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Article

Zinc Complexes of Guanidine– and Amidine–Phenolate Ligands for the Ring-Opening Polymerization of Lactide

by
Víctor Flores-Romero
,
Jesse LeBlanc
and
Gino G. Lavoie
*
Department of Chemistry, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(8), 265; https://doi.org/10.3390/inorganics13080265
Submission received: 7 July 2025 / Revised: 5 August 2025 / Accepted: 12 August 2025 / Published: 13 August 2025
(This article belongs to the Section Organometallic Chemistry)
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Abstract

A series of Zn complexes containing guanidine– and amidine–phenolate ligands were synthesized and evaluated as catalysts for the polymerization of rac-lactide at 130 °C, under solvent-free conditions, giving rate constants in the range of 0.71–4.37 × 10–4 s–1. Polymerization under identical conditions with the guanidine– and amidine–phenol proligands themselves used as catalysts gave values in the range of 0.30–2.45 × 10–4 s–1. The stereoselective production of polylactic acid from either the Zn complexes or the proligands was limited (Pr = 0.47–0.62). The molecular weight of the polymers was lower than expected for living polymerizations due to chain transfer and/or transesterification but were comparable to those obtained in control experiments with Sn(Oct)2.

Graphical Abstract

1. Introduction

Petroleum-based polymeric materials play an essential role in modern daily life due to their high chemical stability and desirable mechanical properties [1]. This however leads to their accumulation in the environment when utilized in single-use applications [2]. On this basis, biodegradable polymers such as polylactic acid (PLA) have generated much attention as a green alternative material. PLA can be synthesized by polycondensation of lactic acid obtained from the fermentation of sugar. Ring-opening polymerization (ROP) of the corresponding lactide dimer using a metal catalyst is, however, by far the preferred process to prepare PLA [3,4,5,6,7,8,9,10,11]. Tin octoate, Sn(Oct)2, in the presence of an alcohol as a co-initiator, has been used in the industry as the main catalyst to produce PLA. This catalyst shows high activity and produces polymer with high molecular weight under solvent-free conditions at temperatures above 130 °C. However, organotin reagents are toxic and Sn(Oct)2 does not impart high polymer tacticity, thereby limiting the application of the final material, especially in relation to the biomedical field [12,13,14,15].
In recent years, catalytic systems containing zinc, a nontoxic biocompatible metal, have been developed as an alternative to tin. Several zinc complexes of nitrogen-donor ligands, such as β-diiminate [16,17,18], imine–phenolate [19,20,21,22,23,24,25], amine–phenolate [26,27,28,29] and amidine–phenolate [30,31], show high activity for the ROP of cyclic esters, including lactide. These zinc-based catalysts operate mainly through two mechanisms: the coordination-insertion mechanism (CIM) and the activated monomer mechanism (AMM). CIM begins with the coordination of the monomer to the metal center followed by ring-opening of the lactide through the migratory insertion of the alkoxide ligand, thereby extending the propagating chain [32,33,34,35]. In contrast, AMM relies on ring-opening of the coordinated lactide by the nucleophilic addition of an external nucleophile, such as an alcohol, including the alcohol chain end of the lactide polymers and oligomers [36,37,38,39]. While several catalysts have emerged, the capability of these initiators to produce PLA under industrial conditions (solvent-free and high temperatures) and using non-purified technical-grade lactide is still limited. The development of new systems capable of satisfying the industry requirements is thus warranted.
N-Heterocyclic imines (NHIs), a class of cyclic guanidines, are of interest due to their ability to easily modulate their electron-donating capacity through the N-heterocyclic carbene (NHC) building block. These NHIs are present in two mesomeric forms, illustrating their strong electron-donating ability (Figure 1). Zinc complexes of anionic guanidinate ligands produce PLA with activities comparable to those of Sn(Oct)2 under solventless conditions at 190 °C, albeit with no reported tacticity data [40]. Incorporating a neutral cyclic guanidine fragment into a bidentate ligand offers great opportunities to vary the electronic and steric properties and thus control the performance of the catalysts. Such neutral ligands have been reported and used in coordination with zinc, leading to active catalysts for the polymerization of lactide under industrially relevant conditions [41,42,43]. However, related monoanionic bidentate ligand scaffolds remain relatively rare and have been used exclusively in coordination with group 4 and group 10 metal and limited to olefin polymerization [44,45,46,47].
Cyclic (alkyl)(amino)carbenes (CAACs) are another important class of carbenes that are stronger σ-donors and π-acceptors than N-heterocyclic carbenes [48]. The corresponding cyclic (alkyl)(amino)imines (CAAIs) are also stronger σ-donors than NHIs (Figure 1) [49,50]. Interestingly, these cyclic amidines have not been used in the ring-opening polymerization of lactide. NHC- and CAAC-based guanidine and amidine donors are thus excellent candidates to incorporate in monoanionic bidentate ligands for coordination with zinc. The ability to tune the electronic and steric properties of the ligand allows for exploring a wide experimental space and thus better understanding the criteria required to enhance the performance of these Zn catalysts for the ROP of lactide. Herein, we report the synthesis of guanidine–phenolate and amidine–phenolate ligands, their coordination with zinc, and the activity of these complexes for the ROP of unpurified lactide in the absence of solvent at 130 °C.

