Phosphazene-Catalyzed Alternating Copolymerization of

May 15, 2017 - Metal-free alternating copolymerization of 3,4-dihydrocoumarin (DHC) and ethylene oxide (EO) was realized by relatively mild phosphazen...
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Phosphazene-Catalyzed Alternating Copolymerization of Dihydrocoumarin and Ethylene Oxide: Weaker Is Better Hongxin Zhang, Shuangyan Hu, 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: Metal-free alternating copolymerization of 3,4dihydrocoumarin (DHC) and ethylene oxide (EO) was realized by relatively mild phosphazene bases (t-BuP2 and tBuP1), which unexpectedly outperformed previously employed t-BuP4 superbase in terms of polymerization rate, monomer conversion, and copolymer molar mass, though macrocycles were still generated when long chains were targeted. Such facts have indicated the occurrence of proton shuttling between phosphazenium cation and alkoxide which reduced chain-end nucleophilicity and thus alleviated side reactions such as backbiting. It was surprising that t-BuP1 whose basicity was too low for the homopolymerization of EO triggered alternating copolymerization, indicating that generation of anionic species (phenoxide) was essential for the epoxide-opening step. Welldefined short-chain diols were subjected to one-pot subsequent chain extension by addition of an aliphatic lactone or a diisocyanate leading to, respectively, block copolymer or polyurethane constituted by alternating segments. Poly(DHC-alt-EO) showed a better thermal stability than those of the substituted epoxides. This study has suggested that mild and non-nucleophilic organobases may be more suitable catalysts for epoxide-based metal-free alternating copolymerization toward well-defined macromolecular structures.

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INTRODUCTION Copolymerization has been a routinely employed method in both industry and fundamental research areas to regulate the structures and optimize the properties of polymers.1−3 Although the more readily achievable random (statistical), gradient, block and graft copolymerizations have been mostly studied and utilized, alternating copolymerization has always held an important and irreplaceable position in polymer chemists’ arsenal because of the charm and challenge of turning two distinct compounds into a uniform main chain structure.4−6 In principle, an alternating copolymer structure is formed by strict kinetic control over the incorporating sequence of two polymerizable monomers.4 What has drawn even more attention is the alternating copolymerization involving one compound which is active toward a certain reaction but lacks the ability to homopolymerize. It has served as a powerful and delicate chemical strategy to enrich the catalogues of monomers and polymers structures. The copolymerization of epoxides with carbon dioxide and its sulfide analogues,7−10 cyclic anhydrides,3,11−13 nonhomopolymerizable lactones,14−18 etc., has been the most appealing of this kind because of the high accessibility of epoxides, the (bio)renewable features of the comonomers, as well as the (bio)degradability and large structural variety of the copolymers. For the last few decades, organometallic catalysis has been dominating the research on epoxide-based alternating copolymerization, accumulating a toolbox of catalysts/initiators and © XXXX American Chemical Society

principles for selecting/designing appropriate catalyst to reach high catalytic efficiency, low polyether content, excellent control over molar mass and stereoregularity of the copolymer.3,7−12 Despite the unprecedentedly fast development of organocatalytic polymerization in the last 15 years, together with the competences and potentials that organocatalysts have exhibited in respect of polymerization rate, selectivity and construction of macromolecular architectures,19−21 efforts devoted to organocatalytic (metal-free) alternating copolymerization have been quite limited particularly in very recent years.14−17,22−24 There are lacks of essential and detailed insights into structure−activity relation of the organocatalysts and suitable catalytic/activating mode for each specific type of monomer combination. Therefore, we have regarded epoxide-based alternating copolymerization as one important future trend for the development of organocatalytic polymerization. The special features and strengths of organocatalysis are expected to bring new opportunities for achieving optimized copolymerization processes and tailored alternating copolymer structures. Recently, we have found that a phosphazene superbase (tBuP4) could trigger ring-opening alternating copolymerization (ROAP) of 3,4-dihydrocoumarin (DHC) and several commonly used monosubstituted epoxides. 24 The method Received: March 21, 2017 Revised: April 28, 2017

