Document not found! Please try again

Dual Organocatalysts for Highly Active and Selective Synthesis of

4 hours ago - Ring-opening polymerization (ROP) of the bioderived and nonstrained γ-butyrolactone (γ-BL) is an emerging approach to produce recyclab...
0 downloads 0 Views 1MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Dual Organocatalysts for Highly Active and Selective Synthesis of Linear Poly(γ-butyrolactone)s with High Molecular Weights Cheng-Jian Zhang, Lan-Fang Hu, Hai-Lin Wu, Xiao-Han Cao, and Xing-Hong Zhang* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou Shi, Zhejiang Sheng 310027, China

Downloaded via UNIV OF SUNDERLAND on October 26, 2018 at 02:12:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Ring-opening polymerization (ROP) of the bioderived and nonstrained γ-butyrolactone (γ-BL) is an emerging approach to produce recyclable polymers. It remains a big challenge to synthesize high molecular weight linear poly(γ-butyrolactone)s (PγBLs) in a highly active and selective manner. In this report, we developed dual organocatalysts for the ROP of γ-BL, using symmetrical (thio)ureas with electron-donating groups to buffer the growing anions that were generared by the superbase phosphazene (P4). Linear PγBLs were selectively obtained with high apparent rate constant (ca. 39 times than single P4), and high number-average molecular weights of up to 64.3 kg/mol (ca. 3 times than single P4). The turnover frequencies of these P4/(thio)urea pair-catalyzed processes were as high as 125 h−1. High molecular weight PγBLs exhibited dramatically improved mechanical properties.

■

INTRODUCTION The majority of commercial polymers are still derived from petrochemicals. These polymers are clearly made for the benefit of our modern life, while most of them are nonbiodegradable or nonrecyclable, resulting in serious white pollution on the land and in the ocean.1,2 Wild animals are often suffered or killed by those long-standing plastic pollutants. The synthesis of polymer from renewable resources with a green life-cycle, which matches or exceeds the properties of nondegradable and/or nonrecylable plastics, is strongly desired, but indeed a grand challenge.3,4 Therefore, efforts must be made to innovate a sustainable synthetic process including seeking renewable monomers5−10 and developing highly active catalysts.11−18 As one of the key downstream chemicals of succinic acid, γbutyrolactone (γ-BL) is a biomass-derived cyclic ester with a nonstrained five-membered ring.19,20 It had been thought to be “nonpolymerizable” for a long time.21 In 2016, a milestone work, reported by Hong and Chen et al., described the ringopening polymerization (ROP) process of γ-BL, using metal (La, Y) complexes as the catalysts at low reaction temperatures (−40 to − 60 °C).5 Successively, a metal-free ROP process of γ-BL was reported by the same research team, using 1-tertbutyl-4,4,4-tris(dimethyla mino)-2,2-bis[tris(dimethylamino) phosphoranyli denamino]-2λ5,4λ5 catenadi (phosphazene) (P4),6 the strongest superbase (pKa: 42.6 in CH3CN; 30.2 in DMSO) as the catalyst.22,23 Both catalytic processes for the ROP of γ-BL afforded mixed linear and macrocyclic poly(γbutyrolactone)s (PγBLs) with number-average molecular weights (Mns, GPC) of 26.7−30.0 kg/mol. Very recently, Li and Liu et al. reported a very interesting work on selectively synthesizing PγBLs with Mns of up to 22.9 kg/mol, using a © XXXX American Chemical Society

newly disclosed cyclic trimeric phosphazene base (pKa of 33.3 in CH3CN), which could dramatically reduce the generation of the macrocyclic PγBL.24,25 It should be noted that the mechanical properties of a polymer are closely related to its molecular weight (MW) and topology. For PγBLs, high MW and linear structure are preferred for improving the mechanical properties, while the macrocyclic polymer causes less chain entanglement and thus weakens mechanical properties. Hence, it is of great significance and a necessity to synthesize high MW PγBLs. Previous studies disclosed that the production of the macrocyclic PγBL, caused by the backbiting reaction of the alkoxide anion to the carbonyl sites in the chain,5,6 decreased the MW. The challenging issue is to develop highly active catalysts capable of inhibiting the backbiting reaction for the ROP of γ-BL. Regarding the above issue, we reviewed the previous P4catalyzed ROPs of γ-BL6 and other monomers.26−28 As seen in Scheme 1, the ion pair including the generated alkoxide anion and the countercation is proposed as the growing species. When P4 is used, a “loose” pair might be formed since [P4H]+ is a cation with a large size of ca. 1.4 nm.29 Hence, only a relatively long distance electrostatic interaction exists in the pair, and the alkoxide anion has strong basicity. We envision that, when a Brønsted acid is inserted into the pair, the formed species may buffer the basicity of the anion. As a result, the backbiting reaction of such anion will be inhibited. Generally, (thio)ureas are used as hydrogen donors for organic basecatalyzed ROP of cyclic monomers (e.g.: latone, lactide).30−33 Received: August 15, 2018 Revised: October 10, 2018

