Article pubs.acs.org/Macromolecules
Ring-Opening Alternating Copolymerization of Epoxides and Dihydrocoumarin Catalyzed by a Phosphazene Superbase Shuangyan Hu, Guoxiong Dai, Junpeng Zhao,* and Guangzhao Zhang Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China S Supporting Information *
ABSTRACT: 3,4-Dihydrocoumarin (DHC), a six-membered phenolic lactone derived from natural resources, which does not undergo homopolymerization, was subject to ring-opening anionic copolymerization with several epoxides (ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide, and 2-ethylhexyl glycidyl ether) from a mono- or dialcohol catalyzed by a phosphazene superbase (t-BuP4). Spectroscopic analysis revealed that the products were aromatic poly(ether− ester)s with perfect alternating monomeric sequences, consisting of both linear alternating copolymers and cyclic ones with relatively lower molar masses caused by intramolecular transesterification reactions. The alternating copolymers showed good thermal stability and higher glass transition temperatures as compared with the homopolymers of the corresponding epoxides. The in situ initiator-activation and chaingrowth mechanism of the t-BuP4-catalyzed polymerization also allowed for copolymerization of DHC and epoxides from multifunctional initiators, such as 1,1,1-trihydroxymethylpropane and poly(4-hydroxystyrene), yielding nonlinear (star and graft) products with alternating copolymer arms or side chains.
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INTRODUCTION Phosphazene bases, a class of nonionic organic superbases,1−3 have been of great interest and concern in the realm of organocatalytic synthesis for both small molecules and polymers.4,5 Because of the highly basic and non-nucleophilic natures, excellent solubility in polar/nonpolar solvents, relatively low air sensitivity, easy handling, and a broad range of working temperatures, phosphazene bases have found applications as effective catalysts/promoters for anionic or quasi-anionic polymerization of various types of monomers, including (meth)acrylates,6−8 cyclic esters,9−11 epoxides,12−17 cyclosiloxanes,18,19 and cyclic carbonates,20,21 etc. Recently, particular attention has been paid to the anionic ring-opening polymerization of epoxides catalyzed/promoted by the extremely basic 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylidenamino]-2λ5,4λ5-catenadi(phosphazene) (t-BuP4) because of the highly enhanced polymerization rate, even with low catalyst loading, and a rich diversity of polymerizable epoxide monomers bearing different functionalities.22−24 More importantly, the capability of t-BuP4 to deprotonate (activate) various weak acids without attacking the electrophilic parts of the monomers has allowed for in situ (co)polymerization of epoxides from substrates (i.e., small molecules, polymers, and biomolecules) carrying protic initiating sites.25 So far, a large variety of polyether-based macromolecular architectures have been achieved in this fashion, including random, 26−29 gradient, 30 block, 31−35 star,36,37 graft,38−40 and end-functionalized13−17 (co)polymers. However, to our knowledge, synthesis of alternating copoly© XXXX American Chemical Society
