Living Radical

Article pubs.acs.org/Macromolecules

Cobalt Bipyridine Bisphenolate Complex in Controlled/Living Radical Polymerization of Vinyl Monomers Yi-Chien Lin, Yi-Liang Hsieh, Yuan-Deng Lin, and Chi-How Peng* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Cobalt(II) bipyridine bisphenolate (CoII(BpyBph)) was applied to mediate the controlled/living radical polymerization of vinyl acetate (VAc), methyl acrylate (MA), and other vinyl monomers such as N-vinylpyrrolidone (NVP) and acrylonitrile (AN). The living characters of linear increased molecular weight with monomer conversion, relatively narrow molecular weight distribution, and the synthesis of block copolymer demonstrated by the formation of PVAc-b-PNVP were all observed. The polymerization of VAc was proposed to be mediated by the degenerative transfer (DT) process, but the polymerization of MA should be mainly controlled by the reversible termination (RT) process since the equilibrium constant (Keq = [CoIII−R]/[CoII][R•]) between cobalt(II) and organocobalt(III) was measured as 8.6 × 107 M−1. The correlation between Keq and the redox potential (E1/2) of cobalt complexes was preliminarily observed as a higher E1/2 leading to a larger Keq.

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INTRODUCTION Cobalt complexes were amazed by their unique performance in polymerization processes such as catalytic chain transfer polymerization1−3 and, more recently, controlled/living radical polymerization (C/LRP). 4 −1 0 Cobalt complexes of CoII(TMP),11−13 CoII(acac)2,5,6,14−17 and CoII(Salen*)18−20 (Figure 1) were applied to control the radical polymerization of

Cobalt complexes mediated radical polymerization (CMRP)5,15 is a subcategory of organometallic mediated radical polymerization (OMRP)26−34 and controls the polymerization through the pathways of reversible termination (RT) and/or degenerative transfer (DT) as shown in Scheme 1.7,12,35 Scheme 1. (a) Reversible Termination (RT) and (b) Degenerative Transfer (DT) Mechanisms That Occurred in Cobalt Mediated Controlled/Living Radical Polymerization

Figure 1. Cobalt complexes used in controlled/living radical polymerization: (a) cobalt(II) tetramesitylporphyrin, CoII(TMP); (b) cobalt(II) acetylacetonate, CoII(acac)2; and (c) cobalt(II) [N,N′bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine], CoII(Salen*).

The RT and DT mechanisms are distinguished by the source and concentration of radicals. When the radicals are dissociated from the dormant species (Co III −P) and the radical concentration is dominated by the equilibrium of deactivator (CoII) and dormant species ([P•] = [CoIII−P]/([CoII] × Keq)), the polymerization is described to be controlled by reversible termination. On the other hand, when the radicals are

vinyl acetate (VAc), methyl acrylate (MA), and other functional monomers. Particularly the control of VAc polymerization is difficult to achieve by using other transition metal catalysts or chain transfer agents except the proper RAFT agent.21 The attraction of poly(vinyl acetate) (PVAc) is mainly from its wide application in adhesive22 and the transformation to poly(vinyl alcohol),23−25 an important biocompatible material, via hydrolysis. © 2014 American Chemical Society

Received: September 10, 2014 Revised: October 16, 2014 Published: October 29, 2014 7362

dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

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Scheme 2. Synthesis of Bipyridine Bisphenolate Ligand (BpyBph-H2) Followed by the Formation of Cobalt(II) Bipyridine Bisphenolate (CoII(BpyBph))

Table 1. Polymerization of Vinyl Acetate Mediated by CoII(BpyBph) with Different Monomer to Cobalt Ratioa entry

[VAc]/[Co]

time (min)

convb (%)

Mn,expc [10−3 g mol−1]

Mn,thd [10−3 g mol−1]

