Living Group Transfer Polymerization of Renewable α-Methylene-γ

Feb 12, 2018 - Here we report the room-temperature group transfer polymerization of conjugated polar alkenes, including linear methyl methacrylate (MM...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Living Group Transfer Polymerization of Renewable α‑Methylene-γbutyrolactones Using Al(C6F5)3 Catalyst Lu Hu,† Jianghua He,† Yuetao Zhang,*,† and Eugene Y.-X. Chen‡ †

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

ABSTRACT: Here we report the room-temperature group transfer polymerization of conjugated polar alkenes, including linear methyl methacrylate (MMA) as well as biorenewable, cyclic γ-methyl-α-methylene-γ-butyrolactone (MMBL) and αmethylene-γ-butyrolactone (MBL), by the silyl ketene acetal (SKA)/Al(C6F5)3 ([Al]) system and the detailed study of its polymerization mechanism. The polymerization of MMA by SKA/[Al] was uncontrolled, while the MMBL polymerization by the bulky SKA (iBuSKA)/[Al] system is living and thus produces well-defined PMMBL with a predicted molecular weight (Mn up to 179 kg mol−1), a narrow molecular weight distribution (Đ as low as 1.02), and a high initiation efficiency (I* ≥ 97%). The living polymerization of MMBL was established through five lines of evidence, including predictable polymer Mn and low Đ values, a linear increase of polymer Mn vs monomer conversion, a linear increase of polymer Mn vs monomer-to-initiator ratio, chain extension experiments, and synthesis of well-defined random, diblock, and triblock copolymers. A combined mechanistic study through isolation and characterization of single-monomeraddition intermediates that simulate the active propagating species, polymerization kinetics, and characterization of polymer chain ends has led to a polymerization mechanism. The polymerization is initiated via intermolecular Michael addition of the SKA enolate group to the vinyl group of the [Al]-activated monomer, while the silyl group is transferred to the carbonyl group of the monomer and [Al] to the oxygen atom of SKA; the coordinated [Al] is released to the incoming monomer, followed by repeated intermolecular Michael additions in the subsequent propagation cycle.

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control.8 Recently, we found that the neutral mono-SKA initiator alone in the polar donor solvent such as DMF promotes the controlled GTP of MMA via a dissociative pathway; in contrast, di-SKA linked by the oxo, ferrocenyl, or binaphthyl bridges mediates extremely rapid, but uncontrolled, polymerization of MMA and bio-derived monomer γ-methyl-αmethylene-γ-butyrolactone (MMBL),9 while incomplete conversion was previously reported for the GTP of MMBL at −78 °C.10 Through the oxidative activation of SKAs with a catalytic amount of [Ph3C][B(C6F5)4] (TTPB), we developed the efficient, living GTP of MMA at room temperature catalyzed by silylium ions (R3Si+).11,12 Such a SKA/TTPB system was also highly active for the living polymerization of α-methylene-γbutyrolactone (MBL) and MMBL.13 Covalently linked, unimolecular silyl enolate/silylium nucleophile/electrophile bifunctional active species rendered a rate enhancement by a factor of >40 and high stereoselectivity (at low temperature) as compared to the mononuclear SKA system.14 R3Si+, formed

INTRODUCTION Group transfer polymerization (GTP) is an important living polymerization method thanks to its ability to control the polymerization of (meth)acrylates monomers, such as methyl methacrylate (MMA), at ambient or higher temperature, affording well-defined acrylic polymers.1,2 A typical GTP uses a silyl ketene acetal (SKA) initiator, which is reductively activated by a nucleophilic, and the polymerization proceeds either through a postulated associative propagation mechanism,1,2 in which the silyl group remains attached to the same polymer chain and is transferred intramolecularly to the incoming monomer through hypervalent anionic silicon species, or through a dissociative mechanism,3−5 which involves the ester enolate anion as propagating species and a rapid, reversible complexation (termination) of small concentrations of enolate anions with SKA or its polymer homologue.6 A 0.1− 1% nucleophilic catalyst loading enables the production of poly(methyl methacrylate) (PMMA) with number-average molecular weight (Mn) of ≤20 kg mol−1 in a controlled fashion at T ≥ ambient temperature,7 but it is difficult for GTP to synthesize PMMA with Mn ≥ 60 kg mol−1.6 The use of a Lewis acidic catalyst requires a much higher catalyst loading (typically 10−20 mol %) to achieve a good polymerization © XXXX American Chemical Society

