Coordination Ring-Opening Copolymerization of Naturally Renewable

Article pubs.acs.org/Macromolecules

Coordination Ring-Opening Copolymerization of Naturally Renewable α‑Methylene-γ-butyrolactone into Unsaturated Polyesters Miao Hong and Eugene Y.-X. Chen* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States S Supporting Information *

ABSTRACT: Reported herein is the first coordination− insertion ring-opening copolymerization of α-methylene-γbutyrolactone (MBL) and ε-caprolactone (ε-CL) catalyzed by f-block lanthanide (Ln) catalysts, Ln[N(SiMe3)2]3, that produce exclusively an unsaturated copolyester PMBL-co-PCL without coproducing any homopolymer PMBL. Accomplishing such synthesis requires effective strategies to meet two key challenges: ring-opening of the γ-butyrolactone (γ-BL) ring in MBLthe five-membered lactone well recognized for its nonpolymerizabilityand shutting down the vinyl-addition pathway via conjugate addition across the double bondthe exocyclic CC moiety in MBL known for its high reactivity toward vinyl addition. Remarkably, the current Ln coordination catalyst system, coupled with judiciously chosen reaction conditions (relatively nonpolar solvent and low temperature, 0 or −20 °C), effectively copolymerizes MBL and ε-CL to produce the ring-opening copolyester PMBL-co-PCL, with the MBL incorporation up to 40 mol % and without any detectable PMBL formation, even when employing a large excess of MBL in feed. Successful ring-opening homopolymerization of γ-BL by the Ln catalyst has also been realized at −78 °C under ambient pressure, producing the polyester PBL with a Tm of 63.0 °C and a Tg of −53.7 °C. Investigation into the thermal property of the resulting copolyester reveals an overall depression of Tm of the copolyester as increasing the MBL incorporation, indicating that the ringopened MBL (unsaturated) polyester incorporated in the random copolymer is noncrystallizable and disrupts the crystallization process of the crystallizable, ring-opened ε-CL (saturated) polyester segment. Mechanistic studies provide key evidence for a coordination−insertion ring-opening copolymerization mechanism.

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INTRODUCTION Tulipaline A, or α-methylene-γ-butyrolactone (MBL), found naturally in tulips or produced chemically from biomass feedstocks, is the simplest member of the naturally occurring sesquiterpene lactone family.1 Thanks to several of its attractive features, including its biorenewability (as a natural substance), higher reactivity (due to the presence of an exocyclic double bond and resonance stabilization of active species), and superior properties of the derived polymers (due to incorporation of the robust five-membered lactone ring into the chain), as compared to its linear analogue methyl methacrylate (MMA) and MMA-derived polymers, MBL has received a renaissance of interest in exploring the prospects of offering a sustainable alternative to the petroleum-based methacrylate monomers for the production of specialty chemicals and acrylic plastics.2 As a result, various types of polymerization processes have been employed or developed to polymerize MBL into low to high molecular weight (MW) polymers or copolymers, including radical,3 anionic,4 zwitterionic,5 group-transfer,6 and coordination7 polymerization methods. These processes, regardless of the polymerization mechanism or method, produce exclusively the addition polymer, poly(α-methylene-γ-butyrolactone) (PMBL), proceeding through conjugate addition across the exocyclic CC double bond without ring-opening of the five-membered γ-butyrolactone (γ-BL) ring (Scheme 1). © 2014 American Chemical Society

Scheme 1. Vinyl Addition vs Ring-Opening Pathways in (Co)polymerization of MBL

The nonpolymerizability of γ-BL toward ring-opening polymerization (ROP) is well recognized due to the relative low strain energy (i.e., high thermodynamic stability) of its fivemembered ring that brings about only a small negative change of enthalpy (ΔHp) for its ROP.8 As entropy changes (ΔS) of polymerization are typically negative, the Gibbs free energy of polymerization, ΔGp = ΔHp − TΔS, would be positive for the ROP of γ-BL as the positive contribution of the −TΔS term is Received: April 14, 2014 Revised: May 14, 2014 Published: May 29, 2014 3614

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not offset by a small negative ΔHp, thus explaining the nonpolymerizability of γ-BL under normal conditions. Drastic changes to the polymerization conditions, such as monomer (M) concentration, temperature, and pressure, can be used to perturb the propagation (P + M) and depropagation (P − M) equilibrium: ΔG = −RT ln(Keq), Keq = kP+M/kP−M. For instance, under an ultrahigh pressure of 20 000 atm at 160 °C, the ringopening polymer PBLthe bacterially derived equivalent poly(4-hydroxybutyrate)with Mn up to 3500 g/mol was achieved.9 A more accessible, thus most commonly employed, strategy to ring-open polymerize γ-BL is to copolymerize it with a lactone with high ring strain energy, such as the four-membered β-butyrolactone and the seven-membered ε-caprolactone (εCL).10 This ring-opening copolymerization (ROC) approach renders the overall negative change of ΔH p of the copolymerization sufficiently large to offset the positive −TΔS term, thus enabling the copolymerization to proceed to form γ-BL-based copolymers (copolyesters). Extending this strategy to a γ-BL derivative carrying an exocyclic double bond (i.e., MBL), copolymerization of neat MBL and ε-CL mediated by Bi(OTf)3 at 130 °C was recently reported to produce the ring-opening copolyester PMBL-co-PCL with variable amounts of MBL incorporation depending on the initial comonomer feed ratio.11 Such resulting unsaturated copolyesters, due to incorporation of the ring-opened MBL units, are of scientific and technological interest for producing tailor-made polyester materials through postfunctionalization and cross-linking.12 However, the obtained polymer product was shown to be actually a mixture of the ring-opening copolyester PMBL-coPCL and the vinyl-addition (ring-retention) homopolymer PMBL (see Results and Discussion), due to the bifunctionality of MBL that promotes competing ring-opening and vinyladdition pathways in the copolymerization. Hence, the synthesis of the unsaturated copolyester PMBL-co-PCL through the direct copolymerization of MBL and ε-CL without coproducing the homopolymer PMBL still remained a challenge before this work. Hence, addressing this challenge, together with mechanistic elucidation of the ROC of MBL and ε-CL, was the central objective of this investigation. Accordingly, this work uncovers the lanthanide (Ln)-based coordination polymerization catalysts and reaction conditions that lead to the exclusive formation of the ring-opening copolyester PMBL-co-PCL (Scheme 1), with the MBL incorporation up to 40 mol % and without any detectable PMBL homopolymer formation. The mechanistic study of this work establishes a coordination−insertion ROC mechanism for the current catalyst and ROC system.

