Highly Active Organic Lewis Pairs for the Copolymerization of

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Highly Active Organic Lewis Pairs for the Copolymerization of Epoxides with Cyclic Anhydrides: Metal-Free Access to Well-Defined Aliphatic Polyesters Lan-Fang Hu, Cheng-Jian Zhang, Hai-Lin Wu, Jia-Liang Yang, Bin Liu, Han-Yi Duan, and Xing-Hong Zhang* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Polyester synthesis from the alternating copolymerization of epoxides with cyclic anhydrides via a metal-free route remains a key challenge. This work reports the development of a highly active organocatalytic route for the copolymerization of a spectrum of epoxides and cyclic anhydrides. Fully alternating polyesters were synthesized by a variety of organic Lewis acid−base pairs including organoboranes and quaternary onium salts. The effect of the acidity, type, and size of Lewis pairs on the catalytic activity and selectivity of the copolymerization is presented. The undesirable transesterification and etherification were effectively suppressed even in the case of complete conversion of the cyclic anhydride. This could be ascribed to the formation of a unique tetracoordinate bond-carboxylate (or alkoxide) anion. The Lewis pairs are highly active, with a turnover frequency of 102 and 303 h−1 for the copolymerization of propylene oxide with maleic anhydride and phthalic anhydride, respectively, at 80 °C. Block polyester with narrow polydispersity of 1.05 was achieved via a sequential addition strategy. This work provides robust organocatalysts for the selective copolymerization of epoxides with cyclic anhydrides.

A

Scheme 1. Alternating Copolymerization of Epoxides with Cyclic Anhydrides: (A) Proposed Hexacoordinate Metal Species [(salen)M Complexes] and Herein (B) Proposed Tetracoordinate Boron Species for the Chain Growth

liphatic polyesters are functional oxygen-rich polymers and have been used in applications from packaging materials to biomedical devices.1−4 The current commercial polyesters are mostly derived from petroleum, commonly prepared by condensation of diacids or diesters with diols, and thereby require the removal of small-molecule byproducts, such as water or alcohol.5 Such a step-growth process necessitates high reaction temperatures and is often highly energy-intensive. Currently, it is of great interest to develop polyesters from fully or partially renewable feedstocks6,7 via a catalytic chain-growth ring-opening polymerization (ROP) process of cyclic esters, such as commodity polylactide and polycaprolactone.8,9 Unfortunately, complex monomer design with multiple synthetic steps is a necessity to obtain functionalized polyesters with targeted chain structures.10 Through the chain-growth alternating copolymerization of epoxides with cyclic anhydrides, polyesters can also be synthesized in an atom-economic manner.11−14 Because most of the epoxides and cyclic anhydrides can be derived from the biomass12,15−19 and are commercially available with excess production capacity, it is a very promising route to get a variety of functionality-rich polyesters. Significant advances have been achieved in developing metal catalysts10,11,13,20 for the copolymerization of epoxides with cyclic anhydrides (Scheme 1). A diverse array of metal complexes, 15,21 such as zinc, 1 1 , 1 9 , 2 2 − 3 0 aluminum, 1 0 , 1 2 , 1 3 , 1 7 , 1 8 , 3 1 − 3 6 chromi© XXXX American Chemical Society

Received: March 8, 2018

A

DOI: 10.1021/acs.macromol.8b00499 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules um, 1 2 , 1 4 , 1 9 , 3 1 − 3 3 , 3 7 − 4 2 cobalt, 1 2 , 1 9 , 3 0 − 3 3 , 3 9 , 4 3 manganese,12,33,44−46 iron,18,47 and magnesium complexes19,27,48 have been reported to produce polyesters. Of great significance, several metal catalysts have been reported for well-controlled copolymerization of epoxides and cyclic anhydrides with high activity,39,40,49 regioselectivity,41 and stereoselectivity20 and inhibited undesirable side transesterification or epimerization reactions for tricyclic anhydrides.50,51 Despite all the progress in metal catalysis for the copolymerization process, metal contamination usually presents a high cost for complete removal and impedes the application of these polyesters in packaging materials and biomedical devices.1,2 Presently, the copolymerization of epoxides with cyclic anhydrides via a metal-free route remains a big challenge. Only a few works have been reported on metal-free catalytic processes for synthesizing polyesters with limited monomers. Quaternary onium salts,51−53 phosphazene,54 and tertiary amines55−57 have been employed to catalyze the copolymerization of the epoxides [i.e., cyclohexene oxide (CHO)51 and ethylene oxide (EO)54] and cyclic anhydrides [i.e., norbornene anhydride (NA)51 and phthalic anhydride (PA)54,55]. The catalytic activity of the above organocatalysts is low even at high reaction temperatures of 100−130 °C.51−53 The possible side reactions, such as transesterification, remain unknown. Of special concern, the copolymerization of propylene oxide (PO) with maleic anhydride (MA), both of which are low-cost monomers and can be mass-produced, could afford poly(propylene maleate) (PPM) with double bonds readily for chemical modification and has been rarely reported with limited success.14,31,38,44,49,53 To seek highly active organic catalysts for the copolymerization of the epoxides with cyclic anhydrides, we referenced the well-established salen-type metal (e.g., Al3+, Cr3+, and Co3+) complexes14,31,49 for the same copolymerization. These metal catalysts often have a hexacoordinate metal species accompanied by Lewis bases (LB, organic bases or salts) as the cocatalysts,15,37,38,40 based on the proposals of Coates,10 Darensbourg,38 Duchateau,37 and Chisholm.40 Remarkably, the Coates group recently discovered that a fluorinated (salcy) Al complex with strong Lewis acidity favored the elimination of the side reactions in the presence of [PPN]Cl even when the cyclic anhydrides was completely consumed.10 Such (salcy)Al/ [PPN]Cl is a ligand-stabilized metal Lewis acid−base pair, and we thus expected to find organic Lewis acid−base pairs with a larger acidity range for regulation for the copolymerization of epoxides with cyclic anhydrides. Because boron and aluminum are a family of elements and the boron atom can be coordinated with heteroatoms such as oxygen and nitrogen owing to its empty p orbital,58−60 we therefore envisioned organoboron compounds, such as triethyl borane (TEB, 1a), triphenyl borane (TPB, 1b), and tris(pentafluorophenyl)borane (TFPB, 1c) (Scheme 2), as the organic Lewis acids for the copolymerization of epoxides with cyclic anhydrides in the presence of the onium salts (e.g.: 2a-2f). Previously, Gnanou, Feng, and co-workers61 and our group59 reported the combination of TEB with quaternary onium salts for the alternating copolymerization of epoxides with carbon dioxide (CO2) and carbonyl sulfide (COS) afforded poly(carbonate)s and poly(monothiocarbonate)s, respectively. In this scenario, the chain end species is proposed as a tetracoordination boron anion (Scheme 1), being different from the hexacoordinate (salen)M complexes with two growing chains, and is still unexplored.