A

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. ROP of γ-BL Using P4 Combined with (Thio)ureasa

Table 1. Results of γ-BL ROP by P4/(thio)ureas Using BnOH as the Initiatora Entry TU(U) 1 2 3 4 5 6 7 8 9 10

TU-1 TU-2 TU-3 TU-4 TU-5 TU-6 TU-7 U-1 U-2

Conversionb (%)

MnTheoc (kg/mol)

MnGPCd (kg/mol)

Đd

26 20 8 80 40 66 80 60 69 78

6.7 5.3 2.2 20.7 10.4 17.2 20.8 15.6 17.9 20.3

11.4 8.9 21.5 10.5 14.9 20.6 13.3 23.7 22.6

1.70 1.58 1.69 1.59 1.74 1.69 1.75 1.57 1.64

Reactions Conditions: [γ-BL] = 10 M (0.42 g, 4.9 mmol); γ-BL: P4: TU (U): BnOH = 300:1:1:1, −40 °C in TOL for 4 h; catalyst and initiator were mixed first, followed by γ-BL. bMonomer conversion measured by the 1H NMR spectroscopy. cTheory moleculer weight: Mn Theo = 86.09 × [γ-BL]/[initiator] × Conv. dMn and Đ(Mw/Mn) were determined by GPC in DMF relative to PMMA standards, 60 °C. a

Scheme 1b) that are more basic (pKas: ca. 20−28 in DMSO) than TU-1(TU-2)37,38 could pair with P4 to afford a new catalytic system for active and selective ROP of γ-BL. The combination of the electron-donating group substituted (thio)ureas (entries 4−10, Table 1) with P4 exhibited much higher catalytic activity than P4 alone or P4/TU-1(or TU-2) pairs for the ROP of γ-BL in the presence of BnOH. Taking TU-3 as an example, the P4/TU-3 pair exhibited high activity toward the ROP of γ-BL with a γ-BL conversion of 80% (entry 4 in Table 1) under the γ-BL/P4/TU-3/BnOH feed ratio of 300/1/1/1 at −40 °C. Mn of the resultant PγBL was 21.5 kg/ mol (GPC) and close to the calculated moleculer weight (20.7 kg/mol). Of interest, with increasing carbon numbers of the alkyl substituents (methyl, ethyl, n-butyl, entries 5−7, Table 1), a clear increase of γ-BL conversion and MW of the resultant PγBLs was observed. However, TU-7 with a large tert-butyl group presented a lower γ-BL conversion (60%) than TU-6 (TU-3, TU-5). Compared with the n-butyl group, the more electron-donating t-Bu group with bigger steric hindrance exhibited less activity for the ROP of γ-BL, suggesting that a suitable steric hinhrance was also considered. Remarkably, two ureas with methyl and ethyl groups (U-1 and U-2) exhibited higher activity than their sulfur-substituted counterparts (TU-4 and TU-5), respectively. It is reasonable that ureas are less acidic than the thioureas with same substituents.37,38 Clearly, the basicity and the steric hinhrance of the alkyl substituents of (thio)ureas can be used to tune the activity for P4-catalyzed ROP of γ-BL. The strong impact of TU-3 on P4-catalyzed ROP of γ-BL is disclosed by the kinetic studies (Figure 1a). The kinetics of the ROP of γ-BL using the P4/TU-3 pair was examined, and single P4 and P4/TU-1 pair were used as the controls. Under the catalysis of P4/TU-3 pair, the ROP of γ-BL proceeded rapidly and the equilibrium monomer concentration [γ-BL]eq was assumed to be 2.0 M at −40 °C (Table S2, Figure S1). Hence, the first-order plot of ln{([M]0 − [M]eq)/([M]t − [M]eq)} (M = γ-BL) vs time shows that the apparent rate constant (kobs) of P4/TU-3 pair-catalyzed ROP of γ-BL is 47.2 × 10−3 min−1 (Figure 1a), which is ca. 39 times bigger than P4 alone (1.2 × 10−3 min−1). In contrast, the use of TU-1 retarded the rate of

a (a) Proposed buffering effect of (thio)ureas for inhibiting the backbiting reaction of the alkoxide anion. (b) Various (thio)ureas. TU-3 − TU-7, U-1, and U-2 with electron-donating groups are better for the ROP of γ-BL.

