Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Control of Molecular Weight and End-Functional Groups of Polyester from A2 + B2 Polycondensation via Cross-Metathesis of Cyclic Unsaturated Polyester with Difunctional Olefin Ryouichi Okabayashi, Yoshihiro Ohta, and Tsutomu Yokozawa* Department of Materials and Life Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan S Supporting Information *
ABSTRACT: As an approach to control the molecular weight and polymer end groups of polyester obtained by A2 + B2 polycondensation, we investigated cross-metathesis of cyclic polyesters containing carbon−carbon double bonds in the backbone with a symmetric olefin having two functional groups as an exchange reagent (ExR). Polycondensation of cis-2-butene-1,4-diol (1) and sebacoyl dichloride (4) did not selectively afford cyclic polyester, but when cis- or trans-4-octene-1,8-diol, instead of 1, was reacted with 4 or isophthaloyl dichloride, we found that cyclic unsaturated polyesters were formed selectively. The obtained cyclic polyesters successfully underwent cross-metathesis reaction with cis-1,4-diacetoxy-2-butene (6) in the presence of second-generation Grubbs catalyst to afford linear polyester bearing acetoxy groups at both ends. The molecular weight of the linear polyester decreased with increasing amount of 6 regardless of the molecular weight of the starting cyclic polyester; the molecular weight of the resulting linear polyester was governed by the molar ratio of 6 to carbon−carbon double bonds in the cyclic ester. Cross-metathesis using other ExRs enabled the introduction of tert-butoxycarbonyl (Boc) amino, bromophenyl, and tert-butyl carboxylate groups at both ends of polyester.
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INTRODUCTION Polyesters are widely used to produce fibers, bottles, films, and even artificial blood vessels, and recently there has been increasing interest in biodegradable polyesters such as polylactide. Synthesis of polyesters is generally conducted either by ring-opening polymerization or by polycondensation of difunctional nucleophilic monomer and difunctional electrophilic monomer (A2 + B2 polycondensation). The molecular weight and polymer end groups of polyesters can be well controlled in living ring-opening polymerization of lactones, but this is more difficult in the case of A2 + B2 polycondensation of diol and dicarboxylic acid derivative. One might think that control of the molecular weight and end groups in A2 + B2 polycondensation could be easily achieved by using excess A2 or B2 monomers or by addition of a monofunctional A or B compound. However, Kricheldorf et al. found that formation of cyclic polymers due to intramolecular reaction of both chain ends occurs in competition with propagation throughout the whole process of polycondensation, eventually affording cyclic polymers if the polycondensation is not accompanied by side reactions,1 and therefore it is difficult to selectively synthesize linear polymers having A or B end groups without contamination by cyclic polymers. Exceptionally, Bi2O3catalyzed polycondensation of dimethyl terephthalate and excess 1,4-butanediol exclusively yielded linear poly(butylene terephthalate) with hydroxyl groups at both ends.2 In order to selectively synthesize linear polymer with functional end groups in A2 + B2 polycondensation, cyclic polymer eventually obtained from the polycondensation should be converted to linear polymer by introducing functional end © XXXX American Chemical Society
groups. We focused on olefin metathesis reaction as a method for introduction of functional end groups into cyclic polymer from the point of view of dynamic covalent chemistry (DCC).3,4 Grubbs et al. conducted ring-opening metathesis polymerization (ROMP) of 1,4-cyclooctadiene in the presence of an alkene with functional groups as a chain transfer agent (CTA), obtaining polymer with end-functional groups derived from the CTA and controlled molecular weight.5−7 Otsuka et al. demonstrated that even carbon−carbon double bonds in the polymer backbone underwent olefin metathesis in scrambling reaction between poly(1,4-butadiene) and unsaturated polyester.8 In this work, we first synthesized cyclic polyesters containing carbon−carbon double bonds in the backbone by means of conventional polycondensation of unsaturated diol and dicarboxylic acid chloride. Then, cross-metathesis of the cyclic polyester with a symmetric olefin having two functional groups as an exchange reagent (ExR) was conducted with Grubbs catalyst. If the olefin metathesis reaction takes place sufficiently fast, the whole cyclic polyester would be converted to linear polyester with the functional groups derived from the ExR at both ends on the basis of DCC (Scheme 1). Moreover, the molecular weight of the linear polymer can be controlled by adjusting the molar ratio of ExR to carbon−carbon double bond in the repeat unit. Received: September 27, 2017 Revised: November 17, 2017