Received: December 13, 2017 Revised: January 29, 2018

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DOI: 10.1021/acs.macromol.7b02647 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

by distillation under reduced pressure. All purified monomers were stored in brown bottles inside a glovebox freezer at −30 °C. Dimethylphenylsilane (Me2PhSiH) and triphenylsilane (Ph3SiH) were purchased from TCI. Triisobutylsilane (iBu3SiH), dimethylethylsilane (Me2EtSiH), and dimethylketene methyl trimethylsilyl acetal (MeSKA) were purchased from Sigma-Aldrich. Butylated hydroxytoluene (BHTH, 2,6-di-tert-butyl-4-methylphenol), triethylsilane (Et3SiH), chlorodimethylsilane (Me2ClSiH), triethylaluminum, and bromopentafluorobenzene were purchased from J&K. Boron trichloride (1.0 M solution in hexanes) and n-BuLi (2.5 M solution in hexanes) were purchased from Energy Chemical. Tris(pentafluorophenyl)borane, B(C6F5)3, was prepared according to literature procedures.21,22 Al(C6F5)3, as a (toluene)0.5 adduct or in its unsolvated form, was prepared by ligand exchange reactions between B(C6F5)3 and AlMe3 or AlEt3 (for preparation of the unsolvated form).23−25 (Extra caution should be exercised when handling these materials, especially the unsolvated Al(C6F5)3, due to its thermal and shock sensitivity!) Literature procedures were employed for the preparation of the following compounds: dimethylketene methyl triethylsilyl acetal Me2C C(OMe)OSiEt3 (EtSKA),10 dimethylketene methyltriphenylsilyl acetal Me2CC(OMe)OSiPh3 (PhSKA),11 dimethylketene methyl triisobutylsilyl acetal Me2CC(OMe)OSi(iBu)3 (iBuSKA),11 dimethylketene methyl dimethylphenylsilyl acetal Me 2 CC(OMe)OSiMe 3 (Me2PhSKA),16 Al(C6F5)3·MMA,26 and Al(C6F5)3·MMBL.27 Synthesis of Dimethylketene Methyl Chlorodimethylsilyl Acetal Me2CC(OMe)OSiMe2Cl (Me2ClSKA). In an argon-filled glovebox, a 100 mL Schlenk flask equipped with a stir bar and charged with CH2Cl2 (50 mL), B(C6F5)3 (128 mg, 0.250 mmol), and MMA (2.65 mL, 25.0 mmol). This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a Schlenk line, and brought to −78 °C. Chlorodimethylsilane (2.78 mL, 25.0 mmol) was added dropwise via syringe to the above 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 vacuum distillation to afford the final product as colorless oil. Yield: 3.84 g (79%). 1H NMR (500 MHz, benzene-d6) δ: 3.35 (s, 3H, OMe), 1.64 (s, 3H, =CMe), 1.61 (s, 3H, =CMe), 0.36 (s, 6H, SiMe2Cl). 13C NMR (126 MHz, benzene-d6) δ: 149.2, 92.8, 57.2, 17.1, 16.4, 2.3. Synthesis of Chloro(ethoxy)dimethylsilane Me2(EtO)SiCl. Literature procedures9 were modified to prepare Me2(EtO)SiCl. In an argon-filled glovebox, a 200 mL Schlenk flask equipped with a stir bar was charged with CH2Cl2 (100 mL), B(C6F5)3 (461 mg, 0.90 mmol), and EtOH (5.08 mL, 90.0 mmol). This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a Schlenk line, and brought to −78 °C for 20 min. Chlorodimethylsilane (10.0 mL, 90.0 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 to afford the product as colorless oil. Yield: 12.48 g (91%). 1H NMR (500 MHz, benzene-d6) δ: 3.61 (q, J = 7.0 Hz, 2H, OCH2), 1.06 (t, J = 7.0 Hz, 3H, OCH2CH3), 0.26 (s, 6H, SiMe2). Synthesis of Dimethylketene Methylethoxydimethylsilyl Acetal Me2CC(OMe)OSiMe2(EtO) (Me2(EtO)SKA). Literature procedures for the general synthesis of ketene trialkylsilyl acetals28,29 were modified to prepare Me2(EtO)SKA. In an argon-filled glovebox, a 200 mL Schlenk flask was equipped with a stir bar and charged with THF (100 mL) and diisopropylamine (7.05 mL, 5.06 g, 50.0 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. nButyllithium (32.0 mL, 1.6 M in hexane, 51.2 mmol) was added dropwise via syringe to the above flask. After being stirred at 0 °C for 30 min, methyl isobutyrate (5.74 mL, 5.11 g, 50.0 mmol) was added to this solution. The resulting mixture was stirred at this temperature for 30 min, after which chloro(ethoxy)dimethylsilane (7.61 g, 50.0 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 in vacuum and hexanes (50 mL) was added. The resulting precipitates were filtered off under an argon atmosphere; the solvent of the filtrate was removed in vacuo. The residue was