2. Results and Discussion

2.1. Synthesis of Ligands

Guanidine–phenols and amidine–phenols (LxH) were synthesized by reacting xCl with an aminophenol (Scheme 1). The 1H NMR spectra of L1[a–d]H showed magnetically equivalent isopropyl substituents, demonstrating significant single-bond character for the formal C=Nexo double bond due to π-electron delocalization over the guanidine functional group. In contrast, the 1H NMR spectrum of the reaction product of 2-aminophenol with the diacylated 2Cl salt presented inequivalent isopropyl groups, indicating greater C=Nexo double-bond character due to electron-withdrawing acyl groups [41]. However, the 1H-13C HMBC spectrum of this product (Figure S9, Supplementary Materials) shows the phenolic and the isopropyl methine protons both correlating to one quaternary carbon, suggesting the formation of the spiro compound L2a′H. The 1H NMR spectrum of the amidine–phenol ligand L3aH is consistent with the structure proposed where two different sets of isopropyl groups were observed due to the restricted rotation about the N–2,6-diisopropylphenyl (N–Dipp) bond caused by the adjacent methyl groups.
Single crystals of L2a′H and L3aH suitable for X-ray diffraction analysis were obtained by slow evaporation of solutions in THF and hexane, respectively. Consistent with the NMR spectra (vide supra), the solid-state structure confirmed the formation of the spiro compound L2a′H, facilitated by the enhanced electrophilicity of the guanidine functional group (Figure 2). The N2–C1 and N3–C1 bonds in L2a′H are longer (~1.449(5) Å) than that of the N1–C1 bond (1.428(5) Å), indicating significant delocalization of electron density towards the acyl groups, further supported by shorter N2–C2 and N3–C3 bond lengths, compared to related nonacylated derivatives (~1.345(5) vs. (~1.390(5) Å, respectively) [51], and longer C2–O2 and C3–O3 bond lengths (~1.225(5) Å) than N-acyl amides (~1.218(1) Å) (Table 1) [52].
The N1–C1 formal double bond in L3aH (1.287(2) Å) is shorter than the N2–C1 single bond (1.362(2) Å) and shorter than the corresponding N=C double bond observed in guanidines bearing alkyl groups on the endocyclic nitrogen, indicative of less π-electron delocalization in the amidine and a lower partial negative charge on its exocylic nitrogen [51,53]. The N2–C1 (1.362(2) Å) formal single bond in L3aH is expectedly slightly longer than the N2–C2 (1.341(5) Å) and N3–C3 (1.349(5) Å) bonds in L2a′H, due to electron delocalization towards the acyl group. The phenolic proton H1 in L3aH was located using the Fourier transform electron density map. The N1 and H1 distance of 2.290 Å indicates a strong intramolecular H-bond. The planes formed by the phenol and Dipp rings are approximately orthogonal (82.05° and 85.45°, respectively) to the amidine functional group, similar to that reported in other non-cyclic amidine–phenol ligands [30,31].

2.2. Synthesis of Zn Complexes

Zn[L1a–d]2 complexes were prepared in excellent yields (>88%) by addition of 2 equivalents of LxH to ZnEt2. Zn[L2a]2 was prepared by reaction of the potassium salt of the spiro compound L2a′H with ZnCl2 at 66 °C in THF (Scheme 1). Coordination of two ligands to the metal causes restricted rotation about the formal C=Nexo double bond, leading to inequivalent resonances in the 1H NMR spectra for the isopropyl groups in L1a–L1d and L2a. While the amidine–phenolate Zn[L3a]2 complex could not be cleanly prepared and isolated, the heteroleptic Zn[L3a]Et was successfully synthesized in good yield by reaction of one equivalent of L3aH with ZnEt2. The related Zn[L1a]Et and Zn[L1b]Et were similarly prepared. All three ethyl complexes displayed broad resonances in the 1H NMR spectra, most likely due to a dynamic equilibrium between monomeric and dimeric structures, as reported in other alkyl zinc complexes [54].
Single crystals of Zn[L1a]2 suitable for X-ray diffraction studies were grown by slow evaporation of an acetone solution at room temperature (Figure 3). Selected bond lengths and angles are shown in Table 2. Both guanidine–phenolate ligands chelate to the metal with an average bite angle of 85.70° and a τ4 value of 0.78, indicating a distorted tetrahedral geometry [55]. Interestingly, the Zn–N and Zn–O bond lengths were comparable, despite the formally neutral nitrogen and the negatively charged oxygen, evidence of the excellent σ-donating properties of the guanidine moiety. Similar Zn–N bond lengths are reported in Zn complexes with a neutral bidentate guanidine donor [41,42]. A structural parameter (ρ) value of 0.98 indicates extensive electron delocalization over the guanidine fragment [56]. This is further reflected by an angle of 60.0° between the best planes formed by the imidazole and phenol rings, allowed by the significant single-bond character of the C=Nexo bond.

