Ring-Opening Polymerization with Lewis Pairs and Subsequent

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

Ring-Opening Polymerization with Lewis Pairs and Subsequent Nucleophilic Substitution: A Promising Strategy to Well-Defined Polyethylene-like Polyesters without Transesterification Bin Wang,† Li Pan,† Zhe Ma,† and Yuesheng Li*,†,‡ †

Tianjin Key Laboratory of Composite & Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300350, China S Supporting Information *

ABSTRACT: Ring-opening polymerization (ROP) of ω-pentadecalactone (PDL) catalyzed by Lewis pairs was thoroughly explored, and a novel approach to well-defined aliphatic long chain polyester with high molecular weight (MW) was developed in the present work. The Zn(C6F5)2/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) Lewis pair was proved to be a promising catalytic system for ROP of PDL, producing cyclic PPDL with high MW (Mw > 100 kg/mol) and relatively low polydispersity index (Mw/Mn = 1.6−1.9). Strikingly, no transesterification occurred in the ROP of PDL by Zn(C6F5)2/DBU. The cyclic topology of the polyester could be switched to linear structure in the presence of alcohol. The feeding mode and the structure of alcohol significantly influence the ROP. Compared with mixing alcohol with Zn(C6F5)2/DBU at first, adding Ph2CHOH with low nucleophilicity after full monomer conversion could afford linear PPDL without transesterification. It was noted that random chain scission or chain extension was not detected after adding Ph2CHOH. Welldefined block copolymer containing polyethylene-like segment can be easily prepared by sequential addition of PDL and lactide (LA) or caprolactone (CL). Cyclic block copolyesters c-poly(PDL-b-CL) and c-poly(PDL-b-LA) were obtained in the absence of alcohol. The blocky structures can be maintained even when prolonging reaction time after full monomer conversion. Similarly, introducing Ph2CHOH before quenching the polymerization led to well-defined linear block copolyesters l-poly(PDL-b-CL) and l-poly(PDL-b-LA).

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ties.10,11 Especially, the thermal stabilities of ALCPEs are improved drastically.12 Meanwhile, the presence of amounts of ester groups makes ALCPEs still have good hydrophilicity, adhesivity, and compatibility with polar polymers.13 ALCPEs can be prepared via polycondensation of α,ωhydroxyl acid or polycondensation between α,ω-diesters and α,ω-diols that derived from fatty acid.12,14,15 Combining acyclic diene metathesis of ester-containing diolefin and subsequent hydrogenation also produces ALCPEs.6,8 However, these strategies usually need multistep transformation and relatively harsh conditions (low pressure or high temperature). In addition, the molecular weights (MWs) of the resultant polyesters are generally restricted. Ring-opening polymerization (ROP) of macrolactones, which can be conducted under relatively mild conditions and shows high degree of control in MW, proved to be a promising approach for efficiently synthesizing ALCPEs with high MW.

INTRODUCTION Aliphatic polyesters, which combine both environmentalfriendly characteristics and sustainable property, have attracted much attention in academic and industrial interests.1 They have always been considered as an important alternatives to petroleum-based polymers. Currently, polylactide (PLA) and polycaprolactone (PCL) have been commercialized and widely used in the fields ranging from biomedical materials to consumer products.2,3 However, the further application of PLA in some specific areas is limited because of its intrinsic brittleness and poor hydrolytic stability.4 Although PCL exhibits excellent elasticity, its thermal stability is unsatisfactory.5 The biodegradability and mechanical property of aliphatic polyesters can be changed significantly by altering the number of methylene units between the two adjacent ester groups along with main chain. Increasing continuous methylene sequence will produce an important class of polyesters with fascinating properties: aliphatic long chain polyesters (ALCPEs).6−9 Increasing methylene/ester ratio results in slow biodegradation rates but also endows the polyesters with polyethylene-like crystalline structure and mechanical proper© XXXX American Chemical Society

Received: November 8, 2017 Revised: January 9, 2018

A

DOI: 10.1021/acs.macromol.7b02378 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules There are two significant types of reactions occurred in the ROP of lactone. The active ends of polymer chains will react with the lactone monomer to give chain growth (propagation, route A in Chart 1). Meanwhile, the active ends of the polymer

Scheme 1. Typical Catalysts for ROP of PDL (A−C) and the Approach to Polyethylene-like (Co)polyester in the Present Work (D)