from the combination of R3SiH (Et3SiH or Me2PhSiH) with [Et3Si(L)]+[B(C6F5)4]−, or in the form of a single component silylium−silane complex, [Et3Si−H−SiEt3]+[B(C6F5)4]−, was shown to exhibit dual functions, first catalyzing 1,4-hydrosilylation of MMA with R3SiH to generate the SKA initiator in situ (via “frustrated Lewis pair (FLP)-type activation) and then catalyzing the subsequent living polymerization of MMA (via classical LA activation of monomer).15 Through such a tandem (FLP and LA) activation, B(C6F5)3 also functions as a dual catalyst in the controlled polymerizations of n-butyl acrylate,16 alkyl methacrylates,17 and N,N-disubstituted acrylamides,18 using R3SiH/B(C6F5)3. On the other hand, the R3SiH/ Al(C6F5)3 system was shown to be much less effective and ill-controlled for MMA polymerization,19 and there is no report on the use of the SKA/Al(C6F5)3 system for polymerization. Here, we report that the SKA/Al(C6F5)3 system promotes the living GTP of MMBL and copolymerization of MMBL and MBL to produce well-defined (co)polymers with predicted molecular weight, narrow molecular weight distribution, and high initiation efficiency (Scheme 1). More importantly, this Scheme 1. Living GTP of (M)MBL by the iBuSKA/Al(C6F5)3 System

SKA/Al(C6F5)3 system enabled us to characterize key reaction intermediates and perform kinetic and mechanistic studies, thereby providing the needed insights into the polymerization mechanism.

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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 or an argon-filled glovebox. Toluene, benzene, THF, and hexane were refluxed over sodium/potassium alloy distilled under a nitrogen atmosphere and then stored over molecular sieves 4 Å. CH2Cl2 was refluxed over CaH2 distilled under a nitrogen atmosphere and then stored over molecular sieves 4 Å. Benzene-d6 and CD2Cl2 were dried over molecular sieves 4 Å. NMR spectra were recorded on a Bruker Avance II 500 (500 MHz, 1H; 126 MHz, 13C; 471 MHz, 19F) instrument at room temperature. Chemical shifts for 1H and 13C spectra were referenced to internal solvent resonances and are reported as parts per million relative to SiMe4, whereas 19F NMR spectra were referenced to external CFCl3. Air-sensitive NMR samples were conducted in Teflon-valve sealed J. Young-type NMR tubes. Methyl methacrylate (MMA) was purchased from J&K, while αmethylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (MMBL) were purchased from TCI. These monomers were first degassed and dried over CaH2 overnight, followed by vacuum distillation. Further purification of MMA involved titration with tri(noctyl)aluminum (Strem Chemicals) to a yellow end point,20 followed B