PDIc

1

700

2

1400

180 240 360 180 210 300

13.8 30.6 53.9 17.3 28.6 55.2

10.2 22.3 30.6 28.0 40.3 61.6

8.3 18.4 32.5 20.9 34.5 66.5

1.12 1.20 1.48 1.14 1.29 1.66

a General conditions: [CoII]0/[AIBN]0 = 1/20, [VAc]0 = 10.68 M in bulk at 60 °C. bConversion was detected by 1H NMR. cMn was determined by gel permeation chromatography (GPC) with polystyrene as standard. dMn,th = ([M]0/[CoII]0) × conversion × MW of monomer.

exclusively from the initiator and the radical concentration is mainly determined by the initiator concentration ([I]), the rate constants for radicals to enter solution (ki), and the radical termination rate constant (kt) ([P•] = (ki[I]/2kt)1/2), the control of polymerization is achieved by degenerative transfer.12 Practically, the radical source may hardly be defined since the radicals came from both external initiators and dormant species in many cases, and thus the radical concentration becomes a better indication to distinguish these two mechanisms. In the literature, C/LRP of MA mediated by CoII(TMP)11,12,36,37 and CoII(Salen*)18,19 was dominated by the RT process, but C/ LRP of VAc mediated by CoII(TMP),38 CoII(Salen*),19 and CoII(acac)25,8 was controlled via the DT process. Herein, cobalt(II) [2,2′-[2,2′]bipyridinyl-6-ylbis-4,6-di-tertbutylphenolate] (CoII(BpyBph)), which has a low spin (S = 1/2) metal center coordinated by two nitrogen and two oxygen with a square planar structure,39 was selected to mediate the polymerization of vinyl acetate, methyl acrylate, and other vinyl monomers for expansion of the system and more comprehensive understanding of CMRP. The cobalt(II) bipyridine bisphenolate complex was synthesized by following the reported method as summarized in Scheme 2.39 The CoII(BpyBph) was generated by the reaction of cobalt acetate and the ligand, which was obtained from a halogen−metal exchange of the bromophenol and tert-butyllithium followed by nucleophilic addition to the bipyridine. The cobalt complex and the ligand were characterized by 1H NMR and UV−vis spectra. The results of polymerization showed that CoII(BpyBph) can control the radical polymerization of vinyl acetate, methyl acrylate, acrylonitrile, and N-vinylpyrrolidone as a new control system for CMRP.

Figure 2. First-order kinetics plots of VAc polymerization mediated by CoII(BpyBph) in bulk at 60 °C under the conditions of [CoII]0/ [AIBN]0/[VAc]0 = (a) 1/20/700 and (b) 1/20/1400; [VAc]0 = 10.68 M.

kinetic plots is proper to the rate of polymerization and is equal to kp[R•].37,40 The polymerization approached 53.9% monomer conversion in 6 h with the molecular weight not only linearly increasing versus conversion but also matching the theoretical values calculated by the assumption of one chain per mediator. The polymerization reaction was stopped due to the high viscosity. The molecular weight distribution was slightly broad as indicated by the polydispersity index (PDI = Mw/Mn) equal to 1.48 (Figure 3a). Poly(vinyl acetate) with higher molecular weight equal to 61 600 was obtained by raising the monomer to cobalt ratio (Table 1, entry 2). The similar length of induction period was due to the same ratio of [CoII]0/[AIBN]0, and the change of slope in kinetic plots was attributed to the different [AIBN]0 (Figure 2b). The control fashion demonstrated by the linearity of increasing molecular weight with conversion and the molecular weight distribution was not significantly affected by the change of monomer to cobalt ratio (Figure 3). The concentration of AIBN is critical to the CMRP of vinyl acetate since it dominates the radical concentration during the polymerization. A higher radical concentration gives a faster, more economical polymerization process,37 but a lower radical concentration provides a better control to the polymeric product.41 The VAc polymerization was thus carried out with different initial concentration of AIBN at 60 °C (Table 2). An

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RESULTS AND DISCUSSION Polymerization of Vinyl Acetate. The CoII(BpyBph) mediated vinyl acetate (VAc) radical polymerization was performed under the condition of CoII(BpyBph)/AIBN/VAc = 1/20/700 at 60 °C in bulk (Table 1, entry 1). A 120 min induction period followed by the first-order polymerization process (ln(M0/Mt) = kp[R•]t) was observed (Figure 2a). This induction period is characteristic of CMRP and was rationalized as the time required to convert cobalt(II) complexes and radicals to organocobalt(III) complexes.12,37 The slope in 7363


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Figure 4. First-order kinetics plots of VAc polymerization mediated by CoII(BpyBph) in bulk at 60 °C under the conditions of [CoII]0/ [AIBN]0 = (a) 1/10, (b) 1/20, and (c) 1/40; [VAc]0 = 10.68 M.

