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Article

Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group †

by
Jiahao Gao
1,2,
Wenjuan Zhang
1,*,
Xing Wang
1,
Rui Wang
1,
Mingyang Han
2,
Furong Cao
1,2 and
Xiang Hao
2
1
Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing Engineering Research Center of Textile Nanofiber, School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China
2
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
*
Author to whom correspondence should be addressed.
†
Dedicated to Prof. Wen-Hua Sun on his 60th birthday.
Catalysts 2023, 13(4), 770; https://doi.org/10.3390/catal13040770
Submission received: 28 February 2023 / Revised: 3 April 2023 / Accepted: 7 April 2023 / Published: 19 April 2023
(This article belongs to the Special Issue Metal-Organic Catalyst for High Performance Materials)
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Abstract

:
The isoselective ring-opening polymerization of rac-LA is a challenging goal. In this work, a series of potassium amidate complexes (K1–K10) were easily prepared and characterized using the 1H/13C NMR spectrum. The molecular structures of potassium complexes K2 and K10 were determined by X-ray diffraction, which showed that both were two-dimensional coordination polymers due to the adjacent π interactions of the aryl. In the presence of benzyl alcohol (BnOH), all of the potassium complexes exhibited a high catalytic activity toward the ring-opening polymerization of L-lactide and rac-LA, yielding linear polylactides capped with BnO or CH3O end groups. A significant solvent effect on the ROP of the L-LA was observed, with a superior efficiency in toluene than in THF and CH2Cl2. These complexes are iso-selective and act as active catalysts for the controlled ring-opening polymerization of rac-lactide, with a Pm from 0.54 to 0.76. This is a rare example of simple alkali metal complexes for the isoselective ROP of rac-lactide. The substituent greatly affected the monomer conversion and isoselectivities.

1. Introduction

Polylactic acid (PLA), which is derived from biomass and can be completely degraded to CO2 and water after use, is the representative biodegradable material at present. The most commonly used method for aliphatic polyesters is the ring-opening polymerization of cyclic esters catalyzed by metal catalysts [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. However, the poor heat resistance, slow crystallization, and brittle properties of PLA materials lead to their narrow processing temperature window and limited application range [16,17,18]. Therefore, improvements to the heat resistance of PLA are crucial to broaden its application [19,20,21,22]. Until today, the stereocomplex PLLA, obtained by the melt blending of PDLA and PLLA in equal amounts (i.e., a melting point of the mixture of up to 230 °C), is the most effective way to improve the heat resistance of polylactide [23,24]. However, D-LA monomers or PDLA polymer are very limited and costly. Therefore, the isoselective ROP of rac-lactide is very attractive and also a challenging goal [25,26]. Presently, Sn(Oct)2 is mostly employed for the industrial production of PLA due to its good moisture stability and solubility; however, it cannot be applied to the stereoselective ROP of rac-LA, as it lacks the defined and bulky stereo configuration around the active center [27,28]. Therefore, there have been many works that have focused on the design of a metal catalyst to initiate the stereoselective ROP of rac-LA [4,25,26,29,30,31,32,33,34].
In 1996, Spassky’s group discovered the first enantiomerically pure Al complexes A (Scheme 1), which showed very good selectivity for the ROP of rac-lactides, producing stereoblock PLA with high melting points (Tm = 187 °C) [35]. Since then, the isoselective ring-opening polymerization of rac-LA has become very popular, and many studies on the stereoselective ROP of rac-LA (Pm up to 0.99) have been realized using Al, Zn, and Y complexes bearing the special ligands that provide high-performance PLAs with good thermal stability (Tm > 200 °C), and Al complexes are the most reported [36,37,38,39,40,41]. However, the limitation of the use of aluminum complexes is the toxicity of the Al elements; for instance, aluminum derivatives are suspected to be involved in Alzheimer’s disease and have also shown a very slow polymerization rate. In order to overcome these issues, many groups have explored initiators with less toxic metals, such as magnesium and alkali metal complexes, for the ring-opening polymerization of lactides [42].
In recent years, the isoselective ROP of rac-lactides using crown ether alkali metal complexes have made great progress [43,44,45,46,47,48,49]. For example, the crown ether alkali metal complexes B (Scheme 1) have shown good efficiency in the ROP of rac-lactide, affording isotactic-enriched polylactide (PLA) [50]. Although quinolin-8-olate ligands exerted a relatively low degree of steric hindrance, the catalysts C still maintained stereoselective polymerization in toluene at room temperature or 0 °C, with the Pm ranging from 0.66 to 0.75 [48]. Moreover, the isoselectivity can also reach the high value of Pm = 0.84 by using complexes D [47]. Ion-paired potassium amidinate complexes E were able to efficiently catalyze the ROP of rac-lactide at low temperature, and the isoselectivity reached its highest value of Pm = 0.88 at −70 °C [51]. These results imply that alkali metal complexes show great potential for realizing the goal of the highly active and highly stereoselective ROP of rac-lactide. In addition, a series of potassium complexes F could initiate the polymerization of rac-LA to achieve high monomer conversions within several minutes in the presence of iPrOH, but it afforded atactic PLA with slightly isotactic-enriched microstructures [52]. Our group developed a family of sodium 2-arylimino-8-quinolates complexes that have shown good activity toward the ring-opening polymerization (ROP) of rac-lactide but without selectivity [53]. Recently, our group reported a series of simple potassium amidate complexes G that showed good efficiency for the ROP of L-lactides [54]. Some crystal structures, especially, clearly showed π interactions of the aryl with the K atom, resembling “CpM”, a sandwich form (Cp: cyclopentyl, M: metal). At the same time, the crown ether alkali metal complexes can also be regarded as “CpM” due to the similar role of the crown ether in the “Cp”. Considering the potentially important role of the “Cp” group, in this paper, a series of potassium amidates with larger conjugated aryls were designed and evaluated for the ROP of L-lactides and rac-lactide. The crystal structure showed π interactions between the K and the aryl ring, and some complexes showed good activity and stereoselectivity for the ROP of rac-lactide. These details are discussed further in this work.

2. Results and Discussion

2.1. Synthesis and Characterization of K1–K10

A series of benzamide containing polycyclic aryls, L1–L10, were readily prepared by the reaction of the corresponding arylamines with benzoyl chloride. Compounds L1–L10 have not been reported before and hence full details of synthesis were shown in the experimental section. Treatment of L1–L10 with 2 equivalents of potassium tert-butoxide (KOtBu) at ambient temperature in THF afforded the corresponding potassium complexes as solid with a good yield (Scheme 2). All complexes were characterized by 1H/13C NMR spectroscopy (showed in Figures S1–S10). As a matter of course, the NMR spectra of the complexes were recorded in d6-DMSO because their solutions in other solvents proved to be unstable (CDCl3 and d6-benzene). The absence of a -NH signal (around δ 10.0) in their 1H NMR spectra gave support for the full conversion of L1–L10 to K1–K10 (K11 was prepared according to the literature [54]).
The single crystal of K2 and the single crystal of K10 were obtained by laying n-hexane to their THF solution, and their structures were shown in Figure 1 and Figure 2. The selected bond lengths and bond angles are shown in Table 1 and Table 2. The crystal structure showed that there was a strong interaction between the K center with Aryl(N) from another molecular, which was clearly demonstrated by the bond distances [3.149–3.496 Å] of C8-K, C9-K, C11-K, C12-K, and C17-K. As shown in Figure 1, due to the interaction between the potassium with the carbon of aryl from another aryl ring, each potassium was high-coordinated by arene, one Caryl, and N from another amidate; this type of alkali metal-π interaction has been reported in the literature on the topic several times [54]. Therefore, the molecules were packed in two-dimensional form, generating a polymeric structure (Figure 1b).
The structure of K10 also showed the similar interaction between the K center with pyrene group from another molecule, in which the interaction distances of K1-C83, K1-C93, K1-C213, and K1-C223 fall in the range of 3.058~3.511 Å. However, unlike K2, these molecular chains are linked by hydrogen bonds, affording one dimension form, such as a polymeric chain.