Chart 1. Chain Propagation and Side Reactions in the ROP of Lactone

chains can also attack to the ester linkages in the intermolecular or intramolecular chains (transesterification, routes B and C in Chart 1). In classical “living” ROP of lactide or ε-caprolactone, the rate of chain propagation is much higher than that of transesterification, assisted by the release of the ring strain.16 In sharp contrast, the rate of propagation is comparable with that of transesterification in the entropy-driven ROP of macrolactones, meaning that the chain propagation is always accompanied by transesterification in the common ROP of macrolactones.17 Many catalytic systems for ROP of macrolactones, including enzymes,18−20 organic bases21−26 (A, Scheme 1), and metalbased catalysts27−35 (C, Scheme 1), have been reported. However, as far as we concern, very limited catalysts can eliminate the transesterification side reactions. For example, competitive intra- and intermolecular transesterifications were observed in the ROP of PDL catalyzed by organic base. Thus, the low MW cyclic side products were usually formed, and preparation of well-defined block copolymers poly(PDL-b-CL) was inaccessible. A similar result was also observed in the (co)polymerization of PDL by using salen−aluminum30 or bis(phenoxy)magnesium catalysts.31 Up to now, only phenoxyimine−amine zinc and calcium catalysts could produce perfect block copolymer poly(PDL-b-CL) by sequential monomer addition.32 Considering that well-defined block copolyesters consisting of a “polyethylene-like” segment are very interesting structures, the development of synthetic methodologies for this purpose is highly important. Lewis pairs catalytic polymerization, in which combination of Lewis acid and Lewis base can improve reactivity significantly and convert small molecule to polymer, has attracted more and more attention in the field of polymer chemistry.36−47 The recently elegant work of Amgoune, Dove, and Naumann demonstrated that Lewis pairs can effectively catalyze the ROP of lactide, ε-caprolactone, and ω-pentadecalactone (ωPDL).39,42,48 Especially, the Zn(C6F5)2-based Lewis pairs could give perfect cyclic diblock copolymer poly(PLA-b-PCL) by sequential feeding, indicating that the transesterification side reactions may be depressed. We further proved that the active species in Lewis pairs catalytic polymerization is the zwitterionic moiety, in which each chain end bonded with amine and Zn(C6F5)2, and Zn(C6F5)2 associated with the

amine closely.49 Inspired by these results, Lewis pairs were selected to catalyze the ROP of macrolactone, and a novel approach to polyethylene-like polyester with well-defined structure was developed in the present work (D, Scheme 1). As expected, Zn(C6F5)2/DBU can efficiently catalyze the ROP of PDL without alcohol (ROH), affording cyclic polyesters with high MW (>100 kg/mol). No transesterification was observed during the polymerization, and perfect cyclic block copolymer c-poly(PDL-b-CL) and c-poly(PDL-b-LA) can be easily prepared by sequential feeding. Introducing ROH with bulky hindrance after full monomer conversion could afford linear polymers without transesterification. Thus, linear diblock copolyesters consisting of polyethylene-like segment could be easily obtained via a cascade of ROP and subsequent nucleophilic substitution, in which the transesterification was eliminated.

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RESULTS AND DISCUSSION Optimization of Lewis Pairs. Considering that the polymerization behavior of ω-PDL is very different from that of lactide, several polymerizations of ω-PDL catalyzed by different Lewis pairs were conducted in the absence of alcohol (Table 1). The control experiment showed that Zn(C6F5)2 or free Lewis base alone could not convert any PDL to polyester under the similar conditions. 1,2,2,6,6-Pentamethylpiperidine (PMP) was first selected because it was proved that the combination of Zn(C6F5)2 and PMP was crucial for efficient and controlled polymerization of lactide. However, the Zn(C6F5)2/PMP Lewis pair only showed relatively low catalytic activity; monomer conversion was only 7.6% after 6 h, and no obvious enhancement of monomer conversion was observed even after 12 h (runs 1 and 2, Table 1). Similarly, substituting B


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Macromolecules Table 1. Ring-Opening Polymerization of PDL with Lewis Pairsa run

Lewis acid

Lewis base

convb (%)

TOF (h−1)

Mn,calcdc (kg/mol)

Mn,expd (kg/mol)

Mw,expd (kg/mol)

PDId

1 2e 3 4 5e 6 7f 8 9

Zn(C6F5)2 Zn(C6F5)2 Zn(C6F5)2 Zn(C6F5)2 Zn(C6F5)2 Zn(C6F5)2 Zn(C6F5)2 Zn(C6H5)2 ZnEt2

PMP PMP DMAP DBU DBU MTBD MTBD DBU DBU

7.6 12.5 14.5 90.5 >99 80.1 92.6 92.9 87.1

1.3 2.1 2.4 15.1 8.3 13.4 7.7 15.5 14.5

1.8 3.0 3.5 21.7 23.8 19.2 22.2 22.3 20.9

16.6 62.8 64.1 74.9 73.3 61.1 53.6

26.5 100.4 102.6 112.4 124.6 91.7 85.8

1.6 1.6 1.6 1.5 1.7 1.5 1.6

The reactions were carried out in xylene at 110 °C for 6 h, [PDL]0 = 1.9 mol/L [PDL]0:[Zn]0:[base] = 100:1:1. bDetermined by 1H NMR. Calculated based on the assumption that each Lewis pair initiate one polymer chain, Mn = [M]0/[Zn] × monomer conversion × Mmono. d Determined by GPC analysis in trichlorobenzene at 150 °C vs polyethylene standard. eReaction time = 10 h. fReaction time = 12 h. a c

