Synthetic Principles Determining Local Organization of Copolyesters

examined samples and peak deconvolution were performed using WAXSFit software designed by M. Rabiej at the University of Bielsko-Biała (AHT) in Po...
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Synthetic Principles Determining Local Organization of Copolyesters Prepared from Lactones and Macrolactones Lidia Jasinska-Walc,*,†,‡ Miloud Bouyahyi,† Artur Rozanski,§ Robert Graf,∥ Michael Ryan Hansen,∥,⊥ and Rob Duchateau*,† †

SABIC Technology & Innovation, STC Geleen, Urmonderbaan 22, Geleen, The Netherlands Department of Polymer Technology, Chemical Faculty, Gdansk University of Technology, G. Narutowicza Str. 11/12, 80-233 Gdansk, Poland § Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ⊥ Department of Chemistry, Interdisciplinary Nanoscience Center (iNANO), Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark ‡

S Supporting Information *

ABSTRACT: A highly effective and facile technique for catalytic ring-opening copolymerization (cROP) of lactones viz. εcaprolactone and ε-decalactone with ω-pentadecalactone is being described. The reactions were mediated by Zn- and Cabased tridentate Schiff base complexes and benzyl alcohol as initiator. The catalysts were successfully employed for the preparation of numerous block and random copolymers. To unravel the composition of the polyesters, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was employed. Furthermore, the combination of solid-state NMR and XRD techniques revealed that not only the presence of branches but also intramolecular transesterification, resulting in low molecular weight cyclic structures, is responsible for a complex ordering of the polymer chain fragments. This paper focuses mainly on the detailed molecular characterization of the synthesized copolymers.

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INTRODUCTION

resembling LLDPE with branches randomly distributed along the polymer backbone. Only a limited number of catalysts are capable of generating polymacrolactones with relatively high molecular weight and narrow molecular weight distributions. The first metal complexes that have been employed as catalysts for the cROP of macrolactones were homoleptic zinc,4 yttrium,5 or alkali metal alkoxides;6a however, they invariably lead to low molecular weight products. Efforts to search for metal-based catalysts affording relatively high molecular weight polymacrolactones resulted in the discovery that aluminum salen catalysts,1a,b terdentate phenoxyimine−amine aluminum, zinc and calcium catalysts,1c and bis(phenoxy)magnesium catalysts6b that are capable of effectively mediating the cROP of macrolactones to high molecular weight products. Whereas the aluminum salen and phenoxyimine−amine catalysts tend to produce random pentadecalactone−caprolactone copolymers as a result of competitive transesterification of poly(pentadecalactone) and poly(caprolactone) blocks,1a the zinc

Catalytic ring-opening polymerization (cROP) is a proven powerful approach toward copolyesters with precisely controlled topology and microstructure.1 Hence, cROP is an ubiquitous tool giving the possibility to prepare a wide variety of products with well-defined monomer configuration and topologies, viz. linear, star, cyclic, or brush type polymers, typically associated with targeted properties.2,3 Contrary to for example organic catalyst-mediated1b ring-opening copolymerization, of lactones, where the transesterification process leads to randomized products with limited control over the compositions and microstructure, metal-catalyzed cROP provides a versatile route to both random and block copolymers.1c An important feature of cROP of lactones and macrolactones is also the ability to produce linear or branched polyesters with tunable distances between ester groups in block copolymers. Specifically, the production of linear polymers with a relatively low concentration of ester groups can give rise to materials resembling HDPE and revealing an enhanced compatibility with hydrophilic materials. This concept can be easily expanded for the branched copolymers like polyester-based systems © XXXX American Chemical Society