2.2. Ring-Opening Polymerization of L-LA by K1–K10

To allow a suitable set of polymerization conditions to evaluate all potassium complexes, the ROP of L-LA was firstly investigated by using K2 under various conditions such as different temperature, run time, [L-LA]:[K] ratio, and solvent. The results are listed in Table 2.
When increasing the temperature from 30 °C to 50 °C, the monomer conversion using K2 was shown to increase from 42% to 96% in 15 min (runs 1–3, Table 2). The molecular weight of the polylactide (PLA) gradually increased from 0.88 to 1.87 × 104 g mol−1 while the molecular weight distribution (Mw/Mn) became broader (from 1.73 to 2.04), suggesting more transesterificationn at a higher temperature [55,56]. The polymerization was then monitored between 5 min and 15 min, with the temperature maintained at 50 °C. The monomer conversion notably increased from 77% to 96% over longer run times (runs 3–5, Table 2) and the molecular weight of the PLA also increased from 1.23 to 1.87 × 104 g mol−1 but with a quite broad distribution (PDI = 1.93~2.59).
By fixing the time of 15 min and the temperature of 50 °C, the BnOH effect was investigated on the polymerization of L-LA. Without BnOH, the K2 showed good activity for ROP of L-LA with 94% conversion, similar to that (96%) with 1 equivalent BnOH, producing the PLLA with a much higher molecular weight (3.85 vs. 1.87× 104 g mol−1).
Table
Table 2. Ring-Opening polymerization of L-lactide by K1–K10 a.
Table 2. Ring-Opening polymerization of L-lactide by K1–K10 a.
RunCatLA:K:BnOHT/°Ct/minSolventConv. b (%)Mncalcdc (×10 4 g mol−1)Mnd (×10 4 g mol−1)PDI d
1K2500:1:13015Toluene423.060.881.73
2K2500:1:14015Toluene886.331.501.77
3K2500:1:15015Toluene966.931.872.04
4K2500:1:15010Toluene876.271.281.93
5K2500:1:1505Toluene775.591.232.59
6K2500:1:05015Toluene946.793.851.49
7K2500:1:25015Toluene1003.621.051.74
8K2500:1:55015Toluene1001.490.961.34
9K2500:1:105015Toluene1000.730.871.32
10K2750:1:15015Toluene687.313.381.93
11K21000:1:15015Toluene557.934.181.65
12K21000:1:15030Toluene598.484.521.79
13K21000:1:15060Toluene618.764.781.75
14K21000:1:150120Toluene689.866.271.64
15K2500:1:15015THF433.147.321.67
16K2500:1:15015Hexane0
17K2500:1:15015DCM412.975.021.87
18K1500:1:15010Toluene785.612.201.70
19K3500:1:15010Toluene926.611.881.90
20K4500:1:15010Toluene715.111.451.54
21K5500:1:15010Toluene856.132.301.68
22K6500:1:15010Toluene956.852.461.81
23K7500:1:15010Toluene785.631.381.67
24K8500:1:15010Toluene735.302.341.63
25K9500:1:15010Toluene977.001.871.93
26K10500:1:15010Toluene825.941.702.02
a Conditions: 20 μmol (pre)cat., 2 mL toluene (total). b Determined by 1H NMR spectroscopy. c Mn (calcd) = M(LA) × conversion × ([LA]0/[BnOH]0) +MBnOH. d GPC data were recorded in THF vs. polystyrene standards using a correcting factor of 0.58 [57].
When increasing the BnOH amount to 10 equivalents, the monomer conversion was almost a 100% conversion, but the molecular weight decreased from 3.85 to 0.87 × 104 g mol−1 and the molecular weight distribution was broad. All the GPC traces showed bimodal distribution and the peak of lower molecular part increased with the increase in the BnOH, indicating the chain transfer role of BnOH.
Subsequently, with the temperature kept at 50 °C and the run time at 15 min, the polymerizations using K2 were performed by varying the molar ratio of [L-LA]:[K]:[BnOH] from 500:1:1 to 1000:1:1. The results showed a sharp decrease in monomer conversion from 96% to 55% but with the molecular weight of the PLLA increasing from 1.87 to 4.18 × 104 g mol−1 (runs 3, 10–11, Table 2), which can be explained by the higher monomer concentration favoring monomer coordination and chain propagation [58,59,60].
As the solvent effect was clearly observed in our previous work [54], in this work, the polymerization using K2 was also carried out in THF, n-hexane, and dichloromethane. The results showed that the monomer conversion in toluene 96% was much higher than that in polar solvents (CH2Cl2 and THF: 41–43%) (runs 3, 15, 17, Table 2). It was probably the coordination competition between the monomer and THF to K that led to the lower activity for ROP [61]. However, there was no monomer conversion in n-hexane (run 16, Table 2), and the reason might be the aggregation form of complexes. Indeed, the GPC traces of the PLLA in these polar solvents showed a broad distribution (Mw/Mn = 1.67–1.87), and the molecular weight was much higher than that in toluene.
By fixing the molar ratio of [LA]:[K] = 500, all potassium complexes K1–K10 were investigated for ring-opening polymerization of L-LA at 50 °C within 10 min. The results showed that the substituents of R1and R2 have a strong influence on their catalytic activity. The conversions by K1–K4 are in the following order: K3(NO2) > K2(Br) > K1(H) > K4(CHPh2), suggesting that the stronger electron withdrawing group of R1 on the naphthyl led to higher activity. In contrast, another interesting point is that the stronger electron donor R2 on the ortho-position of acetyl benzyl will result in higher activity, which was demonstrated by the activity order: K6 (OCH3, 95%) > K5 (CH3, 85%) > K1(H,78%), K7(F, 78%). The reason for this is probably the electron withdrawing effect. The stronger electron withdrawing group R1 on the para-position of naphthyl led to the lower electron density of naphthyl, which resulted in the weak interaction between the aryl group and K, and higher activity was thus achieved. However, the effect of substituent on the aryl (C=O) was reversed. The stronger donor ability of R2 results in the stronger bond of O-K that will also weaken the interaction between the aryl group and K, and, in turn, lead to the higher activity. In addition, the molecular weight of PLLA by K7 was much lower than by K5, K6, and K1. These results may be due to the easy interaction of F with the other aryl group in K7. The conjugated aryl ring effect was also explored by replacing the naphthyl group with flurorenyl (K8), anthracenyl(K9), and the pyrenyl group (K10). The results showed that potassium N-(anthracenyl) benzamidate K9 showed much higher activity (97%) than that (73~82%) by K8, K10, and K1. All obtained polymer possessed much lower molecular weight than the calculated ones, which suggests the stronger side reaction, similar to the most of the literature on potassium complexes for the ROP of cyclic esters.

2.3. Microstructure Analysis of PLLA

In order to obtain more information about the mechanism of L-lactide polymerization, the end group of PLLA catalyzed by K2 was analyzed by using 1H NMR and Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF spectrometry). When BnOH was not used, the MALDI-TOF analysis of PLLA showed that the polymer had two structural compositions, A and B, in which the peak A series can be attributed to linear (C3H4O2) n + C3H4O2 + CH3OH+ Na+/K+, while B can be classified as cyclic (C3H4O2)n + C3H4O2 + Na+/K+ (Figure 3). The interval between the peaks of the two series were 72 Da, which is the molecular weight of half lactide, indicating that the transesterification reaction occurred during the ring-opening polymerization. At the same time, the 1H NMR spectra of the polymer showed a significant -OCH3 signal (as shown in Figure 4), which was consistent with the results of MALDI-TOF. The methoxyl group may come from the quenching solvent of methanol [54].
When one equivalent of BnOH was employed in the polymerization (run 5, Table 2), the MALDI-TOF mass spectrum showed two families of peaks, A and B (Figure S11), in which A was assigned to (C3H4O2)n + CH3OH + Na+/K+ and B to (C3H4O2)n + PhCH2OH + Na+/K+; the presence the PhCH2O protons and CH3O protons in this polymer were confirmed by 1H NMR spectrum (Figure S12). Evidently, intermolecular transesterification is operational in the case of the A and B series. Moreover, when two equivalent BnOH was employed (run 7, Table 2), two families of peaks were observed (Figure S13), in which A was assigned as (C3H4O2)n + CH3OH + Na+/K+ and B to (C3H4O2)n +PhCH2OH + Na+/K+, and the 1H NMR spectrum confirmed the presence of the BnOH methylene protons and the CH3O protons (Figure S14). In order to make the mechanism clearer, 5 equivalent and 10 equivalent benzyl alcohols were added to the reaction. The MALDI-TOF mass spectrum of resulting polymers showed two families of peaks, A and B, in which A was assigned to (C3H4O2)n + PhCH2OH + K+/Na+ and B to (C3H4O2)n +CH3OH + K+/Na+(Figure 5), and the presence of the PhCH2O protons and the CH3O protons in this polymer were confirmed in the 1H NMR spectrum (Figure 6).
Collectively, the results observed using K2 in the L-LA polymerizations that are described above suggest that two mechanisms were involved (as shown in Scheme 3), similar to previous reports [34,42,54,62,63]. One is that the K2 directly initiate the ring-opening polymerization of L-LA, in which the CH3O- end group originated from the methanol used to terminate the polymerization. Conversely, the other one followed the “monomer activated mechanism (BnOH)” seen for most alkali metal systems [64,65,66], in which L-LA can be activated following coordination to the metal center and is then ring-opened via the nucleophilic attack of the benzyl alkoxide. When more BnOH was employed in the catalytic system, polymerizations preferred to proceed in Path 2, producing the linear PLLA (B/B* family).

2.4. Ring-opening Polymerization of rac-Lactide by K1–K11

2.4.1. Polymerization Results of rac-LA in 60 min

As they displayed very good activity for the ROP of L-LA, their selectivity toward ring-opening polymerization of rac-lactide was also investigated, and the results are listed in Table 3. Considering the high molar ratio of monomer to K can lead to poor controllability in polymerization, the ROP of rac-LA was first investigated by using K2/BnOH at [rac-LA]/K = 100 and room temperature. The results showed 100 equivalent rac-lactide can be fully converted into polymer by K2 at 20 °C in 60 min. If polymerization temperature decreased to 0 °C, the conversion decreased to 85%. Further decreasing the temperature to −30 °C and −78 °C, there was no significant change of monomer conversion (83~85%) in 60 min (runs 3, 5, 8, Table 3). However, without BnOH, the conversion sharply decreased to 26% in 60 min at −78 °C, even extending the reaction time to 2 h, and only a 50% conversion rate could be achieved (run 11, Table 3). In addition, the obtained polymer possessed very broad distribution (PDI = 4.52 (120 min) and 3.89 (60 min)).
At −78 °C, the ROP of rac-LA were then investigated from 5 to 120 min, and the results showed conversion gradually increased from 54% to 88%, with molecular weight also increasing from 0.26 to 0.65 × 104 gmol−1 (runs 6–9, Table 3). Especially from 60 to 120 min, there was a slight increase in monomer conversion from 83% to 88%. It is very interesting that the Pm values decreased from 0.76 to 0.61, indicating the selectivity decreased due to more side reaction of transesterification in a longer time. Complex K2, iso-selectivities of Pm = 0.76 (Figure 7) and 0.75 (Figure S28), were achieved when the ratios of [rac-lactide]0:[K2]0: [BnOH]0 were 100:1:1 with 5 min or 10 min, respectively, in toluene at −78 °C (runs 6, 7 in Table 3).
The solvent effect was also investigated within 60 min. The results showed that conversion (54~56%) in other solvents such as hexane, THF, or dichloromethane are much lower than (83%) in toluene (runs 8, 13–15, Table 3). However, the isoselectivity in THF (Pm = 0.67) or dichloromethane (Pm = 0.64) are both higher than in the toluene (Pm = 0.61), suggesting the polarity of the solvent effect on steroselectivity toward the ROP of rac-LA.
In order to better investigate the effect of different substituents on the polymerization selectivity, we studied the catalytic properties of different catalysts for rac-lactide at −78 °C and 60 min. Generally, all these complexes K1–K10 showed better activity than K11 bearing phenyl ring, which to some extent indicates that the bigger conjugated aryl ring may help to increase activity. Among those complexes, K1, K3, and K9 showed good catalytic activity with over 90% conversion, much higher than other complexes. In contrast, K5 and K8 exhibited much lower activity (conversion 67–68%) than others. All the obtained polymers have much lower molecular weights than those calculated, indicating that the polymerization reaction is less controllable. The isoselectivities of ROP of rac-LA were also greatly influenced by the substituents. K9, K10, and K4 showed much better selectivity with Pm values (0.65~0.68) than others (Pm ranging 0.54~0.61). The reason is probably the bigger π-conjugated aryl group of anthracenyl (K9), as the pyrenyl group (K10) favor the monomeric potassium species generation and act as a “CpM” model to influence the coordination of different monomer coordination and insertions. The polymer by K8 and K3 showed almost no selectivity with low Pm values (0.54 and 0.56). The reason may be due to the flurorenyl group of non-conjugated aryl ring that could not support the monomeric “CpM” form of K8 and thus led to the poor selectivity. While for K3 containing the strong electron withdrawing group of NO2, the reason is that they are very easy to form one-dimension polymeric potassium complexes, as in our previous report without π-interaction of K with aryl ring, and this thus led to poor selectivity. Similar to our previous report that the potassium containing NO2 group exhibited good activity for ROP of L-LA [54], in this case, K3 also exhibited much better activity for ROP of rac-LA than others. Complex (K4 and K10) showed higher selectivity (Pm = 0.67) than other complexes (K6, K7, K8) (Pm = 0.59–0.61).

