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lithium heterobimetallic complexes
Inorganic Chemistry Communications 14 (2011) 859–862
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Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
Ring-opening polymerization of L-lactide catalyzed by robust magnesium–sodium/lithium heterobimetallic complexes Lei Wang, Jinfeng Zhang, Lihui Yao, Ning Tang, Jincai Wu ⁎ Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 73000, People's Republic of China
a r t i c l e
i n f o
Article history: Received 29 December 2010 Accepted 2 March 2011 Available online 13 March 2011 Keywords: Heterobimetallic complexes L-lactide Robust catalyst Ring-opening polymerization
a b s t r a c t Two heterobimetallic complexes of [(TBBP)2Mg][(Na)2(THF)4] 1 [TTBP-H2 = 2,2′-dihydroxy-3,3′,5,5′-tetra-tertbutyl-1,1′-diphenyl] and [(TBBP)2Mg][(Li)2(THF)4] 2 can initiate the ring-opening polymerization of L-lactide. And the experiment results show that complex 1 can initiate the ring-opening polymerization of L-lactide under an air atmosphere with high conversion, indicated that 1 is a robust catalyst in the polymerization of L-LA. © 2011 Elsevier B.V. All rights reserved.
Poly(lactide) (PLA) [1], poly(ε-caprolactone) (PCL) [2] and their copolymers are the most promising biodegradable and biocompatible synthetic macromolecules. Due to their biodegradable, biocompatible, and permeable properties [3], these materials have given rise to a broad range of practical applications such as medicine, pharmaceutics, and tissue engineering [4]. Owing to the advantages of well controlled molecular weight and low polydispersity (PDI), metal-based catalytic systems (e.g., Al [5], Li [6], Mg [7], and Zn [8]) have attracted considerable attention for the ring-opening polymerization (ROP) of cyclic esters. However, backbiting reactions leading to the formation of macrocycles always occur as side reactions while using metal alkoxides as initiators for ROP of lactide, the undesired backbiting reactions can be eliminated by using a suitable sterically bulky ligand to interact coordinatively with the active center and therefore provide a steric barrier to a certain extent around that metal center to minimize the side reactions, and Lin's group has synthesized a series of metal aggregates with a bulky ligand, 2,2′ethylidenebis(4,6-di-tert-butyl-phenol) (EDBP-H2)[6c,6d,7b,8b], which have shown great reactivity toward ROP of cyclic ester. Although these metal complexes are efficient catalysts for the preparation of PLA and PCL, it is necessary to note that the majority of initiators reported in the literatures are air- and moisture-sensitive and are not stable to impurities in the monomer. It has been reported that the Zn(II) complexes prepared by taking Zn(OAc)2·2H2O with the corresponding Schiff base precursors, initiating ROP of unsublimed raclactide (rac-LA) in melt conditions at 130 °C in an atmosphere [9]. And Gibson's group has synthesized a dianionic Fe(II) aryloxide, hetero-
⁎ Corresponding author. E-mail address: [email protected] (J. Wu). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.03.011
bimetallic complexes used in the ring-opening polymerization of rac-LA, in which 72% conversion achieved with equivalent of benzoic acid added to the catalyst [10]. However, up to date, there are few reports on ROP initiated by metal-based catalytic systems without protection of N2 or adding water to the run of polymerization to our best knowledge. There is an exigent need to prepare initiators that are stable to the trace impurities, thus negating the need to purify the monomer and reducing processing costs to industry. Based on these people's work above, we herein report a new kind of bulky hetero-bimetallic aryloxides (Scheme 1); the catalytic activities of these complexes toward ROP of L-lactide are also presented. Preliminary results show that complex 1 could initiate ROP of L-LA exposed to air. The synthesis of 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butyl-1,1′diphenyl (TBBPH2) is in accordance with Ref. [11]. [(TBBP)2Mg] [(Na)2(THF)4] (1) was obtained as a colorless crystalline solid in 67% yield from the reaction of TBBPH2 with sodium and Mg(nBu)2 with a molar ratio of 2:2:1 in THF. For the purpose of comparing the cation effects and insensibility to moisture on the ring-opening of lactide, [(TBBP)2Mg][(Li)2(THF)4] (2) can be obtained in yield of 79% with the same procedure when using lithium instead of sodium [12]. X-ray quality crystals of 1 were crystallized from a concentrated solution in THF at ambient temperature [13]. The unit cell of 1 contains two independent molecules, but their structural features (bond distances and angles) are essentially same, so that only one of them is shown in Fig. 1. The mixed sodium–magnesium complex 1 which possesses a single Mg(II) center coordinated by two 2,2′-dihydroxy3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl(TBBP) phenoxide ligands is crystallized in monoclinic space group C2/c. The geometry around Mg(II) is distorted tetrahedral, which coordinated by four oxygen atoms of the phenoxy group with the bond lengths of Mg1―O1 1.941
860
L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 859–862
Scheme 1. Preparation of complexes 1 and 2.
