Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
ppm-Level Thermally Switchable Yttrium Phenoxide Catalysts for Moisture-Insensitive and Controllably Immortal Polymerization of rac-Lactide Changjuan Chen,†,‡ Zhiyong Bai,† Yaqin Cui,† Yong Cong,† Xiaobo Pan,† and Jincai Wu*,†
Macromolecules Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/24/18. For personal use only.
†
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou University, Lanzhou 730000, People’s Republic of China ‡ College of Chemistry and Pharmaceutical Engineering, Huanghuai University, Zhumadian 463000, People’s Republic of China S Supporting Information *
ABSTRACT: Exploring the moisture-insensitive catalysis system for the ring-opening polymerization of cyclic esters is very valuable for the synthesis of polyesters in industry. To reduce metal residue in final polymers, using ppm-level catalysts for the moisture-insensitive and controllable ROP system is still a big challenge until now. In this work, as a general strategy, a series of D3 symmetrical binuclear yttrium phenoxides were applied in the moisture-insensitive ringopening polymerization of rac-lactide. The structures of these yttrium complexes can be reversibly switched between highly active and less active species in the presence of water via changing temperature. These catalysts are so robust in air, the loading of catalyst can be as low as 15 ppm, and one molecular catalyst can mediate the synthesis of more than 1000 polymer chains. The low initiation efficiency of water was proved unambiguously; hence, the polymerizations are controllable with a highly efficient initiator of alcohol in moisture condition, and the high molecular weight of polylactide can reach 78.0 kg/mol. What is more, this moisture-insensitive system can show some heteroselectivity in the ROP of rac-lactide.
■
INTRODUCTION
hand, organocatalysts have not been widely investigated in industrially attractive melt polymerizations. Until now, Sn(Oct)2 (tin(II) (2-ethylhexanoate)2) still serves as a catalyst for industrial production of PLA because of its low sensitivity to air/moisture, low catalytic concentration, and high conversion ratio. However, Sn(Oct)2 also has many disadvantages, such as low polymerization activity, undesirable inter- and intramolecular transesterification reaction, and lack of stereoselectivity. Despite the low sensitivity to air/moisture, Sn(Oct)2 usually needs to be distilled before utilization due to some impurities after exposing to air for a long time,9 likely resulting from hydrolysis and oxidation of tin(II) to tin(IV). Moreover, although Sn(Oct)2 has been approved as a catalyst in the synthesis of polylactide by the U.S. FDA, the toxicity of Sn(Oct)2 is a considerable shortcoming10 in the case of biomedical applications. Hence, some efforts have been devoted to find alternatives to overcome the problems of the commercially used Sn(Oct)2. Although some simple air stable metal salts or complexes, such as MgCO3, CaCO3, Zn(OAc)2, ZnCl2,11,12 zinc complexes with bis-guanidine12 or guanidine−pyridine ligands,13 and
Air/moisture-insensitive catalysis with trace/free metal is the most promising approach in the polymerization industry for economic, environmental, and toxicity concerns, especially in melt polymerization without solvents. However, it is still a big challenge to achieve highly active, productive, and air/ moisture-insensitive catalysts in the controllable ring-opening polymerization (ROP) of cyclic ester, which attracted tremendous attention in industry and academic researches due to the advantages of well-controlled molecular weight and narrow molecular weight distribution (Đ). Polylactide as one of the biorenewable, biocompatible, and biodegradable polyesters has wide applications in daily life and medicine. For the controllable synthesis of polylactide via the ROP method, a great number of catalysts/initiators were developed including metal complexes and organic compounds, some of which can exhibit high activities and outstanding stereoselectivities. However, most of good metal catalysts/initiators are sensitive to air/moisture which limits their industrial applications.1−3 Although organocatalysts display higher tolerances under different catalytic conditions, it is still a challenge to obtain high molecular weight polylactide because the activity and efficiency of organocatalysts are generally much lower than those of metal complexes.4−8 On the other © XXXX American Chemical Society
Received: June 8, 2018 Revised: August 5, 2018
A
DOI: 10.1021/acs.macromol.8b01229 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Synthesis of Binuclear Yttrium Phenoxides, Switchable Reaction between Complex 1 and Complex 2 in the Presence of Water via Changing Temperature, and the Controllable Ring-Opening Polymerization of rac-Lactide Catalyzed by Complexes 1, 3, and 4 with BnOH as a Coinitiator in Air/Moisture Conditions
Figure 1. (a) Crystal structure of complex 1 (tert-butyl groups and hydrogen atoms are omitted for clarity). (b) Crystal structure of complex 2 (tert-butyl groups and some hydrogen atoms are omitted for clarity).
