Ring-Opening Polymerization of Epoxides - ACS Publications

Apr 4, 2017 - production of poly[(epichlorohydrin)-co-(allyl glycidyl ether)- co-(ethylene oxide)] elastomers.22,23 The Vandenberg catalyst was known ...
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Ring-Opening Polymerization of Epoxides: Facile Pathway to Functional Polyethers via a Versatile Organoaluminum Initiator Christina G. Rodriguez, Robert C. Ferrier, Jr., Alysha Helenic, and Nathaniel A. Lynd* McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: We report a new class of organoaluminumbased initiator for anionic ring-opening polymerization of epoxides that is simple to synthesize from readily available precursors. The resultant organometallic initiator was the triethylaluminum adduct of (2-dibenzylamino)ethoxydiethylaluminum (TAxEDA) [(AlEt3)·(O(AlEt2)CH2CH2N(Bn)2)], which was isolated by direct crystallization from the reaction medium and then compositionally and structurally characterized by NMR spectroscopy and XRD. We studied the reactivity and versatility of the new initiator through the polymerization of propylene oxide, butylene oxide, epichlorohydrin, and allyl glycidyl ether into homopolymer, statistical copolymer, and block copolymer architectures with heterobifunctional end-groups consisting of dibenzylamine and hydroxyl functionalities. The TAxEDA-initiated polymerizations were consistent with a controlled, living, anionic mechanism that was tolerant of chemical functionality and exhibited no chain transfer to monomer that limits the traditional anionic ring-opening polymerization of substituted epoxides.

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synthesizing polyethers.19−21 Another example of a widely used catalytic approach in the past was the classical Vandenberg catalyst that was developed originally for the industrial production of poly[(epichlorohydrin)-co-(allyl glycidyl ether)co-(ethylene oxide)] elastomers.22,23 The Vandenberg catalyst was known for its versatility in monomer substrates, ease of preparation, stability, and the high molecular weight polymers that it produced in quantitative yields under relatively unrestricted reaction conditions. Although highly effective, the synthetic route to the Vandenberg catalyst prevented definitive determination of the active catalyst structure as well as the initiation and propagation mechanisms. This impenetrability prevented gaining a mechanistic understanding of the Vandenberg catalyst and further improvement in performance with respect to modern polymerization figures of merit, such as precise control of molecular weight, orthogonal control of endgroup functionality, and narrow molecular weight distribution. In order to further unlock the full multifunctional and modular potential of polyethers, a new polymerization platform for epoxides must be developed that is capable of producing materials by combining the advantages of both AROP and CROP approaches. A united synthetic approach should be capable of producing materials with accurately controlled molecular weights with no chain transfer, few restrictions on monomer structure, controlled chain-end functionality, and facile control of architecture such as block polymer synthesis through sequential addition of monomers. Because of the Earth

INTRODUCTION Polyethers provide the potential for remarkable modularity and multifunctionality in a materials platform based on the variety of readily available, inexpensive, and functional monomer precursors.1,2 The spectrum of methods used to synthesize polyethers each have various advantages and disadvantages, with no single consensus technique for all structures and compositions emerging. For low molecular weight polyethers (50 kg/mol), greater selection of monomer substrates, and/or isotactically enriched materials, a catalytic ring-opening polymerization (CROP) approach can be utilized.9,10 Group 13 and transitionmetal-containing initiators and catalysts have been developed,11−16 which allow for the synthesis of high molecular weight polyethers, are amenable to a wide variety of monomer substrates, and offer perfect regio- and oftentimes stereocontrol of the polymerization under some conditions.17,18 The aluminum−porphyrin catalyzed immortal polymerizations of Inoue et al. represent a controlled and versatile method of © XXXX American Chemical Society

Received: January 25, 2017 Revised: March 27, 2017

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DOI: 10.1021/acs.macromol.7b00196 Macromolecules XXXX, XXX, XXX−XXX

