Multiarm Polycarbonate Star Polymers with a Hyperbranched

Aug 21, 2017 - Multiarm star copolymers, consisting of hyperbranched poly(ethylene oxide) (hbPEO) or poly(butylene oxide) (hbPBO) polyether copolymers...
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Multiarm Polycarbonate Star Polymers with a Hyperbranched Polyether Core from CO2 and Common Epoxides Markus Scharfenberg,†,§ Jan Seiwert,†,§ Maximilian Scherger,† Jasmin Preis,‡ Moritz Susewind,‡ and Holger Frey*,† †

Institute of Organic Chemistry, Organic and Macromolecular Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany ‡ PSS Polymer Standards Service GmbH, In der Dalheimer Wiese 5, 55120 Mainz, Germany S Supporting Information *

ABSTRACT: Multiarm star copolymers, consisting of hyperbranched poly(ethylene oxide) (hbPEO) or poly(butylene oxide) (hbPBO) polyether copolymers with glycerol branching points as a core, and linear aliphatic polycarbonate arms generated from carbon dioxide (CO2) and epoxide monomers, were synthesized via a “core-first” approach in two steps. First, hyperbranched polyether polyols were prepared by anionic copolymerization of ethylene oxide or 1,2-butylene oxide with 8−35% glycidol with molecular weights between 800 and 389,000 g·mol−1. Second, multiple arms were grown via immortal copolymerization of CO2 with propylene oxide or 1,2-butylene oxide using the polyether polyols as macroinitiators and (R,R)-(salcy)-CoCl as a catalyst in a solvent-free procedure. Molecular weights up to 812,000 g·mol−1 were obtained for the resulting multiarm polycarbonates, determined by online viscometry with universal calibration and 1H NMR. Comparing the synthesis of different multiarm star polycarbonates, a combination of a highly reactive macroinitiator with a less reactive epoxide monomer was found to be most suitable to obtain well-defined structures containing up to 88 mol% polycarbonate. The multiarm star copolymers were investigated with respect to their thermal properties, intrinsic viscosity, and potential application as polyols for polyurethane synthesis. Glass transition temperatures in the range from −41 to +25 °C were observed. The intrinsic viscosity could be adjusted between 5.4 and 17.3 cm3·g−1 by varying the ratio of polyether units and polycarbonate units.

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date.14−18 They have been tested successfully with regard to their properties for controlled release of drugs.17 However, the synthesis of the respective cyclic carbonate monomers, commonly substituted trimethylene carbonates, requires multiple steps, presenting a drawback for scale-up and eventual application. This disadvantage motivates the development of a synthesis of multifunctional polycarbonates based on CO2. Branched and linear polymers can be combined in multiarm star copolymers with a branched core and linear arms. Due to the unique architecture, these materials can feature both polymer and colloidal properties19 and possess a large number of functional end groups. Multiarm star copolymers can be generated by “arm-first” or “core-first” strategies.20−24 In the “arm-first” approach, prefabricated linear polymers are either attached to a multifunctional core or interconnected via crosslinking of one end group. “Core-first” approaches rely on multifunctional, branched macroinitiators to graft linear arms. Hyperbranched polyether polyols have been employed as suitable macroinitiators for various types of arms, as they tolerate most polymerization techniques.25−28 Recently, our group introduced hyperbranched aliphatic polyethers based on

INTRODUCTION Carbon dioxide (CO2) is a nontoxic, sustainable, and renewable C1 building block that is available inexpensively in almost unlimited quantities. However, its thermodynamic stability and low reactivity limit the use of CO2. Since the discovery of the immortal copolymerization of carbon dioxide with epoxide monomers by Inoue et al., CO2 was also established as a feedstock for aliphatic polycarbonates.1,2 Currently, the industrial production of these polymers has a modest volume of 1000 tons/year, albeit with an increasing trend.3 Mostly propylene oxide and cyclohexene oxide are used as comonomers in combination with CO2 for the immortal polymerization, resulting in biodegradable materials.3,4 However, the resulting linear poly(propylene carbonate) and poly(cyclohexyl carbonate) exhibit glass transition temperatures (Tg) above room temperature, which are limiting for some areas of application, such as flexible polyols for polyurethanes.4 Besides the addition of flexible side chains, the introduction of branching points into the polycarbonate backbone can be used as a synthetic tool to decrease the Tg and to obtain unusual rheological and mechanical properties.5−13 Except for a single example of branched polycarbonate oligomers based on carbon dioxide and glycidol, only hyperbranched aliphatic polycarbonates prepared by the ringopening of cyclic six-membered carbonates are known to © XXXX American Chemical Society

Received: May 31, 2017 Revised: July 26, 2017

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DOI: 10.1021/acs.macromol.7b01131 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Synthetic scheme for hb(PG-co-PEO)-g-PBC star copolymers from BO and CO2 with hb(PG-co-PEO) core.

