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Oct 24, 2016 - neutral SKA alone mediates either controlled or extremely rapid polymerization of acrylic monomers such as methyl methacrylate (MMA) in...
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Controlled or High-Speed Group Transfer Polymerization by Silyl Ketene Acetals without Catalyst Jiawei Chen,† Ravikumar R. Gowda,† Jianghua He,† Yuetao Zhang,†,‡ and Eugene Y.-X. Chen*,† †

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, China

‡

S Supporting Information *

ABSTRACT: Group transfer polymerization (GTP) is an important ambient-temperature living polymerization method using silyl ketene acetal (SKA) or related initiators. Although several different GTP systems have been developed for polymerizing acrylic monomers, they all require the use of a catalyst to activate the SKA initiator, commonly believed to be ineffective on its own. Now, this work shows that, in fact, the neutral SKA alone mediates either controlled or extremely rapid polymerization of acrylic monomers such as methyl methacrylate (MMA) in polar donor solvents such as DMF, depending on the nuclearity of the SKA and the chelating pendant group on Si. In the case of a mono-SKA such as Me2CC(OMe)OSiMe3, the GTP of MMA in DMF is relatively slow (several hours to completion) but is controlled and remarkably efficient, producing PMMA with Mn values close to those predicted on the basis of the [M]/[I] ratio, low Đ values (≤1.2), and high initiation efficiencies (≥80%). In sharp contrast, the di-SKAs linked by an oxo, ferrocenyl, or binaphthyl bridge, as well as the mono-SKA with a donor chelating methoxy pendant group on Si, mediate extremely rapid polymerization (a few seconds to completion), affording an extremely high turnover frequency up to 1.92 × 105 h−1, but the polymerization is uncontrolled. Several lines of evidence obtained through mechanistic studies indicate that the polymerization by the mono-SKA and di-SKA in DMF proceeds through a dissociative pathway with the released enolate anion being the highly active species and the polymerization characteristics are highly dependent on the amount of free enolate anions in solution. In this mechanism, the donor ability of the solvent plays a critical role in promoting the activity through activation of the Si site of the neutral SKA by forming the pentacoordinate Si intermediate.

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catalysts.4 In the earlier investigations of GTP, anionic bases such as SiMe3F2−,1b,5 HF2−,1b,5,6 F−,5b,6,7 CN−,1b,5b,6,7 N3−,1b,6 oxyanions,8 and hydrogen bioxyanions8,9 were employed. More recently, neutral and organic bases such as N-heterocyclic carbenes,10 phosphorus-based neculeophiles,11 and phosphazene superbases11b,12 have been utilized as well. It is generally accepted that the SKA initiator is activated upon the formation of a pentacoordinate silicon species through the coordination of a specific nucleophilic catalyst to the SKA. The subsequent mechanistic pathways diverge into two possible scenarios (Scheme 1, A). In the associative pathway, the five coordinate species is proposed to undergo repetitive intramolecular Michael addition with the incoming monomer, most likely in a concerted fashion, to form the polymer chain. On the contrary, in the dissociative mechanism, the transient fivecoordinate silicon intermediate releases an enolate anion that promotes the chain propagation. In both cases, the activation of the SKA initiator by nucleophilic catalysts with high basicity to form the hypervalent silicate key intermediate, which is also

INTRODUCTION The concept of the group transfer polymerization (GTP) was first introduced in 1983 by Webster et al.1 at DuPont with the aim to develop a new living polymerization method for polymerizing (meth)acrylate monomers, such as methyl methacrylate (MMA), which could operate at ambient or higher temperatures to produced acrylic polymers with controlled structures including predictable number-average molecular weight (Mn) and narrow molecular weight dispersity (Đ). The typical GTP uses a silyl ketene acetal (SKA) initiator and a nucleophilic or Lewis acid catalyst, and the polymerization was termed such based on the initially postulated associative propagation mechanism in which the silyl group remains attached to the same polymer chain and is simply transferred intramolecularly to the incoming monomer through hypervalent anionic silicon species (path a, Scheme 1). However, this associative mechanism was challenged by a dissociative mechanism,2 which involves the ester enolate anion as the propagating species and a rapid, reversible complexation or termination of small concentrations of the enolate anion with SKA or its polymer homologue (path b, Scheme 1).3 Typical SKA initiators themselves are regarded as inactive, requiring activation by nucleophilic neutral or anionic bases as © XXXX American Chemical Society

Received: July 29, 2016 Revised: October 5, 2016

A

DOI: 10.1021/acs.macromol.6b01654 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Four Types of SKA Activation Pathways, Including the New Solvent-Activation Pathway Reported in This Work, for GTP

termed as reductive activation, has been considered to be a necessary and pivotal step.13 Over the years, great progress had been made to improve such method and the related development has been reviewed extensively.3,4 Brønsted and Lewis acids (LAs) have been employed as activators for GTP as well.4 We have reported earlier that the GTP can also proceed through oxidative activation of SKA with a catalytic amount of [Ph3C][B(C6F5)4] (TTPB) which led to a high-speed living (meth)acrylate polymerization system catalyzed by the silylium ion R3Si+ involving a propagation “catalysis” cycle consisting of a fast step of recapturing the

silylium catalyst from the ester group of the growing polymer chain by the incoming MMA, followed by a rate-determining step (r.d.s.) of the C−C bond formation via intermolecular Michael addition of the polymeric SKA to the silylated MMA (Scheme 1, B).14 By covalently linking two SKA units into close proximity, we developed unimolecular silyl enolate/silylium nucleophile/electrophile bifunctional active species, generated upon oxidative activation with TTPB, which exhibit unique polymerization and kinetic characteristics as well as a rate enhancement by a factor of >40 and high stereoselectivity (at low temperature), as compared to the mononuclear SKA B