2.4.2. Results of ROP toward rac-LA in 5 min

Due to the longer time increasing the occurrence of transesterification and lowering the selectivity, and in order to more clearly clarify the substituent effect of potassium complexes on the selectivity of ROP of rac-LA, all potassium complexes K1–K10 were further investigated for the ROP of rac-LA in the short time of 5 min and results were collected in Table 4.
As shown in Figure 8, generally, the conversions by K1–K10 in 5 min were much lower than that in 10 min, indicating that the longer time increased the monomer conversion. It is very interesting to see that the K9 bearing anthracenyl group exhibited the highest activity toward the ROP of rac-LA in 5 min, with more than 80% conversion. The K10 bearing pyrenyl group, meanwhile, showed lower activity than K9, but still a much higher (conv. 67%) conversion than the K1–K7 bearing naphthyl group (conv. 26~65%), whilst K8 exhibited the lowest activity with only a 8% conversion in 5 min, the same activity order to its result in 60 min. On the other hand, these results further illustrated the bigger pi-conjugated K9, and K10 can quickly initiate the ring-opening polymerization of rac-LA, while the K1–K7 containing naphthyl group gradually initiated the polymerization.
As the substituent of potassium complexes greatly affected their polymerization activity, their influence on isoselectivity toward ROP of rac-LA was also investigated. We found that, generally, the isoselectivity of PLA in 5 min were much better (Pm: 0.61~0.76) than that obtained in 60 min (Pm: 0.54~0.68) (shown in Figure 9), which may be due to the increasing side reaction in the longer time.
Figure 9 also showed that the isoselectivity changes depended on the substituent. For K4 containing the very bulk group of dibenzhydryl on naphthyl, there was almost no change of Pm values (from 0.68 to 0.67), indicating that the bulky substituent greatly affected the geometry of active species and suppressed the side reaction of transesterification. In addition, another interesting thing to note is that the bigger conjugated aryl substituted potassium complexes exhibited better isoselectivity toward the ring-opening polymerization of rac-LA, which was demonstrated by the Pm values in 5 min: K9(An) ≈ K10 (Pyrene) > K1(Quin) > K8 (Flu). Within the longer time of 60 min, K9, K10 still showed better isoselectivity than K1, K8, further indicating that the higher conjugated aryl ring favor the selectivity. Within 5 min, the R2-substituted N-naphthalenyl benzimidate potassium complexes showed similar selectivity (Pm = 0.63 ~ 0.66) towards the ROP of rac-LA, with the slight trend of K6 (OCH3) > K5(CH3) > K1(H) > K7(F), indicating a little electronic effect by p-substituent on the aryl.
As is well documented, the physical, mechanical, and degradation properties of polylactide are intimately related to chain stereochemistry. Typically, isotactic PLLA is a highly crystalline material with a Tm ≈ 170.0 °C, whereas atactic poly(D, L)-LA is amorphous without a clear Tm [26,67]. Hence, melting transition temperatures have been used to characterize the stereoregularity of polylactides to some extent. The DSC curves of polymer obtained by K2, K3, K8, and K10 in 5 min showed that they have an obvious melting point with Tm values from 130~172 °C (shown in Figure S29), indicating that all of them are crystalline polymers, which is in line with their isotactic selectivity.

3. Experimental Section

3.1. General Procedures

All manipulations were performed under high purity nitrogen with a rigorous exclusion of air and moisture using standard Schlenk techniques or glove-boxes. Toluene, n-hexane, diethyl ether, and THF were dried over sodium benzophenone under reflux and then distilled under nitrogen, and finally stored over activated molecular sieves (4 Å) for 24 h in a glove-box prior to use. L-LA was provided by TCI (Tokyo Chemical Industry, Tokyo, Japan). tBuOK was purchased from J&K Scientific (Beijing, China) and used as received. BnOH was provided by Aldrich (Tokyo, Japan). Naphthylamine, 4-brominated naphthalamide, 9-aminofluorene hydrochloride, and 9-nitroanthracene were provided by Ark Pharm(Shanghai, China). 1-pyrenamine was provided by TCI (Tokyo, Japan). 4-nitronaphthylamine, diphenylmethanol, 4-methylbenzoyl chloride, and 4-methoxybenzoyl chloride 4-fluorobenzoyl chloride were provided by J&K Scientific (Beijing, China). Differential scanning calorimetry (DSC, TA2000) was performed under a nitrogen atmosphere to record the melting temperatures of the polyethylenes. The testing program was set as follows: a sample (4.0–6.0 mg) was heated to 200 °C at a heating rate of 20 °C per min for 5 min at the same temperature to remove its thermal history and then cooled to −50 °C at the same heating rate. Elemental analysis was carried out with a Flash EA 1112 microanalyzer. The 1H and 13C NMR spectra were recorded on a Bruker DMX-400 instrument. The MALDI-TOF spectra was recorded on Bruker autoflex Ⅲ by the linear positive ion method. The GPC measurements were performed using a system composed of a 390-LC multidector (MDS), 209-LC pump injection module (PIM), and a PL-GPC 50 plus instrument. THF was used as the eluent (flow rate: 1.0 mL/min, at 40 °C) and the molecular weights and molecular weight distributions were calculated using polystyrene as the standard.