A
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Control of Molecular Weight and End-Functional Groups of Polyesters Obtained by A2 + B2 Polycondensation via Cross-Metathesis of the Obtained Cyclic Polyester with ExR
Scheme 2. Syntheses of Cyclic Polyesters
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RESULTS AND DISCUSSION
When equimolar 1 and 4 were reacted, the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum of the product showed major peaks of cyclic polymer in the low-molecular-weight region and peaks of linear polymers with HO/COOH and HO/OH end groups over the whole molecular weight region (Table 1, entry 1; Figure 1a). Considering that a small amount of carboxylic acid chloride was hydrolyzed to carboxylic acid, resulting in imbalance of the monomer ratio, we next carried out polycondensation by using slight excess amounts of 4 (1.01, 1.03, 1.05, and 1.10 equiv) relative to 1 (Table 1, entries 2−5). Linear polyester having carboxyl groups at both ends was formed even when 1.01 equiv of 4 was used (entry 2; Figure 1b). The use of a larger excess of 4 resulted in formation of polyesters with methoxycarbonyl groups, formed by quenching of the carboxylic acid chloride end with methanol, as main products (entries 3−5). In contrast, polyester with OH/OH end groups was mainly obtained when a slight excess of 1 was used (entries 6 and 7). Consequently, cyclic polyester was not selectively obtained in the polycondensation of equimolar 1 and 4, and linear polyester having end groups derived from excess monomer was formed even when 1 mol % excess either monomer was used. Therefore, it would be difficult to obtain exclusively cyclic polyester in the polycondensation of 1 and 4. Polycondensation of cis- or trans-4-Octene-1,8-diol (2, 3) and Sebacoyl Dichloride (4). We next conducted polycondensation of cis-4-octene-1,8-diol (2)10,11 or the trans counterpart (3)12 with 4 because we considered that the backbone of the polyester derived from 1 and 4 might not be flexible enough to cyclize. The geometry of the carbon−carbon double bond might influence cyclization of the polyester and the desired cross-metathesis of cyclic polyester with ExR. The polycondensation of 2 and 4 was first investigated with varying monomer feed ratio of 2 to 4 under the same conditions as in the case of the polycondensation of 1 and 4 (Scheme 2; Table 2, entries 1−3). When equimolar 2 and 4 were used, the MALDI-TOF mass spectrum of the products showed strong peaks of polymers with HO/COOH and HO/OH end groups in the high-molecular-weight region, as in the case of the
Syntheses of Cyclic Polyesters. Polycondensation of cis2-Butene-1,4-diol (1) and Sebacoyl Dichloride (4). First, we synthesized cyclic polyesters containing carbon−carbon double bonds in the backbone by means of polycondensation of unsaturated diol and dicarboxylic acid chloride in the presence of pyridine, according to the Kricheldorf’s procedure, which afforded cyclic polyesters (Scheme 2).9 Since Kricheldorf reported that the polycondensation of 1,4butanediol and sebacoyl dichloride (4) afforded cyclic polyester, we first conducted polycondensation of cis-2butene-1,4-diol (1) and 4 with various monomer feed ratios (Table 1). Thus, a solution of 4 in dichloromethane (DCM) was added dropwise into a solution of 1 and pyridine in DCM at 0 °C. The mixture was stirred at room temperature for 3 days, and then the reaction was quenched with methanol. Table 1. Polycondensation of 1 and 4a main end groupsd
entry
[1]0/[4]0
Mnc
Mw/Mnc
1
1.00/1.00
12800
1.77
2
1.00/1.01
12100
1.77
3
1.00/1.03
10800
2.04
4 5 6b
1.00/1.05 1.00/1.10 1.01/1.00
3230 4500 7260
1.56 1.56 1.78
7b
1.03/1.00
7310
1.80
low molecular weight
high molecular weight
n ≤ 14 cyclic n ≥ 15 HO/OH, polymer HO/COOH n ≤ 12 cyclic n ≥ 13 HOCO/ polymer COOH n ≤ 14 cyclic n ≥ 15 MeOCO/ polymer COOH MeOCO/COOMe MeOCO/COOMe n ≤ 8 cyclic n ≥ 9 HO/OH polymer n ≤ 8 cyclic n ≥ 9 HO/OH polymer
a
Polycondensation of 1 and 4 was carried out in the presence of pyridine ([pyridine]0/[4]0 = 2/1) in DCM ([1]0 = 0.25 M) at rt for 3 days. b[4]0 = 0.25 M. cEstimated by GPC based on polystyrene standards (eluent: THF). dEstimated from MALDI-TOF mass spectra. B
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. MALDI-TOF mass spectra of PEs14 obtained by the polycondensation of 1 and 4 (Table 1); values of monomer feed ratio [1]0/[4]0 are (a) 1.00/1.00 (entry 1) and (b) 1.00/1.01 (entry 2).
