Macromolecular Research, Vol. 21, No. 4, pp 385-391 (2013) DOI 10.1007/s13233-013-1033-6

www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673

Catalytic Behavior of Silyl-Amide Complexes for Lactide Polymerization Chan Woo Lee*,1, Satoshi Kuno2, and Yoshiharu Kimura*,2 1

Department of Innovative Industrial Technology, Hoseo University, Chungnam 336-851, Korea Department of Biomolecular Engineering, Kyoto Institute of Technology Matsugasaki, Kyoto 606-8585, Japan

2

Received February 10, 2012; Revised April 30, 2012; Accepted May 31, 2012 Abstract: A series of silyl-amide complexes (=M{N(SiMe3)2}2) were examined as the catalysts for the ring-opening polymerization (ROP) of L-lactide. The catalytic activity of the silyl-amide complexes for zinc was compared with that of the silyl-amide complexes of tin in order to clarify the structural effects on the catalytic behavior. In the tin complexes, the addition of 1-dodecanol made the polymerization rate of L-lactide faster and suppressed the initiation with the N(SiMe3)2 ligands. The 1H NMR analysis revealed that the rates of ligand exchange for the N(SiMe3)2 and 1-dodecanol are different between the zinc and tin complexes. Based on this finding, melt polymerizations of L-lactide was conducted with the zinc catalyst to synthesize high-molecular-weight poly(L-lactide). The Mn of the obtained polymer was found to become higher than 60,000 Da, which is required for the industrial application of polylactide, in a short reaction time. Keywords: silyl-amide complex, L-lactide, ring-opening polymerization, initiation, biocompatible catalyst.

Introduction Poly(L-lactic acid) (PLLA) is the most promising biobased polymer that is crystalline with a melting temperature of ca. 180 oC. Since PLLA has excellent mechanical properties that are comparable to those of the commercial oilbased polymers such as polyethylene, polypropylene, and polystyrene, it can be used in various fields as an alternative polymeric material of the conventional oil-based polymers. Ring-opening polymerization (ROP) of L-lactide (L-LA, a cyclic dimer of L-lactic acid) is adopted in the ordinary industrial production of PLLA (Scheme I). The standard catalyst system utilized for synthesizing high-molecularweight PLLA is tin(II) octoate with which lauryl alcohol (1dodecanol) is usually used as an initiator.1,2 This catalyst system has several advantages over the others in terms of solubility in organic solvents and molten lactide in bulk state, stability on storage, and excellent polymerizability up to 180 oC.3 Among the wide application of PLLA, its biomedical use has constituted one of the most important fields until now.4,5 Because of the high biocompatibility and biodegradability of PLLA and its derivatives, bioabsorbable devices have been invented.6,7 For example, sutures, dental devices for orthopedic fixation, medical devices, drug delivery systems, and scaffolds for tissue engineering are made from PLLA

Scheme I. Ring-opening polymerization of L-LA.

materials.8-11 These devices, however, should contain the tin catalyst used for ROP of lactide because it is not eliminated from the raw polymers. Therefore, there has been a concern whether the tin catalyst remaining may cause toxicity problems or undesirable effects on human body when the PLLA polymer is biodegraded in vivo. To eliminate this concern, the catalyst must be removed or replaced by safe biocompatible catalysts. Previous studies reported biocompatible metal catalysts consisting of Zn and Mg. For example, Chisholm et al. used zinc and magnesium alkoxides with tris(pyrazolyl)hydroborate ligands in the ROP of L-lactide in CH2Cl2 and obtained PLLA with narrow molecular weight distribution (PDI=1.1-1.25) at a conversion up to 90%.12,13 Coates et al. reported the polymerization of DLlactide in CH2Cl2 by using a zinc β-diketiminate complex with various initiating groups. They obtained narrow molecular weight distribution (PDI=1.10) of the product when an isopropoxide initiating group was used.14-16 However, as far as we know, the catalytic activity of these biocompatible metal catalysts was not as high as that of the conventional tin catalysts. In order to develop biocompatible metal catalysts that are as active as tin octoate, what is important for increasing the catalytic activity must be known. In the previous ROP of

*Corresponding Authors. E-mails: [email protected] or [email protected] The Polymer Society of Korea

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various cyclic esters, the silyl-amide complexes were used together with cocatalysts that can be true initiating species, but the catalytic activities of themselves in the ROP have not well understood.17-21 In this study, we examined a series of silyl-amide complexes (=M{N(SiMe3)2}2 ) as the catalysts for ROP of L-lactide. Since the silyl-amide complexes can accommodate various metals in the center, comparative study was first conducted with different metal complexes. Then, a silylamide complex of zinc was chosen and compared its catalytic activity with that of tin complex to clarify the structural effects on the catalytic behavior. Finally, we tried to synthesize high-molecular-weight PLLA by melt polymerizations of L-lactide by using the zinc catalyst in view of its industrial application, in particular with their biomedical application in mind.