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General Procedure for Synthesis of Polymers. All polymerizations were performed neat in a septum-capped reaction vial. The vials were cooled to 0 °C to prevent a runaway reaction and charged with a stir bar, monomer, and TAxEDA initiator in an inert environment. After 2 h, the solutions were incrementally heated to the final reaction temperature (50−80 °C, dependent on monomer), and polymerizations were carried out for 2−12 days. Reactions were quenched with methanol and dissolved in acetyl acetone (AcAc) to remove aluminum which interfered with characterization by SEC. The resulting solution was poured into a poor solvent to precipitate the desired polymer product. The supernatant was removed and the polymer was dried in vacuo. Mn was determined by 1H NMR spectroscopy by taking the ratio of the backbone proton signals to the integral of the phenyl proton signal on the dibenzylamine (10H) endgroup. Average molecular weights and dispersities were determined by SEC with refractive index, multiangle light scattering, and viscosity detectors. The reported molecular weights were determined using the multiangle light scattering signal (SEC-MALS). Poly(allyl glycidyl ether) (PAGE). 1H NMR (CD2Cl2, 400 MHz): δ 3.38−3.71 (broad m, −O−CH2−CH(CH2−O−CH2−CHCH2)− O−), 3.99 (d, −O−CH2−CHCH2), 5.16/5.27 (doublet of doublets, −O−CH2−CHCH2), 5.91 (m, −O−CH2−CHCH2), 7.19−7.68 (broad m, −Ph−CH2−N−). 13C NMR (CDCl3, 400 MHz): δ 69.79 (−O−CH2−CH(CH2−O−CH2−CHCH2)−O−, rrm or mrr), 69.97−70.36 (−O−CH2−CH(CH2−O−CH2−CHCH2)−O−, m), 72.38 (−O−CH2−CHCH2), 78.93 (−O−CH2−CH(CH2−O− CH2−CHCH2)−O−), 116.8 (−O−CH2−CHCH2), 128.1/ 128.7 (−Ph−CH2−N−), 134.9 (−O−CH2−CHCH2). Polyepichlorohydrin (PECH). 1H NMR (CD2Cl2, 400 MHz): δ 4.03−3.35 (broad m, −O−CH2−CH(CH2−Cl)−O−), 7.20−7.75 (broad m, −Ph−CH2−N−). 13C NMR (CD2Cl2, 400 MHz): δ 43.8 (−O−CH2−CH(CH2−Cl)−O−), 69.30 (−O−CH2−CH(CH2−Cl)− O−, rrm or mrr), 69.54 (−O−CH2−CH(CH2−Cl)−O−, m), 78.88− 79.15 (−O−CH2−CH(CH2−Cl)−O−), 128.1/128.7 (−Ph−CH2− N−). Poly(butylene oxide) (PBO). 1H NMR (CD2Cl2, 400 MHz): δ 0.92 (t, −CH2−CH3), 1.50 (m, −CH2−CH3), 3.26−3.68 (broad m, −O−CH2−CH(CH2−CH3)−O−), 7.20−7.65 (broad m, −Ph−CH2− N−). 13C NMR (CDCl3, 400 MHz): δ 9.72 (−CH2−CH3), 24.73 (−CH2−CH3), 71.46 (−O−CH2−CH(CH2−CH3)−O−, rrm or mrr), 72.27 (−O−CH2−CH(CH2−CH3)−O−, m), 80.36−80.88 (−O− CH2 −CH(CH2 −CH 3)−O−), 126.7/128.1/128.7/139.8 (−Ph− CH2−N−). Poly(propylene oxide) (PPO). 1H NMR (CD2Cl2, 400 MHz): δ 1.12 (m, −CH3), 3.32−3.60 (broad m, −O−CH2−CH(CH3)−O−), 7.17−7.65 (broad m, −Ph−CH2−N−). 13C NMR (CDCl3, 400 MHz): δ 17.38 (−CH3), 72.88 (−O−CH2−CH(CH3)−O−, rrm or mrr), 73.35 (−O−CH2−CH(CH3)−O−, m), 75.08 (−O−CH2− CH(CH3)−O−, rr), 75.31 (−O−CH2−CH(CH3)−O−, mr + rm), 75.48 (−O−CH2−CH(CH3)−O−, mm) 126.7/128.1/128.7/139.8 (−Ph−CH2−N−). Poly[(allyl glycidyl ether)-co-(Propylene oxide)] (P(AGE-coPO)). 1H NMR (CD2Cl2, 400 MHz): δ 1.12 (m, −CH3), 3.25−3.74 (broad m, (−O−CH2−CH(CH3)−O− and −O−CH2−CH(CH2− O−CH2−CHCH2)−O−), 3.99 (d, −O−CH2−CHCH2), 5.16/ 5.27 (doublet of doublets, −O−CH2−CHCH2), 5.91 (m, −O− CH2−CHCH2), 7.19−7.70 (broad m, −Ph−CH2−N−). 13C NMR (CDCl3, 400 MHz): δ 17.38 (−CH3), 68.81/69.19/70.27 (−O− CH2−CH(CH2−O−CH2−CHCH2)−O−), 72.26 (−O−CH2− CHCH2), 72.83/73.36/74.22 (−O−CH2−CH(CH3)−O−), 75.33 (−O−CH2−CH(CH3 )−O−), 78.77 (−O−CH2−CH(CH2 −O− CH2−CHCH2)−O−), 116.7 (−O−CH2−CHCH2), 126.7/ 128.1/128.7 (−Ph−CH2−N−), 134.9 (−O−CH2−CHCH2). Poly[(allyl glycidyl ether)-co-(Epichlorohydrin)] (P(AGE-coECH)). 1H NMR (CD2Cl2, 400 MHz): δ 3.36−3.84 (broad m, (−O− CH 2 −CH(CH 2 −Cl)−O− and −O−CH 2 −CH(CH 2 −O−CH 2 − CHCH2)−O−), 3.99 (d, −O−CH2−CHCH2), 5.14/5.24 (doublet of doublets, −O−CH2−CHCH2), 5.87 (m, −O−CH2− CHCH2), 7.19−7.69 (broad m, −Ph−CH2−N−). 13C NMR (CD2Cl2, 400 MHz): δ 43.81 (−O−CH2−CH(CH2−Cl)−O−),

abundance of aluminum and the versatility and stability of the Vandenberg catalyst, we chose this system as our initiator design ansatz. In this report, we disclose the synthesis of a new class of aluminum-based initiator and its characteristics as a general platform for the synthesis of functional polyether materials.