Table 2. Polycondensation of 2 or 3 and 4 or 5a main end groupse entry 1 2 3 4b 5 6b 7b,c 8b,c
[diol]0/[diacid chloride]0 [2]0/[4]0 [2]0/[4]0 [2]0/[4]0 [3]0/[4]0 [2]0/[5]0 [2]0/[5]0 [2]0/[5]0 [2]0/[5]0
= = = = = = = =
1.00/1.00 1.00/1.01 1.00/1.03 1.00/1.03 1.00/1.00 1.00/1.03 1.00/1.01 1.00/1.03
reaction time (days)
Mnd
Mw/Mnd
low molecular weight
3 2 2 3 3 4 4 3
9440 9470 12100 20200 5360 1830 2060 1880
1.81 1.69 2.07 2.01 1.68 1.84 2.12 2.03
n ≤ 13 cyclic polymer n ≤ 10 cyclic polymer
high molecular weight
n ≥ 13 HO/OH, HO/COOH n ≥ 11 H3COCO/COOCH3 cyclic polymer cyclic polymer n ≤ 16 cyclic polymer n ≥ 17 HO/OH MeOCO/COOMe cyclic polymer cyclic polymer
a
Polycondensation of diol and diacid chloride was carried out in the presence of pyridine ([pyridine]0/[4 or 5]0 = 2/1) in DCM ([2 or 3]0 = 0.25 M) at rt. b[2 or 3]0 = 0.10 M. c0.05 equiv of DABCO was added. dEstimated by GPC based on polystyrene standards (eluent: THF). eEstimated from MALDI-TOF mass spectra.
Figure 2. MALDI-TOF mass spectra of (a) PEs24 obtained by the polycondensation of 1.00 equiv of 2 and 1.03 equiv of 4 (Table 2, entry 3) and (b) PEs34 obtained by the polycondensation of 1.00 equiv of 3 and 1.03 equiv of 4 (Table 2, entry 4).
2a). Similarly, reaction of trans-diol 3 with 1.03 equiv of 4 selectively afforded cyclic polyester PEs34 (entry 4; Figure 2b). Polycondensation of cis-4-Octene-1,8-diol (2) and Isophthaloyl Dichloride (5). We conducted polycondensation of 2
polymerization of 1 and 4 (entry 1). When 1.03 equiv of 4 was used, however, one series of peaks corresponding to the molecular weight of the repeat unit was observed, indicating successful formation of cyclic polyester PEs24 (entry 3; Figure C
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
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in the ring-opening metathesis polymerization of 1,4-cyclooctadiene,7,14 in the presence of 1 mol % of second-generation Grubbs catalyst (Grubbs cat. II) (Scheme 3 and Table 3). When high-molecular-weight PEs24 (Mn = 18 100, Mw/Mn = 1.90), obtained by precipitation into methanol, and 20 mol % of 6 relative to the carbon−carbon double bonds (CC) in PEs24 were used, 6 was completely consumed and a polymer with Mn of 3270 was obtained (entry 1). The MALDI-TOF mass spectrum of the polymer showed almost a single series of peaks corresponding to linear polyester PEs24a having the acetoxy groups derived from 6 at both ends (designated as AcO/OAc) (Figure 4), indicating that the desired cross-metathesis had taken place to convert cyclic polyester to linear polyester. Since the polymer end groups are derived from 6, the molecular weight should increase with decreasing amount of 6. Indeed, the molecular weight increased until the amount of 6 was decreased to 5 mol % (entries 1−4). However, the use of 1 mol % of 6 did not afford linear polymer, although the Mn of cyclic polymers was decreased from 18 100 to 5250 (entry 5). Cross-metathesis between cyclic polymers presumably occurred, yielding lower-molecular-weight cyclic polymers due to entropy change as a driving force. We next examined the influence of the molecular weight of cyclic polyester. Low-molecular-weight PEs24 (Mn = 3700, Mw/ Mn = 3.14) and oligomeric PEs24 (Mn = 808, Mw/Mn = 1.59) were obtained from the methanol-soluble part of the precipitated crude product after separation by preparative HPLC. The GPC elution curves of the methanol-insoluble and methanol-soluble PEs24 were quite different (Figures 5a and 5b). However, when cross-metathesis of both PEs24 fractions with 20 mol % of 6 relative to the CC in PEs24 was carried out, the GPC elution curves of the products became almost identical (Figures 5c and 5d), and the Mn values and the Mw/ Mn ratios were similar (entry 1 vs 6). It should be noted that the dispersity of the methanol-soluble PEs24 became narrower (from 3.14 to 1.47), despite the similar molecular weight before and after cross-metathesis, implying that cross-metathesis of cyclic polyester with ExR