Received: November 8, 2014 Revised: December 20, 2014

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minoethane, diethylzinc, NaN(SiMe3)2 and CaI2 (Sigma-Aldrich) were used without further purification. Dry ethanol (Biosolve) was used as received. Toluene and THF (Sigma-Aldrich) were dried using a purification column set consisting of BTS and molecular sieves. Zinc and calcium catalysts (1, 2) were synthesized according to the described procedures.10a,b Typical Procedure for PDL Homopolymerization. In the glovebox, PDL (1.0 g, 4.2 mmol), catalyst 1 (3.2 mg, 8.4 μmol), and benzyl alcohol (0.9 mg, 8.4 μmol) and toluene (2 mL) were placed in a small glass crimp cap vial, and the vial was capped. The reaction mixture was removed from the glovebox and stirred for a given time at 100 °C. For all reactions, an aliquot of crude polymer was withdrawn at the end of the polymerization and dissolved in CDCl3 to determine the conversion by 1H NMR spectroscopy. The reaction was then stopped and quenched by addition of excess methanol (∼5 mL). The produced polypentadecalactone (PPDL) was isolated and dried in vacuo at room temperature for 18 h. Typical Copolymerization Reaction for Block Copolymers by Means of Sequential Feed. PDL (500 mg, 2.1 mmol) was transferred into a glass crimp cap vial under an inert nitrogen atmosphere in the glovebox. An appropriate amount of BnOH (1.9 mg, 18.0 μmol) to catalyst 1 (6.9 mg, 18.0 μmol) was added to the mixture, and the vial was then capped and placed in oil bath at 100 °C for a predetermined reaction time. At the end of the reaction period, an aliquot was taken for analysis and the calculated amount of eCL monomer (240 mg, 2.1 mmol) was added; the sealed vial was then placed for an additional predetermined time at 100 °C. At the end, an aliquot was removed and dissolved in CDCl3 for 1H NMR; the mixture was quenched by acidic methanol, and the precipitated polymer was filtered, washed with methanol several times, and dried under vacuum for 24 h before characterization. Typical Copolymerization Reaction for Block or Random Copolymers by Single Feed. A glass crimp cap vial was charged with PDL (500 mg, 2.1 mmol) together with eDL (357 mg, 2.1 mmol) or eCL (240 mg, 2.1 mmol), catalyst 1 (4.5 mg, 0.042 mmol), BnOH (4.43 mg, 0.042 mmol) as an initiator, and toluene. All the manipulations were carried out in the glovebox. Then, the mixture was removed from the glovebox and stirred at 100 °C. The progress of the reaction was followed by 1H NMR. The synthesized copolymer was cooled to room temperature and quenched using methanol (5 mL). The precipitated polymer was filtered, washed with methanol several times, isolated, and dried in vacuo at room temperature for 18 h. Measurements. 1H NMR and 13C NMR spectra were recorded at room temperature using a Varian Mercury Vx spectrometer operating at Larmor frequencies of 400 and 100.62 MHz for 1H and 13C, respectively. For 1H NMR experiments, the spectral width was 6402.0 Hz, acquisition time 1.998 s, and the number of recorded scans equal to 64. 13C NMR spectra were recorded with a spectral width of 24154.6 Hz, an acquisition time of 1.3 s, and 256 scans. Size exclusion chromatography (SEC) was performed at 160 °C on a Polymer Laboratories PLXT-20 Rapid GPC Polymer Analysis System (refractive index detector and viscosity detector) with three PLgel Olexis (300 × 7.5 mm, Polymer Laboratories) columns in series. 1,2,4-Trichlorobenzene was used as eluent at a flow rate of 1 mL min−1. The molecular weights were calculated with respect to polyethylene standards (Polymer Laboratories). A Polymer Laboratories PL XT-220 robotic sample handling system was used as autosampler. MALDI-ToF-MS analysis was performed on a Voyager DE-STR from Applied Biosystems equipped with a 337 nm nitrogen laser. An accelerating voltage of 25 kV was applied. Mass spectra of 1000 shots were accumulated. The polymer samples were dissolved in CHCl3 at a concentration of 1−3 mg mL−1. The cationization agent used was potassium trifluoroacetate or sodium trifluoroacetate (Fluka, > 99%) dissolved in THF at a concentration of 5 mg mL−1. The matrix used was trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) (Fluka) and dissolved in THF at a concentration of 40 mg mL−1. Solutions of matrix, salt, and polymer were mixed in a volume ratio of 4:1:4, respectively. The mixed solution

phenoxyimine−amine catalysts allowed the formation of perfect block copolymers once a sequential feed approach was applied.7 In our preceding communication the overall mechanism of cROP has been elucidated by DFT calculations and clarified the origin of the significant differences in reactivity of lactones, macrolactones, and substituted derivatives thereof.7 Since nonlinear structures of the polymers significantly affects their rheological and mechanical properties, it is of high importance to elucidate their supramolecular arrangement in terms of macromolecules chain organization and dynamics. While liquid-state NMR spectroscopy combined with X-ray diffraction (XRD) provides information about chemical structure and long-range order of the semicrystalline materials, the combination of these techniques with solid-state NMR spectroscopy is particularly suited to discover their kinetically and conformationally controlled primary structure, supramolecular organization, and crystalline arrangements of the macromolecules.8 Such information can be explored by solidstate NMR technique by the analysis of the proton (1H) and carbon (13C) isotropic chemical shifts and short-ranged dipole−dipole (through space) or J-coupling (through bonds) coupling parameters.9 Herein we disclose the report of cROP mediated by Zn- and Ca-based phenoxyimine−amine catalysts 1 and 2 (Scheme 1), Scheme 1. Chemical Structure of the Copolyesters Synthesized via Catalytic Ring-Opening Polymerization of PDL, eCL, and eDL Mediated by 1 and 2

which are applicable to a broad range of ring-strained lactones as well as macrolactones. The difference in reactivity of each of the cyclic esters was successfully applied to promote block or random copolymers revealing linear or branched structures under a single set of standard reaction conditions. The formation of block or random copolymer structures was confirmed by MALDI-ToF-MS and liquid-state NMR spectroscopy. A number of different solid-state NMR techniques combined with X-ray diffraction were applied to provide a detailed insight into the supramolecular and crystalline structure of the polyesters.

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EXPERIMENTAL SECTION

Materials. ω-Pentadecalactone (PDL), ε-decalactone (eDL), εcaprolactone (eCL), and benzyl alcohol (BnOH) (Sigma-Aldrich) were dried over CaH2 and distilled under reduced pressure. 3,5-Di-tertbutyl-2-hydroxybenzaldehyde, 2,2-dimethylethylenediamine, 1,2-diaB

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Table 1. Properties of the Homo- and Copolyesters Synthesized from PDL, EDL, and ECL Catalyzed by 1 and 2 in the Presence of Benzyl Alcohol as an Initiatora entry

complex

mol ratio [M]/[Cat]/[BnOH]

mol ratio [PDL]/[eCL]/[eDL]

reaction time [h]

convb [PDL]/[eCL]/[eDL] [%]

Mnc [kg/mol]

Đc

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

1 1 1 1 1 1 1 1 2 2 2 2 2

500/1/0.5 200/1/1 230/1/1 500/1/0.5 500/1/0.5 500/1/0.5 500/1/0.5 500/1/0.5 100/1/0.5 100/1/1 100/1/1 500/1/1 500/1/1

1/0/0 1/1.3/0 1/1.3/0 0.7/0/0.3 1/0/1 0.3/0/0.7 0/0/1 0/1/1 1/0/0 0/0/1 1/0/1 1/0/0 1/0/1

5 4 4 14 24 24 3 6 5 3 4 5 3

54/−/− 70/90/− 90/50/− 25/−/100 64/−/100 44/−/100 −/−/100 −/100/98 99/0/0 0/0/100 93/0/100 99/0/0 54/0/100

