alternating copolymerization of cyclohexene oxide and carbon dioxide catalyzed by...

Alternating Copolymerization of Cyclohexene Oxideand Carbon Dioxide Catalyzed by NoncyclopentadienylRare-Earth Metal Bis(alkyl) Complexes

ZHICHAO ZHANG,1,2 DONGMEI CUI,1 XINLI LIU1

1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, People’s Republic of China

2Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Received 24 May 2008; accepted 27 July 2008DOI: 10.1002/pola.22989Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The syntheses of several dialkyl complexes based on rare-earth metalwere described. Three b-diimine compounds with varying N-aryl substituents(HL1 ¼ (2-CH3O(C6H4))N¼¼C(CH3)CH¼¼C(CH3)NH(2-CH3O(C6H4)), HL2 ¼ (2,4,6-(CH3)3(C6H2))N¼¼C(CH3)CH¼¼C(CH3)NH(2,4,6-(CH3)3(C6H2)), HL3 ¼ PhN¼¼C(CH3)CH(CH3)NHPh) were treated with Ln(CH2SiMe3)3(THF)2 to give dialkyl complexes L1Ln(CH2SiMe3)2 (Ln ¼ Y (1a), Lu (1b), Sc (1c)), L2Ln(CH2SiMe3)2(THF) (Ln ¼ Y (2a),Lu (2b)), and L3Lu(CH2SiMe3)2(THF) (3). All these complexes were applied to thecopolymerization of cyclohexene oxide (CHO) and carbon dioxide as single-componentcatalysts. Systematic investigation revealed that the central metal with larger radiiand less steric bulkiness were beneficial for the copolymerization of CHO and CO2.Thus, methoxy-modified b-diiminato yttrium bis(alkyl) complex 1a, L1Y(CH2SiMe3)2,was identified as the optimal catalyst, which converted CHO and CO2 to polycarbon-ate with a TOF of 47.4 h�1 in 1,4-dioxane under a 15 bar of CO2 atmosphere(Tp ¼ 130 �C), representing the highest catalytic activity achieved by rare-earthmetal catalyst. The resultant copolymer contained high carbonate linkages ([99%)with molar mass up to 1.9 � 104 as well as narrow molar mass distribution (Mw/Mn

¼ 1.7). VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6810–6818, 2008

Keywords: catalysts; CO2; copolymerization; cyclohexene oxide; polycarbonates;rare-earth metals

INTRODUCTION

The alternating copolymerization of epoxides andcarbon dioxide to produce polycarbonate has beenamong the most important organic syntheses.1

For one thing, the afforded polycarbonate fromthis transformation, a type of biodegradable engi-neering plastics, may act as a potential alterna-

tive for the currently wide used polyolefin mate-rial that has caused the ‘‘white pollution.’’ Foranother, incorporation of carbon dioxide into theepoxides is one of the most promising routes forthe chemical fixation of carbon dioxide. Since theoriginal work of copolymerization of propyleneoxide and carbon dioxide by Inoue in 1969,2 con-tinuous efforts have been focused on designingnew efficient metal-based catalysts. To date,many catalytic systems based on zinc,3 cobalt,4

chromium,5 and aluminum6 have been innovatedand extensively developed including significant

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 6810–6818 (2008)VVC 2008 Wiley Periodicals, Inc.

Correspondence to: D. Cui (E-mail: [email protected])

6810

breakthroughs, whereas the exploration of newmetal catalyst systems still remains attractive.

Rare-earth metal complexes have receivedmore and more attention for their unusually highcatalytic activity on the polymerization of olefinsand polar monomers such as lactones and lactidesand dienes.7,8 However, the application of rare-earth metal catalysts to the copolymerization ofepoxides and carbon dioxide has been exploredless. Since the first report of copolymerization ofpropylene oxide and CO2 catalyzed by a catalystsystem consisting of Y(P204)3-Al(i-Bu)3-glycerine,