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EXPERIMENTAL SECTION

Materials. Ethanolamine (Sigma-Aldrich, >98%), benzyl bromide (Sigma-Aldrich, reagent grade, 98%), sodium bicarbonate (RICCA, ACS reagent grade), acetone (Sigma-Aldrich, ACS reagent, >99.5%) and triethylaluminum (1.0 M in hexanes, Sigma-Aldrich) were all used as received. Anhydrous toluene and hexanes were obtained from a JC Meyer solvent system. Epichlorohydrin (ACROS Organics, 99%), allyl glycidyl ether (Aldrich, >99%), propylene oxide (TCI, >99%), and butylene oxide (TCI, >99%) were dried over calcium hydride. All airand moisture-sensitive reactions were prepared under a dry nitrogen atmosphere inside a glovebox. Acetyl acetone (ACROS Organics, 99+ %), methanol (Fisher, Certified ACS), and hexane (Sigma-Aldrich, anhydrous, 95%) were used for washing polymers and used as received. Equipment. 1H NMR and 13C NMR spectroscopy were performed on a 400 MHz Agilent MR spectrometer at room temperature and referenced to the residual solvent signal of CDCl3 (77.16 and 7.26 ppm, respectively) and CD2Cl2 (5.32 and 53.5 ppm, respectively). Size exclusion chromatography (SEC) was carried out on an Agilent system with a 1260 Infinity isocratic pump, degasser, and thermostated column chamber held at 30 °C containing Aglient PLgel 10 μm MIXED-B and 5 μm MIXED-C columns with a combined operating range of 200−10 000 000 g/mol relative to polystyrene standards. Chloroform with 50 ppm amylene was used as the mobile phase at 0.5 mL/min. A suite of detectors from Wyatt Technologies provided measurement of polymer concentration, molecular weight, and viscosity. Static light scattering was measured using a DAWN HELEOS II Peltier system, with differential refractive index measured with an Optilab TrEX, and differential viscosity measured using a Viscostar II. In situ Fourier transform infrared (FTIR) spectroscopy of polymerizations was performed on a Mettler Toledo ReactIR 15 with an optical range of 4000−650 cm−1. XRD data were collected on a Rigaku AFC12 diffractometer with a Saturn 724+ CCD using a graphite monochromator with Mo Kα radiation (λ = 0.710 73 Å). A total of 1136 frames of data were collected using ω-scans with a scan range of 0.5° and a counting time of 45 s per frame. The data were collected at 173 K using a Rigaku XStream low-temperature device. Differential scanning calorimetric (DSC) tests were conducted on a TA250 instrument with a heating rate of 10 °C min−1 under a N2 atmosphere, and the data from the second heating curve were collected. Synthesis of N,N-Dibenzylethanolamine. Details of the synthesis can be found in ref 24. 1H NMR (CD2Cl2, 400 MHz): δ 2.67 (t, −N−CH2−CH2−O−H), 3.59 (t, −N−CH2−CH2−O−H), 3.64 (s, −Ph−CH2−N−), 7.27/7.33 (m, −Ph−CH2−N−). Synthesis of the Triethylaluminum Adduct of (2Dibenzylamino)ethoxydiethylaluminum [(AlEt 3 )·(O(AlEt 2)CH2CH2N(Bn)2)] (TAxEDA). A reaction vial was charged with a stir bar and triethylaluminum (12.7 mmol, 12.7 mL) and cooled to −78 °C. N,N-Dibenzylethanolamine (4.64 mmol, 1.12 g) was dissolved in 2 mL of toluene before dropwise addition into the reaction vial containing triethylaluminum. The solution was set to stir and warm to room temperature overnight. The solution was then directly cooled to −40 °C to crystallize the desired product. The resultant crystals were washed three times with anhydrous hexanes and dried in vacuo. 1H NMR (CD2Cl2, 400 MHz): δ −0.16 (q, −Al−(CH2−CH3)3), 0.12 (q, −Al−(CH2−CH3)2), 1.08 (m, −Al−(CH2−CH3)3 and −Al−(CH2− CH3)2), 3.02 (t, −N−CH2−CH2−O−), 3.90/4.12 (d, −Ph−CH2− N−), 4.18 (t, −N−CH2−CH2−O−), 7.32/7.48 (m, −Ph−CH2−N−). XRD: empirical formula C26H43Al2NO, triclinic cell with calculated density of 1.09 g/mL. B