73.0 24.6 41.3 15.5 21.8 18.6 13.8 11.0 23.8 3.6 6.1 21.6 4.6

2.3 2.2 1.8 1.9 2.3 1.9 1.3 2.0 2.3 1.1 2.1 2.4 2.3

The polymerizations were carried out in toluene at 100 °C. bDetermined from 1H NMR spectra. cDetermined by HT-SEC in TCB against PE standards.

a

additional pinholes (300 μm in diameter) forming the beam and an imaging plate as a detector and recording medium (Kodak). The camera was coupled to a X-ray source (sealed-tube, fine point Cu Kα filtered radiation, operating at 30 kV and 50 mA; Philips). Exposed imaging plates were read with Phosphor Imager SI scanner and ImageQuant software (Molecular Dynamics). Long periods were determined from one-dimensional sections of 2-D pattern. Background and Lorentz corrections were applied to the curves. Long period was then calculated from position of the maximum of corrected curves using the Bragg law. The lamellar structure of copolymers was examined with a transmission electron microscope (TEM), Tesla BS 500 (Tesla, Czech Republic), operating at the accelerating voltage of 90 kV. Samples for TEM examination, in the form of ultrathin sections 60 nm thick, were prepared by cryo-ultramicrotoming. The cryo-ultramicrotome type PowerTome PC (Boeckeler, USA) equipped with a 35° diamond knife (Diatome, Switzerland) was used for sectioning. The ultrathin sections were placed on standard copper grids for TEM examination.

was hand-spotted on a stainless steel MALDI target and left to dry. The spectra were recorded in the reflection mode. All MALDI-ToFMS spectra were recorded from the crude products. Melting (Tm) and crystallization (Tc) temperatures as well as enthalpies of the transitions were measured by differential scanning calorimetry (DSC) using a DSC Q100 from TA Instruments. The measurements were carried out at a heating and cooling rate of 10 °C min−1 from −60 to 120 °C. The transitions were deduced from the second heating and cooling curves. Solid-state 13C{1H} cross-polarization/magic-angle spinning (CP/ MAS) NMR and 13C{1H} insensitive nuclei enhanced by polarization transfer (INEPT) experiments under fast MAS conditions were carried out on a Bruker AVANCE-III 500 spectrometer employing a doubleresonance H-X probe for rotors with 2.5 mm outside diameter. These experiments utilized a MAS frequency of 25.0 kHz, a 2.5 μs π/2 pulse for 1H and 13C, a CP contact time of 2.0 ms, and TPPM decoupling during acquisition. The CP conditions were preoptimized using Lalanine. The 13C{1H} INEPT MAS spectra were recorded using the refocused-INEPT sequence with a J-evolution period of either 1/ (3JCH) or 1/(6JCH) assuming a 1JCH of 150 Hz; i.e., for a J-evolution time of 1/(3JCH) the signals from CH and CH3 groups are positive, while those of CH2 are negative.11 The 2D 1H−1H double quantumsingle quantum (DQ-SQ) correlation experiments were carried out on a Bruker AVANCE-III 700 spectrometer using a 2.5 mm solid-state MAS double-resonance probe. These experiments employed a spinning frequency of 25.0 kHz. DQ excitation and reconversion were performed using the broadband back-to-back (BaBa) sequence.12 Chemical shifts for 1H and 13C are reported relative to TMS using solid adamantane as an external. Analysis of the crystalline structure of the polyesters was performed using wide-angle X-ray scattering measurements by means of a computer-controlled goniometer coupled to a sealed-tube source of Cu Kα radiation (Philips), operating at 50 kV and 30 mA. The Cu Kα line was filtered using electronic filtering and the usual thin Ni filter. The data were collected at room temperature. The 1D profiles were subsequently background-corrected and normalized. Since reflections from the different crystallographic phases frequently overlap each other, it was necessary to separate them by deconvolution. Analysis of diffraction profiles of the examined samples and peak deconvolution were performed using WAXSFit software designed by M. Rabiej at the University of Bielsko-Biała (AHT) in Poland.13 The software allows to approximate the shape of the peaks with a linear combination of Gauss and Lorentz or Gauss and Cauchy functions and adjusts their settings and magnitudes to the experimental curve with a “genetic” minimizing algorithm. The small-angle X-ray scattering technique was used for determination of long period. The 0.5 m long Kiessig-type camera was equipped with a tapered capillary collimator combined with

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RESULTS AND DISCUSSION Ring-Opening Polymerization of Lactones and Macrolactone. The synthetic pathway consisting of cROP of lactones (eCL, eDL) and the macrolactone ω-pentadecalactone (PDL) using the zinc complex 1 or the calcium complex 2 containing a tridentate Schiff base ligand afforded a family of linear and branched copolyesters (Scheme 1). Since the catalytic testing of 1 and 2 resulted in polymers with diverse polarity and topology, it seemed to be foreseeable that by simple adjusting the feed composition of different branched/ nonbranched, ring-strained/strain-free lactones, the supramolecular arrangement and thus properties of the copolymers can be easily tuned. Furthermore, expecting the significant difference in rate constants of macrolactones and ring-strained lactones, rather blocky structures of the synthesized polymers were anticipated. Interestingly, this phenomenon was observed only for eDL−PDL copolymerization (Table 1, entries 4−6, 11, and 13) while cROP of eCL−PDL and eCL−eDL afforded random products (Table 1, entries 2 and 8). The liquid-state 1H NMR and 13C NMR spectra recorded for the PeDL-block-PPDL copolymer (Table 1, entry 5) display several characteristics that are in agreement with a block copolymer structure (Figures 1 and 2). As evidenced by 1H NMR spectroscopy, eDL readily underwent cROP under applied reaction conditions, which was confirmed by a prompt C