3.2. Synthesis and Characterization of L1–L10

Synthesis of N-naphthalen benzamide (L1): Naphthylamine (1.43 g, 10 mmol) was added to a 100 mL round-bottom flask and dissolved in 50 mL of dichloromethane. The resulting solution was stirred for 10 min with a subsequent dropwise addition of benzoyl chloride (4.20 g, 30 mmol). The reaction was stirred for about 9 h. After it was filtered, the white solid was obtained, and washed with anhydrous diethyl ether. Yield: 0.90 g, 36%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 10.41 (s, 1 H), 8.08 (d, J = 8.0 Hz, 2 H), 7.96 (s, 2 H), 7.85 (d, J = 8.0 Hz, 2 H), 7.62–7.52 (m, 7 H); 13C NMR (100 MHz, d6-DMSO, TMS): 134.1, 129.6, 128.7 127.8, 127.3, 126.7, 125.9, 122.4, 120.2.
Synthesis of N-(4-bromonaphthalen-1-yl)benzamide (L2): The experimental method described for L1 was used in the preparation of L2 but using 4-brominated naphthalamide (1.11 g, 5.0 mmol) and benzoyl chloride (2.10 g, 15 mmol). White solid was obtained. Yield: 0.79 g, 48%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 10.53 (s, 1 H), 8.19 (d, J = 8.0 Hz, 1 H), 8.08 (t, J = 4.0 Hz, 3H) 7.94 (d, J = 8.0 Hz, 1 H), 7.74 (t, J = 8.0 Hz, 1 H), 7.68–7.59 (m, 2 H), 7.57 (t, J = 4.0 Hz, 3H); 13C NMR (100 MHz, d6-DMSO, TMS): 166.5, 134.5, 134.4, 132.0, 130.7, 129.9, 128.7, 128.2, 128.0, 127.2, 126.9, 124.8, 124.4, 119.5.
Synthesis of N-(4-nitronaphthalen-1-yl)benzamide (L3): The experimental method described for L1 was used in the preparation of L3 using 4-nitronaphthylamine (0.99 g, 5 mmol) and benzoyl chloride (2.10 g, 15 mmol). White solid was obtained. Yield: 0.63 g, 43%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 9.03 (d, J = 8.0 Hz, 1 H), 8.94 (s, 1 H), 7.77 (d, J = 8.0 Hz, 1 H), 8.74 (d, J = 8.0 Hz, 1 H), 8.69 (t, J = 8.0 Hz, 1 H), 8.31 (d, J = 8.0 Hz, 1 H), 8.26 (d, J = 8.0 Hz, 1 H) 7.94–7.85 (m, 3 H), 7.72 (t, J = 8.0 Hz, 1 H); 13C NMR (100 MHz, d6-DMSO, TMS): 167.5, 147.4, 136.1, 134.2, 132.0, 129.4, 128.8, 127.5, 126.6, 126.3, 123.5, 121.5, 117.6.
Synthesis of N-(4-dibenzhydrylnaphthalen-1-yl) benzamide (L4): Step 1: A mixture of naphthylamine (4.2 g, 30 mmol), diphenylmethanol (6.7 g, 60 mmol), and a catalytic amount of zinc chloride (0.52 g, 4.0 mmol) in HCl (20 mL) was refluxed for 3 h. After solvent evaporation at reduced pressure, the residue was purified by column chromatography on basic alumina with the eluent of petroleum ether and ethyl acetate (v:v = 30:1) to afford a purple solid in 60% isolated yield. 1H NMR: (400 MHz, CDCl3, TMS): δ 7.95 (d, J = 8 Hz, 1 H), 7.86 (d, J = 8 Hz, 1 H), 7.45–7.37 (m, 2 H), 7.25 (d, J = 8 Hz, 5 H), 7.20 (t, J = 8 Hz, 2 H), 7.12 (d, J = 8 Hz, 5 H), 6.75–6.70 (m, 2 H), 6.18 (s, 1 H).
Step 2: The 4-dibenzhydrylnaphthalen (0.23 g, 1 mmol) was added to a 50 mL round-bottom flask dissolved in 10 mL of toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of benzoyl chloride (0.40 g, 2.0 mmol). The reaction was stirred about 8h at 80 °C. After it was filtered, the filtrate, a yellow solid, was obtained, and it was washed with anhydrous ether. Yield: 0.30 g, 63%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 10.39 (s, 1 H, N-H), 8.13–8.05 (m, 3 H, Ar-H), 8.02–7.98 (m, 1 H, Ph-H), 7.62–7.45 (m, 6 H, Ar-H), 7.33 (t, J = 8.0 Hz, 4 H, Ar-H), 7.23 (t, J = 8.0Hz, 2H, Ar-H), 7.15 (d, J = 8.0 Hz, 4 H, Ph-H), 6.73 (d, J = 8.0 Hz, 1 H, Ar-H), 6.44 (s, 1 H, Ar-H).
Synthesis of 4-methyl-N-(naphthalen-1-yl) benzamide (L5): The naphthylamine (2.8 g, 20 mmol) was added to a 50 mL round-bottom flask dissolved in 30 mL of toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of 4-methylbenzoyl chloride (3.98 g, 25.8 mmol). The reaction was stirred for about 8 h at 80 °C. After it was filtered, a white solid was obtained and washed with anhydrous ether. Yield: 1.95 g, 53%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 10.37 (s, 1 H, N-H), 8.11 (d, J = 8 Hz, 2 H, Ar-H), 7.99–7.95 (m, 1 H, Ar-H), 7.90 (d, J = 8 Hz, 1 H, Ph-H), 7.85 (d, J = 8 Hz, 1 H, Ph-H), 7.68–7.59 (m, 4 H, Ar-H), 7.36 (d, J = 8 Hz, 2 H, Ph-H), 2.41 (s, 3 H, CH3). 13C NMR: (100 MHz, d6-DMSO, TMS): δ166.5, 142.1, 134.2, 132.1, 129.5, 128.8, 128.3, 127.4, 125.2, 125.1, 122.5, 21.5.
Synthesis of 4-methoxy-N-(naphthalen-1-yl) benzamide (L6): The naphthylamine (2.1 g, 15 mmol) was added to a 250 mL round-bottom flask and dissolved in 30 mL toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of 4-methoxybenzoyl chloride (2.8 g, 16.4 mmol). The reaction was stirred for about 8 h at room temperature. After it was filtered, the filtrate, a white solid, was obtained, and it was washed with anhydrous ether. Yield: 1.54 g, 77%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 10.27 (s, 1 H, N-H), 8.09 (d, J = 8 Hz, 2 H, Ar-H), 7.98 (d, J = 8 Hz, 2 H, Ph-H), 7.85 (d, J = 8 Hz, 1 H, Ar-H), 7.59–7.53 (m, 4 H, Ar-H), 7.09 (d, J = 8 Hz, 2 H, Ph-H), 3.86 (s, 3 H, OCH3). 13C NMR: (100 MHz, d6-DMSO, TMS): δ 166.0, 162.4, 134.6, 134.2, 130.2, 129.8, 127.1, 126.6, 126.5, 126.3, 126.0, 124.4, 123.9, 114.1, 55.9.
Synthesis of 4-fluoro-N-(naphthalen-1-yl) benzamide (L7): The naphthylamine (2.1 g, 15 mmol) was added to a 250 mL round-bottom flask dissolved in 30 mL of toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of 4-fluorobenzoyl chloride (2.8 g, 17 mmol). The reaction was stirred for about 4h at 80 °C. After it was filtered, the filtrate, a white solid, was obtained, and it was washed with anhydrous ether. Yield: 1.49 g, 37.4%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 10.32 (s, 1 H, N-H), 8.12(d, J = 8 Hz, 2 H, Ar-H), 8.01 (d, J = 8 Hz, 2 H, Ph-H), 7.85 (d, J = 8 Hz, 1 H, Ar-H), 7.62–7.53 (m, 4 H, Ar-H), 7.11 (d, J = 8 Hz, 2 H, Ph-H). 13C NMR: (100 MHz, d6-DMSO, TMS): δ 168.3, 165.1, 137.6, 137.9, 137.8, 126.6, 126.3, 125.8, 124.7, 122.5, 122.1, 121.6, 120.4, 117.2.
Synthesis of N-(9H-fluoren-9-yl)benzamide (L8): Step 1: 9-aminofluorene hydrochloride (1.27 g, 5.0 mmol) was added to a 100 mL round-bottom flask, dissolved in 10 mL of acetic acid. Adjusting the pH of solvent into pH = 7 with 5% NaOH, excess solvent was removed in a vacuum. 20 mL DCM was added to the flask and stirred for 30 min. The white solid was obtained after it was filtered. Yield: 0.99 g, 91%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 9.24 (s, 1 H), 8.03 (d, J = 8.0 Hz, 2 H), 7.92 (d, J = 8.0 Hz, 2 H), 7.52 (t, J = 8.0 Hz, 2 H), 7.41 (t, J = 8.0 Hz, 2 H), 5.38 (s, 1 H)
Step 2: 9-aminofluorene (0.43 g, 2.0 mmol) was added to a 250 mL round-bottom flask dissolved in 50 mL of toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of benzoyl chloride (0.70 g, 5.0 mmol). The reaction was stirred for about 9 h. After it was filtered, the filtrate, a white solid, was obtained and washed with anhydrous ether. Yield: 0.46 g, 71%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 9.08 (d, J = 8.0 Hz, 1 H, N-H), 7.96 (d, J = 8.0 Hz, 2 H, Ar-H), 7.91 (d, J = 8.0 Hz, 2 H, Ar-H), 7.52 (d, J = 8.0 Hz, 3 H, Ar-H), 7.48–7.42 (m, 4 H, Ar-H), 7.33 (d, J = 8.0 Hz, 2H, Ph-H), 5.40 (s, 1 H, C-H); 13C NMR (100 MHz, d6-DMSO, TMS): 167.5, 145.2, 140.5, 131.7, 130.0, 128.6, 128.2, 127.9, 126.2, 125.1, 120.9, 120.5, 55.0.
Synthesis of N-(anthracen-9-yl)benzamide (L9): Step 1: the suspension of 9-nitroanthracene (0.11 g, 0.50 mmol) in glacial acetic acid (4 mL) was heated at 80 °C for 1.5 h. To the resulting homogeneous solution, a slurry of anhydrous SnCl2 (0.49 g, 2.5 mmol) in conc. HCl (2 mL) was added via a dropping funnel over 10 min at 80 °C and the mixture was stirred for 2 h at 80 °C. The precipitate was filtered and washed with conc. HCl (3 × 20 mL). The solid was washed with 5% aq. NaOH and washed with sufficient water. The orange solid was dried in air shortly and at 50 °C in vacuo for 6 h. The product was obtained as orange powder (0.65 g, 67%). 1H NMR (400 MHz, d6-DMSO, TMS): δ 7.98–7.92 (m, 4 H), 7.88 (s, 1 H), 7.45–7.39 (m, 4 H)
Step 2: 9-aminoanthracene (0.038 g, 0.20 mmol) was added to a 50 mL round-bottom flask and dissolved in 20 mL of toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of benzoyl chloride (0.084 g, 0.6 mmol). The reaction was stirred for about 12 h at 120 °C. After filtration, a yellow solid was obtained, and washed with anhydrous diethyl ether. Yield: 0.031 g, 52%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 10.75 (s, 1 H, N-H), 8.88 (d, J = 8.0 Hz, 2 H, Ph-H), 8.76 (d, J = 8.0 Hz, 2 H, Ar-H), 8.63–8.45 (m, 3 H, Ph-H), 8.08–8.01 (m, 7 H, Ar-H); 13C NMR (100 MHz, d6-DMSO, TMS): 167.7, 139.9, 134.2, 132.1, 128.8, 128.2, 127.5, 125.4, 124.9, 122.1, 118.0, 117.2.
Synthesis of N-(Pyrene-1-yl)benzamide (L10): 1-pyrenamine (0.11 g, 0.50 mmol) was added to a 50 mL round-bottom flask and dissolved in 20 mL of toluene. The resulting solution was stirred for 10 min with a subsequent dropwise addition of benzoyl chloride (0.21 g, 1.5 mmol). The reaction was stirred for about 12 h at 120 °C. After filtration, a white solid was obtained, and it was washed with anhydrous diethyl ether. Yield: 0.03 g, 27%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.14 (d, J = 8.0 Hz, 4 H), 7.94 (d, J = 8.0 Hz, 2H), 7.81 (t, J = 8.0 Hz, 2 H, Ph-H), 7.66–7.60 (m, 4 H, Ar-H), 7.49 (t, J = 8.0 Hz, 2H, Ph-H); 13C NMR (100 MHz, d6-DMSO, TMS): 166.7, 134.2, 132.1, 128.8, 127.7, 127.5, 122.5, 121.2.