DOI: 10.1021/acs.macromol.7b00196 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 68.82−70.71 (−O−CH2−CH(CH2−Cl)−O− and −O−CH2−CH(CH2−O−CH2−CHCH2)−O−), 72.15 (−O−CH2−CHCH2), 78.45−79.58 (−O−CH2−CH(CH2−Cl)−O− and −O−CH2−CH(CH2−O−CH2−CHCH2)−O−), 116.9 (−O−CH2−CHCH2), 128.1/128.7 (−Ph−CH2−N−), 134.7 (−O−CH2−CHCH2). Poly[(allyl glycidyl ether)-b-(Epichlorohydrin)] (P(AGE-bECH)). 1H NMR (CD2Cl2, 400 MHz): δ 3.36−3.84 (broad m, (−O−CH 2 −CH(CH 2 −Cl)−O− and −O−CH 2 −CH(CH 2 −O− CH2−CHCH2)−O−), 3.99 (d, −O−CH2−CHCH2), 5.14/5.24 (doublet of doublets, −O−CH2−CHCH2), 5.87 (m, −O−CH2− CHCH2), 7.19−7.69 (broad m, −Ph−CH2−N−). 13C NMR (CD2Cl2, 400 MHz): δ 43.81 (−O−CH2−CH(CH2−Cl)−O−), 68.91−69.56 (−O−CH2−CH(CH2−Cl)−O−), 69.70 (−CH2−O− CH2−CHCH2), 69.81 (−O−CH2−CH(CH2−O−CH2−CH CH2)−O−, rrm or mrr), 69.99−70.51 (−O−CH2−CH(CH2−O− CH2−CHCH2)−O−, m), 72.10 (−O−CH2−CHCH2), 78.52− 79.24 (−O−CH2−CH(CH2−Cl)−O− and −O−CH2−CH(CH2− O−CH2−CHCH2)−O−), 116.1 (−O−CH2−CHCH2), 128.1/ 128.7 (−Ph−CH2−N−), 135.1 (−O−CH2−CHCH2). In Situ FTIR Spectroscopy Studies. A 100 mL, two-armed reaction flask was charged with TAxEDA initiator and a stir bar under nitrogen. The flask was sealed with a septum on the side arm and the FTIR probe in the center arm. The vessel was cooled to 0 °C, and a background spectrum was collected. The monomer (under nitrogen) was injected into the reaction vessel through the side arm under stirring. The vessel was held at 0 °C for 2 h and then brought up to 80 °C, the final reaction temperature. The reaction was allowed to proceed for 2 days with a spectrum taken every 5 min. A second aliquot of monomer was then injected into the reaction vessel for the chain extension and block copolymer studies.

Scheme 1. Reactivity of the TAxEDA Initiator Was Interrogated via the Polymerization of a Range of Substituted Epoxide Monomers

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RESULTS AND DISCUSSION As a first step toward combining the advantages of AROP and CROP, we sought to develop an initiator with the simplicity

Figure 2. In situ FTIR was used to record the relative intensity (I/I0) of the epoxide band at 852 cm−1 over time, which was proportional to the relative concentration of AGE ([AGE]/[AGE]0). The AGE polymerization was 95% completed after ca. 10 h producing 12 kg/mol PAGE (PAGE-12, Table 1).

decades has provided new insight.25−39 On the basis of this foundational work, we posited that a bis(μ-alkoxoalkylaluminum) ([R2Al(μ-OCH2CH2OMe)]2, R = Me, Et, iBu) motif was likely homologous to the resting-state structure of the Vandenberg catalyst with respect to epoxide polymerizations. While synthesizing a series of bis(μ-alkoxoalkylaluminum)s by tuning the stoichiometry between an Nsubstituted ethanolamine ligand and triethylaluminum (TEAl) at −78 °C in hexane, in one instance we isolated an asymmetric organoaluminum species that proved to be effective for epoxide polymerizations; the triethylaluminum adduct of (2-dibenzylamino)ethoxydiethylaluminum, or TAxEDA, where x refers to the specific N-substitution (here x = 2-dibenzylamino-). Purification of TAxEDA was accomplished by direct crystallization from the reaction medium at −40 °C. The synthesis

Figure 1. Top: The one-step synthesis of TAxEDA is shown. Bottom: Structure determined by XRD of resultant crystals isolated from the reaction mixture. Thermal ellipsoids are shown at a probability level of 50%.

and efficiency of the Vandenberg catalyst, but with a welldefined structure providing control of molecular weight and chain-end functionality. The structure of the Vandenberg catalyst is unknown; however, new insight into compositionally homologous organoaluminum species over the intervening C

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The performance of TAxEDA was first evaluated based on polymerization rate, regioselectivity, and chain-end functionality of the TAxEDA-initiated polymerization of allyl glycidyl ether (AGE). Poly(allyl glycidyl ether) (PAGE) is a wellcharacterized system, which has seen increased usage in the literature due to its inherent chemical flexibility stemming from the pendant allyl groups,42−44 which can be functionalized via a variety of facile transformations.45−52 PAGE is typically synthesized via AROP at low temperature to avoid chain transfer and isomerization; both side-reactions produce known spectroscopic signatures.32,33 As a point of comparison, the AROP of AGE is typically allowed to run over the course of 24−48 h to achieve molecular weights of 5−20 kg/mol. A monomer-to-initiator ratio of 90 was used to target a molecular weight of 10 000 g/mol. The polymerization of AGE was studied using in situ FTIR, 1H NMR spectroscopy, and size exclusion chromatography (SEC) of the resultant PAGE. For low molecular weights (99 97 >99 >99 99 99 >99 90 >99 77 90 90 80 >99

a

Determined by size exclusion chromatography with multiangle light scattering. bDetermined by end-group analysis by 1H NMR spectroscopy. Determined by size exclusion chromatography using the differential refractometer signal. dConversion determined by 1H NMR spectroscopy of the crude reaction mixture.

