Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Well-Defined and Structurally Diverse Aromatic Alternating Polyesters Synthesized by Simple Phosphazene Catalysis Heng Li, Huitong Luo, 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: Alcohol-initiated ring-opening alternating copolymerization (ROAP) of phthalic anhydride (PA) and a variety of mono-, di-, and trisubstituted epoxides has been performed with a weak phosphazene base (t-BuP1) as the catalyst. Each product exhibits a perfectly alternating sequence distribution, controlled molar mass (Mn up to 124 kg mol−1), and low dispersity (ĐM < 1.15, mostly). Full conversion of PA can be reached in 0.5−24 h depending on the substituent of the epoxide, the targeted degree of polymerization, and the amount of t-BuP1 used (0.2−5 mol % of PA) when the reactions are conducted under solvent-free conditions at 100 °C with a small excess of the epoxide (0.5 equiv of PA). The glass transition temperature of the polyester ranges from −14 to 135 °C. The living nature of the ROAP allows one-pot construction of well-defined block-alternating copolymers through sequential addition of two epoxides. Statistical-alternating copolymers have also been synthesized by copolymerization of PA and two mixed epoxides. Thus, the structural diversity of aromatic alternating polyesters synthesized by this simple organocatalysis has been largely enriched.
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INTRODUCTION Aromatic polyesters, especially those constituted by benzoic ester type repeat units, represent one major class of synthetic polymers that are widely produced and used for fibers, resins, films, etc., owing to their inexpensiveness and excellence of their thermal stability and mechanical properties.1 The most commonly used synthetic method for aromatic polyesters is dior multicomponent step-growth polymerization (SGP) of e.g. diols and aromatic dicarboxylic acids or the corresponding diesters. While benefiting from high accessibility and a rich catalogue of the raw materials (monomers), such SGP reactions usually require stringent conditions regarding the efficiency of esterification reaction, the stoichiometric ratio of monomers, and the removal of small-molecule byproducts, and the classic SGP mechanism obstructs paths to controlled molar masses, low dispersities, and well-defined macromolecular structures. On the other hand, the development of monocomponent ringopening polymerization (ROP) for the synthesis of aromatic polyesters has been held back by the poor polymerizability of aromatic cyclic esters.2,3 In this context, efforts have been continuously made, since the middle of the past century and intensively in the recent decade,4−8 to advance catalytic/ initiating systems for ring-opening alternating copolymerization (ROAP) of epoxides and aromatic cyclic anhydride, in particular phthalic anhydride (PA), which have been paving an expedient and highly effective route to aromatic polyesters.9−19 Polymerizing cyclic derivatives of SGP monomers in a chain-growth ROP manner, ROAP integrates, to a large extent, the advantages of both methods, i.e., highly accessible © XXXX American Chemical Society
and structurally abundant monomers, atom economy, relatively less stringent reaction conditions, and high probability of being controlled (even living) polymerization for achieving welldefined structures of aromatic (also aliphatic) polyesters.8,20 For the past few decades, metal complexes have been the leading catalysts investigated and used for epoxide-based alternating copolymerization, building up a powerful toolbox for reaching high efficiency and/or selectivity.8,21−25 For the ROAP of cyclic anhydride and epoxides, controlled molar masses of the resultant alternating polyesters,26−30 chemo-, regio-, or stereoselectivity,14,26−29 