PMP with 4-(dimethylamino)pyridine (DMAP) also led to low monomer conversion (14.5% after 6 h) and tended to produce polymer with low MW (run 3, Table 1). It was envisioned that the catalytic activity of Zn(C6F5)2/DMAP was quenched drastically because of the formation of classical Lewis adduct.49 Remarkably, both Zn(C6F5)2/1,8-diazabicyclo[5,4,0]undec-7ene (DBU) and Zn(C6F5)2/7-methyl-1,5,7-triazabicyclo[4.4.0]decane-5-ene (MTBD) Lewis pairs were much better candidates for the ROP of PDL (Figure S1). High monomer conversion up to 90% was easily achieved after 6 h, and PDL could be consumed almost completely in 10 h by using Zn(C6F5)2/DBU Lewis pair (runs 4 and 5, Table 1). In addition, the resultant PPDL exhibited high MW (100 kg/mol) and relatively narrow molecular weight distributions (Mw/Mn ∼ 1.6). The combination of MTBD with Zn(C6F5)2 also exhibited a propensity to produce PPDL with higher MW (runs 6 and 7, Table 1), albeit a slight drop in monomer conversion. The limitation of monomer diffusion caused by the high viscosity may account for the depressed catalytic activity of Zn(C6F5)2/ MTBD Lewis pair. Some impurities that may induce side reactions, including competitive chain initiation and chain transfer reaction, were found in commercial MTBD and they are hard to be separated and purified. Therefore, DBU was used as Lewis base in the following experiments. PDL polymerization could also be initiated by mixing DBU with other organozinc compounds. The combination of DBU with Zn(C6H5)2 or ZnEt2 can prompt the ROP of PDL effectively as well (runs 8 and 9, Table 1). Thus, the presence of both Lewis acid and Lewis base was crucial for the improved polymerization activity. The topological structures of the resultant PPDL were confirmed by NMR and MALDI-TOF mass spectrometry. No resonance peaks at 3.60−3.70 ppm corresponding to methylene group in the terminal −CH2OH moiety were detected in 1H NMR spectra, even for the PPDL samples with low MW (Figure S2). Meanwhile, the corresponding MALDI-TOF mass spectra exhibited signals assigned to sodium complexed cyclic PPDL and no linear polymer was traced (Figure 1). These results strongly suggested that the ROP of PDL catalyzed by the Lewis pairs involved a similar bifunctional activation mechanism proposed for the ROP of LA in previous work.39,49 The MW of the resultant PPDLs increased almost linearly with monomer conversion and exhibited unimodal distribution, suggesting that the ROP of PDL proceeds in a controlled manner (Figure 2). However, the experimental MWs are much higher than theoretical MWs. It was deduced that only a small fraction of Lewis pairs (∼30%) were converted to active species

Figure 1. MALDI-TOF-MS of PPDL catalyzed by Zn(C6F5)2/DBU. [PDL]0:[Zn(C6F5)2]0:[DBU]0 = 100:1:1, [PDL]0 = 1.9 M/L, xylene, 110 °C, 30 min. The samples used for MALDI-TOF analysis were obtained after quenching the polymerization by HCl/Et2O.

Figure 2. MW and MWDs of the resultant PPDL versus monomer conversion.

which can prompt chain propagation, similar to the case of LA polymerization catalyzed by Lewis pairs.50,51 Optimization of Reaction Conditions. The molar ratio of Lewis acid and Lewis base significantly influences the ROP of PDL. A stepwise changes of Zn(C6F5)2:DBU ratio from 2:1 to 1:2 resulted in a considerable increase of monomer conversion and MW of the resultant polymer (runs 1−3, Table 2). A plot of ln([M]0/[M]t) versus time showed gradually increasing slopes when the polymerizations were carried out at 110 °C C


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Macromolecules Table 2. Ring-Opening Polymerization of PDL with Zn(C6F5)2/DBU in the Absence of Alcohola run

[PDL] (mol/L)

[PDL]0:[Zn]0:[DBU]0

T (°C)

t (h)

convb (%)

TOF (h−1)

Mn,calcdc (kg/mol)

Mn,expd (kg/mol)

PDId

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1.9 1.9 1.9 0.5 1.0 1.5 2.5 3.0 3.9e 3.9e 3.9e 3.9e 3.9e 3.9e 3.9e 3.9e 0.5 1.9 1.9 1.0

100:2:1 100:1:1 100:1:2 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 100:1:1 200:1:1 200:1:1 400:1:1

110 110 110 110 110 110 110 110 110 120 120 120 130 140 150 150 140 120 120 120

6 6 6 6 6 6 6 6 6 6 12 24 1 1 0.5 12 6 6 12 24

31.0 90.5 99.4 8.6 30.6 81.6 97.3 92.3 81.3 95.5 99.0 99.0 88.8 96.4 99.0 99.0 92.9 85.6 98.4 97.6

5.2 15.1 16.6 1.4 5.1 13.6 16.6 15.4 15.7 15.9 8.3 4.2 88.8 96.4 198.0 8.25 15.5 28.5 16.4 16.3