A

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Macromolecules Table 1. Conditions and Results of Polymerizations Initiated by BDM and Catalyzed by PBs entrya

catalyst

[DHC]0/[EO]0/[OH]0/[PB]0b

time (h)

temp (°C)

Mn,SECc (kg mol−1)

ĐM c

convnd (%)

DHCEO1

t-BuP1

105/200/1/0.5

DHCEO2 DHCEO3

t-BuP1 t-BuP2

105/200/1/1 105/200/1/0.5

DHCEO4 DHCEO5 DHCEO6 DHCEO6-VLf DHCEO7 DHCEO7-PUh DHCEO8 DHCEO8-PUh EO1 DHCEO9

t-BuP1 t-BuP2 t-BuP1 t-BuP1 t-BuP1 t-BuP1 t-BuP1 t-BuP1 t-BuP1 t-BuP2

105/200/1/0.5 105/200/1/0.5 5/6/1/0.1 − 2.5/3/1/0.1 − 5/6/1/0.1 − 0/100/1/0.5 20/40/1/0.1

72 168 72 72 132 96 96 22 36 22 24 22 24 168 72 135

60 60 60 60 60 60 60 60 60 60 25 60 25 60 60 60

3.3 5.8 3.9 12.0 (71%)/3.5 (29%)e 15.4 (62%)/5.4 (38%)e 0.9 1.3 2.1 7.8 1.5 46.1 2.1 43.9 − 7.5 4.5

1.06 1.20 1.08 1.05/1.16 1.06/1.12 1.13 1.12 1.07 1.12 1.08 2.03 1.07 2.10 − 1.09 1.42

11 23 15 50 85 99 − >99 − 0 >99 >99

a

Toluene was used as a solvent with [DHC]0 being 1.5−2.0 M, except for DHCEO4 and DHCEO5 which were performed in THF. bMolar feed ratio of DHC, EO, hydroxyl, and catalyst. cNumber-average molar mass and molar mass distribution obtained from SEC analysis (THF, 35 °C, polystyrene standards). dConversion of DHC determined by 1H NMR analysis. eBimodal distribution (number in parentheses denotes the percentage of each population calculated by SEC peak areas). fVL was added to generate triblock terpolymer ([VL]0/[OH] = 22). gConversion of VL. hHDI was added upon cooling at 25 °C in an equal molar amount as BDM to start SGP. ca. 11 °C and toxic by inhalation, we recommend great caution in the process of drying and transferring). Tetrahydrofuran (THF) and toluene (Guangzhou Chemical Reagent, 99%) were successively dried over molecular sieve (4 Å), CaH2 and n-BuLi. 1,4-Benzenedimethanol (BDM; Aladdin, 99%) were dried twice by azeotropic distillation of THF. Phenol (Aladdin, 99%) was sublimated under vacuum. 1-tertButyl-2,2,4,4,4-pentakis(dimethylamino)-2λ 5 ,4λ 5 -catenadi(phosphazene) (t-BuP2; 2.0 M solution in THF, Aldrich) was used after evaporation of THF. Acetic acid (AcOH; Aladdin, 99%), tertbutylimino-tris(dimethylamino)phosphorane (t-BuP1; Aldrich, 97%), and 1,6-diisocyanatohexane (HDI; Aldrich, 99%) were used as received. Instrumentation. Size exclusion chromatography (SEC) coupled with RI and UV 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. Calibration was done with a series of narrowly dispersed polystyrene standards to obtain apparent number-average molar mass (Mn,SEC) and molar mass distribution (ĐM) of the (co)polymers. NMR spectra were recorded at room temperature (RT) or 60 °C on a Bruker AV600 NMR spectrometer using CDCl3 or toluene-d8 as solvents and tetramethylsilane as the internal standard. 1H NMR spectra were used to calculate the monomer conversion using integrals of the characteristic signals from the monomer and polymer. 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 temperature for 10 min to remove thermal history, cooled to −80 °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 nitrogen atmosphere at a heating rate of 10 °C min−1 in the temperature range of 25 to 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 THF solution of matrix (2,5-dihdroxybenzoic acid, 20 mg mL−1) in a volume ratio of 1:10. 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 nearest neighbor positions.

