Mononuclear zinc(II) Schiff base complexes as catalysts for the ring-opening polymerization of lactide

Journal Pre-proofs Mononuclear Zinc(II) Schiff Base Complexes as Catalysts for the Ring-Opening Polymerization of Lactide Martin Fuchs, Sebastian Schmitz, Pascal M. Schäfer, Tim Secker, Angela Metz, Agnieszka N. Ksiazkiewicz, Andrij Pich, Paul Kögerler, Kirill Y. Monakhov, Sonja Herres-Pawlis PII: DOI: Reference:

S0014-3057(19)31716-1 https://doi.org/10.1016/j.eurpolymj.2019.109302 EPJ 109302

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

21 August 2019 7 October 2019 10 October 2019

Please cite this article as: Fuchs, M., Schmitz, S., Schäfer, P.M., Secker, T., Metz, A., Ksiazkiewicz, A.N., Pich, A., Kögerler, P., Monakhov, K.Y., Herres-Pawlis, S., Mononuclear Zinc(II) Schiff Base Complexes as Catalysts for the Ring-Opening Polymerization of Lactide, European Polymer Journal (2019), doi: https://doi.org/10.1016/ j.eurpolymj.2019.109302

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Mononuclear Zinc(II) Schiff Base Complexes as Catalysts for the Ring-Opening Polymerization of Lactide Martin Fuchs,[a] Sebastian Schmitz,[a, b] Pascal M. Schäfer,[a] Tim Secker,[a] Angela Metz,[a] Agnieszka N. Ksiazkiewicz,[c, d, e] Andrij Pich,[c, d, e] Paul Kögerler,[a, f] Kirill Y. Monakhov[b] and Sonja Herres-Pawlis[a]*

a

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen,

Germany. b

Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, 04318 Leipzig,

Germany. Institute of Technical and Macromolecular Chemistry RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany. c

Leibniz-Institute for Interactive Materials (DWI), Forckenbeckstraße 50, 52074 Aachen, Germany. d

Aachen Maastricht Institute for Biobased Materials (AMIBM), Maastricht University, Brightlands Chemelot Campus, Urmonderbaan 22, 6167 RD Geleen, The Netherlands. e

f

Jülich-Aachen Research Alliance (JARA-FIT) and Peter Grünberg Institute (PGI-6),

Forschungszentrum Jülich, 52425 Jülich, Germany.

Keywords: ring opening polymerization, lactide polymerization, Schiff base ligands, zinc catalysts

Abstract Materials combining bio-based resource and degradability aspects such as polylactide (PLA) can represent a sustainable alternative to commercially used oil-based plastics. With tin octanoate (Sn(Oct)2) as the catalyst of choice for industrial scale PLA production, traces of the toxic heavy metal can be released into the environment, rendering PLA unsuitable for specific applications. Herein we present four new homoleptic zinc Schiff base complexes that offer facile synthesis and can be handled under aerobic conditions. These robust, tetra-coordinated complexes have been tested as catalysts for the ring-opening polymerization (ROP) of lactide and are well suited under industrially relevant conditions. In situ Raman spectroscopy has been used to determine the apparent reaction rate constant (kapp) for all complexes at 150 °C in bulk. For the fastest complex a kp of 3.66 ± 0.14 x10–2 L mol–1 s–1 was additionally determined. For this catalyst, 71% conversion and a number-averaged molar mass of above 90000 g mol–1 has been reached within less than 30 minutes at a monomer-to-initiator ratio ([M]/[I]) of 1000:1.

The good polydispersity (Ð = 1.7) indicates a controllable ROP via the coordination-insertion mechanism (CIM).