c

PAGE with a focus on the region between 78.4 and 79.4 ppm that corresponds to the backbone methine (CH) sensitive to the specific regio- and stereoconfiguration of the polymer. The methine signal was consistent with head-to-tail addition of AGE. This regioselectivity was consistent with a primarily anionic epoxide ring-opening mechanism. The methine signal can be further interrogated for the specific tacticity of the allyl substituents along the PAGE backbone by deconvolution per the triad signals associated with particular tacticity (e.g., iso, hetero, syndio).9 The TAxEDA initiated PAGE was characteristically atactic with a statistical distribution of 24.7% isotactic, 27.3% syndiotactic, and 48% heterotactic triads. Conclusively, TAxEDA produced regioregular, atactic PAGE. The full 13C NMR spectrum can be seen in Figure S7. While TAxEDA proved to initiate the controlled polymerization of AGE to quantitative conversion, we next investigated the generality of TAxEDA toward other substituted epoxides. A series of substituted epoxides consisting of epichlorohydrin (ECH), propylene oxide (PO), butylene oxide (BO), and AGE were polymerized with TAxEDA into homopolymer materials at targeted molecular weights of 25 kg/mol. All polymerizations were performed neat. However, the reaction temperature was dependent upon the monomer used. Reaction conditions and characteristics of resultant polymers are summarized in Table 1. It should be noted that not all of these polymerizations went to full conversion as they were terminated after a set period of time. Figure 4 shows the 1H NMR spectra of the resultant (a) poly(epichlorohydrin) (PECH-26, Table 1), (b) poly(propylene oxide) (PPO-24, Table 1), and (c) poly(butylene oxide) (PBO-20, Table 1), while the 1H NMR spectrum for PAGE (PAGE-19, Table 1), which was equivalent to the spectrum shown in Figure 3, can be found in Figure S8. The phenyl protons on the dibenzylamine end-groups (10H) were used for end-group analysis yielding molecular weights of MnPECH = 22.5 kg/mol, MnPPO = 22.8 kg/mol, MnPBO = 24.8 kg/ mol, and MnPAGE = 19.4 kg/mol respectively for PECH, PPO, PBO, and PAGE, which matched well with the respective results from SEC-MALS: MnPECH = 26 kg/mol (Đ = 1.18), MnPPO = 24.3 kg/mol (Đ = 1.04), MnPBO = 20.2 kg/mol (Đ = 1.07), and MnPAGE = 18.2 kg/mol (Đ = 1.40). Dispersities were calculated from the differential refractometer signal of the size

Figure 4. 1H NMR spectra (CD2Cl2) for PECH (a), PBO (b), and PPO (c). The dibenzylamine end-groups at 7−8 ppm are shown magnified for each sample.

chain transfer was evident in the 1H NMR spectra as indicated by the absence of characteristic resonances from excess allyl ether end-groups. Figure 3b shows the 13C NMR spectrum of E

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Figure 6. (a) Plot of the relative ECH concentration as a function of time by monitoring the relative intensity (I/I0) of the epoxide band at 852 cm−1. An aliquot of ECH was polymerized to completion over 24 h, after which another aliquot was injected and polymerized. (b) Plot of the relative intensity at 852 cm−1 as a function of time. AGE was polymerized to completion over 46 h, after which an aliquot of ECH was injected and polymerized to produce a diblock copolymer.

50, and 75 kg/mol. PECH-26 was also included for this analysis, and Table 1 contains a summary of the reaction conditions as well as the results of the polymerizations in entries PECH-14, PECH-26, PECH-34, PECH-40, PECH-43, and PECH-64. All polymerizations were conducted neat. Lower targeted molecular weights at high TAxEDA loading experienced an exotherm when the monomer was added to the initiator, which could lead to autoacceleration and uncontrolled polymerization. To avoid thermal runaway, all polymerizations were initiated at 0 °C and held for 2 h before incremental heating to 80 °C over the course of an additional 2 h. This thermal schedule is admittedly very conservative in order to maximize safety while working with an unknown polymerization platform. PECHs of targeted molecular weights 10−50 kg/mol were terminated after 2 days, while the targeted 75 kg/ mol PECH was terminated after 5 days. All polymerizations in this suite achieved a conversion of at least 97% based upon 1H NMR spectroscopy of the crude reaction mixtures (Figures S12−S17). Molecular weight distributions were measured by SEC (Figures S18−S23). From the SEC analysis, there was a linear trend in molecular weight as measured by SEC-MALS (dn/dc = 0.039 ± 0.004 mL/g in CHCl3) as a function of the monomer-to-initiator ratio, as shown in Figure 5a. We were able to achieve molecular weights of at least 64 kg/mol for PECH. The SEC-MALS results matched well with the molecular weights calculated from 1H NMR spectroscopy (summarized in Table 1), except for the targeted 75 kg/mol sample which measured 93 kg/mol by 1H NMR end-group

Figure 5. (a) Plot of the molecular weight as a function of [M]/[I]. The molecular weight increased linearly with [ECH]/[TAxEDA]. (b) Plot of the molecular weight (SEC-MALS) as a function of the peak elution time from SEC. Full SEC traces can be found in Figures S18− S23.