3.3. Synthesis of Potassium Complexes

Synthesis of N-(naphthalen-1-yl)benzamide potassium (K1): The N-(naphthalen-1-yl)benzamide L1 (0.25 g, 1.0 mmol) and potassium tert-butoxide (0.22 g, 2 mmol) was added into 20 mL tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After that, the mixture was concentrated and n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with toluene and diethyl ether. The final product K1 (0.22 g, 0.75 mmol) was obtained as a white solid in a yield 75%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.72 (d, J = 8.0 Hz, 1 H), 8.25 (d, J = 8.0 Hz, 2 H), 8.13 (d, J = 8.0 Hz, 1 H), 7.62 (d, J = 8.0 Hz, 1 H), 7.30–7.20 (m, 6 H), 7.07 (d, J = 8.0 Hz, 1 H); 13C NMR (100 MHz, d6-DMSO, TMS): 165.6, 150.5, 144.2, 133.8, 130.8, 127.8, 126.8, 126.4, 126.3, 125.1, 123.8, 121.8, 116.3, 115.4.
Synthesis of N-(4-bromonaphthalen-1-yl)benzamide potassium (K2): The N-(4-bromonaphthalen-1-yl)benzamide L2 (0.33 g, 1.0 mmol) and potassium tert-butoxide (0.22 g, 2.0 mmol) was added into 15 mL tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After that, the mixture was concentrated and n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with toluene and diethyl ether. The final product K2 (0.29 g, 0.79 mmol) was obtained as white solid in a yield of 79%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.78 (d, J = 8.0 Hz, 1 H), 8.24–8.21 (m, 2 H), 8.15 (d, J = 8.0 Hz, 1H), 7.92 (d, J = 8.0 Hz, 1 H), 7.57–7.49 (m, 2 H), 7.42–7.31 (m, 4 H); 13C NMR (100 MHz, d6-DMSO, TMS): 167.1, 148.8, 142.9, 132.8, 132.0, 130.5, 128.4, 127.3, 126.6, 126.0, 125.6, 123.9, 118.6, 109.1.
Synthesis of N-(4-nitronaphthalen-1-yl)benzamide potassium K3): The N-(4-nitronaphthalen -1-yl) benzamide L3 (0.29 g, 1.0 mmol) and potassium tert-butoxide (0.22 g, 2.0 mmol) was added into 15 mL tetrahydrofuran, and then the mixture was stirred 12 h at room temperature. After that, the mixture was concentrated and n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with toluene and diethyl ether. The final product K2 (0.21 g, 0.72 mmol) was obtained as black solid in a yield of 72%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.90 (d, J = 8.0 Hz, 1 H), 8.86–8.84 (m, 2 H), 8.75 (d, J = 8.0 Hz, 1 H), 8.64 (s, 1 H), 8.25–8.23 (m, 1 H), 7.82–7.75 (m, 4H), 7.53 (t, J = 8.0 Hz, 1 H), 7.33 (t, J = 8.0 Hz, 3 H); 13C NMR (100 MHz, d6-DMSO, TMS): 159.1, 148.5, 132.6, 131.0, 130.9, 130.0, 129.4, 128.9, 128.2, 127.6, 127.3, 126.2, 124.9, 124.3, 124.1, 124.0, 110.9.
Synthesis of N-(dibenzhydrylnaphthalen-1-yl) benzamide potassium (K4): The N-(dibenzhydrylnaphthalen-1-yl)benzamide L4 (0.30 g, 1.0 mmol) and potassium tert-butoxide (0.22 g, 2.0 mmol) was added into 30 mL tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After concentration, the n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with dichloromethane and diethyl ether. The final product K4 (0.30 g, 0.77 mmol) was obtained as a white solid in a yield of 77%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 8.77 (s, 1 H), 8.21 (d, J = 8.0 Hz, 2 H), 7.95 (t, J = 8.0 Hz, 1 H), 7.81 (d, J = 8.0 Hz, 1 H), 7.31–18 (m, 11 H), 7.13 (d, J = 8.0 Hz, 4 H), 6.64 (d, J = 8.0 Hz, 1 H), 6.20 (s, 1 H). 13C NMR (100 MHz, d6-DMSO, TMS): 166.4, 145.1, 132.6, 129.8, 128.6, 128.0, 127.3, 126.4, 126.2, 125.1, 123.7, 122.8, 52.5.
Synthesis of 4-methyl-N-(naphthalen-1-yl) benzamide potassium (K5): The 4- methyl-N-(naphthalen-1-yl)benzamide L5 (0.50 g, 2.0 mmol) and potassium tert-butoxide (0.44 g, 4.0 mmol) was added into 30 mL of tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After concentration, the n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with dichloromethane and diethyl ether. The final product K5 (0.30 g, 1.12 mmol) was obtained as a white solid in yield of 56%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 8.38 (s, 1 H), 8.08 (s, 2 H), 7.89 (s, 1H), 7.77 (s, 1 H), 7.37 (s, 4 H), 7.21 (s, 2 H). 13C NMR (100 MHz, d6-DMSO, TMS): 166.5, 134.5, 130.6, 128.6, 128.5, 127.9, 126.4, 125.6, 124.8, 124.6, 21.5.
Synthesis of 4-methoxy-N-(naphthalen-1-yl) benzamide potassium (K6): The 4- methoxy-N-(naphthalen-1-yl)benzamide L6 (0.50 g, 2.0 mmol) and potassium tert-butoxide (0.44 g, 4.0 mmol) was added into 30 mL of tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After concentration, the n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with dichloromethane and diethyl ether. The final product K6 (0.39 g, 1.32 mmol) was obtained as a white solid in a yield of 65%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 8.57 (d, J = 8 Hz, 1 H), 8.15 (d, J = 8 Hz, 2H), 8.02 (d, J = 8 Hz, 1 H), 7.65 (d, J = 8 Hz, 1 H), 7.32–22 (m, 2 H), 7.17 (d, J = 8 Hz, 1 H), 6.86 (d, J = 8 Hz, 2 H). 13C NMR: (100 MHz, d6-DMSO, TMS): δ 163.4, 163.2, 161.6, 132.2, 131.7, 131.4, 131.0, 130.7, 129.3, 128.1, 126.5, 126.3, 126.0, 125.4, 124.4, 123.8, 116.1, 112.9.
Synthesis of 4-fluoro-N-(naphthalen-1-yl) benzamide potassium (K7): The 4- fluoro-N-(naphthalen-1-yl)benzamide L7 (0.60 g, 2.0 mmol) and potassium tert-butoxide (0.44 g, 4.0 mmol) was added into 30 mL of tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After concentration, the n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with dichloromethane and diethyl ether. The final product K7 (0.40 g, 1.34 mmol) was obtained as a white solid in a yield of 67%. 1H NMR: (400 MHz, d6-DMSO, TMS): δ 8.71 (d, J = 8 Hz, 1 H), 8.30–8.27 (m, 2 H), 8.15 (d, J = 8 Hz, 1 H), 7.63 (d, J = 8 Hz, 1 H), 7.30–7.21 (m, 3 H), 7.11–7.06 (d, 3 H); 13C NMR: (100 MHz, d6-DMSO, TMS): δ 165.8, 165.5, 163.4, 134.2, 131.4, 131.3, 131.0, 130.9, 129.7, 128.5, 126.8, 126.5, 126.4, 125.9, 124.4, 123.8, 115.9, 115.7.
Synthesis of N-(9H-fluoren-9-yl)benzamide potassium (K8): The N-(9H-fluoren-9-yl)benzamide L8 (0.28 g, 1.0 mmol) and potassium tert-butoxide (0.22 g, 2.0 mmol) was added into 20 mL tetrahydrofuran, and then the mixture was stirred for 9 h at room temperature. After that, the mixture was concentrated and n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with toluene and diethyl ether. The final product K8 (0.26 g, 0.82 mmol) was obtained as a white solid in a yield of 82%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.06 (d, J = 8.0 Hz, 1 H), 7.94 (t, J = 4.0 Hz, 1 H), 7.86 (d, J = 8.0 Hz, 2 H), 7.52 (d, J = 8.0 Hz, 2 H), 7.44–7.37 (m, 2 H), 7.29–7.11 (m, 4 H), 6.83 (s, 1 H), 6.47 (s, 1 H); 13C NMR (100 MHz, d6-DMSO, TMS): 166.1, 128.4, 128.1, 127.2, 124.2, 120.2, 114.0, 113.8, 55.1.
Synthesis of N-(anthracen-9-yl)benzamide potassium (K9): the N-(anthracen-9-yl)benzamide L9 (0.30 g, 1 mmol) and potassium tert-butoxide (0.22 g, 2.0 mmol) was added into 20 mL of tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After that, the mixture was concentrated and n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with toluene and diethyl ether. The final product K9 (0.26 g, 0.82 mmol) was obtained as a yellow solid in a yield of 82%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.56 (s, 4 H), 8.24 (d, J = 8.0 Hz, 2 H), 8.17 (d, J = 8.0 Hz, 2 H), 7.82 (d, J = 8.0 Hz, 2 H), 7.29 (t, J = 8.0 Hz, 1 H), 7.11 (d, J = 8.0 Hz, 2 H); 13C NMR (100 MHz, d6-DMSO, TMS): 167.1, 153.8, 144.1, 128.5, 127.4, 127.1, 125.7, 120.4, 125.4, 55.4
Synthesis of N-(Pyrene-1-yl)benzamide potassium (K10): The N-(Pyrene-1-yl)benzamide L10 (0.32 g, 1.0 mmol) and potassium tert-butoxide (0.22 g, 2.0 mmol) was added into 20 mL of tetrahydrofuran, and then the mixture was stirred for 12 h at room temperature. After that, the mixture was concentrated and n-hexane was added to precipitate the compounds. The white precipitate was obtained and washed with toluene and diethyl ether. The final product K10 (0.28 g, 0.83 mmol) was obtained as a yellow solid in a yield of 83%. 1H NMR (400 MHz, d6-DMSO, TMS): δ 8.93 (d, J = 8.0 Hz, 1 H), 8.78 (d, J = 8.0 Hz, 1 H), 8.33 (d, J = 8.0 Hz, 2 H), 7.93–7.86 (m, 4 H), 7.81–7.78 (m, 2 H), 7.68 (d, J = 8.0 Hz, 1H), 7.36–7.30 (m, 3 H); 13C NMR (100 MHz, d6-DMSO, TMS): 167.0, 132.1, 132.0, 129.1, 128.5, 128.1, 127.9, 127.2, 125.8, 125.6, 124.3, 123.1, 122.4, 122.3, 122.1, 122.0.

3.4. General Procedure for the ROP of L- LA and rac-LA

All procedures using L-LA as the monomer were similar; a typical polymerization was detailed as follows, using K2 as the representative precatalyst. Complex K2 (20 µmol) was introduced to a Schlenk flask and a toluene solution of benzyl alcohol (0.2 mL, 100 µmol/mL in toluene). The reaction mixture was stirred at room temperature for 5 min before the flask was placed in a temperature-controlled oil bath preheated to the designated temperature, and L-LA (0.72 g, 5.00 mmol) and toluene (0.8 mL) was promptly injected. After the mixture was stirred for the designated time, the resulting viscous solution was transferred into a beaker containing cold methanol (100 mL) and stirred. The resulting polymer was collected on filter paper and dried under reduced pressure to provide the PLLA as a white solid. The polymerizations using rac-LA as a monomer were conducted in a similar manner to that described for L-LA.

3.5. Crystal Structure Determination

Single crystals of K2 and K10 suitable for X-ray structural analysis were grown from a mixture of THF and n-hexane at room temperature. The single crystal X-ray diffraction studies was performed on a Rigaku RAXIS Rapid IP diffractometer with graphite mono-chromated Cu-Kα radiation (λ = 1.54184 Å) at 170(10) K. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement [68]. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were solved by direct methods and refined by full matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically. Structure solution was performed by using the OLEX2.solve, and refinement was performed by using the SHELXL package [69,70]. Details of the X-ray structure determination and refinement are provided in Table S1.