mers from epoxides and other types of monomers by the catalysis of phosphazene bases has not been reported yet. Alternating copolymerization has drawn special attention ever since the infancy of polymer science as it results in a uniform composition of the copolymers with two monomeric units occurring in turn repeatedly along the chains, which hence gives rise to substantially different properties than their homo-, random, gradient, or block copolymers.41 Moreover, in many cases alternating copolymerization allows for the production of polymers from compounds that lack the ability or appropriate conditions to homopolymerize and has therefore served as a powerful strategy to enrich the variety of monomers and polymers. In the past decade, the most appealing monomer combinations have been from epoxides and nonhomopolymerizable compounds including carbon dioxide and its sulfur analogues,42−45 cyclic anhydrides,46−50 aromatic lactones51−53 and bicyclic bis(γ-butyrolactone) derivatives.54,55 Especially, organocatalytic alternating copolymerization of epoxides and some of these compounds has been proven feasible with organocatalysts other than phosphazene bases.50−55 Such findings have inspired us to investigate phosphazene-catalyzed anionic ring-opening alternating copolymerization (ROAP) of epoxides with such comonomers and attempt to achieve different topological structures constituted by copolymer chains with alternating monomeric sequences taking advantage of the Received: April 22, 2016 Revised: May 22, 2016
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DOI: 10.1021/acs.macromol.6b00840 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Scheme 1. Mechanistic Pathway for (a) the Ring-Opening Alternating Copolymerization of Epoxides and DHC Catalyzed by tBuP4, (b) β-Scission and α-Scission of Epoxides during the Copolymerization, and (c) Intramolecular Transesterification Yielding a Mixture of Linear and Cyclic Alternating Copolymers
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specialty of phosphazene catalysis as mentioned above, so as to further widen and strengthen the applicability of phosphazene bases in polymer synthesis. 3,4-Dihydrocoumarin (DHC), a biosourced six-membered aromatic (phenolic) lactone, is nonhomopolymerizable but can undergo alternating copolymerization with mono- or disubstituted epoxides by the aid of organic or organometallic catalysts.51,56 Therefore, it was anticipated that addition of tBuP4 into the mixture of a hydroxy compound, DHC, and an epoxide would trigger consecutive and alternating nucleophilic ring-opening reactions of the two heterocycles to grow alternating copolymer chains from the initiator (Scheme 1a). In this study, several commonly used epoxide monomers were employed including ethylene oxide (EO), propylene oxide (PO), 1,2-butylene oxide (BO), styrene oxide (SO), and 2ethylhexyl glycidyl ether (EHG), most of which were copolymerized with DHC for the first time. Mono-, di-, tri-, and multiple hydroxyl initiators were used aiming at different alternating copolymer structures. Size exclusion chromatography (SEC), nuclear magnetic resonance (NMR) spectroscopy, and matrix assisted laser desorption/ionization time-offlight mass spectrometry (MALDI-TOF MS) were used to characterize the copolymers so as to better elucidate the course and mechanism of copolymerization.
EXPERIMENTAL SECTION