DOI: 10.1021/acs.macromol.6b01654 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules system.15 A related activation method for GTP involves the in situ generation of SKA initiators by 1,4-hydrosilylation of a methacrylate monomer. In such a process, the catalysts, typically highly electron deficient silylium cations “R3Si+”16 or strong organo-LAs such as B(C6F5)317 and Al(C6F5)3,18 play a dual role in both hydrosilylation (via “frustrated Lewis pair (FLP)-type activation) and activation of monomers (classical LA activation), thus termed “tandem (FLP & LA) activation” (Scheme 1, C). As can be seen from the above overview, all of the existing GTP methods require addition of external catalysts or activators, such as nucleophilic bases, or Lewis or Brønsted acids. The typical solvents used in the GTP are CH3CN or THF (for nucleophilic base catalysts) and CH2Cl2 or toluene (for Lewis acid catalysts). The research in the GTP has been largely directed at the mechanistic elucidation and extension of catalyst and monomer scopes, without much emphasis on the solvent effect. On the other hand, it is well-known that tetracoordinate silicon compounds can interact with polar donor solvents and accommodate such solvent molecules to achieve hypervalent structures,19 which is the central feature of the reductive activation of SKA. In this context, we reasoned that a suitable donor solvent that is sufficiently nucleophilic could activate the neutral SKA initiator to form a pentacoordinate hypervalent intermediate (scenario D, Scheme 1) that becomes active for GTP via either an associative (through the hexacoordinate species) or a dissociative (through the enolate anion) pathway. Accordingly, the central objective of this work was to develop an active GTP system by using the neutral SKA initiator alone in a polar solvent, without any additional catalyst or activator that the traditional GTP process uses. This contribution presents a full account of our effort in this regard, including the synthesis of five new mono- and diSKA initiators, one of which was structurally characterized, investigation into the characteristics of the polymerization of MMA and the biomass-derived renewable γ-methyl-α-methylene-γ-butyrolactone (MMBL),20 two representative linear and cyclic acrylic monomers, using seven mono- and di-SKAs in solvents with different polarity and donicity, as well as kinetic and mechanistic studies to elucidate the possible mechanisms of the polymerization by the neutral SKA alone in polar solvents.

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(HRMS) data were collected on an Agilent 6220 Accurate time-offlight LC/MS spectrometer. Methyl methacrylate (MMA) was purchased from Sigma-Aldrich Co., while γ-methyl-α-methylene-γ-butyrolactone (MMBL) was purchased from TCI America. These two monomers were first degassed and dried over CaH2 overnight, followed by vacuum distillation. Further purification of MMA involved titration with neat tri(n-octyl)aluminum to a yellow end point,21 followed by distillation under reduced pressure. All purified monomers were stored in brown bottles and stored inside a glovebox freezer at −30 °C. Butylated hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-methylphenol) was purchased from Aldrich Chemical Co. and recrystallized from hexanes prior to use. All other chemicals were purchased from Aldrich Chemical Co. and used as received. Tris(pentafluorophenyl)borane, B(C6F5)3, and triphenylmethyl tetrakis(pentafluorophenyl)borate (TTPB), [Ph3C][B(C6F5)4], were obtained as a research gift from Boulder Scientific Company. B(C6F5)3 was further purified by recrystallization from hexanes at −30 °C, whereas TTPB was used as received. Diisopropylamine, methyl isobutyrate, and methyl trimethylsilyl dimethylketene acetal (MeSKA) were purchased from Sigma-Aldrich and dried over CaH2, followed by vacuum distillation. Literature procedures were employed for the preparation of the following compounds: 1,3-bis([(1-methoxy-2-methyl-1-propenyl)oxy])-1,1,3,3-tetramethyldisiloxane (MeSKA2),15 1,1′-dilithioferrocene· TMEDA,22 1,1′-bis(dimethylsilyl)ferrocene [Fc(SiMe2H)2],23 1,1′bis(chlorodimethylsilyl)ferrocene,24 chlorodimethylsilylferrocene, and dicholorodiferrocenylsilane.25 X-ray diffraction intensities were collected on a Bruker SMART APEX CCD Diffractometer using Mo Kα (0.71073 Å) radiation at 100 K. The structures were solved by direct methods and refined using the Bruker SHELXTL program library by full-matrix least-squares on F2 for all reflections.30 All non-hydrogen atoms were refined with anisotropic displacement parameters, whereas hydrogen atoms were included in the structure factor calculations at idealized positions. Crystallographic data for the structure of Fc2SKA2 (CCDC 1490874) has been deposited with the Cambridge Crystallographic Data Center as supplementary publications. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Typical Procedure for the Preparation Silyl Ketene Acetal (SKA) Derivatives, using 1,3-Bis([(1-methoxy-2-methyl-1propenyl)oxy])-1,1,3,3-tetramethyldisiloxane (MeSKA2) as an Example. Literature procedures15 for general synthesis of SKAs were modified for the preparation of the employed SKA derivatives. In a nitrogen-filled glovebox, a 200 mL Schlenk flask equipped with a stir bar was charged with THF (100 mL) and diisopropylamine (9.88 mL, 7.10 g, 73.0 mmol). This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a Schlenk line, and placed in a 0 °C ice−water bath. nBuLi (44.8 mL, 1.6 M in hexane, 70.0 mmol) was added dropwise via syringe to the flask. Methyl isobutyrate (8.05 mL, 7.19 g, 70.0 mmol) was added to the above solution, after being stirred at 0 °C for 30 min. The resulting mixture was stirred at this temperature for 30 min to generate the corresponding lithium methyl isobutyrate Me2CC(OMe)OLi, after which 1,3-dichloro-1,1,3,3tetramethyldisiloxane (7.10 g, 35.0 mmol) was added. The mixture was allowed to warm slowly to room temperature and stirred overnight at this temperature, after which all volatiles were removed under vacuum. The resulting oil was dissolved in hexanes, and the resulting precipitates were filtered off under an argon atmosphere. The volatiles were once again removed, yielding the final product (8.5 g, 72.4%) as a yellow oil. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 3.52 (s, 6H, OMe), 1.57 (s, 6H, CMe2), 1.53 (s, 6H, CMe2), 0.21 (s, 12H, SiMe2). 13 C NMR (CDCl3, 100 MHz, 25 °C): δ 148.9 [C(OMe)], 91.3 ( CMe2), 56.9 (OMe), 17.0, 16.3 (CMe), −0.5 (SiMe2). 29Si NMR (C6D6, 79 MHz, 25 °C): δ −12.7. HRMS (APCI, + mode) m/z calculated for C14H30NaO5Si2 ([M + Na]+): 357.1524, found: 357.1518, 100%. Synthesis of MeOCH2CH2OSiMe2Cl. In an argon-filled glovebox, a 100 mL Schlenk flask equipped with a stir bar was charged with CH2Cl2 (50 mL), B(C6F5)3 (439 mg, 0.858 mmol) and 2-