indium complex with pyridine bisphenol as ligand,14 are active for the ROP of lactide in air; however, the activities, productivities, or controllabilities of these catalysts are not good. Some air-stable complexes of Al,15 Ti,16 Y,17 Fe,18 Cu,19 and alkali metal carboxylates20 have been reported; it is a pity that the polymerizations in air were not explored. An air/ moisture-stable hydroxyl-bridged indium Salen complex21 can actively catalyze the controlled ROP of cyclic esters in air because of an unexpected fact that this complex does not react with water further; however, similar complexes do not own this characteristic. Additionally, the residual amount of indium in polymers needs to be further reduced to the ppm level for the potential toxicity of indium. Thus, it is necessary to develop a general approach to design air/moisture-stable and even ppmlevel catalysts for the ROP of lactide. Despite a handful of moisture-insensitive systems reported for the ring-opening polymerization of lactide, to the best of our knowledge, there are no reports where just ppm-level moisture-insensitive catalyst was utilized for the controllable, immortal, and even stereoselective ROP of rac-lactide. In this work, a series of air/moisture stable binuclear yttrium
phenoxides due to huge entropy effects, using tridentate ligands, can catalyze the controllable ROP of rac-lactide in air or in the presence of small amounts of water with very high productivity at the ppm level and the molecular weight of polylactide can be high up to 78.0 kg/mol; more than 1000 precise polymer chains can be achieved via one molecular catalyst. In addition, these catalysts still exhibit heteroselectivities.
■
RESULTS AND DISCUSSION Stability and Thermally Switchable Structures of Yttrium Binuclear Complexes. To achieve moisture-stable and active complexes for the ROP of lactide, in this work, three tridentate ligands of bis(2-hydroxyphenyl)methanone were chosen (Scheme 1A). The related yttrium complexes can be readily obtained by treatments of ligands with Y[N(SiMe3)2]3 (Scheme 1A); the crystal structure analysis of complex 1 suggests these complexes are D3 symmetrical binuclear complexes (Figure 1a). 1H/13C NMR spectra as well as elemental analysis of complexes 1, 3, and 4 agree with these D3 symmetrical structures. These complexes are stable in moisture B
DOI: 10.1021/acs.macromol.8b01229 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. (a) 1H NMR spectra for heating complex 2 to recover complex 1 in deuterated o-xylene-d10 solution. (b) Different color pictures and UV−vis spectra of complexes 1 (sample marked as circled 1) and 2 (sample marked as circled 2) in toluene solutions and changes via heating ([1] = [H2L1] = 5.0 × 10−5 M; [2] = 2.5 × 10−5 M).
375 nm, respectively. The yellow solution of complex 2 can quickly change to be an orange color of complex 1 after heating at 110 °C; the color can change back to yellow after cooling this solution. Correspondingly, the regeneration of complex 1 from complex 2 can be supported by the fact that the maximum absorption peak at 426 nm changes to 450 nm in the UV−vis spectrum after heating and changes back to 426 nm after cooling to room temperature again. It is noted that although complex 2 can be almost completely converted to complex 1 in o-xylene-d10 at 120 °C (Figure 2a and Figure S4), the 1H NMR spectrum showed about 82% of complex 1 was converted back to complex 2 and 18% was decomposed to ligand during the cooling progress from 120 to 25 °C (Figure S4), which is consistent with the 75% hydrolysis yield (NMR) of complex 1 to complex 2 in the presence of excess water at room temperature (Figure S3). In the UV−vis spectrum of a solution of complex 2 after cooling from 110 °C to room temperature, a shoulder peak at 375 nm also suggests the appearance of ligand during the hydrolysis progress from 110 to 25 °C (Figure 2b). However, the reactions of hydrolysis and decomposition of complex 1 to complex 2 and