Introduction Synthetic polymer materials have developed into ubiquitous materials of everyday life, resulting in an annual growth rate of 8.4% [1]. Today, most of the commercially used plastics are based on crude oil and are not biodegradable [2]. As the majority of these oil-based plastics, e.g. low density polyethylene (LDPE), is used in single-use items there has been an ongoing and drastic increase in non-degradable plastic waste dumped into landfills and the marine environment [1, 3]. Along with these problems in plastic waste management, a suitable and widely accessible alternative resource for an everyday plastic has to be used, as crude oil resources decline [4]. Bioplastics, combining both natural resource-derived precursors and biodegradability, have become of special interest in industry and academic research over the last years. Especially polylactic acid (PLA) has become more popular as biodegradable and bio-based polyester, reaching a total annual production of around 200000 tons [5]. PLA is used as casing material, in single-use cups and cutlery, or as surgical suture material, but most of all as packaging material [6]. The polymer is accessible via direct polycondensation of lactic acid, derived from corn, or via catalytic ring-opening polymerization (ROP) of lactide, the cyclic dimer of lactic acid [5b, 7]. In industry, ROP using metal-based catalysts is common, as the proceeding coordination-insertion mechanism allows for a polymer with controllable tacticity and high molar mass [5b]. Tin octanoate (Sn(Oct)2) is used as catalyst for the industrial ROP of lactide in bulk [8]. Therefore, PLA is not suitable for some applications, as traces of tin might be released into the environment. To avoid this toxic heavy metal there has been great interest in the development of new catalysts comprising non-toxic metals like Fe [9], Y [10], Hf [11], Zr [11a, 11b, 12], Mg [13], or Al [11c, 14]. In particular, zinc-based catalysts have emerged over the last years and enable a fast polymerization of lactide [13a, 15]. Williams and co-workers developed a bimetallic zinc amido HMDS complex for the polymerization of rac-lactide in THF at room temperature. With this highly active zinc catalyst, a monomer conversion of 73 % is achieved in 30 s reaching a turnover number of 45000 h–1 and a Ð of 1.30 indicating a living character for the ROP with a catalyst loading of 0.1 mol%. The catalyst loading has been decreased to the conditions of an immortal polymerization [16]. For this catalyst, as well as the majority of the fastest zinc-based catalysts, known in literature, anionic ligands are used [13a, 15h, 15m, 17]. Therefore, these catalysts are sensitive to moisture and other impurities in the used lactide. As most of these catalysts are applied for ROP in solution there is a gap in literature for robust, non-toxic, zinc-based catalysts for the ROP of lactide under industrial relevant conditions in bulk [15g, 16-17]. Herres-Pawlis and co-workers have made great effort in lactide polymerization with fast and robust zinc complexes using neutral guanidine-based

ligands [15b, 15c, 15f, 15g, 18]. Recently, we presented highly active guanidine-based zinc catalysts for the ROP of technical grade lactide in bulk, with a kp of 6.10 x 10–2 L mol–1 s–1 at 150 °C. At a monomer-to-initiator ratio ([M]/[I]) of 500:1 a conversion of 69% of technical grade rac-lactide has been achieved within 30 min. With a polydispersity of 1.30 the resulting PLA indicates a living polymerization [19]. Herein we present four new robust zinc complexes for the polymerization of lactide under industrially relevant conditions at 150 °C in bulk. The used anionic Schiff base ligands (Fig. 1) are highly stable against air, moisture and other impurities of lactide and can be synthesized under aerobic conditions. Therefore, our new complexes combine both, a non-toxic and active zinc center and robust anionic ligands. The facile synthesis of these complexes under aerobic conditions, as well as their excellent thermal stability under nitrogen and in air, makes them well suited for lactide polymerization under industrially relevant conditions. S

S

N

N

HO

HO

O

HL1

HL2

F S

N HO HL3

N HO HL4

Fig. 1: Schiff base ligands HL1 – HL4, used for the synthesis of four new zinc Schiff base complexes.

RESULTS AND DISCUSSION Synthesis and stability The facile one-pot synthesis under aerobic refluxing conditions of different Schiff base ligands together with simple zinc(II) salts such as zinc(II) perchlorate hexahydrate (Zn(ClO4)2·6H2O) and zinc(II) chloride (ZnCl2) in the presence of the triethylamine (Et3N) base in organic solvents yields mononuclear complexes with the general formula [Zn(L)2] where L = mono-deprotonated Schiff base (Scheme 1).

O

R1

reflux in EtOH

OH +

 H 2O

R2

N

R1

R3

OH HL

C1: [Zn(L1)2]

R2

reflux in organic solvent + Et3N

R3

+ Zn(ClO4)2·6H 2O (C1, C3 and C4)

C3: [Zn(L3)2]

+ ZnCl2 (C2)

C4: [Zn(L4)2] ·EtOH

H 2N

C2: [Zn(L2)2]

HL1: R1 = R2 = H, R3 = SCH3 HL2: R1 = OCH3, R2 = H, R3 = SCH3 HL3: R1 = H, R2 = SCH3, R3 = H HL4: R1 = R2 = H, R3 = F Scheme 1: Schematic representation of the synthesis of mononuclear [Zn(L)2] complexes C1 – C4.

The molecular structures of the resulting complexes C1 – C4 are shown in Fig. 2.

Fig. 2: Molecular structures of the prepared zinc Schiff base complexes. Colour code: Zn, tan; C, grey; F, green; N, blue; O, red and S, yellow. H atoms and solvent molecules are omitted for clarity.