Chemicals. SO (Aladdin, 99%), DHC (Aladdin, 99%), benzyl alcohol (BA, Aladdin, 99%), and 1,4-butanediol (BDO, Aladdin, 99%) were dried over calcium hydride (CaH2) and distilled under vacuum. SO was dried again over sodium hydride and distilled prior to use. Propionic anhydride (PA, Aladdin, 99%) was dried over phosphorus pentoxide and distilled under vacuum. 1,4-Benzenedimethanol (BDM, Aladdin, 99%) and 1,1,1-trihydroxymethylpropane (TMP, TCI, 98%) were dried by azeotropic distillation of tetrahydrofuran (THF). THF and toluene (Guangzhou Chemical Reagent, 99%) were dried successively by CaH2 and n-butyllithium (n-BuLi). All the other chemicals were purchased from Sigma-Aldrich. EO (99%), PO (99%), BO (99%), and EHG (99%) were dried successively by CaH2 and nBuLi prior to use. Acetic acid (AcOH) and t-BuP4 (0.8 M solution in n-hexane) were used as received. BA was dissolved in purified toluene to prepare a 0.6 M solution. Poly(4-hydroxystyrene) (PHOS) was synthesized by classic anionic polymerization of p-tert-butoxystyrene and postpolymerization hydrolysis as described before.57 Instrumentation. SEC coupled with successively connected UV and RI detectors was conducted in THF at 35 °C using two identical PLgel columns (5 μm, MIXED-C) at a flow rate of 1.0 mL min−1. A series of narrowly dispersed polystyrene standards were used for calibration to obtain apparent number-average molar masses (Mn,SEC) and molar mass distributions (ĐM) of the copolymers. NMR spectra were recorded at room temperature (RT) on a Bruker AV400 NMR spectrometer using CDCl3 as the solvent and tetramethylsilane as the internal standard. Differential scanning calorimetry (DSC) measurements were performed on a NETZSCH DSC204F1 system in a nitrogen flow. The sample was quickly heated to 150 °C, kept at this B
DOI: 10.1021/acs.macromol.6b00840 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Experimental Conditions and Molecular Characteristics of the Alternating Copolymers
entrya
initiator
[epoxide]0/[DHC]0/[OH]0/[t-BuP4]0b
temp (°C)
time (h)
convc (%)
Mn,SECd (kg mol−1)
ĐMd
DHCEO DHCPO DHCBO DHCSO1 DHCSO2 DHCSO3 DHCSO4 DHCSO5 DHCEHG
BA BA BA BA BDM BDM BDM PHOS TMP
500/100/1/0.8 50/50/1/0.5 50/50/1/0.5 100/100/1/0.8 100/100/1/0.8 100/100/1/0.1 100/100/1/0.1 100/100/1/0.1 33/33/1/0.1
50 80 80 80 80 80 120 120 80
72 72 72 72 72 72 48 72 72
25 46 63 29 25 8 −f 88 42
2.5 4.0 4.4 2.5 3.9 2.4 2.7 146.1 7.5
1.29e 1.21e 1.22e 1.26e 1.28e 1.05 1.35 1.14 1.18
a
Type of epoxide used is indicated in the entry name. bFeed (molar) ratio of epoxide, DHC, hydroxyl, and t-BuP4. cConversion of DHC calculated by 1H NMR analysis. dApparent number-average molar mass and molar mass distribution obtained from SEC analysis (THF, 35 °C, polystyrene standards). eBimodal distribution. fCalculation of DHC conversion from 1H NMR spectrum is difficult in this case due to the existence of many cyclic oligomers. NMR measurements. Mn,SEC = 2.5 kg mol−1, ĐM = 1.29 (bimodal distribution). 1H NMR (400 MHz, CDCl3): δ/ppm = 7.15−6.70 (aromatic protons), 5.06−5.04 (C6 H5CH 2OCO−), 4.43−4.33 (−C6H4OCH2CH2OCO−), 4.14−4.03 (−C6H4OCH2CH2OCO−), 2.94−2.87 (−OCOCH2CH2C6H4O−), 2.65−2.56 (−OCOCH2CH2C6H4O−), 1.27−1.09 (−OCOCH2CH3). Poly(3,4-dihydrocoumarin-alt-stryene oxide), P(DHC-alt-SO). Copolymerization of DHC with PO, BO, SO, and EHG followed the same procedure. A typical procedure for DHCSO1 (Table 1) is as follows. 