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Figure 1. Liquid-state 1H NMR spectra recorded during formation of PeDL-block-PPDL copolymers at t = 0 and after 5 h (a), 14 h (b), and 24 h (c) of the reaction. The spectra show the characteristic for H1 and H22 resonances of PDL, PPDL, and PeDL in the spectral range 3.8−5.3 ppm. The assignment follows that of Scheme 1.

disappearance of the eDL α-CH proton signals at 4.28 ppm in comparison to the α-CH2 proton resonances of PDL (at 4.12 ppm). This implies that formation of eDL-derived branched zinc alkoxide hampers the incorporation of a PDL monomer. As demonstrated in Figure 1, the formation of the PPDL block occurs only after most of the eDL has been consumed. This is supported by liquid-state 13C NMR spectrum of PeDL-blockPPDL where the signal assigned to the α-CH2 resonance of the PDL fragment (C1) at 64.37 ppm confirms the formation of a block copolymer (Figure 2a). Clearly, the multiple signals between 73.8 and 73.9 ppm and 63.9−64.2 ppm assigned to αCH resonances of eDL (C22) and eCL (C16) fragments proved the formation of the randomly distributed units (Figure 2b,c). Further evidence for the formation of block copolymers rather than a blend of homopolymers was obtained from SEC, which clearly demonstrated the increase in molecular weight after between the homopolymerization of eDL and the copolymerization of eDL and PDL. Nevertheless, by applying 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) as transesterification catalyst, the PeDL-block-PPDL can easily be randomized. This was clearly evidenced by the appearance of well-resolved signals in the vicinity of 64−74 ppm arising from significant amounts of PDL−eDL and eDL−PDL linkages in the random poly(eDL-co-PDL) copolymer (Figure S1). Certainly, the fact that copolymerization of PDL with eCL as well as eDL with eCL affords random copolymers under identical conditions excludes that only the large difference in reactivity as a result of ring strain of the monomers is responsible for this phenomenon and indicates that the presence of butyl branches of eDL in combination with the different conformation (cis and trans) of the monomers (PDL versus eDL, eCL, respectively) causes this peculiar effect. Therefore, it is most likely that the less reactive secondary alkoxide group attached to zinc (when 1 is used), formed upon insertion of eDL, allows incorporation of another equivalent of eDL despite its steric hindrance, rather than a large and unstrained PDL monomer, which is in addition hampered by the cis-configuration of its ester group. Conversely, only a sequential-feed of PDL followed by eCL leads to a PeCL-block-PPDL copolymer (Table 1, entry 3). As observed, even prolonged heating of the copolymer (16 h at 100 °C) did not change the structure of the block copolymer, revealing that transesterification is insignificant in this system (Figure S2). Furthermore, the copolymerization of eCL and eDL mediated by 1 has been found to result in random copolymers (Table 1, entry 8), indicating that a delicate balance between steric effects and ring strain dictates the formation of

random versus block copolymers. As depicted in Figure 2b, the multiple C16 and C22 resonances originate from randomly distributed eCL units combined with atactic eDL fragments. This delicate difference in catalytic behavior during the cROP of branched, ring-strained monomers with nonbranched, ringstrained lactones or nonstrained macrolactones was also proven by extensive DFT calculations.7 As evidenced by SEC analysis, cROP of PDL mediated by 1 and benzyl alcohol as an initiator afforded polyesters with Mn’s around 73 kg mol−1. As a consequence of the relatively high molecular weight of the formed PPDL and a high viscosity of the reaction mixture, the conversion of PDL stagnated at 54%. An interesting feature of the homopolymerization of PDL was also a relatively high Đ (around 2) for PPDL. The high Đ observed here can be contributed either to heterogeneization and mass transport limitation due to the severely increased viscosity or to transesterification.1b,d MALDI-ToF-MS analysis indeed proved that at least intramolecular transesterification reactions leading to the formation of low molecular weight cyclic structures occurs, and it is likely that intermolecular transesterification affording linear products takes place as well (vide inf ra). The homopolymerization of ring-strained eDL was accomplished already within 3 h (Table 1, entries 7 and 10). Most probably due to water initiation the cROP provided materials with lower Mn values in comparison to PPDL. Remarkably, the low Đ values of PeDL, particularly up to an eDL conversion of 90%, is consistent with a “living” cROP process of eDL. Conversely, the corresponding block copolymers obtained by copolymerization of eDL with PDL have molecular weights of up to 40 kg mol−1 and polydispersity indices close to 2, which strongly suggests that transesterification only takes place in the PPDL block. Catalyst 2 showed a similar selectivity in copolymerization as 1affording a block copolymer when copolymerizing eDL and PDL and a random copolymer when copolymerizing eCL and PDL. Catalyst 2 showed a significantly higher catalytic activity than 1. After 30 min at 100 °C, the cROP of eDL mediated by 2 had already reached an eDL conversion of above 83%, while the homopolymerization of PDL and the copolymerization of PDL and eDL was accomplished after 4 and 5 h, respectively (Table 1, entries 9−13). Despite a high conversion of the monomers, the homo- and copolyesters prepared using 2 revealed only moderate molecular weights. Interestingly, using a double amount of benzyl alcohol as initiator, catalyst 2 afforded homo- and copolyesters with significantly lower molecular weight in comparison to 1. Increasing the D

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Figure 3. Section of MALDI-ToF-MS spectrum of PeDL-block-PPDL copolymer synthesized via catalytic ring-opening copolymerization of eDL and PDL. The spectrum was recorded with sodium trifluoroacetate as an ionization salt.