benefited from the non-nucleophilic nature and in situ activation/initiation mode of phosphazene catalyst, which allowed for expedient synthesis of telechelic, star-like and brush-like alternating copolymers by the use of di- and multihydroxyl initiators. However, the method in general suffered from low monomer conversions, and had nearly no practical use for ethylene oxide (EO) which is one of the most important epoxide monomer. Moreover, large deviations existed between the theoretical molar masses and the measured values, which was similar to the situation of earlier reported imidazole14,25,26 and chromium salen complex18 catalysis. It was shown by us that the DHC-epoxide type alternating copolymer was prone to intramolecular transesterification (backbiting) in the presence of t-BuP4 after a certain chain length was reached which impeded further chain growth.24 It has been well documented that the superbasicity of t-BuP4 endows the alcoholic initiating/propagating species with extremely high nucleophilicity, which is well suited for the ring-opening polymerization (ROP) of epoxides but could be quite detrimental for cyclic esters because of the extensive occurrence of macromolecular transesterification reaction in this case.20,27−31 On the other hand, less basic phosphazene bases (PBs) resulted in lower polymerization rates but could afford much better control for the latter.32−35 Therefore, in this study we were inspired to use such mild PBs (t-BuP1 and tBuP2) for the ROAP of DHC and EO, in the hope that the lowered chain-end nucleophilicity could alleviate side reactions and bring better outcomes, so as to gain a preliminary insight into the relationship between structure and catalytic activity of organocatalysts toward epoxide-based ROAP.

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EXPERIMENTAL SECTION

Chemicals. DHC (Aladdin, 99%) and δ-valerolactone (VL; Energy Chemical, 98%) were dried over calcium hydride (CaH2) overnight and vacuum-distilled. EO (Aldrich, 99%) was first condensed from a metal cylinder into a Schlenk flask and dried by stirring with CaH2 in an ice−water bath for 4 h, then cryo-condensed into a graduated cylindrical flask precharged with n-butyllithium (n-BuLi) and stirred there in an ice−water bath for 1 h (EO is volatile with a boiling point of B