All four complexes are stable against moisture and they are highly thermally stable up to >230 °C. Selected bond lengths and angles of the complexes are listed in Table 1. Table 1: Selected bond lengths and angles of complexes C1 – C4.

complex

C1

C2

C3

C4

Bond lengths in Å Zn–N Zn–O

2.002(2) 2.008(2) 2.029(2) 1.997(2) 2.011(2) 2.008(2) 2.029(2) 2.010(2) 1.918(2) 1.910(2) 1.913(2) 1.915(2) 1.924(2) 1.908(2) 1.913(2) 1.908(2) Angles in °

N–Zn–O

113.3(1) 111.6(1) 121.3(1) 117.7(1) 111.8(1) 111.5(1) 121.3(1) 117.8(1)

N–Zn–N

122.8(1) 121.4(1) 108.0(1) 112.5(1)

O–Zn–O

115.6(1)

τ4 value [20]

0.86

117.5(1) 115.1(1) 117.2(1) 0.86

0.83

0.88

In all four complexes, Zn resides in a distorted tetrahedral environment, binding two of the corresponding Schiff base ligands via the imine nitrogen atom and by the oxygen atom of the hydroxyl group in L1 – L4. Zn–O bond lengths in C1 – C4 are virtually identical (within the experimental error margins). With 2.029 Å, the Zn–N bond in C3 slightly exceeds that of the other complexes. C3 also shows the largest angle between the coordinating atoms of the Schiff base ligand and the zinc atom. Within the error limits, the bite angles are in the same range for the other complexes. Taking into account the τ4 value, all complexes display a slightly distorted tetragonal coordination. All four complexes are highly stable against thermal decomposition under air and nitrogen atmosphere (> 300°C in both cases). Although the thermogravimetric curves of complexes C1 – C4 are similar (see Fig. S10 – Fig. S16 in the Supporting Information), small differences can be detected. The methylthioether-functionalized complexes C1 – C3 show a high thermal stability, as it is expected from literature-reported 3d-thioether-Schiff base-complexes [21]. Under air the decomposition of these complexes starts in a range of 275 – 300 °C. Here the complexes decompose in two differently sized steps. Under nitrogen atmosphere the decompostion of the complexes starts at 270 – 300 °C. For all complexes the decomposition takes place in a sharp step, followed by a sinking plateau. Therefore all four complexes are suitable catalysts for the ROP of lactide under industrially relevant conditions in terms of their thermal stability [8c].

Catalytic activity The catalytic activity of C1 – C4 in the ROP of recrystallized ʟ-lactide has been tested under industrial relevant conditions at 150 °C in bulk without an additional co-initiator. In situ Raman spectroscopy was used to determine the kinetics of the polymerisation. All four complexes are active catalysts in the ROP of lactide. To determine the fastest of these catalysts, recrystallized ʟ-lactide was used to ensure a consistent quality of the monomer and avoid side effects caused by residual water or acid in the used lactide. For all catalysts the measurements were performed for 1 h. To validate the obtained results, the meassurements were performed in again in a shorter run (see Table S1 in the supporting information) An averaged apparent rate

constant kapp was determined for the ROP of ʟ-Lactide using C1 – C4 (Table 3), out of two runs (see Table S2 in the supporting information). Table 2: Catalytic activity of C1 – C4 in the ROP of recrystallised ʟ-Lactide in bulk.a

catalyst C1 C2 C3 C4

Conversionb (%) 66 56 60 54

Mn, calcc (g mol–1) 95000 80600 86000 77700

Mnd (g mol–1) 48000 35000 58000 64000

PD 1.6 1.7 1.3 1.3

a: recrystallized ʟ-lactide was polymerized with a [M]/[I]-ratio of 1000:1 and tracked via in situ Raman spectroscopy in a steel reactor at 150 °C using a stirrer velocity of 260 rpm, for 1 h. b: final conversion determined via 1H-NMR spectroscopy, c: by [M]/[I] x M(LA), d: via GPC, measured against a polystyrene standard and corrected according to literature [22]. Using a [M]/[I]-ratio of 1000:1, all catalysts show a good activity in the ROP of lactide. in 1 h. Here molar masses between 48000 g mol-1 and 64000 g mol-1 were reached. Table 3: Averaged apparent reaction rate constant (kapp) for the polymerisation of recrystallized ʟ-lactide using C1 – C4 with a [M]/[I]-ratio of 1000:1 determined via semilogarithmic plot of lactide conversion vs. time