Scheme 2. Organic Lewis Pairs Used in This Worka

a

TEB, triethylborane; TPB, triphenylborane; TFPB, tris(pentafluorophenyl) borane; DTMeAB, N,N,N-trimethyl-1-dodecanaminium bromide; NBu4Br, tetrabutylammonium bromide; NBu4Cl, tetrabutylammonium chloride; PPh4Br, tetraphenylphosphonium bromide; PPh4Cl, tetraphenylphosphonium chloride; [PPN]Cl, bis(triphenylphosphine) iminium chloride.

Herein, we have reported a fully alternating copolymerization process of a variety of epoxides with cyclic anhydrides, mediated by organic Lewis pairs, including alkyl (aryl) boron 1a−1c, and quaternary onium salts (2a−2f), providing welldefined polyesters, and the side reactions were minimized. The effect of types of boron-containing Lewis acids and the onium salts on the catalytic activity and selectivity of the copolymerization is investigated in detail.

■

RESULTS AND DISCUSSION The copolymerization of PO with MA was investigated using boron-containing Lewis acids (1a−1c, Scheme 2) accompanied by DTMeAB (2a) for the first time, and the results are collected in Table 1. We initially optimized the reaction conditions, such as the feed ratios of PO, MA, and 1a/2a pair; types and feed ratios of the initiators to MA; and reaction temperatures and times (Tables S1 and S2). Unexpectedly, the copolymerization of PO with repeatedly purified MA (via a crystallization−sublimation procedure twice under dried highpurity N2) was slow, and PO conversion was 31% within 19 h using a 1a/2a (1/1) pair (entry 1, Table S1). However, the use of maleic acid, which was formed by adding equimolar water to MA, could effectively initiate the copolymerization with the optimized maleic acid/MA molar ratio of 2.0% (entry 9, Table S1). As seen in Table 1, MA conversion was up to 96% and >99% using the 1a/2a pair at 45 °C within 10 and 12 h, respectively. The resultant PPMs exhibited the close number-average molecular weights (Mn) of 5.5−5.7 kDa and Đ values of 1.29−1.30 (entries 1 and 2, Table 1; Figure S1). No chemical shifts of the polyether were observed at ca. 3.5 ppm in the 1H NMR spectra (Figure S2), demonstrating that the copolymer had a perfectly alternating degree (>99%) even when the copolymerization was run to full conversion with excess PO. In contrast, the sole catalysis of 2a for the copolymerization led to a low MA conversion of only 17%, and 1a could not catalyze the PO/MA copolymerization under the same reaction B


Article

Macromolecules

The dramatic promotion of the catalytic activity of 2a for PO/MA copolymerization by introducing 1a could be ascribed to the possible formation of the 1a/2a pair (species 1 in Scheme 3). As seen in the 1H NMR spectra (Figure 2), the

Table 1. Copolymerization of PO and MA in the Presence of Lewis Acids (1a−1c) with Lewis Base 2aa

entry

cat.

1d 2 3 4 5 6

1a/2a 1a/2a −/2a 1a/− 1b/2a 1c/2a

conv. (%)b ester (%)b H−T (%)b Mn (kDa)c 96 >99 17 − 78 12

>99 >99 >99 − >99 3

71 72 60 − 57 −

5.5 5.7 1.7 − 5.2 −

Scheme 3. Proposed Copolymerization Mechanism Using Lewis Pair

Đc 1.29 1.30 1.14 − 1.30 −

a

The reaction was performed in neat PO (0.25 mL, 3.5 mmol) in a 10 mL autoclave at 45 °C for 12 h; [PO]:[MA]:[1a (1b, 1c)]:[2a] = 350:100:1:1, 2.0 mol % maleic acid, in situ generated by adding equimolar water before adding the catalyst. bConv. (%) is the conversion of the cyclic anhydride, and ester (%) is the percentage of the ester linkage in the polymer, H-T (%) is the content of Head-totail linkage in the polymer. Conv. (%) = A6.26/(A6.26 + A7.04); Ester (%) = (A5.25 + A4.25)/(A5.25 + A4.25 + A3.5).They were all determined by 1 H NMR spectroscopy. cĐ is molecular weight distribution, determined by gel permeation chromatography in THF, calibrated with polystyrene standards. d10 h.

conditions (entries 3 and 4, Table 1). Therefore, the 1a/2a pair exhibited a significant improvement of the catalytic activity over the result of 2a alone for the copolymerization of PO with MA (Tables S3 and S4), as also clearly revealed by the kinetic study in Figure 1. The rate constant (k p,MA1 ) of PO/MA copolymerization catalyzed by the 1a/2a pair was calculated to be 3.4 L·mol−1·h−1 and ca. 14 times that using single 2a (0.24 L·mol−1·h−1, kp,MA2). We also investigated the kinetics of PO with phthalic anhydride (PA), a highly reactive cyclic anhydride, using the 1a/2a pair without using any initiators (Tables S5 and S6). The rate constant (kp,PA1) was 7.3 L·mol−1· h−1 and significantly higher than that of 2a-catalyzed copolymerization (0.29 L·mol−1·h−1, kp,PA2).

peak of the protons Hb of 1a shifted mostly to the high field overlapped with the protons Ha of 1a (curves 1 and 2 in Figure 2 and Figure S3A) based on the variation of integral area of Ha and Hb. We inferred that the equilibrium (i in Scheme 3) tended to generate species 1 owing to the coordination of Br− to boron. When 1a was combined with maleic acid with a molar ratio of 3/1, a clear shielding effect on the protons (Ha, Hb) of 1a is displayed, as seen in curve 4 in Figure 1. Concomitantly, the protons (Hd) of maleic acid at 12.60 ppm shifted to a low field at 13.25 ppm with a sharply decreased intensity relative to the protons (Hc) at the double bond of the maleic acid (integral area ratio of Hd:Hc = 0.11:1.00), suggesting the