(3), Mg1―O1(A) 1.941(3), Mg1―O2 1.951(3), and Mg1―O2(A) 1.951 (3) Å. Two Na counterions bridge phenoxy ligands and are further coordinated by two THF ligands. The average bond length of Na―O of complex 1 is longer than that of 2 (Li―O), possibly owing to the shorter atomic radius of lithium compared with sodium atom. Single crystals of 2 suitable for X-ray structural determination were obtained by slow cooling a hot toluene solution [13]. The ORTEP drawing of the molecular structure of 2, which are crystallized in orthorhombic space group Pbca, are given in Fig. 2. Complex 2 is soluble in toluene, and insoluble in hexane. In the structure, the geometry around center metal atom is a distorted tetrahedral and the atom is firmly fixed by four oxygen atoms of two TBBP ligands. In the solid state of 2, the fourcoordinate lithium atom is surrounded by two oxygen atoms from THF and two phenyl oxygen atoms of two bulky ligands. In this context, ROP of L-lactide employing 1 and 2 (0.02 mmol) as catalyst is systematically examined in toluene at 90 °C, as shown in Table 1. Complex 1 gives 93% conversion of 100 equiv. of L-lactide after 24 h, and the similar conversion should take 48 h by catalyst 2. It can be seen that catalyst 1 has a much higher catalytic activity compared with its counterpart, because Na+ is a weaker Lewis acid than Li+ and complex 1 is more basic complex. The PDIs of the polymers are narrow, ranging from 1.37 to 1.50 (Table 1, entries1–4). Furthermore, epimerization of the chiral centers in PLA-50 does not occur as observed by the homonuclear decoupled 1H NMR studies in the methine region
Fig. 2. Molecular structure of 2 as 30% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Mg1―O1 1.916(4), Mg1―O2 1.932(4), Mg1―O3 1.932(4), Mg1―O4 1.928(4), Li1―O1 1.951(11), Li1―O4 1.939(11), Li1―O7 2.000(11), Li1―O8 2.007(11), Li2―O2 1.945(11), Li2―O3 1.945(10), Li2―O5 2.019(11), and Li2―O6 1.953(11).
Table 1 Ring-opening polymerization of L-LA using complexes 1 and 2a.
Fig. 1. Molecular structure of 1 as 30% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Mg1―O1 1.941(3), Mg1―O1(A) 1.941(3), Mg1―O2 1.951(3), Mg1―O2(A) 1.951 (3), Na1―O1 2.240(4), Na―O1(A) 2.240(4), Na1―O3 2.265(6), Na1―O3(A) 2.265(6), Na2―O2 2.240(3), Na2―O2(A) 2.240(3), Na2―O4 2.260(5), and Na2―O4(A) 2.260 (5).
Entry
Cat.
[M]0/[cat.]0
t(h)
PDI
Mnobsdb
Mncalcdc
Conversn(%)
1 2 3 4 5 6 7 8 9
1 1 1 1 2 1d 1d 1e 2e
100 125 150 175 100 150 200 100 100
24 24 24 24 48 24 24 24 48
1.37 1.38 1.40 1.50 1.40 1.38 1.43 1.76 n.d.f
20,200 21,500 23,300 26,500 21,400 18,000 18,100 3800 n.d.