DOI: 10.1021/acs.macromol.7b02647 Macromolecules XXXX, XXX, XXX−XXX

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Al(C6F5)3·MMA, forming cleanly iBu3Si-MMBL-CMe2C(OMe)O··· Al(C6F5)3 (4). 1H NMR (500 MHz, benzene-d6) δ: 4.30 (ddq, J = 9.4, 6.6, 6.2 Hz, 1H, OCH), 3.24 (s, 3H, OMe), 2.29 (dd, J = 12.9, 9.4 Hz, 1H, CH2), 2.27 (d, J = 14.2 Hz, 1H, CH2), 2.20 (d, J = 14.3 Hz, 1H, CH2), 1.88 (hept, J = 6.7 Hz, 3H, CH), 1.79 (dd, J = 12.8, 6.6 Hz, 1H, CH2), 1.10 (d, J = 6.2 Hz, 3H, Me), 1.04 (s, 3H, Me2), 1.03 (s, 3H, Me2), 1.00 (d, J = 6.6 Hz, 18H, CHMe2), 0.78 (d, J = 7.0 Hz, 6H, SiCH2). 19F NMR (471 MHz, benzene-d6) δ: −122.91 (d, J = 21.3 Hz, 6F, o-F), −151.63 (t, J = 19.8 Hz, p-F), −160.85 (m, 6F, m-F). General Polymerization Procedures. Polymerizations were performed in 20 mL glass reactors inside the glovebox for ambient temperature (ca. 25 °C) runs. In a typical polymerization procedure, a predetermined amount of a Lewis acid (LA), such as Al(C6F5)3, or Al(C6F5)3 adduct, was first dissolved in monomer (500 μL for MMA, 250 μL for MMBL, or 205 μL for MBL, 200 equiv relative to the SKA) and solvent inside a glovebox. The polymerization was started by rapid addition of a solution of a SKA (1 equiv of a LA) in 1.0 mL of solvent via a gastight syringe to the above mixture under vigorous stirring. The amount of the monomer was fixed for all polymerization. After the measured time interval, a 0.1 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, then the reactor was taken out of the glovebox, and the reaction was quenched by addition of 5 mL of 5% HCl-acidified methanol. The quenched mixture was isolated by filtration and dried in a vacuum oven at room temperature to a constant weight. Polymerization Kinetics. Kinetic experiments were carried out in a stirred glass reactor at ambient temperature (ca. 25 °C) inside an argon-filled glovebox using the polymerization procedure already described above, with the [Al(C6F5)3]/[iBuSKA] ratio was 0.5/1, 1/1, 2/1, and 4/1, [MMBL]0 was fixed at 0.936 M, and [iBuSKA]0 was fixed at 2.34 mM, where [Al(C6F5)3] = 1.17, 2.34, 4.68, and 9.36 mM in 2.5 mL mixture solutions. At appropriate time intervals, 0.1 mL aliquots were withdrawn from the reaction mixture using a syringe and quickly quenched into 4 mL septum-sealed vials containing 0.6 mL of undried “wet” CDCl3 mixed with 250 ppm BHT-H. The quenched aliquots were analyzed by 1H NMR for determining the ratio of [MMBL]t at a given time t to [MMBL]0, [MMBL]t:[MMBL]0. Apparent rate constants (kapp) were extracted from the slopes of the best fit lines to the plots of [MMBL]t:[MMBL]0 vs time. Another set of kinetic experiments were carried out to determine the kinetic order with respect to [iBuSKA]. In these experiments, with the [Al(C6F5)3]/ [iBuSKA] ratio was 1/0.5, 1/1, 1/2, and 1/4, [MMBL]0 was fixed at 0.936 M, and [Al(C6F5)3]0 was fixed at 2.34 mM, where [iBuSKA] = 1.17, 2.34, 4.68, and 9.36 mM in 2.5 mL mixture solutions. The rest of the procedure was same as the described above. Polymer Characterizations. For P(M)MBL and copolymers, polymer number-average molecular weight (Mn) and molecular weight distributions (Đ = Mw/Mn) were measured by gel permeation chromatography (GPC) coupled with a Wyatt DAWAN 8+ light scattering (LS) detector at 35 °C and a flow rate of 1 mL/min, with DMF (HPLC grade, containing 50 mmol/L LiBr) as an eluent on a Waters 1515 instrument equipped with Waters 4.6 × 30 mm guard column and three Waters WAT054466, WAT044226, and WAT044223 columns (Polymer Laboratories: linear range of molecular weight = 500−4 × 106). The differential refractive index (DRI) increment (dn/dc) value of 0.0844 mL/g was used for PMMBL and 0.0981 for PMBL. For PMMA, Mn and Đ = Mw/Mn were measured by the GPC instrument calibrated with 10 PMMA standards, and chromatograms were processed with Waters Breeze 2 software. Glass transition temperatures (Tg) of the polymers were measured by differential scanning calorimetry (DSC) on a Q20, TA Instruments. Polymer samples were first heated to 250 °C at 20 °C/min, equilibrated at this temperature for 2 min, then cooled to 0 °C at 20 °C/min, held at this temperature for 2 min, and then reheated to