EXPERIMENTAL SECTION

Materials, Reagents, and Methods. All syntheses and manipulations of air- and moisture-sensitive materials were carried out in flamed Schlenk-type glassware on a dual-manifold Schlenk line, on a high-vacuum line, or in an inert gas-filled glovebox. NMR-scale reactions were conducted in Teflon-valve-sealed J. Young-type NMR tubes. HPLC-grade organic solvents were first sparged extensively with nitrogen during filling 20 L solvent reservoirs and then dried by passage through activated alumina (for Et2O, THF, and CH2Cl2) followed by passage through Q-5 supported copper catalyst (for toluene and hexanes) stainless steel columns. Benzene-d6 and toluened8 were dried over sodium/potassium alloy and vacuum-distilled or filtered, whereas CD2Cl2, CDCl3, DMF-d7 and bromobenzene-d5 were distilled over CaH2 and stored over activated Davison 4 Å molecular sieves. HPLC-grade dimethylformamide (DMF), acetonitrile, DMSO, fluorobenzene and o-difluorobezene were degassed, dried over CaH2 overnight, filtered, and vacuum-distilled. Acetone was degassed and dried over CaSO4, followed by vacuum transfer. NMR spectra were recorded on a Varian 400 MHz (400 MHz, 1H; 100 MHz, 13C; 79 MHz, 29Si) spectrometer. Chemical shifts for 1H, 13C, and 29Si spectra were referenced to internal solvent resonances and are reported as parts per million relative to SiMe4. High-resolution mass spectrometry C