13,400 17,000 19,800 25,200 14,400 19,500 25,800 10,000 n.d.
93.3 94.7 91.8 N 99 N 99 90.6 89.5 70.0 20.0
a Conditions: 0.02 mmol of complexes, 10 mL of toluene solvent, 90 °C, reaction time is 24 h for 1, and 48 h for 2. b Obtained from GPC analysis times 0.58 [15] and calibrated by polystyrene standard. c Mncalcd = ([M]0 / [complex]0) × 144.13 × conv.%. d Polymerization reactions were exposed to air. e 3.6 μL H2O added. f Not determined.
L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 859–862
861
Fig. 3. 1H NMR of PLA-50 in CDCl3.
[14]. End-group analysis of the polymer reveals the existence of terminal hydroxyl group evidenced by the 4.36 ppm peak in 1H NMR spectrum which indicates the polymer is linear chain (Fig. 3). To confirm the components of the end group of the polymer, Electrospray-ionization (+ESI) mass spectrum of PLA-50 after recrystallization which produced from 1 at an initial monomer/complex ratio [LA]0 / [Complex]0 = 50 was also obtained. The analysis exhibited a main set of signals corresponding to oligomers of the formula HO(COCHMeO)nH·M+ (M= Na) (Fig. 4).
It is interesting that complex 1 can initiate polymerization of L-LA under an air atmosphere after 24 h in toluene at 90 °C with high conversion (Table 1, entries 6 and 7). To systemically study the stability of complex 1 as conducting the polymerization of L-LA, equivalent (3.6 μL) water compared to the catalyst was added in the catalysis reaction (100:1). Surprisingly, it was found that the conversion is up to 70% (Table 1, entry 8). The same operation was taken when complex 2 was employed as catalyst, however, the
Fig. 4. + ESI mass spectrum of PLA-50.
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L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 859–862
conversion was just 20% and the Mn and PDI data were not examined (Table 1, entry 9), indicated that complex 1 is a robust compound compared with 2 while catalyzing polymerization of L-LA. In conclusion, two new heterometallic aryloxides were synthesized as catalysts for the ring opening polymerization of L-lactide, and complex 1 even can initiate ROP of L-lactide exposed to air with high conversion. Thus a new strategy for the ROP of lactide in an open atmosphere was presented.
[6] (a) (b) (c) (d) [7] (a) (b) [8] (a)
[9] [10]
Acknowledgments [11]
We thank the financial supports from the National Natural Science Foundation of China (No. 21071069), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and the Fundamental Research Funds for the Central Universities of China.
[12]
Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.03.011.
[13]
References [1] (a) X. Chen, R.A. Gross, Macromolecules 32 (1999) 308; (b) B.M. Chamberlain, Y. Sun, J.R. Hagadorn, E.W. Hemmesch, V.G. Young, M. Pink, M.A. Hillmyer, W.B. Tolman, Macromolecules 32 (1999) 2400. [2] (a) M. Endo, T. Aida, S. Inoue, Macromolecules 20 (1987) 2982–2988; (b) A. Duda, Z. Florjanczyk, A. Hofman, S. Slomkowski, S. Penczek, Macromolecules 23 (1990) 1640–1646. [3] (a) W. Amass, A. Amass, B. Tighe, Polym. Int. 47 (1998) 89; (b) R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. [4] (a) O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147; (b) J.-C. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 250 (2006) 602. [5] (a) Z.Y. Zhong, P.J. Dijkstra, J. Feijen, J. Am. Chem. Soc. 125 (2003) 11291; (b) M.H. Chisholm, N.J. Patmore, Z.P. Zhou, Chem. Commun. (2005) 127.