causes the polymer length to become more uniform, presumably through fast exchange reaction at the CC backbone. Similarly, the reactions of high-molecularweight PEs24 (Mn = 18 100, Mw/Mn = 1.90) and oligomeric PEs24 (Mn = 808, Mw/Mn = 1.59) with 10 mol % of 6 relative to the CC in PEs24 afforded PEs24a of similar molecular weight (entry 3 vs 7). Consequently, the molecular weight of PEs24a obtained by the cross-metathesis of PEs24 with 6 was not dependent on the molecular weight of the starting PEs24 but was governed by the molar ratio of 6 to the CC in PEs24. The reason for the formation of linear polymer with HO/ COOH end groups in the cross-metathesis of oligomeric PEs24 with 6 (entry 7) is not clear at the present time. Because the MALDI-TOF mass spectrum of the starting oligomeric PEs24 only showed peaks of cyclic oligomers, this linear polymer was
and isophthaloyl dichloride (5) with various feed ratios of 2 to 5 under the same conditions in order to synthesize cyclic polyester containing aromatic rings in the backbone (Table 2, entries 5−8). When equimolar 2 and 5 were used, peaks of cyclic polymer were observed in the low-molecular-weight region, while peaks of linear polymers with HO/COOH and HO/OH ends were mainly observed in the high-molecularweight region in the MALDI-TOF mass spectrum. Furthermore, the use of 1.03 equiv of 5 (the corresponding condition selectively afforded cyclic polymer in the case of 2 and 4) resulted in linear polyester with MeOCO/COOMe ends as the main products. Brunnel et al. found that a catalytic amount of 1,4diazabicyclo[2.2.2]octane (DABCO) accelerated esterification of aromatic carboxylic acid chloride with diol and applied this finding to the polycondensation of aromatic dicarboxylic acid chloride and diol.13 Accordingly, we re-examined the polycondensation of 2 and 5 in the presence of a catalytic amount (5 mol % to 3) of DABCO (entries 7 and 8). In this case, cyclic polyester PEs25 was selectively obtained when 1.01−1.03 equiv of 5 was used, as shown by a single series of peaks in the MALDI-TOF mass spectrum (Figure 3). The molecular weight was rather lower than that of cyclic polymer obtained from 2 or 3 and 4 (entries 7 and 8).
Figure 3. MALDI-TOF mass spectrum of PEs25 obtained by the polycondensation of 1.00 equiv of 2 and 1.03 equiv of 5 in the presence of 0.05 equiv of DABCO (Table 2, entry 8).
Cross-Metathesis Reactions of Cyclic Unsaturated Polyesters with ExRs. cis-1,4-Diacetoxy-2-butene (6) as ExR. We then investigated cross-metathesis of the obtained cyclic polyesters (PEs24, PEs34, and PEs25) with cis-1,4diacetoxy-2-butene (6), which Grubbs et al. used as a CTA
Scheme 3. Cross-Metathesis of Cyclic Unsaturated Polyesters with 6
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DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 3. Cross-Metathesis of PEs24, PEs25, or PEs34 with 6a PEs entry 1 2 3b 4 5 6 7c 8 9 10
PEs24 PEs24 PEs24 PEs24 PEs24 PEs24 PEs24 PEs34 PEs34 PEs25
PEsa
Mnd
Mw/Mnd
mol % of 6 relative to CC in PEs
conv of 6e (%)
Mnd
Mw/Mnd
main end groupsf
18100 18100 18100 18100 18100 3700 808 19700 19700 3700
1.90 1.90 1.90 1.90 1.90 3.14 1.45 2.50 2.50 1.99
20 15 10 5 1 20 10 20 10 10
100 100 100 100 100 100 100 100 100 100
3270 3560 4250 4670 5250 3010 3920 3300 4420 2530
1.49 1.67 1.66 1.89 2.26 1.47 1.59 1.38 1.59 2.13
AcO/OAc AcO/OAc AcO/OAc AcO/OAc cyclic polyester AcO/OAc AcO/OAc, HO/COOH AcO/OAc AcO/OAc AcO/OAc
a
Cross-metathesis of PEs with 6 was carried out in the presence of 1 mol % of Grubbs cat. II in DCM ([CC in PEs]0 = 0.17 M) at rt for 5 days. b2 days. c4 days. dEstimated by GPC based on polystyrene standards (eluent: THF). eEstimated from 1H NMR spectra. fEstimated from MALDI-TOF mass spectra.
Figure 4. MALDI-TOF mass spectrum of PEs24a obtained by crossmetathesis of PEs24 (insoluble part in methanol) with 20 mol % of 6 (Table 3, entry 1).
Figure 6. MALDI-TOF mass spectrum of PEs34a obtained by crossmetathesis of PEs34 with 20 mol % of 6 (Table 3, entry 8).