exclusion chromatograms. As seen in Figure 4, the 1H NMR spectroscopy associated with the phenyl protons of the dibenzylamine end-groups appeared characteristically different for each polymer, which may be due to the interaction between the benzyl group and the various substituent groups in the polymer pendant as observed previously.53 The full 13C NMR spectra for PPO, PBO, and PECH can be found in Figures S9− S11. The performance of TAxEDA for ECH, PO, and BO was consistent with the behavior of the previous AGE polymerizations. The performance of TAxEDA with ECH, PO, BO, and AGE suggested that control of molecular weight could be accomplished by reaction stoichiometry. In order to investigate the efficiency of TAxEDA to control molecular weight, a series of ECH polymerizations were conducted at varying monomerto-initiator ratios targeting five molecular weights: 10, 30, 40, F

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Figure 7. 1H NMR spectrum (CD2Cl2) of P(AGE-b-ECH) (a) revealed block molecular weights of MnPAGE = 14 kg/mol and MnPECH = 6.6 kg/mol. The 13C NMR spectrum (CD2Cl2) (b) showed sharp peaks associated with PAGE and PECH homotriad sequences, consistent with a diblock copolymer architecture. No cross-peaks were present. The 1H NMR spectrum (CD2Cl2) (c) of P(AGE-co-PO) was used to calculate a molecular weight of 15.2 kg/mol with weight fraction wPO = 0.66. The 13C NMR spectrum (CDCl3) (d) revealed broad peaks associated with PAGE and PPO as well as mixed heterotriad peaks from adjacent AGE and PO repeat units, consistent with a statistical copolymer.

analysis and 64 kg/mol by SEC-MALS. This discrepancy could be attributed to increased error of end-groups analysis as molecular weight was increased. Figure 5b is a plot of the molecular weight as a function of the peak elution time, which shows that the retention time decreased proportionally to the targeted molecular weight, consistent with linear control of molecular weight with reaction stoichiometry. The dispersities were low (Đ = 1.15−1.25) for all polymerizations. Significantly, TAxEDA provided for direct, linear control of molecular weight of PECH with relatively low Đ. We note that traditional alkoxide initiators do not allow for routine control of molecular weights above 20 kg/mol with select substituted epoxides such as PO, BO, or ECH.7,8,54 A lack of innate self-termination (i.e., livingness) is an important characteristic to enable precise control of macromolecular architecture, composition, and size. The livingness of a TAxEDA-initiated polymerization was investigated by chain extension experiments monitored by in situ FTIR spectroscopy. The in situ FTIR allowed us to monitor the relative amount of ECH monomer present in the reaction as a function of time as shown in Figure 6a. The experiment proceeded by first

injecting a large aliquot (ca. 12 mL) of ECH into a roundbottom flask containing TAxEDA and heating the vessel to 80 °C over the course of several hours. Once 80 °C was reached, a rapid consumption of ECH occurred as seen in Figure 6a. The polymerization was allowed to complete over the course of 24 h. At 24 h, a smaller aliquot (ca. 2.5 mL) of ECH was injected into the same flask containing the dormant polymerization. The second addition of ECH was characterized by an increase in the ECH signal, followed by an exponential decrease in signal as the ECH was consumed and converted into PECH. SECMALS analysis of the resulting polymer revealed a molecular weight of 30 kg/mol with a Đ = 1.22 of a unimodal molecular weight distribution (Figure S24). The molecular weight was calculated to be 31 kg/mol by end-group analysis from 1H NMR spectroscopy (Figure S25). The effective propagation rate constant of the ECH polymerization (kpECH) was calculated to be 9.0 × 10−4 ± 1.1 × 10−5 M−1 s−1 and 1.2 × 10−3 ± 1.0 × 10−5 M−1 s−1 for the first and second additions of ECH, respectively. The difference in kpECH between the first and the second addition was most likely due to variation in temperature during initiation. The maximum rate of polymerization G

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present. As a comparison for the diblock copolymer synthesis, ECH and AGE were simultaneously copolymerized at an initial molar feed of nAGE = 0.5.55 13C NMR spectroscopy was performed and is shown in Figure S27 for comparison to Figure 7b. The spectrum contains a number of additional cross-peaks from mixed heterotriad sequences and methylene dyads as a result of neighboring ECH and AGE repeat units that are not present in the spectrum of the BCP. Differential scanning calorimetry (DSC) was performed on the P(AGE-co-ECH) to measure a TgP(AGE‑co‑ECH) = −56 °C which compared well to the predicted value of −63 °C based on the Fox equation. The DSC trace can be found in Figure S28. As further expression of the generality of TAxEDA to control polymer architecture, the statistical copolymerization PO and AGE was carried out. 1H and 13C NMR spectroscopies are shown in Figures 7c and 7d. The overall molecular weight was calculated from end-group analysis to be 15.2 kg/mol, with a weight fraction of PO (wPO) of 0.66. The 13C NMR spectrum presents significant populations of mixed AGE/PO heterotriad sequences. The heterotriad peaks are indicated as follows. The s and t refer to the methylene(s) diad and methine(t) triad groups,56 while A refers to AGE and P refers to PO. Therefore, t-XAX corresponds to AGE-centered triads associated with the methine (CH) carbon on the AGE repeat units. DSC analysis further supported the architecture of the statistical copolymer. As a control, we measured the Tg of PPO and PAGE to be TgPPO = −72 °C and TgPAGE = −77 °C. The DSC of the P(AGE-co-PO) revealed a single Tg at −74 °C, which compared favorably with the Fox equation prediction of −74 °C for a random copolymer of identical composition to our P(AGE-coPO). The DSC traces can be found in the Supporting Information for PPO (Figure S29), PAGE (Figure S30), and P(AGE-co-PO) (Figure S31). The accepted polymerization mechanism for epoxides proposes that the epoxide undergoes a coordination−insertion type mechanism in AROP,57,58 CROP,9,59−61 and activatedmonomer strategies.62 The regioselectivity and lack of stereoselectivity of the TAxEDA-initiated polymerization suggest that the polymerization follows an anionic coordination−insertion mechanism in a relatively nonsterically hindered environment. We posit that the trialkylaluminum adduct functions to activate the incoming epoxide in a similar manner to trialkylaluminumactivated monomer polymerization.57 However, we have observed no increase in polymerization rate with added triethylaluminum (TEAl), which suggests that the TEAl adduct alone provides the site of monomer activation. Based on these observations, Scheme 2 presents a proposed mechanism for the polymerization of substituted epoxides with the TAxEDA initiator. Briefly, the epoxide coordinates to the highly activating TEAl adduct, which breaks and re-forms a dative O−Al bond. The dialkylaluminum translates forward on the chain re-forming the mono-μ-alkoxo-dialuminum chain end and enchaining the epoxide. Propagation proceeds in a similar manner until all available monomer is depleted. The mono-μoxo architecture of TAxEDA allows the incoming epoxide to attack with few geometric restrictions leading to atactic polyethers. Future work will interpret the proposed polymerization mechanism in light of structural variation on the TAxEDA motif and its relationship to existing systems such as the classical Vandenberg catalyst.