DOI: 10.1021/acs.macromol.6b01654 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules methoxyethanol (1.35 mL, 17.2 mmol). This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a Schlenk line, and placed in a −78 °C dry ice−acetone bath. Dimethylchlorosilane (2.00 mL, 18.0 mmol) was added dropwise via syringe to the flask. The resulting mixture was allowed to warm up to room temperature over a period of 30 min and then subjected to vacuum. The residue was purified by distillation under vacuum to afford the product as colorless oil. Yield: 2.34 g (81%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 3.71 (t, J = 4.8 Hz, 2H, CH2), 3.21 (t, J = 4.8 Hz, 2H, CH2), 3.05 (s, 3H, OMe), 0.30 (s, 6H, SiMe2). 13C NMR (C6D6, 100 MHz, 25 °C): δ 73.0 (OMe), 62.5 (CH2), 58.1 (CH2), 1.61 (SiMe2). 29 Si NMR (C6D6, 79 MHz, 25 °C): δ 14.0. Synthesis of MeOCH2CH2OSiMe2OC(OMe)CMe2 (MeOSKA). To the MeOCH2CH2OSiMe2Cl (0.70 g, 4.15 mmol) solution in THF (5 mL) was added a solution of in situ generated Me2C C(OMe)OLi (4.15 mol) in THF (5 mL) at 0 °C. The mixture was allowed to warm slowly to room temperature and stirred overnight at this temperature, after which all volatiles were removed under vacuum. The resulting oil was dissolved in hexanes, and the resulting precipitates were filtered off under an argon atmosphere. The volatiles were once again removed, yielding the final product as pale yellow oil. Yield: 0.923 g (95%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 3.83 (t, J = 4.8 Hz, 2H, CH2), 3.41 (s, 3H, OMe), 3.30 (t, J = 4.8 Hz, 2H, CH2), 3.31 (s, 3H, OMe), 1.72 (s, 3H, CMe2), 1.69 (s, 3H, CMe2), 0.25 (s, 6H, SiMe2). 13C NMR (C6D6, 100 MHz, 25 °C): δ 149.3 [ C(OMe)], 90.2 (CMe2), 73.6 (OMe), 62.1 (CH2), 58.2 (CH2), 56.2 (OMe), 16.7 (CMe), 16.1 (CMe), −2.93 (SiMe2). 29Si NMR (C6D6, 79 MHz, 25 °C): δ −4.05. HRMS (APCI, + mode) m/z: calculated for C10H22NaO4Si ([M + Na]+), 257.1185; found, 257.1178. Synthesis of FcSKA. To the in situ generated solution of Me2C C(OMe)OLi (3.59 mol) in THF (5 mL) at 0 °C was added a solution of chlorodimethylsilylferrocene (1.00 g, 3.59 mmol) in THF (5 mL). The mixture was allowed to warm slowly to room temperature and stirred overnight at this temperature, after which all volatiles were removed under vacuum. The resulting oil was dissolved in hexanes, and the resulting precipitates were filtered off under an argon atmosphere. The volatiles were once again removed, yielding the final product as a red-orange oil. Yield: 1.14 g (92%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 4.19 (pst, J = 1.8 Hz, 2H, Cp), 4.13 (pst, J = 1.8 Hz, 2H, Cp), 3.99 (s, 5H, free Cp), 3.31 (s, 3H, OMe), 1.73 (s, 3H,  CMe2), 1.69 (s, 3H, CMe2), 0.49 (s, 6H, SiMe2). 13C NMR (C6D6, 100 MHz, 25 °C): δ 150.5 [C(OMe)], 90.5 (CMe2), 73.6 (Cp), 71.7 (Cp), 68.8 (free Cp), 56.7 (OMe), 17.4, 16.6 (CMe), −0.83 (SiMe2), ipso-Cp-Si not observed. 29Si NMR (C6D6, 79 MHz, 25 °C): δ 10.6. HRMS (APCI, + mode) m/z: calculated for C17H24FeO2Si ([M]+), 344.0890; found, 344.0887, 100%. Synthesis of FcSKA2. To the in situ generated solution of Me2C C(OMe)OLi (5.37 mol) in THF (5 mL) at 0 °C was added a solution of 1,1′-bis(chlorodimethylsilyl)ferrocene (1.00 g, 2.68 mmol) in THF (5 mL). The mixture was allowed to warm slowly to room temperature and stirred overnight at this temperature, after which all volatiles were removed under vacuum. The resulting oil was dissolved in hexanes, and the resulting precipitates were filtered off under an argon atmosphere. The volatiles were once again removed, yielding the final product as a red-orange oil. Yield: 1.20 g (89%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 4.22 (pst, J = 1.8 Hz, 2H, Cp), 4.14 (pst, J = 1.8 Hz, 2H, Cp), 3.30 (s, 6H, OMe), 1.73 (s, 6H, CMe2), 1.68 (s, 6H, CMe2), 0.49 (s, 12H, SiMe2). 13C NMR (C6D6, 100 MHz, 25 °C): δ 150.4 [C(OMe)], 90.6 (CMe2), 73.9 (Cp), 72.3 (Cp), 69.3 (Cp), 56.7 (OMe), 17.4, 16.5 (CMe), −0.74 (SiMe2). 29Si NMR (C6D6, 79 MHz, 25 °C): δ 11.0. HRMS (APCI, + mode) m/z: calculated for C24H38FeO4Si2 ([M]+), 502.1653; found, 502.1650, 16%. HRMS (APCI, + mode) m/z: calculated for C24H38FeNaO4Si2 ([M + Na]+), 525.1551; found, 525.1545, 100%. Synthesis of Fc2SKA2. To the in situ generated solution of Me2CC(OMe)OLi (5.04 mol) in THF (5 mL) at 0 °C was added a solution of dicholorodiferrocenylsilane (0.900 g, 2.52 mmol) in THF (5 mL). The mixture was allowed to warm slowly to room temperature and stirred overnight at this temperature, after which all volatiles were removed under vacuum. The resulting oil was dissolved