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bis(trimethylsilyl)amide]lanthanide complexes, Ln[N(SiMe3)2]3 (Ln = La, 1; Sm, 2; Nd, 3), and trifluoromethanesulfonate bismuth [Bi(OTf)3] were purchased from Sigma-Aldrich and used as received. Tris[(trimethylsilyl)methyl]bis(tetrahydrofuran)yttrium [Y(CH2SiMe3)3(THF)2 (4)] was prepared according to a literature procedure.13 Polymer Characterizations. Polymer weight-average molecular weights (Mw), number-average molecular weights (Mn), and molecular weight distributions or polydispersity indices (PDI = Mw/Mn) were measured by gel permeation chromatography (GPC) analyses carried out at 40 °C and a flow rate of 1.0 mL/min, with DMF as the eluent on a Waters University 1500 GPC instrument equipped with one PLgel 5 μm guard and three PLgel 5 μm mixed-C columns (Polymer Laboratories; linear range of molecular weight = 200−2 000 000). The instrument was calibrated with 10 PMMA standards, and chromatograms were processed with Waters Empower software (version 2002). Melting temperatures (Tm) and crystallization temperatures (Tc) were measured by differential scanning calorimetry (DSC) on a DSC 2920, TA Instruments. All Tm values were obtained from the second scan after the thermal history was removed from the first scan. Maximum rate decomposition temperatures (Tmax) and decomposition onset temperatures (Tonset) of the polymers were measured by thermal gravimetric analysis (TGA) on a TGA 2950, TA Instruments. Polymer samples were heated from ambient temperatures to 600 °C at a rate of 20 °C/min. 1H NMR, 13C NMR, and 1H−13C HMQC spectra were recorded on a Varian Inova 400 MHz spectrometer. Chemical shifts for 1H and 13C spectra were referenced to internal solvent resonances and are reported as parts per million relative to SiMe4. The MBL incorporation in the MBL/ε-CL ring-opening copolymer was determined by 1H NMR spectra (DMSO-d6) and calculated according to the formula MBL mol % = [2I6.12 ppm/(2I6.12 ppm + I3.97 ppm)] × 100%, where Ix ppm is the peak area of proton at x ppm. The PMBL content in the polymer produce was also determined by 1H NMR spectra (DMSO-d6) and calculated according to the formula PMBL mol % = [I4.34 ppm/(2I6.12 ppm + I3.97 ppm + I4.34 ppm)] × 100%. The γ-BL incorporation in the γ-BL/ε-CL ring-opening copolymer was determined by 1H NMR spectra (bromobenzene-d5) and calculated according to the formula γ-BL mol % = (I1.87 ppm/(I3.99−4.06 ppm) × 100%. The isolated low-MW sample was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS); the experiment was performed on an Ultraflex MALDI-TOF mass spectrometer (Bruker Daltonics) operated in positive ion, reflector mode using a Nd:YAG laser at 355 nm and 25 kV accelerating voltage. A thin layer of a 1% NaI solution was first deposited on the target plate, followed by 0.6 μL of both sample and matrix (dithranol, 10 mg/mL in 50% ACN, 0.1% TFA). External calibration was done using a peptide calibration mixture (4−6 peptides) on a spot adjacent to the sample. The raw data were processed in the FlexAnalysis software (version 2.4, Bruker Daltonics). General Polymerization Procedures. Polymerizations were performed either in 25 mL flame-dried Schlenk flasks interfaced to the dual-manifold Schlenk line for runs using external temperature bath or in 20 mL glass reactors inside the glovebox for room temperature (RT) runs. The reactor was charged with 4.0 mL of solvent and a predetermined amount of initiator or catalyst. After equilibration at the desired polymerization temperature for 10 min, the polymerization was initiated by rapid addition of monomer via a gastight syringe. After a desired period of time, the polymerization was immediately quenched by addition of 5.0 mL of 5% HCl-acidified methanol. The quenched mixture was precipitated into 100 mL of cold methanol, filtered, washed with methanol to remove any unreacted monomer, and dried in a vacuum oven at RT to a constant weight.

EXPERIMENTAL SECTION

Materials, Reagents, and Methods. All synthesis and manipulations of air- and moisture-sensitive materials were carried out in flamed Schlenk-type glassware on a dual-manifold Schlenk line or in an argon-filled glovebox. HPLC-grade organic solvents were first sparged extensively with nitrogen during filling 20 L solvent reservoirs and then dried by passage through activated alumina (for CH2Cl2) followed by passage through Q-5 supported copper catalyst (for toluene) stainless steel columns. HPLC-grade dimethylformamide (DMF) was degassed and dried over CaH2 overnight, followed by vacuum distillation (CaH2 was removed before distillation). Monomers α-methylene-γ-butyrolactone (MBL), γ-butyrolactone (γBL), and ε-caprolactone (ε-CL) were purchased from TCI America; the monomers were dried over CaH2 overnight, vacuum-distillated, and stored at −40 °C in the glovebox for further use. Tris[N,N-