Figure 1. First-order plot of MA (or PA) conversion vs time for the copolymerization process at 45 °C. (A) [PO]:[MA]:[maleic acid]:[2a] = 350:100:2:1, [MA]0 = 0.08 mol/L and (B) [PO]:[PA]:[2a] = 350:100:1, [PA]0 = 0.02 mol/L in the mixture of THF and PO. Blue dot, single 2a catalysis; red dot, 1a/2a (1/1) pair catalysis. Rate constants (kp) can be calculated by the following equation: ln([M]0/[M]) = kp[C]twhere [M] is the concentration of MA (or PA), [C] the concentration of the initiator, and t the reaction time. For PO/MA copolymerization, maleic acid is initiator, 8 × 10−3 mol/L; for PO/PA copolymerization, Br− acts as the initiator, 2 × 10−3 mol/L. C


Article

Macromolecules

Figure 2. Analysis of 1H NMR spectra of (1) 1a, (2) 1a/2a (1/1), (3) maleic acid, (4) 1a/maleic acid (3/1), and (5) 1a/2a/maleic acid (3/1/1) in CDCl3, using TMS as the peak reference and equal volume of THF in each sample because commercial TEB has THF as the solvent.

deprotonation of −COOH by 1a owing to strong coordination between the electron-rich COO− with the electron-poor 1a (Scheme 3), generating species 2. Another example in which phenethyl alcohol with less Brønsted acidity was combined with 1a also led to a deprotonation process to form species 3 (Scheme 3 and Figure S3B). Of interest, introducing 2a to species 2 (1a/2a/maleic acid of 3/1/1) resulted in a clear split (6.82, 6.48 ppm) of the two protons (Hc) of the double bond of maleic acid, indicating an asymmetric chemical environment around protons (Hc) caused by 2a. As a result, we assumed that the deprotonation of the protic initiators was assisted by the coordination process of TEB and the carboxylate (or alkoxide) groups of the initiators (Scheme 3) rather than H-bonding interaction.62 Such TEB-bonded carboxylate (alkoxide) anions may be more nucleophilic than the corresponding protic initiators. Simultaneously, HX (X = Br or Cl) could be probably generated and even reacted with PO, giving X-substituted propanol,63 which could also act as a protic initiator (or chaintransfer agent) in the polymerization.36,64 The structure of the Lewis acid has a strong impact on the catalytic activity for the PO/MA copolymerization. Upon change of 1a to TPB (1b) with stronger acidity and bigger steric hindrance, MA conversion was lowered to 78% under the same reaction conditions (entry 5, Table 1). When 1a was changed to TPFB (1c), the strongest Lewis acid with the biggest steric hindrance reportedherein led to only 12% MA conversion and even 3% polyester produced (entry 6, Table 1). Clearly, the catalytic activity of the 1a/2a, 1b/2a, and 1c/2a pairs decreased sharply with the order of the acidity and steric hindrance of 1a < 1b < 1c. Correspondingly, the increasing upfield effect of the protons at NCH3 of 2a with increasing the acidity and steric hindrance was revealed by the 1H NMR spectra (Figure S4). This result is also consistent with aluminum salen complex with stronger Lewis acidity showing relatively low activity.10 We next examined the impact of the structure of the onium salts on the copolymerization of PO with MA. NBu4Br (2b), NBu4Cl (2c), PPh4Br (2d), PPh4Cl (2e), and [PPN]Cl (2f)

were employed in the copolymerization in the presence of 1a or 1b, as summarized in Table 2. The 1a/2b−2f pairs exhibited >99% conversion of MA within 12 h (Table 2), which was remarkably higher than those when only onium salts were used. When the copolymerization was quenched at 6 h, the order of Table 2. Copolymerization of PO and MA in the Presence of Various Lewis Pairsa

entry

cat.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

−/2b 1a/2b 1b/2b −/2c 1a/2c 1b/2c −/2d 1a/2d 1b/2d −/2e 1a/2e 1b/2e −/2f 1a/2f 1b/2f

conv. (%)b ester (%)b 16 >99 89 22 >99 83 26 >99 98 22 >99 80 16 >99 >99

>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99

H−T (%)b

Mn (kDa)c

Đc

57 70 60 58 69 60 60 76 62 52 71 60 61 72 68

1.0 7.0 6.2 2.1 6.2 7.4 1.8 5.4 6.1 1.0 6.8 5.5 0.8 8.0 4.3

1.24 1.30 1.30 1.21 1.15 1.30 1.27 1.31 1.25 1.23 1.28 1.26 1.23 1.39 1.30

a

The reaction was performed in neat PO (0.25 mL, 3.5 mmol) in a 10 mL autoclave at 45 °C for 12 h; [PO]:[MA]:[1a(1b)]:[2b(2c−2f)] = 350:100:1:1, 2.0 mol % maleic acid, in situ generated by adding equimolar water before adding the catalyst. bConv. (%) is the conversion of the cyclic anhydride, and ester (%) is the percentage of the ester linkage in the polymer. Conv. (%), ester (%), and H−T content (%) were determined by 1H NMR spectroscopy (see Table 1). cDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards. D


Article

Macromolecules the observed activity was 2d (87%) > 2e (59%) and 2b (84%) > 2c (68%) (Table S7). Concurrently, the change of 1a to 1b for the copolymerization exhibited relatively low catalytic activity but presented the similar order of 2d (98%) > 2e (80%) and 2b (89%) > 2c (83%) (Table 2). Clearly, the onium salts with Br− exhibited catalytic activity higher than that of those with Cl−, which could be attributed to Br− being a better leaving group than Cl−.65,66 However, it appears that the influence of the types and sizes of the cations on the catalytic activity of these Lewis pairs for the copolymerization of PO with MA was not obvious. We suspected that, in contrast to 1a which was closely attached with the carboxylate (alkoxide) anion (species 2 and 3 in Scheme 3), the cation was in the periphery of the boron-carboxylate (alkoxide) anion as the counterion. Boron-containing Lewis acids had a direct influence on the growing anions. To further identify the proposed boron-bonded growing anions and expecting the structure of the Lewis acid (1a,1b) may affect the regioselective ROP of PO, we examined the regioregularity of the resultant PPMs, which involved the selective attacking of the boron-carboxylate anion to PO. It was observed that the content of the H−T diad of PPM from the 1a/2a pair catalysis at 45 °C was ca. 72% and higher than that

