Mechanistic Aspects of Initiation and Deactivation in N-Heterocyclic

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Mechanistic Aspects of Initiation and Deactivation in N‑Heterocyclic Olefin Mediated Polymerization of Acrylates with Alane as Activator Yin-Bao Jia, Yan-Bo Wang, Wei-Min Ren, Tieqi Xu, Jing Wang, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: Highly active catalyst systems based on Nheterocyclic olefin (NHO) as initiator were developed for polymerizing various polar monomers such as methyl methacrylate (MMA), n-butyl methacrylate (BMA), N,Ndimethylacrylamide (DMAA) and N,N-diphenylacrylamide (DPAA) at ambient temperature. The preactivation of these polar monomers by Lewis acid such as Al(C6F5)3 or AlCl3 is a prerequisite for their rapid transformation, since a stable adduct easily forms by the interaction of nucleophilic NHO and Lewis acidic activator. The formation of NHO/Al(C6F5)3 adduct was confirmed by 1H NMR spectroscopy and X-ray single crystal analysis. The length of AlAl(C6F5)3−CNHO bond is in the range of 2.002−2.018 Å, dependent on the substitute groups of N-heterocyclic ring. Highly molecular weight (Mn = 7.50 × 105 g/mol) and narrow molecular weight distribution (Mw/Mn = 1.04−1.27) were achieved with dissymmetry NHO as initiator and Al(C6F5)3 as activated agent. Although binary NHO/Al(C6F5)3 catalyst system could polymerize both MMA and BMA with high activity, the attempt to synthesize their block copolymers proved to be unsuccessful. Electrospray ionization time-of-flight mass spectrometry (ESI−TOF MS) study provided important information on the polymer-chain ends. It was found that NHO as the initiation group bounded to one end of a polymer chain and an unexpected six-membered lactone ring appeared at another chain end. The formation of lactone end is ascribed to the nucleophilic backbiting of the polymeric anion to the carboxyl carbon of the adjacent unit, in companion with the release of the methoxyl group. The low initiation efficiency of NHO is attributed to the formation of the stable NHO/alane adduct during the polymerization, while the production of lactone end results in complete deactivation in polymer chain propagation.

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INTRODUCTION Since Stephan and Erker first introduced the “frustrated Lewis pairs” (FLPs) concept to describe sterically encumbered Lewis acid and Lewis base pairs that are sterically precluded from forming classical donor−acceptor adducts,1 the application of FLP in activating a variety of inert molecules such as CO2 and N2O has received much attention in recent years.2−7 In 2010, Chen’s group first reported the alane-based classical or frustrated Lewis pairs for highly active polymerization of various polar monomers,8,9 in which PtBu3, Ph3P or Nheterocyclic carbene (NHC) was used as Lewis base. The structure of Lewis base has a drastic effect on the catalytic activity and polymerization behaviors. Although X-ray singlecrystal analysis and computational study have revealed the formation of the zwitterionic adduct from FLPs, the polymerization process particularly regarding deactivation and polymerchain end is still unclear. More recently, a promising progress came from Amgoune and co-workers, concerning the application of Zn(C6F5)3-based Lewis pairs in ring-opening polymerization of lactide and εcaprolactone.10 The Lewis pairs cooperated to activate the monomers, affording well-defined high molecular weight cyclic polyesters or cyclic block copolymers. © 2014 American Chemical Society

N-Heterocyclic olefin (NHO) possesses a strongly polarized CC double bond and thus made the charge of olefin separation. This can be described through their resonance structure (Scheme 1), making the terminal carbon atom of the olefin of NHO more electronegative.11,12 As a result, NHOs were considered as potent nucleophiles and strong donor ligands.13 Recently, Tamm and co-workers reported the synthesis of classical or abnormal Lewis acid/base adducts by the treatment of NHOs with B(C6F5)3.14 In a recent study, we Scheme 1. Resonance Structures of N,N′-Disubstituents-2methylene Imidazoline