7.4 21.7 23.9 2.1 7.3 19.6 23.8 22.2 22.6 22.9 23.8 23.8 21.3 23.1 23.8 23.8 22.3 41.1 47.2 93.7

23.4 62.8 65.9 6.9 24.5 59.2 69.1 58.3 55.9 52.5 53.2 54.0 47.1 39.5 30.0 31.0 29.3 103.9 118.1 234.3

1.5 1.6 1.8 1.5 1.5 1.8 1.9 1.9 2.1 1.8 1.8 1.8 1.8 1.8 1.9 1.9 1.7 2.0 1.9 1.9

a

The reactions were carried out in xylene or in bulk. bDetermined by 1H NMR. cCalculated based on the assumption that each Lewis pair initiate one polymer chain, Mn = [M]0/[Zn] × monomer conversion × Mmono. dDetermined by GPC analysis in trichlorobenzene at 150 °C vs polyethylene standards. eIn bulk.

lactone.49 These results also indicated that the Lewis base component played a dominant role in the ROP of PDL, which may be ascribed to its intrinsic nucleophilicity. The critical concentration of PDL for polymerization was 0.5 mol/L at 110 °C (run 4, Table 2). The monomer conversion and the MW of the resultant PPDL increased significantly with increasing [PDL]0 from 0.5 to 2.5 mol/L (runs 5−7, Table 2). These observations supported that high concentration was more preferred in the entropy-driven ROP, while further increasing [PDL]0 caused a depressed monomer conversion (runs 8 and 9, Table 2). This results may be ascribed to viscosity effects. The medium became rather viscous above 40% of monomer conversion, and the diffusion of the monomer was limited severely. The kinetics studies clearly showed that the initiation rate has almost a first order dependence on the concentration of monomer (Figures S3 and S4). Besides [PDL]0, the reaction temperature was another important factor in affecting the ROP of PDL (runs 9−16, Table 1). Only a small amount of monomer (8.6%) can be converted to polymer at 110 °C with the initial PDL concentration of 0.5 mol/L, whereas high monomer conversion (92.9%) could be easily achieved at 140 °C. It was rational that the critical concentration for PDL polymerization was relevant to temperature, and higher temperature was beneficial for the polymerization according to the equation ΔG = ΔH − TΔS ≈ −TΔS. Increasing the temperature led to a significant increase in initiation rate (Figure S5). Meanwhile, there was an obvious decrease in the MW of the resultant PPDL, indicating that much more Lewis pairs could be converted to active species at higher temperature. We envisioned that the dissociation of Lewis pairs became easier at high temperature, which facilitated monomer activation and finally resulted in high initiation efficiency and rapid initiation rate. PPDL with MW more than 100 kg/mol could be easily prepared by increasing the monomer feed ratio (runs 18−20, Table 2).

with different Zn(C6F5)2/DBU ratio (Figure 3), suggesting the presence of induction period in the ROP of PDL with Lewis

Figure 3. Polymerization kinetics with different Zn(C6F5)2:DBU molar ratios.

pairs. The slope of the early stage equals the initiation rate constant (ki), while the slope of the latter stage reflects the propagation rate constant (kp).52 The kp is about 7.8-fold higher than ki (ki = 0.13 h−1, kp = 1.01 h−1) with 1:1 of Zn(C6F5)2:DBU molar ratio. Increasing Zn(C6F5)2:DBU molar ratio to 1:2 resulted in a faster initiation rate (ki = 0.37 h−1) and decreased kp/ki value (kp/ki = 4.2). Similar results were reported by Dove, who found that stepwise increase of Lewis base/Lewis acid ratio resulted in a considerable increase of monomer conversion.42 We envisioned that the molar ratio of Lewis acid to Lewis base may affect the decoupling of the Lewis pairs and subsequent activation of monomer, which was proved to be a decisive step in the initiation reaction of Lewis pairs catalytic ROP of D