9

several other reports of the copolymerization ofepoxides and CO2 by rare-earth metal catalystsystems have been released.10 Nonetheless, allthese lanthanide element based catalytic systemssuffered from inactivity in the absence of excesscocatalysts, such as alkyl zinc or aluminum, sug-gesting that the rare-earth metal compoundsmight not be the real active species. Thus, the sin-gle-component rare-earth metal polyhydride andalkyl catalysts bearing cyclopentadienyl ligand11

and a series of bridged b-diiminato rare-earthmetal amino complexes12 have been innovated forthe copolymerization of cyclohexene oxide (CHO)and CO2. Recently, our group has developed aseries of rare-earth metal complexes bearing non-cyclopentadienyl ligands, which have showndistinguished catalytic activities or/and regio- orstereoselectivities for the polymerizations of olefinand dienes and especially lactide of the stable six-membered cyclic ester.13 These results stronglyintrigued us to exploit the capability of these com-plexes as single-component catalysts for copoly-merization of carbon dioxide and epoxides.Herein, we wish to report the syntheses and char-acterization of several new b-diiminates ligatedrare-earth metal bis(alkyl) complexes and theircatalytic behavior for the copolymerization. Theinfluences of the ligand framework, such as thetype and coordination-site of the chelating atoms,and the reaction conditions on the catalytic activ-ities of the related complexes will be explicated.The effect of 1,4-dioxane that played a crucial rolein the aspect of enhancing the carbonate linkagesof the polymer was elucidated as well.

EXPERIMENTAL

General Methods

All reactions were carried out under a dry andoxygen-free argon atmosphere by using Schlenktechniques or under a nitrogen atmosphere in a

glovebox. Solvents were purified by an MBraunSPS system. All ligands were synthesized accord-ing to modified literature procedures. Acetyl ace-tone, 2,4,6-trimethylaniline, and 2,6-diisopropyla-niline were purchased from Aldrich. All liquidswere dried over 4 A molecular sieves for a weekand distilled before use, and solid materials wereused without purification. The synthesis of lan-thanide tris(alkyl)s followed the establishedmethod with a little modification.

Instruments and Measurements

Organometallic samples for NMR spectroscopicmeasurements were prepared in a glovebox byuse of NMR tubes and then sealed by paraffinfilm. 1H, 13C NMR spectra were recorded on aBruker AV400 (FT, 400 MHz for 1H; 100 MHz for13C) spectrometer. The molar mass and molarmass distribution of the polymers were measuredby TOSOH HLC 8220 GPC at 40 �C using THF aseluent against polystyrene standards. Elementalanalyses were performed at National AnalyticalResearch Centre of Changchun Institute ofApplied Chemistry (CIAC). Crystals for X-rayanalysis were obtained as described in the Experi-mental section. The crystals were manipulated ina glovebox. Data collections were performed at�86.5 �C on a Bruker SMART APEX diffractome-ter with a CCD area detector, using graphite-monochromated Mo KR radiation (k ¼ 0.71, 073 A).The determination of crystal class and unit cellparameters was carried out by the SMART pro-gram package. The raw frame data were proc-essed using SAINT and SADABS to yield thereflection data file. The structures were solved byusing the SHELXTL program. Refinement wasperformed on F2 anisotropically for all nonhydro-gen atoms by the full-matrix least-squaresmethod. The hydrogen atoms were placed at thecalculated positions and were included in thestructure calculation without further refinementof the parameters. Ligands and complexes 1a,1b, and 1c (Chart 1) were synthesized accordingto the literature methods and structurally char-acterized by 1H NMR.13(g)

Synthesis of Complex 2a

In a glovebox 2-(2,4,6-trimethyl)phenylamino-4-(2,4,6-trimethyl)phenyliminopent-2-ene (HL2)(0.13 g, 0.40 mmol) in hexane (10 mL) was addeddropwise to a solution of Y(CH2SiMe3)3(THF)2(0.20 g, 0.40 mmol) in hexane (10 mL) at ambient

COPOLYMERIZATION OF CHO AND CARBON DIOXIDE 6811

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

temperature. The mixture was kept stirring for4 h followed by concentrating to a saturated solu-tion and cooling to �30 �C. Complex 2a, L2Y(CH2-

SiMe3)3(THF)2, was collected as white powders af-ter washing three times with hexane and driedin vacuo at ambient temperature (0.17 g, 62%).Colorless single crystals suitable for X-ray analy-sis precipitated overnight.