2.55 mL of DHC (20.0 mmol), 2.35 mL of SO (20.0 mmol), 0.34 mL of BA solution (0.20 mmol of BA), and 0.20 mL of t-BuP4 solution (0.16 mmol of t-BuP4) were injected successively into a reaction flask in an argon flow upon stirring. The flask was then sealed by a stopcock and immersed in an oil bath preheated at 80 °C. After heating at this temperature and stirring for 72 h, the flask was cooled down to RT. Then 1 mL of AcOH was injected into the reaction mixture in an argon flow. A small amount of the solution was withdrawn and diluted with CDCl3 for 1H NMR analysis to determine the conversion of the comonomers. The rest was diluted with THF (or toluene in other cases) and poured into cold methanol to precipitate the copolymer. The yellowish pasty product was then collected, dried under vacuum, and used for SEC and 1H NMR measurements. Mn,SEC = 2.5 kg mol−1, ĐM = 1.26 (bimodal distribution). 1H NMR (400 MHz, CDCl3): δ/ppm = 7.35−6.65 (aromatic protons), 6.20−6.08 (−C6H4OCH2CH(C6H5)OCO−), 5.35−5.23 (−C6H4OCH(C6H5)C H 2 OCO−) , 5 . 0 9−5 . 0 3 ( C 6 H 5 CH 2 O C O−), 4.38−4.25 (−C6H4OCH(C6H5)CH2OCO−), 4.20−3.97 (−C6H4OCH2CH(C6H5)OCO−), 3.05−2.60 (−OCOCH2CH2C6H4O−), 2.58−2.35 (−OCOCH2CH2C6H4O−). Poly(3,4-dihydrocoumarin-alt-propylene oxide), P(DHC-alt-PO) (DHCPO in Table 1). 4.50 mL of DHC (35.7 mmol), 2.50 mL of PO (35.7 mmol), 1.18 mL of BA solution (0.71 mmol of BA), and 0.45 mL of t-BuP4 solution (0.36 mmol of t-BuP4) were used, and the copolymerization was quenched after heating at 80 °C for 72 h. Mn,SEC = 4.0 kg mol−1, ĐM = 1.21 (bimodal distribution). 1H NMR (400
temperature for 3 min to remove thermal history, cooled to −150 °C at a cooling rate of 10 °C min−1, and then heated again to 150 °C at a heating rate of 10 °C min−1. Thermogravimetric analysis (TGA) was conducted on a NETZSCH STA449C thermal analyzer under a nitrogen atmosphere at a heating rate of 10 °C min−1 in the temperature range of 25−600 °C. Matrix assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF MS) measurements were performed on a Bruker Autoflex III Smartbeam MALDI-TOF mass spectrometer (Bruker, Germany). Samples were dissolved in THF (10 mg mL−1) and mixed with a solution of sodium trifluoroacetate (NaTFA) in THF (10 mg mL−1) in a volume ratio of 5:1. This solution was then mixed with a solution of matrix, 2,5dihdroxybenzoic acid in THF (20 mg mL−1), in a volume ratio of 1:20. Then, 0.4 μL of the final solution was spotted on the target plate (dried-droplet method). The reflective positive ion mode was used to acquire the mass spectra of the samples. Calibration was done externally with poly(methyl methacrylate) standards using the nearestneighbor positions. Polymer Synthesis. Poly(3,4-dihydrocoumarin-alt-ethylene oxide), P(DHC-alt-EO) (DHCEO in Table 1). 2.55 mL of DHC (20.0 mmol), 0.34 mL of BA solution (0.20 mmol of BA), 0.20 mL of t-BuP4 solution (0.16 mmol of t-BuP4), and 10 mL of clean toluene were charged into a reaction flask. 5.1 mL of EO (100 mmol, [EO]0 = 5.5 M) was slowly condensed into the flask at −20 °C. The flask was then sealed by a stopcock, and the temperature was slowly elevated to 50 °C. After stirring and heating at this temperature for 72 h, the flask was cooled down to RT. Then 1 mL of PA was injected into the reaction mixture in an argon flow. After heating at 50 °C and stirring for another 24 h, the flask was cooled down to RT followed by addition of 1 mL of AcOH (in other cases quenching was done by adding AcOH directly after the copolymerization). A small amount of the solution was withdrawn and diluted with CDCl3 for 1H NMR analysis to determine the conversion of DHC. The rest was poured into cold methanol to precipitate the copolymer. The yellowish viscous liquid was then collected, dried under vacuum, and used for SEC and 1H C
