1800 cm−1 assigned to cyclic propylene carbonate appeared. Following the induction period, the growth of a νCO2 absorption at 1740 cm−1 due to poly(propylene carbonate) was observed. Of importance, during the induction period for copolymer production there was a dramatic change in the infrared region between 3000−3800 cm−1. That is, the broad absorption band due to H2O around 3600 cm−1 smoothly merged into a sharp single band at 3500 cm−1 at a much faster rate, reaching a maximum at the onset of copolymer formation. For comparison, a control experiment was carried out under the same reaction conditions (complex 1/PPNTFA/PO = 1/1/ 1000 molar ratio, CO2 at 0.3 MPa) but without the addition of water. In this instance, PPC was produced without a significant induction period (see Figure 3). These observations led us to draw two scenarios: (i) The added water was first involved in generating an active species initially during the first 0.5 h, followed by C

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consumed in 0.5 h with no PO/CO2 copolymer being formed. In order to identify the reaction product resulting from the loss of water, the reaction mixture was subjected to column chromatography analysis. The species isolated other than starting reagents proved to be 1,2-propanediol, as characterized by 1H/13C NMR and IR spectra. That is, the characteristic infrared band (3500 cm−1) and 1H NMR resonances (δ 3.82, 3.57, 3.34, and 2.05 ppm) of the isolated 1,2-propanediol were completely consistent with the observations shown in Figures 2 and 4, confirming that 1,2-propanediol was produced during the copolymer induction period.14 Indeed, the formation of 1,2propanediol during the catalytic coupling of CO2/PO in the presence of water is anticipated based on studies of the hydrolytic kinetic resolution (HKR) of epoxides as pioneered by Jacobsen.15 That is, in 1997 the use of a chiral Co(III)X salen complex for HKR of racemic terminal epoxides was reported to afford both enantio enriched epoxides and diols with high activity and selectivity. Considering the high concentration of complex 1 (1/PO/water =1/1000/50) used in this study, it is not surprising that the hydration of propylene oxide was much faster than the CO2/PO copolymerization process.16 To further probe the postulated CO2/epoxide copolymerization process, the coupling of the other most studied oxirane, cyclohexene oxide (CHO), was carried out in the presence of water. The polymerization process was performed in 6.0 mL of neat CHO at ambient temperature under 0.3 MPa of carbon dioxide in the presence of 25 equiv of water using the 1/ PPNTFA catalyst system (1/PPNTFA/PO/water =1/1/500/ 25 molar ratio). The three-dimensional profiles of the infrared absorbances vs time observed during the process are provided in Figure 6, where the focus is again on the infrared regions between 1700−1850 and 3100−3800 cm−1. As noted in Figure 6, approximately a 2 h induction period is required for the production of poly(cyclohexene carbonate) (PCHC) as indicated by the infrared band at 1740 cm−1. During the induction period, PCHC production proceeded very slowly with a weak peak around 1800 cm−1 assigned to trans-cyclic cyclohexene carbonate. The broad absorption band due to water at 3600 cm−1 gradually merged into a sharp single peak at 3450 cm−1 during the first 2 h of the reaction. This infrared band was identified as the characteristic absorbance of 1,2cyclohexene diol, which was isolated and purified from the reaction mixture via column chromatography (see Figure S2 in the Supporting Information). Unlike in the instance of the copolymerization of PO/CO2, because the hydrolysis of CHO proceeded much slower than PO, the two processes, hydrolysis and copolymerization, took place concurrently to some extent during the overall reaction. Nevertheless, as indicated for the PO/CO2 coupling reaction in the presence of complex 1/PPNTFA and water, the hydrolysis of CHO prior to the onset of its significant copolymerization with CO2 was evident.17 The above observations enable us to update our understanding of the mechanistic pathways operative during the copolymerization reactions of CO2/epoxides in the presence of water. This is summarized in Scheme 2, where water is first involved in the (salen)CoTFA-catalyzed hydrolysis of the epoxide to produce the corresponding diol. Subsequently, the diol can serve directly as a chain-transfer reagent in the CO2/ epoxide copolymerization process. The hydroxyl groups of the diol can be deprotonated by the growing anionic polycarbonate chain to afford new initiators to incorporate CO2/epoxide. The

Figure 4. 1H NMR of the reaction solution of 1/PPNTFA/PO/H2O (1/1/1000/50, molar ratio) under 0.3 MPa CO2 at various times (A, 3 min; B, 15 min; C, 30 min) at 22 °C in CDCl3.