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RESULTS AND DISCUSSION

Behavior of MBL/ε-CL Copolymerization by Bi(OTf)3. Copolymerization of neat MBL and ε-CL mediated by Bi(OTf)3 at 130 °C was recently reported to produce the ring-opening copolymer PMBL-co-PCL.11 In our hands, 3615

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coordination and initiate the polymerization via the nucleophilic ligand attached to the metal center,14 we hypothesized that such Ln complexes are suitable candidates for achieving the desired coordination−insertion copolymerization of MBL and εCL, thus potentially producing the desired ring-opening copolyester without coproducing the homopolymer PMBL. Characteristics of MBL/ε-CL Copolymerization by Ln Coordination Catalysts. The following four Ln coordination catalystsLa[N(SiMe3)2]3 (1), Sm[N(SiMe3)2]3 (2), Nd[N(SiMe3)2]3 (3), and Y(CH2SiMe3)3(THF)2 (4)were chosen for the study of copolymerization of MBL and ε-CL, due to the combination of the high Lewis acidity and coordination number of the Ln center (desirable for monomer coordination and activation) and the high nucleophilicity of the ligand (desirable for chain initiation). As expected, homopolymerization of MBL at 25 or 0 °C by any of the above catalysts (0.2 mol %) in a solvent (4.0 mL) such as CH2Cl2, DMF, or toluene led to exclusive formation of the vinyl-addition product PMBL through enchainment of double bonds. As an example, complex 1 rapidly polymerizes MBL (500 equiv) at room temperature in DMF, achieving 74% yield of PMBL in 1 min, 1H NMR (see Supporting Information Figure S2) and 13C NMR (see Supporting Information Figure S3) of which clearly showed formation of PMBL without formation of any detectable amount of the ring-opening polymer. Next, we investigated the copolymerization of MBL and εCL by the above four Ln catalysts, the results of which are summarized in Table 1. When the copolymerization of MBL and ε-CL (1000 equiv relative to Ln) in a feed ratio of 1:1 was carried out at room temperature in dichloromethane (DCM, 4.0 mL), all three amide Ln catalysts 1−3 produced the desired ring-opening copolyester without any vinyl addition homopolymer PMBL (runs 1−3, Table 1). For example, La catalyst 1 afforded the clean copolyester PMBL-co-PCL, free of any PMBL contamination (Figure 2). The copolymer produced had 4.8 mol % the ring-opened MBL units incorporated and Mw = 43.7 kg/mol with a unimodal MW distribution (PDI = 1.54, run 1). The other two amide catalysts performed similarly, albeit lower activity (runs 2 and 3). In contrast, the alkyl Ln catalyst 4, although exhibiting the highest activity in the series, produced a mixture of polymer products composing the desired ring-opening copolyester and the undesired homopolymer PMBL (18 mol %), as clearly shown by both 1H NMR (see Supporting Information Figure S4) and GPC trace that revealed a bimodal MW distribution in a ratio of about 84% (Mw = 8.35 kg/mol, PDI = 1.33 for the copolymer) and 16% (Mw = 145 kg/mol, PDI = 1.24 for PMBL, run 4). Effects of Reaction Conditions on Behavior of Copolymerization by Catalyst 1. Considering that the La catalyst 1 not only leads to the desired ring-opening copolymer free of the homopolymer contamination but also exhibits a higher polymerization activity than the Sm and Nd amide catalysts (2 and 3), catalyst 1 (0.1 mol % loading) was chosen for our subsequent investigation into effects of reaction conditions (feed ratio, solvent, temperature) on the copolymerization behavior. Keeping the reaction temperature (25 °C) and solvent (DCM) fixed, we gradually increased the MBL/ε-CL feed ratio from 1:1 to 10:1 (runs 5−8) and then decreased to 1/10 (run 9). With increasing MBL in the feed, the MBL incorporation in the ring-opening copolymer first increased accordingly and then reached a plateau of 14 mol %, as shown in Figure 3. However, when the MBL/ε-CL feed ratio was increased to 3/1

however, the obtained polymer product was shown to be a mixture of the ring-opening copolyester and the vinyl-addition (ring-retention) homopolymer PMBL, with the amount of the latter component becoming more pronounced as the amount of MBL in feed increases. The PMBL homopolymer actually present in the copolymer product was previously undetected by 1 H NMR analysis in CDCl3, simply due to the fact that PMBL is insoluble in CHCl3 (or THF, but soluble in DMSO or DMF). For the same reason, GPC analysis using CHCl3 or THF as eluent would not be able to reveal the presence of PMBL in the polymer product by observing a unimodal MW distribution when using such solvents as the eluent. However, when DMSO-d6 was used for 1H NMR analysis of the polymer product, which was prepared from the copolymerization of neat ε-CL and MBL (1:1) by Bi(OTf)3 ([M]/[Bi] = 2000) at 130 °C for 1 h, the presence of the homopolymer PMBL (20 mol %) became apparent with observing an extra set of resonances (in addition to the copolyester resonances) at 4.33, 2.07, and 1.71 ppm in the 1H NMR spectrum (Figure 1), characteristic of

Figure 1. 1H NMR spectra of the polymer product derived from the copolymerization of neat ε-CL and MBL (1:1) at 130 °C by Bi(OTf)3 in CDCl3 (bottom) and DMSO-d6 (top), showing that the polymer product contained 20 mol % of PMBL and 80 mol % of the desired copolyester, in that 14 mol % MBL was incorporated.