1H NMR (400 MHz, C6D6, 25�C): d ¼ 6.79 (s,

4H, ArAH), 5.07, (s, 1H, HAC(C(CH3)NAr)2), 3.35(m, 4H, THF), 2.28 (s, 12H, o-CH3AAr), 2.16 (s,6H, p-CH3AAr), 1.58 (s, 3H, ipso-CACH3), 1.57 (s,3H, ipso-CACH3), 1.14–1.12 (m, 4H, THF), 0.22(d, 18H, Si(CH3)3), –0.56 (s, 2H, SiACH2), –0.57(s, 2H, SiACH2).

13C NMR (100 MHz, C6D6, 25�C): d ¼ 166.5 (2C, HC(C(CH3)NAr)2)), 145.5 (2C,ipso-NAC6H2), 134.1 (2C, p-C6H2), 132.7 (4C, m-C6H2), 129.9 (4C, o-C6H2), 98.3 (1C, HC(C(Me)-NAr)2), 69.9 (2C, THF), 37.0 (1C, LuACH2), 36.5(1C, LuACH2), 25.5 (2C, THF), 24.1(2C, ipso-CACH3), 20.9 (2C, p-CH3A(C6H2)), 20.0 (4C, o-CH3A(C6H2)), 4.6 (6C, Si(CH3)3). Anal. Calcd forC35H59N2OSi2Y (%): C, 62.84; H, 8.89; N, 4.19.Found: C, 62.81; H, 8.84; N, 4.18.

Systhesis of Complex 2b

Following the same procedure as that for prepara-tion of 2a, 2b, L2Lu(CH2SiMe3)3(THF), was iso-lated as white powders in 73% yield by treatmentof HL2 (0.13 g, 0.40 mmol) with Lu(CH2Si-Me3)3THF2 (0.23 g, 0.40 mmol).

1H NMR (400 MHz, C6D6, 25�C): d ¼ 6.79 (s,

4H, o-HAAr), 5.05 (s, 1H, HC(C(CH3)NAr)2),3.38–3.34 (m, 4H, THF), 2.29 (s, 12H, o-CH3AAr),2.16 (s, 6H, p-CH3AAr), 1.56 (d, 6H, ipso-CCH3),1.16–1.12 (m, 4H, THF), 0.21 (s, 9H, Si(CH3)3),0.20 (s, 9H, Si(CH3)3), �0.73 (s, 4H, SiACH2).

13CNMR (100 MHz, C6D6, 25 �C): d ¼ 167.1 (2C,HC(C(CH3)N)2), 145.8 (2C, ipso-NAC6H2), 134.2(2C, p-C6H2), 132.7(4C, m-NAC6H2), 129.8 (4C, 2-

NAC6H2), 98.7 (1C, HC(C(Me)NAr)2), 70.3 (2C,THF), 43.6 (2C, LuACH2), 25.3 (2C, THF), 24.3(2C, ipso-CACH3), 21.0 (2C, p-CH3AC6H2), 20.0(4C, o-CH3A(C6H2)), 4.8 (6C, Si(CH3)3). Anal.Calcd for C35H59N2OSi2Lu (%): C, 55.68; H, 7.88;N, 3.71. Found: C, 55.63; H, 7.86; N, 3.67.

Synthesis of Complex 3

Following the aforementioned workup procedure,the reaction of 2-phenylamino-4-phenylimino-pent-2-ene (HL3) (0.10 g, 0.40 mmol) withLu(CH2SiMe3)3(THF)2 (0.23 g, 0.40 mmol) in tolu-ene afforded complex 3, L2Lu(CH2SiMe3)3(THF)2(0.12 g, 45%).

1H NMR (400 MHz, C6D6, 25�C): d ¼ 7.12–7.11

(m, 8H, PhAH), 6.94–6.91 (m, 2H, PhAH), 4.94 (s,1H, HC(MeCPhN)2), 3.37–3.35 (m, 4H, THF), 1.72(s, 6H, ipso-CACH3), 1.10–1.09 (m, 4H, THF),0.24 (s, 18H, Si(CH3)3), �0.58 (s, 4H, SiCH2).