Figure 3. Plots of conversion vs Mn vs PDI for VAc polymerization mediated by CoII(BpyBph) in bulk at 60 °C under the conditions of [CoII]0/[AIBN]0/[VAc]0 = (a) 1/20/700 and (b) 1/20/1400; [VAc]0 = 10.68 M.

induction period followed by a linear first-order kinetic plots was observed in all polymerizations, but the length of induction period decreased and the slope of kinetic plots raised with the higher AIBN concentration (Figure 4), which can be rationalized as that the increased radical concentration speeded up the conversion of cobalt(II) to organocobalt(III) species and enhanced the propagation rate. The control of VAc polymerization demonstrated by the linearity of molecular weight growth, deviation of molecular weight from the theoretical values, and the PDI values was not significantly influenced by the change of AIBN concentration (Figure 5). Comparing to Co I I (TMP) 3 8 , 4 2 and Co I I (Salen*), 1 9 CoII(BpyBph) can mediate VAc polymerization with a higher radical concentration and thus gives a more rapid and economical polymerization process. The temperature effect was also considered, and therefore, vinyl acetate radical polymerization was mediated by CoII(BpyBph) at different temperature from 50 to 70 °C (Table 3). An induction period followed by a linear first-order kinetic plots was still observed in all polymerizations, but the length of induction period decreased and the slope of kinetic plots raised with the increased temperature (Figure 6), which should be mainly attributed to the higher radical concentration generated by the accelerated dissociation of AIBN. The half-life of AIBN is shortened from 74 to 4.8 h when the temperature changed from 50 to 70 °C.40,43 The increased temperature also changed the kinetic parameters such as formation rate constant of organocobalt(III) and propagation rate constant that would

Figure 5. Plots of conversion vs Mn vs PDI for VAc polymerization mediated by CoII(BpyBph) in bulk at 60 °C under the conditions of [CoII]0/[AIBN]0 = (a) 1/10, (b) 1/20, and (c) 1/40; [VAc]0 = 10.68 M.

affect the induction time and polymerization rate as well. Besides, the raised temperature led to a lower viscosity, and thus the polymerization can approach a higher monomer conversion. The temperature effect was not significant to the molecular weight distribution but caused the molecular weight deviation to the polymeric products formed at 50 and 70 °C (Figure 7). The positive Mn deviation that occurred at 50 °C was suspected to be relevant to the slow exchange process

Table 2. Polymerization of Vinyl Acetate Mediated by CoII(BpyBph) with Different Initial Concentration of AIBNa entry

[AIBN]/[Co]

time (min)

convb (%)



PDIc

1

10

2

20

3

40

300 420 630 180 240 360 90 120 200

20.0 32.4 53.0 13.8 30.6 53.9 20.6 33.3 59.7

14.5 22.0 29.0 10.2 22.3 30.6 13.6 22.0 35.5

12.1 19.5 31.9 8.3 18.4 32.5 12.4 20.1 36.0

1.14 1.34 1.55 1.12 1.20 1.48 1.18 1.40 1.56

General conditions: [VAc]0 = 10.68 M in bulk at 60 °C. bConversion was detected by 1H NMR. cMn was determined by gel permeation chromatography (GPC) with polystyrene as standard. dMn,th = ([M]0/[CoII]0) × conversion × MW of monomer.

a

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Table 3. Polymerization of Vinyl Acetate Mediated by CoII(BpyBph) at Different Temperaturea t1/2,AIBN (h)

entry

temp (°C)

1

50

74

2

60

15

3

70

4.8

time (min)

convb (%)