PMBL. Likewise, using DMF instead of THF in GPC analysis revealed a bimodal MW distribution (see Supporting Information Figure S1), consistent with the findings by NMR analysis. Owing to their large solubility differences, the copolyester and the homopolymer PMBL can be readily separated by solvent fractionation in CHCl3 or THF. Overall, the above results clearly showed that the copolymerization of MBL and ε-CL by Bi(OTf)3 produced a mixture of products containing the desired copolyester PMBLco-PCL and the undesired homopolymer PMBL. Hence, the synthesis of the ring-opening copolyester without coproducing the homopolymer through the direct copolymerization of MBL and ε-CL still remained a challenge before the present work. On the basis of the reasoning that metal triflates such as Bi(OTf)3 are not known for being effective initiators/catalysts to promote coordination−insertion ROP, we focused on our investigation using Ln complexes such as {Ln[N(SiMe3)2]3 and Y(CH2SiMe3)3(THF)2. Owing to the ability of such Ln complexes to activate the monomer through strong monomer 3616

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Table 1. Results of MBL/ε-CL Copolymerization by Ln Coordination Catalystsa run

Cat.

MBL/ε-CL

Sol.

temp (°C)

t (min)

yield (g)

convb (MBL%)

convb (CL%)

PMBLb (mol %)

MBLb (mol %)

Mwc (kg/mol)

PDIc

1 2 3 4 5 6 7 8 9 10 11e 12 13 14 15 16e 17e 18 19 20 21 22g 23 24g 25 26

1 2 3 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1/1 1/1 1/1 1/1 2/1 3/1 5/1 10/1 1/10 3/1 1/3 1/1 3/1 5/1 10/1 3/1 3/1 3/1 5/1 10/1 3/1 3/1 5/1 5/1 10/1 10/1

DCM DCM DCM DCM DCM DCM DCM DCM DCM DMF neat TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL

25 25 25 25 25 25 25 25 25 25 130 25 25 25 25 50 80 0 0 0 −20 −20 −20 −20 −20 −20

10 10 10 10 10 10 20 30 10 1 1 10 10 20 30 3 3 60 60 60 120 180 120 240 240 480

0.16 0.12 0.07 0.36 0.10 0.07 0.11 0.11 0.50 0.70 0.43 0.40 0.17 0.15 0.12 0.48 0.60 0.17 0.13 0.10 0.11 0.12 0.09 0.08 0.05 0.09

1.44 1.10 0.65 15.3 1.23 1.20 4.63 7.20 5.67 78.0 82.4 5.84 3.80 5.00 7.01 28.6 46.0 4.40 4.35 4.26 3.71 6.87 3.08 5.53 2.18 3.60

27.1 20.3 11.8 51.2 24.2 21.9 40.3 48.5 48.1 0 21.5 65.9 50.9 60.5 59.7 100 100 49.5 52.3 62.6 29.9 68.1 35.8 63.6 30.9 58.2

0 0 0 18.1 0 3.10 24.8 51.1 0 100 50.0 0 0 8.60 35.0 32.2 50.0 0 4.30 11.4 0 0 0 0 0 0

4.75 4.58 4.81 4.44 8.72 10.4 13.6 14.0 1.10 0 3.32 7.70 17.3 21.4 27.0 16.0 13.3 20.0 25.2 31.2 26.0 22.2 29.1 26.7 40.0 36.3

43.7 31.1 40.2 8.35 (84%) 27.5 24.6 50.2 61.4 73.0 50.8 108f 76.0 50.3 46.2 38.1 92.3 47.8f 51.9 48.8 36.5 80.0 96.0 49.8 72.4 38.8 65.1

1.54 1.81 1.79 1.33 1.50 1.45 2.77 2.00 1.68 2.33 2.34f 1.54 1.47 1.66 1.75 1.79 2.19f 1.67 1.76 1.68 1.65 1.90 1.66 1.70 1.68 1.72

Tmd (°C)

Tcd (°C)

56.7

31.8

48.7 41.8 38.9 26.7

37.2 11.4 1.58 −18.4

36.6 35.8 23.1 39.1

7.12 1.84 −11.1 −2.70

29.8

−2.43

16.5

−32.6

a Conditions: Cat. = 10 μmol, [MBL+ ε-CL]/cat. = 1000, Vsol = 4.0 mL. bMonomer conversions, PMBL homopolymer content, and MBL incorporation of the copolymer were measured by 1H NMR. cWeight-average molecular weights (Mw) and polydispersity indices (PDI = Mw/Mn) were determined by GPC at 40 °C in DMF relative to PMMA standards. dMelting-transition temperature (Tm) and crystallization temperatures (Tc) were measured by DSC. eCat. = 5 μmol, [MBL+ ε-CL]/Cat. = 2000. fSome polymer fractions were not soluble in DMF due to cross-linking at high temperature. gCat. =10 μmol, [MBL+ ε-CL]/Cat. = 500.

Figure 2. 1H NMR spectra of the MBL/ε-CL copolymer produced by La[N(SiMe3)2]3 (1) (run 1) in DMSO-d6 (top) and CDCl3 (bottom), showing no homopolymer PMBL formation.

Figure 3. Effects of MBL/ε-CL feed ratio and solvent on MBL incorporation in the ring-opening copolymer PMBL-co-PCL produced by catalyst 1 at 25 °C (data taken from runs 1, 6−8, and 12−15).

or higher, the copolymerization became less controlled as a result of competing homopolymerization of MBL through vinyl addition, thus starting to coproduce an increasing amount of PMBL with increasing MBL in the feed (runs 6−8). For example, the copolymerization in a 10/1 MBL/ε-CL feed ratio yielded a mixture of polymer products containing 51 mol % PMBL; upon removal of the homopolymer by solvent

fractionation, the ring-opening copolyester was shown to have 14 mol % MBL incorporated and a Mw of 61.4 kg/mol and a PDI of 2.00 (run 8). Changing the solvent from DCM to the much more polar DMF for the copolymerization of MBL and ε-CL in a 3/1 ratio resulted in the rapid polymerization of MBL via the vinyl addition pathway, producing exclusively the homopolymer 3617