and block copolymers with alternating polyester segments12,26,29−36 have been successfully achieved. Recently, there has been a rapidly growing interest in implementing epoxide-based alternating copolymerization with metal-free catalysts. Attempts have been made on the comonomers that are most frequently involved with metallic catalysts, including carbon dioxide,37 carbonyl sulfide,38 and cyclic anhydrides.39−41 The competence and potential of organocatalysts for such reactions have been strongly suggested by the achievements of strictly alternating sequence distribution (or very low polyether content), high catalytic efficiency, and regio-/stereoregularity of the obtained alternating copolymers. Phosphazene bases have been one prominent type of organocatalyst for ROP of heterocycles owing to the high and tunable basicity, non-nucleophilic nature, and favorable in Received: January 23, 2018 Revised: February 22, 2018
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DOI: 10.1021/acs.macromol.8b00159 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Conditions and Results of ROAP of PA and Epoxides Catalyzed by t-BuP1a entry g
[OH]0/[PA]0/[EP]0/[tBuP1]0b
temp (°C)
time (h)
convc (%)
Mn,thd (kg mol−1)
Mn,NMRe (kg mol−1)
Mn,SECf (kg mol−1)
ĐM f
72 48 72 48 48 96 24 6.5 2 5 12 40 12 1 3.5 3.5 2 4 24 24 0.5 3 8 18 2 5 5 30 24 12 24 12 18 2.5
>99 52 75 35 83 >99 90 95 96 85 86 76 >99 >99 >99 >99 97 >99 95 93 >99 >99 >99 >99 >99 >99 >99 >99 48 >99 50 >99 >99 >99
19.3 10.9 15.6 8.7 20.6 24.7 23.0 23.5 23.8 21.0 19.1 167.3 26.9 5.5 26.9 13.5 26.5 6.1 28.6 14.1 5.7 27.9 55.7 139.1 26.3 52.5 33.5 66.9 16.2 29.8 16.8 29.1 30.0 25.8
15.5
14.1 13.1 16.4 6.5 14.5 17.0 18.2 18.3 18.4 19.9 16.6 124.3 18.8 5.0 14.3 9.2 26.1 6.2 18.3 10.5 6.6 31.1 61.1 117.3 26.9 49.2 26.4 67.4 14.7 23.0 15.9 26.4 27.5 23.6
1.09 1.05 1.04 1.14 1.11 1.10 1.07 1.06 1.06 1.10 1.08 1.10 1.09 1.10 1.19 1.11 1.05 1.12 1.19 1.14 1.08 1.05 1.08 1.15 1.06 1.05 1.09 1.10 1.06 1.07 1.06 1.08 1.09 1.09
PAEO PAPOg
1/50/75/0.5 1/50/75/0.5
60 60
PACHO1g
1/50/75/0.5
PACHO2 PACHO3 PACHO4 PACHO5 PABO1 PABO2 PASO1 PASO2 PASO3 BAPASOh PAVCHO PALO1 PALO2 BAPALOh PATBGE1 PATBGE2 PATBGE3 PATBGE4 PAAGE1 PAAGE2 PAEHGE1 PAEHGE2 PAEHGE3i PAEHGE3-AGEi PAEHGE4i PAEHGE4-CHOi PA(BO/EHGE)j PA(AGE/CHO)j
1/50/75/0.5 1/50/75/0.5 1/50/75/0.5 1/50/75/0.1 1/50/75/0.5 1/500/750/0.5 1/50/75/0.5 1/10/15/0.5 1/50/75/0.5 1/50/75/0.5 1/50/75/0.5 1/10/15/0.5 1/50/75/0.5 1/50/75/0.5 1/10/15/0.5 1/50/75/0.5 1/100/150/0.5 1/250/375/0.5 1/50/75/0.5 1/100/150/0.5 1/50/75/0.5 1/100/150/0.5 1/50/25/0.5 1/26/75/0.5 1/50/25/0.5 1/25/75/0.5 1/50/(37/37)/0.5 1/50/(37/37)/0.5
60 80 80 60 80 100 100 60 80 80 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 80 100
14.9
20.3 25.5 26.0 24.8 27.5 17.6 19.4 4.4 20.0 11.9 26.5 6.4 22.0 10.9 5.7 31.3
27.9 33.5
29.9 32.9 26.6
Tg (°C) 36.1 55.0
134.8
44.0 45.6
91.3 119.4 103.2 51.0 56.1 59.2 59.3 11.6 16.4 −13.8 −13.4 −3.1
−9.0 41.5
a
Performed with or without a solvent, [PA]0 ranging from 1.5 to 7.6 M. bMolar feed ratio of hydroxyl, PA, epoxide, and t-BuP1. cConversion of PA calculated by 1H NMR analysis of the crude product. dTheoretical number-average molar mass calculated from the feed and monomer (mostly PA) conversion. eCalculated from the 1H NMR spectrum of the isolated product by comparing signal integrals of the end group (mostly −C6H4CH2O−) and polymer main body. fObtained from SEC analysis (THF, 35 °C, PS standards). gPerformed in THF with [PA]0 ranging from 1.5 to 1.7 M; PACHO1 was heated at 60 °C for 48 h and then 80 °C for 96 h. hInitiated by BA, other entries by BDM. iOne-pot synthesis of triblock terpolymer by sequential addition of two epoxides with a small amount of toluene added. jSynthesis of statistical-alternating copolymer by copolymerizing PA and two epoxides simultaneously.