2.3. Ring-Opening Polymerization of Rac-Lactide: Kinetics

All zinc complexes were assessed for their performance in the ROP of rac-lactide under solvent-free conditions at 130 °C using a monomer-to-catalyst stoichiometric ratio of 100:1 with or without benzyl alcohol (BnOH) as co-initiator (Table 3). The rate constants (kapp) for the Zn catalysts were measured by monitoring the monomer consumption over time by 1H NMR spectroscopy and ranged from 0.71 to 4.37 × 10–4 s–1 (Table 3, entries 1–9), with Zn[L1a]2 being the most active and approximately a third as active as Sn(Oct)2 (12.4–15.1 × 10–4 s–1; entries 17 and 18). The addition of BnOH to Zn[L1a]2 (entry 2) did not markedly affect the activity of the catalyst.
The activity of the bulkier tert-butyl derivative Zn[L1b]2 is comparable to that of the parent complex Zn[L1a]2, albeit giving markedly a larger weight-average molecular weight (Mw; entry 4). Surprisingly, installing either an electron-withdrawing nitro group or an electron-donating methoxy group on the phenoxide ring, as in L1cH and L1dH, respectively, had a severe deleterious effect, with an approximately 75% decrease in polymerization activity (entries 6 and 7) relative to that observed with Zn[L1a]2. Similarly, the use of the zinc complex of the diacylated guanidine ligand (Zn[L2a]2) gave the lowest activity among all catalysts tested (entry 8). This illustrates the criticality of being able to tune the electronics of the ligand through a thoughtful modular design. While an electron-poor ligand might increase the Lewis acidity of the metal and thus increase its activity, it might also enhance poisoning of the catalyst by polar impurities and possibly even dissociation of the ligand itself. Conversely, an electron-rich ligand might mitigate such poisoning and ligand dissociation, albeit at the expense of poorer monomer activation.
Zn[L3a]2 of the cyclic (alkyl)(amino)imine ligand could not be cleanly prepared and thus used in polymerization studies. The corresponding heteroleptic Zn[L3a]Et complex does however catalyze the ring-opening polymerization of lactide in the presence of benzyl alcohol at a rate twice as fast as Zn[L1a]Et (4.08 × 10–4 s–1, entry 9 vs. 2.01 × 10–4 s–1, entry 3). This offers future opportunities to further improve the performance of zinc catalysts containing an amidine fragment.
The 50% decrease in activity of Zn[L1a]Et relative to that of Zn[L1a]2 is possibly due to the disproportionation of in situ-generated Zn[L1a]OBn into 0.5 equivalent Zn[L1a]2 (and 0.5 equivalent Zn(OBn)2), also reported in analogous zinc complexes of phenoxy–imine ligands [21] and guanidine–ethenolate ligands [51]. The disproportionation was confirmed by the addition of one equivalent of benzyl alcohol to Zn[L1a]Et and the immediate quantitative formation of a product that is spectroscopically identical to Zn[L1a]2 and Zn(OBn)2). Disproportionation was also confirmed with the tert-butyl-substituted Zn[L1b]Et analog. The contribution of Zn(OBn)2 to the activities was therefore assessed by performing a polymerization with ZnEt2 and two equivalents of benzyl alcohol (to generate Zn(OBn)2 in situ). A rate constant of 1.41 × 10–4 s–1 (entry 16) was obtained, suggesting its possible participation as an active species in polymerizations with Zn[Lx]Et and BnOH as co-initiator.
The performance of the guanidine– and amidine–phenols (LxH) themselves as catalysts for the ROP of lactide was also investigated. The monomer-to-catalyst ratio was chosen to allow for a direct comparison to the homoleptic zinc complexes, assuming complete dissociation of the ligands in Zn[Lx]2 and Zn[Lx]Et complexes under reaction conditions. Thus, a lactide-to-LxH ratio of 50:1 was used for L1aH, L1bH, L1cH, L1dH and L2a’H, and 100:1 for L3aH. Proligands L1aH, L1bH, and L2a’H catalyzed the reaction with rate constants approximately half that of their corresponding homoleptic complexes (entries 10, 11, and 14), suggesting that dissociation of the ligand in the homoleptic zinc complex occurs, with the ligands themselves catalyzing the polymerization of lactide, possibly via a hydrogen bonding mechanism [57].
The activity of L1cH itself is 1.9 times faster (entry 12) than that observed with the corresponding zinc complex, further supporting that the strong electron-withdrawing nitro group enhances the poisoning of the metal center by polar impurities, lowering the activity. Although the electron-donating methoxy group in L1d should mitigate dissociation from the metal, the activity of L1dH in fact showcased a 1.7-fold increase in activity (entry 13) compared to the corresponding bischelate zinc complex. The lower activity observed with the complex must then be associated with either a slower coordination of the monomer and/or poorer activation due to a less electrophilic Zn center. Interestingly, the cyclic (alkyl)(amino)imine-based system again stands out, with L3aH now being 3 times slower than Zn[L3a]Et (entry 15). This further demonstrates the uniqueness of these CAAI-based complexes, with a more tightly bound amidine–phenolate ligand and yet a more activating zinc center. Regardless, data from these experiments clearly further demonstrate that these proligands have the ability to polymerize lactide under reaction conditions, to different extents depending on the nature of their substituents.

2.4. Ring-Opening Polymerization of Rac-Lactide: Molecular Weight and Tacticity

The number-average (Mn) and weight-average molecular weight (Mw) of the polymers generated by the zinc complexes and the phenols LxH, in the absence of BnOH, were measured by gel permeation chromatography (400–3300 Da and 700–4900 Da, respectively). The molecular weights of these oligomers and polymers are comparable to those obtained using Sn(Oct)2 and lower than predicted based on the 100:1 monomer-to-initiator ratio (Table 3), indicating chain transfer and/or transesterification due to impurities in the unpurified lactide used and further supported by dispersity values (Ð) ranging from 1.2 to 2.3.
Addition of BnOH to the reaction mixture expectedly further decreased the molecular weight of the polymer, with incorporation of BnO as a chain end, supported by both 1H NMR spectroscopy and MALDI-TOF mass spectrometry (entry 2). The latter also showed hydroxyl end-capped polymers, suggesting reaction of water present in the unpurified lactide with the ester group in the lactide monomer (as initiator) or in the polymer itself (as chain-transfer agent). This also suggests that the Zn[Lx]2 catalysts operate through an activated monomer mechanism, as reported for another homoleptic Zn-based catalyst [26,37]. Interestingly, peaks associated with Zn[Lx]2-complexed cyclic polymers were observed, indicating the presence of back-biting reactions. The data did not provide evidence of incorporation of the guanidine–phenolate ligand in the polymer, which rules out its role as initiator.
Polymerization with zinc complex Zn[L1b]2 of the tert-butyl-substituted ligand gave polymers with greater weight-average molecular weights (entry 4). Conversely, a decrease in the molecular weights was observed when an electron-withdrawing nitro group was present (entry 6). A similar effect was observed in PLA obtained from the zinc complex Zn[L2a]2 of the diacylated ligand (entry 8), with molecular weights lower than those generated by Zn[L1a]2 (entry 1). Again, the amidine L3a-based ethyl zinc complex stood out (entry 9), producing PLA with larger molecular weights than the guanidine–phenolate L1a-based analog (entry 3). The phenol with the least donating capability (L1cH, entry 12) generated larger polymers than its corresponding Zn[Lx]2 complex. In contrast, L1dH (entry 13), with a more nucleophilic oxygen, and L3aH (entry 15), with a stronger N-donor, both gave polymers with lower molecular weights than their corresponding bischelate zinc complexes. These results are consistent with electron-poor ligands enhancing the Lewis acidity of the metal, leading to more transesterification and lower molecular weights for PLA compared to the free ligand.
The tacticity of polymers was determined by homonuclear-decoupled 1H NMR spectroscopy. PLA from all Zn catalysts showed a slight heterotactic bias with Pr values from 0.52 to 0.62, with the exception of Zn[L1c]2 (Pr = 0.47) [58,59,60]. In contrast, all free ligands gave mainly atactic PLA (Pr = 0.48–0.51). The poor tacticity control of the Zn-based catalysts might be further evidence of the ligands themselves catalyzing the polymerization. The presumably lower binding constant for the electron-poor L1c and thus its more favorable dissociation might explain the poorer stereoselectivity observed with Zn[L1c]2.