Table 1. Overview of the Characterization Data for All Multiarm Star Copolymers sample

compositiona

Mnb (g·mol−1)

Đb

Mna (g·mol−1 )

unit ratioc,d

1 2 3 4 5 6 7 8 9

hb(PG9-co-PEO16)-g-PBC168 hb(PG9-co-PEO16)-g-PBC92 hb(PG4-co-PEO15)-g-PBC50 hb(PG5-co-PEO10)-g-PBC25 hb(PG5-co-PEO57)-g-PBC20 hb(PG1250-co-PEO6750)-g-PBC3640 hb(PG4-co-PEO45)-g-PPC349 hb(PG4-co-PBO10)-g-PPC146 hb(PG4-co-PBO10)-g-PBC66

8,800 6,500 10,600 3,300 3,900 68,800 21,100 8,900 5,200

1.69 1.55 1.37 1.64 1.62 1.13 1.55 1.90 2.32

20,800 12,000 6,800 3,800 5,300 812,000 38,000 16,000 8,700

7.40 4.05 2.99 2.02 0.33 0.46 7.53 10.98 4.93

a Terminology: Indices represent the absolute number of the respective repeating units (rounded to integer), determined by 1H NMR spectroscopy and online viscometry with universal calibration. bDetermined by SEC in DMF calibrated with a PEO standard. cDetermined by 1H NMR spectroscopy. dRatio of polycarbonate (arms) and polyether (core) repeating units; polyether units are normalized to 1.

polyether cores (hbPEO and hbPBO) and two epoxide monomers (propylene oxide and butylene oxide) for the copolymerization with CO2. We investigated the resulting multiarm star copolymers with respect to their thermal properties, intrinsic viscosity, and their potential as polyols for polyurethane synthesis. Both low and high molecular weight star copolymers in the range from 3,800 to 812,000 g·mol−1 were synthesized, demonstrating the broad scope of this approach.

the anionic ring-opening copolymerization of the inimer glycidol with ethylene oxide, propylene oxide, and 1,2-butylene oxide, respectively. The one-step synthesis procedures provide access to hyperbranched polyether polyols with an adjustable branching pattern, solubility, and number of addressable hydroxyl end groups.12,29−32 In contrast, the few examples of star- and H-shaped aliphatic polycarbonates synthesized from CO2 and epoxides known to date are purely hydrophobic materials. Furthermore, starshaped polycarbonates with low numbers of arms (3, 4, and 6) exhibit glass transitions above room temperature, comparable to those of linear poly(propylene carbonate) (PPC).33−36 Multiarm star polycarbonates have hardly been studied. In a first communication, our group reported a multiarm star with PPC arms. In this work, the star copolymers had to be fractionated to characterize the properties of the pure starshaped copolymer separately from polycarbonate homopolymer side product.5 In the current work, the core-first synthesis of amphiphilic star-shaped polyether polycarbonates from hyperbranched poly(ethylene oxide) (hbPEO) cores and poly(butylene carbonate) (PBC) arms is introduced (Figure 1). We present a systematic comparison of two types of hyperbranched

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EXPERIMENTAL SECTION

Materials, instrumentation, and further synthetic procedures are described in the Supporting Information. Synthesis of Polyether−Polycarbonate Star Copolymers. A typical polymerization was performed as follows for both types of cores as well as for both epoxides. A 100 mL Roth autoclave was dried under a vacuum at 50 °C. hb(PG9-co-PEO16) (190 mg) was dried in a high vacuum for 2 d, and BO (1.5 mL, 17 mmol), (R,R)-(salcy)-CoCl (9.1 mg, 0.014 mmol), and [PPN]Cl (8.4 mg, 0.014 mmol) were combined with a stir bar inside the autoclave. The mixture was stirred under 50 bar and 30 °C for 3 d. The crude product was dissolved in 5 mL of acetone and quenched with 1.0 mL of 5% HCl solution in methanol. Subsequently, the solution was precipitated in cold methanol. The solid product was dried in a vacuum for 24 h; yield B