4. Conclusions

In summary, a series of potassium amidate complexes K1–K10 were prepared and characterized by 1H/13C NMR. The crystal structure of K2, K10 showed that the interaction between K and aryl ring generated a “CpM” form. All potassium complexes exhibited high catalytic activity towards ring-opening polymerization of L-lactide, and the substituent greatly affected their activity. When the molar ratio of [LA]:[K] was 500, at 50 °C, in the presence of BnOH, the monomer conversion by these potassium complexes K1–K10 is higher than 70% within 10 min, and the monomer conversion could reach 96% within 15 min. The structural analysis of the obtained PLLA shows that all of them had a linear structure with CH3O- as the end group. In addition, these potassium complexes exhibited moderate to good selectivity, with Pm ranging from 0.56 to 0.76 for ROP of rac-LA, and substituent on the ligand and the reaction time greatly affected the monomer conversion and isoselectivities.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13040770/s1, Figures display the NMR spectra of potassium complexes as well as Homonuclear-decoupled 1H NMR spectroscopy of polymer of ROP of rac-LA by different potassium complexes. X-ray crystallographic CIF files for CCDC 2244714 (K2), 2244715 (K10) are available free of charge from the Cambridge Crystallographic Data Centre. See supporting information.

Author Contributions

Design of the study by W.Z.; synthesis of the organic compounds and the potassium complexes by J.G.; characterization by J.G. and X.W.; X-ray study by M.H. and X.H.; catalytic study by J.G.; characterization of polymer by J.G., F.C. and R.W.; writing and editing by W.Z. and J.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (51973005) and Beijing Scholar Program (Project No. RCQJ20303).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Iwata, T. Biodegradable and Bio-Based Polymers: Future Prospects of Eco-Friendly Plastics. Angew. Chem. Int. Ed. 2015, 54, 3210–3215. [Google Scholar] [CrossRef] [PubMed]
  2. Zhang, X.; Fevre, M.; Jones, G.O.; Waymouth, R.M. Catalysis as an Enabling Science for Sustainable Polymers. Chem. Rev. 2018, 118, 839–885. [Google Scholar] [CrossRef] [PubMed]
  3. Schwach, G.; Coudane, J.; Engel, R.; Vert, M. Zn lactate as initiator of DL-lactide ring opening polymerization and comparison with Sn octoate. Poly. Bull. 1996, 37, 771–776. [Google Scholar] [CrossRef]
  4. Wu, J.; Yu, T.L.; Chen, C.T.; Lin, C.C. Recent developments in main group metal complexes catalyzed/initiated polymerization of lactides and related cyclic esters. Coord. Chem. Rev. 2006, 250, 602–626. [Google Scholar] [CrossRef]
  5. Cabaret, O.D.; Vaca, B.M.; Bourissou, D. Controlled Ring-Opening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 6147–6176. [Google Scholar] [CrossRef]
  6. Guillaume, S.M.; Kirillov, E.; Sarazin, Y.; Carpentier, J.F. Beyond Stereoselectivity, Switchable Catalysis: Some of the Last Frontier Challenges in Ring-Opening Polymerization of Cyclic Esters. Chem. Eur. J. 2015, 21, 7988–8003. [Google Scholar] [CrossRef]
  7. Sarazin, Y.; Carpentier, J.-F. Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides. Chem. Rev. 2015, 115, 3564–3614. [Google Scholar] [CrossRef] [PubMed]
  8. Santoro, O.; Zhang, X.; Redshaw, C. Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters. Catalysts 2020, 10, 800. [Google Scholar] [CrossRef]
  9. Wu, L.-J.; Lee, W.; Ganta, P.K.; Chang, Y.-L.; Chang, Y.-C.; Chen, H.-Y. Multinuclear metal catalysts in ring-opening polymerization of ε-caprolactone and lactide: Cooperative and electronic effects between metal centers. Coord. Chem. Rev. 2023, 475, 214847. [Google Scholar] [CrossRef]
  10. Lyubov, D.M.; Tolpygin, A.O.; Trifonov, A.A. Rare-earth metal complexes as catalysts for ring-opening polymerization of cyclic esters. Coord. Chem. Rev. 2019, 392, 83–145. [Google Scholar] [CrossRef]
  11. Strianese, M.; Pappalardo, D.; Mazzeo, M.; Lamberti, M.; Pellecchia, C. Salen-type aluminum and zinc complexes as two faced Janus compounds: Contribution to molecular sensing and polymerization catalysis. Dalton Trans. 2020, 49, 16533–16550. [Google Scholar] [CrossRef] [PubMed]
  12. Gadomska-Gajadhur, A.; Ruśkowski, P. Biocompatible Catalysts for Lactide Polymerization Catalyst Activity, Racemization Effect, and Optimization of the Polymerization Based on Design of Experiments. Org. Process Res. Dev. 2020, 24, 1435–1442. [Google Scholar] [CrossRef]
  13. Chen, M.; Chen, C. Controlling the Ring-Opening Polymerization Process Using External Stimuli. Chin. J. Chem. 2020, 38, 282–286. [Google Scholar] [CrossRef]
  14. Kaler, S.; Jones, M.D. Recent advances in externally controlled ring-opening polymerisations. Dalton Trans. 2022, 51, 1241–1256. [Google Scholar] [CrossRef]
  15. Kricheldorf, H.R.; Weidner, S.M. Syntheses of polylactides by means of tin catalysts. Polym. Chem. 2022, 13, 1618–1647. [Google Scholar] [CrossRef]
  16. Farah, S.; Anderson, D.G.; Langer, R. Physical and mechanical properties of PLA, and their functions in widespread applications —A comprehensive review. Adv. Drug Deliver. Rev. 2016, 107, 367–392. [Google Scholar] [CrossRef]
  17. Lima, L.-T.; Aurasb, R.; Rubinob, M. Processing technologies for poly(lactic acid). Prog. Poly. Sci. 2008, 33, 820–852. [Google Scholar] [CrossRef]
  18. Albertsson, A.-C.; Varma, I.K. Recent Developments in Ring Opening Polymerization of Lactones for Biomedical Applications. Biomacromolecules 2003, 4, 1466–1486. [Google Scholar] [CrossRef]
  19. Auras, R.; Harte, B.; Selke, S. An Overview of Polylactides as Packaging Materials. Macromol. Biosci. 2004, 4, 835–864. [Google Scholar] [CrossRef]
  20. Middleton, J.C.; Tipton, A.J. Synthetic biodegradable polymers as orthopedic devices. Biomaterials 2000, 21, 2335–2346. [Google Scholar] [CrossRef]
  21. Oudega, M.; Gautier, S.E.; Chapon, P.; Fragoso, M.; Bates, M.L.; Parel, J.-M.; Bunge, M.B. Axonal regeneration into Schwann cell grafts within resorbable poly(α-hydroxyaciguidance channels in the adult rat spinal cord. Biomaterials 2001, 22, 1125–1136. [Google Scholar] [CrossRef] [PubMed]
  22. Wei, G.; Ma, P.X. Structure and properties of nano-hydroxyapatite/polymer composite scaffolds for bone tissue engineering. Biomaterials 2004, 25, 4749–4757. [Google Scholar] [CrossRef] [PubMed]
  23. Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S.-H. Stereocomplex formation between enantiomeric poly(lactides). Macromolecules 1987, 20, 904–906. [Google Scholar] [CrossRef]
  24. Stanford, M.J.; Dove, A.P. Stereocontrolled ring-opening polymerisation of lactide. Chem. Soc. Rev. 2010, 39, 486–494. [Google Scholar] [CrossRef]
  25. Thomas, C.M. Stereocontrolled ring-opening polymerization of cyclic esters: Synthesis of new polyester microstructures. Chem. Soc. Rev. 2010, 39, 165–173. [Google Scholar] [CrossRef]
  26. Dijkstra, P.J.; Du, H.; Feijen, J. Single site catalysts for stereoselective ring-opening polymerization of lactides. Polym. Chem. 2011, 2, 520–527. [Google Scholar] [CrossRef]
  27. Kowalski, A.; Duda, A.; Penczek, S. Mechanism of Cyclic Ester Polymerization Initiated with Tin(II) Octoate. 2. Macromolecules Fitted with Tin(II) Alkoxide Species Observed Directly in MALDI−TOF Spectra. Macromolecules 2000, 33, 689–695. [Google Scholar] [CrossRef]
  28. Dove, A.P.; Gibson, V.C.; Marshall, E.L.; Rzepa, H.S.; White, A.J.P.; Williams, D.J. Synthetic, Structural, Mechanistic, and Computational Studies on Single-Site β-Diketiminate Tin(IInitiators for the Polymerization of rac-Lactide. J. Am. Chem. Soc. 2006, 128, 9834–9843. [Google Scholar] [CrossRef]
  29. Kremer, A.B.; Mehrkhodavandi, P. Dinuclear catalysts for the ring opening polymerization of lactide. Coord. Chem. Rev. 2019, 380, 35–37. [Google Scholar] [CrossRef]
  30. Carpentier, J.-F. Rare-Earth Complexes Supported by Tripodal Tetradentate Bis(phenolatLigands: A Privileged Class of Catalysts for Ring-Opening Polymerization of Cyclic Esters. Organometallics 2015, 34, 4175–4189. [Google Scholar] [CrossRef]
  31. Marin, P.; Venditto, V.; Thomas, C.M. Polymerization of rac-Lactide Using Achiral Iron Complexes: Access to Thermally Stable Stereocomplexes. Angew. Chem. Int. Ed. 2019, 58, 12585–12589. [Google Scholar] [CrossRef] [PubMed]
  32. Pang, X.; Duan, R.; Li, X.; Hu, C.; Wang, X.; Chen, X. Breaking the Paradox between Catalytic Activity and Stereoselectivity: Rac-Lactide Polymerization by Trinuclear Salen–Al Complexes. Macromolecules 2018, 51, 906–913. [Google Scholar] [CrossRef]