Received: January 7, 2014 Revised: February 23, 2014 Published: March 6, 2014 1966

dx.doi.org/10.1021/ma500047d | Macromolecules 2014, 47, 1966−1972

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to −155.3 ppm, indicating that the electronegativity of terminal carbon of NHO exocyclic has a significant effect on the property of Al(C6F5)3. Furthermore, the adducts of 1a· Al(C6F5)3, 1b·Al(C6F5)3, and 1c·Al(C6F5)3 were also characterized by single-crystal X-ray crystallography (Figure 2). Selected bond lengths and bond angles are shown in Table 1. The C1−C2 bond lengths of the adducts (1.450(4)−1.455(6) Å) are significantly longer than that of the exocyclic C−C bond of free NHO 1a (1.332 Å),11b indicating a strong interaction with Al(C6F5)3. On the other hand, the exocyclic double bond of NHO is extremely polarized, allowing for the most resonance stabilization of the zwitterionic structure consisting of an imidazolium cation and Al(C6F5)3 anion. The lengths of C1−Al bond in the three NHO·Al(C6F5)3 adducts are slightly different (2.002(6)-2.034(5)Å). Initially, the isolated 1b· Al(C6F5)3 adduct was used to initiate the polymerization of MMA, but only 19% yield of PMMA was got after 19 h. This result implies that NHO·Al(C6F5)3 adducts are highly stable in the reaction system, and difficultly used as active species for initiating polymerization. Polymerization of MMA with Lewis Pairs of NHO/ Alane. Because of the strong reactivity of nucleophilic NHO toward Lewis acidic alane, Lewis NHO/alane pairs could be not applied directly to the polymerizations of acrylate or acrylamide monomers. As a result, just as the previously reported polyacrylates formation mediated by Lewis pairs consisted of Al(C6F5)3 and NHC or phosphine,8 the monomer was simply premixed with Lewis acidic alane and thus activated by the weak interaction. The following addition of NHO solution rapidly initiates the polymerization of the activated monomers. Usually, the ratio of Lewis acid and NHO was fixed at 2:1, with varied ratios of monomer to base from 200:1 to 1600:1. No polymerization occurred within 24 h when B(C6F5)3/1a Lewis pair was used for MMA polymerization.16 On the contrary, replaced B(C6F5)3 with stronger Lewis acid Al(C6F5)3, the polymerization reaction quickly happened and complete conversion was observed within several minutes (Table 2), as similar to alane-based classical and FLP systems reported by Chen et al.8,9 For example, a high yield up to 99% was achieved

found that NHOs showed a strong tendency for CO2 sequestration to afford CO2 adducts (NHO−CO2), which exhibited excellent activity in catalyzing the coupling reaction of CO2 with propargylic alcohols at mild conditions.15 On the basis of the strong nucleophilicity and easily tunable stereochemitry property of NHOs, further exploration of their new application is highly desired. Herein, we communicate the recent efforts in the use of isolated NHOs 1a−1c (Scheme 2) Scheme 2. Structures of N-Heterocyclic Olefins 1a−1c

as neutral nucleophilic Lewis base in conjunction with Al(C6F5)3 for the polymerization of polar acrylate monomers. With the help of Electrospray ionization time-of-flight mass spectrometry (ESI−TOF MS), the polymer-chain ends were indentified, and thus proposing a polymerization mechanism concerning initiation and deactivation. This study also provides a reasonable explanation for the failure of preparing PMMAbased block copolymers using the present systems and previously reported Lewis pairs.

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RESULTS AND DISCUSSION Interaction of NHOs and Al(C6F5)3. The reactivities of NHOs with various structures (Scheme 2) toward equivalent Al(C6F5)3 were investigated in the mixture solution of hexane/ toluene. The off-white solid was immediately precipitated from the mixture solvents. Similar observations were reported in the previous systems of 2-alkylidene-1,3,4,5-tetramethylimidazolines with B(C6F5)3 or BH3.12,14 19F NMR spectroscopy study demonstrated that the chemical shift of F atoms of Al(C6F5)3 shifted to the high field after the interaction with NHO. Take 1c·Al(C6F5)3 adduct as an example (see Figure 1), the para position F atoms of one C6F5 unit has even shifted from −150.5

Figure 1. 19F NMR spectra of Al(C6F5)3 and 1c·Al(C6F5)3 adduct. 1967


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Figure 2. Molecular structures of 1a·Al(C6F5)3, 1b·Al(C6F5)3, and 1c·Al(C6F5)3 adducts with thermal ellipsoids drawn at 30% probability. Hydrogen atoms have been omitted for clarity.

Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for 1a·Al(C6F5)3, 1b·Al(C6F5)3, and 1c·Al(C6F5)3 adducts adduct

C1−C2 (Å)

Al−C1 (Å)

Al−C8 (Å)

Al−C7 (Å)

Al−C9 (Å)

C1−Al−C9 (deg)

C1−Al−C7 (deg)

C1−Al−C8 (deg)

1a·Al(C6F5)3 1b·Al(C6F5)3 1c·Al(C6F5)3

1.451(9) 1.455(6) 1.450(4)

2.002(6) 2.034(5) 2.018(3)

1.980(9) 2.002(5) 2.030(3)

2.018(8) 2.016(5) 2.012(3)

2.018(9) 2.013(5) 2.017(3)

104.3(3) 107.0(2) 103.7(13)

107.6(4) 106.8(2) 108.8(12)

120.0(4) 114.3(2) 112.9(13)

Table 2. Polymerization of Polar Monomers Mediated by Lewis Pairs of Al(C6F5)3/Basea run

alane (2 equiv)

base (1 equiv)

1 2 3 4 5 6 7 8 9 10 11 12

B(C6F5)3 Al(C6F5)3 Al(C6F5)3 AlCl3 Al(C6F5)3 Al(C6F5)3 Al(C6F5)3 Al(C6F5)3 Al(C6F5)3 Al(C6F5)3 Al(C6F5)3 Al(C6F5)3

1a 1a 1b 1b Mes

NHC

1c 1c 1c 1c 1c 1c 1c

monomer MMA MMA MMA MMA MMA MMA BMA BMA BMA DMAA DPAA MMA/ BMA

monomer/base (molar ratio)

time (min)

yieldb (%)

Mnc (×104 g mol−1)

400 800 800 800 800 800 400 800 1600 800 200 400/400/1

1440 3 3 1440 2 3 5 5 15 4 150 25

0 96 99 46 100 94 100 94 91 100 90 100

n.d 71 82 8 61 29 14 28 75 17 n.d 48

Mw/Mnc rrd (%) n.d 1.30 1.41 1.99 1.54 1.17 1.23 1.27 1.27 1.05 n.d 1.31

n.d 73 71 n.d 70 72 79 79 79 n.d n.d n.d

mr (%)

mm (%)

n.d 26 26 n.d 27 24 20 20 20 n.d n.d n.d

n.d 1 3 n.d 3 4 1 1 1 n.d n.d n.d

Polymerization was performed at 25 °C in the 10 mL total solution volume (solvent toluene (TOL)) + monomer). bYields of the isolated polymer determined by gravimetric methods. n.d. = not determined. cPolymer molecular weight and distribution was determined by GPC relative to polystyrene standards. drr, mr, mm = polymer methyl triads measured by 1H NMR spectroscopy. a

ratio from 400 to 800 (yield 100% and 94%; runs 7 and 8), and even for the ratio of 1600:1, a prolonged reaction time of 15 min afforded 94% yield (run 9). Notably, all the resulted PBMAs have narrow molecular weight distribution (Mw/Mn = 1.23−1.27; runs 7−9). Similarly, polymerization of N,Ndimethylacrylamide (DMAA) (800 equiv) achieved 100% yield in 4 min, providing a polymer with Mn = 1.72 × 105 g/ mol and a very narrow molecular weight distribution (Mw/Mn = 1.05; run 10). Al(C6F5)3/1c Lewis pair was found efficiently in polymerizing sterically encumbered N,N-diphenylacrylamide (DPAA), with a yield of 90% within 150 min (run 11). Furthermore, the copolymerization of MMA and BMA using Al(C6F5)3/1c pair was carried out in a fixed [MMA]/[BMA]/ [1c] ratio of 400/400/1 at room temperature. The reaction proceeded smoothly, and all the monomers were transformed to copolymer within 25 min. (Table 2, run 12). The molar composition of the two monomer units in the copolymers obtained is close to the monomer molar feed ratio, and 1H