in hexanes, and the resulting precipitates were filtered off under an argon atmosphere. The volatiles were once again removed, yielding the final product as an orange solid. Single-crystal X-ray diffraction quality crystals were obtained by recrystallization in hot hexanes. Yield: 1.24 g (82%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 4.60 (pst, J = 1.8 Hz, 4H, Cp), 4.27 (pst, J = 1.8 Hz, 4H, Cp), 4.22 (s, 10H, free Cp), 3.39 (s, 6H, OMe), 1.86 (s, 6H, CMe2), 1.72 (s, 6H, CMe2). 13C NMR (C6D6, 100 MHz, 25 °C): δ 150.5 [C(OMe)], 91.4 ( CMe2), 74.6 (Cp), 71.6 (Cp), 69.4 (free Cp), 65.7 (Cp), 58.3 (OMe), 17.5, 16.6 (CMe), −0.83 (SiMe2). 29Si NMR (C6D6, 79 MHz, 25 °C): δ 0.00. HRMS (APCI, + mode) m/z: calculated for C30H36Fe2O4Si ([M]+), 600.1077; found, 600.1071, 100%; HRMS (APCI, + mode) m/z: calculated for C30H36Fe2NaO4Si ([M + Na]+), 623.0975; found, 623.0965, 58%. Synthesis of rac-2,2′-Bis(chlorodimethylsiloxy)-1,1′-binaphthalene. In an argon-filled glovebox, a 100 mL Schlenk flask equipped with a stir bar was charged with CH2Cl2 (20 mL), dimethylchlorosilane (0.946 g, 10.0 mmol), and B(C6F5)3 (51.2 mg, 0.1 mmol). This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a Schlenk line, and placed in a −78 °C dry ice−acetone bath. rac-1,1′-Bi-2-naphthol (1.432 g, 5.0 mmol) was added dropwise via syringe to the flask. The resulting mixture was allowed to warm up to room temperature over a period of 30 min and then subjected to high vacuum. The residue was then washed with hexanes (2 × 10 mL) and the product was dried under vacuum. Yield: 2.22 g (94%). 1H NMR (CD2Cl2, 400 MHz, 25 °C): δ 7.95−7.18 (12H, binaphthyl), 0.19 (s, 3H, SiMe), 0.11 (s, 3H, SiMe). 13C NMR (CD2Cl2, 100 MHz, 25 °C): δ 149.9, 134.4, 130.4, 129.9, 128.5, 126.8, 126.1, 124.7, 122.3, 121.1 (binaphthyl), 2.41 (SiMe2). 29Si NMR (CD2Cl2, 79 MHz, 25 °C): δ 13.0. Synthesis of BinapSKA2. To the in situ generated solution of Me2CC(OMe)OLi (4.24 mol) in THF (5 mL) at 0 °C was added a solution of rac-2,2′-bis(chlorodimethylsiloxy)-1,1′-binaphthalene (1.00 g, 2.12 mmol) in THF (5 mL). The mixture was allowed to warm slowly to room temperature and stirred overnight at this temperature, after which all volatiles were removed under vacuum. The resulting oil was dissolved in hexanes, and the resulting precipitates were filtered off under an argon atmosphere. The volatiles were once again removed, yielding the final product as a pale yellow oil. Yield: 1.04 g (81%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 7.71−7.00 (12H, binaphthyl), 3.31 (s, 6H, OMe), 1.69 (s, 6H, CMe2), 1.60 (s, 6H, CMe2), 0.01 (s, 6H, SiMe), −0.05 (s, 6H, SiMe). 13C NMR (C6D6, 100 MHz, 25 °C): δ 150.0 [C(OMe)], 149.5, 134.2, 131.0, 130.5, 128.5, 127.5, 126.6, 124.5, 122.2, 122.0 (binaphthyl), 91.4 (CMe2), 56.8 (OMe), 17.1, 16.5 (CMe), −1.84 (SiMe), −3.14 (SiMe). 29Si NMR (C6D6, 79 MHz, 25 °C): δ 12.2. HRMS (APCI, + mode) m/z: calculated for C34H42NaO6Si2 ([M + Na]+), 625.2412; found, 625.2395, 100%. General Polymerization Procedures. Polymerizations were performed either in 25 mL flame-dried Schlenk flasks interfaced to the dual-manifold Schlenk line for runs using external temperature bath, or in 20 mL glass reactors inside the glovebox for ambient temperature (ca. 25 °C) runs. In a normal addition polymerization procedure, the monomer was dissolved in DMF inside a glovebox, and the predetermined amount of SKA complex (stock solution in DMF) was added to the monomer solution to start the polymerization under rapid stirring. In the reverse addition polymerization, a predetermined amount of SKA complex (stock solution) was dissolved in DMF, and the polymerization was started by rapid addition of the corresponding monomer via a gastight syringe to the above solution under vigorous stirring. The amount of the monomer was fixed (0.5 mL, 0.939 mmol for MMA, 0.935 mmol for MMBL) for all polymerization. The overall volume of the polymerization solution was set to 5.0 mL. After the measured time interval, a 0.2 mL aliquot was taken from the reaction mixture via syringe and quickly quenched into a 4 mL vial containing 0.6 mL of undried “wet” CDCl3 stabilized by 250 ppm of BHT-H; the quenched aliquots were later analyzed by 1H NMR to obtain the percent monomer conversion data. After the polymerization was stirred for the stated reaction time, the reactor was taken out of the glovebox and the reaction was quenched by addition of 5 mL of 5% (w/w) HCl-acidified methanol. The quenched mixture was D

DOI: 10.1021/acs.macromol.6b01654 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Chart 1. Mono- and Di-SKA Initiators Employed Directly for the GTP in This Study

Figure 1. X-ray structure (50% thermal displacement) of Fc2SKA2. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) for Fc2SKA2: Si1−C1 1.841 (2), Si1−C11 1.846 (2), Si1−O1 1.642 (2), Si1−O2 1.654 (2), O1−C21 1.361 (3), C21−O3 1.370 (3), C21−C22 1.332 (3), C22− C23 1.504 (3), C22−C24 1.501 (3), O2−C26 1.370 (3), C26−O4 1.382 (3), C26−C27 1.324 (4), C27−C28 1.519 (5), and C27−C29 1.483 (4).