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favors the ROC over the vinyl addition to form exclusively the ring-opening copolymer. This preference is so pronounced that even the copolymerization with a large excess of MBL in feed (MBL/ε-CL = 5/1 and 10/1) produced exclusively the ringopening copolymer with no detectable PMBL formation when the copolymerization was carried out in toluene at −20 °C (runs 23 and 25). The conversion of MBL and ε-CL at −20 °C can be enhanced by prolonging the polymerization time and increasing the catalyst loading (run 21 vs 22, 23 vs 24, 25 vs 26), and the obtained copolymers still possess a homogeneous composition and exhibit relatively narrow, unimodal MWD distributions (Mw/Mn = 1.5−1.7, Figure 5). Furthermore, such

PMBL (run 10) and thus behaving much like the homopolymerization of MBL in this solvent. We also examined the copolymerization in neat MBL and ε-CL in a 1/3 ratio at 130 °C. Although the copolymerization proceeded rapidly and gelled in 1 min, the obtained polymer product was not homogeneous in composition, containing 50 mol % PMBL (run 11) and also some insoluble fractions presumably due to cross-linking at high temperature (130 °C) through the double bond in the copolymer. The resulting ring-opening copolymer incorporated only 3.3 mol % MBL. Most interestingly, using toluene as the solvent in place of DCM enhanced not only the copolymerization activity but also significantly the MBL incorporation (runs 12−15). As shown in Figure 3, the MBL incorporation increased gradually from 7.7 to 27 mol % when the MBL/ε-CL feed ratio was increased from 1/1 to 10/1. Noteworthy also for the copolymerization carried out in toluene at 25 °C are that the ring-opening copolymer with a MBL incorporation up to 17 mol % (Mw = 50.3 kg/mol and PDI = 1.47, run 13) can be produced without forming any PMBL and that there is no plateau established for reaching a maximum MBL incorporation even when the MBL incorporation reached 27 mol % (Figure 3). In comparison, the copolymerization of MBL and ε-CL (1/1) by Bi(OTf)3 under the same conditions (toluene, 25 °C, 10 min) yielded only a trace amount of polymer products. Overall, for the current Ln catalyst system, the relatively nonpolar solvent toluene favors the ROC pathway while effectively suppressing the MBL homopolymerization via the conjugate-addition pathway. The contamination by PMBL occurs only when MBL is used in large excess in feed (MBL/ε-CL = 5/1, run 14 and 10/1, run 15). Lastly, potential effects of temperature on the behavior of the copolymerization by catalyst 1 in toluene were also investigated and are summarized in Table 1 (runs 12−26). As shown in Figure 4, raising temperature of the copolymerization with a

Figure 5. Selected GPC traces of MBL/ε-CL ring-opening copolyesters produced by catalyst 1 in toluene at −20 °C: (green) Mw = 80.0 kg/mol, PDI = 1.65 (run 21); (gray) Mw = 49.8 kg/mol, PDI = 1.66 (run 23); (blue) Mw = 38.8 kg/mol, PDI = 1.68 (run 25).

PMBL-co-PCL copolyester structure free of the PMBL homopolymer has been confirmed by 1H NMR (Figure 2), 13 C NMR (Figure 6), and 1H−13C HMQC (Figure 7) analyses.

Figure 6. 13C NMR spectrum (DMSO-d6) of PMBL-co-PCL (29.1 mol % MBL, run 23). The starred peak was originated for the NMR solvent. Figure 4. Effect of polymerization temperature on MBL incorporation and PMBL content (with a fixed MBL/ε-CL ratio of 3/1; data taken from runs 13, 16−18, and 21).

Noteworthy are the shoulder peaks within the two main carbonyl resonances (labeled as peaks 1 and 2 in Figure 6), which can be attributed to homo- and heterosequences of ε-CL and MBL units in the copolymer, according to the analysis of the 13C NMR spectrum of the related copolymer PBL-coPCL.10a 1 H NMR spectra of the MBL/ε-CL copolymers obtained with a MBL/ε-CL feed ratio of 10 at different temperatures were compared in Figure 8. As can be clearly seen from this figure, the content of PMBL gradually decreased with decreasing the temperature. When the temperature was

fixed MBL/ε-CL ratio of 3/1 from −20 to 25 °C, the composition of the copolymer remained homogeneous without forming any PMBL polymer. Further raising temperature to 50 or 80 °C, ROC still proceeded but accompanied by vinyl addition polymerization of MBL and the content of PMBL enhanced with increasing temperature (runs 16 and 17 vs runs 13 and 18). These results clearly showed that low temperature 3618

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Figure 7. 1H−13C HMQC spectrum (DMSO-d6) of PMBL-co-PCL (29.1 mol % MBL, run 23).

lowered to −20 °C, the characteristic peaks attributed to PMBL at 4.33, 2.07, and 1.71 ppm were completely absent. Moreover, low temperature also has an obvious advantage in enhancing MBL incorporation due to the absence of competing vinyladdition homopolymerization. As shown in Figure 4, MBL incorporation increases with decreasing the temperature. Thus, the ring-opening copolyester with a high MBL incorporation up to 40 mol % and without any PMBL contamination (Figure 8) has been successfully achieved (run 25). Mechanistic Aspects of Ring-Opening Copolymerization. To determine the chain initiation and termination end groups, we prepared a low-MW copolymer by terminating a copolymerization of MBL and ε-CL (with a 2/1 ratio) by catalyst 1 in toluene (under which conditions no homopolymer PMBL was formed) within 10 s of reaction time. The MALDITOF mass spectrum of this low-MW copolymer is depicted in Figure 9. The peaks in the spectrum can be assigned to different sets of isomerides and the peaks in the same set represent the same total degrees of copolymerization (but different degrees of polymerization of the two comonomers). Therefore, the masses m/z can be expressed as m/z = Mend + 23 (Na+) +

Figure 8. 1H NMR spectra (DMSO-d6) of MBL/ε-CL copolymers obtained in a fixed MBL/ε-CL feed ratio of 10/1 at different temperatures: 25 °C, top (35 mol % PMBL, 27 mol % MBL incorporation in the copolymer, run 15); 0 °C, middle (11.4 mol % PMBL, 31.2 mol % MBL incorporation in the copolymer, run 20); −20 °C, bottom (0 mol % PMBL, 40 mol % MBL incorporation in the copolymer, run 25).