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H.R. Kricheldorf, C. Boettcher, Makromol. Chem. 194 (1993) 1665; J.E. Kasperczyk, Macromolecules 28 (1995) 3937; B.-T. Ko, C.-C. Lin, J. Am. Chem. Soc. 123 (2001) 7973; M.-L. Hsueh, B.-H. Huang, J. Wu, C.-C. Lin, Macromolecules 38 (2005) 9482. T. Chivers, C. Fedorchuk, M. Parvez, Organometallics 24 (2005) 580; J. Wu, Y.-Z. Chen, W.-C. Hung, C.-C. Lin, Organometallics 27 (2008) 4970. A.P. Dove, V.C. Gibson, E.L. Marshall, A.J.P. White, D.J. Williams, Dalton Trans. (2004) 570; (b) H.Y. Chen, H.Y. Tang, C.-C. Lin, Macromolecules 39 (2006) 3745. D.J. Matthew, M.G. Davidson, G.K. Callum, M.H. Laura, F.M. Mary, C.A. David, Eur. J. Inorg. Chem. (2009) 635–642. D.S. McGuinness, E.L. Mashall, V.C. Gibson, J.W. Steed, J. Polym. Sci. A Polym. Chem. 41 (2003) 3798. J.I. Vlugt, A.C. Hewat, S. Neto, R. Sablong, A.M. Mills, M. Lutz, A.L. Spek, C. Müller, D. Vogta, Adv. Synth. Catal. 346 (2004) 993. a) Data for complex 1: (0.87 g, 67% Yield). 1H NMR (200 MHz, CDCl3, ppm): δ 7.09 (s, 4H, Ph), 6.75 (m, 4H, Ph), 3.60 (s, 16H, O―CH2), 1.74 (s, 16H, CH2), 1.25 (s, 36H, 13 C(CH3)3), 1.18 (s, 36H, C(CH3)3). C NMR (CDCl3, ppm): 160.12, 135.28, 134.79, 134.04, 128.44, 121.66 (Ph); 67.93 (O―CH2); 34.65, 33.88,31.94, 30.71 (t-Bu); 25.43 (CH2). Anal. calcd for C72H112Na2O8Zn: C, 71.05; H, 9.28. Found: C, 71.09; H, 9.31. b) Data for complex 2: (1.02 g, 79% Yield). 1H NMR (200 MHz, CDCl3, ppm): δ 7.13 (s, 4H, Ph), 6.73 (m, 4H, Ph), 3.56 (s, 16H, O―CH2), 1.68 (s, 16H, CH2), 1.25 13 (s, 36H, C(CH3)3), 1.18 (s, 36H, C(CH3)3). C NMR (CDCl3, ppm): 157.93, 136.13,135.94, 133.43, 128.17, 122.09 (Ph); 68.08 (O―CH2); 34.86, 33.88, 32.10, 31.50 (t-Bu); 25.30 (CH2). Anal. calcd for C72H112Li2O8Zn: C, 72.98; H, 9.53. Found: C, 72.94; H, 9.49. a) Crystal data for complex 1: C78H125MgNa2O9, M =1297.04, monoclinic, space group C2/c, a= 25.4945(8) Å, b= 26.2967(8) Å, c =25.0617(9) Å, α=90.00, β=103.384 (2), γ=90.00, V=16345.6(9) Å3, T=293(2) K, Z= 8, Dc =1.054 g/cm3, F000 = 5674, θmax = 26.05, 45830 reflections collected, 15885 unique (Rint = 0.0885), no. of observed reflections 5542 (IN 2σ(I)); R1 =0.0873, wR2 =0.2268. b) Crystal data for complex 2: C86H128MgLi2O8, M = 1328.07, orthorhombic, space group Pbca, a = 18.9268(16) Å, b = 20.4262(18) Å, c = 43.3010(4) Å, α= 90.00, β= 90.00, γ=90.00, V=16740(3) Å3, T= 293(2) K, Z=8, Dc =1.054 g/cm3, F000 = 5808, θmax =20.82, 55562 reflections collected, 8768 unique (Rint =0.0850), no. of observed reflections 4338 (IN 2σ(I)); R1 = 0.0666, wR2 =0.1914. K.A.M. Thakur, R.T. Kean, E.S. Hall, J.J. Kolstad, T.A. Lindgren, M.A. Doscotch, J.I. Siepmann, E.J. Munson, Macromolecules 30 (1997) 2422. The Mn (GPC) value is multiplied by a factor of 0.58, giving the actual Mn of the polylactide. (a) J. Baran, A. Duda, A. Kowalski, R. Szymanski, S. Penczek, Macromol. Rapid Commun. 18 (1997) 325; (b) T. Biela, A. Duda, S. Penczek, Macromol. Symp. 183 (2002) 1.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
Ring-opening polymerization of L-lactide catalyzed by robust magnesium–sodium/lithium heterobimetallic complexes Lei Wang, Jinfeng Zhang, Lihui Yao, Ning Tang, Jincai Wu ⁎ Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 73000, People's Republic of China
a r t i c l e
i n f o
Article history: Received 29 December 2010 Accepted 2 March 2011 Available online 13 March 2011 Keywords: Heterobimetallic complexes L-lactide Robust catalyst Ring-opening polymerization