3. Experimental Section

3.1. General Considerations

All syntheses of transition metal complexes and all polymerizations were performed under inert conditions, in either a nitrogen-filled MBraun glovebox or under an atmosphere of argon. ZnEt2, Sn(Oct)2, KOtBu, 2-amino-4-nitrophenol and 2-amino-4-methoxyphenol and rac-lactide (98%) were purchased from Sigma-Aldrich (Oakville, ON, Canada) and used as received. 2-Amino-4,6-di-tert-butylphenol [61], 1Cl [44], 2Cl [62], 3Cl [63] and L1aH [44] were synthesized according to procedures from the literature. All protio-solvents were dried using an mBraun solvent purification system fitted with alumina columns and stored over molecular sieves and under argon prior to use. CDCl3 was purchased from Sigma-Aldrich, and dried over CaH2, degassed by freeze–pump–thaw cycles, vacuum-transferred into a dry ampule and stored over molecular sieves prior to use. Characterization by 1H and 13C NMR spectra was recorded on a Bruker 400 or Bruker UltraShield 300 MHz NMR spectrometer (Bruker Corporation, Billerica, MA, USA) at room temperature. NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, q = quadruplet, quint = quintet, sext = sextet, sept = septet, m = multiplet, br = broad. J-Couplings are expressed in Hertz (Hz).

3.2. Solid-State Structure Determination

Diffraction data for ligands and complexes were collected on a Bruker APEX-II CCD diffractometer with a Mo Kα (λ = 0.71073 Å) radiation source (Bruker AXS, Karlsruhe, Germany). Crystals were selected under Paratone oil and mounted under a stream of N2 and kept at 173K during data collection. Structures were solved in Olex2 [64] using direct methods and refined with SHELXL [65] refinement package using least-squares minimization.

3.3. Polymers’ Molecular Weight Determination by Gel Permeation Chromatography

Number-average molecular weight (Mn), weight-average molecular weight (Mw) and dispersity (Đ; Mw/Mn) of samples were determined by gel permeation chromatography (GPC) using two PLgel miniMIX-B columns (4.6 mm × 250 mm) and two PLgel miniMIX-C columns (4.6 mm × 250 mm) from Agilent on elution with tetrahydrofuran (0.4 mL/min) at room temperature, equipped with an Agilent multi-detector suite composed of an Agilent G7801A refractive index detector, an Agilent G7803A dual-angle light scattering detector and an Agilent G7802A viscometer (Agilent Technologies, Santa Clara, CA, USA). Columns were calibrated using seven polystyrene standards with a conventional approach (Mp = 482,000, 125,900, 66,600, 27,060, 9310, 5610, and 1180 g/mol). Data were analyzed using the Agilent GPC software version A.02.01 software (Santa Clara, CA, USA).