Experimental Materials. L-Lactide (L-LA:Mp=95 oC, 99.8%ee) was supplied by Musashino Chemical Laboratory, Ltd. (Tokyo) and dried in vacuum for more than 6 h at 45 oC before use. Silyl-amide complexes Zn{N(SiMe3)2}2 and Sn{N(SiMe3)2}2 were purchased from Aldrich. 1-Dodecanol (DDN) was purchased from Nacalai Tesque Co. (Kyoto). These materials were used as received. All manipulations were conducted under an inert atmosphere of nitrogen using the standard Schlenk-line and glove-box techniques. Measurements. 300 MHz 1H NMR spectra were recorded in deuterated chloroform (CDCl3) containing 0.03 vol% tetramethylsilane on a Bruker AV-300 NMR spectrometer and were referenced to the residual solvent resonance (CDCl3, δ 7.26). Gel permeation chromatography (GPC) was carried out using a Shimadzu (Kyoto) system consisting of a LC10A pump, a refractive-index detector, and a C-R7A plus Chromatopac data processor equipped with a set of two Tosoh TSK gel-GMHHR-M columns. The sample was eluted with 1,3-dioxolane as the solvent at 45 oC and at 0.75 mL/ min in flow rate. The number (Mn) and weight (Mw) molecular weights of the sample polymers were calibrated with polystyrene standards. Solution Polymerizations. L-LA (0.20 g, 1.39 mmol) was dissolved in 2.5 mL of CH2Cl2, and a CH2Cl2 solution of DDN (0.02 mmol) was added as the initiator, if necessary. Then, a predetermined amount of a catalyst (0.01 mmol) was added to this solution ([L-LA]/[cat.]=139/1), and the reaction mixture was stirred at room temperature or 50 oC. After a desired time, the polymerization was quenched by adding an excess of methanol. The polymer precipitates obtained were then filtered and dried under vacuum to constant weight. The polymerization was also conducted in CHCl3 and tetrahydrofuran (THF) for which the reaction temperature was set at 60 oC with other condition being identical. Kinetic Studies. L-LA (0.20 g, 1.39 mmol) was dissolved 386

in 2.5 mL of CH2Cl2 and added with a solution of DDN (0.02 mmol) as an initiator. A catalyst (0.01 mmol) was then added to this solution ([L-LA]/[cat.]=139:1), and the reaction mixture was stirred at 45 oC. At desired time intervals, an aliquot of the solution was taken out and evaporated to dryness in a hood. The residue was then subjected to 1H NMR measurement to determine the monomer conversion based on the integral ratios of the methine signals of the polymer and monomer shown at δ 5.16 (q, H) and δ 5.03 (q, H) ppm, respectively. The consumption of the N(SiMe3)2 group was also monitored by the increase of signal intensity at δ 0.07 and 0.15 ppm. Melt Polymerization. L-LA (1.00 g, 6.95 mmol) was mixed with a certain amount of a CH2Cl2 solution of DDN (0.01 mmol) in a test tube and dried in vacuum at room temperature for 1 h. Then, a predetermined amount of Zn{N (SiMe3)2}2 (5.00×10-3 mmol) was added to the mixture. The resultant solid mixture was heated at 120 oC for melt-polymerization. After a desired time, the whole product was dissolved in a CH2Cl2 and reprecipitated into an excess of methanol. The polymer precipitates obtained were filtered and dried in vacuum to constant weight.