13CNMR (100 MHz, C6D6, 25

�C): d ¼ 165.9 (2C, ipso-C(CH3)N)2), 149.8 (2C, C6H5), 129.3 (4C, C6H5),126.5 (4C, C6H5), 124.8 (2C, C6H5), 99.1 (1C,HC(C(CH3)NAr)2), 70.0 (2C, THF), 41.2 (2C,LuACH2), 25.0 (2C, THF), 24.4 (2C, ipso-CACH3),4.7(6C, Si(CH3)3). Anal. Calcd for C29H47Lu-N2OSi2 (%): C, 51.92; H, 7.06; N, 4.18. Found: C,51.89; H, 7.04; N, 4.15.

Copolymerization of CHO/CO2

A typical procedure for the copolymerization ofCHO/CO2: a 50-mL stainless steel autoclaveequipped with a magnetic stirring bar was placedin a 110 �C oil bath, dried under vacuum for 2 h.After cooled to room temperature, the autoclavewas charged with carbon dioxide. A 1,4-dioxanesolution of 1a (28.6 mg, 50 lmol) was loaded intothe autoclave with a syringe through the injectionport under the protection of CO2 followed by theaddition of CHO (3 mL). Then the reactor was

Chart 1. Rare-earth metal dialkyl complexes.

6812 ZHANG, CUI, AND LIU


pressurized with CO2 to 15 bar and then heatedto 110 �C with stirring for 10 h. After cooling toroom temperature, CO2 was ventilated and excessCHO and the solvent were removed in vacuo at40 �C to a constant weight. The carbonate link-ages were determined from the relative intensityof the cyclohexyl methine proton signals (4.6 ppmfor carbonate linkages, 3.4 ppm for ether link-ages) in the 1H NMR spectrum of the polymer.The molar mass and the molar mass distributionof the resulting polymer were determined byGPC.

RESULTS AND DISCUSSIONS

Syntheses of Complexes 2a, 2b, and 3

Treatment of Ln(CH2SiMe3)3(THF)2 with HL2

yielded complexes L2Ln(CH2SiMe3)2(THF) (Ln¼ Y (2a); Lu (2b)) via alkane elimination. The 1HNMR spectra of both complexes showed similartopology, indicative of bis(alkyl)s with a coordinat-ing THF molecule. The amino proton of the ligandoriginally centered at d 12.14 ppm disappeared,suggesting the completeness of the reaction. Themethylene protons of the metal alkyl species, Y-CH2SiMe3 (Lu-CH2SiMe3) gave a doublet reso-nance at d �0.57 ppm (�0.73 ppm), shifting down-

field when compared with �0.61 ppm in its pre-cursor Y(CH2SiMe3)3(THF)2 (d �0.78 ppm inLu(CH2SiMe3)3(THF)2). Two broad peaks at d3.35 and 1.13 ppm were assigned to the protons ofthe coordinating THF molecule. The resonance atd 5.07 ppm was assigned to the olefinic proton ofthe ligand framework. X-ray diffraction analysisconfirmed further the overall molecular structurein the solid-state of 2a and 2b was in consistentwith that in the solution-state (Figs. 1 and 2,Table 1). Each metal ion coordinates to ligand L2

via two N atoms and two alkyl groups and a THFmolecule, adopting a trigonal bipyramidal geome-try with the N(2), C(24), and C(28) atoms formingthe base whilst the O(1) and N(1) atoms generat-ing the apices. Two N-aryl rings are perpendicularto the ‘‘nacnac’’ plane, respectively, with respect tothe spacial environment of the molecule. TheYAC bond lengths of 2.394 and 2.406 A in 2aare slightly shorter than those of 1a (2.420 and2.425 A),13(g) but reasonably longer than the LuACbond lengths of 2.345 and 2.350 A in 2b owing tothe larger ionic radius of Y3þ than that of Lu3þ.