DOI: 10.1021/acs.macromol.7b01131 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 75%. Dichloromethane was used as a solvent for samples prepared from cores with a molecular weight of 389,000 g·mol−1. Purification was carried out by dialysis in CH2Cl2 (MWCO = 1000 Da). hb(PG-co-PEO)-g-PBC (Table 1, samples 1−6): 1H NMR (CD3CN-d3, 400 MHz): δ (ppm) = 4.87−4.76 (CH PBC backbone), 4.38−3.92 (CH2 PBC backbone), 3.79−3.40 (polyether core), 3.01 (OH PBC), 1.74−1.55 (CH2), 1.53−1.32 (CH2 terminal unit), 1.09− 0.83 (CH3). 13C NMR (CD3CN-d3, 100 MHz): δ (ppm) = 155.88− 155.38 (−O−CO−O−), 77.90−77.65 (CH PBC backbone), 72.41 (CH2 terminal PBC backbone), 71.79−70.06 (polyether core), 69.44− 68.15 (CH2 PBC backbone), 26.97 (CH2 terminal unit), 24.26 (CH2), 10.13−9.69 (CH3). hb(PG-co-PEO)-g-PPC (Table 1, sample 7): 1H NMR (CD3CN-d1, 400 MHz): δ (ppm) = 5.30−4.69 (CH PPC backbone), 4.32−3.88 (CH2 PBC backbone), 3.74−3.42 (polyether core), 1.33−1.18 (CH3), 1.12−1.07 (CH3 terminal group). hb(PG-co-PBO)-g-PPC (Table 1, sample 8): 1H NMR (CD3CN-d3, 400 MHz): δ (ppm) = 5.02−4.89 (CH PPC backbone), 4.77−4.63 (CH PPC backbone terminal unit), 4.30−4.04 (CH2 PPC backbone), 4.02−3.88 (CH2 PPC backbone terminal unit), 3.77−3.19 (polyether core), 3.02 (OH PPC) 1.78−1.38 (CH2 polyether core), 1.36−1.14 (CH3 PPC), 1.14−1.06 (CH3 PPC terminal unit), 0.99−0.81 (CH3 polyether core). hb(PG-co-PBO)-g-PBC (Table 1, sample 9): 1H NMR (CD3CN-d3, 400 MHz): δ (ppm) = 4.88−4.75 (CH PBC backbone), 4.75−4.63 (CH PBC backbone terminal unit), 3.40−3.86 (CH2 PBC backbone), 3.77−3.18 (polyether core), 2.98 (OH PBC) 1.75−1.56 (CH2 PBC), 1.56−1.25 (CH2 polyether core), 0.99−0.79 (CH3 PBC and polyether core).

reactivity on the success of the grafting of the polycarbonate arms. It is known that 1,2-butylene oxide reacts significantly slower than propylene oxide in the anionic homopolymerization.40−42 Differences in reactivity can also be observed for the catalytic copolymerization with CO2. It has to be emphasized that, due to the low glass transition temperatures, amorphous nature, and high flexibility of the hyperbranched polyether polyols, the catalytic epoxide/CO2 polymerizations were performed in bulk, with the epoxide monomers representing good solvents for both the hyperbranched polyether polyols and the multiarm star polycarbonates formed. Only in the case of the copolymerization of CO2 and BO using a hbPEO core with a very high molecular weight of 389,000 g·mol−1, dichloromethane had to be added as a solvent. This high molecular weight polyether polyol was soluble in PO but not in BO. (R,R)-(salcy)-CoCl and bis(triphenylphosphine)iminium chloride ([PPN]Cl) were used as a catalyst and cocatalyst. (Salcy)-CoX catalyst systems are well-established for the immortal polymerization.43,44 All multiarm star polycarbonates were precipitated in ice-cold methanol. The insolubility in methanol demonstrates the transformation of the hydrophilic hbPEO cores to hydrophobic products or, more precisely, to an amphiphilic star-like structure with a hydrophilic core and a hydrophobic shell. By changing the ratio of monomer and hyperbranched polyol initiator, different polycarbonate arm lengths were targeted. Furthermore, the glycidol content of the cores was varied, also resulting in different monomer/initiator ratios, which is due to the different numbers of hydroxyl end groups. SEC and 1H NMR spectroscopy characterization data as well as the calculated composition of the multiarm star copolymers prepared are summarized in Table 1. The formation of cyclic carbonates, a typical side reaction in the copolymerization of CO2 with epoxides, occurred only to a very low extent, when using butylene oxide. In the case of propylene oxide, no cyclic carbonates were formed at all. The small amount of this side product was confirmed by FT-IR spectra of the crude products (Figures S1−S4, Supporting Information). Besides the band at 1740 cm−1 that can be clearly assigned to the linear polycarbonate backbone, a small signal at 1804 cm−1 occurred when using BO as a monomer. This signal results from a small amount of cyclic carbonate byproduct. These impurities were conveniently removed by precipitation in methanol, as demonstrated by FT-IR and other characterization techniques (Supporting Information). In addition to the FT-IR analysis, the samples were also characterized by 1H NMR, 13C NMR, and 2D NMR spectroscopy (correlation spectroscopy (COSY), hetero single quantum coherence (HSQC), and heteronuclear multiple bond correlation (HMBC)) with regard to their composition and their structure. Figure 2 shows a typical 1H NMR spectrum of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2). Besides the hbPEO backbone signal (3.79−3.40 ppm), the signals of aliphatic 1,2-poly(butylene carbonate) are visible at 4.87−4.76 ppm (methine backbone), 4.38−3.92 ppm (methylene backbone), 1.74−1.55 ppm (methylene side chain), and 1.09−0.83 ppm (methyl side chain). Furthermore, the methylene group in the side chain (1.53−1.32 ppm) and the hydroxyl group (3.01 ppm) can be assigned. Capitalizing on the signals of the polycarbonate units, the signals of the polyether core, and the known absolute molecular weight of the core, determined by online viscometry, both the ratio of polycarbonate units in the arms and polyether units in the core and the absolute number