situ initiator activation and chain growth catalytic mechanism.42−44 PBs exhibit excellent catalytic efficiency toward ROP of aliphatic lactones provided with appropriate reaction conditions. However, narrow molar mass distributions are usually difficult to obtain (or maintain) because of the lability of aliphatic esters under basic conditions and the consequent occurrence of macromolecular transesterification reactions, especially at high monomer conversions.45−49 This also holds true for ROAP of epoxides and 3,4-dihydrocumarin (a phenolic lactone) catalyzed by a PB superbase, in which backbiting (intramolecular transesterification) occurs inevitably after a certain monomer conversion or polymer chain length is reached.50 Reducing the basicity of PB is found to significantly suppress backbiting reaction and thus promote the elongation of linear chains.51,52 This basicity effect is even more pronounced for PB-catalyzed ROAP of PA and ethylene oxide (EO). The transesterification reaction, as well as selfpropagation of EO, is fully inhibited when the weakest
commercially available PB (t-BuP1) is used so that the copolymerization proceeds in a perfectly alternating manner yielding aromatic alternating polyesters with well-controlled molar masses, very low dispersities, and readily tunable macromolecular structures.41 The living nature of this organocatalytic ROAP is primarily attributable to the pKa of protonated t-BuP1 which lies very appropriately between those of benzoic carboxyl and aliphatic hydroxyl. In such a case, the catalyst alternately and repeatedly acts as an activator and a deactivator for the PA- and EO-derived end groups, respectively, by shuttling the proton that originates from a hydroxy or carboxy initiator (“self-buffering” mechanism), so that no reaction other than the desired ROAP may occur. In consideration of the high volatility of EO, we performed its ROAP with PA in tetrahydrofuran (THF) solution at a moderate temperature (60 °C); hence, 24 h or longer reaction time was needed to ensure high or full conversion of PA.41 We herein report the successful implementation of t-BuP1-catalyzed B
DOI: 10.1021/acs.macromol.8b00159 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
a cooling rate of 10 °C min−1, kept at this temperature for 10 min, and then heated again to 200 °C at a heating rate of 10 °C min−1. The glass transition temperature (Tg) was acquired on the second heating run of a DSC measurement. Polymer Synthesis. Poly(phthalic anhydride-alt-epoxide), P(PAalt-EP), Synthesized in Bulk. A typical procedure for PATBGE2 (Table 1) is given as follows. PA (1.48 g, 10.0 mmol) and BDM (13.8 mg, 0.10 mmol) were charged in a reaction flask and dissolved in cryodistilled THF, followed by slow removal of THF by cryo-evaporation. Then TBGE (2.12 mL, 15.0 mmol) and t-BuP1 (23.4 μL, 0.10 mmol) were successively added in a glovebox ([PA]0 = 4.7 M). The flask was then heated at 100 °C and stirred for 3 h. (For most of the experiments, aliquots were withdrawn in an argon flow in the process of the ROAP after temporarily cooling the reaction flask to RT, so as to monitor monomer conversions and evolution of molar masses.) After addition of 1 mL of AcOH for quenching, a small amount of the crude product was withdrawn and diluted with CDCl3 for 1H NMR analysis to determine the conversion of PA and further diluted with THF for SEC analysis to obtain Mn,SEC and ĐM. The rest was diluted with chloroform and poured into cold methanol to precipitate the copolymer which was then collected and dried in a vacuum. Conv(PA) > 99%, Mn,th = 27.9 kg mol−1. Mn,SEC = 31.1 kg mol−1, ĐM = 1.05. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.78−7.36 (aromatic protons), 5 . 4 8− 5 .3 8 ( − C 6 H 4 C O O C H 2 C H ( C H 2 O R ) OC O− ) , 5 .2 7 (−C6H4CH2OCO−), 4.68−4.48 (−C6H4COOCH2CH(CH2OR)OCO−), 3.68−3.56 (−COOCH2CH(CH2OR)OCO−), 1.22−1.08 (−OC(CH3)3); R = −C(CH3)3; Mn,NMR = 31.3 kg mol−1. P(PA-alt-EP) Synthesized in Solution. A typical procedure for PAPO (Table 1) is given as follows. PA (1.48 g, 10.0 mmol) and BDM (13.8 mg, 0.10 mmol) were charged in a reaction flask and dissolved in cryo-distilled THF, followed by slow removal of THF by cryoevaporation. Then THF (5 mL), PO (1.05 mL, 15.0 mmol), and tBuP1 (23.4 μL, 0.10 mmol) were successively added in a glovebox ([PA]0 = 1.7 M, [PO]0 = 2.5 M). The mixture was then heated at 60 °C and stirred for 72 h. After addition of 1 mL of AcOH for quenching, a small amount of the crude product was withdrawn for 1H NMR and SEC analysis. The rest was poured into cold methanol to precipitate the copolymer which was then collected and dried in a vacuum. Conv(PA) = 75%, Mn,th = 15.6 kg mol−1. Mn,SEC = 16.4 kg mol−1, ĐM = 1.04. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.78−7.34 (aromatic protons), 5.48−5.36 (−C6H4COOCH2CH(CH3)OCO−), 5.26 (−C6H4CH2OCO−), 4.46−4.26 (−C6H4COOCH2CH(CH3)OCO−), 1.44−1.28 (−COOCH2CH(CH3)OCO−); Mn,NMR = 14.9 kg mol−1. Block-Alternating Copolymer, Typically for Poly(phthalic anhydride-alt-allyl glycidyl ether)-block-poly(phthalic anhydride-alt-2ethylhexyl glycidyl ether)-block-poly(phthalic anhydride-alt-allyl glycidyl ether), P(PA-alt-AGE)-b-P(PA-alt-EHGE)-b-P(PA-alt-AGE). PAEHGE3-AGE in Table 1. The experimental procedure for the P(PA-alt-EHGE) precursor (PAEHGE3, Table 1) is similar to PATBGE2, using PA (1.48 g, 10.0 mmol), BDM (13.8 mg, 0.1 mmol), t-BuP1 (23.4 μL, 0.1 mmol), EHGE (1.05 mL, 5.0 mmol), and toluene (1 mL) ([PA]0 = 4.9 M). After stirring and heating at 100 °C for 24 h, the reaction flask was cooled to RT and transferred in a glovebox. A small amount of the crude product was withdrawn for 1H NMR and SEC analysis. Conv(PA) = 48%, conv(EHGE) > 99%, Mn,th = 16.2 kg mol−1. Mn,SEC = 14.7 kg mol−1, ĐM = 1.06. Then the second epoxide, AGE (1.78 mL, 15.0 mmol; in a large excess to the remaining PA), was added into the reaction flask. After stirring and heating at 100 °C for another 12 h, AcOH (1 mL) was added for quenching, and a small aliquot was withdrawn for 1H NMR and SEC analysis. The crude product was diluted with chloroform, precipitated in cold methanol, collected, and dried in a vacuum. Conv(PA) > 99%, Mn,th = 29.8 kg mol−1. Mn,SEC = 23.0 kg mol−1, ĐM = 1.07. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.78−7.36 (aromatic protons), 5.92−5.80 (−CH2OCH2CHCH2), 5.56−5.44 (−C6H4COOCH2CH(CH2OR)OCO−), 5.30−5.12 (−CH2OCH2CHCH2), 4.70− 4 . 4 8 ( −C 6 H 4 C O O C H 2 C H ( C H 2 O R ) O C O −) , 4 . 0 6− 3 .9 4 (−CH 2 OCH 2 CHCH 2 ), 3.76−3.62 (−C 6 H 4 COOCH 2 CH(CH 2 OR)OCO−), 3.38−3.28 (−CH 2 OCH 2 CH(CH 2 CH 3 )-
ROAP of PA and various mono-, di-, or trisubstituted epoxides. Because of the much higher boiling points of these epoxides (bp >100 °C, mostly), the reactions can be carried out in bulk at higher temperatures so that polymerization rates can be greatly elevated and the use of volatile solvents is avoided in most cases. The variety of substituted epoxides has encouraged us to apply this ROAP on convenient synthesis of block- and statistical-alternating copolymers through respectively sequential and simultaneous addition of two epoxide monomers. The strength and versatility of this living ROAP catalyzed by a simple organocatalyst for synthesis and enrichment of welldefined aromatic alternating polyesters have therefore been well demonstrated.