Herein, (thio)ureas are preferentially selected, because they can be deprotonated by strong bases and oxyanions,11,12,34,35 to generate (thio)urea anions that are less nucleophilic and hard to initiate the ROP of γ-BL, and the ionized (thio)ureas can also effectively activate the cyclic monomers.11,12

■

RESULTS AND DISCUSSION Initially, two popular thioureas (TUs) with electron-withdrawing groups,11,13,14,30−33,36 TU-1 and TU-2 (Scheme 1b), were mixed with P4 in a molar ratio of P4/TU-1(TU-2) of 1/1 for the ROP of γ-BL at −40 °C, using equivalent benzyl alcohol (BnOH) as the initiator. Surprisingly, the P4/TU1(TU-2) pairs exibihited lower activity than single P4 (entires 1−3, Table 1). Since TU-1 and TU-2 have Brønsted acidity (TU-1: pKa = 13.0; TU-2: pKa = 14.6, in DMSO)37,38 close to BnOH (pKa = 11.09, in DMSO),37 lower amounts of BnOH were deprotonated for initiation when TU-1 (or TU-2) was paired with P4. In addition, the generated anions combined with TU-1 (or TU-2) (Scheme 1a) were presumably less nucleophilic toward γ-BL. This result intrigued us to explore the (thio)ureas with relative less acidity for pairing with P4, expecting to generate growing anions with suitable basicity for the ROP of γ-BL. It was pleasing to observe that (thio)ureas with electron-donating groups (TU-3 − TU-7, U-1, and U-2, B

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. (a) First-order plot of ln{([BL]0 − [BL]eq)/([BL]t − [BL]eq)} vs time for the ROP of γ-BL at −40 °C by γ-BL/P4/BnOH = 300/1/1 (square); γ-BL/P4/TU-3/BnOH = 300/1/1/1 (circle); γBL/P4/TU-1/BnOH = 300/1/1/1 (uptriangle). (b) The plot of the chemical shifts of binding systems with the concentration of BnOH and P4 (-TU), [BnOH] = [P4 (-TU)], 25 °C in C6D6. Figure 2. (a) 31P NMR spectra of P4, P4/TU-3 (1/0.5), P4/TU-3 (1/1); (b) 1H NMR spectra of BnOH, BnOH/P4 (1/1), BnOH/P4/ TU-3 (1/1/1), BnOH/P4/TU-1 (1/1/1) in C6D6.

P4-catalyzed ROP of γ-BL, as revealed by the kobs value of only 0.8 × 10−3 min−1. Unlike the popular (thio)ureas with at least one electronwithdrawing group for the ROP of cyclic esters, the above (thio)ureas with electron-donating groups are structurally symmetrical and simple, commercially available, and lowcost.30−33 They (excluding TU-3) also are still not reported for the ROP of the cyclic esters.11,12,30 pKa values of these (thio)ureas are ca. 20−28 (in DMSO) and significantly higher than those of TU-1 and TU-2.37,38 We hence proposed that these (thio)ureas are well matched with P4 to generate a wellbalanced acid−base catalytic system, which can buffer the growing anion in a very suitable way and activate γ-BL simultaneously, as shown in Scheme 1a. To verify the above idea, we further probed the role of TU-3 on P4-catalyzed ROP of γ-BL by detecting the interactions of TU-3, P4, and BnOH via the NMR technique. The 31P NMR (Figure 2a) and 1H NMR spectra (Figure S3) show a complete deprotonation of TU-3 by P4 in a stoichiometric way; half amounts of protons of N−H in TU-3 were deprotonated by P4. Hence, a P4/TU-3 ion pair could be formed as species (1) in Scheme 2. When BnOH with a quite smaller pKa than TU-3 and P4 was introduced (red line, Figure 2b), it could be deprotonated to afford BnO−, which was bound with TU-3 via the hydrogen bond interaction, generating species (2) in Scheme 2. This was supported by a reduced deshielding effect of the methylene protons (A in Figure 2b) of BnOH in the BnOH/TU-3/P4 (1/1/1) mixture (5.40 ppm) than that in the BnOH/P4(1/1) mixture (5.61 ppm) in C6D6. In contrast, the deshielding effect of the methylene protons of BnOH in the