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Figure 1. Evolution of SEC traces (left) of ROAP of DHC and EO catalyzed by t-BuP2 at 60 °C in toluene (DHCEO3 in Table 1), and the 1H NMR spectrum (right) of the isolated product. Polymer Synthesis. Poly(3,4-dihydrocoumarin-alt-ethylene oxide), P(DHC-alt-EO). A typical procedure for DHCEO3 (Table 1) is as follows. BDM (0.014 g, 0.10 mmol) and t-BuP2 (50.0 μL of 2.0 M THF solution, 0.10 mmol) were charged in a reaction flask and dissolved in purified THF cryo-distilled from another flask, followed by slow cryo-evaporation of THF. The azeotropic distillation of THF was repeated before toluene (5.0 mL) and DHC (2.66 mL, 21.0 mmol) were successively added in a glovebox. The mixture was carefully shaken to ensure complete mixing and dissolving. The flask was sealed by a stopcock, taken out of the glovebox and docked back on the Schlenk line. EO (2.0 mL, 40.0 mmol) was slowly condensed at −20 °C into the flask ([EO]0 = 4.0 M, [DHC]0 = 2.0 M), which was then slowly warmed up to 60 °C. At different time intervals, the flask was cooled down at 0 °C for a short period, and aliquots were withdrawn (ca. 0.1 mL each), injected into a mixture of CDCl3 (1.0 mL) and AcOH (two drops), and used for NMR analysis to determine the conversion of DHC. A bit of this solution was diluted with THF (1.0 mL) for SEC measurement. After stirring and heating for a final reaction time of 132 h, the flask was cooled down to RT and AcOH (0.5 mL) was added for quenching. A small amount of the solution was collected for SEC and NMR measurements and the rest was poured into cold methanol to precipitate the copolymer, which was then collected and dried in vacuum to afford a slightly yellowish paste. Convn(DHC) = 85%. Mn,SEC = 10.2 kg mol−1, ĐM = 1.38 (bimodal distribution, Table 1). 1H NMR (600 MHz, CDCl3): δ/ppm = 7.22− 6.70 (aromatic protons), 5.03−5.00 (−C6H4CH2OCO−), 4.43−4.30 (−C6H4OCH2CH2OCO−), 4.11−4.00 (−C6H4OCH2CH2OCO−), 2.96−2.83 (−OCOCH2CH2C6H4O−), 2.67−2.56 (−OCOCH2CH2C6H4O−). Poly(δ-valerolactone)-block-poly(3,4-dihydrocoumarin-alt-ethylene oxide)-block-poly(δ-valerolactone), PVL-b-P(DHC-alt-EO)-b-PVL (DHCEO6-VL in Table 1). DHCEO6 was synthesized similarly as described above with BDM (0.205 g, 1.50 mmol), t-BuP1 (70.0 μL, 0.30 mmol), DHC (1.88 mL, 15.0 mmol), toluene (8.0 mL), and EO (0.9 mL, 18.0 mmol) ([EO]0 = 1.8 M, [DHC]0 = 1.5 M). After stirring and heating at 60 °C for 22 h, the flask was cooled down to RT and half of the solution (ca. 5.0 mL) was withdrawn with a syringe in an argon flow and injected into another flask precharged with AcOH (0.5 mL) for quenching. A small aliquot was withdrawn to be analyzed by NMR and SEC, the rest was poured into cold methanol to precipitate the P(DHC-alt-EO) diol. Convn(DHC) > 99%. Mn,SEC = 2.1 kg mol−1, ĐM = 1.07. VL (3.0 mL, 33.0 mmol) was added in a glovebox to the rest of the solution containing P(DHC-alt-EO) diol (ca. 1.5 mmol hydroxyl) and t-BuP1. The mixture was heated again to 60 °C and stirred for 36 h, after which the reaction was finally quenched by addition of AcOH (0.5 mL). A small aliquot was withdrawn for NMR and SEC analysis, the rest was poured into cold methanol to precipitate the triblock terpolymer. Convn(VL) = 92%. Mn,SEC = 7.8 kg mol−1, ĐM = 1.12. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.22−6.70 (aromatic protons), 5.03−5.00 (−C6H4CH2OCO−), 4.47−4.32

(−C6H4OCH2CH2OCO−), 4.20−3.92 (−C6H4OCH2CH2OCO− and −(CH2)3CH2OCO−), 3.67−3.62 (−(CH2)3CH2OH), 2.97− 2.86 (−OCOCH2CH2C6H4O−), 2.72−2.57 (−OCOCH 2 CH 2 C 6 H 4 O−), 2.40−2.25 (−OCOCH 2 (CH 2 ) 3 −), 1.75−1.55 (−OCOCH2CH2CH2CH2−). Poly(3,4-dihydrocoumarin-alt-ethylene oxide)-Based Polyurethane, P(DHC-alt-EO)-PU. A typical procedure for DHCEO8-PU (Table 1). DHCEO8 was synthesized in exactly the same way as DHCEO6. After being stirred and heated at 60 °C for 22 h, the reaction mixture was cooled down to RT and an small aliquot was withdrawn in an argon flow to be analyzed by NMR and SEC. Convn(DHC) > 99%. Mn,SEC = 2.1 kg mol−1, ĐM = 1.07. Then HDI (0.24 mL, 1.50 mmol, [HDI]0 = [hydroxyl]0 = 0.15 M) was added in a glovebox into the flask containing P(DHC-alt-EO) diol and t-BuP1. The reaction mixture was stirred at RT (ca. 25 °C) for 24 h, during which an apparent viscosity increase was observed, then quenched by addition of AcOH (0.5 mL). The solution was diluted with THF and poured into methanol. The slightly yellowish and pasty product was collected and dried under vacuum. Convn(−CH2OH) > 99%. Mn,SEC = 43.9 kg mol−1, ĐM = 2.10. 1H NMR (600 MHz, CDCl3): δ/ppm = 7.22−6.70 (aromatic protons), 5.08−5.00 (−C6H4CH2OCO−), 4.45− 4.30 (−C6H4OCH2CH2OCO−), 4.15−4.00 (−C6H4OCH2CH2OCO−), 3.24−3.05 (−OCONHCH2−), 2.96− 2.86 (−OCOCH2CH2C6H4O−), 2.68−2.58 (−OCOCH2CH2C6H4O−), 1.50−1.39 (−OCONHCH2CH2CH2−), 1.32−1.23 (−OCONHCH2CH2CH2−).