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benzyl alcohol nucleophilically attacked carbonyl carbon (Figure S9).42 The MALDI-TOF mass spectra suggested the resultant PPDL was composed of major linear structure (92.5%) endcapped by benzyloxy groups, and minor cyclic structure (7.5%) (Figure S10). There are two possible reasons that produce cyclic PPDL in the presence of BnOH. First, a small amount of PDL was competitively initiated by Zn(C6F5)2/DBU, yielding cyclic PPDL. Another possibility is the intramolecular backbiting of active −O* terminal group (intramolecular transesterification) in the polymer chain initiated by DBU-activated BnOH (Figure S9). Increasing BnOH loading dosage could significantly suppress the competitive ROP of PDL catalyzed by Zn(C6F5)2/DBU, while the transesterification was irrelevant to the BnOH feeding content. Experimentally, further increase of the feed ratio of benzyl alcohol to 10 equiv, there were still a small fraction of cyclic PPDL (4.5%) (Figure S11), suggesting the cyclic PPDL was produced by intramolecular transesterification. Meanwhile, GPC profiles suggested the formation of low MW PPDLs and a broadened MWDs when prolonging reaction time after full conversion (Figure S12). These results strongly suggested that the transesterification was accompanied by chain propagation when introducing BnOH as an initiator. In addition, the linear structure was usually obtained at the expense of catalytic activity and MW (run 3, Table 3). According to our previous report, the polymerization mechanism involved a bifunctional activation process, and the active species was zwitterionic intermediate, in which DBUbonded CO (acylazolium species) and Zn-bonded C−O terminal groups were formed.49 Theoretically, adding alcohol after full monomer conversion will also result in linear PPDL because of the nucleophilic attack of alcohol to the acylazolium group. Namely, linear PPDL could be prepared through a cascade of ROP and subsequent nucleophilic substitution. If this strategy is feasible, it is a very promising alternative for preparing ALCPEs with high MW. The transesterification and adverse effect of excessive ROH on MW can be circumvented. Introducing 1 equiv of BnOH after full conversion resulted in a mixture of major linear PPDL (about 90%) and minor cyclic PPDL (about 10%) (Figures S13 and S14). Further increasing the amount of BnOH (5−20 equiv) could produce totally linear polymer. However, the MW of the PPDL decreased sharply with the increasing the dosage of BnOH, suggesting that the excessive BnOH could induce chain scission by random attacking the ester group along the polymer chain (Figure S15). As known, the DBU-bonded CO (acylazolium species) have a higher reactivity than ester group in the polymer chain.56,57 We hypothesized that an alcohol ROH with low activity and bulky steric would attack the acylazolium species exclusively rather than ester group in polymer chain; thus, the chain scission could be eliminated. Based on this consideration, a variety of alcohols including t-BuOH, CF 3 CH 2 OH, (CF3)2CHOH, and Ph2CHOH were selected as the nucleophilic agent. The changes of the MW of the resultant PPDL with the amount of the ROH were traced by GPC (Figure 4). Obviously, the Mw of the PPDL was decreased with the increasing of ROH feed ratio, when t-BuOH, CF3CH2OH, or (CF3)2CHOH was added into polymerization solution after the full monomer conversion. It was clearly that excessive t-BuOH, CF3CH2OH, and (CF3)2CHOH could induce random chain scission. By sharp contrast, the PPDL exhibited apparently

If transesterification side reactions could be eliminated in the ROP of PDL catalyzed by Lewis pairs, we can prepare aliphatic long-chain (co)polyesters with high MW (>100 kg/mol) and well-defined blocky structure using this catalytic system. The transesterification side reactions could be judged by tracing the changes of the MW and the MWDs with prolonged reaction time after full monomer conversion. Intramolecular transesterification usually produces oligomer, thus increasing the total number of chains and significantly broadening MWDs. Intermolecular transesterification only results in broadened MWDs.53 There were no obvious differences in the MW and the MWDs of the PPDL obtained at 6 h (run 10, Table 2) and 24 h (run 12, Table 2) (Figure S6). The MW and the MWD of the polymer still remain almost constant after full conversion even at 150 °C (runs 15 and 16 in Table 2). The results clearly suggest that the transesterification side reactions could be ignored in the Zn(C6F5)2/DBU Lewis pairs catalytic ROP of PDL. ROP of PDL with Zn(C6F5)2/DBU in the Presence of Alcohol. It was well-known that the topological structure dramatically influences on the final properties of the polymer. The linear counterparts were more desired in most case because of their superior mechanical property derived from efficient chain entanglement.54,55 To prepare linear PPDL with high MW, we introduced alcohol (ROH) into the reaction systems (Table 3). The feeding mode and structures of ROH Table 3. Ring-Opening Polymerization of PDL with Zn(C6F5)2/DBU in the Presence of Alcohola run

ROH

[PDL]: [ROH]:[Zn]: [DBU]

1 2 3 4 5 6 7 8 9

PhCH2OH PhCH2OH PhCH2OH PhCH2OH PhCH2OH Ph2CHOH Ph2CHOH Ph2CHOH Ph2CHOH

100:1:1:1 100:1:1:1 100:5:1:1 100:5:1:1 100:20:1:1 100:1:1:1 100:5:1:1 100:10:1:1 100:20:1:1

convb (%)

Mn,calcdc (kg/mol)

Mn,expd (kg/mol)

PDId

99.2 99.2 82.6 91.5 95.0 90.6 91.5 92.3 91.8

23.9 23.9 0.41 22.1 22.1 21.9 22.1 22.3 22.2

21.7 20.8 0.50 54.6 42.1 65.3 65.5 65.3 65.2

1.7 1.9 1.6 1.8 1.9 1.7 1.8 1.7 1.7

a

The reactions was carried out in xylene, [PDL] = 1.9 mol/L, [Zn]: [DBU] = 100:1:1, T = 110 °C, the alcohol was mixed with Zn(C6F5)2/ DBU for runs 1−3, and the alcohol was added after full monomer conversion for runs 4−9. bDetermined by 1H NMR. cCalculated from Mn = [M]0/[ROH] × monomer conversion × Mmono + MROH for runs 1−3 and Mn = [M]0/[Zn] × monomer conversion × Mmono + MROH for runs 4−9. dDetermined by GPC analysis in trichlorobenzene at 150 °C vs polyethylene standard.