Scheme 2. Proposed Mechanism of ROP of γ-BL with P4/ TU-3 Pair

BnOH/TU-1/P4(1/1/1) mixture was little compared with the single BnOH (4.58 vs 4.47 ppm, Figure 2b), suggesting a very weak deprotonation of BnOH by P4/TU-1 pair in C6D6. Indeed, BnO− generated by the P4/TU-1 pair presented low nucleophilicity and thus had quite low activity, as the result noted in Table 1. As a result, compared with the single P4, an intramolecularly nucleophilic attack of BnO− to the activated γ-BL could occur and maintained a fast enchainment, as shown C

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. MALDI-TOF MS spectra of PγBLs synthesized with (a) γ-BL/P4/TU-3/BnOH = 300/1/1/1(Mn: 3.5 kg/mol, GPC), (b) γ-BL/P4/ BnOH = 300/1/1(Mn: 3.8 kg/mol, GPC).

Table 2. Results of ROP of γ-BL by P4/TU-3 Pair under Different Conditionsa Entry

T (°C)

t (h)

Initiator (I)

M/P4/TU-3/I

Conversionb (%)

MnTheoc (kg/mol)

MnGPCc (kg/mol)

Đd

1 2 3 4 5 6 7 8 9 10 11 12 13

−40 −40 −40 −55 −55 −25 0 −40 −40 −40 −40 −40 −40

12 12 12 12 12 4 4 4 4 4 4 4 4

BnOH BnOH BnOH t-BuOH Ph(CH2)2OH C2H5C(CH2OH)3 BnOH BnOH

100/1/0.5/0 100/1/1/0 100/1/1.5/0 100/1/1/0 100/1/1.5/0 100/1/1/1 100/1/1/1 300/1/1/3 300/1/1/1 300/1/1/1 300/1/1/1 500/1/5/10 1000/1/10/20

80 80 79 78 74 35 0 78 65 77 42 75 60

3.0 6.7 16.8 19.9 10.9 3.2 2.6

22.4 33.8 42.3 55.1 64.3 8.4 8.9 37.3 26.5 14.3 7.0 7.6

1.62 1.66 1.67 2.23 2.35 1.59 1.55 1.54 1.68 1.50 1.43 1.38

Reactions Conditions: [γ-BL] = 10 M (0.42 g, 4.9 mmol); catalyst and initiator were mixed first, followed by γ-BL, TOL was used as the solvent. Measured by the 1H NMR spectroscopy. cTheoretical moleculer weight: MnTheo = 86.09 × [γ-BL]/[initiator] × Conv. dMn and Đ(Mw/Mn) were determined by GPC at 60 °C in DMF relative to PMMA standards. The copolymerization results under other conditions are shown in Table S1. Representative GPC traces are shown in Figure S12.

a

b

(see detail in Supporting Information). Keqs of BnOH were 84.6 ± 9.2 for P4 alone, 30.1 ± 2.3 for the P4/TU-3 pair, and 16.7 ± 2.7 for the P4/TU-1 pair in equilibrium in C6D6 at 25 °C (Figure 1b), suggesting a basicity order of the generated BnO−: P4 > P4/TU-3 pair > P4/TU-1 pair. Clearly, the P4/ TU-3 pair provided locally confined BnO− that has just the right basicity to depress the backbiting reaction, which was firmly supported by the MALDI-TOF MS spectroscopy and TGA results. The employment of TU-3 led to unprecedentedly selective production of linear PγBL for P4-catalyzed ROP of γ-BL. PγBLs with low Mns were synthesized with the γ-BL/P4/TU3/BnOH feed ratios of 300/1/0/1, 300/1/1/1, and 300/1/1/

as species (3) and (4). This process was also revealed by the 13 C NMR spectra of γ-BL in the presence of TU-3, P4, and BnOH (γ-BL/P4/TU-3/BnOH: 1/10/10/10) (Figure S5).11 It exhibited a clear shift of the resonance signal of the carbonyl group of γ-BL from 176.78 to 178.41 ppm (Δδ = 1.63 ppm) when P4, TU-3, and BnOH were added, while the 13C NMR trace of ethyl acetate (EA) showed a little Δδ of 0.26 ppm under the same conditions. This result suggests that the species (2) could selectively activate monomer (species (3)) instead of the polymer chain, leading to minimal transesterifications. We further quantitatively determined the binding constants (Keqs) of BnOH in the presence of P4, P4/TU-3 pair, and P4/ TU-1 pair, respectively, using the 1H NMR dilution experiment D