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precipitated into 100 mL of methanol, stirred for 1 h, filtered, washed with methanol, and dried in a vacuum oven at 50 °C overnight to a constant weight. Polymer Characterizations. Polymer number-average molecular weights (Mn) and molecular weight dispersity (Đ = Mw/Mn) were measured by gel permeation chromatography (GPC) analyses carried out at 40 °C and a flow rate of 1.0 mL/min, with DMF as the eluent, on a Waters University 1500 GPC instrument equipped with one PLgel 5 μm guard and three PLgel 5 μm mixed-C columns (Polymer Laboratories; linear range of molecular weight = 200−2 000 000). The instrument was calibrated with 10 PMMA standards, and chromatograms were processed with Waters Empower software (version 2002). The isolated low MW polymer samples were analyzed by matrixassissted laser desorption/ionization time-of-flight mass spectroscopy (MALDI−TOF MS); the experiment was performed on a Ultraflex MALDI−TOF mass spectrometer (Bruker Daltonics) operated in positive ion, reflector mode using a Nd:YAG laser at 355 nm and 25 kV accelerating voltage. A thin layer of a 1% NaI solution was first deposited on the target plate, followed by 0.6 μL of both sample and matrix (dithranol, 10 mg/mL in 50% CAN, 0.1% TFA). External calibration was done using a peptide calibration mixture (4−6 peptides) on a spot adjacent to the sample. The raw data were processed in the FlexAnalysis software (version 2.4, Bruker Daltonics).

RESULTS AND DISCUSSION Synthesis and Characterization of Mono- and Di-SKA Initiators. The parent mono-SKA MeSKA, and di-SKA MeSKA2 are known initiators that have been investigated, in combination with a catalyst or activator, for GTP reactions in previous studies.14b,15 To further extend the SKA scope to be employed directly for the polymerization in this study, we synthesized five new mono- and di-SKA initiators, MeOSKA, Fc SKA, FcSKA2, Fc2SKA2, and BinapSKA2 (Chart 1), with unique electronic properties and structural features. In MeOSKA, the silicone atom is attached with a flexible pendant group with a terminal -OMe moiety that could potentially stabilize the pentacoordinate silicate intermediate. The ferrocenyl (Fc) group is electron-rich and also provides a geometrically rigid framework. In the di-SKA case, FcSKA2 and Fc2SKA2 offer a different geometrical feature when compared to the oxygenbridged, flexible MeSKA2. In addition, we also introduced a (racemic) binaphthyl backbone to investigate the effect of the chiral platform on the possible tacticity control of such GTP polymerization. E

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Macromolecules Table 1. Results of MMA Polymerization by Using SKA Alone in DMFa run no. 1 2 3 4 5 6 7

initiator Me

SKA SKA2 MeO SKA Fc SKA Fc SKA2 Fc2 SKA2 Binap SKA2 Me

[M]:[I]

time

convnb (%)

TOF (h−1)

rrc (%)

Mnd (kg/mol)

Đd (Mw /Mn)

I*e (%)

100:1 200:1 200:1 50:1 200:1 50:1 200:1

9h 15 s 15 s 24 h 15 s 15 s 15 s

96.2 100 100 0 100 100 100

11 48000 48000 − 48000 12000 48000

60 59 57 − 58 58 57

12.2 62.4 78.0 − 83.4 119 55.8

1.14 3.49 2.77 − 3.53 3.24 3.99

80 32 26 − 24 4.3 36

a Carried out at ambient temperature (∼25 °C) in 4.5 mL of DMF and 0.5 mL of MMA solution, where [MMA]0 = 0.939 M and [MeSKA]0 = 9.39 mM for a [MMA]:[MeSKA] ratio of 100:1. The SKA solution in DMF was added to the MMA solution in DMF. bMonomer conversions measured by 1H NMR. cTacticity determined by 1H NMR. dNumber-average molecular weight (Mn) and dispersity (Đ) determined by GPC relative to PMMA standards in DMF. eInitiation efficiency (I*) = Mn(calcd)/Mn(exptl), where Mn(calcd) = MW(MMA) × [MMA]0: [Initiator]0 × conversion % + MW of chain-end groups.

species promoted by the coordination solvents to the silicon center by Lambert et al. in 1986.19a,h Cremer et al. further studied such type of solvated silylium cations by 29Si NMR analysis and computational methods.19b−d We reasoned that the coordination of a donor solvent molecule to the tetracoordinate silicon center could resemble the activation of the SKA initiator by nucleophilic catalysts in the GTP process. In the scenario where polar donor solvents such as DMF are sufficiently nucleophilic to activate the SKA species (Scheme 1), an effective GTP could be brought about in the absence of the catalyst that the GTP typically uses, thus creating a GTP system by using the SKA initiator alone in a polar solvent. To test this hypothesis, we first investigated the GTP of MMA in DMF with the mono-SKA MeSKA and the di-SKA Me SKA2. Gratifyingly, MMA (100 equiv) was gradually polymerized by MeSKA, achieving a MMA conversion of 96.2% after 9 h and yielding PMMA with a Mn of 12.2 kg/mol and a low dispersity Đ value of 1.14 (Table 1, run 1). The calculated initiation efficiency (I*) was 80%, indicating a rather efficient GTP process. In fact, this GTP in DMF by the SKA alone is controlled, as evidenced by the observed clearly linear increase (R2 = 0.999) of the PMMA Mn as an increase in the [M]/[I] ratio (50 to 800) while maintaining the low Đ value ≤1.2 (Figure 2). An approximate linear relationship between the PMMA Mn as a function of MMA conversion was also established (Figure S29).