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EXPERIMENTAL SECTION
Chemicals. Propylene oxide (PO; Aldrich, 99%), 1,2-butylene oxide (BO; Aldrich, 99%), and cyclohexene oxide (CHO; Aladdin, 98%) were stirred with sodium hydride overnight and then cryocondensed under vacuum into a graduated flask precharged with nbutyllithium (n-BuLi) and stirred there at room temperature (RT) for 1 h before finally cryo-condensed into a storage flask. Styrene oxide (SO; Aladdin, 99%), tert-butyl glycidyl ether (TBGE; Aldrich, 99%), 2ethylhexyl glycidyl ether (EHGE; Aladdin, 98%), 4-vinyl-1-cyclohexene-1,2-epoxide (VCHO; TCI, 98%), (+)-cis/trans-limonene oxide (LO; Aldrich, 97%), allyl glycidyl ether (AGE; Aladdin, 99%), and benzyl alcohol (BA; Aladdin, 99%) were dried over calcium hydride (CaH2) overnight and vacuum-distilled. THF and toluene (Guangzhou Chemical Reagent, AR) were successively dried over molecular sieve (4 Å), CaH2, and n-BuLi before cryo-distilled under vacuum. PA (Aladdin, 99%) was first sublimated under vacuum, then dissolved in acidic anhydride, and stirred at 80 °C overnight before being recrystallized at RT. All purified chemicals are stored in a glovebox. 1,4-Benzenedimethanol (BDM; Aladdin, 99%) were dried by azeotropic distillation of THF prior to use. Acetic acid (AcOH; Aladdin, 99%) and tert-butyliminotris(dimethylamino)phosphorane (tBuP1; Aldrich, 97%) were used as received. Instrumentation. Size exclusion chromatography (SEC) coupled with successively connected UV, refractive index (RI), and light scattering (15° and 90°) detectors was conducted in THF at 35 °C using two identical PL gel columns (5 μm, MIXED-C) at a flow rate of 1.0 mL min−1. A series of narrowly dispersed polystyrene (PS) standards were used for calibration to obtain apparent number-average molar masses (Mn,SEC) and molar mass distributions (ĐM; or dispersity) of the copolymers. NMR spectra were recorded at RT on a Bruker AV600 NMR spectrometer utilizing CDCl3 as the solvent and tetramethylsilane as the internal standard. Monomer conversions were calculated from 1H NMR spectra of the crude products by comparison of the integrals of characteristic signals from the remaining monomer and the corresponding signals from the polymer. Numberaverage molar masses of the isolated polymers (Mn,NMR) were calculated from 1H NMR spectra through comparison of signal integrals for the end group (mostly −C6H4CH2O−) and polymer main body. 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 10:1. This solution was then mixed with a solution of matrix, 2,5-dihdroxybenzoic acid in THF (20 mg mL−1), in a volume ratio of 1:5. Then, 0.4 μL of the final solution was spotted on the target plate (dried-droplet method). The reflective positive ion mode was utilized to acquire mass spectra of the samples. Calibration was done externally with poly(methyl methacrylate) standards using the nearest-neighbor positions. Differential scanning calorimetry (DSC) measurements were performed on a NETZSCH DSC204F1 system in a nitrogen flow. The sample was quickly heated to 200 °C, kept at this temperature for 5 min to remove thermal history, cooled to −80 °C at C
DOI: 10.1021/acs.macromol.8b00159 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Schematic Illustration for ROAP of PA and Epoxides Catalyzed by t-BuP1