PEs24a, obtained by cross-metathesis of PEs24 containing cisCC with 20 mol % of 6, (entry 1) and of PEs34a, obtained by cross-metathesis of PEs34 containing trans-CC with the same amount of 6, (entry 8) indicated that the cis:trans ratios in the backbone were 27:73 and 25:75, respectively (Figure 7). Therefore, the cross-metathesis of the cyclic polyesters with ExR 6 probably reached an equilibrium state during the reaction time (5 days), irrespective of the geometry of CC in the starting cyclic unsaturated polyester. The cross-metathesis reaction also proceeded with PEs25 containing aromatic rings in the backbone, yielding PEs25a with acetoxy groups at both ends (entry 10). Consequently, many kinds of cyclic unsaturated polyesters can be used for this crossmetathesis with ExR, and it should be straightforward to incorporate other functional groups into the aromatic rings in the polyesters. Examination of Other ExRs. In order to introduce a variety of functional groups at both ends, cross-metathesis of PEs24 with ExRs 7-10 was carried out (Scheme 4 and Table 4). We first examined ExR 715 having tert-butoxycarbonyl (Boc)protected amino groups, which can introduce amino groups at both ends after treatment with a weak acid (Table 4, entries 1 and 2). The molecular weight was decreased as in the case of 6. However, the MALDI-TOF mass spectra of the products showed many peaks which could not be assigned as well as the
Figure 5. GPC elution curves of PEs24: (a) insoluble part in methanol; (b) soluble part in methanol. GPC elution curves of polyester obtained by cross-metathesis with 20 mol % of 6: (c) from polyester (a) (Table 3, entry 1) and (d) from polyester (b) (Table 3, entry 6).
presumably formed by hydrolysis during cross-metathesis. However, hydrolyzed products were not observed in other cases. We next conducted cross-metathesis of PEs34 containing trans-CC with 6 under the same conditions (entries 8 and 9) and obtained linear polyester PEs34a with acetoxy end groups derived from 6 at both ends (Figure 6). The molecular weights of the resulting PEs34a obtained with 10 and 20 mol % of 6 were almost identical with those of PEs24a obtained with 10 and 20 mol % of 6, respectively (entry 8 vs 1; 9 vs 3). Therefore, the geometry of the CC in the starting cyclic unsaturated polyester did not influence cross-metathesis of cyclic ester with ExR. Moreover, the 1H NMR spectra of E
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
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and 4). Therefore, it turned out that the conjugated CC of 8 did not effectively undergo cross-metathesis. However, the MALDI-TOF mass spectrum showed that linear polyester PEs24c having BrPh at both ends was the main product (Figure 8b). Furthermore, 9 having tert-butyl ester moieties18 was used as an ExR for introduction of carboxyl groups at the polymer ends. The conversion of 9 could not be determined from the 1H NMR spectrum because the signals of 9 overlapped with the signals of the backbone. Linear polyester PEs24d having tertbutyl ester moieties at both ends was mainly obtained in the reaction with 20 mol % of 9 (Figure S47a), while other linear polymers with different polymer ends were formed in the reaction with 10 mol % of 9 (Figure S47b). Since ExRs 6, 7, and 9 have heteroatoms at the β-position of CC, we examined 1019 having a longer methylene chain between CC and the heteroatom to see whether the β-heteroatom of ExR would influence the cross-metathesis reaction (entries 7 and 8). The molecular weights of the products obtained by using 20 and 10 mol % of 10 were almost identical with those of the products obtained by using 20 and 10 mol % of 6, respectively (entry 7 in Table 4 vs entry 1 in Table 3; entry 8 in Table 4 vs entry 3 in Table 3). The MALDI-TOF mass spectra clearly showed formation of linear polyester PEs24e with both 20 and 10 mol % of 6 (Figure S51). Consequently, ExRs bearing isolated CC can be used for the cross-metathesis reaction of cyclic unsaturated polyesters irrespective of the position of heteroatoms.
Figure 7. 1H NMR spectra of PEs24, PEs34, PEs24a, and PEs34a in CDCl3 at 25 °C.