The desired SKA species were synthesized in a straightforward fashion by silicon−lithium exchange between the corresponding in situ generated Li enolate, Me2CC(OMe)OLi, with the specific cholorosilane precursors (see the Supporting Information). The incorporation of the ferrocene moiety is reflected by the red-orange color of the SKA products, as well as by the corresponding characteristic 1H and 13 C NMR resonances (see the Supporting Information). Introduction of the more rigid ferrocenyl group rendered higher crystallinity of the resulting SKA derivatives. While Fc SKA and FcSKA2 are viscous liquids with the tendency to solidify upon standing at −40 °C, Fc2SKA2 is a solid which can be recrystallized from a hot hexanes solution. The electron richness around the silicon center of this series of SKA compounds was probed by the 29Si NMR chemical shifts. The 29 Si NMR resonances from downfield (electron-poor) to upfield (electron-rich) follows the order of: MeSKA (19.0 ppm), BinapSKA2 (12.2 ppm), FcSKA2 (11.0 ppm), FcSKA (10.6 ppm), Fc2SKA2 (0.0 ppm), MeOSKA (−4.05 ppm), and MeSKA2 (−12.7 ppm). All of the current SKA initiators except Fc2SKA2 were isolated as viscous oil, which precludes the structural characterization of these compounds. It is worth noting that the solid-state characterizations for SKA compounds are extremely rare since very few exist in the crystalline form.26 Fortunately, Fc2SKA2, installed with two rigid ferrocene units, can be recrystallized from hot hexanes and its molecular structure was determined by the single-crystal X-ray diffraction method. The molecule of Fc2 SKA2 adopts a geometrical orientation in which two of the ferrocene units are positioned in an orthogonal arrangement to one another, minimizing steric congestion (Figure 1). As a result, the two ester enolate moieties are quite extended out. The ketene acetal functionality is evident from the single C−O bond distance for O1−C21 of 1.361 (3) and O2−C26 of 1.370 (3), as well as the double CC bond distance for C21−C22 of 1.332 (3) and C26−C27 1.324 (4) Å. Overall, the metric parameters of the silyl enolate moieties are rather similar to those of the hexameric lithium enolate species and the monomeric lithium enolate-alane complex that we reported earlier.27 Polymerization of MMA and MMBL by Mono-SKA and Di-SKA Initiators in Polar Solvents. The ability of tetracoordinate silicon species to form hypervalent silicate complexes with solvent molecules was induced by the evidence that silyl salts such as silyl perchlorates showed increased conductivity in polar solvents due to the ionization of such

Figure 2. Plot of the PMMA Mn and Đ values as a function of the [MMA]/[MeSKA] ratio. F

DOI: 10.1021/acs.macromol.6b01654 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Results of MMA Polymerization by FcSKA2 in Different Solventsa run no. 1 2 3 4 5 6 7 8 9 10

conditions 15 24 15 24 24 24 24 24 24 24

s, DMF h, neat s, DMSO h, THF h, acetone h, CH3CN h, C6H4F2 h, C6H5F h, C7H8 h, DCM

dielectric constant (ε)

donor number (kcal/mol)

convnb (%)

TOF (h−1)

Mnc (kg/mol)

Đc (Mw/Mn)

36.1 6.53 46.7 7.6 20.7 38.0 9.9 5.4 2.41 9.1

26.6 n.a. 29.8 20.0 17.0 14.1 ∼3f 3 0.1 1

100 90.5 100 90.0 0 0 0 0 0 0

48 000 7.5 48 000 7.5 − − − − − −

119 30.5 79.9 19.9 − − − − − −

3.24 1.76 4.10 2.00 − − − − − −

Carried out at ambient temperature (∼25 °C) in 4.5 mL of solvent and 0.5 mL of MMA solution, with a [MMA]:[FcSKA2] ratio of 200:1. Monomer conversions measured by 1H NMR. cMn and Đ values determined by GPC relative to PMMA standards in DMF. fEstimated from the value for o-dichlorobenzene (DN = 3 kcal/mol).

a b

Table 3. Results of MMBL Polymerization by Neutral SKA Systema run no. 1 2 3 4 5 6 7

initiator Me

SKA SKA Me SKA2 Me SKA2 Fc SKA Fc SKA2 Binap SKA2 Me

[M]/[I]

solvent

time

convnb (%)

TOF (h−1)

Mnc (kg/mol)

Đc (Mw/Mn)

100:1 100:1 200:1 200:1 100:1 200:1 200:1

neat DMF neat DMF DMF DMF DMF

5m 1h 15 s 15 s 24 h 15 s 15 s

87.6 100 90.0 100 0 100 100

1050 100 43200 48000 − 48000 48000

47.1 80.8 276 215 − 309 163

2.75 2.48 2.35 2.83 − 1.68 2.66

Carried out at ambient temperature (∼25 °C) in 4.5 mL of toluene (TOL) or CH2Cl2 (DCM) or DMF and 0.5 mL of MMBL solution, where [MMBL]0 = 0.935 M and [MeSKA]0 = 9.35 mM for a [MMBL]:[MeSKA] ratio of 100:1. bMonomer conversions measured by 1H NMR. cMn and Đ determined by GPC relative to PMMA standards in DMF. a