Table 3. Copolymerization of PO with MA (and PA) Catalyzed by 1a with Various LBs at 80 °Ca

entry anhyd. 1 2d 3 4d 5 6d

MA MA MA MA PA PA

LB

time (h)

conv. (%)b

TOF (h−1)b

Mn (kDa)c

Đc

2a 2a 2d 2d 2d 2d

0.5 0.5 0.5 0.5 0.3 0.3

21 7 51 8 91 5

42 14 102 16 303 15

1.3 − 2.8 0.7 20.0 −

1.27 − 1.29 1.17 1.12 −

a

Reaction conditions: [PO]:[MA (PA)]:[1a]:[2a(2d)] = 350:100:1:1, 80 °C; for MA, 2.0 mol % maleic acid for entries 1−4. bConv. (%) is the conversion of the cyclic anhydride, determined by 1H NMR spectroscopy, see Table 1; TOF = (moles of anhydride consumed)/ (moles of LB). cDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards. dWithout TEB (1a).

more nucleophilic. This is also in agreement with the result that the use of 1b with stronger acidity and bigger steric hindrance exhibited lower regioregularity due to it being more steric and less nucleophilic (Figure S6). The Lewis pairs exhibited surprisingly improved activity upon elevating the reaction temperatures without observing side reactions, as shown in Table 3. The two examined copolymerizations of PO/MA and PO/PA afforded perfectly alternating polyesters at 80 °C in a short reaction time (0.3 or 0.5 h). For the copolymerization of PO/MA, the 1a/2a pair exhibited a TOF of 42 h−1, while the 1a/2d pair exhibited a dramatically improved TOF of 102 h−1 (entries 1 and 3, Table 3). To the best of our knowledge, this is the highest TOF value for PO/MA copolymerization.49 By way of contrast, the sole catalysis of 2a and 2d exhibited TOFs of 14 and 16 h−1, respectively (entries 2 and 4 in Table 3). Remarkably, for the copolymerization of PO with PA, the 1a/2d pair had a high TOF of 303 h−1(entry 5, Table 3), much higher than that using 2d alone as the catalyst (15 h−1, entry 6, Table 3); the resultant poly(PO-alt-PA) had a Mn of 20.0 kDa, a Đ of 1.12, and >99% alternating degree (Figure S7). The improved catalytic activity by elevating reaction temperature meant that the growing species composed of a Lewis pair was stable and effective to the chain propagation. The chain microstructures of PPMs obtained under various reaction conditions were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) spectra (Figures 4 and S8−S11). Figure 4a shows a PPM synthesized using the 1a/2a pair and 2.0% maleic acid as the initiator (entry 2, Table 1). Of interest, two major distributions, α-Br,ω-OH-terminated [Br + (PO + MA)n + PO + H + K+] (A) and α-OH,ω-OH-terminated [OH + (PO + MA)n + PO + H + K+](B) copolymers were observed in the low and high m/z regions, respectively. The former was generated by Br− initiation, while the latter was produced by the initiation of maleic acid [X(Br)-(PO-MA)n-PO-H, HO(PO-MA)n-PO-H in Scheme 3]. Because copolymer B could grow at two ends simultaneously, it had about 2 times the

Figure 3. Analysis of the regiochemistry of poly(propylene maleate) (PPM) using 1H NMR. Black curve: PPM (Mn = 2.3 kDa, Đ = 1.17) from the catalysis of single 2a at 45 °C for 24 h. Blue curve: PPM (Mn = 5.7 kDa, Đ = 1.30) from the catalysis of 1a/2a at 45 °C for 12 h. Red curve: PPM (Mn = 3.8 kDa, Đ = 1.15) from the catalysis of 1a/2a at −1 °C for 48 h.

from 2a (ca. 60%), as revealed by the 1H NMR (Figure 3) and 13 C NMR spectra (Figure S5). Meanwhile, the H−T diad content of PPMs was unvaried with increased reaction time, i.e., ca. 70% for 1a/2a pair and ca. 60% for 2a alone (Tables S3 and S4). These results indicated that the 1a/LB pair could promote the regioregularity of the resultant PPMs. As our expectation, the size and type of the cations derived from 2a−2f exhibited minor influence on the regioregularity (65−71%, Figure S6). In addition, decreasing the polymerization temperature to −1 °C led to a slight increase of the H−T diad content (77%), while the high temperature of 80 °C (entry 1, Table 3) led to a low H−T diad content (63%). It was possible that low temperatures favored the formation of an intimate 1a-carboxylate anion. Therefore, the regioselective ring-opening reaction of PO was very likely affected by the structure of the Lewis acid, rather than the onium salts. Presumably, the H−T diad mostly resulted from the consecutive ROP of PO at the methylene (CH2) site because the 1a-carboxylate anion was less steric and E


Article

Macromolecules

less reactivity to attack PO for forming ether linkages (step iii, Scheme 3). Although nearly all polyesters displayed bimodal GPC curves (and MALDI-TOF MS spectra) caused by at least two kinds of initiations, most of the PPMs synthesized by a Lewis pair generally exhibited narrow Đ values in the range of 1.05−1.30 (Tables 1, 2, and S2). In addition, the kinetic result for forming PPM using the 1a/2a pair showed that Mn increased linearly with increasing reaction time while Đ values varied a little from 1.23 to 1.30 (Figure S12). By introducing more initiators into the copolymerization system, the Br-initiation could be dramatically depressed. For example, the copolymerization of PO/MA catalyzed using the 1a/2a pair in the presence of 5.0 mol % phenethyl alcohol (PhCH2CH2OH) exhibited a major distribution (A′ in Figure 4b) [PhCH2CH2O + (MA + PO)n + H + K+][RO-(MA-PO)n-H in Scheme 3], and two distributions of [PhCH2CH2O + (MA + PO)n + PO + H + K+] (B′) and [Br + (MA + PO)n + H + K+] (C′) were observed with minor intensity. Another example using benzyl alcohol (PhCH2OH, 4.0 mol %) as the initiator showed similar major distribution of [PhCH2O + (MA + PO)n + H + K+] in the MALDI-TOF MS spectrum with minimal distributions similar to B′ and C′ (Figure S11). Other epoxides were examined for copolymerizing with various cyclic anhydrides using the 1a/2a pair. Succinic anhydride (SA) and diglycolic anhydride (DGA) could copolymerize with PO, affording fully alternating polyesters with Đ values of 1.19−1.27 (1H and 13C NMR spectra, seen in Figures S13 and S14). The complete suppression in production of the ether linkage and transesterification is noted in these instances (entries 1 and 2, Table 4). On the other hand, although polyester synthesis from the copolymerization of cyclic anhydride with phenyl glycidyl ether (PGE) and ECH was very rare,14,41,49 the 1a/2a pair was effective at copolymerizing MA with ECH and PGE with electron-withdrawing