Following the similar procedure for prepara-tion of complexes 2a and 2b, complex 3(L3Lu(CH2SiMe3)2(THF)) was isolated from thereaction of b-di(phenylimine) (HL3) with Lu(CH2-

SiMe3)3(THF)2 albeit in toluene rather than inhexane. The Lu ion coordinates to the two alkylligands and one THF molecule as well as two N

Figure 1. ORTEP drawing of complex 2a with ther-mal ellipsoids at 35% probability levels. The hydrogenatoms are omitted for clarity. Selected bond distances(A) and angles (deg): Y(1)–C(28) 2.406(3), Y(1)–C(24)2.394(3), Y(1)–N(1) 2.376(2), Y(1)–N(2) 2.346(2), Y(1)–O(1) 2.4230(19); N(2)–Y(1)–N(1) 78.50(8), N(2)–Y(1)–C(24) 120.67(9), N(1)–Y(1)–C(24) 96.48(9), N(2)–Y(1)–C(28) 124.07(9), N(1)–Y(1)–C(28) 98.65(9),C(24)–Y(1)–C(28) 115.17(10), N(2)–Y(1)–O(1) 91.41(7),N(1)–Y(1)–O(1) 169.53(7), C(24)–Y(1)–O(1) 86.22(9),C(28)–Y(1)–O(1) 89.27(9).

Figure 2. ORTEP drawing of the complex 2b withthermal ellipsoids at 35% probability levels. Thehydrogen atoms are omitted for clarity. Selected bonddistances (A) and angles (deg): Lu(1)–N(1) 2.326(2),Lu(1)–N(2) 2.304(2), Lu(1)–C(24) 2.350(2), Lu(1)–C(28) 2.345(2), Lu(1)–O(1) 2.3763(18); N(2)–Lu(1)–N(1) 80.38(7), N(2)–Lu(1)–C(28) 123.99(8), N(1)–Lu(1)–C(28) 98.25(9), N(2)–Lu(1)–C(24) 121.53(8),N(1)–Lu(1)–C(24) 95.52(8), C(28)–Lu(1)–C(24)114.36(9), N(2)–Lu(1)–O(1) 90.18(7), N(1)–Lu(1)–O(1)170.01(7), C(28)–Lu(1)–O(1) 89.71(8), C(24)–Lu(1)–O(1) 86.60(8).



atoms from ligand L3 (Fig. 3), generating trigonal

bipyramidal geometry similar to those of com-

plexes 2a and 2b. The LuAC bond distances in 3

are 2.341 and 2.348 A comparable to those in 2b.

While the bond angle of 113.23� for C(18)–Lu(1)–

C(19) are also within the normal range for

those of rare-earth metal bis(alkyl)s reported

previously.13(g)

Copolymerization of CHO and CO2

Complexes 1a, 1b, 1c, 2a, 2b, and 3 were eval-uated as catalysts for the copolymerization ofCHO and CO2 under various reaction conditions,which exhibited versatile catalytic activities. Asshown in Table 2, complexes 1a and 1b bearingligand with methoxy functionalized N-aryl ringsexhibited slightly higher catalytic activity than

Table 1. Summary of Crystallographic Data for Compounds 2a, 2b, and 3

2a 2b 3

Empirical formula C35H59N2OSi2Y C35H59N2OSi2Lu C29H47N2OSi2LuFormula weight 668.93 754.99 670.84T (K) 185 (2) 187 (2) 185 (2)Wavelength 0.71073 0.71073 0.71073Crystal system Monoclinic Monoclinic MonoclinicSpace group P2(1)/C P2(1)/C P2(1)/nUnit cell dimensionsa (A) 10.6845 (7) 10.6273 (6) 9.9082 (7)b (A) 14.0488 (9) 14.0127 (8) 21.6838 (16)c (A) 25.9308 (17) 26.0177 (15) 14.9511 (11)a (�) 90 90 90b (�) 93.0560 (10) 92.9320 (10) 95.0240 (10)c (�) 90 90 90V (A3) 3886.8 (4) 3869.4 (4) 3199.9 (4)Z 4 4 4Dc (Mg/m3) 1.143 1.296 1.392Absorption coefficient (nm–1) 1.588 2.639 3.182F(0 0 0) 1432 1560 1368Crystal size (mm) 0.12 � 0.12 � 0.11 0.16 � 0.15 � 0.12 0.12 � 0.12 � 0.11Theta range for data collection (�) 1.57–26.02 1.57–26.01 1.66–26.01Index ranges �12 � h � 13,