  33. Ren, F.; Li, X.; Xian, J.; Han, X.; Cao, L.; Pan, X.; Wu, J. Bench-stable potassium complexes for living and isoselective ring-opening polymerization of rac-lactide. J. Polym. Sci. 2022, 60, 2847–2854. [Google Scholar] [CrossRef]
  34. Katalin, D.-P.; Oldenburg, F.J.; Menzel, J.P.; Springer, M.; Dawe, L.N.; Kozak, C.M. Lithium, sodium, potassium and calcium amine-bis(phenolatcomplexes in the ring-opening polymerization of rac-lactide. Dalton Trans. 2020, 49, 1531–1544. [Google Scholar]
  35. Spassky, N.; Wisniewski, M.; Pluta, C.; Borgne, A.L. Highly stereoelective polymerization of rac-(D, L)-lactide with a chiral Schiff’s base/aluminium alkoxide initiator. Macromol. Chem. Phys. 1996, 197, 2627–2637. [Google Scholar] [CrossRef]
  36. Meduri, A.; Fuoco, T.; Lamberti, M.; Pellecchia, C.; Pappalardo, D. Versatile Copolymerization of Glycolide and rac-Lactide by Dimethyl(salicylaldiminato)aluminum Compounds. Macromolecules 2014, 47, 534–543. [Google Scholar] [CrossRef]
  37. Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. Stereoselective Ring-Opening Polymerization of Racemic Lactide Using Aluminum-Achiral Ligand Complexes:  Exploration of a Chain-End Control Mechanism. J. Am. Chem. Soc. 2002, 124, 5938–5939. [Google Scholar] [CrossRef]
  38. Hormnirun, P.; Marshall, E.L.; Gibson, V.C.; White, A.J.P.; Williams, D.J. Remarkable Stereocontrol in the Polymerization of Racemic Lactide Using Aluminum Initiators Supported by Tetradentate Aminophenoxide Ligands. J. Am. Chem. Soc. 2004, 126, 2688–2689. [Google Scholar] [CrossRef]
  39. Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Stereoselective Ring-Opening Polymerization of a Racemic Lactide by Using Achiral Salen–and Homosalen–Aluminum Complexes. Chem. Eur. J. 2007, 13, 4433–4451. [Google Scholar] [CrossRef]
  40. Hormnirun, P.; Marshall, E.L.; Gibson, V.C.; Pugh, R.I.; White, A.J.P. Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators. Proc. Natl. Acad. Sci. USA 2006, 103, 15343–15348. [Google Scholar] [CrossRef]
  41. Tang, Z.; Chen, X.; Pang, X.; Yang, Y.; Zhang, X.; Jing, X. Stereoselective Polymerization of rac-Lactide Using a Monoethylaluminum Schiff Base Complex. Biomacromolecules 2004, 5, 965–970. [Google Scholar] [CrossRef] [PubMed]
  42. Gao, J.; Zhu, D.; Zhang, W.; Solan, G.A.; Ma, Y.; Sun, W.-H. Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters. Inorg. Chem. Front. 2019, 6, 2619–2652. [Google Scholar]
  43. Dai, Z.; Sun, Y.; Xiong, J.; Pan, X.; Wu, J. Alkali-Metal Monophenolates with a Sandwich-Type Catalytic Center as Catalysts for Highly Isoselective Polymerization of rac-Lactide. ACS Macro Lett. 2015, 4, 556–560. [Google Scholar] [CrossRef]
  44. Sun, Y.; Xiong, J.; Dai, Z.; Pan, X.; Tang, N.; Wu, J. Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(rac-lactidSynthesis. Inorg. Chem. 2016, 55, 136–143. [Google Scholar] [CrossRef]
  45. Dai, Z.; Sun, Y.; Xiong, J.; Pan, X.; Tang, N.; Wu, J. Simple sodium and potassium phenolates as catalysts for highly isoselective polymerization of rac-lactide. Catal. Sci. Technol. 2016, 6, 515–520. [Google Scholar] [CrossRef]
  46. Wu, B.-B.; Tiana, L.-L.; Wang, Z.-X. Ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium iminophenoxides. RSC Adv. 2017, 7, 24055–24063. [Google Scholar] [CrossRef]
  47. Chen, C.; Cui, Y.; Mao, X.; Pan, X.; Wu, J. Suppressing Cyclic Polymerization for Isoselective Synthesis of High-Molecular-Weight Linear Polylactide Catalyzed by Sodium/Potassium Sulfonamidate Complexes. Macromolecules 2017, 50, 83–96. [Google Scholar] [CrossRef]
  48. Wu, B.-B.; Wang, Z.-X. Crown ether complexes of potassium quinolin-8-olates: Synthesis, characterization and catalysis toward the ring-opening polymerization of rac-lactide. RSC Adv. 2017, 7, 11657–11664. [Google Scholar] [CrossRef]
  49. Fernández-M, M.; Ortega, P.; Cuenca, T.; Cano, J.; Mosquera, M.E.G. Alkali-Metal Compounds with Bio-Based Ligands as Catalysts for Isoselective Lactide Polymerization: Influence of the Catalyst Aggregation on the Polymerization Control. Organometallics 2020, 39, 2278–2286. [Google Scholar] [CrossRef]
  50. Zhang, J.; Xiong, J.; Sun, Y.; Tang, N.; Wu, J. Highly Iso-Selective and Active Catalysts of Sodium and Potassium Monophenoxides Capped by a Crown Ether for the Ring-Opening Polymerization of rac-Lactide. Macromolecules 2014, 47, 7789–7796. [Google Scholar] [CrossRef]
  51. Chen, C.; Jiang, J.; Mao, X.; Cong, Y.; Cui, Y.; Pan, X.; Wu, J. Isoselective Polymerization of rac-Lactide Catalyzed by Ion-Paired Potassium Amidinate Complexes. Inorg. Chem. 2018, 57, 3158–3168. [Google Scholar] [CrossRef]
  52. Yao, C.; Yang, Y.; Ma, H. Potassium complexes supported by monoanionic tetradentate amino-phenolate ligands: Synthesis, structure and catalysis in the ring-opening polymerization of rac-lactide. Dalton Trans. 2017, 46, 6087–6097. [Google Scholar] [CrossRef] [PubMed]
  53. Zhang, Q.; Zhang, W.; Wang, S.; Solan, G.A.; Liang, T.; Rajendrana, N.M.; Sun, W.-H. Sodium iminoquinolates with cubic and hexagonal prismatic motifs: Synthesis, characterization and their catalytic behavior toward the ROP of rac-lactide. Inorg. Chem. Front. 2016, 3, 1178–1189. [Google Scholar] [CrossRef]
  54. Gao, J.; Zhang, W.; Cao, F.; Solan, G.A.; Zhang, X.; Jiang, Y.; Hao, X.; Sun, W.-H. Potassium N-arylbenzimidates as readily accessible and benign (pre)catalysts for the ring opening polymerization of ε-CL and L-LA. Mol. Catal. 2020, 498, 111280. [Google Scholar] [CrossRef]
  55. Montaudo, G.; Montaudo, M.S.; Puglisi, C.; Samperi, F.; Spassky, N.; LeBorgne, A.; Wisniewski, M. Evidence for Ester-Exchange Reactions and Cyclic Oligomer Formation in the Ring-Opening Polymerization of Lactide with Aluminum Complex Initiators. Macromolecules 1996, 29, 6461–6465. [Google Scholar] [CrossRef]
  56. Spassky, N.; Simic, V.; Montaudo, M.S.; Hubert-Pfalzgraf, L.G. Inter- and intramolecular ester exchange reactions in the ring-opening polymerization of (D,L)-lactide using lanthanide alkoxide initiators. Macromol. Chem. Phys. 2000, 201, 2432–2440. [Google Scholar] [CrossRef]
  57. Save, M.; Schappacher, M.; Soum, A. Controlled Ring-Opening Polymerization of Lactones and Lactides Initiated by Lanthanum Isopropoxide, 1. General Aspects and Kinetics. Macromol. Chem. Phys. 2002, 203, 889–899. [Google Scholar] [CrossRef]
  58. Iwasa, N.; Katao, S.; Liu, J.; Fujiki, M.; Furukawa, Y.; Nomura, K. Notable Effect of Fluoro Substituents in the Imino Group in Ring-Opening Polymerization of ε-Caprolactone by Al Complexes Containing Phenoxyimine Ligands. Organometallics 2009, 28, 2179–2187. [Google Scholar] [CrossRef]
  59. Cayuela, J.; Bounor-Legaré, V.; Cassagnau, P.; Michel, A. Ring-Opening Polymerization of ε-Caprolactone Initiated with Titanium n-Propoxide or Titanium Phenoxide. Macromolecules 2006, 39, 1338–1346. [Google Scholar] [CrossRef]
  60. Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S. Intermolecular chain transfer to polymer with chain scission: General treatment and determination of kp/ktr in L, L-lactide polymerization. Macromol. Rapid Commun. 1997, 18, 325–333. [Google Scholar] [CrossRef]
  61. Huang, B.-H.; Ko, B.-T.; Athar, T.; Lin, C.-C. Synthesis, Characterization, and Structural Determination of Polynuclear Lithium Aggregates and Factors Affecting Their Aggregation. Inorg. Chem. 2006, 45, 7348–7356. [Google Scholar] [CrossRef]
  62. Maudoux, N.; Roisnel, T.; Carpentier, J.-F.; Sarazin, Y. Aluminum, Indium, and Mixed Yttrium−Lithium Complexes Supported by a Chiral Binap-Based Fluorinated Dialkoxide: Structural Features and Heteroselective ROP of Lactide. Organometallics 2014, 33, 5740–5748. [Google Scholar] [CrossRef]
  63. García-Valle, F.M.; Estivill, R.; Gallegos, C.; Cuenca, T.; Mosquera, M.E.G.; Tabernero, V.; Cano., J. Metal and Ligand-Substituent Effects in the Immortal Polymerization of rac-Lactide with Li, Na, and K Phenoxo-imine ComplexesVanessa Tabernero. Organometallics 2015, 34, 477–487. [Google Scholar] [CrossRef]
  64. Roşca, S.-C.; Roşca, D.-A.; Dorcet, V.; Kozak, C.M.; Kerton, F.M.; Carpentier, J.-F.; Sarazin, Y. Alkali aminoether-phenolate complexes: Synthesis, structural characterization and evidence for an activated monomer ROP mechanism. Dalton Trans. 2013, 42, 9361–9375. [Google Scholar] [CrossRef] [PubMed]