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RESULTS AND DISCUSSION A. Synthesis and Characterization of Polycarbonate− Polyether Multiarm Stars. We aimed at the synthesis of welldefined multiarm star architectures exclusively derived from simple epoxide monomers and carbon dioxide. Four series of aliphatic multiarm star polycarbonates were targeted by grafting two types of polycarbonate arms from two different multifunctional hyperbranched polyether polyol cores. Control over the star architectures was targeted by using the so-called “immortal copolymerization” of CO2 with epoxides, with the polyether polyols acting as transfer agents in the way of multifunctional initiators.37,38 Hydrophilic hyperbranched poly(ethylene oxide) (hbPEO) and hydrophobic hyperbranched poly(1,2-butylene oxide) (hbPBO) copolymers with molecular weights ranging from 800 to 389,000 g·mol−1 and moderate dispersities in view of the batch synthesis (Đ = 2.00−3.40) were employed as initiators, respectively. These branched polyether copolymers were synthesized via anionic ring-opening multibranching copolymerization of the respective alkylene oxide monomer with glycidol as a branching unit, relying on procedures reported recently.31,32 Both types of polyether polyols were highly soluble in propylene oxide and 1,2-butylene oxide. Variation of the glycidol content yielded polyols with different degrees of branching (DB) and thus different hydroxyl functionality. It should be noted that hbPEO polyols with low glycidol content mainly possess primary hydroxyl moieties from terminal ethylene oxide units, whereas hbPBO polyols merely contain secondary hydroxyl groups from terminal butylene oxide units.39 The different repeating units of the hyperbranched polyether polyols (i.e., linear, dendritic, and terminal structures) can be determined from inverse gated (IG) 13C NMR spectroscopy (Supporting Information, Table S1).29,32 Propylene oxide and 1,2-butylene oxide, respectively, were used as comonomers for the immortal polymerization with CO2 to study the influence of the respective epoxide and its C

DOI: 10.1021/acs.macromol.7b01131 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. 1H NMR spectrum of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2) (300 MHz, CD3CN). Figure 4. SEC results of (A) hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2), (B) hb(PG4-co-PEO45)-g-PPC349 (Table 1, sample 7), (C) hb(PG4-co-PBO10)-g-PBC66 (Table 1, sample 9), and (D) hb(PG4-coPBO10)-g-PPC146 (Table 1, sample 8) and their macroinitiators in DMF.

of polycarbonate units were calculated. An overview of the calculated characteristics is given in Table 1. The 13C NMR spectrum of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2) is shown in Figure 3. Signal h (155.88−155.38 ppm), signal g (77.90−77.65 ppm), signal d (69.44−68.15 ppm), signal b (24.26 ppm), and signal a (10.13−9.69 ppm) can be assigned to the poly(butylene carbonate) chains. The signals in the range 71.79−70.06 ppm result from the hyperbranched polyether core. Similar to the 1H NMR spectra, resonances of the terminal carbonate unit are visible in the 13C NMR spectra. Signal f (72.41 ppm) and signal c (26.97 ppm) can be clearly distinguished from other signals, whereas the methine group signal of the terminal unit overlaps with the polyether signal. The 2D NMR spectra of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2) and the NMR spectra of further multiarm star copolymers with different core and epoxide combinations (hb(PG4-co-PEO45)-g-PPC349, hb(PG4-co-PBO10)-g-PPC146, hb(PG4-co-PBO10)-g-PBC66) are listed in the Supporting Information (Figures S5−S10). Size exclusion chromatography (SEC, linPEO standards) yielded molecular weights between 3,300 and 68,800 g·mol−1 with Đ (M̅ w/M̅ n) values between 1.13 and 2.32. All polyether polycarbonate star copolymer samples exhibit a lower elution volume than the macromolecular initiator, corresponding to a higher molecular weight. Figure 4A shows the distribution of