�17 � k � 16,�31 � l � 27

�11 � h � 13,�17 � k � 16,�27 � l � 32

�12 � h � 11,�17 � k � 26,�17 � l � 18

Reflections collected 21584 21269 17639Independent reflections (Rint) 7632 (0.0450) 7587 (0.0181) 6270 (0.0157)Completeness to y 99.6% (26.02) 99.6% (26.01�) 99.5% (26.01)Absorption correction Semiempirical

from equivalentsSemiempirical

from equivalentsSemiempirical

from equivalentsMaximum and minimumtransmission

0.8447 and 0.8202 0.7424 and 0.6775 0.7210 and 0.7014

Refinement method Full-matrixleast-squares on F2

Full-matrixleast-squares on F2

Full-matrixleast-squares on F2

Data/restraints/parameters 7632/78/403 7587/6/384 6270/45/334Goodness-of-fit on F2 0.990 1.055 1.053Rotation twin N/A N/A N/AFinal R indices [I[ 2r]R1 0.0440 0.0227 0.0199wR2 0.0978 0.0549 0.0476R indices (all data)R1 0.0752 0.0267 0.0230wR2 0.1095 0.0570 0.0489Largest difference peakand hole (e A�3)

0.362 and �0.207 0.633 and �0.420 0.856 and �0.352



complexes 2a and 2b stabilized by ligand withmethyl substituted N-aryl rings. While complexesbased on rare-earth metal with larger ionic radiuswere superior in catalytic activity than thosebased on rare-earth metal with smaller ionic ra-dius as comparing 1a with 1b and 2a with 2b,respectively. Complex 3 bearing the sterically lessbulky ligands showed medium activity betweenits analogues 1b and 2b (entries 1–6). These dif-ferences in ligand framework and central metalion also reflected on the microstructure of the re-sultant polymer. With complexes 1a and 1b, thecopolymerization afforded products with about90% carbonate linkages. While switching to 2a,2b, and 3 led to copolymers possessing lower car-bonate linkages (85%).

The influence of reaction temperature on thecopolymerization of CHO and CO2 was investi-gated by using 1a and keeping other conditionsinvariable. Great increase in catalytic activitywith temperature was observed, and the turnoverfrequency (TOF) at 80 �C (17.0 h�1) almostdoubled that at 60 �C (9.0 h�1) (entries 7 and 8),which reached to 31.0 h�1 when the copolymeriza-tion was performed at 110 �C (entry 9). The high-est catalytic activity (39.0 h�1) was achieved at

130 �C (entry 10). In agreement with the increaseof TOF, the molar mass (Mn) of the resultant poly-mer increased correspondingly from 7.0 � 103 g/mol at 60 �C to 15.5 � 103 g/mol at 130 �C. In themeantime, the molar mass distribution remainednarrow and almost constant (Mw/Mn \ 1.7).Nevertheless, the polymerization temperaturegave rise to little impact on the carbonate link-ages of the resultant copolymer. Moreover,although the copolymerization of epoxides andcarbon dioxide is plagued by the concomitant for-mation of cyclic product and the cyclic productsare the main products at elevated temperaturesfor some systems,14 by using complex 1a, theselective formation of polycarbonate over cyclo-hexene carbonate was more than 98% when thetemperature was below 110 �C, which remainedat a high level of 95% at a much higher tempera-ture (130 �C).

The monomer-to-catalyst ratio was also impor-tant for the copolymerization of CHO and CO2.With the increase of CHO-to-1a ratio from 200 to600, the catalytic activity increased drasticallyfrom a TOF of 18.7 to 30.0 h�1 at 130 �C (entries10–12), which reached up to 39.0 h�1 at 130 �Cwithin the same period of reaction time (10 h).Whereas, when the ratio was over 1000, bothdrops in catalytic activity and carbonate linkagesof the resultant product were observed (entry 13),which could be attributed to the lowing of the cat-alytic concentration when overloading monomer.