significantly affected the polymerization behavior. When mixing the benzyl alcohol (1 equiv) with Zn(C6F5)2/DBU at first, the initiation rate was comparable with propagation rate, and the kinetic plot exhibited excellent linearity (Figure S7). The benzyl alcohol might facilitate the decoupling of the Lewis pairs and the activation of the PDL. In 1H NMR spectra (Figure S8), new signals assigned to the methylene protons of terminal hydroxyl end groups (−CH2OH) and aromatic rings appeared at 3.65 and 7−8 ppm, respectively, confirming the formation of linear PPDL. We envisioned the ROP of PDL might involve a “cooperative catalysis”, in which Zn(C6F5)2 electrophilically coordinated with carbonyl oxygen and the DBU-activated E


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transesterification could be circumvented in the Zn(C6F5)2/ DBU Lewis pairs catalytic ROP of PDL, it was expected to prepare well-defined block copolymer by sequential addition of PDL followed by lactone with small ring. To explore the possibility, cyclic block copolymer c-poly(PDL-b-CL) was first prepared via sequential addition of PDL (100 equiv) and CL (100 equiv) by using Zn(C6F5)2/DBU (1:1) Lewis pairs as the catalyst. As a reference, the random copolymer c-poly(PDL-rCL) was also synthesized by one-pot polymerization. Obviously, the MW of the initial PPDL segment increased from Mn = 61.8 to Mn = 78.1 kg/mol after the formation of block copolymer (Figure 6). The random copolymer c-

Figure 4. Variations of Mw (ΔM) for PPDL with the increase of ROH dosage after full monomer conversion.

higher MW after introducing 1 equiv of Ph2CHOH. It was noteworthy that the MW of the PPDL remained almost constant, even when further increasing Ph2CHOH dosage (20 equiv). This result clearly suggested that Ph2CHOH could attack the DBU-bonded CO (acylazolium species), and the random chain scission could be eliminated. It may be ascribed to the less nucleophilicity of Ph2CHOH than other alcohols because of bulky hindrance and electronic withdrawing property of the benzene rings. Furthermore, MALDI-TOF MS also indicated that only linear PPDL was obtained (Figure 5).

Figure 6. GPC profiles of PPDL, cyclic block copolymer c-poly(PDLb-CL), and l-poly(PDL-b-CL) obtained from cascade of ring-opening polymerization and subsequent nucleophilic substitution.

poly(PDL-r-CL) exhibited only one melting peak at about 78.4 °C. In contrast, the block copolymer c-poly(PDL-b-CL) showed two distinct melting temperatures at 54.8 and 93.1 °C, which corresponded to the PCL block and PPDL block, respectively (Figure S16). Strikingly, the blocky structure could be remained even after 24 h at 110 °C, indicating that the Zn(C6F5)2/DBU Lewis pairs did not catalyze the inter- or intramolecular transesterification of the block copolymer. These results strongly suggested that Zn(C6F5)2/DBU Lewis pair had more superior catalytic performance than the organic catalyst and some metal complexes. Having confirmed the successful preparation of cyclic block copolymer c-poly(PDL-b-CL), we further investigated the possibility of preparing linear block copolymer l-poly(PDL-bCL) in the presence of Ph2CHOH with a feed ratio [PDL]0: [CL]0:[Zn]0:[DBU]0:[OH] = 100:100:1:1:1. However, analyzing the copolymer at defined intervals (1, 6, and 12 h) by DSC with longer reaction time clearly showed a gradual transformation from blocky structure to random counterpart (Figure 7A). This structural transformation could also be determined by tracing the resonance peaks of α-methylene groups CH2O (δ ∼ 64 ppm) in 13C NMR spectra (Figure 8A). The copolymer with blocky structure exhibited two major peaks at 64.3 and 64.1 ppm, which corresponded to the sequential PDL−PDL and CL−CL linkages, respectively. Extra two peaks of PDL−CL (64.4 ppm) and CL−PDL (63.9 ppm) linkages were detected after 12 h, confirming the random microstructure of the copolymers. The well-defined linear block copolymer l-poly(PDL-b-CL) with high MW can be obtained via a cascade of ROP and subsequent nucleophilic substitution, in which ROP of PDL

Figure 5. MALDI-TOF MS analysis for the isolated low MW PPDL obtained from cascade of ROP and subsequent nucleophilic substitution. 110 °C, [PDL]0 = 1.9 mol/L, [PPDL]:[Zn]:[DBU]: [Ph2CHOH] = 100:1:1:5; the product was diluted after full conversion, and then Ph2CHOH was added into the mixture.