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CONCLUSION Conventional A2 + B2 type polycondensation of diol and diacid chloride monomers affords cyclic unsaturated polyesters, and we show here that cross-metathesis of these cyclic polyesters with a symmetric olefin having two functional groups as an exchange reagent (ExR) provides a new way to control the molecular weight and end groups of the linear polymer products. Synthesis of cyclic unsaturated polyester was first investigated. Polycondensation of cis-2-butene-1,4-diol (1) and sebacoyl dichloride (4) did not selectively afford cyclic polyester, probably due to the low flexibility of the backbone. Therefore, we tried polycondensation of cis- or trans-4-octene1,8-diol (2 or 3) and 1.03 equiv of 4 or isophthaloyl dichloride (5) and found that cyclic polyesters PEs24, PEs34, and PEs25 were selectively formed. Cross-metathesis of the obtained cyclic polyesters with ExR in the presence of second-generation Grubbs catalyst was then investigated. When cis-1,4-diacetoxy-2-butene (6) was used as
peaks of linear polyester PEs24b having BocNH end groups derived from 7 at both ends (Figure 8a). Since the signals of end groups other than the BocNH groups were not observed in the 1H NMR spectra of the products (Figure S37), the unknown peaks in the MALDI-TOF mass spectra might have arisen from decomposition of the BocNH groups in the matrix bearing phenolic hydroxyl groups under laser irradiation during measurement. Next, 4,4′-dibromo-trans-stilbene (8) was tried to see whether conjugating olefins could serve as effective ExRs. Bromophenyl terminal groups can be connected with πconjugated polymers.16,17 The time course of the reaction was followed by means of GPC, and it was found that the rate of decrease of molecular weight was slower than in the cases of other ExRs (Figures S39 and S40); consequently, the molecular weight at 5 days was higher than in the case of the other ExRs. ExR 8 remained even after 5 days (54 and 65% conversion in the reaction with 20 and 10 mol % of 8, respectively) (entries 3
Scheme 4. Cross-Metathesis of PEs24 with a Variety of ExRs 7−10
F
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 4. Cross-Metathesis of PEs24 with ExRs 7−10a PEs24
PEs24b−e
entry
Mnb
Mw/Mnb
ExR
mol % of ExR relative to CC in PEs24
conv of ExR (%)c
Mnb
Mw/Mnb
main end groupsd
1 2 3 4 5 6 7 8
15100 15100 13600 13600 13600 13600 15100 15100
2.20 2.20 2.36 2.36 2.36 2.36 2.44 2.44
7 7 8 8 9 9 10 10
20 10 20 10 20 10 20 10
100 100 54 65 e e e e
3110 3720 3850 4690 3130 4140 3250 4440
1.44 1.54 1.69 1.89 1.42 1.62 1.35 1.62
BocNH/NHBoc, BocNH/COOH BocNH/NHBoc, BocNH/COOH BrPh/PhBr BrPh/PhBr t Bu/tBu t Bu/tBu, tBu/COOH AcO/OAc AcO/OAc
a
Cross-metathesis of PEs24 with ExR was carried out in the presence of 1 mol % of Grubbs cat. II in DCM ([CC in PEs24]0 = 0.17 M) at rt for 5 days. bEstimated by GPC based on polystyrene standards (eluent: THF). cEstimated from 1H NMR spectra. dEstimated from MALDI-TOF mass spectra. eNot determined.
Figure 8. MALDI-TOF mass spectra of (a) PEs24b obtained by cross-metathesis of PEs24 with 20 mol % of 7 (Table 4, entry 1) and (b) PEs24c obtained by cross-metathesis of PEs24 with 20 mol % of 8 (Table 4, entry 3). sublimation before use. Other starting materials were used without further purification. Synthesized cis- and trans-4-octene-1,8-diol (2 and 3) were dried in vacuo over P2O5 immediately before use. Commercially available dehydrated dichloromethane (Kanto) was used as a dry solvent. The synthetic procedures for cis-4-octene-1,8diol (2), trans-4-octene-1,8-diol (3), cis-di-tert-butyl 2-butene-1,4diyldicarbamate (7), di-tert-butyl 3,3′-(cis-but-2-ene-1,4-diyloxy)dipropionate (9), and cis-4-octene-1,8-diyl diacetate (10) are described in the Supporting Information. General. 1H and 13C NMR spectra were obtained on JEOL ECA500 and ECA-600 spectrometers. The internal standard for 1H NMR spectra in CDCl3 was tetramethylsilane (0.00 ppm), and the internal standard for 13C NMR spectra in CDCl3 was the midpoint of CDCl3 (77.0 ppm). IR spectra were recorded on a JASCO FT/IR-410. The Mn and Mw/Mn values of polymers were measured on a Shodex GPC101 gel permeation chromatography unit (eluent, THF; calibration, polystyrene standards) with two Shodex KF-804L columns, Shodex UV-41, and Shodex RI-71S. Purification of polyester after the crossmetathesis was conducted with a Shimadzu LC-6AD preparative HPLC (eluent: THF, flow rate: 5.0 mL/min) equipped with Shimadzu SPD-10A, Shimadzu RID-10A, and two TOSOH TSKgel columns (2 × GMHHR-H) and with a JAI LC-908 preparative HPLC (eluent: CHCl3, flow rate: 6.0 mL/min) equipped with a JAI UV detector 310, JAI RI detector RI-5, and two TOSOH TSKgel columns (2 × G2000HHR). Matrix-assisted laser desorption ionization time-of-flight
an ExR, cyclic polyesters PEs24, PEs34, and PEs25 were converted into linear polyesters bearing acetoxy groups at both ends. The molecular weight of the resulting linear polyester was controlled by the molar ratio of 6 to CC in the cyclic polyester, regardless of the molecular weight of the starting cyclic polyester. Cross-metathesis by using other ExRs enabled the introduction of tert-butoxycarbonyl (Boc) amino, bromophenyl, and tert-butyl carboxylate groups at both ends of polyester. It should be easily possible to introduce polymerization-initiating sites or other polymers at the polymer ends of polyester to obtain triblock copolymer consisting of polyester and conventional polymer, which would otherwise be difficult to synthesize. Further studies along this line are in progress.