Scheme 2. Proposed Mechanism for the Ring-Opening Polymerization of Epoxides by TAxEDA

(−d[ECH]/dt) was also calculated and found to be 9.2 × 10−4 M/s for the first addition and 1.8 × 10−4 M/s for the second addition of ECH. The difference in maximum rate between the first and second addition is due primarily to decreased concentration of monomer. The performance of TAxEDA as measured by in situ FTIR was consistent with a living polymerization. The living nature of TAxEDA-initiated polymerization enables control of polymer architecture and composition by sequential addition of different monomers to produce block or statistical copolymers. A sequential copolymerization of AGE and ECH was monitored with in situ FTIR spectroscopy. Figure 6b is a plot of the relative monomer concentration over time obtained by monitoring the epoxide signal at 852 cm−1. We first injected an aliquot (ca. 10 mL) of AGE into a round-bottom flask charged with TAxEDA. The AGE monomer was fully converted to PAGE after ca. 46 h, as evidenced by the plateau lasting ca. 5 h as seen in Figure 6b. At 48 h, an aliquot (ca. 3.3 mL) of ECH was injected, which was characterized by an increase in intensity of the epoxide signal. The relative monomer concentration decreased over time until all epoxide signal had vanished. The effective propagation rate constants for AGE (kpAGE) and ECH (kpECH) were calculated from the data to be kpAGE = 2.6 × 10−4 ± 2.1 × 10−5 M−1 s−1 and kpECH = 4.9 × 10−4 ± 1 × 10−5 M−1 s−1. These effective rate constants compared favorably to those calculated for the respective homopolymerizations: kpAGE = 7.7 × 10−4 M−1 s−1 and kpECH = 4.3 × 10−4 M−1 s−1. The maximum polymerization rate was also determined to be −d[AGE]/dt = 2.0 × 10−4 M/s for AGE and −d[ECH]/dt = 2.8 × 10−4 M/s for ECH, which compare favorably to the rates for the homopolymerizations. The final diblock copolymer (BCP) was characterized by NMR spectroscopy. Figure 7 contains a 1H (a) and 13C (b) NMR spectrum of the BCP. The block molecular weights were determined by end-group analysis to be MnPAGE = 14 kg/mol and MnPECH = 6.6 kg/mol, which matched well with the targeted molecular weights of 14.8 and 5 kg/mol for PAGE and PECH. The 13C NMR spectrum revealed sharp peaks associated with PAGE and PECH homotriad sequences, with no cross-peaks associated with mixed heterotriad sequences H