a b s t r a c t Two heterobimetallic complexes of [(TBBP)2Mg][(Na)2(THF)4] 1 [TTBP-H2 = 2,2′-dihydroxy-3,3′,5,5′-tetra-tertbutyl-1,1′-diphenyl] and [(TBBP)2Mg][(Li)2(THF)4] 2 can initiate the ring-opening polymerization of L-lactide. And the experiment results show that complex 1 can initiate the ring-opening polymerization of L-lactide under an air atmosphere with high conversion, indicated that 1 is a robust catalyst in the polymerization of L-LA. © 2011 Elsevier B.V. All rights reserved.
Poly(lactide) (PLA) [1], poly(ε-caprolactone) (PCL) [2] and their copolymers are the most promising biodegradable and biocompatible synthetic macromolecules. Due to their biodegradable, biocompatible, and permeable properties [3], these materials have given rise to a broad range of practical applications such as medicine, pharmaceutics, and tissue engineering [4]. Owing to the advantages of well controlled molecular weight and low polydispersity (PDI), metal-based catalytic systems (e.g., Al [5], Li [6], Mg [7], and Zn [8]) have attracted considerable attention for the ring-opening polymerization (ROP) of cyclic esters. However, backbiting reactions leading to the formation of macrocycles always occur as side reactions while using metal alkoxides as initiators for ROP of lactide, the undesired backbiting reactions can be eliminated by using a suitable sterically bulky ligand to interact coordinatively with the active center and therefore provide a steric barrier to a certain extent around that metal center to minimize the side reactions, and Lin's group has synthesized a series of metal aggregates with a bulky ligand, 2,2′ethylidenebis(4,6-di-tert-butyl-phenol) (EDBP-H2)[6c,6d,7b,8b], which have shown great reactivity toward ROP of cyclic ester. Although these metal complexes are efficient catalysts for the preparation of PLA and PCL, it is necessary to note that the majority of initiators reported in the literatures are air- and moisture-sensitive and are not stable to impurities in the monomer. It has been reported that the Zn(II) complexes prepared by taking Zn(OAc)2·2H2O with the corresponding Schiff base precursors, initiating ROP of unsublimed raclactide (rac-LA) in melt conditions at 130 °C in an atmosphere [9]. And Gibson's group has synthesized a dianionic Fe(II) aryloxide, hetero-
⁎ Corresponding author. E-mail address: [email protected] (J. Wu). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.03.011
bimetallic complexes used in the ring-opening polymerization of rac-LA, in which 72% conversion achieved with equivalent of benzoic acid added to the catalyst [10]. However, up to date, there are few reports on ROP initiated by metal-based catalytic systems without protection of N2 or adding water to the run of polymerization to our best knowledge. There is an exigent need to prepare initiators that are stable to the trace impurities, thus negating the need to purify the monomer and reducing processing costs to industry. Based on these people's work above, we herein report a new kind of bulky hetero-bimetallic aryloxides (Scheme 1); the catalytic activities of these complexes toward ROP of L-lactide are also presented. Preliminary results show that complex 1 could initiate ROP of L-LA exposed to air. The synthesis of 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butyl-1,1′diphenyl (TBBPH2) is in accordance with Ref. [11]. [(TBBP)2Mg] [(Na)2(THF)4] (1) was obtained as a colorless crystalline