Catalyst kapp x 10–4 (s–1) C1 2.63 C2 2.60 C3 1.36 C4 4.44

The determined apparent reaction rates are in a similar range, with values varying between kapp = 1.36 s–1 (C3) and kapp = 4.44 s–1 (C4). Regarding the apparent reaction rate the fluorinesubstituted catalysts C4 showed the highest catalytic activity, reaching a monomer conversion of 54% in 1 h. Taking into account the obtained molar masses, the catalysts are relevant for the industrial use, since industry calls for a polylactide with high molar masses [8c]. As demonstrated previously, an increased Lewis acidity at the zinc center leads to a rise in the catalytic activity of zinc complexes in lactide ROP [15f]. This is the case for C4, as the thioether in the backbone of the ligand was substituted by a fluorine atom, increasing the electron pulling effect of the ligand L4. In all cases, the theoretical molar masses of the polymer are slightly higher than the experimental molar masses. Therefore ROP is not proceeding in an ideal controlled way (see Table S1 in the Supporting Information). Regarding the polymerization in bulk, it can thus be classified a living polymerization, taking into account the predictable molar masses. Considering Ð, a living character of the ROP of ʟ-lactide in bulk is indicated.

To prove the catalyst´s relevance for industrial lactide polymerization, the fastest catalyst C4 was used for the ROP of technical grade ʟ-lactide at 150 °C. Different [M]/[I]-ratios were tested to determine the rate constant kp of the polymerization (Table 3). Table 4: Catalytic activity of C4 in the ROP of technical grade ʟ-Lactide under industrial relevant conditions in bulk.a

Runb

[M]/[I]

time (min)

Conversionc (%)

Mn, calc.d (g mol–1)

Mne (g mol–1)

PDe

A B C D E

1000:1 1250:1 1500:1 1750:1 2000:1

73 45 39 51 99

46 20 13 14 17

65880 36720 28944 35280 48528

68000 -

1.4 -

kappf (10–4 s– 1) 16.5 5.36 1.02 3.96 2.69

a: Technical grade ʟ-lactide, tracked via in situ Raman spectroscopy in a steel reactor at 150 °C using a stirrer velocity of 260 rpm. b: The average was formed for runs A and E c: final conversion determined via 1H-NMR spectroscopy, d: by [M]/[I] x M(LA), e: via GPC, measured against a polystyrene standard and corrected according to literature [22] f: Via semilogarithmic plot of lactide conversion vs. time Via a semilogarithmic plot of kapp against the concentration of C4 (Fig. 4), a rate constant of kp = 3.66 ± 0.14 x10–2 L mol–1 s–1 has been determined for the polymerisation of technical grade ʟ-lactide, using C4 under industrial relevant conditions in bulk.

Fig. 3: Semilogarithmic plot of kapp vs. [C4]

Herres-Pawlis and co-workers used the same conditions to characterize the activity of robust Zn and Fe guanidine catalysts, as well as Sn(Oct)2, for the polymerization of lactide [9a, 15c, 19]. Regarding different substituents on the guanidine backbone, these highly active catalysts can reach a rate constant between kp = 9.48 x10–3 L mol–1 s–1 and 6.10 x 10–2 L mol s–1 (Zn) or

kp = 9.20 x10–2 L mol–1 s–1 (Fe). For Sn(Oct)2 a rate constant of kp = 8.40 x10–2 L mol–1 s–1 could be determined. Hence C4 ranges in a comparable activity span in lactide ROP, compared to the zinc-guanidine catalysts. Combined with its facile synthesis and high stability under aerobic conditions C4 is a well-suited catalyst for lactide polymerization under industrial conditions. A dispersity of 1.4 indicates a living character of the ROP of technical grade ʟ-lactide under industrial relevant conditions in melt. To further investigate the proceeding mechanism, MALDI-TOF MS has been used to characterize the end groups of the obtained polymer chain. To ensure the polymerization of short PLA chains, BnOH was added as a co-initiator and a [M]/[BnOH]/[I]-ratio of 100:1:1 was used. As expected, BnOH is found as an endgroup of the polymer chains, as well as the complex C4 and the anionic ligand L4. Here 2.5 % of these chains have BnOH as an endgroup, 4.6 % the complex C4 and 6.8 % the ligand L4. This indicates that the ROP proceeds via a coordination-insertion mechanism. As the chain growth is started by L4, C4 might act as a well-defined catalyst. For all complexes, the use of recrystallized ʟ-lactide as well as of technical grade ʟ-lactide leads to slightly smaller molar masses than the theoretical molar masses which might be caused by chain-starts initiated by the ligand and by BnOH. The polydispersity is slightly enlarged by transesterification effects due to the higher polymerization temperature in melt. However, the achieved high molar masses make the complexes highly relevant for the use in industrial lactide polymerization.