hb(PG9 -co-PEO 16 )-g-PBC 92 (Table 1, sample 2). It is monomodal, apart from a small shoulder at lower molecular weights. The elugrams of other hbPEO-g-PBC samples are listed in the Supporting Information (Figure S12) and exhibit comparable molecular weight distributions. In contrast, grafting poly(propylene carbonate) arms from hbPEO cores generally resulted in bimodal distributions (Figure 4B). Using hbPBO as a macroinitiator leads to polymodal distributions independent of whether poly(propylene carbonate) or poly(butylene carbonate) are grafted from it (Figure 4C and D). There is an obvious correlation between the nature of the macroinitiators or monomers for the immortal polymerization and the resulting polymers. The hydrophilic hbPEO possess mainly primary hydroxyl groups, and the hydrophobic hbPBO has exclusively secondary hydroxyl groups.39 Consequently, the macroinitiators are different with respect to the necessary exchange at the catalyst center. Fast exchange is crucial for the polymerization to operate with low amounts of homopolymers.45 Furthermore, the monomers differ in their reactivity with respect to the catalyst employed. Our results show that a combination of mainly primary hydroxyl groups and a less reactive monomer like BO is the best combination for welldefined multiarm polycarbonate structures (Figure 4A). DOSY NMR spectra were measured to evidence that the polycarbonate chains formed are actually attached to the hyperbranched polyether polyol core and linear, nonattached chains are absent. Figure 5 reveals only a single signal with low diffusion coefficient, corresponding to the 1H NMR signals of both the polyether as well as the polycarbonate for hb(PG9-coPEO16)-g-PBC92 (Table 1, sample 2). The formation of a significant amount of polycarbonate homopolymer and unreacted polyether core is thus unlikely, and successful grafting is confirmed. However, DOSY NMR spectroscopy cannot distinguish star copolymers and polycarbonate homopolymers with the same diffusion coefficient. Furthermore, the DOSY NMR data do not explain the existing lower molecular weight shoulder found in the SEC traces. The DOSY NMR spectra of further multiarm star copolymers with different core and polycarbonate arms (hb(PG4-co-PEO45)-g-PPC349, hb(PG4-

Figure 3. 13C NMR spectrum of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2) (100 MHz, CD3CN). D

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Figure 5. DOSY spectrum of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2) (CD3CN, 400 MHz).

co-PBO10)-g-PPC146, hb(PG4-co-PBO10)-g-PBC66) are displayed in the Supporting Information (Figures S13−S15). For hb(PG4-co-PBO10)-g-PPC146, the mass fraction of the PPC arms is too high to permit resolution of the polyether signals. All other spectra are comparable to Figure 5. Preparative size exclusion chromatography was used to shed light on the molecular-weight-dependent composition of the different copolymers. The respective samples were dissolved in CHCl3 and fractionated with respect to their hydrodynamic volume by preparative SEC. For the discussion, fractions are labeled in ascending order with decreasing molecular weight according to SEC. These fractions were analyzed by 1H NMR spectroscopy and SEC individually. The 1H NMR and SEC results of hb(PG9-co-PEO16)-g-PBC92 are shown exemplarily in Figure 6 and Figure 7. The 1H NMR spectra in Figure 6 demonstrate that polyether structures (signal 3.79−3.40 ppm) are present in each fraction. The ratio of core and polycarbonate chains is the same in the first 11 fractions. Larger polyether polyol cores contain more hydroxyl groups, and thus, a larger number of polycarbonate chains can be grafted from them. However, the polyether to polycarbonate ratio exhibits decreases from the lower molecular weight fraction 12 to fraction 18. This hints at the formation of short polycarbonate homopolymer. From comparison of SEC (Figure 7) and NMR data (Figure 6), it can be deduced that the small shoulder in the monomodal distribution of hb(PG9co-PEO16)-g-PBC92 (Table 1, sample 2) is a nongrafted, bimodal homopolymer. It is known that the catalyst (R,R)(salcy)-CoCl is capable of copolymerizing carbon dioxide and epoxides also without macroinitiator, resulting in bimodal distributions.46 Detailed characterization of fractionated samples was also performed for hb(PG4-co-PEO45)-g-PPC349, hb(PG4-co-PBO10)-g-PPC146, and hb(PG4-co-PBO10)-g-PBC66) (Supporting Information, Figures S16−S22). In analogy to hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2), modes corresponding to lower molecular weight polycarbonate homopolymers are found at higher elution volume. All data show that the amount of nongrafted homopolycarbonate is higher when using PO instead of BO. PO is more reactive than

Figure 6. 1H NMR spectra of hb(PG9-co-PEO16)-g-PBC92 (Table 1, sample 2) after separation via preparative SEC (300 MHz, CD3CN), demonstrating a similar composition of all fractions of the material. The polycarbonate backbone signals are highlighted in yellow and green and the polyether core signals in blue.