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RESULTS AND DISCUSSION As we previously reported, the major issue associated with tBuP4-catalyzed ROAP of DHC and epoxides is the intramolecular transesterification (backbiting) reaction which caused bimodal molar mass distributions of the product (coexistence of linear and cyclic alternating copolymers) and limitation to the achievable chain length.24 Therefore, to ensure clear analysis on this matter, a dihydroxyl initiator (BDM) is used in this study to leave out the influence of adventitious water or hydrolyzed forms of DHC and epoxide which would act as difunctional initiators and thus an extra cause for bimodal molar mass distribution in the case where a monoalcohol was used as the initiator. Having speculated that the low monomer conversion and backbiting reaction might be related to the high basicity of t-BuP4, we were interested to explore on the effect of relatively mild PBs. Table 1 lists the experimental conditions and results of ROAP of DHC with EO catalyzed by t-BuP1 and t-BuP2.36 As one of the most important epoxide monomers with respect to commercial applications, EO has been scarcely involved in the study of alternating copolymerization.37 In our C

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Scheme 1. Plausible Mechanistic Pathways for (a) ROAP of DHC and EO Catalyzed by t-BuP1 or t-BuP2 (in General) and (b) Occurrence of a Backbiting Reaction and Evolution of the Macrocycles

distribution is again demonstrated by 1H NMR spectrum (Figure 1), and SEC shows a bimodal distribution indicating the occurrence of backbiting reaction and the formation of macrocycles. However, DHC conversion reaches 50% in 72 h and 85% in 132 h, and the linear population has reached a molar mass of >15 kg mol−1 and a weigh fraction (calculated by the peak areas) of ≥60%, which are significant improvements over the results obtained with t-BuP4 and t-BuP1. In the 1H NMR spectrum, signal of −CH2OH protons that would be derived from terminal EO units and centered at ca. 3.85 ppm is barely visible. It then can be inferred that the reaction between DHC terminals and EO is much slower than that between EO terminals and DHC, so that most of the linear chains are terminated with DHC before a high conversion of DHC is reached which can be further supported by the results presented below (e.g., MALDI−TOF MS spectra). To explain for the better outcomes obtained with t-BuP2, a proton shuttling mechanism is proposed here. Namely, t-BuP2 is basic enough to deprotonate DHC-derived terminal phenol group so that it gets ionized and can react efficiently with EO (Scheme 1a). But after the ring-opening of EO, the highly basic alkoxide tends to recapture proton from the phosphazenium cation ([t-BuP2H]+), which causes a proton exchange equilibrium between the two species (probably involving formation of hydrogen bonding). Thus, the chain-end nucleophilicity is reduced overall to partly suppress transesterification reaction (as shown previously, it is the epoxidederived alkoxide terminal other than the DHC-derived

previous study with t-BuP4, the conversion of DHC in its ROAP with EO was too low (25% in 72 h) to have practical use in spite of the fact that 5 equiv of EO and almost 1 equiv of tBuP4, with regard to DHC and hydroxyl respectively, were used. In the present study, t-BuP1 was first used to catalyze the ROAP of DHC and EO at 60 °C in toluene with ca. 1 equiv excess of EO (DHCEO1 in Table 1) and a targeted degree of polymerization (Dp) of 105. 1 H NMR analysis shows that the conversion of DHC reaches 11% and 23% in 72 and 168 h respectively, and that the product is a copolymer of DHC and EO with perfectly alternating sequence distribution and absence of EOEO connections (Figure S1). Evolution of SEC traces shows that molar mass distribution is unimodal and narrow when the P(DHC-alt-EO) chains are short (72 h), whereas a small amount of macrocycles are formed when the copolymer chains grow longer at 132 and 168 h (also see Figure S2 for MALDI− TOF MS spectrum of the final product). Increasing the feed of t-BuP1 from 0.5 to 1 equiv of hydroxyl did not make a remarkable improvement on the polymerization rate or monomer conversion (DHCEO2 in Table 1). It has then appeared to us that t-BuP1 is not sufficiently active to afford high conversion in a reasonable scale of reaction time when high Dp is targeted. Higher temperature was not applied in consideration of the volatility and toxicity of EO. Copolymerization of DHC and EO catalyzed by t-BuP2 was performed with the same feed ratio and experimental conditions (DHCEO3 in Table 1). Alternating sequence D