Conclusion In summary, we demonstrated the facile synthesis of four new Schiff base ligands and their zinc complexes, which are highly stable against air and moisture. All complexes are active catalysts in the ROP of recrystallized ʟ-lactide under industrial relevant conditions at 150 °C in bulk. A conversion of around 50% has been reached in less than 1 hour for all catalysts. A molar mass of above 90000 g mol–1 (Ð = 1.8) was obtained using C4. In situ Raman spectroscopy was used to evaluate the apparent reaction rates of the complexes for the ROP of recrystallised ʟ-lactide using a monomer-to-initiator ratio of 1000:1 at the afore-mentioned polymerization conditions. For the fastest catalysts C4, a reaction rate of kp = 3.66 ± 0.14 x10– 2

L mol–1 s–1 has been determined, ranging C4 in a comparable activity span in lactide ROP, to

the robust and active zinc guanidine catalysts [15c, 19]. MALDI-TOF MS was used to investigate the end groups of the polymer chain, classifying the mechanism of the ROP as a coordination-insertion-mechanism, having a slightly diminished control over the molar mass of

the polymer chain. With a non-toxic metal center and robust anionic ligands, the complexes are well suited catalysts in the ROP of lactide under industrial relevant conditions.

Acknowledgment The authors wish to thank Total Corbion PLA for lactide donations and Monika Paul for the GPC measurements. Pascal M. Schäfer thanks the Hanns-Seidel-Foundation (fellowship) for funding (Bundesministerium für Bildung und Forschung, BMBF).

Experimental Materials and methods Catalysts C1 – C4 were synthesized under aerobic conditions. The Schiff base ligands HL1, HL2 and HL4 were freshly prepared following the procedure described in literature [21b, 23]. HL3 was synthesized using a similar procedure. All other starting materials were from commercial sources and used as received. Solvents were used without further purification. To determine the crystal structures of C1 – C4 Suitable crystals were selected and mounted on a Bruker APEX CCD diffractometer. The crystal was kept at 273 K during data collection if not indicated otherwise. Using Olex2 [24], the structure was solved with the ShelXT [25] structure solution program using Intrinsic Phasing and refined with the ShelXL [26] refinement package using least-squares minimization. Displacement parameters of carbon connected hydrogen atoms were fixed at 1.2 times the value of the carbon atom for methyl groups 1.5 was used. Aromatic and amide H atoms were refined using a riding model. Full crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary no. CCDC 1946940 for C1, CCDC 1946942 for C2, CCDC 1946939 for C3 and CCDC 1946941 for C4. The IR spectra of Ligand HL3 was recorded on a Bruker TENSOR II FTIR spectrometer using a KBr pellet (ca. 250 mg) in a range of 400-4000 cm-1. The IR spectra of complexes C1 – C4 were recorded on a Shimadzu IRTracer 100 using a CsI beam splitter in combination with an ATR unit (Quest model from Specac utilizing a robust monolithic crystalline diamond) in a resolution of 2 cm–1. Elemental Analysis of all compounds was performed on a Elementar varioEL, a Elementar varioEL cube or a Elementar Vario EL III. NMR spectra were recorded at room temperature on a Bruker Avance II (400 MHz) and a Bruker Avance III (400 MHz) or a Bruker AVANCE II+600. The NMR signals were calibrated to the residual signals of the deuterated solvent [δH(CDCl3) = 7.26 ppm, δC(CDCl3) = 77.16 ppm] Data for 1H NMR are reported as follows: chemical shift ( ppm) (multiplicity, coupling constants (Hz), integration).

Couplings are expressed by: s = singlet, d = doublet, m = multiplet or combinations thereof. 13C

NMR spectra are also expressed in parts per million (ppm) and reported as aforementioned.