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules 3, respectively. In the presence of 1.0 equiv TU-3, one predominant distribution of α-OBn, ω-OH-terminated polymer (BnO−[C(O)CH2CH2 CH2O]n + H + K+) (Figure 3a), i.e., linear PγBL, was observed, while the distribution of the macrocyclic polymers was inconspicuous in the low MW region. After increasing the amounts of BnOH (3.0 equiv to P4), only linear PγBL was collected, as revealed by the MALDI-TOF MS and 1H NMR spectra (Figures S6−S7). By way of contrast, in the absence of TU-3, considerable amounts of macrocyclic polymers ([C(O)CH2CH2CH2O]n + K+) were observed in the low MW region (Figure 3b). In addition, the use of 1.5 equiv TU-3 for P4-catalyzed ROP of γ-BL without using BnOH also produced linear PγBL with the acylated lactone/OH chain ends selectively. In contrast, lower amounts of TU-3 (P4/TU-3 = 1/0.5) led to obvious production of the macrocyclic PγBLs (Figure S8). Therefore, by combining ≥1.0 equiv TU-3 with P4, the undesired backbiting reaction of the growing anion (species (5) in Scheme 2) could be significantly depressed. Comparative studies of TGA results of the samples (entries 1, 4−5 in Table 1 and entries 1−3 in Table 2, Figure S9) also supported the sole production of linear PγBL under the above optimized conditions since macrocyclic PγBL has better thermal properties.5,6 The selective production of linear PγBLs could be attributed to the buffering effect of TU-3 on the growing anion. Of significance, high MW PγBLs were obtained from the P4/TU-3 pair-catalyzed ROP of γ-BL, especially in the absence of BnOH (Table 2). Even in a P4/TU-3 feeding ratio of 1/0.5, the conversion of γ-BL was up to 80% at −40 °C within 12 h (entry 1, Table 2). Mn of the resultant PγBL was 22.4 kg/mol. After increasing the loading of TU-3, the γ-BL conversion remained nearly the same (79−80%, entries 2−3, Table 2), while Mns of the generated PγBLs increased to 33.8 and 42.3 kg/mol, respectively. Since TU-3 acted as a Brønsted acid and consumed P4 via deprotonation, greater amounts of TU-3 reduced the numbers of the deprotonated γ-BL that could initiate the ROP,6 thus producing high MW PγBLs. Elevating the reaction temperature to −25 or 0 °C was unfavorable to the ROP of γ-BL, causing 35% and 0% of conversion, respectively (entries 6−7, Table 2). Further lowering the reaction temperature to −55 °C produced PγBLs with much high Mns of 55.1 and 64.3 kg/mol (entries 4−5, Table 2) that were ca. 2−3 times the previously reported values.5,6,24,25 At the same time as our work, Chen et al. just reported the synthesis of PγBLs with high Mn of up to 83.2 kg/mol by an yttrium-based catalyst and the thermal properties of PγBLs with various end groups.39 Herein, to demonstrate the impact of the MWs of PγBLs on the mechanical properties, the stress−strain behavior of PγBLs with different MWs were comparatively studied. As shown in Figure 4, the ultimate tensile strength of PγBL with a Mn of 42.5 kg/mol presents an elongation at break of 183.5% and a tensile strength of 16.3 MPa at room temperature, while PγBL with a Mn of 18.5 kg/ mol exhibits a strain to failure of only 8.5% and a tensile strength of 8.6 MPa. As a result, it is of importance to obtain high MW PγBLs for improving their mechanical properties. The effect of the γ-BL concentration (using solvent) and the types of initiators on the ROP of γ-BL was also investigated. The use of polar solvent such as THF and DMF led to a decrease of γ-BL conversion (68% and 44%, respectively) (entries 4−5, Table S1). Indeed, the monomer concentration also has a strong effect on the equilibrium of the ROP of γ-BL

Figure 4. Stress−strain curves of PγBLs with a Mn of 18.5 kg/mol (blue line) and 42.5 kg/mol (red line) at room temperature, 50 mm/ min. The synthetic conditions were the same with that of entries 2 and in Table 2. Note that repeated thermal processing reduced the Mn for pure PγBLs.