Figure 2. Liquid-state 13C NMR spectra of PeDL-block-PPDL (a), poly(eDL-co-eCL) (b), and poly(eCL-co-PDL) (c). The spectra show the C1, C16, and C22 of PPDL, PeCL, and PeDL nuclei next to the ester groups. The labeling follows that of Scheme 1.

molecular fraction again is dominated by cyclic structures (Figure S4). All isotope patterns visible in MALDI-ToF-MS spectrum of poly(eCL-co-eDL) are separated in the distances of 114 m/z or 170 m/z, which match to exchange of eCL and eDL units. Based on MALDI-ToF-MS analysis, it is assumed that the potential occurrence of intramolecular transesterification and formation of low molecular weight cyclic structures occur for both block and random copolymers and is mainly responsible for the relatively low molecular weight and high Đ of these materials. As solid-state magic-angle spinning (MAS) NMR gives complementary information about conformations and chain dynamics present in the synthesized bulk materials, the technique was used to characterize packing and mobility of different segments of the copolymers. By performing 1D and 2D NMR correlation experiments of these macromolecules, taking advantage of the short-ranged dipole−dipole (through space) or J-coupling (through bonds), we could further broaden the information concerning supramolecular arrangements of the polymers.9c Although NMR pulse sequences exist utilizing the J-coupling in the solid state, we have here chosen to apply the liquid-state sequences, 13C{1H} INEPT and DEPT without 1H−1H homonuclear decoupling but under fast MAS conditions, together with solid-state 13C{1H} cross-polarization CP/MAS analysis. This combination of techniques allowed us to distinguish two different phases of the semicrystalline materials, viz. rigid and flexible sections of the analyzed samples. Thus, we have exploited both types of NMR couplings and Figures 4 and 5 present the recorded spectra using the assignment shown in Scheme 1. For the homopolymer PPDL (Figure 4a and Table 1, entry 1), the intense signals of the methylene segments are present at ∼31 ppm, while the macromolecular fragments closer to the ester group, C1 and C14, appear at higher frequencies. The signal deriving from carbonyl groups of PPDL, centered at ∼173 ppm, reveals both a sharp (∼172.5 ppm) and a broader underlying 13C resonance, demonstrating significant heterogeneity in the sample caused by local changes in the bonding environments of the ester groups. Thus, the appearance of 13C signals from the carbonyl groups with both broad and narrow line widths illustrates the semicrystalline nature of the PPDL sample in line with the

monomer/catalyst molar ratio did not lead to a significant improvement of the polymers molecular weight (Table 1, entries 12 and 13), which strongly indicates that intramolecular transesterification affording low molecular weight cyclic structures is a prominent side reaction for this catalyst and might suggest that the product mixtures obtained with either 1 or 2 has not yet reached the thermodynamic equilibrium but still is a kinetic product mixture.1d The topology and end-group analysis of the synthesized homo- and copolyesters was determined by MALDI-ToF-MS spectroscopy. With a difference of 240 m/z between the consecutive isotope patterns, the isotope distribution in MALDI-ToF-MS for PPDL corresponded exclusively to cyclic structures (PDL)nNa+ (Figure S3). However, this technique is restricted to the relatively low molecular weight fraction of polymer samples and the ability of the polymer to desorb from the matrix.14 Hence, the cyclic species simply appear to be the most abundant ones in the recorded m/z segment. However, as end-group analysis by 1H NMR spectroscopy shows, the higher molecular weight fraction of the product consists of linear macromolecules. In agreement with the observation that PeDL did not show any tendency to undergo transesterification (vide supra), the highest intensity isotope distribution A in the MALDI-ToF-MS of PeDL-block-PPDL copolymers (Figure 3) corresponds to cyclic PPDL ((PDL)nNa+). Analysis of the isotope distributions B in Figure 3 revealed the presence of cyclic products containing both PDL and eDL units. For example, the isotope patterns at 1014.4 m/z corresponding to (eDL)3(PDL)2Na+ and the one at 1184.6 m/z corresponding to (eDL)4(PDL)2Na+ are clearly visible. This trend is represented by multiple B signals with a consecutive m/z difference of 170 or 240 and corresponding to eDL and PDL residues, respectively. However, isotope patterns from pure PeDL were not observed. It is therefore likely that the PeDLblock-PPDL copolymer has a somewhat tapered structure allowing the (intramolecular) transesterification to take place at a nonbranched ester function. Ring-opening polymerization of eCL and eDL afforded random copolymers where the low E

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Figure 4. 1D 13C{1H} CP/MAS NMR spectra of (a) PPDL, (b) PeCL-block-PPDL, and (c) PeDL-block-PPDL. (d) 13C{1H} INEPT MAS NMR spectrum of PeDL-block-PPDL. Note that the spectrum in (d) displays negative signals for methylene groups, while those from methine and methyl groups are positive. The 13C signals in (c) and (d) were assigned on the basis of the (e) 2D 13C{1H} DEPT-45 NMR correlation spectrum of PeDL-block-PPDL recorded at 11.75 T using a spinning frequency of 5.0 kHz. The assignment follows that of Scheme 1. All spectra were recorded at ambient conditions.

results from WAXS (vide inf ra, Figure 6). For the PeCL-blockPPDL copolymer with linear methylene segments of different lengths of the blocks the spectrum in Figure 4b similarly includes a broad underlying carbonyl component in addition to two sharper signals from the ester functionalities. Moreover, the PeCL-block-PPDL sample clearly displays both broad and wellresolved signals corresponding to the methylene groups (gray highlighted signals 16 and 20, Figure 4b) of the eCL part. These groups are covalently bonded to the ester groups and show that the semicrystallinity of PeCL-block-PPDL originates from both the eCL and PPDL part of the sample. The 13C{1H} CP/MAS NMR spectra of the copolyesters formed by incorporation of the butyl-branched eDL are shown in Figure 4c,d, where the assignment has been performed on the basis of a 2D 13C{1H} DEPT-45 spectrum (Figure 4e). Compared to the linear PeCL-block-PPDL sample, the 13C{1H} CP/MAS spectrum of PeDL-block-PPDL in Figure 4c include relatively sharp resonances related to the eDL part and narrow and low intensity signals from the butyl group. From the 13C{1H} INEPT spectrum in Figure 4d it is evident that the flexible methylene segments are well resolved and located at ∼27 ppm (marked by the black arrow). This demonstrates that PeDLblock-PPDL contains semirigid regions related to both the linear and butyl-branched polymer chains, but also highly flexible regions of both. From our previous studies7 we also learned that an enhanced flexibility of the copolymers in part is arising from the presence of small quantities of macrocyclic