Various 2D NMR experiments (COSY, HSQC, HMBC, DEPT135) were used to assign the 1Hand

13C-NMR

spectra. TG/DT analyses of C1 – C4 were performed under N2 and dry air flow

and a heating rate of 5 K min−1 in the temperature range of 25–800 °C on a Mettler Toledo TGA/SDTA 851e instrument. The APCI-MS spectra of complexes C1 – C4 were recorded on a Bruker maXis II mass spectrometer using a set capillary voltage of 4000 V with an end plate offset of -500 V. A nebulizer pressure of 2.0 bar was applied, using a temperature of 200 °C and a N2 flow rate of 4.0 mL min–1. For all samples, MeCN was used as a solvent. The average molar masses and the mass distributions of the obtained polylactide samples were determined by gel permeation chromatography (GPC) in THF as the mobile phase at a flow rate of 1 mL min–1, utilizing an Agilent 1100 Series HPLC, G1310A isocratic pump, an Agilent 1100 Series refractive index detector and 8 × 600 mm, 8 × 300 mm, 8 × 50 mm PSS SDV linear M columns. Conventional calibration was applied to evaluate the chromatographic results, using a correction factor of 0.58 as described in literature [22]. For MALDI-TOF mass spectrometry, a Bruker ultrafleXtreme equipped with a 337 nm Smartbeam laser in the reflective mode was used. THF solutions of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) (5 μL of a 20 mg mL–1 solution), sodium trifluoroacetate (0.1 μL of a 10 mg mL–1 solution), and analyte (5 μL of a 10 mg mL–1) were mixed and a droplet thereof applied on the sample target. Protein 1 calibration standard is the name of the protein mixture used for calibration. For spectra 4000 laser shots with 24 % laser power were collected. The laser repetition rate was 1000 Hz. The homopolymer analysis was performed using Polymerix software (Sierra Analytics). Raman spectra were measured with a RXN1 spectrometer of Kaiser Optical Systems. The laser was used at a wavelength of 785 nm and 450 mW through a probe head with sapphire lenses (d = 0.1 mm). Kinetic data were obtained by integration of the Raman spectrum with Peaxact 4.6 [27], boundaries were 627 – 713 cm–1 for lactide.

Synthesis of [Zn(L1)2] (C1) Freshly prepared 2-(((4-(methylthio)phenyl)imino)methyl)phenol (HL1) (0.108 g; 0.5 mmol) and Zn(ClO4)2·6H2O (0.093 g; 0.25 mmol) were dissolved in a 10 mL mixture of MeCN and MeOH with a volume ratio of 1:1 together with triethylamine (0.08 mL; 0.58 mmol). The resulting intense yellow solution was stirred under refluxing conditions for two hours, then it was filtered off and the filtrate was stored in a capped vial under ambient conditions. Yellow single crystals of complex C1 were obtained after one day. They were collected by filtration and washed with ice cold MeOH. Yield of the air-dried product: 0.087 g (63 %).

1H{13C}-NMR

(400 MHz, CDCl3): δ = 8.33 (s, 2Himine),

7.19 (dd, J = 8.0, 1.8 Hz, 2Harom.),

7.37 (td, J = 7.8, 6.8, 1.8 Hz, 2Harom..),

7.11 (m, 4Harom.),

7.02 (d, J = 8.5 Hz, 2Harom.),

6.93 (d, J = 8.5 Hz, 2Harom.), 6.66 (td, J = 8.1, 6.9, 1.2 Hz, 2Harom.), 2.43 (s, 6Hthioether) ppm. 13C-NMR

(101 MHz, CDCl3): δ = 171.4 (Carom.), 169.0 (Carom.), 146.0 (Carom.), 137.6 (Carom.),

136.7 (Carom.), 136.3 (Carom.), 127.8 (Carom.), 127.7 (Carom.), 123.9 (Carom.), 121.9 (Carom.), 121.8 (Carom.), 118.7 (Carom.), 115.5 (Carom.), 16.0 (Cthioether), 8.30 (Cimine) ppm. Elemental analysis, calcd. for C28H24N2O2S2Zn (550.02 g·mol–1): C, 61.14; H, 4.40 and N, 5.09 %. Found: C, 60.98; H, 4.34 and N, 5.08 %. HRMS (APCI+, MeCN): m/z (%): calcd.: 549.06434 ([Zn(L1)2]+H; C28H25N2O2S2Zn), found: 549.06378 ([Zn(L1)2]+H; C28H25N2O2S2Zn) (100). FT-IR (AT-IR): 𝜐 = 1600 (m), 1577 (m), 1529 (m), 1492 (m), 1460 (m), 1460 (s), 1389 (m), 1349 (w), 1331 (m), 1314 (m), 1301 (m), 1252 (w), 1173 (m), 1145 (s), 1122 (m), 1096 (m), 981 (w), 966 (w), 955 (w), 926 (w), 958 (m), 948 (w), 822 (m), 812 (s), 785 (w), 756 (vs), 586 (m) cm-1. Synthesis of [Zn(L2)2] (C2) Freshly prepared 2-methoxy-6-(((4-(methylthio)phenyl)imino)methyl)phenol (HL2) (0.137 g; 0.5 mmol) and Zn(ClO4)2·6H2O (0.093 g; 0.25 mmol) were dissolved in a 10 mL mixture of MeCN and MeOH with a volume ratio of 1:1 together with triethylamine (0.08 mL; 0.58 mmol). The intense yellow solution was stirred under refluxing conditions for two hours, then it was filtered off and the filtrate was stored in a capped vial under ambient conditions. Yellow single crystals of complex C2 were obtained after one day. They were collected by filtration and washed with ice cold MeOH. Yield of the air-dried product: 0.043 g (28 %). 1H{13C}-NMR