DOI: 10.1021/acs.macromol.7b00196 Macromolecules XXXX, XXX, XXX−XXX

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(6) Hans, M.; Keul, H.; Moeller, M. Chain Transfer Reactions Limit the Molecular Weight of Polyglycidol Prepared via Alkali Metal Based Initiating Systems. Polymer 2009, 50, 1103−1108. (7) Ding, J.; Price, C.; Booth, C. Use of Crown Ether in the Anionic Polymerization of Propylene Oxide−1. Rate of Polymerization. Eur. Polym. J. 1991, 27, 891−894. (8) Yu, G.-E.; Heatley, F.; Booth, C.; Blease, T. G. Anionic Copolymerisation of Ethylene Oxide and Propylene Oxide. Investigation of Double-Bond Content by NMR Spectroscopy. Eur. Polym. J. 1995, 31, 589−593. (9) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129−8152. (10) Sarazin, Y.; Carpentier, J.-F. Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides. Chem. Rev. 2015, 115, 3564−3614. (11) Mason, M. R.; Perkins, A. M. AlkylaluminophosphonateCatalyzed Ring-Opening Homopolymerization of Epichlorohydrin and Propylene Oxide. J. Organomet. Chem. 2000, 599, 200−207. (12) Billouard, C.; Carlotti, S.; Desbois, P.; Deffieux, A. Controlled” High-Speed Anionic Polymerization of Propylene Oxide Initiated by Alkali Metal Alkoxide/Trialkylaluminum Systems. Macromolecules 2004, 37, 4038−4043. (13) Brocas, A.-L.; Deffieux, A.; Le Malicot, N.; Carlotti, S. Combination of Phosphazene Base and Triisobutylaluminum for the Rapid Synthesis of Polyhydroxy Telechelic Poly(Propylene Oxide). Polym. Chem. 2012, 3, 1189−7. (14) Rejsek, V.; Sauvanier, D.; Billouard, C.; Desbois, P.; Deffieux, A.; Carlotti, S. Controlled Anionic Homo- and Copolymerization of Ethylene Oxide and Propylene Oxide by Monomer Activation. Macromolecules 2007, 40, 6510−6514. (15) Gervais, M.; Labbé, A.; Carlotti, S.; Deffieux, A. Direct Synthesis of α-Azido,Ω-Hydroxypolyethers by Monomer-Activated Anionic Polymerization. Macromolecules 2009, 42, 2395−2400. (16) Carlotti, S.; Labbé, A.; Rejsek, V.; Doutaz, S.; Gervais, M.; Deffieux, A. Living/Controlled Anionic Polymerization and Copolymerization of Epichlorohydrin with Tetraoctylammonium Bromide− Triisobutylaluminum Initiating Systems. Macromolecules 2008, 41, 7058−7062. (17) Peretti, K. L.; Ajiro, H.; Cohen, C. T.; Lobkovsky, E. B.; Coates, G. W. A Highly Active, Isospecific Cobalt Catalyst for Propylene Oxide Polymerization. J. Am. Chem. Soc. 2005, 127, 11566−11567. (18) Widger, P. C. B.; Ahmed, S. M.; Hirahata, W.; Thomas, R. M.; Lobkovsky, E. B.; Coates, G. W. Isospecific Polymerization of Racemic Epoxides: A Catalyst System for the Synthesis of Highly Isotactic Polyethers. Chem. Commun. 2010, 46, 2935−2937. (19) Aida, T.; Mizuta, R.; Yoshida, Y.; Inoue, S. Polymerization of Epoxides Catalysed by Metalloporphine. Makromol. Chem. 1981, 182, 1073−1079. (20) Yasuda, T.; Aida, T.; Inoue, S. Synthesis of Polyester-Polyether Block Copolymer with Controlled Chain Length From β-Lactone and Epoxide by Aluminum Porphyrin Catalyst. Macromolecules 1984, 17, 2217−2222. (21) Aida, T.; Wada, K.; Inoue, S. Copolymerization of Epoxides by Aluminum Porphyrin - Reactivity of (Porphinato)Aluminum Alkoxide as Growing Species. Macromolecules 1987, 20, 237−241. (22) Vandenberg, E. J. Organometallic Catalysts for Polymerizing Monosubstituted Epoxides. J. Polym. Sci. 1960, 47, 486−489. (23) Vandenberg, E. J. Some Aspects of the Bimetallic μ-OxoAlkoxides for Polymerizing Epoxides to Polyether Elastomers. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 1423−1431. (24) Vojkovsky, T.; Sullivan, B.; Sill, K. N. Synthesis of Heterobifunctional Polyethylene Glycols: Polymerization From Functional Initiators. Polymer 2016, 105, 72−78. (25) Van Aelstyn, M. A.; Keizer, T. S.; Klopotek, D. L.; Liu, S.; Munoz-Hernandez, M.-A.; Wei, P.; Atwood, D. A. Bimetallic Aluminum and Gallium Chelates with N2O2 Ligands. Organometallics 2000, 19, 1796−1801.

CONCLUSION We presented a new, mono-μ-alkoxo-dialuminum initiator for epoxide polymerization, which we call the trialkylaluminum adduct of (2-dibenzylamino)ethoxydialkylaluminum (TAxEDA). The TAxEDA initiator provided control over molecular weight, composition, architecture, and end-group functionality resulting in well-defined, heterobifunctional poly(ether)s. The TAxEDA initiator combines many of the advantages of traditional anionic and catalytic or activated ring-opening polymerization of epoxides. Further investigation of the polymerization mechanism, tolerance to chemical functionality, optimization of polymerization conditions and initiator structure, and the exploration of new materials synthesis enabled by TAxEDA is currently underway.

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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.7b00196. 1 H NMR spectroscopy of the initiator, its components, and resultant polymers, and 13C NMR spectra of resultant polymers; GPC traces for polymers; CIF information for the XRD of the initiator (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (N.A.L.). ORCID

Christina G. Rodriguez: 0000-0001-8681-4002 Nathaniel A. Lynd: 0000-0003-3010-5068 Author Contributions

C.G.R. and R.C.F. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge partial support of this work through the Welch Foundation (Grant No. F-1904). Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (56387-DNI7). The authors thank Martin Wolffs of DSM (Netherlands) for the synthesis of N,N-dibenzylethanolamine used in this work.

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REFERENCES

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