in thermodynamics, which was well demonstrated by Chen et al.5 Concurrently, upon increasing the amounts of BnOH (BnOH/P4 = 3/1, entry 8 in Table 2), the conversion of γ-BL reached 78% within 4 h, and the resultant PγBL had a Mn of 8.9 kg/mol, suggesting that the excess BnOH participated in the initiation owing to high efficiency of deprotonation by P4. In contrast, no polymers were collected when the P4/TU-3 pair or single P4 were used in the absence of BnOH under the same P4 concentration (entry 8 in Table 2 vs entries 1−2, Table S1). Concurrently, the acidity of the initiator could also affect the catalytic activity of the P4/TU-3 pair. The γ-BL conversion order of BnOH (80%) > PhCH2CH2OH (77%) > t-BuOH (65%) was observed with decreasing their acidity (entry 4 in Table 1, entries 9−10 in Table 2). Note that the use of a trifunctional initiator, trimethylolpropane [C2H5C(CH2OH)3], for the ROP of γ-BL generated a PγBL with a Mn of 14.3 kg/mol with a Đ of 1.50 (entry 11, Table 2; Figure S12d) that was narrower than those of most samples in Table 2, suggesting a minimal intermolecular transesterification. Remarkably, the conversion of γ-BL could be up to 60% and 75% under the γ-BL/P4/TU-3/BnOH feed ratios of 1000/1/ 10/20 and 500/1/5/10, respectively (entries 12−13, Table 2). That is, a maximum turnover frequency (TOF) of P4 for the ROP of γ-BL was up to 125 h−1, much higher than previously reported TOFs (ca. ≤ 25 h−1).5,6 Clearly, using low-cost (thio)ureas dramatically reduced the dosage of expensive P4. We finally investigated the types of organic bases on the ROP of γ-BL in the presence of TU-3. The combination of TU-3 with 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi (phosphazene) (P2) or tert-butylimino-tris(dimethyl-amino) phosphorene (P1) exhibited TOFs of 37 or 6 h−1, respectively (entries 7−8, Table S1). Compared with P4 (Figure 2), a little bit of variation of the chemical shifts of N− H of TU-3 in the presence of P2 (or P1) in the 1H NMR spectra (Figure S10) suggests a weak deprotonation of TU-3 by P2 (or P1) in C6D6. It could be attributed to their low basicity (pKas of 26.9 and 33.5 for P1 and P2 in CH3CN).22,23

■

CONCLUSION In summary, we have demonstrated highly active and selective ROP of γ-BL for synthesizing high MW linear PγBLs, using commercially available P4 with various electron-donating groups substituted (thio)ureas that are also commercially available and low-cost. It is a metal-free ROP of γ-BL with high E