(CH2)3CH3), 1.50 (−CH2OCH2CH(CH2CH3)(CH2)3CH3), 1.38− 1.14 (−CH 2 OCH 2 CH(CH 2 CH 3 )(CH 2 ) 3 CH 3 ), 0.88−0.76 (−CH2OCH2CH(CH2CH3)(CH2)3CH3); R = −CH2CH(CH2CH3)(CH2)3CH3 or −CH2CHCH2; Mn,NMR = 29.9 kg mol−1. Statistical-Alternating Copolymer, Typically for Poly(phthalic anhydride-alt-allyl glycidyl ether/cyclohexene oxide), P(PA-alt-AGE/ CHO). PA(AGE/CHO) in Table 1. The experimental procedure is similar to PATBGE2, except that two epoxides were added and reacted together. PA (1.48 g, 10.0 mmol), BDM (13.8 mg, 0.1 mmol), t-BuP1 (23.4 μL, 0.1 mmol), AGE (0.89 mL, 7.5 mmol), and CHO (0.76 mL, 7.5 mmol) were used ([PA]0 = 6.1 M). The reaction mixture was stirred and heated at 100 °C. Small aliquots were withdrawn in an argon flow, upon cooling the flask to RT, at different time intervals and used for 1H NMR analysis to monitor the evolution of monomer conversions. The reaction was quenched with AcOH (1 mL) after 2.5 h; then the crude product was diluted with chloroform, precipitated in cold methanol, collected, and dried in a vacuum. Conv(PA) > 99% (62% for PA-AGE units, 38% for PA-CHO units), Mn,th = 25.8 kg mol−1. Mn,SEC = 23.6 kg mol−1, ĐM = 1.09. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.86−7.32 (aromatic protons), 5.94−5.80 (−CH 2 OCH 2 CHCH 2 ), 5.58−5.46 (−C 6 H 4 COOCH 2 CH(CH2OR)OCO−), 5.32−5.08 (−CH2OCH2CHCH2, −C6H4COOCH(CH2CH2CH2CH2)CHOCO−), 4.72−4.48 (−C6H4COOCH2CH(CH2OR)OCO−), 4.12−3.92 (−CH2OCH2CHCH2), 3.82− 3.64 (−C6H4COOCH2CH(CH2OR)OCO−), 2.36−1.18 (−C6H4COOCH(CH2CH2CH2CH2)CHOCO−), R = −CH2CHCH2.
occurrence of polymerization. SEC data shown in Table 1 and all the figures with SEC traces are obtained from crude products withdrawn from the reaction mixtures, while the values of Mn,NMR in Table 1 and NMR spectra shown in the figures are mostly acquired from the isolated (precipitated and dried) products. As the conversion of PA increases, the product of PAPO maintains a narrow and unimodal molar mass distribution (ĐM ≤ 1.05; Table 1 and Figure S1). In the NMR spetra of the crude and isolated products (Figures S1−S3; 1H and 13C spectra), signals of linear aliphatic ethers (POPO diads; 3.60−3.30 ppm in the 1H NMR spectrum53) are absent. This is readily understandable since t-BuP1 cannot afford homopolymerization of epoxides.41,51,52 Signal integrals of protons from PO- and PA-derived units, i.e., benzoic ester and aromatic protons, respectively, as well as those from the remaining PO and PA demonstrate that the two monomers are incorporated in the polymer in a ratio of 1/1 (Figure S2). Such results have confirmed the formation of strictly alternating sequence distribution. The sole signals from BDM-derived central group (−C6H4CH2OCO− at 5.26 ppm in 1H NMR spectrum and −C6H4CH2OCO− at 135.67 ppm in the 13C NMR spectrum) indicate that the initiation step occurs exclusively on PA rather than PO, which implies that the reaction between hydroxyls and PO can barely occur thus further leaves out the possibility for the self-propagation of PO (Scheme 1). Mn,NMR calculated by comparing signal integrals of methylene protons originating from BDM and PO agrees well with the theoretical molar mass (Mn,th) and Mn,SEC (Table 1). It needs to be noted that Mn,NMR calculated in this way can be somewhat overestimated because a certain amount of P(PA-alt-EP) chains may be initiated by impurities such as phthalic acid, especially when high DPs are targeted and BDM is added in relatively low quantities.40,41 The conversion of PA reaches 75% after 72 h while complete PA conversion is reached in the case of PAEO with the same reaction time, which is attributable to the lower reactivity of methyl-substituted epoxy ring of PO compared with nonsubstituted EO and