PDIc

675 840 1020 180 240 360 60 80 140

17.3 28.6 36.7 13.8 30.6 53.9 20.0 32.9 62.0

18.2 27.6 33.6 10.2 22.3 30.6 9.7 17.8 27.4

10.5 17.2 22.1 8.3 18.4 32.5 12.1 19.8 37.4

1.18 1.37 1.45 1.12 1.20 1.48 1.17 1.37 1.68

a

General conditions: [CoII]0/[AIBN]0 = 1/20, [VAc]0 = 10.68 M in bulk. bConversion was detected by 1H NMR. cMn was determined by gel permeation chromatography (GPC) with polystyrene as standard. dMn,th = ([M]0/[CoII]0) × conversion × MW of monomer.

concentration did not significantly impact the control process is still being studied. Polymerization of Methyl Acrylate. The CoII(BpyBph) was also applied to the C/LRP of methyl acrylate since CoII(TMP) and CoII(Salen*) showed a good control to MA polymerization. CoII(BpyBph) mediated MA polymerization was performed under the condition of CoII(BpyBph)/AIBN/ MA = 1/10/700 at 60 °C in benzene with [MA]0 = 5.47 M and reached 82.6% monomer conversion in 240 min with a 90 min induction period (Figure 8). Instead of a linear increased

Figure 6. First-order kinetics plots of VAc polymerization mediated by CoII(BpyBph) in bulk under the conditions of [CoII]0/[AIBN]0/ [VAc]0 = 1/20/700, [VAc]0 = 10.68 M, at (a) 50, (b) 60, and (c) 70 °C.

Figure 8. First-order kinetics plots of MA polymerization mediated by CoII(BpyBph) in benzene at 60 °C under the conditions of [CoII]0/ [AIBN]0/[MA]0 = 1/10/700, [MA]0 = 5.47 M.

kinetic plots in VAc polymerization, the kinetic plots of MA polymerization increased as a curvature, which was also observed in CoII(TMP)37 and CoII(Salen*)19 mediated MA polymerization and correlated to the equilibrium constant (Keq) of cobalt(II) and organocobalt(III). The kinetic simulation indicated that the curvature becomes more obvious when Keq decreases.37 The living characters of a linear increased molecular weight with conversion and narrow molecular weight distribution demonstrated by PDI values below 1.17 were observed (Figure 9). However, the molecular weight measured by GPC was slightly larger than the theoretical values calculated by the assumption of one chain per mediator. The molecular weight deviation associated with the curvature of kinetic plots suggested that CoII(BpyBph) could mediate the MA polymerization via the reversible termination pathway, and possibly not all cobalt(II) complexes were converted to organocobalt(III) species during the induction period. UV−vis spectroscopy was thus used to follow the transformation of cobalt species during the induction period (Figure

Figure 7. Plots of conversion vs Mn vs PDI for VAc polymerization mediated by CoII(BpyBph) in bulk under the conditions of [CoII]0/ [AIBN]0/[VAc]0 = 1/20/700, [VAc]0 = 10.68 M, at (a) 50, (b) 60, and (c) 70 °C.

between the propagating radicals in solution and the dormant radicals on the organocobalt(III) species. A negative molecular weight deviation was observed after 44% conversion in the VAc polymerization at 70 °C and could be due to the chain transfer process that was accelerated by the increasing of temperature. CoII(BpyBph) has been demonstrated to be able to mediate the controlled/living radical polymerization of vinyl acetate with molecular weight that increased linearly with conversion and matched the theoretical values. The mediation of VAc polymerization with high radical concentration is the feature of the CoII(BpyBph) system, but why the increased radical 7365


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Figure 11. Change of [CoII(BpyBph)] and [CoIII(BpyBph)−R] during the induction period in the polymerization of methyl acrylate under the conditions of [CoII]0/[AIBN]0/[MA]0 = 1/10/700, [AIBN]0 = 0.078 M, in benzene at 60 °C.