solid in 67% yield from the reaction of TBBPH2 with sodium and Mg(nBu)2 with a molar ratio of 2:2:1 in THF. For the purpose of comparing the cation effects and insensibility to moisture on the ring-opening of lactide, [(TBBP)2Mg][(Li)2(THF)4] (2) can be obtained in yield of 79% with the same procedure when using lithium instead of sodium [12]. X-ray quality crystals of 1 were crystallized from a concentrated solution in THF at ambient temperature [13]. The unit cell of 1 contains two independent molecules, but their structural features (bond distances and angles) are essentially same, so that only one of them is shown in Fig. 1. The mixed sodium–magnesium complex 1 which possesses a single Mg(II) center coordinated by two 2,2′-dihydroxy3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl(TBBP) phenoxide ligands is crystallized in monoclinic space group C2/c. The geometry around Mg(II) is distorted tetrahedral, which coordinated by four oxygen atoms of the phenoxy group with the bond lengths of Mg1―O1 1.941
860
L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 859–862
Scheme 1. Preparation of complexes 1 and 2.
(3), Mg1―O1(A) 1.941(3), Mg1―O2 1.951(3), and Mg1―O2(A) 1.951 (3) Å. Two Na counterions bridge phenoxy ligands and are further coordinated by two THF ligands. The average bond length of Na―O of complex 1 is longer than that of 2 (Li―O), possibly owing to the shorter atomic radius of lithium compared with sodium atom. Single crystals of 2 suitable for X-ray structural determination were obtained by slow cooling a hot toluene solution [13]. The ORTEP drawing of the molecular structure of 2, which are crystallized in orthorhombic space group Pbca, are given in Fig. 2. Complex 2 is soluble in toluene, and insoluble in hexane. In the structure, the geometry around center metal atom is a distorted tetrahedral and the atom is firmly fixed by four oxygen atoms of two TBBP ligands. In the solid state of 2, the fourcoordinate lithium atom is surrounded by two oxygen atoms from THF and two phenyl oxygen atoms of two bulky ligands. In this context, ROP of L-lactide employing 1 and 2 (0.02 mmol) as catalyst is systematically examined in toluene at 90 °C, as shown in Table 1. Complex 1 gives 93% conversion of 100 equiv. of L-lactide after 24 h, and the similar conversion should take 48 h by catalyst 2. It can be seen that catalyst 1 has a much higher catalytic activity compared with its counterpart, because Na+ is a weaker Lewis acid than Li+ and complex 1 is more basic complex. The PDIs of the polymers are narrow, ranging from 1.37 to 1.50 (Table 1, entries1–4). Furthermore, epimerization of the chiral centers in PLA-50 does not occur as observed by the homonuclear decoupled 1H NMR studies in the methine region
Fig. 2. Molecular structure of 2 as 30% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Mg1―O1 1.916(4), Mg1―O2 1.932(4), Mg1―O3 1.932(4), Mg1―O4 1.928(4), Li1―O1 1.951(11), Li1―O4 1.939(11), Li1―O7 2.000(11), Li1―O8 2.007(11), Li2―O2 1.945(11), Li2―O3 1.945(10), Li2―O5 2.019(11), and Li2―O6 1.953(11).
Table 1 Ring-opening polymerization of L-LA using complexes 1 and 2a.
Fig. 1. Molecular structure of 1 as 30% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Mg1―O1 1.941(3), Mg1―O1(A) 1.941(3), Mg1―O2 1.951(3), Mg1―O2(A) 1.951 (3), Na1―O1 2.240(4), Na―O1(A) 2.240(4), Na1―O3 2.265(6), Na1―O3(A) 2.265(6), Na2―O2 2.240(3), Na2―O2(A) 2.240(3), Na2―O4 2.260(5), and Na2―O4(A) 2.260 (5).