purified by distillation under vacuum to afford the product as colorless oil. Yield: 8.79 g (86%). 1H NMR (500 MHz, benzene-d6) δ: 3.72 (q, J = 7.0 Hz, 2H, OCH2CH3), 3.39 (s, 3H, OMe), 1.72 (s, 3H, =CMe), 1.68 (s, 3H, =CMe), 1.13 (t, J = 7.0 Hz, 3H, OCH2CH3), 0.21 (s, 6H, SiMe2). 13C NMR (126 MHz, benzene-d6) δ: 149.8, 90.7, 58.6, 56.6, 18.5, 17.1, 16.5, −2.5. NMR Reaction of Me2CC(OMe)OSiMe3 (MeSKA) with Al(C6F5)3. In an argon-filled glovebox, a Teflon-valve-sealed J. Young-type NMR tube was charged with 1.74 mg (0.01 mmol) of Me SKA and 0.3 mL of C6D6. A 0.3 mL C6D6 solution of Al(C6F5)3 (5.28 mg, 0.01 mmol) was added to this tube via pipet at room temperature. The mixture was allowed to react for ∼15 min at room temperature before the NMR spectra were recorded, which showed the clean formation of the species Me3SiO(OMe)CC(Me)2· Al(C6F5)3. 1H NMR (500 MHz, benzene-d6) δ: 3.43 (s, 3H, OMe), 1.10 (s, 3H, =CMe), 0.99 (s, 3H, =CMe), −0.13 (s, 9H, SiMe3). 19F NMR (471 MHz, benzene-d6) δ: −121.35 (dd, J = 27.3, 10.9 Hz, 6F, o-F), −151.56 (t, J = 19.8 Hz, 3F, p-F), −161.46 (m, 6F, m-F). NMR Reaction of MeSKA with Al(C6F5)3·MMA. In an argon-filled glovebox, a Teflon-valve-sealed J. Young-type NMR tube was charged with 1.74 mg (0.01 mmol) of MeSKA and 0.3 mL of C6D6. A 0.3 mL C6D6 solution of Al(C6F5)3·MMA (6.28 mg, 0.01 mmol) was slowly added to this tube via pipet at room temperature. The mixture was allowed to react for ∼15 min at room temperature before the NMR spectra were recorded, which showed the clean formation of the species Me 3 SiO(OMe)CC(Me)CH 2 CMe 2 C(OMe)O···Al(C6F5)3 (1) as two isomers (Z/E) in 1:0.9 ratio: major isomer 1A and minor isomer 1B, plus a small amount of unreacted starting materials. 1A: 1H NMR (500 MHz, benzene-d6) δ: 3.22 (s, 3H, OMe), 3.08 (s, 3H, COOMe), 2.16 (s, 2H, CH2), 1.35 (s, 3H, Me), 0.97 (s, 6H, Me2), 0.058 (s, 9H, SiMe3). 1B: δ: 3.27 (s, 3H, OMe), 3.24 (s, 3H, COOMe), 2.18 (s, 2H, CH2), 1.43 (s, 3H, Me), 1.01 (s, 6H, Me2), 0.063 (s, 9H, SiMe3). 