Figure 5. 2D 1H−1H DQ-SQ correlation spectra of (a) poly(eCL-coPDL) and (b) PeDL-block-PPDL recorded at 16.45 T using a spinning frequency of 25.0 kHz. Both spectra were recorded at ambient conditions and employed the broadband back-to-back (BaBa)12 sequence for dipolar recoupling of 8 and 128 rotor periods, respectively. The insets in (a) and (b) show the end-groups corresponding to benzyl alcohol used as an initiator of cROP. The blue and red labeled correlations in (b) mark the connectivities for the PPDL and eDL fragments, respectively.

structures, formed via intramolecular transesterification of the polyesters. To further characterize the flexible and blocky nature of the samples we have recorded 1H−1H DQ-SQ correlation spectra as shown in Figure 5a,b for the samples poly(eCL-co-PDL) (Table 1, entry 2) and PeDL-block-PPDL (Table 1, entry 5). These spectra were recorded using the newly introduced broadband back-to-back (BaBa) sequence capable of observing weak 1H−1H correlations caused by motional averaging.15 For F

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Macromolecules

X-ray measurements have been performed for materials catalyzed by complex 1 only, both for as-synthesized and nonisothermal recrystallized (in DSC; heating/cooling rate: 10 °C min−1) samples. The as-synthesized and recrystallized materials reveal identical crystallographic structuressimilar value of degree of crystallinity (WAXS) and long period (SAXS)hence, only the detailed results obtained for the recrystallized materials will be presented further. The degrees of crystallinity of the analyzed materials were estimated using WAXS profiles, according to the ratio between the surfaces of peaks and the amorphous halo, corresponding to crystalline and amorphous fractions. Figure 6 presents selected WAXS diffraction profiles. In the case of the PPDL homopolymer we can observe three characteristic signals coming from the crystallographic planes 001, 110, and 200. Therefore, the PDL homopolymer crystallizes into a pseudo-orthorhombic form in a manner corresponding to PCL or polyethylene. The degree of crystallinity of the PDL homopolymer amounts to 47%. The diffraction profile of the PeCL-block-PPDL copolymer (entry 3) contains signals originating from the same population of crystallographic planes. The location of the signals is similar to the location of analogueical signals in the PPDL homopolymer. It may indicate the lack of cocrystallization phenomenon, which manifests itself with a change of interplanar spacing (displacement of the signals coming from the 110 or 200 planes toward higher or lower values of 2theta) or loss of regularity along the chain (loss of the signal coming from the 001 plane). During the crystallization of the copolymer two populations of crystals containing PDL or eCL fragments are created. The degree of crystallinity of the copolymer amounts to 55%. Such an effect, higher value of degree of crystallinity of the copolymer in comparison to the homopolymer, was observed earlier in random poly(eCL-co-PDL) copolymers at relatively small content of eCL units.16 Also, PeDL-block-PPDL copolymers are characterized by similar crystallographic structures, analogous to PPDL. It is confirmed by the presence and location of three characteristic signals coming from the same population of crystallographic planes. The degree of crystallinity of particular PeDL-block-PPDL copolymers is dependent on their composition and amounts respectively to Table 1, entry 4 = 20%, Table 1, entry 5 = 21%, and Table 1, entry 6 = 8%. Introducing PeDL block sequences seems to substantially diminish the ability of PDL fragments to crystallize despite the fact that PeDL homopolymer is an amorphous material. However, the presented values of the degrees of crystallinity for particular copolymers do not include the actual content of PDL comonomer. Taking the molar mass of the monomers (PDL: 254 g mol−1; eDL: 184 g mol−1) into consideration, the molar ratio of monomers used for the synthesis and the degree of conversion may easily establish the actual weight fraction of the PDL and eDL in the polymers. The PDL weight fraction in the analyzed copolymers amounts to entry 4 = 44%, entry 5 = 47%, and entry 6 = 21%. Taking the weight fraction of the PDL comonomer and the degree of crystallinity obtained as a result of the WAXS profiles analysis into account, the actual degree of crystallinity of the PPDL blocks in the examined copolymers amounts to entry 4 = 45%, entry 5 = 45%, and entry 6 = 40%. Therefore, we can see that the presence of the eDL comonomer actually does not have a substantial impact on the crystallization of the fragments containing PDL units (entries 4 and 5)the total degree of crystallinity of the sample is lower, but the degree of crystallinity of the PPDL block in the block copolymer is not changed. Only at relatively

Figure 6. WAXS profiles of analyzed polyesters. The curves have been displaced along ordinate for better visualization and comparison.