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EXPERIMENTAL SECTION
Materials. All starting materials were purchased from commercial suppliers (TCI, Aldrich, Wako, and Kanto). cis-2-Butene-1,4-diol (1) and pyridine were dehydrated at reflux over CaH2 followed by distillation under reduced pressure and stored in a Schlenk flask under argon until use. Sebacoyl dichloride (4) and isophthaloyl dichloride (5) were refluxed with thionyl chloride followed by distillation under reduced pressure and stored under argon (4 was stored in a Schlenk flask). 1,4-Diazabicyclo[2.2.2]octane (DABCO) was purified by G
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules (MALDI-TOF) mass spectra were recorded on a Shimadzu/Kratos AXIMA-CFR plus in the reflectron mode by use of a laser (λ = 337 nm). Dithranol (1,8-dihydroxy-9[10H]-anthracenone) was used as the matrix for the MALDI-TOF mass measurements. All reactions were conducted under an inert gas atmosphere. Syntheses of Cyclic Polyesters. Polymerization of cis-2Butene-1,4-diol (1) and Sebacoyl Dichloride (4). The atmosphere in a round-bottomed flask was replaced with argon. A solution of 1 (0.1762 g, 2.000 mmol) and pyridine (0.33 mL, 4.1 mmol) in dry dichloromethane (4 mL) was added to the flask via a syringe, and the mixture was cooled to 0 °C. To the stirred reaction mixture, a solution of 4 (0.4778 g, 1.998 mmol) in dry dichloromethane (4 mL) was added dropwise via a syringe at 0 °C. The reaction mixture was stirred at room temperature for 3 days, and then the reaction was quenched with methanol (aliquots of the reaction mixture were taken at 24 and 45 h during the reaction). The mixture was evaporated, and the residue was dissolved in dichloromethane. The solution was washed with 1 M HCl twice and with water twice sequentially. The organic layer was dried over anhydrous MgSO4 and concentrated in vacuo to afford crude PEs14 as a colorless translucent solid (0.4563 g; total including that in the aliquots taken, 0.4803 g, 95%). Polymerization of cis-4-Octene-1,8-diol (2) and Sebacoyl Dichloride (4). The atmosphere in a round-bottomed flask was replaced with argon. A solution of 2 (0.1441 g, 0.9992 mmol) and pyridine (0.17 mL, 2.1 mmol) in dry dichloromethane (2 mL) was added to the flask via a syringe, and then the mixture was cooled to 0 °C. To the stirred reaction mixture, a solution of 4 (0.2461 g, 1.029 mmol) in dry dichloromethane (2 mL) was added dropwise via a syringe at 0 °C. The reaction mixture was stirred at room temperature for 2 days, and then the reaction was quenched with methanol. The mixture was washed with 1 M HCl twice and with water twice sequentially, dried over anhydrous MgSO4, and concentrated in vacuo to afford a crude product as a pale yellow viscous liquid (0.2896 g, 93%). The crude product was dissolved in dichloromethane and precipitated into methanol. The supernatant liquid was removed by decantation. The precipitated viscous liquid was taken up in dichloromethane, and the solution was concentrated in vacuo to afford PEs24 as a colorless viscous liquid (0.1981 g, 64%). Furthermore, the supernatant liquid, removed during the precipitation, was concentrated in vacuo to afford low-molecular-weight PEs24 as a pale yellow viscous liquid (0.0903 g, 29%). Polymerization of trans-4-Octene-1,8-diol (3) and Sebacoyl Dichloride (4). A round-bottomed flask was flame-dried, and the atmosphere was replaced with argon. A solution of 3 (0.2886 g, 2.001 mmol) and pyridine (0.340 mL, 4.20 mmol) in dry dichloromethane (10.0 mL) was added to the flask via a syringe, and then the mixture was cooled to 0 °C. To the stirred reaction mixture, a solution of 4 (0.4927 g, 2.060 mmol) in dry dichloromethane (10.0 mL) was added dropwise via a syringe at 0 °C. The reaction mixture was stirred at room temperature for 3 days, and then the reaction was quenched with methanol (1 mL) (aliquots of the reaction mixture were taken at 4, 24, and 48 h during the reaction). The mixture was washed with 1 M HCl twice and with water twice sequentially, dried over anhydrous MgSO4, and concentrated in vacuo to afford a crude product as a colorless solid (0.6070 g; total including that in the aliquots, 0.6314 g, 102%). The crude product was dissolved in dichloromethane, and the solution was added to methanol to precipitate the product. The supernatant liquid was removed, and the sticky precipitate