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Macromolecules phenoxide terminal that is responsible for the transesterification reaction) but remains adequate to trigger the ring-opening of DHC.24 On the other hand, the basicity of t-BuP4 is too high to allow for the proton shuttling so that the alkoxide species maintains a high nucleophilicity throughout and induces more extensive transesterification reactions, and the basicity of t-BuP1 is too low to efficiently deprotonate phenolic hydroxyls or activate aliphatic hydroxyls so that the polymerization rate is brought down (see Figure S3 for the pKa values of PBs and hydroxy compounds). It is also noteworthy that the higher monomer conversion in the case of t-BuP2 compared with tBuP4 may indicate that there exists an opening-closing equilibrium of the six-membered phenolic lactone ring which causes a lower equilibrium conversion of DHC with higher basicity of the catalyst. In Figure 1, a left shift of the low-molar-mass population with prolonged reaction time is presented, indicating that the macrocycles become larger as monomer conversion and the lengths of linear chains increase. Therefore, it can be inferred that the initially formed macrocycles can be reopened giving rise to a dynamic cyclization-reopening process and the chances for ring expansion (Scheme 1b). It also needs to be pointed out that there is still a noticeable difference between the molar mass determined by SEC (Mn,SEC) and the theoretical value (Mn,theor). For instance, Mn,SEC of DHCEO1 and the linear part of DHCEO3 at 72 h are respectively 3.3 kg mol−1 and 12.0 kg mol−1, while the corresponding Mn,theor values are 4.4 kg mol−1 and 14.3 kg mol−1 as calculated by the feed ratio of monomers to initiator, DHC conversion and the weight fraction of the linear part (for DHCEO3, Table 1). Except for experimental errors, this may be understood as the deviation caused by using polystyrene standards. To gain an insight into the influence of solvent, THF was used in association with t-BuP1 and t-BuP2 (DHCEO4 and DHCEO5 in Table 1, to compare with DHCEO1 and DHCEO3). DHC conversion reached only 99%) are converted to ester groups (for DHCEO8, −C6H4CH2OH is not observed any more) indicating sufficiently high initiation efficiency. After cooling to RT, HDI with an equal molar amount as BDM was added to start the SGP. After 24 h, Mn,SEC becomes >40 kg mol−1 with broadened distributions (ĐM ≈ 2). 1H NMR spectra of the isolated polyurethanes present all the characteristic signals from P(DHC-alt-EO) segments and carbamate linkages (Figure 4). Signal integrals in 1H NMR spectra of the polyurethanes again indicate that DHC units outnumbers EO units (Figures S6 and S7). This means that P(DHC-alt-EO) precursors had a considerable number of phenol end groups when subjected to SGP with HDI, which did not seem to have

an impact on SGP implying that phenol also participated in the reaction with HDI maintaining a numeric equality between hydroxyls and isocyanate groups. The participation of phenol is also evidenced by the existence of two adjacent NMR signals for − OCONHCH2− protons centered at 3.11 and 3.19 ppm (Figures 4, S6, and S7) representing isocyanate groups connected to EO- and DHC-derived terminals, respectively. It is also worth noting that the excess of EO did not affect the SGP either. It was previously demonstrated that t-BuP2 is strong enough to trigger homopolymerization of EO, which though appeared to be slower than the case with t-BuP4.34 In the present study, the attempt to homopolymerize EO with t-BuP1 did not achieve success (EO1 in Table 1). Therefore, it was somewhat surprising to see that EO copolymerized with DHC through the catalysis of t-BuP1. Such a result indicates that phenolic hydroxyl is more effective than aliphatic hydroxyl to attack and open EO in the presence of a non-nucleophilic base. Partial deprotonation (ionization) of the more acidic phenol groups may be the key (Figure S8), which may be helpful to guide the selection/design of organocatalyst in future study on epoxidebased metal-free alternating copolymerization. The fact that EO does not homopolymerize in the presence of t-BuP1 is beneficial because timely quenching is unnecessary even if excess EO is used. On the other hand, it is crucial in the case of t-BuP2 to quench the reaction before or close to the F