Figure 4. MALDI-TOF MS spectra of PPMs produced using maleic acid and phenethyl alcohol as the initiators. (a) Mn, 5.7 kDa; Đ, 1.30 (entry 2, Table 1); (b) Mn, 3.2 kDa; Đ, 1.05 (entry 7, Table S2). GPC curves are in Figure S1.

Table 4. Copolymerization of Various Epoxides and Cyclic Anhydrides by 1a/2a Paira

molecular weight of copolymer A, as revealed by MALDI-TOF MS spectra (Figure 4a). Concurrently, no α-Br,ω-Br-terminated or cyclic polyesters were detected in all MALDI-TOF MS spectra (Figures 4 and S8−S11); we could thus conclude that the transesterification was minimal, and even MA was run to full conversion. The proposed tetrahedral borane-alkoxide anion (Scheme 3) could reduce the concentration of the alkoxide-[C+] or free alkoxide, which was largely responsible for depressing the undesirable transesterifications. Certainly, the application of TPB (1b) with stronger acidity than 1a also led to the complete inhibition of the transesterification (Figure S10). Of interest, a minor distribution of [OH + (PO + MA)n + PO2 + H + K+] (C), which was likely copolymer B plus one more PO unit, was observed in Figure 4a. In contrast, the similar copolymerization quenched before full conversion of MA (79%) showed both distributions A and B but no any traces of the copolymer C. Meanwhile, the distribution of αCOOH,ω-COOH-terminated copolymer [H + MA + (PO + MA)n + H + K+] was observed (Figure S8). Hence, copolymer C in Figure 4a could be produced when MA was completely consumed. The reason lies in that the 1a-alkoxide anion had

entry

epoxide/ anhy.

time (h)

conv. (%)b

ester (%)b

Mn (kDa)c

Đc

1 2 3 4

PO/SA PO/DGA PGE/MA ECH/MA

10 10 12 12

79 85 62 95

>99 >99 >99 >99

5.1 6.0 6.1 10.1

1.19 1.27 1.26 1.37

a

The reaction was performed in neat epoxide (0.25 mL, 3.5 mmol) in a 10 mL autoclave, 45 °C; [ECH(PGE)]:[MA]:[maleic acid]:[1a]: [2a] = 200:100:2:1:1, [PO]:[SA(DGA)]:[1a]:[2a] = 350:100:1:1. b Conv. (%) is the conversion of the cyclic anhydride, and ester (%) is the percentage of ester linkage in the polymer, determined by 1H NMR spectroscopy, similar to Table 1. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards. F


Article

Macromolecules

PO/PA (TOF: 303 h−1) at 80 °C. The block polyesters could be achieved via a sequential addition strategy. Importantly, the use of such Lewis pairs for this copolymerization process provides an alternative route to avoid metal residues and is helpful to their wide application. For the mechanism, a tetracoordinate TEB-bonded carboxylate (or alkoxide) anion was proposed for the copolymerization, being different from the classic coordination copolymerizations. Our ongoing efforts seek to establish an intensive understanding of the mechanism of the catalytic process and to develop Lewis pairs with tailored structures for making functionalized polyesters from various epoxides with cyclic anhydrides.

substitutes at 45 °C, affording perfectly alternating polyesters (entries 3 and 4, Table 4; Figures S15 and S16). MA conversions were 62% and 95% for PGE/MA and ECH/MA copolymerization, respectively. The 1a/2a pair is effective for a variety of epoxides and cyclic anhydrides. As discussed above, the Lewis pair-catalyzed copolymerization process is well-controlled and robust. To address this advantage, we further explored the chain extension reaction via tandem syntheses based on the resultant PPM having the mono hydroxyl group and narrow Đ. PO and MA were copolymerized first using the 1a/2a pair in the presence of PhCH2CH2OH ([PO]:[MA]:[PhCH2CH2OH]:[1a]:[2a] = 350:100:4:1:1) under 35 °C for 20 h. MA was consumed completely, and the resultant PPM exhibited a unimodal peak with a Mn of 4.6 kDa (calculated: 4.0 kDa) and a Đ of 1.09 (Figure 5B). After 4

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00499. Experimental Section, Tables S1−S7, GPC curves, NMR spectra, and MALDI-TOF MS spectra (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.Z. 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).

Figure 5. (A) Tandem synthesis of poly[(PO-alt-MA)-b-(PO-alt-SA)] using 1a/2a pair at 35 °C. (B) GPC traces of the PPM (in blue) and the poly[(PO-alt-MA)-b-(PO-alt-SA)] (in red). (C) 13C NMR spectra of the poly[(PO-alt-MA)-b-(PO-alt-SA)], PPM, and poly(PO-alt-SA).