in 3 min with the pair of Al(C6F5)3/1b (run 3). For a comparison purpose, the Lewis pair of Al(C6F5)3/MesNHC for MMA polymerization was also performed, affording a quantitative yield within 2 min. The polymer resulted from NHO 1b as Lewis base has narrower molecular weight distribution (Mw/Mn = 1.41 vs. 1.54) and higher molecular weight (Mn = 8.20 × 105 g/mol vs. 6.10 × 105 g/mol, runs 3 and 5) than that obtained with MesNHC as initiator. The similar activity was also observed in the catalyst system regarding Al(C6F5)3/1c Lewis pair (Run 6). However, when Al(C6F5)3 was changed to AlCl3, only 46% yield of PMMA was got even the reaction time was elongated to 24 h. The resultant polymer has a small molecular weight and broader molecular weight distribution (Mn = 8.0 × 104 g/mol, Mw/Mn =1.99; run 4). Lewis 1c/Al(C6F5)3 pair was found to be highly active in polymerizing other acrylate or acrylamide monomers (runs 7− 11). For example, with monomer n-butyl methacrylate (BMA), high yields were achieved within 5 min for the [BMA]/[1c] 1968


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Figure 3. 1H NMR spectrum of a representative sample of MMA/BMA copolymer.

1a in frustrated Lewis pairs was just replaced with 1b at the same reaction condition. It should be noted that 84.1, the deduction value of m/z 671.4 and m/z 587.3 was equivalent to the molecular weight discrepancy of 1a (M = 402.3) and 1b (M = 318.2). This analysis indicates that the initiator role of Nheterocyclic olefin in the frustrated Lewis pairs mediated polymerization of MMA. Additionally, this result also suggests that the NHO initiator was covalently bound to the propagating polymer-chain in a very stable state after the reaction. As a result, the species of m/z 671.4 should include 1a (m/z 402.3) as chain end. A new question arises: what components establish the residual segment (m/z 269.1). In other word, what is another chain end of the species of m/z 671.4? According to the polymerization mechanism involving zwitterionic active species formed by the reaction of monomer/Al(C6F5)3 mixture with Lewis base, we can reasonably assume the occurrence of the nucleophilic backbiting of the polymeric anion to the carboxyl carbon of the adjacent unit, resulting in the formation of six-membered-ring lactone chain end, in companion with the release of the methoxy group and the adduct [Al(C6F5)3−OMe−Al(C6F5)3], which crystal structure was previously reported by Chen and co-worker.18 Therefore, the unknown residual segment (m/z 269.1) in the species of m/z 671.4 can be tentatively ascribed to a repeated MMA unit (m/z 100) and a six-membered lactone ring (m/z 169.1). If the occurrence of the backbiting reaction during or/and after polymerization is a fact, the leaving group should be found in the filtrate. Considering the difficult determination of the release of the methoxyl group in MMA polymerization, N,N-diphenylacrylamide (DPAA) was chosen as model monomer for investigating deactivation with a low ratio of DPAA/1b of 50. The reaction mixtures was quenched by BHT (2,6-di-tert-butyl-4-methylphenol), and further treated by a certain amount of methanol to precipitate the polymer. The spectrum exhibited the existence of the species at m/z 819.5, 1042.5, 1265.6 etc which can be disassembled into m/z 318.5 (1b as initiation group), m/z 223n (repeated units of DPAA) and m/z 278 (a six-membered ring) based on our proposed hypothesis above. More importantly, we succeeded in obtaining the diphenylamine from the filtrate, confirmed by