BO in the immortal copolymerization with CO2, rendering the formation of PPC homopolymer more likely. In addition, the characterization data evidence that hbPEO is a more efficient initiator than hbPBO. The molecular weight distribution of the grafted polymers is considerably narrower (dispersity 1.13− 1.64) with the polyfunctional hbPEO macroinitiator than with hbPBO, regardless of whether PO or BO was used for polycarbonate synthesis. This is most likely related to the more E

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homopolycarbonate byproduct was not possible using fractionated precipitation or dialysis. Therefore, the amount of grafted aliphatic polycarbonate was not determinable by 1H NMR. B. Properties of the Multiarm Stars. In the study of the materials properties of the hbPEO-g-PBC copolymers, the glass transitions and intrinsic viscosities were of particular interest. The thermal behavior of the multiarm star polymers was studied by differential scanning calorimetry (DSC). All DSC results are summarized in Table 2. As expected, the polymers reveal no melting point due to the amorphous nature of both the hyperbranched polyether core and the polycarbonate chains prepared from racemic PO and BO. Linear poly(butylene carbonate) homopolymer shows a glass transition temperature (Tg) around 8 °C, whereas hyperbranched poly(ethylene oxide) exhibits a Tg at −60 °C. Hyperbranched poly(ethylene oxide) is known to exhibit a melting point only if a small amount of glycerol units are incorporated.29 The Tg of hyperbranched poly(ethylene oxide) is largely independent of the degree of branching.29 A blend of hbPEO and PBC exhibits two distinct glass transitions, because these polymers are immiscible (Figure 8, top). The synthesized multiarm star copolymers, however, exhibit merely a single Tg between the values of the homopolymers due to coupling of the different polymers. This arm-governed behavior was also described by Sunder et al. for multiarm stars based on a polyglycerol core and poly(propylene oxide) arms.28 Figure 8 shows the DSC curves of different hbPEO-g-PBC copolymers, revealing increasing glass transitions with increasing content of polycarbonate units. As expected, a higher polycarbonate fraction results in an increase of Tg. A clear shift from −41 to 4 °C is visible. The other three multiarm star combinations also exhibit single Tgs between those of the homopolymers. The stars comprising PPC arms showed values ranging from 21 °C (hb(PG4-coPEO45)-g-PPC349) (Table 2, sample 7) to 25 °C (hb(PG4-coPBO10)-g-PPC146) (Table 2, sample 8) because of their high amount of polycarbonate. Furthermore, the DSC data clearly reveal that the Tg is independent of the molecular weight of the multiarm stars but depends on the ratio of polyether units in the core and polycarbonate units in the arms. hb(PG5-coPEO57)-g-PBC20 and hb(PG1250-co-PEO6750)-g-PBC3640 (Table 2, samples 5 and 6) possess significantly different molecular weights, but their ratio of polycarbonate to polyether units and their glass transition temperatures are almost identical. Their cores also show similar Tgs (Table 2). In contrast to a

Figure 7. SEC results for selected fractions of hb(PG9-co-PEO16)-gPBC92 (Table 1, sample 2) star polymers in DMF with PEG calibration.

reactive primary hydroxyl groups in hbPEO. In line with these observations, the amount of PBC homopolymer using hbPEO as an initiator is very low compared to the other three combinations. In summary, it can be concluded that the copolymerization of BO and CO2 with hbPEO as a polyfunctional macroinitiator represents the best combination to provide the targeted well-defined multiarm star polyether− polycarbonate polyols. Capitalizing on the polyether polycarbonate combination yielding the best defined star copolymers, hbPEO and butylene oxide were used to create a high molecular weight multiarm star polymer with a molecular weight average of 812,000 g·mol−1. This material is based on a hbPEO macroinitiator with a molecular weight of 389,000 g·mol−1. Due to the poor solubility of hb(PG1250-co-PEO6750) in butylene oxide, grafting of the PBC arms was carried out in dichloromethane as a solvent. The resulting multiarm star copolymer was soluble in solvents like acetonitrile, dichloromethane, DMF, or acetone. The synthesis of even larger multiarm stars by using a higher monomer to macroinitiator ratio or the more reactive PO monomer instead of BO, however, resulted in poorly soluble materials and a high fraction of linear homopolycarbonate byproduct. The SEC elugram of these copolymers compared with hb(PG1250-coPEO6750)-g-PBC3630 suggests a significantly higher degree of polymerization (Supporting Information, Figure S22). A clear shift to lower elution volumes is visible. Separation of the linear