IR (at 1740 cm−1) and 1H NMR (at 5.0 ppm) would appear first. This encouraging observation led us to call into question the validity of the mechanistic interpretation of the role of water during the epoxide/CO2 coupling reaction. As previously noted in both the infrared and 1H NMR spectra (Figures 2 and 4), water is shown to be essentially

Figure 5. IR (A), 1H NMR (B), and 13C NMR (C) spectra of the 1,2propane diol isolated from the reaction mixture of PO/CO2 coupling reaction in the presence of water after the reaction proceeded half an hour (Reaction condition: 1/PPNTFA/PO = 1/1/1000/50, molar ratio, CO2 0.3 MPa). D

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Figure 6. Partial three-dimensional stack plot of IR spectra and the resulting reaction profile during the CHO/CO2 copolymerization in the presence of water under 1/PPNTFA, (CHO/H2O/1/PPNTFA = 500/25/1/1, molar ratio). Deconvolution was performed; the absorbance around 3450 cm−1 is the hydroxy group from 1,2-cyclohexanediol, and 1740 cm−1 is the absorbance of the carbonyl group from the resulting poly(cyclohexene carbonate) copolymer. The absorbance between 3100−1900 cm−1 is omitted for clarity.

Scheme 2. Proposed Mechanistic Pathway of the Role of Water in the Predesigned Metal-Catalyzed CO2/Epoxide Copolymerization and Organocatalytic Ring-Opening Polymerization Tandem Strategy

Table 1. Tandem Strategy To Prepare ABA Poly(phosphate-b-carbonate-b-phosphate)sa

entry

Epo

[Cat]/[H2O]/[Epo] (molar ratio)

MP (mmol)

t (h)

convnb (%)

PC/PPE (molar ratio)

Tgc (°C)

Mnd (g/mol)

PDId

1 2 3 4 5

PO PO CHO CHO CHO

1/15/1000 1/15/1000 1/20/500 1/20/500 1/20/500

10 25 22 32 46

0.5 1.5 1.0 2.0 3.5

97.9 99.0 98.0 99.7 99.6

1/1.9 1/5.0 1/4.4 1/6.4 1/9.2

−40/32 −48/30 −35/80 −38/81 −42/81

13000 26000 16000 22000 31000

1.15 1.12 1.08 1.18 1.17

a

Experimental procedure: 1/PPNTFA = 1/1, molar ratio, 0.005 mmol, epoxide (5 mmol) and 1.0 mL toluene/CH2Cl2 (1/1, volume ratio) where a certain amount of water was added into a 10 mL predried autoclave which was pressurized to 1 MPa CO2. The reaction proceeded until full conversion of epoxide took place, then excess CO2 was released, and the autoclave was cooled to 0 °C. After that, 1 mL of CH2Cl2 containing a certain amount of MP was added into the reaction solution, and after 5 min, a certain amount of DBU (0.12 mmol) was in situ added to prepare the triblock polymers at 0 °C. bDetermined by 1H NMR spectroscopy. cDetermined by DSC. dMeasured values by GPC and 1H NMR.

terminated polymer chains can also rapidly recover its chain growth via fast proton exchange with the alkoxide anions on the cobalt catalyst center. Thus, the so-called “immortal polymerization” continues until all of the epoxide monomer is incorporated into the copolymer chain.5,11,18 Given the equal opportunity of the two −OH groups of the diol to incorporate into the chain-growing anion for propagation, the α,ωdihydroxyl end-capped polycarbonates also benefited from the use of the TFA anion in the catalyst system, for it is proven to undergo hydrolysis more easily than other initiating groups.8−10 Once these α,ω-dihydroxyl end-capped polycarbonates are produced, they can serve as macroinitiators for the ring-opening of cyclic ester catalyzed by organic bases, thereby affording predesigned triblock copolymers. These block copolymers could efficiently expand the scope of application of these polymers produced from renewable resources. There have been

key advances in the development of CO2-based block copolymers by introducing polylactides, polylactones, or anhydrides/epoxides as the second block.8,19−23 Nevertheless, these blocks often strongly interact with one another, thereby making them poor choices for microphase separation for the fabrication of functional nanomaterials. Previously, we have reported the preparation of CO2-based amphiphilic triblock copolymers by sequential copolymerization of different epoxides with CO2.10 These amphiphilic block copolymers exhibit attractive properties for biomedical applications; however, their manufacturing process requires complementary chemical transformations for introducing the functional groups. In this report, we have directly constructed CO2-based amphiphilic triblock copolymers using the tandem strategy shown in Scheme 2, where the cyclic phosphate monomer, 2methoxy-2-oxo-1,3,2-dioxaphospholene (MP) is employed as E

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Figure 7. MALDI−TOF mass spectrum of poly(cyclohexene carbonate) end-capped with −OH groups. The polymer was obtained from cyclohexene oxide/CO2 copolymerization with 15 equiv water under catalyst 1/PPNTFA, 96 h, and 1.5 MPa CO2.