Entry
Cat.
[M]0/[cat.]0
t(h)
PDI
Mnobsdb
Mncalcdc
Conversn(%)
1 2 3 4 5 6 7 8 9
1 1 1 1 2 1d 1d 1e 2e
100 125 150 175 100 150 200 100 100
24 24 24 24 48 24 24 24 48
1.37 1.38 1.40 1.50 1.40 1.38 1.43 1.76 n.d.f
20,200 21,500 23,300 26,500 21,400 18,000 18,100 3800 n.d.
13,400 17,000 19,800 25,200 14,400 19,500 25,800 10,000 n.d.
93.3 94.7 91.8 N 99 N 99 90.6 89.5 70.0 20.0
a Conditions: 0.02 mmol of complexes, 10 mL of toluene solvent, 90 °C, reaction time is 24 h for 1, and 48 h for 2. b Obtained from GPC analysis times 0.58 [15] and calibrated by polystyrene standard. c Mncalcd = ([M]0 / [complex]0) × 144.13 × conv.%. d Polymerization reactions were exposed to air. e 3.6 μL H2O added. f Not determined.
L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 859–862
861
Fig. 3. 1H NMR of PLA-50 in CDCl3.
[14]. End-group analysis of the polymer reveals the existence of terminal hydroxyl group evidenced by the 4.36 ppm peak in 1H NMR spectrum which indicates the polymer is linear chain (Fig. 3). To confirm the components of the end group of the polymer, Electrospray-ionization (+ESI) mass spectrum of PLA-50 after recrystallization which produced from 1 at an initial monomer/complex ratio [LA]0 / [Complex]0 = 50 was also obtained. The analysis exhibited a main set of signals corresponding to oligomers of the formula HO(COCHMeO)nH·M+ (M= Na) (Fig. 4).
It is interesting that complex 1 can initiate polymerization of L-LA under an air atmosphere after 24 h in toluene at 90 °C with high conversion (Table 1, entries 6 and 7). To systemically study the stability of complex 1 as conducting the polymerization of L-LA, equivalent (3.6 μL) water compared to the catalyst was added in the catalysis reaction (100:1). Surprisingly, it was found that the conversion is up to 70% (Table 1, entry 8). The same operation was taken when complex 2 was employed as catalyst, however, the
Fig. 4. + ESI mass spectrum of PLA-50.
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L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 859–862
conversion was just 20% and the Mn and PDI data were not examined (Table 1, entry 9), indicated that complex 1 is a robust compound compared with 2 while catalyzing polymerization of L-LA. In conclusion, two new heterometallic aryloxides were synthesized as catalysts for the ring opening polymerization of L-lactide, and complex 1 even can initiate ROP of L-lactide exposed to air with high conversion. Thus a new strategy for the ROP of lactide in an open atmosphere was presented.
[6] (a) (b) (c) (d) [7] (a) (b) [8] (a)
[9] [10]
Acknowledgments [11]
We thank the financial supports from the National Natural Science Foundation of China (No. 21071069), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and the Fundamental Research Funds for the Central Universities of China.
[12]
Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.03.011.