Preparation of Well-Defined Block Copolyesters Containing Polyethylene-like Segments. Block copolyesters having a “polyethylene-like” block and a polar polymer block are very interesting structures, which can be prepared via the sequential ROP of macrolactones and lactones with small ring, such as lactide or ε-caprolactone.26,58 However, seldom well-defined block copolymers were prepared by the sequential addition method32 because the transesterification would convert a block copolymer to a random one. Since the F


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Macromolecules

Figure 7. DSC tracing of the transformation of block copolymer poly(PDL-b-CL) into random copolymer by transesterification side reactions (A) and comparison of DSC curves of c-poly(PDL-b-CL) and l-poly(PDL-b-CL) obtained from cascade of ring-opening polymerization and subsequent nucleophilic substitution (B).

(100 equiv) and CL (100 equiv) was sequentially initiated with Zn(C6F5)2/DBU (1:1), and the Ph2CHOH (5 equiv) was added after the full conversion of CL. The apparent MW of the resultant copolymer was slightly higher than the cyclic block copolymer c-poly(PDL-b-CL) because of the larger hydrodynamic volume of linear copolymer than the cyclic counterpart with the same MW (Figure 6). Meanwhile, the resultant copolymer exhibited relative narrow MWDs (Mw/Mn ∼ 1.9). These results also indicated that no random chain scission or chain extension occurred after adding Ph2CHOH. Two melting peaks assigned to PPDL block and PCL block respectively were observed at 54.5 and 93.6 °C in the DSC curve of the lpoly(PDL-b-CL) (Figure 7B). Only two major peaks at 64.3 and 64.1 ppm corresponding to the sequential PDL−PDL and CL−CL linkages were detected in 13C NMR, further confirming a well-defined blocky structure (Figure 8B). The resonance peaks of PDL−PDL and CL−CL segments were still intact (Figure S17) even when prolonging the polymerization time to 24 h after addition of ε-CL, indicating the absence of transesterification side reaction. Finally, to prove the versatility of this novel approach, linear block copolymer l-poly(PDL-b-LA) with well-defined structure was further prepared by using the same approach to lpoly(PDL-b-CL). The resultant block copolymer was determined by GPC, NMR, and DSC analysis. Obviously, the MW of polyester increased from Mn = 67.9 to Mn = 87.9 kg/mol after formation of block copolymer (Figure S18). Only two resonances corresponded to PPDL block and PLA block were observed in both the carbonyl region (between 160 and 180 ppm) and the methylene region (between 60 and 70 ppm) (Figure 9). The lack of the further signal peaks within this two regions suggested that the transesterification has been eliminated. Besides, the block copolymer l-poly(PDL-b-LA) displayed two melting temperature at 92.6 °C (PPDL block) and 165.0 °C (Figure 10), further confirming the blocky

Figure 8. 13C NMR (in CDCl3) tracing of the transformation of block copolymer poly(PDL-b-CL) into random copolymer (A) by transesterification side reactions and comparison of 13C NMR of cpoly(PDL-b-CL) and l-poly(PDL-b-CL) obtained from a cascade of ring-opening polymerization and subsequent nucleophilic substitution (B).

Figure 9. 13C NMR spectrum (in CDCl3) of the block copolymer lpoly(PDL-b-PLA) magnified in the carbonyl region (166−176 ppm) and the methine region (60−70 ppm).

structure was not disturbed by transesterification and the epimerization of the PLLA segment could be avoided.26

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CONCLUSIONS The Zn(C6F5)2/DBU Lewis pair exhibited excellent performance in catalyzing the ROP of PDL, affording high MW cyclic cG


Macromolecules

Article

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ACKNOWLEDGMENTS The authors are grateful for financial support by the National Natural Science Foundation of China (No. 21604061) and the Natural Science Foundation of Tianjin (No. 17JCQNJC02500).

