Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Isoselective Polymerization of rac-Lactide Catalyzed by Ion-Paired Potassium Amidinate Complexes Changjuan Chen,†,‡ Jinxing Jiang,† Xiaoyang Mao,† Yong Cong,† Yaqin Cui,† Xiaobo Pan,† and Jincai Wu*,† †
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: Three potassium crown ether complexes supported with bulky amidinate ligands were synthesized for the ring-opening polymerization (ROP) of rac-lactide. The side polymerization reaction initiated directly by ligand anion was suppressed well in the presence of alcohol as our design, and the synthesis of linear polylactide with a molecular weight as high as 117.7 kg/mol was successful together with an isoselectivity value of Pm = 0.88 at −70 °C. In this system, lactide can be deprotonated by amidinate anion to give lactide enolate, which can initiate the ROP of lactide as a side reaction in the absence of alcohol; however, this side reaction can also be suppressed well in the presence of alcohol by a decrease in temperature. An interesting anti-Arrhenius-like behavior in the polymerization was discovered, which can be attributed to the fact that the active catalyst can be converted to a less active lactide enolate potassium complex at a high temperature.
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side reaction,11 which can offer a high-molecular-weight polylactide with molecular weight high up to 107 kg/mol and Pm can also be kept at a high value. In this work, we report another approach to minimize the nucleophilicity of the ligand anion to suppress the side polymerization reaction initiated by the ligand anion itself: some new ion-paired potassium complexes were designed (Scheme 1) with the aim of increasing the stereo hindrance of the ligand and consequently avoiding ligand anion attack lactide monomer directly, but with the basicity of ligand anion remaining strong enough to activate alcohol to initiate the ROP reaction.
INTRODUCTION The ring-opening polymerization (ROP) of lactide has attracted much attention in the past several decades, because biorenewable, biocompatible, and biodegradable polylactide is an alternative to replace slowly degrading petrochemical-based polymeric material. Due to the influence of stereoregularity on the physical and chemical properties of polylactide, the catalytically stereocontrolled ROP of rac-lactide is considered to be very important.1−6 In the past several years, a series of sodium/potassium crown ether complexes were reported by us7−11 and Wang and co-workers12,13 which can isoseletively catalyze the ROP of rac-lactide (A and B in Chart 1); the isoselectivity even can reach Pm = 0.94 at −70 °C.10 These types of alkali-metal complexes seem to be comparably good catalysts for the ROP of rac-lactide in comparison to other isoselective metal complex systems of aluminum,14−32 indium,33−37 zinc,38−43 lanthanides,44−48 and other complexes.49−52 While in this sodium/potassium system the ligand anion itself can initiate the ROP reaction, directly giving a cyclic polymer which is a detrimental side reaction; sometimes the reaction is not serious upon addition of 1 equiv of alcohol, but in some cases, this side reaction cannot be neglected. To suppress this side reaction, the addition of excess alcohol as initiator is useful, but it gives rise to a large obstacle in obtaining high-molecular-weight polymers. The reduction of the nucleophilicity of the ligand anion is a viable method: for example, a series of bis-chelated sulfonamidate sodium/ potassium complexes exhibited the ability to suppress this © XXXX American Chemical Society
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RESULTS AND DISCUSSION With the aim of decreasing the nucleophilicity of the ligand anion in mind, amidine ligands of HL1−HL3 with different substituent groups of phenyl, naphthyl, and anthryl53,54 were designed and chosen because the active center is surrounded by bulky substituted groups but the strong basicity of amidinate anion center can still be maintained (Scheme 1). Complexes 1−3 were synthesized by treatment of ligands with potassium salt and 18-crown-6 ether in high yields (72−85%). Crystals of complex 1 suitable for diffraction data collection were obtained from a deuterated benzene solution. As shown in Figure 1, there are two types of potassium crown cations: the K1-crown Received: December 20, 2017
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DOI: 10.1021/acs.inorgchem.7b03184 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Chart 1. Alkali-Metal Crown Ether Complexes
which suggests that the surroundings of the negatively charged centers of two nitrogen atoms are crowded. The ROP of rac-LA using complexes 1−3 as catalysts was performed in toluene because the polymerization rates are very slow in CH2Cl2 and THF with complexes 1−3 as catalysts (Table 1, entries 1, 2, and 4) due to the competitive coordination of THF and the solvent polarities, which was also found in our previous works.7−11 When 100 equiv of lactide solid was added to a solution mixture of a 1:1 ratio of catalyst and BnOH with a catalyst concentration of 2 mM at room temperature in toluene (please note the addition sequence of reactants), 90% and 56% rac-lactide can be converted into polymer within 60 min with complexes 1 and 2 as catalysts, respectively, affording polymers with molecular weights slightly lower than expected (Table 1, entries 3 and 4); the isotacticities of the resulting polymers are modest with Pm values of 0.63 and 0.64, respectively. However, under identical reaction conditions, the polymerization almost cannot happen within 480 min using a catalyst of complex 3 (conversion