More strikingly, when switched to the di-SKA MeSKA2, the rate of MMA polymerization with the same ratio of [MMA]/ [MeSKA2] (or [M]/[I], note that the di-SKA is effectively a single enolate initiator, vide inf ra) was drastically enhanced. For instance, in a [M]/[I] ratio of 200, the polymerization by Me SKA2 achieved completion in 15 s, affording an extremely high turnover frequency (TOF) of 48,000 h−1 (run 2), which represents 2,670-fold rate enhancement over the same polymerization by MeSKA. The polymerization by MeSKA2 produced PMMA with a similar syndiotacticity of 59% rr, relative to the 60% rr observed for the PMMA by MeSKA. However, the di-SKA initiator MeSKA2 led to an uncontrolled polymerization, affording PMMA with a much higher-than-thepredicted Mn of 62.4 kg/mol (thus a low initiation efficiency of 32%) and also a high Đ value of 3.49. Noteworthy is that the mono-SKA MeOSKA that bears the donor methoxy pendant group also exhibits exceedingly high activity and generates PMMA (run 3) that is comparable to that produced by MeSKA2. For the ferrocenyl-substituted SKAs, the mono-SKA FcSKA is ineffective for MMA polymerization up to 24 h, even with a high initiator loading of 2% (run 4). However, similarly, the diSKA initiators FcSKA2 and Fc2SKA2 are effective (runs 5 and 6). In fact, FcSKA2 exhibits essentially identical activity when compared to the initiator MeSKA2 (run 5 vs 2). The PMMA produced also has the same syndiotacticity of 58% rr, but the molecular weight is much higher, thus giving rise to a much lower initiation efficiency of only 24% (run 5). Furthermore, the di-SKA initiator bridged by the chiral binaphthyl platform is also extremely active for the GTP (run 7), performing as well as the other two di-SKA initiators MeSKA2 and FcSKA2 and achieving noticeably better initiation efficiency of 36%. On the other hand, the tacticity of the resulting PMMA (57% rr) is

basically the same as the PMMA by other di-SKA initiator, indicating that the introduction of the chiral binaphthyl backbone has no impact on the stereoselectivity of the GTP. Overall, we observed the drastically enhanced rate of the polymerization by the di-SKA initiators over the mono-SKA initiator, at the expense of the polymerization control. Next, we investigated the MMA polymerization by FcSKA2 in several solvents with different polarity (as defined by dielectric constant ε) and donor ability (as defined by donor number, DN, a quantitative measure of Lewis basicity),28 the result of which are summarized in Table 2. First of all, the polymerization occurred under neat conditions, yielding a gel with a monomer conversion of 90.5% after 24 h (Table 2, run 2). This result indicates that the MMA monomer itself is basic enough to attack the silicon center of the neutral SKA even without the assist of polar donor solvent molecules. The polymerization carried out in DMSO (ε = 46.7, DN = 29.8 kcal/mol) matched the high activity of the polymerization carried out in DMF (ε = 36.1, DN = 26.6 kcal/mol), run 3 vs 1. Since both DMSO and DMF have high ε and DN values, the next set of experiments was designed to delineate which parameter is more important to enable the polymerization activity of the neutral SKA. In this context, we observed no polymerization activity in either acetonitrile (ε = 38.0, DN = 14.1 kcal/mol) or acetone (ε = 20.7, DN = 17 kcal/mol), although they both have high dielectric constants, especially CH3CN which has a higher ε value than DMF. On the other hand, the polymerization carried out in THF (ε = 7.6, DN = 20 kcal/mol) was effective, which achieved a 90.0% conversion in 24 h and gave a high initiation efficiency of 91% (run 4). As THF is a much less polar solvent (ε = 7.6) then acetonitrile (ε = 38.0), but it has a greater DN value (20.0 kcal/mol) than acetonitrile (14.1 kcal/mol), this G

DOI: 10.1021/acs.macromol.6b01654 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules study suggests that the donor ability of the solvent plays a critical role in promoting the activity of the GTP by the neutral SKA alone. None of the solvents with both low dielectric constant and donor number values promoted the polymerization (Table 2, runs 7−10), including the solvents with high silicophilic fluorine atoms such as fluorobenzene and odifluorobenzene. To examine the efficacy of the current neutral SKA system for polymerization of other monomers, we performed the polymerization of the renewable MMBL monomer by different SKA initiators. With a 1.0 mol % loading of MeSKA, the MMBL polymerization without any solvent occurred rapidly, forming a gel within 5 min at 25 °C and thus preventing further efficient stirring of the reaction mixture (Table 3, run 1). Accordingly, the polymerization did not reach quantitative conversion (87.6%) and produced PMMBL with Mn = 47.1 kg/mol and Đ = 2.75. The polymerization by the di-SKA (MeSKA2, 0.5 mol % loading) was even more rapid, achieving 90.0% monomer conversion in 15 s and yielded PMMBL with a high Mn of 276 kg/mol (Table 3, run 3). When carried out in DMF, both polymerizations by MeSKA and MeSKA2 achieved quantitative monomer conversion in short times, 1 h (Table 3, run 2) and 15 s (Table 3, run 4), respectively. Switching the solvent to acetone, no polymerization activity was observed. For the ferrocenyl- and binaphthyl-substituted SKA initiators, di-SKA species FcSKA2 and BinapSKA2 are highly active for MMBL polymerization (Table 3, runs 6 and 7), while FcSKA is inactive (Table 3, run 5), mirroring the results obtained from the MMA polymerization (vide supra). Again, these fast polymerization reactions produced the polymer with the much-higher-thancalculated Mn value, thus yielding low initiation efficiency of 3 orders of magnitude) polymerization rate over the monoSKA system, and remarkably similar performances with essentially identical polymerization activity and stereoselectivity for all di-SKAs investigated in this study, regardless of the diSKA structure and chirality, as the active species involved in this dissociative mechanism is the same ester enolate anion.

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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.6b01654. Additional figures and structural data of Fc2SKA2 (PDF) Cif file for Fc2SKA2 (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*(E. Y.-X.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF-1507702). We thank Boulder Scientific Co. for the research gift of B(C6F5)3 and [Ph3C][B(C6F5)4] and Dr. Brian Newell for assistance on X-ray structural analysis.

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REFERENCES

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