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(5) Hong, M.; Chen, E. Y. X. Completely Recyclable Biopolymers with Linear and Cyclic Topologies via Ring-opening Polymerization of γ-Butyrolactone. Nat. Chem. 2016, 8, 42−49. (6) Hong, M.; Chen, E. Y. X. Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained γ-Butyrolactone. Angew. Chem., Int. Ed. 2016, 55, 4188−4193. (7) De Hoe, G. X.; Zumstein, M. T.; Tiegs, B. J.; Brutman, J. P.; McNeill, K.; Sander, M.; Coates, G. W.; Hillmyer, M. A. Sustainable Polyester Elastomers from Lactones: Synthesis, Properties, and Enzymatic Hydrolyzability. J. Am. Chem. Soc. 2018, 140, 963−973. (8) Nakano, R.; Ito, S.; Nozaki, K. Copolymerization of Carbon Dioxide and Butadiene via a Lactone Intermediate. Nat. Chem. 2014, 6, 325−331. (9) Tang, X.; Hong, M.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y. X. The Quest for Converting Biorenewable Bifunctional α-Methylene-γ-Butyrolactone into Degradable and Recyclable Polyester: Controlling Vinyl-Addition/Ring-Opening/Cross-Linking Pathways. J. Am. Chem. Soc. 2016, 138, 14326−14337. (10) Tang, X.; Chen, E. Y. X. Chemical Synthesis of Perfectly Isotactic and High Melting Bacterial Poly(3-hydroxybutyrate) from Bio-Sourced Racemic Cyclic Diolide. Nat. Commun. 2018, 9, 2345− 2355. (11) Zhang, X.; Jones, G. O.; Hedrick, J. L.; Waymouth, R. M. Fast and Selective Ring-opening Polymerizations by Alkoxides and Thioureas. Nat. Chem. 2016, 8, 1047−1053. (12) Coates, G. W.; Moore, D. R. Discrete Metal-Based Catalysts for the Copolymerization CO2 and Epoxides: Discovery, Reactivity, Optimization, and Mechanism. Angew. Chem., Int. Ed. 2004, 43, 6618−6639. (13) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Waymouth, R. M.; Hedrick, J. L. Thiourea-Based Bifunctional Organocatalysis: Supramolecular Recognition for Living Polymerization. J. Am. Chem. Soc. 2005, 127, 13798−13799. (14) Lin, B.; Waymouth, R. M. Urea Anions: Simple, Fast, and Selective Catalysts for Ring-Opening Polymerizations. J. Am. Chem. Soc. 2017, 139, 1645−1652. (15) Wang, Q.; Zhao, W.; Zhang, S.; He, J.; Zhang, Y.; Chen, E. Y. X. Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs. ACS Catal. 2018, 8, 3571−3578. (16) Li, H.; Zhao, J.; Zhang, G. Self-Buffering Organocatalysis Tailoring Alternating Polyester. ACS Macro Lett. 2017, 6, 1094−1098. (17) Tan, C.; Xiong, S.; Chen, C. Fast and Controlled Ring-Opening Polymerization of Cyclic Esters by Alkoxides and Cyclic Amides. Macromolecules 2018, 51, 2048−2053. (18) Li, Y.; Zhang, Y.-Y.; Liu, B.; Zhang, X.-H. HCAII-Inspired Catalysts for Making Carbon Dioxide-Based Copolymers: The Role of Metal-Hydroxide Bond. Chin. J. Polym. Sci. 2018, 36, 139−148. (19) Bozell, J. J.; Petersen, G. R. Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates-the US Department of Energy’s ″Top 10″ Revisited. Green Chem. 2010, 12, 539−554. (20) Bomgardner, M. M. Biobased Polymers. Chem. Eng. News 2014, 92, 10−14. (21) Houk, K. N.; Jabbari, A.; Hall, H. K.; Alemán, C. Why δValerolactone Polymerizes and γ-Butyrolactone Does Not. J. Org. Chem. 2008, 73, 2674−2678. (22) Boileau, S.; Illy, N. Activation in Anionic Polymerization: Why Phosphazene Bases are Very Exciting Promoters. Prog. Polym. Sci. 2011, 36, 1132−1151. (23) Hu, S.; Zhao, J.; Zhang, G.; Schlaad, H. Macromolecular Architectures through Organocatalysis. Prog. Polym. Sci. 2017, 74, 34−77. (24) Zhao, N.; Ren, C.; Li, H.; Li, Y.; Liu, S.; Li, Z. Selective RingOpening Polymerization of Non-Strained γ-Butyrolactone Catalyzed by A Cyclic Trimeric Phosphazene Base. Angew. Chem., Int. Ed. 2017, 56, 12987−12990. (25) Shen, Y.; Zhang, J.; Zhao, N.; Liu, F.; Li, Z. Preparation of Biorenewable Poly(γ-butyrolactone)-b-Poly(l-lactide) Diblock Copo-

activity and selectivity even in low loading of P4. High MW PγBLs exhibited greatly improved mechanical properties. It is proposed that an organic acid−base buffering system was generated through the interaction of (thio)ureas with P4, which can both buffer the growing anions and activate γ-BL synchronously. The dual organocatalysts reflect the Yin and Yang philosophy (two factors that restrict and promote each other for balance) perfectly. This strategy is supposed to widely apply to regulating similar anionic polymerizations. Extending the variety of (thio)ureas and developing commercially available and low-cost (thio)ureas (especially ureas) are competitive relative to the metal catalysts and worthy of special focus.

■

EXPERIMENTAL SECTION

■

ASSOCIATED CONTENT

Polymerizations were performed in 10 mL flame-dried Schlenk tubes interfaced to the dual-manifold Schlenk line using an external cooling bath. The reactor was charged with a predetermined amount of catalyst, initiator, and solvent, and kept stirring for 10 min in the glovebox. The reactor was then sealed, taken out of the glovebox, and immersed into a cooling bath under the predetermined temperature for 10 min, the polymerization was started by rapid addition of monomer via a gastight syringe. After a set time, the reaction was quenched by adding 3 mL the benzoic acid/CHCl3 solution (10 mg/ mL), the precipitated polymer dissolved, and a 0.2 mL aliquot was then taken from the reaction mixture for the 1H NMR analysis. The quenched mixture was then precipitated into 100 mL of cold methanol, filtered, washed with methanol to remove unreacted monomer, and dried in a vacuum oven at room temperature to a constant weight. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01757. Full experimental details, NMR spectra, and additional polymerization data (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xing-Hong Zhang: 0000-0001-6543-0042 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS X.-H. Zhang gratefully acknowledges the financial support of the National Science Foundation of the People’s Republic of China (no. 21774108) and the Distinguished Young Investigator Fund of Zhejiang Province (LR16B040001).