Figure 9. MALDI-TOF mass spectrum of a low-molecular-weight MBL/ε-CL copolymer. 3619

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(98.10 × m + 114.14 × n), where m and n are the degrees of polymerization for MBL and ε-CL, respectively, and Mend is the MW of chain ends. Take the fourth set as an example, the peaks in this set are attributed to the 15th-mer copolymer, where the peaks at 1815.953, 1831.629, and 1848.003 represent the copolymers containing 5 units of MBL and 10 units of ε-CL, 4 units of MBL and 11 units of ε-CL, and 3 units of MBL and 12 units of ε-CL, respectively. According to this analysis, the MW of the chain ends is calculated to 161 g/mol, corresponding to H/N(SiMe3)2. Collaboratively, the peak for the −N(SiMe3)2 chain end was also observed at 0.07 ppm in the 1H NMR spectrum (Figure 10), further confirming the analysis of

especially when the copolymerization is carried out in toluene under relatively low temperature (−20 °C). The same strategies can be applied to the ROC of γ-BL and ε-CL as well as the ROP of γ-BL directly. For example, copolymerization of γ-BL and ε-CL in a 10/1 γ-BL/ε-CL ratio in toluene (3.0 mL) at 25 °C by catalyst 1 (0.2 mol % loading) produced a ring-opening copolyester, PBL-co-PCL, with 42 mol % γ-BL incorporation and Mw = 20.6 kg/mol, PDI = 1.55. The γ-BL incorporation can be further enhanced by two ways: lowering the temperature to −20 °C (53.8 mol % γ-BL incorporation, Mw = 75.7 kg/mol, PDI = 1.92, Figure 11) or

Figure 10. 1H NMR spectrum of a low-molecular-weight MBL/ε-CL copolymer.

Figure 11. 1H NMR spectra (bromobenzene-d5) of γ-BL (53.8 mol %)/ε-CL copolymer (4.05, 3.99, 2.26, 2.17, 1.87, 1.50, 1.24 ppm), top spectrum; PBL (4.05, 2.26, 1.86 ppm), middle spectrum; γ-BL monomer (3.82, 2.04, 1.64 ppm) for comparison, bottom spectrum.

MALDI-TOF mass spectrum. On the basis of the result of this chain-end analysis, we proposed the copolymerization of MBL and ε-CL by La[N(SiMe3)2]3 and its analogues to proceed through a “coordination−insertion” mechanism, as outlined in Scheme 2. This work employed two strategies to enable ring-opening of the γ-BL ring in MBL: (a) copolymerizing MBL with ε-CL to make the overall negative change of ΔHp sufficiently large to offset the positive −TΔS term and (b) low reaction temperature to reduce the positive contribution of the −TΔS term. In addition, we employed the relative nonpolar solvent toluene to suppress the competing vinyl-addition polymerization of MBL. Collectively, combing these strategies effectively promote the polymerization of MBL through the ring-opening pathway, rather than the typical vinyl-addition pathway, thereby enabling us to produce the ring-opening copolymer PMBL-co-PCL, free of any PMBL homopolymer,

running the copolymerization at 25 °C in neat (51.0 mol % γBL incorporation, Mw = 21.0 kg/mol, PDI = 1.62). The 13C NMR spectrum of the copolymer PBL-co-PCL with a 53.8 mol % of γ-BL incorporation (Figure 12) clearly showed homo- and heterosequences of ε-CL and γ-BL units. Likewise, while homopolymerization of γ-BL by catalyst 1 at 25 °C yielded no polymer formation after 24 h, lowering the temperature to −78 °C started to produce PBL (Figure 11), although the γ-BL conversion was limited to only ∼3% after 24 h. The resulting PBL had a Tm of 63 °C and a Tg of −53.7 °C (Figure 13), somewhat higher than those thermal transitions of PCL. Thermal Property of Ring-Opening Copolymers. The thermal property of the ring-opening copolyester PMBL-coPCL was examined by DSC and TGA analyses. Typical second heating scans of DSC curves are depicted in Figures 14 and 15.

Scheme 2. Proposed “Coordination−Insertion” Mechanism for the ROC of MBL/ε-CL by the Ln Amide Catalyst