Table 2. Overview of the Tg, [η], and α-Parameters for All Multiarm Star Copolymers sample

compositiona

unit ratiob,c

Tgd (°C)

[η]e (cm3·g−1)

αe

1 2 3 4 5 6 7 8 9 10 11

hb(PG9-co-PEO16)-g-PBC168 hb(PG9-co-PEO16)-g-PBC92 hb(PG4-co-PEO15)-g-PBC50 hb(PG5-co-PEO10)-g-PBC25 hb(PG5-co-PEO57)-g-PBC20 hb(PG1250-co-PEO6750)-g-PBC3640 hb(PG4-co-PEO45)-g-PPC349 hb(PG4-co-PBO10)-g-PPC146 hb(PG4-co-PBO10)-g-PBC66 PPC147 PBC207

7.40 4.05 2.99 2.02 0.33 0.46 7.53 10.98 4.93

4 1 −2 −11 −41 −40 21 25 −12 31 8

9.9 8.1 n.d. 6.5 5.4 7.8 17.3 10.5 6.9 18.2 22.8

0.11 0.08 n.d. 0.20 0.58 0.16 0.21 0.10 0.22 1.11 0.72

a

Terminology: Indices represent the absolute number of the respective repeating unit (rounded to integer), determined by 1H NMR spectroscopy and online viscometry with universal calibration. bDetermined by 1H NMR spectroscopy. cRatio between polycarbonate (arms) and polyether (core) repeating units; polyether units are normalized to 1. dDetermined by DSC. eDetermined by online viscometry in DMF. n.d. = not determined. F

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Figure 9. Intrinsic viscosities of the series of hb(PG-co-PEO)-g-PBC copolymers in DMF plotted versus the ratio of monomer units N(carbonate)/N(ether).

the correlation between the α-parameter, the intrinsic viscosity, and the molecular mass. Low α-values translate to a low dependence of the intrinsic viscosity on the total molecular mass of the copolymers. These results provide additional support for the successful synthesis of multiarm star copolymers. The intrinsic viscosity shows a clear dependence on the ratio of the building units N (carbonate)/N (ether). In summary, the synthesis of hb(PG-co-PEO)-g-PBC copolymers enables one to adjust the intrinsic viscosity in a broad range by varying the ratio of monomer units N(carbonate)/N(ether). The low α-parameters determined in this work confirm the compact, three-dimensional structure of the multiarm star polycarbonates. C. Urethane Functionalization. Polyester polyol and polyols based on aliphatic polycarbonates from CO2 can be used to create partially biodegradable polyurethanes. Polylactide- and polycarbonate-based diols and triols are wellestablished for this purpose.33,48,49 However, polyesters form carboxylic acids upon degradation. This is a serious issue for biomedical applications, because local acidity may cause necrosis. Degradation of polycarbonates on the other hand only produces carbon dioxide and neutral diols.50 Furthermore, the low glass transition temperatures and the low viscosity of the multiarm star polyether−polycarbonate polyols reported herein enable the synthesis of polyurethanes without any solvent or heating. The addressability of their hydroxyl groups was confirmed by functionalization with phenylisocyanate as a model compound to obtain urethanes. hb(PG9-co-PEO16)-gPBC92 (Table 1, sample 2) was dissolved in phenylisocyanate and stirred for 4 h under an argon atmosphere at room temperature. Excess phenylisocyanate was conveniently removed by precipitation. Successful conversion was proved by 1 H NMR spectroscopy and SEC. Figure S24 (Supporting Information) shows the 1H NMR spectrum in CD3CN of hb(PG9-co-PEO16)-g-PBC92 after functionalization with phenylisocyanate. New signals of the aromatic protons and the NHgroup appear between 7.88 and 7.03 ppm. Furthermore, Figure S25 (Supporting Information) shows the SEC elugram of functionalized hb(PG9-co-PEO16)-g-PBC92 based on both RI and UV detection. Due to the phenyl groups attached, the multiarm stars show a UV signal after functionalization. Both measurements prove successful transformation without degradation of the polymer, rendering these polyether−polycar-

Figure 8. (a) DSC results of the series of hb(PG-co-PEO)-g-PBC copolymers with decreasing PBC amount from top to bottom. Top diagram: blend of both homopolymers. Temperature range from −70 to +20 °C, heating rates of 10 K·min−1. (b) Glass transition temperatures of the series of hb(PG-co-PEO)-g-PBC copolymers plotted versus monomer unit ratio N(carbonate)/N(ether).