■

REFERENCES

(1) Rabnawaz, M.; Wyman, I.; Auras, R.; Cheng, S. A roadmap towards green packaging: the current status and future outlook for polyesters in the packaging industry. Green Chem. 2017, 19, 4737− 4753. (2) Zhu, Y.; Mao, Z.; Gao, C. Aminolysis-based surface modification of polyesters for biomedical applications. RSC Adv. 2013, 3, 2509− 2519. (3) Auras, R.; Harte, B.; Selke, S. An overview of polylactides as packaging materials. Macromol. Biosci. 2004, 4, 835−864. (4) Kai, D.; Loh, X. J. Polyhydroxyalkanoates: Chemical Modifications Toward Biomedical Applications. ACS Sustainable Chem. Eng. 2014, 2, 106−119. (5) East, A. J. Polyesters, Thermoplastic. In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, 2004; Vol. 20, pp 34−95. (6) Hong, M.; Chen, E. Y. Completely recyclable biopolymers with linear and cyclic topologies via ring-opening polymerization of gammabutyrolactone. Nat. Chem. 2016, 8, 42−9. (7) Hong, M.; Chen, E. Y. Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained gamma-Butyrolactone. Angew. Chem., Int. Ed. 2016, 55, 4188−93. (8) Masutani, K.; Kimura, Y. PLA Synthesis: From the Monomer to the Polymer. In Poly(lactic acid) Science and Technology: Processing, Properties, Additives and Applications; Royal Society of Chemistry: Cambridge, 2015; pp 1−36.10.1039/9781782624806-00001 (9) Labet, M.; Thielemans, W. Synthesis of polycaprolactone: a review. Chem. Soc. Rev. 2009, 38, 3484−504.

h, the flask was transferred to the glovebox, and 0.25 mL of product mixture was syringed and added to 0.25 mL of PO with 1.0 equiv. of SA ([PO]:[SA] = 350:1); the flask was sealed for another 48 h of reaction time at 35 °C. SA was fully converted, and the resultant copolymer had a Mn of 6.5 kDa (calculated: 8.0 kDa) with a Đ of 1.05, as revealed by the GPC curve shifting overall to high molecular weight (Figure 5B). The block structure of the final polyester was further evident by the 1 H(13C) NMR spectra (Figure S17). As seen in Figure 5C, the carbonyl region of the block polyester (curve 2) displays two groups of peaks that are in good agreement with the stacked curves of poly(PO-alt-MA) and poly(PO-alt-PA). Uniquely, the success of the chain extension reaction also demonstrated that the 1a/2a pair has a long lifespan.

■

CONCLUSIONS We have described the synthesis of perfectly alternating polyesters from a variety of epoxides and cyclic anhydrides employing metal-free Lewis pairs. The copolymerization process could be carried out over a range of reaction temperatures (−1 to 80 °C) with minimal side transesterifications and ether formation even in the case of complete conversion of the cyclic anhydride. The Lewis pair is highly active to the copolymerization of PO/MA (TOF: 102 h−1) and G


Article

Macromolecules (10) Van Zee, N. J.; Sanford, M. J.; Coates, G. W. Electronic Effects of Aluminum Complexes in the Copolymerization of Propylene Oxide with Tricyclic Anhydrides: Access to Well-Defined, Functionalizable Aliphatic Polyesters. J. Am. Chem. Soc. 2016, 138, 2755−61. (11) Jeske, R. C.; DiCiccio, A. M.; Coates, G. W. Alternating Copolymerization of Epoxides and Cyclic Anhydrides: An Improved Route to Aliphatic Polyesters. J. Am. Chem. Soc. 2007, 129, 11330− 11331. (12) Robert, C.; de Montigny, F.; Thomas, C. M. Tandem synthesis of alternating polyesters from renewable resources. Nat. Commun. 2011, 2, 586. (13) Aida, T.; Inoue, S. Catalytic Reaction on Both Sides of a Metalloporphyrin Plane. Alternating Copolymerization of Phthalic Anhydride and Epoxypropane with an Aluminum PorphyrinQuaternary Salt System. J. Am. Chem. Soc. 1985, 107, 1358−1364. (14) DiCiccio, A. M.; Coates, G. W. Ring-opening copolymerization of maleic anhydride with epoxides: a chain-growth approach to unsaturated polyesters. J. Am. Chem. Soc. 2011, 133, 10724−7. (15) Longo, J. M.; Sanford, M. J.; Coates, G. W. Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure-Property Relationships. Chem. Rev. 2016, 116, 15167−15197. (16) Peña Carrodeguas, L.; Martín, C.; Kleij, A. W. Semiaromatic Polyesters Derived from Renewable Terpene Oxides with High Glass Transitions. Macromolecules 2017, 50, 5337−5345. (17) Van Zee, N. J.; Coates, G. W. Alternating copolymerization of propylene oxide with biorenewable terpene-based cyclic anhydrides: a sustainable route to aliphatic polyesters with high glass transition temperatures. Angew. Chem., Int. Ed. 2015, 54, 2665−8. (18) Sanford, M. J.; Peña Carrodeguas, L.; Van Zee, N. J.; Kleij, A. W.; Coates, G. W. Alternating Copolymerization of Propylene Oxide and Cyclohexene Oxide with Tricyclic Anhydrides: Access to Partially Renewable Aliphatic Polyesters with High Glass Transition Temperatures. Macromolecules 2016, 49, 6394−6400. (19) Winkler, M.; Romain, C.; Meier, M. A. R.; Williams, C. K. Renewable polycarbonates and polyesters from 1,4-cyclohexadiene. Green Chem. 2015, 17, 300−306. (20) Li, J.; Liu, Y.; Ren, W. M.; Lu, X. B. Asymmetric Alternating Copolymerization of Meso-epoxides and Cyclic Anhydrides: Efficient Access to Enantiopure Polyesters. J. Am. Chem. Soc. 2016, 138, 11493−11496. (21) Paul, S.; Zhu, Y.; Romain, C.; Brooks, R.; Saini, P. K.; Williams, C. K. Ring-opening copolymerization (ROCOP): synthesis and properties of polyesters and polycarbonates. Chem. Commun. 2015, 51, 6459−79. (22) Ji, H.; Wang, B.; Pan, L.; Li, Y. Lewis Pairs for Ring-Opening Alternating Copolymerization of Cyclic Anhydrides and Epoxides. Green Chem. 2018, 20, 641−648. (23) Garden, J. A.; Saini, P. K.; Williams, C. K. Greater than the Sum of Its Parts: A Heterodinuclear Polymerization Catalyst. J. Am. Chem. Soc. 2015, 137, 15078−81. (24) Zhu, L.; Liu, D.; Wu, L.; Feng, W.; Zhang, X.; Wu, J.; Fan, D.; Lü, X.; Lu, R.; Shi, Q. A trinuclear [Zn3(L)2(OAc)2] complex based on the asymmetrical bis-Schiff-base ligand H2L for ring-opening copolymerization of CHO and MA. Inorg. Chem. Commun. 2013, 37, 182−185. (25) Liu, D.-F.; Wu, L.-Y.; Feng, W.-X.; Zhang, X.-M.; Wu, J.; Zhu, L.-Q.; Fan, D.-D.; Lü, X.-Q.; Shi, Q. Ring-opening copolymerization of CHO and MA catalyzed by mononuclear [Zn(L2)(H2O)] or trinuclear [Zn3(L2)2(OAc)2] complex based on the asymmetrical bis-Schiff-base ligand precursor. J. Mol. Catal. A: Chem. 2014, 382, 136−145. (26) Liu, Y.; Xiao, M.; Wang, S.; Xia, L.; Hang, D.; Cui, G.; Meng, Y. Mechanism studies of terpolymerization of phthalic anhydride, propylene epoxide, and carbon dioxide catalyzed by ZnGA. RSC Adv. 2014, 4, 9503−9508. (27) Saini, P. K.; Romain, C.; Zhu, Y.; Williams, C. K. Di-magnesium and zinc catalysts for the copolymerization of phthalic anhydride and cyclohexene oxide. Polym. Chem. 2014, 5, 6068−6075.