DOI: 10.1021/acs.macromol.6b00840 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules MHz, CDCl3): δ/ppm = 7.21−6.70 (aromatic protons), 5.31−5.20 (−C6H4OCH2CH(CH3)OCO−), 5.08−5.02 (C6H5CH2OCO−), 4.58−4.50 (−C6H4OCH(CH3)CH2OCO−), 4.27−4.03 (−C 6H4 OCH(CH3)CH 2OCO−), 4.00−3.75 (−C6 H4OCH2 CH(CH3)OCO−), 2.94−2.83 (−OCOCH2CH2C6H4O−), 2.66−2.53 (−OCOCH2CH2C6H4O−), 1.35−1.23 (−C6H4OCH2CH(CH3)OCO−). Poly(3,4-dihydrocoumarin-alt-1,2-butylene oxide), P(DHC-altBO) (DHCBO in Table 1). 3.65 mL of DHC (28.8 mmol), 2.50 mL of BO (28.8 mmol), 0.95 mL of BA solution (0.57 mmol of BA), and 0.36 mL of t-BuP4 solution (0.29 mmol of t-BuP4) were used, and the copolymerization was quenched after heating at 80 °C for 72 h. Mn,SEC = 4.4 kg mol−1, ĐM = 1.22 (bimodal distribution). 1H NMR (400 MHz, CDCl3): δ/ppm = 7.20−6.70 (aromatic protons), 5.18−5.09 (−C6H4OCH2CH(CH2CH3)OCO−), 5.08−5.06 (C6H5CH2OCO−), 4.40−4.34 (−C 6 H 4 OCH(CH 2 CH 3 )CH 2 OCO−), 4.25−4.09 (−C6H4OCH(CH2CH3)CH2OCO−), 4.00−3.86 (−C6H4OCH2CH(CH2CH3)OCO−), 2.95−2.83 (−OCOCH2CH2C6H4O−), 2.68− 2.52 (−OCOCH2CH2C6H4O−), 1.80−1.55 (−C6H4OCH2CH(CH2CH3)OCO−), 0.96−0.85 (−C6H4OCH2CH(CH2CH3)OCO−). Poly(4-hydroxystyrene)-graf t-poly(3,4-dihydrocoumarin-altstryene oxide), PHOS-g-P(DHC-alt-SO) (DHCSO5 in Table 1). 24.5 mg of PHOS (0.20 mmol of phenolic hydroxyl groups), 2.55 mL of DHC (20.0 mmol), 2.35 mL of SO (20.0 mmol), and 25 μL of t-BuP4 solution (0.02 mmol of t-BuP4) were used and the copolymerization was quenched after heating at 120 °C for 72 h. The crude product was fractionated with toluene as the solvent and methanol as the precipitant to obtain pure graft alternating copolymer. The fractionated product was precipitated in methanol. The white powder was then collected, dried under vacuum, and used for 1H NMR measurement. Mn,SEC = 146.1 kg mol−1, ĐM = 1.14. 1H NMR (400 MHz, CDCl3): δ/ppm = 7.35−6.50 (aromatic protons), 6.14−6.06 (−C6H4OCH2CH(C6H5)OCO−), 5.33−5.20 (−C6H4OCH(C6H5)CH2OCO−), 4.43−4.21 (−C6H4OCH(C6H5)CH2OCO−), 4.20− 3.93 (−C6H4OCH2CH(C6H5)OCO−), 3.05−2.60 (−OCOCH2CH2C6H4O−), 2.58−2.30 (−OCOCH2CH2C6H4O−). Poly(3,4-dihydrocoumarin-alt-2-ethylhexyl glycidyl ether), P(DHC-alt-EHG) (DHCEHG in Table 1). 27.2 mg of TMP (0.20 mmol), 2.55 mL of DHC (20.0 mmol), 4.25 mL of EHG (20.0 mmol), and 75 μL of t-BuP4 solution (0.06 mmol of t-BuP4) were used, and the copolymerization was quenched after heating at 80 °C for 72 h. Mn,SEC = 7.5 kg mol−1, ĐM = 1.18. 1H NMR (400 MHz, CDCl3): δ/ ppm = 7.16−6.70 (aromatic protons), 5.36−5.26 (−C6H4OCH2CH(CH2OCH2CH(CH2CH3)CH2CH2CH2CH3)OCO−), 4.12−4.00 (−C 6 H 4 OCH 2 CH(CH 2 OCH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 2 CH 3 )OCO−), 3.93−3.90 (CH 3 CH 2 C(CH 2 OCO−) 3 ), 3.67−3.55 (−C 6 H 4 OCH 2 CH(CH 2 OCH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 2 CH 3 )OCO−), 3.36−3.25 (−C6H4OCH2CH(CH2OCH2CH(CH2CH3)CH2CH2CH2CH3)OCO−), 2.96−2.83 (−OCOCH2CH2C6H4O−), 2.70−2.54 (−OCOCH2CH2C6H4O−), 1.53−1.41 (−C6H4OCH2CH(CH2 OCH2CH(CH2CH3)CH2CH2CH2CH3)OCO−), 1.37−1.16 (−C 6 H 4 OCH 2 CH(CH 2 OCH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 2 CH 3 )OCO−, CH3CH2C(CH2OCO−)3), 0.90−0.70 (−C6H4OCH2CH(CH2OCH2CH(CH2CH3)CH2CH2CH2CH 3)OCO−, CH 3CH2C(CH2OCO−)3).