Results and Discussion Synthesis of PLLA with Silyl-Amide Complexes. The ROP of L-LA was conducted in CH2Cl2 with silyl-amide complexes Zn{N(SiMe3)2}2 and Sn{N(SiMe3)2}2 in the presence and absence of DDN at 45 oC. The L-LA-to-silylamide complex ratio was 139 with which the theoretical Mn ought to be about 10,000 Da if all the silylamido ligands are involved in the initiation. The DDN-to-silyl-amide complex ratio was 2.0, assuming their ligand exchange reaction. Table I summarizes the results of the polymerization. The both silyl-amide complexes were found to be active in CHCl3 and CH2Cl2, whereas inactive in THF. Since THF is a coordinating solvent, it may hinder the coordination of Llactide to the metal centers. In the presence of DDN, the polymerization was induced even at room temperature, whereas in the absence of DDN the polymerization was possible at higher temperature. The polymer yield became much higher in the presence of DDN. It was therefore evident that the silyl-amide complexes shows catalytic activity by themselves although the silyl-amido ligands are less reactive than the alkoxide ligands (=OC12H25) that may be formed by the ligand exchange between DDN and the silylamide complexes. The Mn values of the PLLA samples synthesized with the Zn complex in the absence of DDN were slightly higher than the values (Mn(th)) estimated from the [L-LA]0/[cat.]0 ratios in the feed and the monomer conversion. On the other hand, the Mn values obtained with the Zn complex in the presence of DDN became significantly lower than the Mn(th). The Mn value increased with decreasing the reaction Macromol. Res., Vol. 21, No. 4, 2013

Catalytic Behavior of Silyl-Amide Complexes for Lactide Polymerization

Table I. Results of Solution Polymerization of L-Lactide with M{N(SiMe3)2}2 as the Catalystsa Run 1 2 3 4 5 6 7 8 9 10 11 12

M

Zn

Sn

Initiator

Solvent

Temp. (οC)

Time (h)

Yield (%)

Mn (103 Da)

Mw/Mn

DDN DDN DDN DDN DDN DDN -

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CHCl3 CHCl3 THF THF CHCl3 CHCl3 THF THF

r.t. r.t. 50 50 60 60 60 60 60 60 60 60

42 42 71 71 90 90 93 93 70 70 70 70

91.8 82.3 61.1 93.8 62.6 72.0 64.2 -

8.25 7.56 9.71 6.47 13.0 16.1 15.4 -

1.24 1.17 1.46 1.16 1.29 1.09 1.30 -

a [L-LA]0/[cat.]0=1.39 (Mn (th)=10,000 Da), [DDN]0/[cat.]0=2. [L-LA]0: The initial monomer concentration of L-lactide. [cat.]0: The initial concentration of M{N(SiMe3)2}2. [DDN]0: The initial concentration of 1-dodecanol.

Figure 1. A typical 1H NMR spectrum of PLLA polymerized with Zn{N(SiMe3)2}2 in the presence of DDN (Run 5).

temperature (Entries 1, 3, 5). The Sn complex gave relatively higher Mn values than the Zn complex. These results suggest that the initiation efficiency of the silyl-amido ligands in regard to the propagation rate is responsible for determination of the Mn values of the PLLA products. Figure 1 shows the 1H NMR spectrum of the PLLA synthesized in Run 5. The signals of the 1-dodecanoyl group which has been introduced to the terminal of the polymer

can be detected. The signals of the silyl-amide groups are also detected near δ 0 ppm. It is assumed that the peak h is the signal of the N(SiMe3)2 groups introduced as the polymer tails whereas the peak i is the signal of the hexamethyldisiloxane which is generated by the hydrolysis of the N(SiMe3)2 terminals. As a result of this hydrolysis, a part of the polymer terminals may become amide groups. To confirm this assumption, we hydrolyzed the polymer obtained in Run 6 with dilute aqueous hydrochloric acid (dil-HCl) to remove the whole N(SiMe3)2 terminal groups. Figure 2 compares the 1H NMR spectra of the PLLA sample and its hydrolysis product. In the latter, the peak h is absent with other peaks unchanged, supporting the above assignments of the peaks h and i. This analysis reveals that there are two possible initiation paths in the presence of DDN, depending on the silyl-amido and alkoxide ligands. Kinetic Studies. The catalytic behavior of the silyl-amide complexes in the ROP of L-lactide was evaluated by kinetic studies in which the reaction conditions were identical with

Figure 2. 1H NMR spectra of the PLLA sample (Run 6) (left) and that hydrolyzed with dil-HCl (right). Macromol. Res., Vol. 21, No. 4, 2013