NMR spectrum of a representative copolymer is shown in Figure 3. The resulting copolymer has only one Tg value at 66 °C. Sequential block polymerization of MMA and BMA was also performed with the use of Lewis Al(C6F5)3/1c pair. After all of MMA monomer was completely consumed within 20 min (confirmed by 1H NMR analysis of an aliquot reaction mixture), BMA or BMA/Al(C6F5)3 mixture was added to stir another 20 min. However, no PBMA was found in the resultant products based on 1H NMR analysis, and BMA monomer remained. The same result was observed in the systems that BMA was first injected and then MMA or MMA/Al(C6F5)3 mixture was added after BMA was completely consumed within 20 min. These results indicate the deactivation of frustrated Lewis pairs of Al(C6F5)3/1c before the addition of the second monomer. Mechanistic Investigations. Previously, Chen and coworkers reported an elegant mechanism regarding classical and frustrated Lewis pairs mediated polymerizations of conjugated polar alkenes,9 suggesting that the polymerization conformed to a bimolecular, activated monomer propagation mechanism. However, this mechanism did not address chain termination or catalyst deactivation events during the late stage of polymerization or under conditions where chain termination competes with chain propagation. For understanding the deactivation of frustrated Lewis pairs of Al(C6F5)3/1c before the addition of the second monomer, it is necessary to investigate the polymer chain-end structures. In previous studies, we have applied ESI−TOF MS method for determining the propagating polymer-chain ends in the CO2/ epoxide copolymerization.17 Therefore, ESI−TOF MS method was also used to investigate the polymer chain-end structures in the Al(C6F5)3/NHO mediated polymerization of acrylates. At first, a relative low-molecular-weight PMMA was produced by frustrated Lewis pairs of Al(C6F5)3/1a in toluene at room temperature. When all the monomer was consumed by 1H NMR study the reaction mixture was performed in positive mode of ESI−MS for determining the transient cationic species (Figure 4A). A series of species based on m/z 671.4 at an interval of 100 (which is equivalent to a repeat unit of MMA) were observed. The species at m/z 671.4 was changed to 587.3 (Figure 4B) with the same interval of 100 when the Lewis base 1969


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Figure 4. ESI−TOF MS spectra of the PMMAs produced by Lewis pairs: (A) Al(C6F5)3/1a, (B) Al(C6F5)3/1b, and (C) AlCl3/1b.

both 1H NMR and MS analysis (Supporting Information, Figures S3 and S4). It is worthwhile noting here parenthetically that the NHO-bound polymer chains with a six-membered lactone ring end could not initiate polymerization of the following added monomers. On the basis of the facts mentioned above, we can reasonably assume the polymerizations of acrylates accompanied by a competition reaction regarding the backbiting reaction to form a six-membered-ring lactone chain end (Scheme 3). With Al(C6F5)3/NHO catalyst system, NHO as nucleophilic agent attacked the Al(C6F5)3-activiated monomer to initiate polymerization and meanwhile the NHO covalently bounded to the propagated polymer chain. At initiation and polymer-chain propagation steps, the rate of polymerization is significantly higher than that of the backbiting reaction to form a sixmembered-ring lactone chain end. On the contrary, when the monomer was nearly complete consumed or the polymerization was performed at a very dilute solution, the rate of the backbiting reaction is close to that of the polymer-chain growth, and thus ultimately resulting in complete deactivation in

polymer chain propagation due to the formation of sixmembered-ring lactone chain ends. Additionally, when polar monomers were not enough activated and thus cause a very low polymerization rate, it is possible to observe two kinds of polymer chains based NHO. To demonstrate this assumption, AlCl3/1b Lewis pair initiated MMA polymerization process was investigated by ESI−TOF MS. That is, as illustrated in Figure 4C, two kinds of 1b-based polymer chains, one concerning sixmembered-ring lactone chain end and another regarding the incorporated MMA end are clearly seen. This observation also provides a reasonable explanation why AlCl3/1b Lewis pair has low activity for the formation of polymer with very low molecular weight. Similar results were observed in the Al(C 6F5) 3/MesNHC or Al(C6F 5) 3/PPh3 mediated MMA polymerization (Supporting Information, Figures S5 and S6). On the whole, the low initiation efficiency of NHO is attributed to the formation of the stable NHO/alane adduct during the polymerization, while the production of lactone end results in complete deactivation in polymer chain propagation. 1970