polycarbonate-b-polyphosphoester (PPE-b-PC-b-PPE) copolymers. These block copolymers were also well characterized by differential scanning calorimetry (DSC), 1H, 13C, and 31P NMR spectroscopy (see Supporting Information). With these well-defined amphiphilic triblock copolymers in hand, the self-assembly behavior of these materials in water was examined. It was found that when the molar fraction of the PPE exceeded 80% of resulting triblock copolymer, the polymer was well dispersed in deionized water by sonication for 10 min at ambient temperature (entries 2, 4, and 5). The morphology of the resulting nanostructures was characterized by scanning electron microscopy (SEM) and dynamic light scattering (DLS). A solution of the resulting amphiphilic PC−PPE block copolymers samples with a higher concentration than critical micelle concentration (CMC) was spin-coated onto a freshly cleaned silicon wafer for SEM imaging. The PPE-b-PPCb-PPE block copolymer (PPC/PPE = 1/5.0, Table 1, entry 2) self-assembled into near-monodisperse spherical micelles with a homogeneous diameter of 150 nm determined by SEM statistics over 50 particles (Figure 8A), which was in accordance with the DLS results (the average hydrodynamic radius, Rh = 180 ± 15 nm, polydispersity =0.059, Figure 8B). A similar sphere morphology but with much smaller size (20 nm, polydispersity =0.255, Figure 8C) was found for PPE-b-PCHCb-PPE copolymer (PCHC/PPE = 1/9.2, molar ration, Table 1 entry 5), which was further supported by DLS in Figure 8D. With the increase of PPE content (PCHC/PPE = 1/6.4, molar ratio, Table 1 entry 4), a rod shape micelles with hundreds of nanometers in length and a diameter of 35 ± 10 nm with a polydispersity of 0.377 appeared in Figure 8E and 8F. Because of the ready biodegradability of both of the CO2−PC and PPE blocks, as well as the facile nanomaterial construction shown here, we anticipate that these types of block copolymers should be useful in biomedical and other applications.

CO2, various quantities of MP and 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) were added in situ to the reactor to complete the synthesis. These triblock copolymers displayed a unimodal molecular weight distribution with a narrow PDI, indicative of the successful synthesis of polyphosphoester-b-

CONCLUSIONS In summary, an improved understanding of the mechanistic details of water’s role in the PO/CO2 and CHO/CO2 coupling reactions was carried out in this study employing the extensively studied (salen)CoX/PPNTFA binary catalyst

the second monomer. The cyclic phosphate monomer was selected for the construction of amphiphilic triblock copolymers with CO2 polycarbonates because of its ease of preparation, excellent water solubility, degradability, and controlled polymerization behavior in the presence of organic bases.24 The details for the synthesis and resulting copolymers are summarized in Table 1. After quantitative conversion of the epoxide, the polycarbonate polyols with >99% polycarbonate linkages were obtained exclusively. This was demonstrated by the extremely narrow PDI and only one series of peaks in the MALDI−TOF mass spectra of the resulting polymers (Figure 7). In a subsequent step following the careful release of excess

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Figure 8. SEM and DLS results of the self-assemply behavior of the obtained poly(phosphoester-b-polycarbonate-b-phosphoester) in water solution: (A, B) from the block copolymer in Table 1, entry 2, Dh(intensity) = 180 ± 25 nm; (C, D) from the block copolymer in Table 1, entry 5, Dh(intensity) = 22 ± 6 nm; (E, F) from the block copolymer in Table 1, entry 4, Dh(intensity) = 780 ± 15 nm. F

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Macromolecules system. Mechanistic studies using IR and 1H NMR revealed that the CO2/epoxide copolymerization process followed the initial formation of diol via a hydrolysis process. Consequently, water was not the true chain-transfer reagent during the epoxides/CO2 coupling reaction, instead the generated diol served as the true chain-transfer reagent for generating the α,ωdihydroxyl end-capped polycarbonates. Presumably, this mechanistic role of water during the CO2/epoxide coupling reaction should apply to other closely related catalyst systems. On the basis of this understanding, our predesigned tandem strategy for the one-pot synthesis of CO2-based triblock copolymers essentially consisted of three catalysis reaction: hydrolysis, CO2/epoxide coupling reaction, and ring-opening polymerization. By using this tandem strategy, the direct synthesis of amphiphilic polyphosphoester-b-polycarbonate-bpolyphosphoester copolymers with various nanostructures was carried out in one reactor. Given the biocompatibility and biodegradability via hydrolysis under physiological conditions of both polymeric components, a biomedical study of these types of triblock copolymers are currently underway in our laboratories. We also expect that the mechanistic insights into water’s role in CO2/epoxide copolymerization processes and the versatile nanostructures of the CO2−based copolymers provided will facilitate the development of improved catalyst systems, as well as the generation of new materials in the future.

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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.5b02752. Experimental procedures and characterization of polymers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(G.-P.W.) E-mail: [email protected]. *(D.J.D.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Foundation (CHE-1057743) and the Robert A. Welch Foundation (A-0923) are acknowledged for their support. G.-P.W. gratefully acknowledges the support of the “Hundred Talents Program” of Zhejiang University from China.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02752 Macromolecules XXXX, XXX, XXX−XXX