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the above. A portion of the reaction mixture was sampled at desired time intervals, and the polymer conversions were monitored by the 1H NMR spectroscopy as described in the Experimental. Figure 3 shows the plots of the conversion with reaction time. For each catalyst, faster polymerization was recorded in the presence of DDN. The Sn catalyst showed a slightly faster polymerization rate in the presence of DDN, while in the absence of it the Zn complex was faster than the Sn complex. Figure 4 shows the semi-logarithmic plots based on Figure 3. From the initial linear parts of the plots, the apparent propagation rate constants were calculated and are listed in Table II. The propagation rate constant in the presence of DDN (kMOR) was 26 times larger than that in the absence of DDN (kM) with the Sn complex, whereas the ratio (kMOR/kM) with the Zn complex was 9.9, suggesting that the catalytic activity is highly enhanced by the tin alkoxide species that may be formed by the ligand exchange. The initiation efficiency of the silyl-amido ligands was also monitored by the change in its signal intensity in reference to the methine signals of the polymer and monomer whose total intensity was constant throughout the polymerization. The initial content [N(SiMe3)2]0 was also determined from the [L-LA]0/[cat.]0 ratio of 139. Figure 5 shows the consumption of the N(SiMe3)2 ligands versus the mono-

Figure 3. Polymer conversion vs. reaction time in the L-LA polymerization with M{N(SiMe3)2}2 in the presence and absence of DDN.

Figure 4. Quasi-first order plots for the polymerizations of L-LA with M{N(SiMe3)2}2 in the presence and absence of DDN. 388

Table II. Propagation Rate Constants for the ROP of L-LA with M{N(SiMe3)2}2 M Zn Sn

Propagation Rate Constants kZnOR=8.8×10-2 (h-1) kZn=8.9×10-3 (h-1) kSnOR=9.0×10-2 (h-1) kSn=3.5×10-3 (h-1)

Rate Ratio at 45 oC kZnOR/kZn=9.9 kSnOR/kSn=26

Figure 5. Consumption of N(SiMe3)2 group as a function of conversion.

mer conversion that is correlated with the polymerization time. The Zn complex showed similar consumptions both in the presence and absence of DDN whenthe polymer conversion was over 70%. On the contrary, the Sn complex showed significantly different consumptions in the presence and absence of DDN up to 90% in the conversion; much lower value in the absence of DDN. The lower initiation efficiency of the silyl-amido ligands in the presence of DDN may be attributed to the faster initiation by the alokoxide ligands and the following fast propagation. Therefore, in the final stage of propagation where its rate becomes slow, the consumption of the silyl-amido ligands increased in high degree. Structure and Reactivity of Silyl-Amide Complexes. Figure 6 shows the 1H NMR spectra of the silyl-amide complexes. The signal at δ 0.06 ppm is assigned to the HN(SiMe3)2 which was liberated by the hydrolysis of the silyl-amide complex with the H2O existing in CDCl3. The signals of the SiMe3 groups of Zn{N(SiMe3)2}2 and Sn{N (SiMe3)2}2 appear around δ 0.1 and 0.4 ppm, respectively. Westerhausen reported that the signal of the SiMe3 group appears at higher magnetic field with increasing the ionic character of M-N bonds, because of the increased (pN-dSi)p back-bonding followed by the shielding of the silicon atom.22 Accordingly, the polarization of the M-N bonds should be in the order of Zn>Sn. Considering this fact together with Figure 4, the degree of polarization may reasonably be correlated with the catalytic activity of the silylamide complexes. In the absence of DDN, the Zn complex, Macromol. Res., Vol. 21, No. 4, 2013

Catalytic Behavior of Silyl-Amide Complexes for Lactide Polymerization

Figure 6. 1H NMR spectra of the two M{N(SiMe3) 2}2.