3.4. Synthesis of Ligands and Metal Complexes

General procedure for the synthesis L1bH, L1cH and L1dH. A Schlenk flask was charged with the proper 2-aminophenol derivative (approximately 1.08 mmol, 1.0 equiv.) and triethylamine (2.0 equiv.) with 10 mL of DCM. A solution of compound 1Cl (1.0 equiv.) in 10 mL of DCM was added, and the mixture was stirred for 16 h at room temperature (with the exception of L1bH where MeCN was used, and the mixture was stirred under reflux for 3 days). The solvent was removed under vacuum, and 5 mL of EtOH was added followed by the addition of KOtBu (2.0 equiv.) in 10 mL of EtOH; the mixture was stirred at room temperature for 1 h. The volatiles were removed in vacuo, and the product was extracted with DCM (3 × 10 mL). The combined fractions were dried in vacuo. L1bH, 208 mg, 0.56 mmol, 77%. 1H NMR (400 MHz, CDCl3): δ 6.72 (d, J = 4.0 Hz, 1H, CH-Ar), 6.64 (d, J = 4.0 Hz, 1H, CH-Ar), 6.41 (d, 2H, N(CH)2N), 4.47 (sept, J = 8.0 Hz, 2H, CHMe2) 1.44 (s, 9H, CMe3), 1.27 (s, 9H, CMe3), 1.22 (d, J = 8.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 146.9 (s, C=N), 145.3 (s, C-OH), 139.9 (s, C-N), 136.8 (s, CtBu), 132.7 (s, CtBu), 113.7 (s, CH-Ar), 112.1 (s, CH-Ar), 109.7 (s, N(CH)2N), 45.9 (s, CHMe2), 34.8 (s, CMe3), 34.5 (s, CMe)3), 32.0 (s, CMe3), 29.8 (s, CMe3) 21.9 (s, CHMe2). Anal. Calcd for C23H37N3O (%): C, 74.35; H, 10.04; N, 11.31. Found (%): C, 74.12; H, 9.88; N, 11.60. L1cH, 276 mg, 0.91 mmol, 81%. 1H NMR (400 MHz, CD3OD): δ 7.79 (dd, J = 8.0, 4.0 Hz, 1H, CH-Ar), 7.40 (d, J = 4.0 Hz, 1H, CH-Ar), 7.39 (s, 2H, N(CH)2N), 6.42 (d, J = 8.0 Hz, 1H, CH-Ar) 4.51 (sept, J = 8.0 Hz, 2H, CHMe2) 1.40 (d, J = 8.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 171.2 (s, C–NO2), 143.0 (s, C=N), 134.8 (s, C-OH), 133.3 (s, C-N), 124.0 (s, CH-Ar), 118.3 (s, CH-Ar), 115.9 (s, N(CH)2N), 113.2 (s, CH-Ar), 50.3 (s, CHMe2), 22.3 (s, CHMe2). Anal. Calcd for C15H20N4O2 (%): C, 59.20; H, 6.62; N, 18.41. Found (%): C, 58.92; H, 6.57; N, 18.20. L1dH, 151 mg, 0.522 mmol, 83%. 1H NMR (400 MHz, CDCl3): δ 7.77 (d, J = 8.0 Hz, 1H, CH-Ar), 6.49 (s, 2H, N(CH)2N), 6.23–6.18 (m, 3H, CH-Ar), 4.48 (sept, J = 8.0 Hz, 2H, CHMe2) 3.70 (s, 3H, OCH3), 1.26 (d, J = 8.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 153.3 (s, C-OCH3) 147.8 (s, C=N), 143.4 (s, C-OH), 139.2 (s, C-N), 111.7 (s, CH-Ar), 110.2 (s, N(CH)2N), 102.9 (s, CH-Ar), 102.1 (s, CH-Ar), 55.9 (s, C-OMe) 46.4 (s, CHMe2), 21.9 (s, CHMe2). Anal. Calcd for C15H20N4O2 (%): C, 59.20; H, 6.62; N, 18.41. Found (%): C, 58.92; H, 6.57; N, 18.20.
L2a′H. A 20 mL vial was charged with 2-aminophenol (132 mg, 1.21 mmol, 1.0 equiv.) and triethylamine (0.50 mL, 3.6 mmol, 3.0 equiv.) in 5 mL of THF. A solution of 2Cl (310 mg, 1.22 mmol, 1.0 equiv.) in 10 mL of THF was added, and the mixture was stirred for 16 h at room temperature. The THF soluble fraction was filtered, and the solvent was removed under reduced pressure. Solids were washed with pentane and dried under vacuum to yield a pale-yellow solid (242 mg, 0.82 mmol, 66%). 1H NMR (400 MHz, CDCl3): δ 6.90–6.95 (m, 2H, CH-Ar), 6.85–6.81 (m, 2H, CH-Ar) 4.98 (s, 1H, NH), 3.59 (sept, J = 8.0 Hz, 2H, CHMe2) 1.43 (d, J = 8.0 Hz, 12H, CHMe2). 1.41 (d, J = 8.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 156.8 (s, C=O), 146.6 (s, C-O), 134.6 (s, C-N), 122.5 (s, C-Ar), 119.2 (s, C-Ar), 113.2 (s, CH-Ar), 108.2 (s, CH-Ar), 108.2 (s, N-C-N), 45.6 (s, CHMe2), 20.1 (s, CHMe2). Anal. Calcd for C15H19N3O3 (%): C, 62.27; H, 6.62; N, 14.52. Found (%): C, 62.21; H, 6.77; N, 14.77.
L3aH. A 20 mL vial was charged with 2-aminophenol (81.6 mg, 0.748 mmol, 1.0 equiv.) and triethylamine (0.30 mL, 2.1 mmol, 2.8 equiv.) in 5 mL of DCM. A solution of 3Cl (263 mg, 0.738 mmol, 1 equiv.) in 10 mL of DCM was added and the mixture was stirred for 16 h at room temperature. The solvent was removed under vacuum and the product extracted with diethyl ether. Volatiles were removed under reduce pressure and solids washed with cold hexane and dried to yield an orange solid (260 mg, 0.66 mmol, 88%). 1H NMR (400 MHz, CDCl3): δ 7.34–7.30 (m,1H, Ar-CH), 7.24–7.22 (m, 2H, Ar-CH), 6.82–6.78 (m, 2H, Ar-CH), 6.70–6.69 (m, 2H, Ar-CH), 3.16 (sept, J = 7.0 Hz, 2H, CHMe2), 2.09 (s, 2H, CH2), 1.35 (d, J = 7.0 Hz, 6H, CHMe2), 1.32 (s, 6H, CH3), 1.27 (d, J = 7.0 Hz, 6H, CHMe2), 1.26 (s, 6H, CH3). 13C{1H} NMR (100 MHz, CDCl3) δ 169.3 (s, C=N), 148.6 (s, C-iPr), 148.1 (s, C-N-Dipp), 137.4 (s, C-OH), 132.2 (s, C-N), 128.4 (s, CH-Dipp) 124.2 (s, CH-Dipp), 122.5 (s, CH-Ar), 122.4 (s, CH-Ar), 119.3 (s, CH-Ar), 113.5 (s, CH-Ar), 62.0 (s, CMe2), 54.9 (s, CMe2), 43.0 (s, CH2), 29.6 (s, CHMe2), 29.2 (s, CMe2), 29.0 (s, CMe2), 26.8 (s, CHMe2), 23.3 (s, CHMe2). Anal. Calcd for C26H36N2O (%): C, 79.55; H, 9.24; N, 7.14. Found (%): C, 79.29; H, 8.89; N, 7.01.