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crylamide (DMAA), were purchased from TCI America. These monomers were degassed and then dried with CaH2 overnight, followed by vacuum distillation under reduced pressures. The purified monomers were stored in brown bottles with 5 Å molecular sieves inside a glovebox freezer at −30 °C. N,N-Diphenylacrylamide (DPAA) was synthesized according to the literature.19 B(C6F5)3 was purchased from Innochem (Beijing) Technology Co., Ltd. and used as received. Al(C6F5)3 was prepared by ligand exchange reactions between B(C6F5)3 and AlEt3.20 Hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4methylphenol) was purchased from Aladdin and recrystallized prior to use. General Polymerization Procedures. MMA polymerizations were performed either in 20 mL oven-dried glass reactors inside the glovebox under ambient conditions (ca. 25 °C). A predetermined amount of Lewis acid, such as B(C6F5)3 and Al(C6F5)3 etc. was first dissolved in the monomer MMA (1.00 mL, 9.35 mmol) and 5 mL of toluene inside glovebox, and the polymerization was started by rapid addition of a solution of N-heterocyclic olefins in 4 mL of Toluene via a gastight syringe to the above Lewis acid/MMA mixture solution under vigorous stirring. The molar ratio of Lewis acid to NHO was fixed into 2/1 for all polymerizations, whereas the [monomer]/ [NHO] ratio was varied in some experiments. After the measured time, the reaction mixture was immediately quenched at by addition of 2 mL 5% HCl-acidified methanol. The quenched mixture was precipitated into 100 mL of methanol, stirred for 1 h and filtered. The polymer was further washed with methanol and dried in a vacuum oven at 50 °C overnight to a constant weight. The isolated yield was got by the measuring the dried polymer. Polymer Characterizations. Polymer molecular weights (Mn) and molecular weight distributions were measured by gel permeation chromatography (GPC) analysis carried out at 35 °C and a flow rate of 1.0 mL min−1, with CHCl3 as the eluent, on a Agilent 1260 instrument coupled with a Agilent RI detector and equipped with four PL gel 5 μm mixed-C columns. The sample concentration was about 0.1%, and the injection volume was 50 uL. The curve was calibrated using monodisperse polystyrene standards covering the molecular weight rage from 580 to 460000 g/mol. 1H NMR and 13C NMR spectra for the analysis of PMMA microstructures were recorded and analyzed according to the literature methods. Low-molecular-weight PMMA produced by NHO and Al(C6F5)3 or AlCl3 in toluene was analyzed by electrospray ionization mass spectrometry (ESI−MS) in positive mode, using a Aglient 6224 TOF LC/MS or matrix-assisted laser.

Scheme 3. Possible Mechanism of MMA Polymerization Mediated by Alane/1b Lewis Pair

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CONCLUSIONS In summary, we have established new frustrated Lewis pairs consisted of N-heterocyclic olefins (NHOs) and Al(C6F5)3 or AlCl3. As similar to the previously reported Al(C6F5)3/PtBu3 or Al(C6F5)3/NHCs systems, a strong interaction of NHO and Al(C6F5)3was observed and rapidly formed their adduct. The exocyclic C−C bond lengths of the adducts (1.450(4) −1.455(6) Å) are significantly longer than that of the free NHOs (∼1.332 Å). The lengths of C1−Al bond in the three NHO·Al(C6F5)3 adducts are slightly different (2.002(6)− 2.034(3)Å). Furthermore, the frustrated Lewis pairs based on NHOs and Al(C6F5)3 were shown to be highly active in polymerizing conjugated polar alkenes such as MMA, BMA, and acrylamides at ambient temperature, affording high molecular weight polymers with relatively narrow distributions. These Lewis pairs were also found to be highly efficient in copolymerizing two different conjugated polar alkenes, but they could not synthesize block copolymers by stepwise addition of two different monomers. ESI−TOF MS study suggested that NHO as the initiation group bound to one end of a polymer chain and an unexpected six-membered lactone ring appeared at another chain end after polymerization of conjugated polar alkenes. The formation of six-membered ring lactone chain-end in the NHO/Al(C6F5)3 mediated polymerization of MMA was ascribed to the nucleophilic backbiting of the polymeric anion to the carboxyl carbon of the adjacent unit, in companion with the release of the methoxyl group. The formation of sixmembered ring lactone at the low conversion of the polymerization of MMA mediated by NHO/AlCl3 well explained why this Lewis pair has low activity.