[13]
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H.R. Kricheldorf, C. Boettcher, Makromol. Chem. 194 (1993) 1665; J.E. Kasperczyk, Macromolecules 28 (1995) 3937; B.-T. Ko, C.-C. Lin, J. Am. Chem. Soc. 123 (2001) 7973; M.-L. Hsueh, B.-H. Huang, J. Wu, C.-C. Lin, Macromolecules 38 (2005) 9482. T. Chivers, C. Fedorchuk, M. Parvez, Organometallics 24 (2005) 580; J. Wu, Y.-Z. Chen, W.-C. Hung, C.-C. Lin, Organometallics 27 (2008) 4970. A.P. Dove, V.C. Gibson, E.L. Marshall, A.J.P. White, D.J. Williams, Dalton Trans. (2004) 570; (b) H.Y. Chen, H.Y. Tang, C.-C. Lin, Macromolecules 39 (2006) 3745. D.J. Matthew, M.G. Davidson, G.K. Callum, M.H. Laura, F.M. Mary, C.A. David, Eur. J. Inorg. Chem. (2009) 635–642. D.S. McGuinness, E.L. Mashall, V.C. Gibson, J.W. Steed, J. Polym. Sci. A Polym. Chem. 41 (2003) 3798. J.I. Vlugt, A.C. Hewat, S. Neto, R. Sablong, A.M. Mills, M. Lutz, A.L. Spek, C. Müller, D. Vogta, Adv. Synth. Catal. 346 (2004) 993. a) Data for complex 1: (0.87 g, 67% Yield). 1H NMR (200 MHz, CDCl3, ppm): δ 7.09 (s, 4H, Ph), 6.75 (m, 4H, Ph), 3.60 (s, 16H, O―CH2), 1.74 (s, 16H, CH2), 1.25 (s, 36H, 13 C(CH3)3), 1.18 (s, 36H, C(CH3)3). C NMR (CDCl3, ppm): 160.12, 135.28, 134.79, 134.04, 128.44, 121.66 (Ph); 67.93 (O―CH2); 34.65, 33.88,31.94, 30.71 (t-Bu); 25.43 (CH2). Anal. calcd for C72H112Na2O8Zn: C, 71.05; H, 9.28. Found: C, 71.09; H, 9.31. b) Data for complex 2: (1.02 g, 79% Yield). 1H NMR (200 MHz, CDCl3, ppm): δ 7.13 (s, 4H, Ph), 6.73 (m, 4H, Ph), 3.56 (s, 16H, O―CH2), 1.68 (s, 16H, CH2), 1.25 13 (s, 36H, C(CH3)3), 1.18 (s, 36H, C(CH3)3). C NMR (CDCl3, ppm): 157.93, 136.13,135.94, 133.43, 128.17, 122.09 (Ph); 68.08 (O―CH2); 34.86, 33.88, 32.10, 31.50 (t-Bu); 25.30 (CH2). Anal. calcd for C72H112Li2O8Zn: C, 72.98; H, 9.53. Found: C, 72.94; H, 9.49. a) Crystal data for complex 1: C78H125MgNa2O9, M =1297.04, monoclinic, space group C2/c, a= 25.4945(8) Å, b= 26.2967(8) Å, c =25.0617(9) Å, α=90.00, β=103.384 (2), γ=90.00, V=16345.6(9) Å3, T=293(2) K, Z= 8, Dc =1.054 g/cm3, F000 = 5674, θmax = 26.05, 45830 reflections collected, 15885 unique (Rint = 0.0885), no. of observed reflections 5542 (IN 2σ(I)); R1 =0.0873, wR2 =0.2268. b) Crystal data for complex 2: C86H128MgLi2O8, M = 1328.07, orthorhombic, space group Pbca, a = 18.9268(16) Å, b = 20.4262(18) Å, c = 43.3010(4) Å, α= 90.00, β= 90.00, γ=90.00, V=16740(3) Å3, T= 293(2) K, Z=8, Dc =1.054 g/cm3, F000 = 5808, θmax =20.82, 55562 reflections collected, 8768 unique (Rint =0.0850), no. of observed reflections 4338 (IN 2σ(I)); R1 = 0.0666, wR2 =0.1914. K.A.M. Thakur, R.T. Kean, E.S. Hall, J.J. Kolstad, T.A. Lindgren, M.A. Doscotch, J.I. Siepmann, E.J. Munson, Macromolecules 30 (1997) 2422. The Mn (GPC) value is multiplied by a factor of 0.58, giving the actual Mn of the polylactide. (a) J. Baran, A. Duda, A. Kowalski, R. Szymanski, S. Penczek, Macromol. Rapid Commun. 18 (1997) 325; (b) T. Biela, A. Duda, S. Penczek, Macromol. Symp. 183 (2002) 1.