having a larger polarization, can give a higher polymer conversion, while the Sn complex, having a smaller polarization, gives a lower conversion. Therefore, the former with higher polarization of the silyl-amide bonds may have higher catalytic activity in the absence of DDN. Ligand Exchange Reaction. In order to analyze the effect of DDN on the ROP of L-LA, the reaction of the silylamide complexes and DDN was studied by 1H NMR spec-

troscopy. Figure 7 shows the spectra obtained for the reaction systems of the respective complexes. In the Zn complex, two signals (a and e) are shown for the α-methylene of dodecanyl group. Since the signal e appearing in the higher magnetic field increases with reaction time, it can be assigned to the DDN interacting with the metal center. Goel et al. reported that a dimeric half-exchanged silyl-amide alkoxyl complex is formed by coordination of an alcohol to the Zn silyl-amide complex.23 Based on this report, the Zn metal center has formed a bridged dimer by insertion of DDN. Ma et al. proposed a similar dimeric structure for the yttrium silyl-amide complex with 2-propanol in their use of this system to ROP of lactide.20 As they indicated, the dimeric alkoxide conjugate must be cleaved by lactide in order to induce the polymerization, although it is a slow process (Scheme II). In the Sn complex, the similar two signals are shown with a wider separation in magnetic field, indicating that the silyl-amide ligands have been substituted by the tin alkoxide bonds by the ligand exchange reaction. This ligand exchange is followed by the fast polymerization of L-LA to suppress the initiation by the silyl-amido ligands (Scheme III). The ligand exchange may correlate with the degree of the polarization of the M-N bond. In the highly polarized Zn complex, the DDN molecules are allowed to coordinate to the metal center instead of the ligand exchange occurring because of the ionic character of Zn. In the less polarized Sn complex, in return, the hydroxyl molecules are involved in the ligand exchange reaction to have a more polarized state. Accordingly, in the former Zn complex both amide and alkoxyl ligands can be used for the initiation (Scheme II), while the alkoxyl ligands preferentially initiate the polymerization in the latter Sn complex (Scheme III). The molecular weight and molecular weight distribution of the resultant polymers may be more easily controlled with the single ligand initiation with complexes consisting of highly polarized metal-ligand bonds such as the single Zn

Scheme II. Plausible mechanisms of ROP of L-LA with Zn{N(SiMe3)2}2 in the absence of DDN.

Figure 7. The ligand exchange of M{N(SiMe3) 2}2 with DDN as monitored by 1H NMR spectroscopies. Macromol. Res., Vol. 21, No. 4, 2013

Scheme III. Plausible mechanisms of ROP of L-LA with Sn{N(SiMe3)2}2 in the presence of DDN. 389

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Table III. Results of Melt-Polymerization of L-LA Catalyzed by Zn{N(SiMe3)3}a Time (min)

[L-LA]0/[cat.]0b,c

Initiatord

Conversion (%)

Yield (%)

Mn (10-4 Da)

Mw/Mn

20

1390

DDN

63.4

60.2

6.98

1.82

20

1390

-

59.6

56.3

7.42

2.00

120

1390

DDN

85.7

76.0

8.54

1.81

120

1390

-

84.1

80.4

6.20

2.11

a

o

b

c

Reaction temperature=120 C. [L-LA]0: The initial monomer concentration of L-LA. [cat.]0: The initial concentration of Zn{N(SiMe3)3}. [DDN]0/[cat.]0=2.0.

d

{N(SiMe3)2}2 system or the binary Sn{N(SiMe3)2}2/DDN system. Melt Polymerization with Silyl-Amide Complexes. Melt polymerization of L-LA was examined with the Zn {N(SiMe3)2}2 system in the presence and absence of DDN. Here, the [L-LA]0/[cat.]0 ratio was set at 1390 with which the theoretical molecular weight of the obtained polymer ought to be 100,000 Da. Table III summarizes the results. The polymerization proceeded fast, and the polymer conversion reached over 80% within 2 h to give a Mn ranging from 60,000 to 80,000 Da which is not so far from the Mn(th). The high monomer concentration in bulk state may have cleaved the dimeric alkoxide form to make the polymerization rate faster. Although the Mn value was in good correlation with the polymer conversion in the presence of DDN, there was no relationship between them in the absence of DDN. This may be because the slower initiation by the silyl-amido ligands has lowered the controllability of the polymerization. The molecular weight distribution was generally wider than that recorded in the solution polymerization. The catalytic species show low specificity due to the enhanced propagation with both the alkoxyl and amide routes allowed, probably because the metal complexes may not be so well stabilized by solvation in bulk state. For practical use of the Zn silyl-amide complex, the specificity should preferably be improved.