General procedure for the synthesis of Zn[L1a]Et, Zn[L1b]Et and Zn[L3a]Et. A solution of LxH (approximately 0.49 mmol) 1.0 equiv.) in DCM (10 mL) was added to a solution of ZnEt2 (1.0 M in n-hexane, 1.0 equiv.) in DCM (5 mL). The mixture was stirred for 6 h at room temperature. Thereafter, the solvent was removed under reduced pressure. The solid was washed with pentane to yield a tan solid. Zn[L1a]Et. 335 mg, 0.957 mmol, 82%. 1H NMR (400 MHz, CDCl3) δ 6.78–6.69 (br,1H, Ar-CH), 6.72 (s, 1H, N(CH)2N) 6.60–6.54 (br, 1H, Ar-CH), 6.46–6.40 (m, 1H, Ar-CH), 6.32–6.27 (m, 1H, Ar-CH) 4.67–4.56 (br, 2H, CHMe2), 1.33–1.32 (br, 6H, CHMe2), 1.25 (d, J = 8.0 Hz, 6H, CHMe2), 0.95–0.86 (br, 3H, CH2Me), 0.0–0.15 (br, 2H, CH2Me). 13C{1H} NMR (100 MHz, CDCl3) δ 156.8 (s, C=N), 150.3 (s, C-O), 118.9 (s, CH-Ar), 117.4 (s, C-N), 117.4 (s, CH-Ar), 115.6 (s, CH-Ar), 112.9 (s, CH-Ar), 111.8 (s, N(CH)2N), 47.4 (s, CHMe2) 22.7 (s, CHMe2), 22.1 (s, CHMe2), 13.1 (s, CH2CH3), 1.9 (CH2CH3). Anal. Calcd for C17H25N3OZn (%): C, 57.88; H, 7.14; N, 11.91. Found (%): C, 58.10; H, 7.34; N, 12.16. Zn[L1b]Et. 157 mg, 0.337 mmol, 79%. 1H NMR (400 MHz, CDCl3): δ 6.72 (d, J = 4.0 Hz, 1H, CH-Ar), 6.64 (d, J = 4.0 Hz, 1H, CH-Ar), 6.41 (s, 2H, N(CH)2N), 4.47 (sept, J = 8.0 Hz, 2H, CHMe2) 1.44 (s, 9H, CMe3), 1.27 (s, 9H, CMe3), 1.22 (d, J = 8.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 150.4 (s, C=N), 149.9 (s, C-OH), 142.1 (s, C-N), 139.2 (s, CtBu), 136.7 (s, CtBu), 115.6 (s, CH-Ar), 112.9 (s, N(CH)2N), 109.3 (s, CH-Ar), 48.6 (s, CHMe2), 48.2 (s, CHMe2), 35.4 (s, CMe3), 34.3 (s, CMe3), 31.9 (s, CMe3), 31.4 (s, CMe3), 21.0 (s, CHMe2), 21.9 (s, CHMe2), 20.9 (CH2Me). Anal. Calcd for C25H41N3OZn (%): C, 64.57; H, 8.89; N, 9.04. Found (%): C, 64.84; H, 9.10; N, 9.31. Zn[L3a]Et. (86 mg, 0.17 mmol, 85%) 1H NMR (400 MHz, CDCl3): δ 7.45–7.43 (br,1H, Ar-CH), 7.36–7.34 (br, 2H, Ar-CH), 6.93–6.91 (br, 2H, Ar-CH), 6.74 (br, 1H, Ar-CH), 6.37 (br, 1H, Ar-CH) 3.05 (br, 2H, CHMe2), 2.19 (s, 2H, CH2), 1.49 (s, 6H, CH3) 1.33–1.30 (br,18H, CHMe2, CH3). 13C{1H} NMR (100 MHz, CDCl3) δ 169.3 (s, C=N), 148.6 (s, C-iPr), 148.1 (s, C-N-Dipp), 137.4 (s, C-OH), 132.2 (s, C-N), 128.4 (s, CH-Dipp) 124.2 (s, CH-Dipp), 122.5 (s, CH-Ar), 122.4 (s, CH-Ar), 119.3 (s, CH-Ar), 113.5 (s, CH-Ar), 62.0 (s, CMe2), 54.9 (s, CMe2), 43.0 (s, CH2), 29.6 (s, CHMe2), 29.2 (s, CMe2), 29.0 (s, CMe2), 26.8 (s, CHMe2), 23.3 (s, CHMe2). Anal. Calcd for C28H40N2OZn C, 69.20; H, 8.30; N, 5.76. Found (%): C, 69.47; H, 8.07; N, 6.02.
General procedure for the synthesis of Zn(L1[a–d])2. ZnEt2 (1.0 M in n-hexane, 1.0 equiv.) was added to a solution of L1[a–d]H (approximately 0.58 mmol), 2.0 equiv.) in DCM (10 mL). The reaction mixture was stirred for 6 h at room temperature. The solvent was decanted, and the solids formed were washed with pentane. The product was dried under reduced pressure to yield a solid. Zn[L1a]2. 253 mg, 0.436 mmol, 87%. 1H NMR (400 MHz, CD3OD) δ 7.12 (s, 2H, N(CH)2N), 6.69 (dd, J = 8.0, 2.0 Hz, 1H, CH-Ar) 6.56 (td, J = 8.0, 2.0 Hz, 1H, CH-Ar) 6.35 (td, J = 8.0, 2.0 Hz, 1H, CH-Ar) 6.29 (dd, J = 8.0, 2.0 Hz, 1H, CH-Ar) 4.58 (sept, J = 8.0, 7.0 Hz, 2H, CHMe2) 1.33 (d, J = 7.0 Hz, 6H, CHMe2) 0.97 (d, J = 7.0 Hz, 6H, CHMe2). 13C{1H} NMR (100 MHz, CD3OD) δ 157.6 (s, C=N), 150.0 (s, C-O), 140.7 (s, C-N), 121.0 (s, CH-Ar), 116.3 (s, CH-Ar), 115.9 (s, CH-Ar), 114.3 (s, N(CH)2N) 113.6 (s, CH-Ar) 23.0 (s, CHMe2), 21.8 (s, CHMe2). Anal. Calcd for C30H40N6O2Zn (%): C, 61.90; H, 6.93; N, 14.44. Found (%): C, 62.15; H, 7.11; N, 14.18. Zn[L1b]2.177 mg, 0.219 mmol, 88%. 1H NMR (400 MHz, CDCl3) δ 6.76 (s, 1H, CH-Ar), 6.60 (s, 2H, N(CH)2N), 6.33 (s, 1H, CH-Ar), 4.87 (br, 2H, CHMe2), 1.46 (s, 9H, CMe3), 1.30 (d, J = 6.0 Hz, 6H, CHMe2), 1.24 (s, 9H, CMe3), 0.91 (d, J = 6.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 155.1 (s, C=N), 149.3 (s, C-OH), 137.1 (s, C-N), 133.5 (s, CtBu), 131.6 (s, CtBu), 113.6 (s, CH-Ar), 110.5 (s, N(CH)2N), 107.1 (s, CH-Ar) 46.2 (s, CHMe2), 34.4 (s, CMe3), 33.2 (s, CMe3), 32.0 (s, CMe3), 21.8 (s, CMe3) 21.3 (s, CHMe2). Anal. Calcd for C46H72N6O2Zn (%): C, 68.51; H, 9.00; N, 10.42. Found (%): 68.44; H, 8.79; N, 10.17. Zn[L1c]2. 167 mg, 0.248 mmol, 95%. 1H NMR (400 MHz, CD3OD) δ 7.61 (dd, J = 9.0, 3.0 Hz, 1H, CH-Ar),7.38 (s, 2H, N(CH)2N), 6.81 (d, J = 3.0 Hz, 1H, CH-Ar) 6.41 (d, J = 9.0 Hz, 1H, CH-Ar), 4.42 (sept, J = 7.0 Hz, 2H, CHMe2) 1.34 (d, J = 7.0 Hz, 6H, CHMe2) 1.27 (d, J = 7.0 Hz, 6H, CHMe2). 13C{1H} NMR (100 MHz, CD3OD) δ 168.0 (s, C–NO2), 133.6 (s, C=N), 132.0 (s, C-OH), 122.6 (s, C-N), 116.9 (s, N(CH)2N), 114.4 (s, CH-Ar), 114.4 (s, CH-Ar), 111.9 (s, CH-Ar), 20.9 (s, CHMe2). Anal. Calcd for C30H38N8O2Zn (%): C, 53.62; H, 5.70; N, 16.67. Found (%): C, 53.90; H, 5.55; N, 16.38. Zn[L1d]2. 166 mg, 0.258 mmol, 92%. 1H NMR (400 MHz, CDCl3) δ 7.34 (d, J = 3.0 Hz, 1H, CH-Ar), 6.87 (d, J = 3.0 Hz, 1H, N(CHCH)N), 6.80 (d, J = 3.0 Hz, 1H, N(CHCH)N), 6.01 (dd, J = 9.0, 3.0 Hz, 1H, CH-Ar), 5.29 (d, J = 3.0 Hz, 1H, CH-Ar), 4.95 (sept, J = 8.0 Hz, 1H, CHMe2), 4.05 (sept, J = 8.0 Hz, 1H, CHMe2), 3.58 (s, 3H, OCH3), 1.49 (d, J = 7.0 Hz, 3H, CHMe2), 1.30 (d, J = 7.0 Hz, 3H, CHMe2), 1.19 (d, J = 7.0 Hz, 3H, CHMe2), 0.86 (d, J = 7.0 Hz, 3H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) δ 154.0 (s, C-OCH3) 149.5 (s, C=N), 148.7 (s, C-O), 139.7 (s, C-N), 114.3 (s, CH-Ar), 111.9 (s, N(CH)2N), 103.6 (s, CH-Ar), 101.7 (s, CH-Ar), 56.3 (s, C-OMe) 47.7 (s, CHMe2), 22.4 (s, CHMe2), 22.1 (s, CHMe2). Anal. Calcd for C32H44N6O2Zn (%): C, 59.86; H, 6.91; N, 13.09. Found (%): C, 59.59; H, 7.15; N, 12.84.
Zn[L2a]2. A solution of L2a′H (136 mg, 0.470 mmol, 2.0 equiv.) in 15 mL of THF was added to a Schlenk flask containing a solution of ZnCl2 (32.4 mg, 0.240 mmol, 1.0 equiv.) and KOtBu (54.2 mg, 0.483 mg, 2.0 equiv.) in THF. The mixture was left stirring under reflux; after 24 h, volatiles were removed under reduced pressure. The product was dissolved in MeCN and filtered; thereafter, the solvent was removed under vacuum to yield a white solid (120 mg, 0.186 mmol, 77%). 1H NMR (400 MHz, CD3OD): δ 7.36 (d, J = 8.0 Hz, 1H, CH-Ar), 7.30 (d, J = 8.0 Hz, 1H, CH-Ar), 7.15 (t, J = 8.0 Hz, 1H, CH-Ar), 7.04 (t, J = 8.0 Hz, 1H, CH-Ar), 4.00 (sept, J = 6.0 Hz, 2H, CHMe2) 1.32 (d, J = 6.0 Hz, 12H, CHMe2). 1.21 (d, J = 6.0 Hz, 12H, CHMe2). 13C{1H} NMR (100 MHz, CDCl3) Anal. Calcd for C30H36N6O6Zn (%): C, 56.12; H, 5.65; N, 13.09. Found (%): C, 56.37; H, 5.80; N, 13.25.