the poly(eCL-co-PDL) sample, the 2D 1H−1H DQ-SQ spectrum show relative narrow 1H signals, and it includes several characteristic 1H−1H cross-correlation signals (marked by horizontal dashed lines) between the methylene groups next to the ester functionalities (1, 16) and those of the methylene segments located in between subsequent ester groups on either side. These observations prove that the eCl−PDL copolymer is indeed a poly(eCL-co-PDL) random copolymer. The 2D 1 H−1H DQ-SQ spectrum for PeDL-block-PPDL shown in Figure 5b display even narrower 1H line widths when compared to the random poly(eCL-co-PDL) copolymer sample in Figure 5a. This includes intense and sharp 1H−1H cross-correlation signals between the branched fragments (22) and the short methylene segment (23−26) of the eDL fragments (marked in red). Interestingly, the PPDL part only display a few resolved 1 H−1H cross-correlation signals (marked in blue) with no clear correlations to the eDL fragment. Thus, the result presented in Figure 5b for PeDL-block-PPDL supports the earlier statements that this sample is a block copolymer sample. Moreover, PeDLblock-PPDL appears to include more flexible polymer chains that surprisingly is related to the cyclic PPDL fragments and not eDL as one might anticipate based on the branched butyl group. To evaluate the effect of the distance between ester groups in the polyester backbone and the influence of butyl branches on thermal properties of the polymers, DSC analysis was performed. As evidenced by DSC thermograms (Figure S5), the PeCL-block-PPDL (Table 1, entry 3) displays two wellresolved melting point at 55 and 95 °C, which can be easily attributed to PeCL and PPDL domains. This observation proves that the crystalline domains in the block copolyesters are well separated which supports our investigation concerning formation of block copolymers. Subsequently, the PeDL-blockPPDL (entries 4−6) copolymer reveals a remarkable decay of the PPDL melting temperature and enthalpy of the transition in comparison to pure PPDL. As nicely observed from solid-state NMR and WAXS analysis, butyl branches, although not randomly distributed along the copolymers backbone, significantly influence the ordered structure of PPDL. This supports the above-mentioned assumption based on MALDI-ToF-MS that the polymer is not a pure block but contains a tapered structure formed once most of the eDL had been consumed and before the PDL homopolymerization took place. G

DOI: 10.1021/ma502262e Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

of ε-decalactone. Since low molecular weight PPDL still has a Tm close to or above 90 °C, the observed drop in Tm is clearly an effect of the amorphous PeDL block attached to the PPDL, hampering the crystallization of the PPDL block. The presence of cyclic structures in the final product was detected by MALDI-ToF-MS spectroscopy while their influence on supramolecular arrangement, enhanced mobility, and crystallization ability of the materials was analyzed using a combination of 13 C{1H} CP/MAS NMR with 13C{1H} INEPT spectra. WAXS analysis of the PPDL-block-PCL copolymers exhibited that during the crystallization of the copolymer two populations of crystals containing PDL or eCL fragments are created. The WAXS profiles for different PeDL-block-PPDL samples demonstrated that the degree of crystallinity of the copolymer is lower while the overall degree of crystallinity of the PPDL blocks is not changed.

high content of the eDL comonomer (entry 6) a significant though still relatively small effect on the degree of crystallinity of the PPDL block in the block copolymers can be observed. For selected materials the SAXS patterns have also been recorded. Only in the case of the PPDL homopolymer and the PeCL-block-PPDL block copolymer, the presence of regular lamellar structures was observed. The value of the long period of the lamellar system for both materials was calculated using Bragg’s law: PPDL: 19.0 nm;PeCL-block-PPDL: 15.2 nm. The long period designated for the PeCL-block-PPDL copolymer is clearly lower than for the PDL homopolymer. According to the literature data,17 the value of the long period for the eCL homopolymer ranges between 11 and 12 nm. Figure 7 presents

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ASSOCIATED CONTENT

S Supporting Information *

Liquid-state and solid-state NMR spectra, MALDI-ToF-MS analysis, and DSC analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. Transmission electron micrographs of PeCL-block-PPDL copolymer lamellae.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (R.D.). *E-mail [email protected] (L.J.-W.).

the TEM micrographs collected for a stained sample of the PeCL-block-PPDL copolymer (Table 1, entries 1 and 3−6). It can clearly been seen that during crystallization only one fraction of regular lamellae (similar like usually observed in a case of polyolefins) was created with the average thickness in the range of 6.5−8.5 nm. As elucidated from X-ray analysis, the PDL homopolymer crystallizes into a pseudo-orthorhombic form in a manner corresponding to PCL or polyethylene. Moreover, the degree of crystallinity of the copolymers depends on their composition while the eDL units do not have a significant influence on the crystallization of PDL blocks.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from SABIC is gratefully acknowledged. REFERENCES

(1) (a) van der Meulen, I.; Gubbels, E.; Huijser, S.; Koning, C. E.; Heise, A.; Duchateau, R. Macromolecules 2011, 44, 4301−4305. (b) Bouyahyi, M.; Pepels, M. P. F.; Heise, A.; Duchateau, R. Macromolecules 2012, 45, 3356−3366. (c) Martin, E.; Dubois, P.; Jerome, R. Macromolecules 2003, 36, 5934−5941. (c) Dove, A. P. Chem. Commun. 2008, 6446−6470. (d) Bouyahyi, M.; Duchateau, R. Macromolecules 2014, 47, 517−524. (e) Pepels, M. P. F.; Souljé, P.; Peters, R.; Duchateau, R. Macromolecules 2014, 47, 5542−5550. (2) (a) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813−5840. (b) Lebedev, B.; Yevstropov, A. Makromol. Chem. 1984, 185, 1235− 1253. (c) Nakayama, Y.; Watanabe, N.; Kusaba, K.; Sasaki, K.; Cai, Z.; Shiono, T.; Tsutsumi, C. J. Appl. Polym. Sci. 2011, 121, 2098−2103. (d) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Macromol. Chem. Phys. 2000, 201, 1329−1333. (3) (a) Matsumura, S.; Mabuchi, K.; Toshima, K. Macromol. Rapid Commun. 1997, 18, 477−482. (b) Uyama, H.; Suda, S.; Kikuchi, H.; Kobayashi, S. Chem. Lett. 1997, 11, 1109−1110. (c) Kobayashi, S.; Takeya, K.; Suda, S.; Uyama, H. Macromol. Chem. Phys. 1998, 199, 1729−1736. (d) Kumar, A.; Kalra, B.; Dekhterman, A.; Gross, R. A. Macromolecules 2000, 33, 6303−6309. (4) Lebedev, B.; Yevstropov, A. Makromol. Chem. 1984, 185, 1235− 1253. (5) Zhong, Z.; Dijkstra, P. J.; Feijen. J. Macromol. Chem., Phys. 2000, 201, 1329−1333. (6) (a) Jedliriski, Z.; Juzwa, M.; Adarnus, G.; Kowalczuk, M. Macromol. Chem. Phys. 1996, 197, 2923−2929. (b) Wilson, J. A.; Hopkins, S. A.; Wright, P. M.; Dove, A. P. Polym. Chem. 2014, 5, 2691−2694.