ligand do not seriously happen at room temperature in air in the absence of excess water (Figure S1), and most complex 2 can be recovered back to complex 1 at a melt polymerization temperature (>120 °C, Figure 2a); thus, the slight decomposition of complex 1 to ligand in the presence of small amounts of water does not affect the following catalysis remarkably, in which complex 1 just acts as a catalyst in the ROP of lactide (vide infra), and consequently molecular weights of polymers mainly depend on the ratio of monomer/ BnOH (vide infra: the initiation efficiency of water is very low compared with BnOH). In the presence of water, similar thermally switchable reactions of complexes 3 and 4 also were found which can be verified by UV−vis spectra (Figures S5− S8; see more details and discussions in the Supporting Information). Controllably Immortal and Heteroselective ROP of rac-Lactide in Air and the Synthesis of High Molecular Weight Polylactide. Because these binuclear complexes are stable in air, the ROP of rac-lactide was conducted in a system opening to air (Scheme 1B). The polymerizations mediated by complex 1 with 1 equiv of BnOH as an initiator in THF, toluene, and CH2Cl2 at room temperature can proceed
condition because the colors of these complexes can always remain to be orange in air, which can also be confirmed by almost identical NMR spectra collected at different times after samples exposing in air for about 4 months (Figure S1). To verify the stabilities of these complexes further, the NMR titration experiments of complex 1 with water in CDCl3 were performed. Complex 1 almost does not hydrolyze in the presence of 1 equiv of water in CDCl3 (Figure S2), which supports again the stability of complex 1 in air. Upon addition of 10 equiv of water in CDCl3, large new suspending solids arise. But, it is interesting that the new species do not change further when excess water was added, for example 500 equiv of water, and the new species was separated after stirring the mixture for 5 h (isolated yield 62%, yield in NMR 75%, Figure S3). The crystal structure (Figure 1b) shows that it is a partial hydrolysis product of complex 1 (Scheme 1A), where two tridentate ligands each acquire a proton from two water molecules and change to just bidentately coordinate to yttrium atom. An additional two water molecules coordinate to two yttrium atoms. Because of the high basicity of hydroxyl group, the partial hydrolysis product dimerizes to form the new species of complex 2 (Scheme 1A). To our astonishment, complex 2 can be converted back to complex 1 completely just via heating at 120 °C in deuterated o-xylene-d10 solution (Figure 2a). That is to say, complex 1 is stable in less water conditions; although complex 1 can hydrolyze in the presence of excess of water, it can be regenerated just by heating. We believe the reason for the thermally switchable reaction can be attributed to a huge entropy effect because two molecules of complex 1 react with eight molecules of water to form just one molecule of complex 2. DFT calculations at the M06/def2svp level (to save the computing sources, tert-butyl groups in complexes 1 and 2 were replaced with methyl groups; more details in the Supporting Information) show the hydrolysis reaction entropy is a huge negative value of −338.16 cal mol−1 K−1, enthalpy is −105.51 kcal mol−1, and free energy is −3.10 kcal mol−1 at 25 °C. The free energy of this hydrolysis reaction changes to be 27.39 kcal mol−1 at 120 °C, which clearly indicates that the regeneration of complex 1 becomes favorable at a high temperature. The above thermally reversible progress can also be verified by color changes and UV−vis spectra (Figure 2b). The maximum absorption peaks of toluene solutions of complexes 1, 2, and ligand H2L1 are 450, 426, and C
DOI: 10.1021/acs.macromol.8b01229 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Controllable rac-Lactide Polymerization Catalyzed by Complexes 1, 3, and 4 in Aira entry
Cat.