■

REFERENCES

(1) Hillmyer, M. A. The Promise of Plastics from Plants. Science 2017, 358, 868−870. (2) Garcia, J. M.; Robertson, M. L. The Future of Plastics Recycling. Science 2017, 358, 870−872. (3) Zhu, J.-B.; Watson, E. M.; Tang, J.; Chen, E. Y. X. A Synthetic Polymer System with Repeatable Chemical Recyclability. Science 2018, 360, 398−403. (4) Zhu, Y.; Romain, C.; Williams, C. K. Sustainable Polymers from Renewable Resources. Nature 2016, 540, 354. F

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules lyesters via One-Pot Sequential Metal-Free Ring-Opening Polymerization. Polym. Chem. 2018, 9, 2936−2941. (26) Chen, J.; Li, M.; He, W.; Tao, Y.; Wang, X. Facile Organocatalyzed Synthesis of Poly(ε-lysine) under Mild Conditions. Macromolecules 2017, 50, 9128−9134. (27) Zhao, J.; Pahovnik, D.; Gnanou, Y.; Hadjichristidis, N. Phosphazene-Promoted Metal-Free Ring-Opening Polymerization of Ethylene Oxide Initiated by Carboxylic Acid. Macromolecules 2014, 47, 1693−1698. (28) Hu, S.; Dai, G.; Zhao, J.; Zhang, G. Ring-Opening Alternating Copolymerization of Epoxides and Dihydrocoumarin Catalyzed by a Phosphazene Superbase. Macromolecules 2016, 49, 4462−4472. (29) Leito, I.; Rodima, T.; Koppel, I. A.; Schwesinger, R.; Vlasov, V. M. Acid−Base Equilibria in Nonpolar Media. 1. A Spectrophotometric Method for Acidity Measurements in Heptane. J. Org. Chem. 1997, 62, 8479−8483. (30) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.; Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Guanidine and Amidine Organocatalysts for Ring-Opening Polymerization of Cyclic Esters. Macromolecules 2006, 39, 8574−8583. (31) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Organocatalysis: Opportunities and Challenges for Polymer Synthesis. Macromolecules 2010, 43, 2093−2107. (32) Chan, J. M. W.; Zhang, X.; Brennan, M. K.; Sardon, H.; Engler, A. C.; Fox, C. H.; Frank, C. W.; Waymouth, R. M.; Hedrick, J. L. Organocatalytic Ring-Opening Polymerization of Trimethylene Carbonate To Yield a Biodegradable Polycarbonate. J. Chem. Educ. 2015, 92, 708−713. (33) Blake, T. R.; Waymouth, R. M. Organocatalytic Ring-Opening Polymerization of Morpholinones: New Strategies to Functionalized Polyesters. J. Am. Chem. Soc. 2014, 136, 9252−9255. (34) Xia, Y.; Chen, Y.; Song, Q.; Hu, S.; Zhao, J.; Zhang, G. Base-toBase Organocatalytic Approach for One-Pot Construction of Poly(ethylene oxide)-Based Macromolecular Structures. Macromolecules 2016, 49, 6817−6825. (35) Ladelta, V.; Kim, J. D.; Bilalis, P.; Gnanou, Y.; Hadjichristidis, N. Block Copolymers of Macrolactones/Small Lactones by a ″Catalyst-Switch″ Organocatalytic Strategy. Thermal Properties and Phase Behavior. Macromolecules 2018, 51, 2428−2436. (36) Zhang, C.-J.; Wu, H.-L.; Li, Y.; Yang, J.-L.; Zhang, X.-H. Precise Synthesis of Sulfur-Containing Polymers via Cooperative Dual Organocatalysts with High Activity. Nat. Commun. 2018, 9, 2137− 2146. (37) Jakab, G.; Tancon, C.; Zhang, Z.; Lippert, K. M.; Schreiner, P. R. (Thio)urea Organocatalyst Equilibrium Acidities in DMSO. Org. Lett. 2012, 14, 1724−1727. (38) Ho, J.; Zwicker, V. E.; Yuen, K. K. Y.; Jolliffe, K. A. Quantum Chemical Prediction of Equilibrium Acidities of Ureas, Deltamides, Squaramides, and Croconamides. J. Org. Chem. 2017, 82, 10732− 10736. (39) Tang, J.; Chen, E. Y.-X. J. Polym. Sci., Part A: Polym. Chem. 2018, DOI: 10.1002/pola.29180.

G

DOI: 10.1021/acs.macromol.8b01757 Macromolecules XXXX, XXX, XXX−XXX