3.5. General Procedure for the Polymerization of rac-Lactide

Otherwise specified, all polymerizations were performed using technical-grade rac-lactide. In a typical experiment, a flask was charged with 50 μmol of catalyst and 5.0 mmol of rac-lactide under inert conditions. The mixture was submerged in an oil bath at 130 °C and stirred until full conversion of the monomer. The polymerization was cooled down to room temperature and terminated by the addition of 10 mL of wet CHCl3. Volatiles were removed under vacuum at 50 °C for several hours. The remaining material was analyzed without further purification. Aliquots were removed from the reaction mixture at different times to monitor the monomer conversion by 1H NMR spectroscopy. Molecular weight averages were determined by GPC and tacticity was determined by homonuclear-decoupled 1H{1H} NMR spectroscopy [58,59,60].

4. Conclusions

The guanidine– and amidine–phenolate Zn complexes are active for the ROP of rac-lactide under solventless conditions and produced polymers with molecular weights comparable to the industrial standard Sn(Oct)2 under the conditions studied (130 °C and 100:1 lactide-to-catalyst ratio). Homoleptic complexes Zn[Lx]2 were approximately twice as active as the corresponding heteroleptic complexes Zn[Lx]Et, shown to undergo disproportionation to form 0.5 equivalent of Zn[Lx]2 and 0.5 equivalent of Zn(OBn)2. The electronic nature of the ligand has a profound effect on the activity of the catalyst, with electron-rich systems poorly activating the monomer and electron-poor ones leading to both ligand dissociation and possibly enhanced poisoning of the more electrophilic metal. The proligands themselves, LxH, proved to be effective catalysts, demonstrating that dissociated free ligands cannot be ruled out as active species under the studied reaction conditions, especially for the guanidine–phenolate system. Interestingly, the amidine–phenolate-based system (L3a) stands out amongst the other systems for both the activity of the catalyst and the molecular weight of the polymer and is thus being further studied. Polymers generated from Zn complexes showed a heterotactic bias except for Zn[L1c]2, which gave atactic PLA, consistent with dissociation of the ligand from the metal under reaction conditions; all LxH gave atactic PLA. This new class of ligands and the corresponding Zn complexes offer good potential, with future work aimed at further exploring, under a greater range of experimental conditions (such as, higher [LA]:[cat] ratios), amidine–phenolates as spectator ligands for the ROP of lactide to better understand and address the observed transesterification and mitigate the participation of the proligands in the polymerization of lactide.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13080265/s1, 1H and 13C NMR spectra, plots to calculate kapp, and crystal and structure refinement data for L2a’H and L3aH.

Author Contributions

Conceptualization, V.F.-R. and G.G.L.; Methodology, V.F.-R., J.L. and G.G.L.; Formal analysis, V.F.-R., J.L. and G.G.L.; Investigation, V.F.-R., J.L. and G.G.L.; Resources, G.G.L.; Writing—original draft, V.F.-R.; Writing—review & editing, V.F.-R., J.L. and G.G.L.; Supervision, G.G.L.; Project administration, G.G.L.; Funding acquisition, G.G.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Sciences and Engineering Research Council of Canada, grant number RGPIN-2017-05665.

Data Availability Statement

Full crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre CCDC 2467961–2467963 and can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures (accessed on 6 July 2025) or by contacting the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk).