[Cat.]0/[rac-LA]0/[BnOH]0
temp (°C)
time
convb (%)
Mn,obsdc (g/mol)
Mn,calcdd (g/mol)
Đ
Pre
f
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 4 4 4 4 4 4 4 4
1/100/1 1/100/1 1/100/1 1/100/2 1/100/4 1/200/2 1/400/2 1/800/2 1/1000/1 1/10000/20 1/10000/50 1/10000/100 1/10000/1000 1/20000/200 1/40000/400 1/50000/500 1/50000/500 1/50000/500 1/70000/700 1/70000/700 1/100000/1000 1/100000/1000 1/130000/1300 1/50000/500 1/50000/500
r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. 130 130 130 130 130 130 130 150 150 150 150 150 150 150 150 150 150
48 h 48 h 48 h 3h 2h 10 h 18 h 48 h 2h 12 h 5h 80 min 40 min 5h 10 h 24 h 24 h 24 h 48 h 72 h 48 h 72 h 48 h 48 h 72 h
82 85 98 70 85 87 72 72 74 85 78 94 92 76 70 58 70 83 90 96 80 82 77 85 98
10800 10200 12900 4900 2700 12100 19700 38700 78000 56800 24300 12800 1100 9800 9900 8300 9800 11500 11700 16100 9000 9100 9900 11100 16500
11900 12300 14200 5100 3200 12600 20800 41600 106700 61300 22600 13600 1400 11100 10200 8500 10200 12100 13100 13900 11600 11900 11200 12300 14200
1.25 1.36 1.26 1.05 1.05 1.05 1.07 1.17 1.23 1.25 1.26 1.29 1.30 1.26 1.28 1.42 1.43 1.42 1.50 1.56 1.46 1.51 1.54 1.46 1.48
0.69 0.68 0.72 0.72 0.69 0.72 0.73 0.74 0.63 0.62 0.62 0.61 0.60 0.61 0.60 0.59 0.58 0.60 0.58 0.58 0.57 0.57 0.58 >0.99 >0.99
1 2g 3 4 5 6 7 8 9h 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24i 25i
a Conditions: polymerization reactions were performed in air, with 2 mL of CH2Cl2 unless melt polymerization and otherwise specified, 0.01 mmol of catalyst, and rac-LA used as received. bDetermined by 1H NMR spectroscopy. cExperimental Mn and Đ determined by GPC in THF against polystyrene standards and corrected using the factor 0.58.22 dCalculated from Mrac‑LA × [LA]0/[BnOH]0 × conversion + MBnOH. eDetermined by analysis of all of the tetrad signals in the methine region of the homonuclear-decoupled 1H NMR spectrum. fIn air with 2 mL of THF. gIn air with 2 mL of toluene. hUsing rac-LA recrystallized from dry toluene. iUsing L-lactide.
Figure 3. (a) MALDI-TOF mass spectrum (matrix: DCTB; ionization salt: CF3CO2Na; solvent: THF) of poly(rac-LA) obtained in solution polymerization (Table 1, entry 4). (b) Plots of Mn and molecular weight distribution Đ of poly(rac-LA) versus [rac-LA]0/[BnOH]0 ratio (Table 1, entries 4−8).
NMR spectrum of final polymers obtained with complex 1 as a catalyst is 1:1, and the molecular weights calculated with NMR spectrum agree with calibrated GPC values, which prove these polymers are linear and BnOH acts as a real initiator (Figure S9). The MALDI-TOF spectrum confirmed this further by a series of primary peaks at 144m + 108 + 23 with a charge of +1, which can be assigned to m(C6H8O4) + BnOH + Na+ (Table 1, entry 4, and Figure 3a). A series of weak peaks with a difference in molecular mass of 72 Da suggest some transesterification reaction happens during this polymerization
smoothly and afford polymers with desirable molecular weights (Table 1, entries 1−3), while the polymerization of lactide mediated by commercial Sn(Oct)2 usually happens at a high temperature, which suggests complex 1 is more active than Sn(Oct)2.23 Because of the better solubility of these complexes and fast reaction rates, CH2Cl2 seems to be a better solvent. When more BnOH was added as chain transfer reagents, the reaction becomes faster and the molecular weight distributions (Đ) change to be narrower (Table 1, entries 4−8). The ratio between a benzyl ester group and a hydroxyl group in the 1H D
DOI: 10.1021/acs.macromol.8b01229 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 2. Different Initiation Efficiencies of Water and BnOH in rac-Lactide Polymerizationa entry
Cat.
[Cat.]0/[rac-LA]0/[BnOH]0/[H2O]0 or [acid]0
1 2 3 4 5f 6 7 8 9 10 11 12 13 14 15 16
1 2 1 2 2 1 2 1 2 1 1 2 1 1 1 1
1/100/0/0 1/100/0/0 1/100/5/0 1/100/5/0 1/100/5/0 1/100/0/5 1/100/0/5 1/500/0/0 1/500/0/0 1/500/20/0 1/500/20/4 0.5/500/20/0 1/500/20/20 1/500/2/2 1/500/0/20 1/500/20/4
initiator
BnOH BnOH BnOH H2O H2O
BnOH BnOH, BnOH BnOH, BnOH, H2O BnOH, acid
H2O H2O H2O lactic
temp (°C)
time
convb (%)
r.t. r.t. r.t. r.t. 90 r.t r.t 130 130 130 130 130 130 130 130 130
48 h 48 h 1h 48 h 12 h 48 h 48 h 4h 4h 2 min 2h 2h 12 h 2h 24 h 3h