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(1) Hillmyer, M. A.; Tolman, W. B. Aliphatic Polyester Block Polymers: Renewable, Degradable, and Sustainable. Acc. Chem. Res. 2014, 47 (8), 2390−2396. (2) Rasal, R. M.; Janorkar, A. V.; Hirt, D. E. Poly(lactic acid) modifications. Prog. Polym. Sci. 2010, 35 (3), 338−356. (3) Tian, H.; Tang, Z.; Zhuang, X.; Chen, X.; Jing, X. Biodegradable synthetic polymers: Preparation, functionalization and biomedical application. Prog. Polym. Sci. 2012, 37 (2), 237−280. (4) Södergård, A.; Stolt, M. Properties of lactic acid based polymers and their correlation with composition. Prog. Polym. Sci. 2002, 27 (6), 1123−1163. (5) Labet, M.; Thielemans, W. Synthesis of polycaprolactone: a review. Chem. Soc. Rev. 2009, 38 (12), 3484−3504. (6) Pepels, M. P. F.; Hansen, M. R.; Goossens, H.; Duchateau, R. From Polyethylene to Polyester: Influence of Ester Groups on the Physical Properties. Macromolecules 2013, 46 (19), 7668−7677. (7) Stempfle, F.; Ortmann, P.; Mecking, S. Long-Chain Aliphatic Polymers To Bridge the Gap between Semicrystalline Polyolefins and Traditional Polycondensates. Chem. Rev. 2016, 116 (7), 4597−4641. (8) Ortmann, P.; Mecking, S. Long-Spaced Aliphatic Polyesters. Macromolecules 2013, 46 (18), 7213−7218. (9) Stempfle, F.; Ortmann, P.; Mecking, S. Which Polyesters Can Mimic Polyethylene? Macromol. Rapid Commun. 2013, 34 (1), 47−50. (10) Menges, M. G.; Penelle, J.; Le Fevere de Ten Hove, C.; Jonas, A. M.; Schmidt-Rohr, K. Characterization of Long-Chain Aliphatic Polyesters: Crystalline and Supramolecular Structure of PE22,4 Elucidated by X-ray Scattering and Nuclear Magnetic Resonance. Macromolecules 2007, 40 (24), 8714−8725. (11) Jasinska-Walc, L.; Hansen, M. R.; Dudenko, D.; Rozanski, A.; Bouyahyi, M.; Wagner, M.; Graf, R.; Duchateau, R. Topological behavior mimicking ethylene-hexene copolymers using branched lactones and macrolactones. Polym. Chem. 2014, 5 (10), 3306−3310. (12) Liu, C.; Liu, F.; Cai, J.; Xie, W.; Long, T. E.; Turner, S. R.; Lyons, A.; Gross, R. A. Polymers from Fatty Acids: Poly(ω-hydroxyl tetradecanoic acid) Synthesis and Physico-Mechanical Studies. Biomacromolecules 2011, 12 (9), 3291−3298. (13) Genovese, L.; Gigli, M.; Lotti, N.; Gazzano, M.; Siracusa, V.; Munari, A.; Dalla Rosa, M. Biodegradable Long Chain Aliphatic Polyesters Containing Ether-Linkages: Synthesis, Solid-State, and Barrier Properties. Ind. Eng. Chem. Res. 2014, 53 (27), 10965−10973. (14) Papageorgiou, D. G.; Guigo, N.; Tsanaktsis, V.; Exarhopoulos, S.; Bikiaris, D. N.; Sbirrazzuoli, N.; Papageorgiou, G. Z. Fast Crystallization and Melting Behavior of a Long-Spaced Aliphatic Furandicarboxylate Biobased Polyester, Poly(dodecylene 2,5-furanoate). Ind. Eng. Chem. Res. 2016, 55 (18), 5315−5326. (15) Stempfle, F.; Ritter, B. S.; Mulhaupt, R.; Mecking, S. Long-chain aliphatic polyesters from plant oils for injection molding, film extrusion and electrospinning. Green Chem. 2014, 16 (4), 2008−2014. (16) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107 (12), 5813−5840. (17) Hodge, P. Entropically Driven Ring-Opening Polymerization of Strainless Organic Macrocycles. Chem. Rev. 2014, 114 (4), 2278− 2312. (18) van der Meulen, I.; Li, Y.; Deumens, R.; Joosten, E. A. J.; Koning, C. E.; Heise, A. Copolymers from Unsaturated Macrolactones: Toward the Design of Cross-Linked Biodegradable Polyesters. Biomacromolecules 2011, 12 (3), 837−843. (19) Jiang, Z. Lipase-Catalyzed Synthesis of Aliphatic Polyesters via Copolymerization of Lactone, Dialkyl Diester, and Diol. Biomacromolecules 2008, 9 (11), 3246−3251.

Figure 10. DSC curves of the poly(PDL-b-LA) block polymer and poly(PDL-r-LA) random polymer.

PPDL (Mw > 100 kg/mol) with a satisfying catalytic activity (TOF = 15.1 h−1). Further detailed investigation showed that reaction conditions significantly impact the polymerization kinetics and MWs of the resultant PPDL. Excitingly, the transesterification side reaction can be avoided in the ROP of PDL catalyzed by the Zn(C6F5)2/DBU Lewis pair, as evidenced by the invariable MW and the MWD of the resultant PPDL even when prolonging the reaction time. Well-defined cyclic diblock copolyesters containing polyethylene-like blocks can be easily prepared by sequential adding PDL and lactone with small rings, such as LA and CL. No structural transformation was observed even for prolong reaction time. The ROPs of PDL with Lewis pairs were further conducted in the presence of Ph2CHOH. When mixing Ph2CHOH with Zn(C6F5)2/DBU first, cyclic PPDLs derived from intramolecular backbiting were detected, suggesting the presence of transesterification. By contrast, initiating the ROP of PDL with Zn(C6F5)2/DBU and adding the Ph2CHOH after the full monomer conversion also produce linear PPDL with high MW, in which the transesterification could be circumvented. Based on these results, linear diblock copolymers containing polyethylene-like blocks were also prepared by a cascade of ROP of and subsequent nucleophilic substitution.

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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.7b02378. Detailed experimental section, characterization of the polymer samples by DSC, NMR, and MALDI-TOF MS (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.L.). ORCID

Li Pan: 0000-0002-9463-6856 Yuesheng Li: 0000-0003-4637-4254 Notes

The authors declare no competing financial interest. H


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Ring-Opening Polymerization with Lewis Pairs and Subsequent

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