19F NMR (471 MHz, benzene-d6) δ: −122.88 (d, J = 18.8 Hz, 6F, o-F), −151.75 (t, J = 19.7 Hz, 3F, p-F), −160.94 (m, 6F, m-F). NMR Reaction of MeSKA with Al(C6F5)3·MMBL. This reaction was carried out in the same manner as the reaction of MeSKA with Al(C6F5)3·MMA, forming cleanly MMBL-based enolaluminate Me3SiO-MMBL-CMe2C(OMe)O···Al(C6F5)3 (2), plus a small amount of unreacted starting materials. 1H NMR (500 MHz, benzene-d6) δ: 4.26 (ddq, J = 9.5, 6.4, 6.2 Hz, 1H, OCH), 3.25 (s, 3H, COOMe), 2.29 (dd, J = 13.0, 9.5 Hz, 1H, CH2), 2.19 (d, J = 14.2 Hz, 1H, CH2), 2.09 (d, J = 14.3 Hz, 1H, CH2), 1.77 (dd, J = 12.9, 6.4 Hz, 1H, CH2), 1.05 (d, J = 6.2 Hz, 3H, Me), 0.99 (br, 6H, Me2), 0.14 (s, 9H, SiMe3). 19F NMR (471 MHz, benzene-d6) δ: −122.96 (br, 6F, o-F), −151.74 (t, J = 18.8 Hz, 3F, p-F), −160.92(br, 6F, m-F). NMR Reaction of Me2CC(OMe)OSi(iBu)3 (iBuSKA) with Al(C6F5)3. This reaction was carried out in the same manner as the reaction of MeSKA with Al(C6F5)3, forming cleanly iBu3SiO(OMe)CC(Me)2·Al(C6F5)3. 1H NMR (500 MHz, benzene-d6) δ: 3.52 (s, 3H, OMe), 1.66 (br, 3H, CHMe2), 1.19 (br, 3H, =CMe), 1.14 (br, 3H, =CMe), 0.87 (d, J = 6.6 Hz, 18H, CHMe2), 0.61 (d, J = 7.0 Hz, 6H, SiCH2). 19F NMR (471 MHz, benzene-d6) δ: −120.95 (dd, J = 27.3, 11.0 Hz, 6F, o-F), −151.59 (t, J = 19.8 Hz, 3F, p-F), −161.33 (m, 6F, m-F). NMR Reaction of iBuSKA with Al(C6F5)3·MMA. This reaction was carried out in the same manner as the reaction of MeSKA with Al(C 6 F 5 ) 3 ·MMA, forming cleanly i Bu 3 SiO(OMe)CC(Me)CH2CMe2C(OMe)O···Al(C6F5)3 (3) as two isomers (Z/E) in 2:1 ratio: major isomer 3A and minor isomer 3B. 3A: 1H NMR (500 MHz, benzene-d6) δ: 3.32 (s, 3H, OMe), 3.17 (s, 3H, COOMe), 2.18 (s, 2H, CH2), 1.87 (m, 3H, CHMe2), 1.46 (s, 3H, Me), 0.994 (d, J = 6.6 Hz, 18H, CHMe2), 0.987 (s, 6H, CMe2), 0.74 (d, J = 6.9 Hz, 6H, SiCH2). 3B: δ: 3.33 (s, 3H, OMe), 3.25 (s, 3H, COOMe), 2.35 (s, 2H, CH2), 1.85 (m, 3H, CHMe2), 1.40 (s, 3H, Me), 1.10 (s, 6H, CMe2), 0.98 (d, J = 7.5 Hz, 18H, CHMe2), 0.74 (d, J = 6.9 Hz, 6H, SiCH2). 19F NMR (471 MHz, benzene-d6) δ: −122.83 (d, J = 19.7 Hz, 6F, o-F), −151.68 (t, J = 19.8 Hz, p-F), −160.89 (m, 6F, m-F). NMR Reaction of iBuSKA with Al(C6F5)3·MMBL. This reaction was carried out in the same manner as the reaction of MeSKA with C