Conclusions In this study, we revealed the catalytic activities of the silyl-amide complexes in the ROP of L-LA. The highly polarized nature of the M-N bonds of the silyl-amide complexes was found to give the higher catalytic activity. However, the initiation efficiency of the silyl-amido ligands was lower than that of the alkoxyl ligands formed by the ligand exchange with DDN, particularly in the Sn complex. The different initiation activities of the two ligands caused the wider molecular weight distribution of the resultant polymers. The monomeric alkoxide species formed by the ligand exchange reaction was found out to be favorable for controlling the ROP, whereas the dimeric species formed between the highly polarized metal center and the bridging alkoxy ligands lowered the specificity of the initiating spe390

cies. Therefore, with the Zn complex, the generation of monomeric species must be required in order to obtain a well-controlled polymer having narrow molecular distribution. Although the PLLA polymers obtained by the melt polymerization catalyzed by the Zn complex had a wide molecular weight distribution, the Mn became higher than 60,000 Da in a short reaction time. This result indicated that the Zn silyl-amide complex has high potential for use as a practical catalyst for the lactide polymerization. Further studies will be needed to create active biocompatible catalysts that can be utilized for the lactide polymerization. Acknowledgment. This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20114010203130).

References (1) M. H. Chisholm, J. C. Gallucci, and C. Krempner, Polyhedron, 26, 4436 (2007). (2) A. Grafov, S. Vuorinen, T. Repo, M. Kemell, M. Nieger, and M. Leskela, Eur. Polym. J., 44, 3797 (2008). (3) A. P. Gupta and Vimal Kumar, Eur. Polym. J., 43, 4053 (2007). (4) M. J. Stanford and A. P. Dove, Chem. Soc. Rev., 39, 486 (2010). (5) C. M. Thomas, Chem. Soc. Rev., 39, 165 (2010). (6) H. I. Kim, K. Ishihara, S. Lee, J. H. Seo, H. Y. Kim, D. Sub, M. U. Kim, T. Konno, M. Takai, and J. S. Seo, Biomaterials, 32, 2241 (2011). (7) D. Ishii, T. H. Ying, A. Mahara, S. Murakami, T. Yamaoka, W. K. Lee, and T. Iwata, Biomacromolecules 10, 237 (2009). (8) M. Penco, R. Donetti, R. Mendrichi, and P. Ferruti, Macromol. Chem. Phys., 199, 1737 (1998). (9) A. Smith and I. M. Hunneyball, Int. J. Pharm., 30, 215 (1986). (10) A. P. Pego, B. Siebum, M. J. A. Van Luyn, K. J. Gellego, Y. Van Seijen, A. A. Poot, D. W. Grijpma, and J. Feijen, Tissue Eng., 9, 981 (2003). (11) J. J. Marler, J. Upton, R. Langer, and J. P. Vacanti, Adv. Drug Deliv. Rev., 33, 165 (1998). (12) M. H. Chisholm and N. W. Eilerts, Chem. Commun., 853 (1996). (13) M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer, Macromol. Res., Vol. 21, No. 4, 2013

Catalytic Behavior of Silyl-Amide Complexes for Lactide Polymerization

(14) (15) (16)

(17)

M. Pacold, and K. Phomphrai, J. Am. Chem. Soc., 122, 11845 (2000). M. Cheng, T. M. Ovitt, P. D. Hustad, and G. W. Coates, Polymer Prepr., 40, 542 (1999). M. Cheng, A. B. Attygalle, E. B. Lobkovsky, and G. W. Coates, J. Am. Chem. Soc., 121, 11583 (1999). B. M. Chamberlain, M. Cheng, and D. R. Moore, T. M. Ovitt, E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc., 123, 3229 (2001). M. H. Chisholm, J. C. Gallucci, and K. Phomphrai, Inorg. Chem., 44, 8004 (2005).

Macromol. Res., Vol. 21, No. 4, 2013

(18) M. H. Chisholm, J. C. Gallucci, and K. Phomphrai, Inorg. Chem., 43, 6717 (2004). (19) D. J. Darensbourg, W. Choi, O. Karroonnirun, and N. Bhuvanesh, Macromolecules, 41, 3493 (2008). (20) H. Ma and J. Okuda, Macromolecules, 38, 2665 (2005). (21) A. Amgoune, C. M. Thomas, T. Roisnel, and J. -F. Carpentier, Chem. Eur. J., 12, 169 (2006). (22) M. Westerhausen, Inorg. Chem., 30, 96 (1991). (23) S. C. Goel, M. Y. Chiang, and W. B. Buhro, Inorg. Chem., 29, 4646 (1990).

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