As it has been reported that solvent also playedsignificant role in the copolymerization of CHOand CO2,

15 1,4-dioxane was employed as a reac-tion medium instead of toluene when the otherconditions were kept constant. When the copoly-merization reaction was performed at 130 �C, thecatalytic activity reached up to 47.4 h�1, the high-est value achieved to date by a single-componentcatalyst based on rare-earth metal (entry 14).Strikingly, the carbonate linkages dramaticallyincreased up to 99%. Examination of this copoly-merization at 110 �C also exhibited similar result,the catalytic activity increased to 36.2 h�1 whencompared with 31.0 h�1 when the polymerizationwas performed in toluene (entries 9 and 15). Like-wise, the carbonate linkages of the resultant poly-mer were over 99%.

This positive effect originated from 1,4-dioxanestimulated us to reinvestigate the copolymeriza-tion of CHO and CO2 by 1b, 1c, 2a, 2b, and 3. Asexpected, both the catalytic activities and the car-bonate linkages of the resultant polymers fromcopolymerization by 1b, 2a, 2b, and 3 were

Figure 3. ORTEP drawing of complex 3 with ther-mal ellipsoids at 35% probability levels. The hydrogenatoms are omitted for clarity. Selected bond distances(A) and angles (deg): Lu(1)–N(1) 2.2720(19), Lu(1)–N(2) 2.2975(19), Lu(1)–O(1) 2.3450(16), C(18)–Lu(1)2.341(2), C(19)–Lu(1) 2.348(3); N(1)–Lu(1)–N(2)77.92(7), N(1)–Lu(1)–C(18) 119.52(8), N(2)–Lu(1)–C(18) 103.43(8), N(1)–Lu(1)–O(1) 86.42(7), N(2)–Lu(1)–O(1) 162.53(7), C(18)–Lu(1)–O(1) 90.94(8),N(1)–Lu(1)–C(19) 126.97(8), N(2)–Lu(1)–C(19)95.57(8), C(18)–Lu(1)–C(19) 113.23(8), O(1)–Lu(1)–C(19) 87.63(8).



enhanced to some extent when compared withthose using toluene as solvent, in agreement withprevious report (entries 16–20).15

On the basis of these results, we postulatedthat 1,4-dioxane might not only act as solvent butalso, in all probability, participates in the catalyticreaction as reported that THF molecule couldcoordinate to metal center and dissociate from themetal center.16 1,4-dioxane was a weak base likeTHF, coordinating to the metal center to form anintermediate adduct, which effectively inhibitedthe consecutive coordination of CHO, whereas onreceiving the more electrophilic CO2 molecule, theactive metal center released 1,4-dioxane to formthe carbonate unit. Thereby, the carbonate link-ages of polycarbonate in 1,4-dioxane solution werehigh. In contrast, toluene cannot coordinate to themetal center and the chance of consecutive inser-tion of CHO to the metal center increased, leadingto low carbonate enchainments.17 This postula-tion was partially supported by successful separa-tion of 1,4-dioxane/metal adduct by treatment ofcomplex 2b with 1,4-dioxane (complex 4).18 In theNMR spectra of complex 4, no proton and carbonsignals of THF molecule can be detected. Instead,

only a singlet peak positioned at 3.44 ppm in 1HNMR spectrum and a singlet resonance at 67.9ppm19 in 13C NMR spectrum showed up, suggest-ing that a 1,4-dioxane molecule coordinated totwo complex 2b molecules to generate a symmet-ric 1,4-dioxane bridged binuclear complex.

Complex 1a could not initiate the copolymer-ization of propylene oxide (PO) and CO2 in neatPO (2000 eq.) either at room temperature or60 �C. Other complexes were inactive as well.This result was in agreement with the mono-Cpligated rare-earth metal bis(alkyl) complexes,which are also inert to the coplymerization of POand CO2.

11(a)

CONCLUSIONS

In summary, we have synthesized a variety ofrare-earth metal bis(alkyl) complexes, which cancatalyze the copolymerization of CHO and carbondioxide without the assistance of cocatalysts. ATOF of as high as 47.4 h�1 can be achieved undermild conditions, which represents the highestvalue for the single-component catalyst systems

Table 2. Copolymerization of CHO and CO2

Entry Cat [CHO]/[Cat] Solvent

Temp(�C)

Time(h)