DOI: 10.1021/acs.macromol.7b02647 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules 280 °C at 20 °C/min. All Tg values were obtained from the second scan, after the removal of the thermal history. The isolated low-MW polymer samples were analyzed by matrixassisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS); the experiment was performed on a Bruker Autoflex speed TOF/TOF mass spectrometer in linear, positive ion, reflector mode using a Nd:YAG laser at 355 nm and 25 kV accelerating voltage. A thin layer of 1% CF3COONa solution was first deposited on the target plate, followed by 0.6 μL of both sample and matrix (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propylidene]malonitrile (DCTB), 20 mg/mL in THF). 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.

On the other hand, the high Lewis acidity and oxophilicity of Al(C6F5)3 should be beneficial for the activation of acrylic monomers and thus facilitate the polymer chain propagation via the activated monomer mechanism. In this context, we have demonstrated advantages of Al(C6F5)3 over B(C6F5)3 in several different types of catalytic reactions.19,30,31 On the basis of this line of reasoning, we anticipated the direct use of SKA as initiator and Al(C6F5)3 as catalyst would achieve enhancement in polymerization activity and control. As expected, polymerization rate enhancement was indeed observed for most of the SKA/Al(C6F5)3 system, but overall the polymerization of MMA was still largely uncontrolled with high Đ but low initiator efficiency values (runs 5−25, Table 1). Controlled MMBL Polymerization by R3SiH/Al(C6F5)3 and SKA/Al(C6F5)3. Next, we investigated the effectiveness of the R3SiH/Al(C6F5)3 and SKA/Al(C6F5)3 systems for MMBL polymerization, a cyclic analogue of MMA, the results of which are summarized in Table 2. Among the silanes screened, only Me2EtSiH and Me2PhSiH were found to be effective for MMBL polymerization and achieved quantitative monomer conversion in 6 h for the Me2EtSiH/Al(C6F5)3 system and 24 h for the Me2PhSiH/Al(C6F5)3 system. Compared to the MMA polymerization, the polymerization of MMBL was more controlled, achieving noticeably better initiation efficiency and narrower molecular weight distribution (e.g., Mn = 64.9 kg mol−1, Đ = 1.32, I* = 35% for run 1, Table 2; Mn = 58.6 kg mol−1, Đ = 1.26, I* = 38% for run 2, Table 2). The rest R3SiH/ Al(C6F5)3 systems only yielded negligible polymer up to 24 h, further indicating the difficulty in the in situ formation of the SKA initiator through the Al(C6F5)3-catalyzed 1,4-hydrosilylation of the monomer. Recognizing this deficiency of the [Al]-based system toward the in situ hydrosilylation approach, we switched to use SKA/ Al(C6F5)3 system and observed drastically enhanced both the rate and control of the MMBL polymerization. Under our current standard polymerization conditions {[MMBL]0: [SKA]0:[Al(C6F5)3]0 = 200:1:1, 0.25 mL of MMBL, 2.25 mL of CH2Cl2, RT}, the polymerization by Me2PhSKA/Al(C6F5)3 reached completion in 2 h, affording PMMBL with a Mn of 29.9 kg mol−1 and a dispersity Đ value of 1.20 (run 3, Table 2). Accordingly, the initiation efficiency was also improved to 75%. Interestingly, replacing the Ph group in the SKA structure with the electron-withdrawing Cl (i.e., Me2ClSKA) rendered the polymerization ill-controlled, achieving only 69.8% monomer conversion even after 24 h and producing bimodal PMMBL (run 4, Table 2). On the other hand, replacing the Ph group with the electron-donating ethoxy group (i.e., Me2(EtO)SKA) produced a polymerization system that is comparable with that of Me2PhSKA/Al(C6F5)3 (run 5 vs 3, Table 2). Noteworthy is the SKA with three Ph substituents, PhSKA, when combined with Al(C6F5)3, exhibited a better control over the MMBL polymerization, producing PMMBL with a Mn of 22.1 kg mol−1 and a Đ value of 1.15, thus yielding a significantly enhanced initiation efficiency of 102% (run 6, Table 2). More strikingly, by replacing the three Ph groups in SKA with three Me groups (i.e., MeSKA), we observed an 18-fold polymerization rate enhancement, thus achieving quantitative monomer conversion in only 30 min and producing PMMBL with a low Đ value of 1.06. Furthermore, the measured Mn value of 23.1 kg mol−1 is close to the calculated MW, thus giving rise to a relatively high initiation efficiency of 98% (run 7, Table 2). Following this significant lead, we found that both EtSKA and iBuSKA are also highly effective for MMBL polymerization, and in fact, they