preceding communication,5 the current findings reveal a more pronounced effect of the amount of polycarbonate. As a consequence, it is possible to tune the Tg in a broad temperature range from −41 to 25 °C, reflecting the impact of the flexibility of the respective polyether cores. In addition to the thermal characterization of the multiarm stars, also their intrinsic viscosities and α-parameters were investigated. The results are listed in Table 2 and Table S1 in the Supporting Information. Linear poly(butylene carbonate) homopolymer shows an intrinsic viscosity around 22.8 cm3·g−1 and poly(propylene carbonate) around 18.2 cm3·g−1. The αparameters of PBC and PPC are 0.72 and 1.11, respectively. These are typical values for linear polymers.47 Hyperbranched polyethers exhibit very low intrinsic viscosities and αparameters, due to their compact structure.31,32,47 The synthesized multiarm star copolymers exhibit generally intrinsic viscosities lower than the linear homopolymers. Figure 9 shows the intrinsic viscosity [η] of different hbPEO-g-PBC copolymers, revealing an increase of [η] with increasing polycarbonate fraction. This can be ascribed to the linear arms constituting a larger part of the multiarm star structures. The consequence of the increasing chain length of the arms of the stars is an increase from 5.4 to 9.9 cm3·g−1. The stars comprising PPC arms generally showed higher [η] values ranging from 17.3 cm3 ·g−1 (hb(PG4-co-PEO45)-g-PPC349) (Table 2, sample 7) to 10.5 cm3·g−1 (hb(PG4-co-PBO10)-g-PPC146) (Table 2, sample 8) because of their high polycarbonate content. The α-parameters of all multiarm star copolymers are around 0.1−0.2. The Kuhn−Mark−Houwink−Sakurada equation ([η] = KMα) gives G

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Macromolecules

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bonate multiarm star copolymers suitable polyol components for polyurethane chemistry.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

CONCLUSION Flexible multiarm stars based on a hyperbranched polyether polyol core and linear aliphatic polycarbonate arms of varied degree of polymerization were synthesized in a two-step coref irst approach. First, hyperbranched polyether polyols of different molecular weights were prepared by anionic ringopening copolymerization of glycidol with EO or BO, thereby varying the nature of the end groups from primary to secondary hydroxyl groups.39 In the second step, hbPEO and hbPBO polyols were employed as macroinitiators for the copolymerization of CO2 with propylene oxide and butylene oxide, respectively. The star architecture was confirmed by IR spectroscopy, 1H, 13C and 2D NMR spectroscopy, SEC, and DSC. The influence of both types of macroinitiators (hbPEO and hbPBO) and the two different epoxide monomers employed for the immortal copolymerization on the formation of multiarm star copolymers was investigated systematically. Molecular weight distributions were found to be much narrower when using hbPEO instead of hbPBO as initiators, independent of whether PO or BO was used for the grafting of the polycarbonate arms. Furthermore, a larger amount of linear polycarbonate homopolymer is formed in the case of hbPBO. These findings demonstrate the significant difference in the reactivity of the mainly primary hydroxyl groups of hbPEO and the exclusively secondary hydroxyl groups of hbPBO.39 Comparing the copolymerization of PO and BO with CO2, it becomes obvious that butylene oxide is more suitable for the preparation of well-defined multiarm star polymers, because of the lower amount of homopolymer formed. A series of multiarm star polymers based on hbPEO and PBC were prepared in solvent-free procedures with molecular weights of 3,800 and 20,800 g·mol−1 with moderate dispersity Đ between 1.37 and 1.69. Depending on the ratio of polyether units and polycarbonate units, the glass transition temperature as well as the intrinsic viscosity can be tuned in a broad range between the values of the respective homopolymers. The low α-parameters support compact, three-dimensional structures. The successful addressability of the hydroxyl groups for urethane chemistry was shown as well. Furthermore, the synthesis of amphiphilic highmolecular-weight hbPEO-g-PBC star copolymer (Mn = 812,000 g·mol−1) was possible as well. The multiarm star copolymers presented herein are obtained from common epoxide monomers and CO2, in most cases in a solvent-free synthesis procedure. This renders them conveniently available materials with potential applications as inverse unimolecular micelles with a degradable shell and as highly flexible, degradable cross-linkers for polyurethanes. Because of their low intrinsic viscosity, they are also conceivable processing additives for linear polycarbonates.

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Article

ORCID

Holger Frey: 0000-0002-9916-3103 Author Contributions §

M.S. and J.S. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S. is grateful for the financial support through the Max Planck Graduate Center (MPGC) with the Johannes Gutenberg University Mainz. The authors thank Pia Winterwerber and Maria Golowin for technical assistance.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01131. Additional information on materials and instrumentation, synthesis and characterization, and the Mark−Houwink α-parameter and functionalization (PDF) H

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DOI: 10.1021/acs.macromol.7b01131 Macromolecules XXXX, XXX, XXX−XXX