TOFb

(h–1)Selectivityc

(% Polymer)Carbonate

Linkagesc (%)Mn

d � 10�3

(g mol�1) Mw/Mnd

1 1a 600 Toluene 80 20 15.6 [99 90 12.9 1.42 1b 600 Toluene 80 20 14.9 [99 90 12.4 1.63 1c 600 Toluene 80 20 nde [99 47 nd nd4 2a 600 Toluene 80 20 13.6 [99 85 14.2 2.15 2b 600 Toluene 80 20 12.3 [99 84 12.7 2.26 3 600 Toluene 80 20 13.7 [99 85 13.6 2.07 1a 600 Toluene 60 10 9.0 [99 93 7.0 1.68 1a 600 Toluene 80 10 17.0 [99 92 13.5 1.79 1a 600 Toluene 110 10 31.0 98 91 15.5 1.5

10 1a 600 Toluene 130 10 39.0 95 91 15.5 1.511 1a 200 Toluene 130 10 18.7 91 93 8.9 1.812 1a 500 Toluene 130 10 30.0 94 92 9.7 1.613 1a 1000 Toluene 130 10 37.2 95 83 21.3 1.614 1a 600 Dioxane 130 10 47.4 96 99 19.0 1.715 1a 600 Dioxane 110 10 36.2 98 99 16.0 1.316 1b 600 Dioxane 130 10 37.9 94 95 17.5 2.017 1c 600 Dioxane 110 10 9.6 98 52 4.4 1.418 2a 600 Dioxane 130 10 35.6 98 93 14.6 2.019 2b 600 Dioxane 130 10 31.7 96 91 20.0 6.620 3 600 Dioxane 130 10 36.4 97 91 18.5 2.0

aConditions: catalyst amount: 50 lmol; P(CO2): 15 bar; the volume of solvent: 1 mL.b Turnover frequency: mole of CHO consumed by per mole of catalyst per hour.cDetermined by 1H NMR spectrum.dDetermined by GPC against polystyrene standard.e nd: Not determined.



based on rare-earth metals. In addition, the re-sultant polymer had high molecular weight andhigh carbonate linkages as well as narrow molarmass distribution. The carbonate linkages of theresultant polymer were deeply affected by thetype of solvent added. These results might shednew lights on designing new catalysts for thecopolymerization of epoxide and CO2.

The authors are thankful for financial support fromNational Natural Science Foundation of China forProject Nos. 20571072 and 20674081; The Ministry ofScience and Technology of China for Project No.2005CB623802; and ‘‘Hundred Talent Program’’ ofCAS.

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18. Synthesis and characterization of complex 4[(L2Lu(CH2SiMe3)2)-l-OC4H8O-l-(L2Lu(CH2-SiMe3)2)]. Complex 2b was dissolved in 1,4-diox-ane and stirred at ambient temperature for 4 h.Removal all the volatile compounds underreduced pressure to give yellow solids. Yellowcrystals were grown from a toluene/hexane solu-

tion. Decanted off the supernatants, washed thecrystals with hexane for several times, and driedunder reduced pressure at ambient temperatureto give complex 4 as yellow solids. 1H NMR (400MHz, C6D6, 25

�C): d ¼ 6.96 (s, 8H, o-HAAr), 5.14(s, 2H, HC(C(CH3)NAr)2), 3.44 (s, 8H, 1,4-diox-ane), 2.37 (s, 24H, o-CH3AAr), 2.27 (s, 12H, p-CH3AAr), 1.68 (s, 12H, ipso-CCH3), 0.28 (s, 18H,Si(CH3)3), 0.27 (s, 18H, Si(CH3)3), �0.51 (s, 8H,SiACH2).

13C NMR (100 MHz, C6D6, 25 �C): d¼ 167.7 (4C, HC(C(CH3)N)2), 142.7 (4C, ipso-NAC6H2), 135.6 (4C, p-C6H2), 132.6 (8C, m-NAC6H2), 130.6 (8C, 2-NAC6H2), 97.9 (2C,HC(C(Me)NAr)2), 67.9 (4C, 1,4-dioxane), 46.2 (4C,LuACH2), 24.1 (4C, ipso-CACH3), 21.3 (4C, p-CH3-C6H2), 20.1 (8C, o-CH3-(C6H2)), 4.7 (12C, Si(CH3)3).

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alternating copolymerization of cyclohexene oxide and carbon dioxide catalyzed by...

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