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RESULTS AND DISCUSSION Uncontrolled MMA Polymerization by R3SiH/Al(C6F5)3. In the absence of Al(C6F5)3, control experiments with the five hydrosilanes, including Ph3SiH, iBu3SiH, Me2ClSiH, Me2EtSiH, and Me2PhSiH, as well as the seven SKAs (Scheme 2) Scheme 2. SKA Structures Employed in This Study

employed in this study all showed no activity for the polymerization of MMA performed in CH2Cl2 at room temperature. Using Al(C6F5)3 as the catalyst, two approaches were employed to investigate the polymerization of MMA. The first was the in situ generation of the initiator SKA via [Al]catalyzed hydrosilylation of the monomer MMA with R3SiH using various ratios of [MMA]0/[R3SiH]0/[Al(C6F5)3]0, while the second approach was to use the preformed SKA with various ratios of [MMA]0/[SKA]0/[Al(C6F5)3]0. It can be seen from the results summarized in Table 1 that the polymerization with the first approach was sluggish, requiring long reaction times (48 h) to achieve quantitative or near-quantitative monomer conversion (runs 1 and 4, Table 1). More significantly, the PMMA formed exhibited a high molecular weight (MW) and a relatively broad molecular weight distribution (e.g., in a monomer (M):initiator (I) ratio of 50:1, Mn = 342 kg mol−1, Đ = 1.47); this measured MW is much higher than the MW calculated based on the [M]/[I] ratio, thus giving an extremely low initiation efficiency of less than 1%. Variations of the silane and the [M]/[I] ratio did not significantly alter such polymerization characteristics (runs 1−4, Table 1). These results indicated the initiation by R3SiH/ Al(C6F5)3 is largely ineffective, attributable to the slow and incomplete 1,4-hydrosilylation of MMA, which resulted in the formation of only a small concentration of the in situ generated initiator SKA. Owing to the high oxophilicity and Lewis acidity of Al(C6F5)3, both of which impair the dissociation of the monomer (via ester carbonyl)−Al(C6F5)3 adduct to release the catalyst Al(C6F5)3 that in turn activates the silane in the form of the silane−Al(C6F5)3 intermediate, which is responsible for the FLP-type hydrosilylation.19 D

DOI: 10.1021/acs.macromol.7b02647 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Al(C6F5)3-Catalyzed MMA Polymerizationa run no.

initiator

[M]:[I]:[Al]

time (h)

convb (%)

Mnc (kg mol−1)

Đ

I*d (%)

1 2e 3 4e 5 6 7e 8 9 10e 11 12 13e 14 15 16e 17 18 19e 20 21 22e 23 24 25e

Me2PhSiH Me2PhSiH Me2EtSiH Me2EtSiH Me SKA Me SKA Me SKA Et SKA Et SKA Et SKA iBu SKA iBu SKA iBu SKA Ph SKA Ph SKA Ph SKA Me2Cl SKA Me2Cl SKA Me2Cl SKA Me2(EtO) SKA Me2(EtO) SKA Me2(EtO) SKA Me2Ph SKA Me2Ph SKA Me2Ph SKA

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

48 48 48 48 24 6 6 24 24 24 24 48 48 24 0.08 0.17 24 24 24 24 2 2 24 6 6

96.8 9 25.8 100 32.4 100 100 17.5 92.8 98.4 2.09 55.8 69.4 4.96 100 100 6.27 13.7 11.9 59.1 100 100 41.0 100 100

342 353 208 491 133 16.4 37.7 n.d. 21.0 31.1 n.d. 56.7 115 n.d. 505 580 n.d. n.d. n.d. 40.0 51.9 64.1 39.3 24.3 62.5

1.47 1.48 1.80 1.30 1.23 1.36 1.30 n.d. 1.34 1.40 n.d. 1.42 1.46 n.d. 1.27 1.22 n.d. n.d. n.d. 1.31 1.66 2.07 1.23 1.46 1.45