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CONCLUSIONS Catalytic ring-opening polymerization of lactones, viz. εcaprolactone and ε-decalactone, with ω-pentadecalatone mediated by Zn- and Ca-containing tridentate Schiff base complexes and benzyl alcohol as an initiator provided a variety of linear and branched copolyesters. Liquid-state NMR spectroscopy has been successfully used to determine the blockiness versus random microstructure of the copolymers, the origin of which was unambiguously illustrated by DFT calculations in our previous studies.7 Our calculations confirmed that the copolymerization of eCL with eDL and eCL with PDL leads to random copolymers while a high-energy barrier for insertion of PDL into a branched alkoxide during eDL + PDL copolymerization results in block copolymers. Further, the mobility of macromolecules and their thermal properties are significantly affected by the local microstructure of the materials, the presence of branches in the polyesters backbone, and macrocyclic structures formed by transesterification. With an increasing block length of the amorphous poly(ε-decalactone) fragment in PeDL-block-PPDL their melting point decreased from 93 °C for PPDL homopolymer down to around 70 °C for the copolymer containing 70 mol % H

DOI: 10.1021/ma502262e Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (7) Jasinska-Walc, L.; Hansen, M. R.; Dudenko, D.; Rozanski, A.; Bouyahyi, M.; Wagner, M.; Graf, R.; Duchateau, R. Polym. Chem. 2014, 5, 3306−3310. (8) (a) Gitsas, A.; Floudas, G.; Mondeshki, M.; Lieberwirth, I.; Spiess, H. W.; Iatrou, H.; Hadjichristidis, N.; Hirao, A. Macromolecules 2010, 43, 1874−1881. (b) Schreiber, R.; Veeman, W. S.; Gabrielse, W.; Arnauts, J. Macromolecules 1999, 32, 4647−4657. (c) Wendoloski, J. J.; Gardner, K. H.; Hirschinger, J.; Miura, H.; English, A. D. Science 1990, 247, 431−436. (d) Vanderhart, D. L.; Earl, W.; Garroway, A. J. Magn. Reson. 1981, 44, 361−401. (e) Vanderhart, D. L. J. Magn. Reson. 1981, 44, 117−126. (f) Spiess, H. W. Macromol. Symp. 2010, 298, 10− 16. (9) (a) Dudenko, D.; Kiersnowski, A.; Shu, J.; Pisula, W.; Sebastiani, D.; Spiess, H. W.; Hansen, M. R. Angew. Chem., Int. Ed. 2012, 51, 11068−11072. (b) Morris, G.; Freeman, R. J. Am. Chem. Soc. 1979, 101, 760−762. (c) Burum, D. P.; Ernst, R. R. J. Magn. Reson. 1980, 39, 163−168. (d) Brown, S. P. Macromol. Rapid Commun. 2009, 30, 688− 716. (e) Sardo, M.; Siegel, R.; Santos, S. M.; Rocha, J.; Gomes, J. R. B.; Mafra, L. J. Phys. Chem. A 2012, 116, 6711−6719. (10) (a) Chen, H. Y.; Tang, H. Y.; Lin, C. C. Macromolecules 2006, 39, 3745−3752. (b) Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh, N. Macromolecules 2008, 41, 3493−3502. (11) (a) Morris, G.; Freeman, R. J. Am. Chem. Soc. 1979, 101, 760− 762. (b) Burum, D. P.; Ernst, R. R. J. Magn. Reson. 1980, 39, 163−168. (c) Doddrell, D. M.; Pegg, D. T.; Bendall, M. R. J. Magn. Reson. 1982, 48, 323−327. (12) Saalwachter, K.; Lange, F.; Matyjaszewski, K.; Huang, C.-F.; Graf, R. J. Magn. Reson. 2011, 212, 204−215. (13) Rabiej, M. J. Appl. Crystallogr. 2014, 47, 1502−1511. (14) Huijser, S.; Staal, B. B. P.; Huang, J.; Duchateau, R.; Koning, C. E. Angew. Chem., Int. Ed. 2006, 45, 4104−4108. (15) (a) Saalwächter, K. ChemPhysChem 2013, 14, 3000−3014. (b) Hansen, M. R.; Graf, R.; Spiess, H. W. Acc. Chem. Res. 2013, 46, 1996−2007. (16) Ceccorulli, G.; Scandola, M.; Kumar, A.; Kalra, B.; Gross, R. A. Biomacromolecules 2005, 6, 902−907. (17) (a) Lefevre, C.; Villers, D.; Koch, M. H. J.; David, C. Polymer 2001, 42, 8769−8777. (b) Neppalli, R.; Causin, V.; Marega, C.; Saini, R.; Mba, M.; Marigo, A. Polym. Eng. Sci. 2011, 51, 1489−1496.

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DOI: 10.1021/ma502262e Macromolecules XXXX, XXX, XXX−XXX