(28) Wu, L.-Y.; Fan, D.-D.; Lü, X.-Q.; Lu, R. Ring-opening copolymerization of cyclohexene oxide and maleic anhydride catalyzed by mononuclear [Zn(L)(H2O)] or binuclear [Zn2(L)(OAc)2(H2O)] complex based on the Salen-type Schiff-base ligand. Chin. J. Polym. Sci. 2014, 32, 768−777. (29) Thevenon, A.; Garden, J. A.; White, A. J.; Williams, C. K. Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides. Inorg. Chem. 2015, 54, 11906−15. (30) Yu, C. Y.; Chuang, H. J.; Ko, B. T. Bimetallic bis(benzotriazole iminophenolate) cobalt, nickel and zinc complexes as versatile catalysts for coupling of carbon dioxide with epoxides and copolymerization of phthalic anhydride with cyclohexene oxide. Catal. Sci. Technol. 2016, 6, 1779−1791. (31) Bernard, A.; Chatterjee, C.; Chisholm, M. H. The influence of the metal (Al, Cr and Co) and the substituents of the porphyrin in controlling the reactions involved in the copolymerization of propylene oxide and cyclic anhydrides by porphyrin metal(III) complexes. Polymer 2013, 54, 2639−2646. (32) Hosseini Nejad, E.; van Melis, C. G. W.; Vermeer, T. J.; Koning, C. E.; Duchateau, R. Alternating Ring-Opening Polymerization of Cyclohexene Oxide and Anhydrides: Effect of Catalyst, Cocatalyst, and Anhydride Structure. Macromolecules 2012, 45, 1770−1776. (33) Nejad, E. H.; Paoniasari, A.; van Melis, C. G. W.; Koning, C. E.; Duchateau, R. Catalytic Ring-Opening Copolymerization of Limonene Oxide and Phthalic Anhydride: Toward Partially Renewable Polyesters. Macromolecules 2013, 46, 631−637. (34) Aida, T.; Sanuki, K.; Inoue, S. Well-controlled polymerization by metalloporphyrin. Synthesis of copolymer with alternating sequence and regulated molecular weight from cyclic acid anhydride and epoxide catalyzed by the system of aluminum porphyrin coupled with quaternary organic salt. Macromolecules 1985, 18, 1049−1055. (35) Takasu, A.; Bando, T.; Morimoto, Y.; Shibata, Y.; Hirabayashi, T. Asymmetric Epoxidation of α-Olefins Having Neighboring Sugar Chiral Templates and Alternating Copolymerization with Dicarboxylic Anhydrides. Biomacromolecules 2005, 6, 1707−1712. (36) Sanford, M. J.; Van Zee, N. J.; Coates, G. W. Reversibledeactivation anionic alternating ring-opening copolymerization of epoxides and cyclic anhydrides: access to orthogonally functionalizable multiblock aliphatic polyesters. Chem. Sci. 2018, 9, 134−142. (37) Huijser, S.; Hosseininejad, E.; Sablong, R.; de Jong, C.; Koning, C. E.; Duchateau, R. Ring-Opening Co- and Terpolymerization of an Alicyclic Oxirane with Carboxylic Acid Anhydrides and CO2 in the Presence of Chromium Porphyrinato and Salen Catalysts. Macromolecules 2011, 44, 1132−1139. (38) Darensbourg, D. J.; Poland, R. R.; Escobedo, C. Kinetic Studies of the Alternating Copolymerization of Cyclic Acid Anhydrides and Epoxides, and the Terpolymerization of Cyclic Acid Anhydrides, Epoxides, and CO2 Catalyzed by (salen)CrIIICl. Macromolecules 2012, 45, 2242−2248. (39) Hosseini Nejad, E.; Paoniasari, A.; Koning, C. E.; Duchateau, R. Semi-aromatic polyesters by alternating ring-opening copolymerisation of styrene oxide and anhydrides. Polym. Chem. 2012, 3, 1308. (40) Harrold, N. D.; Li, Y.; Chisholm, M. H. Studies of RingOpening Reactions of Styrene Oxide by Chromium Tetraphenylporphyrin Initiators. Mechanistic and Stereochemical Considerations. Macromolecules 2013, 46, 692−698. (41) Liu, J.; Bao, Y. Y.; Liu, Y.; Ren, W. M.; Lu, X. B. Binuclear chromium−salan complex catalyzed alternating copolymerization of epoxides and cyclic anhydrides. Polym. Chem. 2013, 4, 1439−1444. (42) Biermann, U.; Sehlinger, A.; Meier, M. A. R.; Metzger, J. O. Catalytic copolymerization of methyl 9,10-epoxystearate and cyclic anhydrides under neat conditions. Eur. J. Lipid Sci. Technol. 2016, 118, 104−110. (43) Longo, J. M.; DiCiccio, A. M.; Coates, G. W. Poly(propylene succinate): a new polymer stereocomplex. J. Am. Chem. Soc. 2014, 136, 15897−900. H