Figure 1. SEC traces of alternating copolymers (isolated products) from DHC and EO/PO/BO, corresponding to respectively DHCEO, DHCPO, and DHCBO in Table 1.
distribution with a Mn,SEC of 2.5 kg mol−1 (Figure 1, Figure S1, and Table 1). The 1H NMR spectrum of the precipitated product reveals that it is a perfect alternating copolymer without trace of EO homopolymerization as evidenced by the absence of typical signals from aliphatic ethers (EOEO diads) that would have located at 3.85−3.45 ppm (Figure S2).16,27,40 The sole signal from BA-derived α-end group indicates that the first monomeric units are exclusively from DHC, and quenching of this reaction with PA helps prove that the alternating copolymer chains are mostly ended with DHC units (Figure S2). The copolymerization of PO and DHC was first attempted at 50 °C in bulk. SEC measurement indicated that no polymer was formed after 72 h. Then another copolymerization was conducted at 80 °C (DHCPO in Table 1). 1H NMR analysis revealed that the conversion of DHC reached 46% after 72 h, and the product was an alternating copolymer of DHC and PO without trace of aliphatic ethers (POPO diads) that would have shown signals at 3.65−3.35 ppm (Figure S3).39 A bimodal molar mass distribution is also shown in the SEC trace of DHCPO (Figure 1 and Figure S1) with a Mn,SEC of 4.0 kg mol−1. The copolymer of DHC and BO was synthesized in the same manner, with a DHC conversion of 63% reached after 72 h (DHCBO in Table 1). The integrals of the characteristic signals in the 1H NMR spectrum of the isolated product, e.g., a 3:2 ratio of the integral area of −CH2CH(CH2CH3)OCO− in BO units (Figure 2, left, h) and −OCOCH2CH2C6H4O− in DHC units (Figure 2, left, e), together with the absence of signals from aliphatic ethers (BOBO diads) that would have located at 3.65−3.30 ppm (also see Figure S4),34 once again demonstrate the achievement of an alternating copolymer. Similar to DHCEO and DHCPO, a bimodal molar mass distribution is also presented in the SEC trace of DHCBO (Figure 1 and Figure S1) with a Mn,SEC of 4.4 kg mol−1, which is lower than the theoretical number-average molar mass (Mn,theor = 6.7 kg mol−1) calculated from the feed ratio and monomer conversion. The lower comonomer conversion reached in the case of DHCPO, compared with DHCBO, is most probably due to extensive gasification of PO at 80 °C, leading to a more profoundly reduced concentration of PO in the reaction mixture. The identical intensity ratios of RI and UV peaks in the SEC traces (Figure 1 and Figure S1) indicate that the two populations have the same composition; namely, they are both
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RESULTS AND DISCUSSION Copolymerization of EO and DHC was performed with a monohydroxyl initiator (BA) and 0.8 equiv of t-BuP4 as the catalyst (DHCEO in Table 1). Concerning the high volatility of EO, we used toluene as the solvent and applied a mild temperature to this reaction. As a complement, a large excess of EO was used (5 equiv of DHC) in the hope to promote the conversion of DHC. Upon a certain reaction time, the viscosity of the reaction mixture apparently increased, suggesting the occurrence of polymerization. However, the conversion of DHC calculated from 1H NMR analysis reached only 25% after 72 h. SEC measurement shows a bimodal molar mass D
DOI: 10.1021/acs.macromol.6b00840 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. 1H NMR spectra of representative alternating copolymers (isolated products) from DHC and BO (left) or SO (right), corresponding to DHCBO and DHCSO1 in Table 1, respectively.
Figure 3. SEC traces of alternating copolymers (isolated products) from DHC and SO synthesized at 80 °C, corresponding to respectively DHCSO3, DHCSO2, and DHCSO1 in Table 1. Figure 4. MALDI-TOF MS spectrum (inset: magnified area) of a P(DHC-alt-SO) alternating copolymer (DHCSO3 in Table 1) synthesized at 80 °C with the feed ratio of [SO]0/[DHC]0/[OH]0/ [t-BuP4]0 = 100/100/1/0.1 and BDM as the initiator. NaTFA was used as cationizer. Calculated exact masses for [BDM-(DHC+SO)8(DHC)2]Na+ and [BDM-(DHC+SO)8-DHC]Na+ are 2602.4 and 2454.2 Da, respectively, which are in good agreement with the measured values presented in the magnified area (leftmost isotopic signals).
alternating copolymers of DHC and the corresponding epoxide. The weak signals from 4.1 to 4.4 ppm in the 1H NMR spectrum (Figure 2, left, inset) implies that the ring-opening of BO, when attacked by the phenoxide species at 80 °C, also occurs in a α-scission manner, which, however, has a much lower probability (