(400 MHz, CDCl3): δ = 8.29 (s, 2Himine),

6.97 (m, 4Harom.),

6.94 – 6.90 (m, 2Harom.),

7.75 (dd, J = 7.86, 7.86 Hz, 4Harom.), 6.81 (dd, J = 8.1, 1.4 Hz, 2Harom.),

6.59 (t, J= 7.9 Hz, 2Harom.), 3.88 (s, 3Hthioether), 2.42 (s, 3Hether) ppm. 13C-NMR

(101 MHz, CDCl3): δ = 168.8 (Carom.), 162.5 (Carom.), 152.9 (Carom.), 146.1 (Carom.),

137.3 (Carom.), 127.8 (Carom.), 127.7 (Carom.), 127.6 (Carom.), 122.0 (Carom.), 121.8 (Carom.), 118.7 (Carom.),

118.3 (Carom.),

115.2 (Carom.),

114.3 (Carom.),

56.2 (Cether),

16.1 (Cthioether),

8.2 (Cimine) ppm. Elemental analysis, calcd. for C30H28N2O4S2Zn (610.07 g·mol–1, disregarding solvent): C, 59.06; H, 4.63 and N, 4.59 %. Found: C, 59.45; H, 4.58 and N, 4.57 %. HRMS (APCI+, MeCN): m/z (%): calcd.: 609.08547 ([Zn(L2)2]+H; C30H29N2O4S2Zn), found: 609.08458 ([Zn(L2)2]+H; C30H29N2O4S2Zn) (100).

FT-IR (AT-IR): 𝜐 = 1581 (m), 1540 (m), 1497 (m), 1431 (s), 1421 (s), 1396 (s), 1335 (w), 1306 (w)1247 (m), 1229 (s), 1202 (w), 1181 (vs), 1109 (m), 1078 (w), 982 (m), 953 (w), 854 (w), 821 (s), 802 (m), 749 (m), 738 (m), 731 (vs), 705 (w), 682 (w)616 (w), 584 (m), 578 (m) cm-1. Synthesis of [Zn(L3)2] (C3) Freshly prepared 2-(((3-(methylthio)phenyl)imino)methyl)phenol (HL3) (0.122 g; 0.5 mmol) and Zn(ClO4)2·6H2O (0.093 g; 0.25 mmol) were dissolved in a 10 mL mixture of MeCN and MeOH with a volume ratio of 1:1 together with triethylamine (0.08 mL; 0.58 mmol). The intense yellow solution was stirred under refluxing conditions for two hours, then it was filtered off and the filtrate was stored in a capped vial under ambient conditions. Yellow single crystals of complex C3 were obtained after one day. They were collected by filtration and washed with ice cold MeOH. Yield of the air-dried product: 0.024 g (17 %). 1H{13C}-NMR

(400 MHz, CDCl3): δ = 8.34 (s, 2Himine), 7.38 (ddd, J = 8.7, 6.9, 1.9 Hz, 1Harom.),

7.21 – 7.17 (m, 1Hariom.), 6.97 -6.91 (m, 2Harom.),

7.17 – 7.12 (m, 1Harom.),

7.05 (ddd, J = 7.9, 1.7, 0.9 Hz, 1Harom.),

6.81 (ddd, J = 7.9, 1.3 Hz, 2Harom.),

6.70 – 6.60 (m, 2Harom.),

2.20 (s, 6Hthioether) ppm. 13C-NMR

(101 MHz, CDCl3): δ = 171.6 (Carom.), 169.7 (Carom.), 149.5 (Carom.), 136.8 (Carom.),