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nated/ionized to ensure effective reaction with EO, and EOderived alkoxide species remains protonated to keep a low nucleophilicity thus alleviate side reactions. Although backbiting and generation of macrocycles has not been fully suppressed when a long chain is targeted, well-defined P(DHCalt-EO) diols with low molar masses can be readily achieved and subjected to one-pot subsequent block copolymerization with an aliphatic lactone or SGP with a diisocyanate to afford P(DHC-alt-EO)-based novel macromolecular structures. The insights gained in this study are believed to be essential for the selection/design of organocatalysts and strategies toward exquisite synthesis of epoxide-based metal-free alternating copolymers.

point that full conversion of DHC is reached. For example, DHCEO9 (Table 1) was performed with 0.1 equiv of t-BuP2 with regard to hydroxyls (to compare with DHCEO3). At 72 h, an almost full conversion of DHC was reached and a unimodal molar mass distribution (ĐM = 1.09) was maintained (Figure S9). But after reaction time was prolonged to 135 h, Mn,SEC decreased, ĐM increased, and 1H NMR spectrum exhibited the formation of polyether segments (Figure S9). Such results indicate that excess EO homopolymerizes in the presence of tBuP2 after DHC is completely consumed. Meanwhile, aliphatic hydroxyl end groups can be largely preserved which causes extensive transesterification reactions sabotaging the alternating copolymer chains. Interestingly, the consequence in this case is not a bimodal molar mass distribution but a broad SEC peak suggesting a high probability for the concurrence of inter- and intramolecular transesterification. Therefore, the preference for intramolecular transesterification observed previously seems to be associated with the low concentration of aliphatic hydroxyls at incomplete DHC conversions, which also further supports the conclusion that most of the copolymer chains are terminated by DHC units in the process of the ROAP. Thermal properties of alternating copolymers of DHC and EO were briefly characterized by DSC and TGA (Figure 5).

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEC traces, 1H NMR and MALDI−TOF spectra, structural formula and pKa values. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00599. Additional SEC traces, 1H NMR and MALDI−TOF spectra, and structural formulas and pKa values (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(J.Z.) E-mail: [email protected]. ORCID

Junpeng Zhao: 0000-0002-2590-0027 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of National Natural Science Foundation of China (21504024, 21674038) is acknowledged. REFERENCES

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Figure 5. DSC (left, second heating curves) and TGA (right) traces of P(DHC-alt-EO) with different molar masses and the polyurethane derived from a short P(DHC-alt-EO) diol.

None of the P(DHC-alt-EO) samples appears to be crystalline. Glass transition temperature (Tg) increases slightly with higher molar mass (from DHCEO1 to DHCEO3). The temperature for 5% weight loss (Td) does not show a remarkable dependence on molar mass either (369 °C for DHCEO1 and 375 °C for DHCEO3), indicating that P(DHC-alt-EO) is more thermally stable than the alternating copolymers of DHC and monosubstituted epoxides whose Tds are lower by at least 40 °C.24 P(DHC-alt-EO)-based polyurethane does not show a noticeably different Tg, but Td gets lower (335 °C) indicating that introduction of carbamate linkages reduces overall thermal stability.

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CONCLUSIONS This study has shown that relatively mild phosphazene bases (tBuP2 and t-BuP1) serve better as organocatalysts for ROAP of DHC and EO, as compared with previously employed t-BuP4 superbase. Proton shuttling between the catalyst and the hydroxyl species is proposed for explanation. Namely, provided with appropriate basicity of the catalyst and copolymerization conditions, DHC-derived phenolic species becomes deprotoG

DOI: 10.1021/acs.macromol.7b00599 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b00599 Macromolecules XXXX, XXX, XXX−XXX