Acknowledgments

V.F.R thanks the Consejo Nacional de Humanidades, Ciencias y Tecnologías of Mexico for a graduate scholarship and York University for a Carswell scholarship.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AMMActivated monomer mechanism
BnOHBenzyl alcohol
CAACCyclic (alkyl)amino carbene
CAAICyclic (alkyl)amino) imine
CCDCharge-coupled device
CIMCoordination-insertion mechanism
DCMDichloromethane
Dipp2,6-Diisopropylphenyl
EtEthyl
HMBCHeteronuclear multiple bond correlation
kappApparent (pseudo-first-order) rate constant
LALactide
MnNumber-average molecular weight
MwWeight-average molecular weight
MALDI-TOFMatrix-assisted laser desorption/ionization time-of-flight
MeCNAcetonitrile
MHzMegahertz
NHCN-Heterocyclic carbene
NHIN-Heterocyclic imine
NMRNuclear magnetic resonance
ORTEPOak Ridge thermal-ellipsoid plot
PLAPolylactic acid
ROPRing-opening polymerization
THFTetrahydrofuran

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Inorganics 13 00265 g001
Figure 1. Mesomeric forms of N-heterocyclic imines (NHIs) and cyclic alkyl(amino) imines (CAAIs).
Figure 1. Mesomeric forms of N-heterocyclic imines (NHIs) and cyclic alkyl(amino) imines (CAAIs).
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Inorganics 13 00265 sch001
Scheme 1. Synthesis of Zn complexes and their precursors.
Scheme 1. Synthesis of Zn complexes and their precursors.
Inorganics 13 00265 sch001
Inorganics 13 00265 g002
Figure 2. Solid-state structures of L2a′H (left) and L3aH (right). For clarity, only the NH and OH protons in L2a′H and L3aH, respectively, located on the Fourier difference map, are included. Solvent molecules are omitted for clarity. ORTEP drawn at 50% probability. Only one of the independent molecules in the unit cell is drawn for L2a′H.
Figure 2. Solid-state structures of L2a′H (left) and L3aH (right). For clarity, only the NH and OH protons in L2a′H and L3aH, respectively, located on the Fourier difference map, are included. Solvent molecules are omitted for clarity. ORTEP drawn at 50% probability. Only one of the independent molecules in the unit cell is drawn for L2a′H.
Inorganics 13 00265 g002
Inorganics 13 00265 g003
Figure 3. Solid-state structure of Zn[L1a]2. Hydrogen atoms have been omitted for clarity. ORTEP drawn at 50% probability.
Figure 3. Solid-state structure of Zn[L1a]2. Hydrogen atoms have been omitted for clarity. ORTEP drawn at 50% probability.
Inorganics 13 00265 g003
Table 1. Selected bond distances and angles for ligands L2a′H a and L3aH.
Table 1. Selected bond distances and angles for ligands L2a′H a and L3aH.
L2′aHL3aH
Bond lengths (Å)
N1–C11.428(5)N1–C11.287(2)
N2–C11.453(5)N2–C11.362(2)
N3–C11.444(5)
N2–C21.341(5)
N3–C31.349(5)
O2–C21.226(5)
O3–C31.223(5)
Bond angles (deg)
N1–C1–N2114.7(3)N1–C1–N2120.3(2)
N1–C1–N3115.1(3)N1–C1–C4131.2(2)
N2–C1–N3103.2(3)N2–C1–C4108.6(1)
O1–C1–N3110.0(3)
O1–C1–N2109.3(3)
O1–C1–N1104.6(3)
a Average bond lengths and angles of independent molecules present in the unit cell.
Table 2. Selected bond lengths and angles for complex Zn[L1a]2.
Table 2. Selected bond lengths and angles for complex Zn[L1a]2.
Selected Bond Lengths (Å)
Zn1–N12.002(11)Zn1–N41.949(11)
Zn1–O11.938(10)Zn1–O21.943(10)
N1–C11.336(17)N4–C161.337(17)
N2–C11.360(18)N5–C161.359(17)
N3–C11.394(18)N6–C161.358(17)
Selected Bond Angles (deg)
N1–Zn–O185.73(4)N4–Zn–O285.67(4)
N1–Zn–O2125.59(5)N4–Zn–O1124.19(5)
O1–Zn–O2117.40(5)N1–Zn–N4122.82(5)
N1–Zn–O185.73(4)N4–Zn–O285.67(4)
Table 3. Solvent-free polymerization of rac-lactide a.
Table 3. Solvent-free polymerization of rac-lactide a.
EntryCatalyst[LA]:[Cat]:[BnOH]kapp b
(10–4 s–1)		Mn c
(Da)		Mw c
(Da)		РdPr
1Zn[L1a]2100:1:04.26180025001.40.58
2Zn[L1a]2100:1:14.37140021001.50.58
3Zn[L1a]Et100:1:12.01170026001.60.55
4Zn[L1b]2100:1:04.01320049001.50.54
5Zn[L1b]Et100:1:12.24110016001.40.54
6Zn[L1c]2100:1:01.114007001.80.47
7Zn[L1d]2100:1:01.41300038001.30.52
8Zn[L2a]2100:1:00.7180012001.50.54
9Zn[L3a]Et100:1:14.08270037001.40.62
10L1aH50:1:02.45170025001.50.48
11L1bH50:1:02.26170020001.20.50
12L1cH50:1:02.11190027001.40.50
13L1dH50:1:02.41110016001.40.51
14L2a’H50:1:00.30130019001.50.50
15L3aH100:1:01.22100013001.30.49
16ZnEt2200:1:21.41120015001.30.56
17Sn(Oct)2100:1:115.1210031001.50.68
18Sn(Oct)2100:1:012.4330047001.40.68
a All polymerizations were carried out at 130 °C to full conversion. b Determined from the slope of the plots of ln ([LA]o/[LA]t) vs. time, with an average standard error of 5%. c Determined by GPC in THF. d Ð = Mw ÷ Mn as determined by GPC.
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Flores-Romero, V.; LeBlanc, J.; Lavoie, G.G. Zinc Complexes of Guanidine– and Amidine–Phenolate Ligands for the Ring-Opening Polymerization of Lactide. Inorganics 2025, 13, 265. https://doi.org/10.3390/inorganics13080265

AMA Style

Flores-Romero V, LeBlanc J, Lavoie GG. Zinc Complexes of Guanidine– and Amidine–Phenolate Ligands for the Ring-Opening Polymerization of Lactide. Inorganics. 2025; 13(8):265. https://doi.org/10.3390/inorganics13080265

Chicago/Turabian Style

Flores-Romero, Víctor, Jesse LeBlanc, and Gino G. Lavoie. 2025. "Zinc Complexes of Guanidine– and Amidine–Phenolate Ligands for the Ring-Opening Polymerization of Lactide" Inorganics 13, no. 8: 265. https://doi.org/10.3390/inorganics13080265

APA Style

Flores-Romero, V., LeBlanc, J., & Lavoie, G. G. (2025). Zinc Complexes of Guanidine– and Amidine–Phenolate Ligands for the Ring-Opening Polymerization of Lactide. Inorganics, 13(8), 265. https://doi.org/10.3390/inorganics13080265

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