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incorporation (Figure 15a−c, runs 13−15). It is interesting to see that the copolymer obtained at −20 °C with a MBL incorporation of 26% exhibited a somewhat higher Tm value (39.1 °C, run 21) than the one obtained at 0 °C (36.6 °C, run 18) with a MBL incorporation of 20%, presumably caused by the much higher molecular weight of the copolymer produced at −20 °C (80.0 vs 51.9 k/mol). Overall, there appeared to have approximately a linear correlation between the Tm value of the copolymer and the MBL mol % incorporation (Figure 16). Hence, the Tm of the copolymer with 1.1 mol % of MBL incorporation is 56.7 °C (vs 58 °C for the PCL homopolymer prepared by catalyst 1, Mw = 33.0 kg/mol, PDI = 1.68), while the Tm of the copolymer with 40 mol % of MBL incorporation is decreased to only 16.5 °C. The depression of Tm of the copolyester PMBL-co-PCL with an increase in the MBL incorporation can be understood by the Flory theory of crystallization in copolymers.15 According to this theory, for random copolymers such as PMBL-co-PCL consisting of noncrystallizable component 1 (MBL) and crystallizable component 2 (ε-CL), increasing the amount of the noncrystallizable component 1 will decrease the Tm value of the copolymer by disrupting the crystallization process of the crystallizable component 2. This relationship is expressed as follows: (1/Tm − 1/T0m) ∝ −ln(χ2), where Tm is the Tm value at a given χ2 (the mole fraction of the crystallizable component, i.e., ε-CL in this example) and T0m is the Tm value of the homopolymer PCL in this example. Thus, a plot the reciprocal of Tm vs −ln(χ2) of the seven copolyesters prepared from this study gave a linear relationship (R2 = 0.97; Figure 17). From the intercept of this linear plot, T0m was derived to be 57 °C, which compares well with the experimental Tm value of 58 °C for the PCL prepared by catalyst 1 (vide supra). Figure 18 compares TGA curves and relative derivative thermogravimetry (DTG) curves of the copolymer PMBL-coPCL as well as the homopolymers PCL and PMBL produced by catalyst 1 for comparison. The onset degradation temperatures (Td) are defined by the temperatures of 5% weight loss in TGA curves, and the maximum degradation temperatures (Tmax) are measured by the peaks in DTG curves. A high Td at around 360 °C was observed, showing the high thermal stability of the copolyester. As shown in Figure 18, the copolymer displays a one-step degradation profile with a Tmax of about 405 °C, attributed to the degradation of main chain. The overlay plots (Figure 18) show that the copolyester exhibits rather similar thermal stability to the ring-opening homopolymer PCL and the vinyl-addition homopolymer PMBL.

Figure 12. 13C NMR spectrum (bromobenzene-d5, starred peaks) of γBL (53.8 mol %)/ε-CL copolymer consisting of homo- and heterosequences of ε-CL and γ-BL units.

Figure 13. DSC curve of γ-BL ring-opening polymer PBL.

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CONCLUSIONS In summary, this contribution reports the first coordination− insertion ring-opening copolymerization of MBL and ε-CL catalyzed by f-block Ln catalysts, Ln[N(SiMe3)2]3, that produce exclusively the unsaturated copolyester PMBL-co-PCL without coproducing the homopolymer PMBL. Having achieved this feat, we have met two challenges in such ROC: ring-opening of the γ-BL ring in MBLthe five-membered lactone that is well recognized for its nonpolymerizabilityand completely shutting down the vinyl-addition pathway through conjugate addition across the double bondthe exocyclic CC moiety in MBL that is known for its remarkable reactivity toward vinyl addition. Hence, the current Ln catalyst system consisting of the strong coordinating metal center (desirable for monomer coordination and activation) and the strong nucleophilic ligand (desirable for chain initiation), coupled with judiciously chosen

Figure 14. DSC curves: (a) vinyl-addition homopolymer PMBL and (b) ring-opening copolymer PMBL-co-PCL (MBL mol % = 26.0%, run 21).

Take the copolyester PMBL-co-PCL (26 mol % MBL) produced at −20 °C for example; a single Tm of 39.1 °C was observed, while a Tg typically at 195.5 °C attributed to the ringretention, vinyl-addition homopolymer PMBL was not observed (Figure 14), which further demonstrated the homogeneous composition of the obtained ring-opening copolymer. Moreover, Tm values of the copolymers produced at the same temperature decrease with increasing the MBL 3621

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Figure 15. DSC curves of MBL/ε-CL copolymers: (a) MBL mol % = 17.3% (run 13), (b) MBL mol % = 21.4% (run 14), (c) MBL mol % = 27.0% (run 15), (d) MBL mol % = 40% (run 25).

to achieve the ring-opening copolyester PMBL-co-PCL, with the MBL incorporation up to 40 mol % and without any detectable PMBL formation. On the basis of the mechanistic study, the ring-opening copolymerization mediated by the present Ln catalyst is proposed to proceed through a coordination−insertion mechanism, as outlined in Scheme 1. To enable ring-opening of the γ-BL ring of MBL in the copolymerization scheme, this work employed two strategies: (a) copolymerizing MBL with ε-CL to make the overall negative change of ΔHp sufficiently large to offset the positive −TΔS term and (b) low reaction temperature to reduce the positive contribution of the −TΔS term. Both strategies contribute to enabling of the Gibbs free energy of polymerization (ΔGp) in favor of ring-opening polymerization. The low-temperature strategy even enabled successful ring-opening homopolymerization of γ-BL by catalyst 1 at −78 °C under ambient pressure, producing the polyester PBL with a Tm of 63.0 °C and a Tg of −53.7 °C. To completely shut down the competing vinyl-addition polymerization of MBL, we employed the relatively nonpolar solvent (toluene) and low temperatures of polymerization (0 °C for a MBL/ε-CL feed ratio up to 3/1, or −20 °C for all the feed ratios investigated in this study). In fact, when the copolymerization was carried out at −20 °C in toluene, the preference for MBL ring-opening over vinyl addition is so pronounced that even the copolymerization employing a large excess of MBL in feed (MBL/ε-CL = 5/1 and 10/1) still produces exclusively the ring-opening copolyester with up to 40 mol % MBL incorporation and no detectable PMBL formation. There is an apparent linear correlation between the Tm value of the copolymer PMBL-co-PCL and the MBL mol %

Figure 16. Plot of Tm values of MBL/ε-CL ring-opening copolymers and MBL incorporation (data taken from runs 9, 12−15, 20, and 25).