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ASSOCIATED CONTENT

* Supporting Information S

General experimental procedures, full experimental details on the preparation of compounds 1a−1c, characterizations and preparation of NHO·Al(C6F5)3 adducts; representative GPC traces of the resulting polymers, and 1H NMR and MS spectra of diphenylamine formed in Al(C6F5)3/1b mediated DPAA polymerization, and .cif files of the compounds 1a−1c. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

EXPERIMENTAL SECTION

Notes

The authors declare no competing financial interest.

Materials. All syntheses and manipulations of air- and moisturesensitive materials were carried out in flamed Schlenk-type glassware on a dual-manifold Schlenk line, a high-vacuum line, or an argon-filled glovebox. Toluene and hexane were refluxed over sodium and fractionally distilled under nitrogen atmosphere prior to use. Benzene-d6 was dried over sodium/potassium alloy and vacuumdistilled. AlEt3 (Acros, 2 M in toluene) were used as received. Anhydrous AlCl 3 were purchased from Alfa Aesar. Methyl methacrylate (MMA), n-butyl methacrylate (BMA), N,N-dimethyla-

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ACKNOWLEDGMENTS This work is supported by the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, Grant 20130041130004), and Program for Changjiang Scholars and Innovative Research Team in University (IRT13008). X.-B.L. gratefully acknowledges the Chang Jiang Scholars Program 1971


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Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.; McDonald, R.; Rivard, E. Chem. Commun. 2011, 47, 6987−6989. (d) Maji, B.; Horn, M.; Mayr, H. Angew. Chem., Int. Ed. 2012, 51, 6231−6235. (14) Kronig, S.; Jones, P. G.; Tamm, M. Eur. J. Inorg. Chem. 2013, 2301−2314. (15) Wang, Y. B.; Wang, Y. M.; Zhang, W. Z.; Lu, X. B. J. Am. Chem. Soc. 2013, 135, 11996−12003. (16) It is worthwhile noting here parenthetically that this observation is reminiscent of the previous finding regarding the high activity of enolaluminate vs. inactivity of enolboratespecies toward conjugateaddition polymerization of acrylates, attributed to the inability of the enolborate/borane pair to effect the bimolecular, activated-monomer anionic polymerization as does the enolaluminate/alane pair. See: (a) Ning, Y.; Zhu, H.; Chen, E. Y.-X. J. Organomet. Chem. 2007, 692, 4535−4544. (b) Chen, E. Y.-X. Chem. Rev. 2009, 109, 5157−5214. (17) (a) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang, Y.-J.; Peng, X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664− 1674. (b) Li, B.; Zhang, R.; Lu, X.-B. Macromolecules 2007, 40, 2303− 2307. (c) Ren, W.-M.; Liu, Z.-W.; Wen, Y.-Q.; Zhang, R.; Lu, X.-B. J. Am. Chem. Soc. 2009, 131, 11509−11518. (d) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.; Xu, T.-Q.; Darensbourg, D. J. J. Am. Chem. Soc. 2011, 133, 15191−15199. (e) Ren, W.-M.; Wang, Y.-M.; Zhang, R.; Jiang, J.-Y.; Lu, X.-B. J. Org. Chem. 2013, 78, 4801−4810. (18) Ning, Y.; Chen, E. Y.-X. Macromolecules 2006, 39, 7204−7215. (19) Kim, Y. C.; Jeon, M.; Kim, S. Y. Macromol. Rapid Commun. 2005, 26, 1499−1503. (20) Feng, S. G.; Roof, G. R.; Chen, E. Y.-X. Organometallics 2002, 21, 832−839.

(T2011056) from Ministry of Education of the People’s Republic of China.

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NOTE ADDED AFTER ASAP PUBLICATION This article posted ASAP on March 6, 2014. The reference citations after reference 10 were renumbered in the text to correspond correctly to the reference list. The correct version posted on March 25, 2014.

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Mechanistic Aspects of Initiation and Deactivation in N-Heterocyclic

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