was taken up in dichloromethane. This solution was concentrated in vacuo to afford PEs34 as a colorless solid (0.4510 g, 73%). Polymerization of cis-4-Octene-1,8-diol (2) and Isophthaloyl Dichloride (5) with DABCO. A round-bottomed flask was flame-dried, and the atmosphere was replaced with argon. A solution of 2 (0.1440 g, 0.9985 mmol), DABCO (5.7 mg, 0.051 mmol), and pyridine (0.170 mL, 2.10 mmol) in dry dichloromethane (5.00 mL) was added to the flask via a syringe, and then the mixture was cooled to 0 °C. To a stirred reaction mixture, a solution of 5 (0.2092 g, 1.030 mmol) in dry dichloromethane (5.00 mL) was added dropwise via a syringe at 0 °C. The reaction mixture was stirred at room temperature for 3 days, and then the reaction was quenched with methanol (0.5 mL) (aliquots of
the reaction mixture were taken at 2, 29.5, and 49 h during the reaction). The mixture was washed with 1 M HCl twice and with water twice sequentially, dried over anhydrous MgSO4, and concentrated in vacuo to afford a crude product as a white sticky solid (0.2596 g; total including that in the aliquots, 0.2653 g, 97%). The crude product was dissolved in dichloromethane, and the solution was added to methanol to precipitate the product. The supernatant liquid was removed, and the precipitated viscous liquid was taken up in dichloromethane. The solution was concentrated in vacuo to afford PEs25 as a colorless sticky liquid (0.0660 g, 24%). Cross-Metatheses of Cyclic Unsaturated Polyesters with ExRs. General Procedure of Cross-Metathesis of Cyclic Polyester with ExR. Cyclic polyester (0.1 mmol of repeat unit) was added to a round-bottomed flask, and the atmosphere in the flask was replaced with argon. Degassed solution of an ExR in dry dichloromethane (0.30 mL) was added to the flask via a syringe (the ratio of the added ExR to polyester was varied). A degassed solution of second-generation Grubbs catalyst (0.001 mmol) in dry dichloromethane (0.280 mL) was added to the flask via a syringe, and the reaction mixture was stirred at room temperature for 5 days (aliquots of the reaction mixture were taken five times during the reaction). The reaction was quenched by addition of ethyl vinyl ether (0.02 mL). The mixture was stirred for 2 h and then concentrated in vacuo to afford a crude product. The crude product was purified by preparative HPLC with SEC columns to remove low-molecular-weight compounds. Cross-Metathesis of PEs24 with 4,4′-Dibromo-trans-stilbene (8). PEs24 (31.1 mg, 0.1 mmol of repeat unit) and 8 (6.63 mg, 0.0196 mmol) were added to a round-bottomed flask, and the atmosphere in the flask was replaced with argon. Dry, degassed dichloromethane (0.30 mL) was added to the flask via a syringe. A degassed solution of second-generation Grubbs catalyst (0.84 mg, 0.000 99 mmol) in dry dichloromethane (0.280 mL) was added to the flask via a syringe, and the reaction mixture was stirred at room temperature for 5 days (aliquots of the reaction mixture were taken at 1, 22, 45, and 72 h during the reaction). The reaction was quenched by addition of ethyl vinyl ether (0.02 mL). The mixture was stirred for 2 h and then concentrated in vacuo to afford a crude product as a purple-brown sticky solid (0.0293 g; total including that in the aliquots, 0.0365 g, 97%). The crude product was purified by preparative HPLC with SEC columns (eluent: CHCl3) to remove low-molecular-weight compounds, affording PEs24c as a purple viscous liquid (0.0125 g, 33%).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02089.
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Synthetic procedure of monomers and ExRs, and GPC elution curves, 1H NMR spectra and MALDI-TOF mass spectra of synthesized polyesters (PDF)
AUTHOR INFORMATION
Corresponding Author
*(T.Y.) E-mail:
[email protected]. ORCID
Tsutomu Yokozawa: 0000-0003-2536-6271 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities (No. S1311032), 2013−2018. H
DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
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(19) Bouquillon, S.; Muzart, J. Palladium(0)-Catalyzed Isomerization of (Z)-1,4-Diacetoxy-2-butene Dependence of η1- or η3Allylpalladium as a Key Intermediate on the Solvent Polarity. Eur. J. Org. Chem. 2001, 2001 (17), 3301−3305.
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DOI: 10.1021/acs.macromol.7b02089 Macromolecules XXXX, XXX, XXX−XXX