the fact that the reactions between activated carboxyls and the epoxide are the rate-determining steps of the ROAP. A strong evidence for this assumption is the absence of −OCH2CH(CH3)OH signals that would be given by POderived hydroxy chain ends (Figure S1; 3.8−4.0 ppm). Namely, the reaction between carboxyl and PO is greatly slower than that between hydroxyl and PA so that almost all the copolymer chains are ended with carboxyls before full or a substantially high PA conversion is reached (Scheme 1).41 It is therefore predictable that the ROAP would be even slower for epoxides with larger and/or more substituents if the
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RESULTS AND DISCUSSION We have subjected nine different substituted epoxides, including two monoalkylated epoxides (PO, BO), one phenylated epoxide (SO), three glydcidyl ethers (TBGE, AGE, and EHGE), two 2,3-disubstituted epoxides (CHO and VCHO), and one 2,3,3-trisubstituted epoxide (LO), to copolymerization with PA catalyzed by t-BuP1. The experimental conditions and characterization data of the products are listed in Table 1, and the general reaction scheme is drawn in Scheme 1. A dihydroxy initiator (BDM) is used in most cases and monohydroxy BA is also used in two experiments. For most of the reactions, the epoxide is added in a slight excess with regard to PA ([PA]0/[EP]0 = 1/1.5), and the ratio of hydroxyl to t-BuP1 ([OH]0/[t-BuP1]0) is kept at 1/0.5. An entry for the ROAP of PA and EO performed in our previous study41 under comparable conditions is given as a reference experiment (PAEO in Table 1). Considering the low boiling point of PO, we conducted its ROAP with PA in THF at 60 °C (PAPO in Table 1; [PA]0 = 1.7 M) as we did for PAEO, with the targeted degree of polymerization being 100 (DP = 50 per hydroxyl). A distinct viscosity increase of the reaction mixture is visualized, and the apparent molar mass (Mn,SEC) of the product gradually increases with prolonged reaction time, suggesting the D
DOI: 10.1021/acs.macromol.8b00159 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. SEC traces (left; crude products; THF, 35 °C) and 1H NMR spectrum (right; isolated products) of representative P(PA-alt-EP)s synthesized by use of BDM as the initiator and t-BuP1 as the catalyst with [OH]0/[PA]0/[EP]0/[t-BuP1]0 being 1/50/75/0.5, corresponding to (from top to bottom) PABO1, PASO1, PATBGE2, and PAVCHO in Table 1, respectively.
contrast to the situation with a high but incomplete PA conversion (PACHO4; Figure S4). High boiling points of CHO and most of the epoxides used in this study encourage us to perform the ROAP in bulk aiming at further enhanced polymerization rate. PACHO2, -3, and -4 (Table 1) are carried out without an added solvent at different temperatures with the same ratio of [OH]0/[PA]0/[CHO]0/[tBuP1]0 as that for PACHO1. In these cases, [PA]0 is greatly elevated to 6.6 M, yet homogeneous reaction mixtures can be ensured by heating. Indeed, the polymerization is remarkably faster (PACHO2 vs PACHO1), as indicated by the reaction time and achieved PA conversions, and gets even faster as temperature futher increases. At 100 °C, a nearly complete PA conversion is reached in only 2 h (DP = 100). Reducing the amount of t-BuP1 from 0.5 to 0.1 equiv of hydroxyl leads to a lower polymerization rate (PACHO5 vs PACHO4), but a high PA conversion (85%) is still achieved in 5 h at 100 °C. All the products exhibit strictly alternating sequence distributions, controlled molar masses, and low ĐMs (