136.6 (Carom.), 129.9 (Carom.), 125.0 (Carom.), 123.9 (Carom.), 119.0 (Carom.), 118.6 (Carom.), 117.3 (Carom.), 115.5 (Carom.), 15.1 (Cthioether), 8.3 (Cimine) ppm. Elemental analysis, calcd. for C28H24N2O2S2Zn·0.75H2O (550.02 g·mol–1, disregarding solvent): C, 59.68; H, 4.56 and N, 4.97 %. Found: C, 59.40; H, 4.40 and N, 5.05 %. HRMS (APCI+, MeCN): m/z (%): calcd.: 549.06434 ([Zn(L3)2]+H; C28H25N2O2S2Zn), found: 549.06389 ([Zn(L3)2]+H; C28H25N2O2S2Zn) (100). FT-IR (AT-IR): 𝜐 = 1603 (m), 1577 (m), 1570 (m), 1529 (m), 1518 (m), 1459 (m), 1444 (vs), 1388 (m), 1354 (m), 1322 (m), 1248 (w), 1180 (s), 1147 (s), 1128 (m), 1089 (w), 1025 (m), 958 (w), 932 (m), 899 (m), 873 (m), 802 (w), 778 (m), 767 (m), 761 (vs), 692 (vs), 604 (m), 570 (m) cm-1. Synthesis of [Zn(L4)2]∙EtOH (C4) Freshly prepared 2-(((4-(fluorophenyl)imino)methyl)phenol (HL4) (0.108 g; 0.5 mmol) and Zn(ClO4)2·6H2O (0.093 g; 0.25 mmol) were dissolved in a 10 mL of EtOH together with triethylamine (0.08 mL; 0.58 mmol). The intense yellow solution was stirred under refluxing conditions, it became cloudy after several minutes and was stirred for two hours. Then the reaction mixture was filtered off, and the filtrate was stored in a capped vial under ambient conditions. Yellow single crystals of complex C4 were obtained after one week. They were

collected by filtration and washed with ice cold EtOH. Yield of the air-dried product: 0.062 g (50 %). 1H{13C}-NMR

(400 MHz, CDCl3): δ = 8.30 (s, 2Himine),

7.19 (dd, J = 8.0, 1.9 Hz, 4Harom.),

7.40 (ddd, J =8.7, 6.9 1.9 Hz, 2Harom.), 7.03 (ddt, J =8.1, 5.7, 2.8 Hz, 2Harom.),

6.94 (ddt, J =8.1, 6.6, 2.8 Hz, 6Harom.), 6.67 (ddd, J = 8.0, 6.9, 1.1Hz, 2Harom.) ppm. 13C-NMR

(101 MHz, CDCl3): δ = 171.5 (Carom.), 169.8 (Carom.), 162.8 (Carom.), 160.3 (Carom.),

145.0 (Carom.), 136.7 (Carom.), 136.6 (Carom.), 123.9 (Carom.), 123.0 (Carom.), 122.9 (Carom.), 118.6 (Carom.), 116.8 (Carom.), 116.5 (Carom.), 115.6 (Carom.), 8.2 (Cimine) ppm. Elemental analysis, calcd for C26F2H18N2O2Zn·0.2H2O (493.82 g·mol–1; disregarding solvent): C, 62.78; H, 3.73 and N, 5.63 %. Found: C, 62.75; H, 3.67 and N, 5.61 %. HRMS (APCI+, MeCN): m/z (%): calcd.: 493.07005 ([Zn(L4)2]+H; C26H19F2N2O2Zn), found: 493.06961 ([Zn(L4)2]+H; C26H19F2N2O2Zn) (100). FT-IR (AT-IR): 𝜐 =1617 (w), 1608 (m), 1601 (m), 1586 (m), 1532 (m), 1509 (m), 1462 (m), 1438 (m), 1396 (m), 1347 (w), 1324 (m), 1304 (w), 1251 (w), 1234 (m), 1176 (m), 1169 (m), 1160 (w), 1147 (vs), 1127 (m), 1113 (w), 1031 (m), 986 (w), 923 (w), 866 (w), 852 (w), 830 (s), 809 (m), 770 (m), 752 (s), 742 (m), 601 (m), 584 (m) cm-1.

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Highlights     

Four new Schiff base zinc complexes are reported All complexes show high activity in the ring-opening polymerization of lactide under industrial conditions. In situ Raman monitoring in melt reveals semi-logarithmic behavior of the polymerization. The polymerization rate constant reaches almost the speed of SnOct2. MALDI-ToF analysis shows the ligand and co-initiator as end-group.

Graphical abstract

Mononuclear Zinc(II) Schiff Base Complexes as Catalysts for the Ring-Opening Polymerization of Lactide Martin Fuchs, Sebastian Schmitz, Pascal M. Schäfer, Tim Secker, Angela Metz, Agnieszka N. Ksiazkiewicz, Andrij Pich, Paul Kögerler, Kirill Y. Monakhov and Sonja Herres-Pawlis*

We have no conflicts to declare.