  65. Gallegos, C.; Tabernero, V.; Mosquera, M.E.G.; Cuenca, T.; Cano, J. Comparative Study of Lactide Polymerization with Lithium, Sodium, Potassium, Magnesium, Calcium, and Zinc Azo naphthoxide Complexes. Eur. J. Inorg. Chem. 2015, 30, 5124–5132. [Google Scholar] [CrossRef]
  66. Cui, Y.; Chen., C.; Sun, Y.; Wu, J.; Pan., X. Isoselective mechanism of the ring-opening polymerization of rac-lactide catalyzed by chiral potassium binolates. Inorg. Chem. Front. 2017, 4, 261–269. [Google Scholar] [CrossRef]
  67. Stirling, E.; Champouret, Y.; Visseaux, M. Catalytic metal-based systems for controlled statistical copolymerisation of lactide with a lactone. Polym. Chem. 2018, 9, 2517–2531. [Google Scholar] [CrossRef]
  68. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Cryst. 2015, C71, 3–8. [Google Scholar]
  69. Bourhis, L.J.; Dolomanov, O.V.; Gildea, R.J.; Howardd, J.A.K.; Puschmann, H. The anatomy of a comprehensive constrained, restrained refinement program for the modern computing environment—Olex2 dissected. Acta Cryst. 2015, A71, 59–75. [Google Scholar]
  70. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339–341. [Google Scholar] [CrossRef]
Catalysts 13 00770 sch001 550
Scheme 1. Alkali metal complexes with conjugated structure used for the ROP of cyclic esters.
Scheme 1. Alkali metal complexes with conjugated structure used for the ROP of cyclic esters.
Catalysts 13 00770 sch001
Catalysts 13 00770 sch002 550
Scheme 2. Synthetic route of potassium amidates K1–K10.
Scheme 2. Synthetic route of potassium amidates K1–K10.
Catalysts 13 00770 sch002
Catalysts 13 00770 g001 550
Figure 1. (a) Ortep drawing of structure of K2, ellipsoids are shown at the 30% probability level (left). (b) A section of the packing diagram in K2 (right).
Figure 1. (a) Ortep drawing of structure of K2, ellipsoids are shown at the 30% probability level (left). (b) A section of the packing diagram in K2 (right).
Catalysts 13 00770 g001
Catalysts 13 00770 g002 550
Figure 2. (a) Ortep drawing of structure of K10, with ellipsoids shown at the 30% probability level (left). (b) Mercury drawing dimer of K10 through interaction (middle). (c) A section of the packing diagram in K10 (right).
Figure 2. (a) Ortep drawing of structure of K10, with ellipsoids shown at the 30% probability level (left). (b) Mercury drawing dimer of K10 through interaction (middle). (c) A section of the packing diagram in K10 (right).
Catalysts 13 00770 g002
Catalysts 13 00770 g003 550
Figure 3. MALDI-TOF spectrum of polylactide prepared by the ROP of L-LA (run 6, Table 2).
Figure 3. MALDI-TOF spectrum of polylactide prepared by the ROP of L-LA (run 6, Table 2).
Catalysts 13 00770 g003
Catalysts 13 00770 g004 550
Figure 4. 1H NMR spectrum of polylactide prepared by the ROP of L-LA (run 6, Table 2).
Figure 4. 1H NMR spectrum of polylactide prepared by the ROP of L-LA (run 6, Table 2).
Catalysts 13 00770 g004
Catalysts 13 00770 g005 550
Figure 5. MALDI-TOF spectrum of PLLA obtained by K2 and 10 equiv. BnOH (run 9, Table 2).
Figure 5. MALDI-TOF spectrum of PLLA obtained by K2 and 10 equiv. BnOH (run 9, Table 2).
Catalysts 13 00770 g005
Catalysts 13 00770 g006 550
Figure 6. MALDI-TOF spectrum of PLLA prepared by K2 and 10 equivalent BnOH (run 9, Table 2).
Figure 6. MALDI-TOF spectrum of PLLA prepared by K2 and 10 equivalent BnOH (run 9, Table 2).
Catalysts 13 00770 g006
Catalysts 13 00770 sch003 550
Scheme 3. Possible mechanisms for the ROP of L-LA using K2/BnOH.
Scheme 3. Possible mechanisms for the ROP of L-LA using K2/BnOH.
Catalysts 13 00770 sch003
Catalysts 13 00770 g007 550
Figure 7. (a) 1H NMR spectrum of rac-LA obtained by K2 (run 6, Table 3); (b) Homonuclear-decoupled 1H NMR spectroscopy of rac-LA obtained by K2 (run 6, Table 3). The Pm values were determined for all tetrads based on Bernoulli statistics, and their average value was used.
Figure 7. (a) 1H NMR spectrum of rac-LA obtained by K2 (run 6, Table 3); (b) Homonuclear-decoupled 1H NMR spectroscopy of rac-LA obtained by K2 (run 6, Table 3). The Pm values were determined for all tetrads based on Bernoulli statistics, and their average value was used.
Catalysts 13 00770 g007
Catalysts 13 00770 g008 550
Figure 8. Comparing the conversion by K1–K10 in 5 or 60 min.
Figure 8. Comparing the conversion by K1–K10 in 5 or 60 min.
Catalysts 13 00770 g008
Catalysts 13 00770 g009 550
Figure 9. Comparing the isoselectivity of ROP by K1–K10 in 5 or 60 min.
Figure 9. Comparing the isoselectivity of ROP by K1–K10 in 5 or 60 min.
Catalysts 13 00770 g009
Table
Table 1. The selected bond length and bond angles of K2 and K10.
Table 1. The selected bond length and bond angles of K2 and K10.
Bond Lengths of K2 (Å)Bond Lengths of K10 (Å)
K1-O12.598(2)K1-C113.413(3)K1-O12.593(4)K1-C21 33.058(5)
K1-O1 12.598(2)K1-C123.232(3)K1-N1 12.693(4)K1-C22 33.239(5)
K1-C83.249(3)K1-C173.149(3)K1-C8 33.180(5)K1-O1 32.633(4)
K1-C93.496(3)K1-N1 12.733(2)K1-C9 33.511(5)K1-N1 42.693(4)
Bond angles of K2 (°)Bond angles of K10 (°)
O1-K1-O1 180.39(7)O1-K1-K1 344.94(8)
O1 1-K1-N1 1139.25(8)O1-K1-O1 389.01(11)
O1-K1-C853.33(7)O1-K1-N1 4132.82(13)
O1-K1-K1764.14(7)O1 3-K1-N1 4111.91(12)
O1-K1-C970.39(7)O1-K1-C8 3119.49(13)
O1-K1-C11102.57(7)O1-K1-C9 3141.63(13)
O1-K1-C1289.92(7)O1-K1-C21 399.66(13)
O1-K1-C22 3101.49(13)
The atoms labelled superscript ‘1′ ‘3′ and ‘4′ have been generated through symmetry.
Table
Table 3. Ring-opening polymerization of rac-lactide a.
Table 3. Ring-opening polymerization of rac-lactide a.
RunCatLA:K:BnOHT/°Ct/minSolventConv. b
(%)		Mncalcdc
(×10 4gmol−1)		Mnd
(×10 4 g mol−1)		PDI dPme
1K2100:1:12060Toluene1001.450.642.10
2K2250:1:12060Toluene772.781.271.81
3K2100:1:1060Toluene851.240.432.23
4K2100:1:10120Toluene1001.450.611.85
5K2100:1:1−3060Toluene851.230.452.03
6K2100:1:1−785Toluene540.780.261.790.76
7K2100:1:1−7810Toluene670.970.431.350.75
8K2100:1:1−7860Toluene831.210.581.720.61
9K2100:1:1−78120Toluene881.280.652.12
10K2100:1:0−7860Toluene260.380.384.52
11K2100:1:0−78120Toluene500.720.483.89
12K2100:1:2−7860Toluene540.790.251.33
13K2100:1:1−7860hexane540.790.561.410.58
14K2100:1:1−7860THF560.820.861.580.67
15K2100:1:1−7860DCM640.930.791.670.64
16K1100:1:1−7860Toluene951.381.211.910.59
17K3100:1:1−7860Toluene951.380.871.240.54
18K4100:1:1−7860Toluene821.191.231.170.67
19K5100:1:1−7860Toluene680.990.811.990.60
20K6100:1:1−7860Toluene831.210.751.830.61
21K7100:1:1−7860Toluene781.130.931.650.61
22K8100:1:1−7860Toluene670.970.661.210.56
23K9100:1:1−7860Toluene941.360.611.420.65
24K10100:1:1−7860Toluene851.230.601.560.68
25K11 f100:1:1−7860Toluene630.920.631.690.54
a Conditions: 20 μmol (pre)cat., 1 mL toluene (total). b Determined by 1H NMR spectroscopy. c Mn (calcd) = M(LA) × conversion × ([LA]0/[BnOH]0) +MBnOH. d GPC data were recorded in THF vs. polystyrene standards using a correcting factor of 0.58 [57]. e Determined by homonuclear-decoupled 1H NMR spectroscopy. f This complex is from our previous work [54].
Table
Table 4. Ring-opening polymerization of rac-lactide a.
Table 4. Ring-opening polymerization of rac-lactide a.
RunCatLA:K:BnOHConv(%) bMnCalcd cMn d PDI dPme
1K1100:1:1260.380.191.130.66
2K2100:1:1540.780.261.790.76
3K3100:1:1540.780.891.440.65
4K4100:1:1650.940.611.830.68
5K5100:1:1350.510.391.770.66
6K6100:1:1410.600.291.340.67
7K7100:1:1300.440.461.820.63
8K8100:1:180.120.291.320.61
9K9100:1:1801.160.371.450.72
10K10100:1:1670.970.291.070.72
a Conditions: 20 μmol (pre)cat., 1 mL toluene (total), 5 min, −78 °C, b Determined by 1H NMR spectroscopy. c Mn (calcd) = M(LA) × conversion × ([LA]0/[BnOH]0) +MBnOH. Unit: ×104 g mol−1, d GPC data were recorded in THF vs. polystyrene standards using a correcting factor of 0.58 [57]. e Determined by homonuclear-decoupled 1H NMR spectroscopy.
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Gao, J.; Zhang, W.; Wang, X.; Wang, R.; Han, M.; Cao, F.; Hao, X. Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group. Catalysts 2023, 13, 770. https://doi.org/10.3390/catal13040770

AMA Style

Gao J, Zhang W, Wang X, Wang R, Han M, Cao F, Hao X. Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group. Catalysts. 2023; 13(4):770. https://doi.org/10.3390/catal13040770

Chicago/Turabian Style

Gao, Jiahao, Wenjuan Zhang, Xing Wang, Rui Wang, Mingyang Han, Furong Cao, and Xiang Hao. 2023. "Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group" Catalysts 13, no. 4: 770. https://doi.org/10.3390/catal13040770

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