Article

Macromolecules (44) Robert, C.; Ohkawara, T.; Nozaki, K. Manganese-corrole complexes as versatile catalysts for the ring-opening homo- and copolymerization of epoxide. Chem. - Eur. J. 2014, 20, 4789−95. (45) Liu, D.-F.; Zhu, L.-Q.; Wu, J.; Wu, L.-Y.; Lü, X.-Q. Ring-opening copolymerization of epoxides and anhydrides using manganese(iii) asymmetrical Schiff base complexes as catalysts. RSC Adv. 2015, 5, 3854−3859. (46) Liu, D.; Zhang, Z.; Zhang, X.; Lü, X. Alternating Ring-Opening Copolymerization of Cyclohexene Oxide and Maleic Anhydride with Diallyl-Modified Manganese(iii)−Salen Catalysts. Aust. J. Chem. 2015, 69, 47−55. (47) Mundil, R.; Hošt’álek, Z.; Šeděnková, I.; Merna, J. Alternating ring-opening copolymerization of cyclohexene oxide with phthalic anhydride catalyzed by iron(III) salen complexes. Macromol. Res. 2015, 23, 161−166. (48) Takasu, A.; Ito, M.; Inai, Y.; Hirabayashi, T.; Nishimura, Y. Synthesis of Biodegradable Polyesters by Ring-Opening Copolymerization of Cyclic Anhydrides Containing a Double Bond with 1, 2Epoxybutane and One-Pot Preparation of the Itaconic Acid-Based Polymeric Network. Polym. J. 1999, 31, 961−969. (49) DiCiccio, A. M.; Longo, J. M.; Rodríguez-Calero, G. G.; Coates, G. W. Development of Highly Active and Regioselective Catalysts for the Copolymerization of Epoxides with Cyclic Anhydrides: An Unanticipated Effect of Electronic Variation. J. Am. Chem. Soc. 2016, 138, 7107−7113. (50) Han, B.; Zhang, L.; Yang, M.; Liu, B.; Dong, X.; Theato, P. Highly Cis/Trans-Stereoselective (ONSO)CrCl-Catalyzed RingOpening Copolymerization of Norbornene Anhydrides and Epoxides. Macromolecules 2016, 49, 6232−6239. (51) Han, B.; Zhang, L.; Liu, B.; Dong, X.; Kim, I.; Duan, Z.; Theato, P. Controllable Synthesis of Stereoregular Polyesters by Organocatalytic Alternating Copolymerizations of Cyclohexene Oxide and Norbornene Anhydrides. Macromolecules 2015, 48, 3431−3437. (52) Kameyama, A.; Ueda, K.; Kudo, H.; Nishikubo, T. The First Synthesis of Alternating Copolymers of Oxetanes with Cyclic Carboxylic Anhydrides Using Quaternary Onium Salts. Macromolecules 2002, 35, 3792−3794. (53) Hošt’álek, Z.; Trhlíková, O.; Walterová, Z.; Martinez, T.; Peruch, F.; Cramail, H.; Merna, J. Alternating copolymerization of epoxides with anhydrides initiated by organic bases. Eur. Polym. J. 2017, 88, 433−447. (54) Li, H.; Zhao, J.; Zhang, G. Self-Buffering Organocatalysis Tailoring Alternating Polyester. ACS Macro Lett. 2017, 6, 1094−1098. (55) Fischer, R. F. Polyesters from Epoxides and Anhydrides. J. Polym. Sci. 1960, 44, 155−172. (56) Lustoň, J.; Vašs,̌ F. Anionic copolymerization of cyclic ethers with cyclic anhydrides. Adv. Polym. Sci. 1984, 56, 91−133. (57) Ikeda, I.; Simazaki, Y.; Suzuki, K. Synthesis of Graft Polyesters by Ringopening Copolymerization of Epoxy-Terminated Poly(ethylene Glycol) with Acid Anhydrides. J. Appl. Polym. Sci. 1991, 42, 2871−2877. (58) Diaz, D. B.; Yudin, A. K. The versatility of boron in biological target engagement. Nat. Chem. 2017, 9, 731−742. (59) Yang, J. L.; Wu, H. L.; Li, Y.; Zhang, X. H.; Darensbourg, D. J. Perfectly Alternating and Regioselective Copolymerization of Carbonyl Sulfide and Epoxides by Metal-Free Lewis Pairs. Angew. Chem., Int. Ed. 2017, 56, 5774−5779. (60) Sharif, S.; Day, J.; Hunter, H. N.; Lu, Y.; Mitchell, D.; Rodriguez, M. J.; Organ, M. G. Cross-Coupling of Primary Amides to Aryl and Heteroaryl Partners Using (DiMeIHeptCl)Pd Promoted by Trialkylboranes or B(C6F5)3. J. Am. Chem. Soc. 2017, 139, 18436−18439. (61) Zhang, D.; Boopathi, S. K.; Hadjichristidis, N.; Gnanou, Y.; Feng, X. Metal-Free Alternating Copolymerization of CO2 with Epoxides: Fulfilling “Green” Synthesis and Activity. J. Am. Chem. Soc. 2016, 138, 11117−20. (62) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Organocatalysis: Opportunities and Challenges for Polymer Synthesis. Macromolecules 2010, 43, 2093−2107.

(63) Stewart, C. A.; Vanderwerf, C. A. Reaction of Propylene Oxide with Hydrogen Halides. J. Am. Chem. Soc. 1954, 76, 1259−1264. (64) Hu, L. F.; Li, Y.; Liu, B.; Zhang, Y. Y.; Zhang, X. H. Alternating and regioregular copolymers with high refractive index from COS and biomass-derived epoxides. RSC Adv. 2017, 7, 49490−49497. (65) Kember, M. R.; Buchard, A.; Williams, C. K. Catalysts for CO2/ epoxide copolymerisation. Chem. Commun. 2011, 47, 141−63. (66) Wei, R.-J.; Zhang, X.-H.; Du, B.-Y.; Fan, Z.-Q.; Qi, G.-R. Synthesis of bis(cyclic carbonate) and propylene carbonate via a onepot coupling reaction of CO2, bisepoxide and propylene oxide. RSC Adv. 2013, 3, 17307.

I


Highly Active Organic Lewis Pairs for the Copolymerization of

Recommend Documents