Figure 17. Plot of 1/Tm values of the copolyester PMBL-co-PCL as a function of −ln(χ2) (data taken from runs 9, 12−15, 20, and 25).

reaction conditions (i.e., using the relatively nonpolar solvent and low temperature), effectively copolymerizes MBL and ε-CL 3622

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Figure 18. TGA (left) and DTG (right) curves of PMBL-co-PCL (19.1 mol % MBL, run 23), PCL (prepared by catalyst 1, Mw = 33.0 kg/mol, PDI = 1.68) and PMBL (prepared by catalyst 1, Mw = 62.5 kg/mol, PDI = 2.31). Macromolecules 2008, 41, 5509−5511. (h) Pickett, J. E.; Ye, Q. Tulipalin Copolymers, U.S. Pat. 0122625 A1, 2007. (i) Bandenburg, C. J. U.S. Pat. 6,841,627 B2, 2005. (j) PCT Int. Appl. WO. 2003048220, 2003. (k) Gridnev, A. A.; Ittel, S. D. WO 035960 A2, 2000. (l) Stansbury, J. W.; Antonucci, J. M. Dent. Mater. 1992, 8, 270−273. (m) Ueda, M.; Takahashi, M.; Imai, Y.; Pittman, C. U. J. Polym. Sci., Part A: Polym. Chem. 1982, 20, 2819−2828. (n) Koinuma, H.; Sato, K.; Hirai, H. Makromol. Chem., Rapid Commun. 1982, 3, 311−315. (o) Akkapeddi, M. K. Polymer 1979, 20, 1215−1216. (p) Akkapeddi, M. K. Macromolecules 1979, 12, 546−551. (q) McGraw, W. J.; Morristown, N. J. U.S. Pat. 2,624,723, 1953. (4) By the anionic polymerization method: (a) Hu, Y.; Gustafson, L. O.; Zhu, H.; Chen, E. Y.-X. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 2008−2017. (b) Suenaga, J.; Sutherlin, D. M.; Stille, J. K. Macromolecules 1984, 17, 2913−2916. (5) By the zwitterionic polymerization method: (a) Chen, E. Y.-X. Top. Curr. Chem. 2013, 334, 239−260. (b) Miyake, G. M.; Chen, E. Y.X. Angew. Chem., Int. Ed. 2012, 51, 2465−2469. (c) Zhang, Y.; Miyake, G. M.; John, M. G.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y.X. Dalton Trans. 2012, 41, 9119−9134. (d) Zhang, Y.; Miyake, G. M.; Chen, E. Y.-X. Angew. Chem., Int. Ed. 2010, 49, 10158−10162. (6) By the group-transfer polymerization method: (a) Zhang, Y.; Gustafson, L. O.; Chen, E. Y.-X. J. Am. Chem. Soc. 2011, 133, 13674− 13684. (b) Miyake, G. M.; Zhang, Y.; Chen, E. Y.-X. Macromolecules 2010, 43, 4902−4908. (c) Sogah, D. Y.; Hertler, W. R.; Webster, O. W.; Cohen, G. M. Macromolecules 1987, 20, 1473−1488. (7) By the coordination polymerization method (selected examples): (a) Gowda, R. R.; Chen, E. Y.-X. Dalton Trans. 2013, 42, 9263−9273. (b) Chen, X.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. J. Am. Chem. Soc. 2012, 134, 7278−7281. (c) Hu, Y.; Miyake, G. M.; Wang, B.; Cui, D.; Chen, E. Y.-X. Chem.Eur. J. 2012, 18, 3345−3354. (d) Hu, Y.; Xu, X.; Zhang, Y.; Chen, Y.; Chen, E. Y.-X. Macromolecules 2010, 43, 9328−9336. (e) Miyake, G. M.; Newton, S. E.; Mariott, W. R.; Chen, E. Y.-X. Dalton Trans. 2010, 39, 6710−6718. (8) (a) Alemán, C.; Betran, O.; Casanovas, J.; Houk, K. H.; Hall, H. K., Jr. J. Org. Chem. 2009, 74, 6237−6244. (a) Houk, K. H.; Jabbari, A.; Hall, H. K., Jr; Alemán, C. J. Org. Chem. 2008, 73, 2674−2678. (b) Saiyasombat, W.; Molloy, R.; Nicholson, T. M.; Johnson, A. F.; Ward, I. M.; Poshyachinda, S. Polymer 1998, 39, 5581−5585. (c) Duda, A.; Biela, T.; Libiszowski, J.; Penczek, S.; Dubois, P.; Mecerreyes, D.; Jérôme, R. Polym. Degrad. Stab. 1998, 59, 215−222. (9) For a recent review, see: Moore, T.; Adhikari, R.; Gunatillake, P. Biomaterials 2005, 26, 3771−3782. (10) Selected examples: (a) Agarwal, S.; Xie, X. Macromolecules 2003, 36, 3545−3549. (b) Lee, C. W.; Urakawa, R.; Kimura, Y. Eur. Polym. J. 1998, 34, 117−122. (c) Duda, A.; Penczek, S. Macromol. Chem. Phys. 1996, 197, 1273−1283. (11) Zhou, J.; Schmidt, A. M.; Ritter, H. Macromolecules 2010, 43, 939−942. (12) (a) Johnson, K. G.; Yang, L. S. Preparation, Properties and Applications of Unsaturated Polyester. In Modern Polyesters; Scheirs, J.,

incorporation, showing the overall depression of Tm of the copolyester with an increase in the MBL incorporation. For example, the Tm of the copolyester with 1.1 mol % of MBL incorporation is 56.7 °C (vs 58 °C for PCL), while the Tm of the copolyester with 40 mol % of MBL incorporation is decreased to only 16.5 °C. This Tm depression shown for PMBL-co-PCL can be explained by the Flory theory of crystallization in copolymers, indicating that the ring-opened MBL (unsaturated) polyester component incorporated in the random copolymer PMBL-co-PCL is noncrystallizable and disrupts the crystallization process of the crystallizable component, the ring-opened ε-CL saturated polyester segment.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.Y.-X.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF-1300267). REFERENCES

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