Figure 9. Plots of conversion vs Mn vs PDI for MA polymerization mediated by CoII(BpyBph) in benzene (MA:benzene = 1:1 (v/v)) at 60 °C under the conditions of [CoII]0/[AIBN]0/[MA]0 = 1/10/700, [MA]0 = 5.47 M. The black dotted line was the theoretical molecular weight calculated by the assumption of one chain per cobalt complex ([CoII]0), and the red dashed line was the theoretical molecular weight calculated by the equilibrium concentration of organocobalt(III) species ([CoIII]eq).

slightly larger than that for CoII(Salen*) (2.4 × 107 M−1)19 but smaller than the value for CoII(TMP) (8.7 × 109 M−1).37 The equilibrium of cobalt(II) and organocobalt(III) species (eq 1) is crucial to CMRP and determines the control mechanisms of the polymerization. When the equilibrium extremely favors the site of organocobalt(III) (Keq > 1012),8,19,42 the reverse reaction, Co−C bond homolysis that is the key step for reversible termination, can hardly happen, and thus DT becomes the only possible control mechanism. When the equilibrium constant declines to 1010−107,19,37 the radical could be dissociated from the organocobalt(III) so that the contribution of RT to the control process becomes more significant. The discussion of the change of equilibrium was focused on the stability of radical species,37,44 but the transformation of the oxidation state of cobalt species should also play an important role so that the redox potentials of selected cobalt complexes used in CMRP were compared to the cobalt(II)/organocobalt(III) equilibrium constants in MA polymerization (Table 4). The complexes of CoII(Salen*)

10). The concentration of CoII(BpyBph) was estimated by the absorbance at 550 nm with extinction coefficient equal to 1723

Table 4. Equilibrium Constant and Redox Potential of Cobalt Complexes in CH2Cl2 Solution Figure 10. UV−vis spectrum illustrating the formation of the CoIII(BpyBph)−R complex during the induction period of the methyl acrylate polymerization ([CoII]0/[AIBN]0/[MA]0 = 1/10/700, [AIBN]0 = 0.078 M in benzene at 60 °C).

complex II

Co (BpyBph) CoII(Salen*) CoII(TMP)

Keq (M−1)

E1/2 (V)

ref

8.6 × 107 2.4 × 107 8.7 × 109

−0.01a 0.01a 0.60b

39 19, 45 37, Supporting Information

a In the presence of 0.1 M n-Bu4NClO4. bIn the presence of 0.1 M nBu4NPF6. Potential values given in V versus the Fc/Fc+ reference electrode, T = 298 K.

M−1 cm−1, and the concentration of CoIII(BpyBph)−R was calculated by the absorbance at 550 nm with extinction coefficient equal to 4557 M−1 cm−1 and at 839 nm with extinction coefficient equal to 962 M−1 cm−1 (Supporting Information). Changes in the electronic spectra during the induction period for CoII(BpyBph) mediated MA polymerization demonstrated that part of CoII(BpyBph) was converted to the CoIII complex (PMA−CoIII(BpyBph)) and the ratio of [CoII]eq/[CoIII]eq was 0.38 (Figure 11), which gave an estimate for the theoretical molecular weight calculated by [CoIII]eq (Mn,th = ([M]0/[CoIII]eq) × conversion × MW of monomer) highly agreeing with the results of MA polymerization mediated by CoII(BpyBph) complexes (Figure 9, red dashed line). The value of [CoII]eq/[CoIII]eq combined with a radical concentration of 3.1 × 10−8 M obtained from the kinetic plots ([R•] = slope/kp(333 K) of MA = 3.98 × 10−4/13000)37,43 indicates the equilibrium constant of cobalt(II) and organocobalt(III) species in MA polymerization as 8.6 × 107 M−1, which is

and CoII(BpyBph) have the E1/2 as 0.0145 and −0.01 V39 with the Keq equal to 2.4 × 107 and 8.6 × 107 M−1, respectively.19 However, the values of E1/2 and Keq for CoII(TMP) are much larger as 0.60 V (Supporting Information) and 8.7 × 109 M−1.37 Although the order of E 1/2 values (Co II (BpyBph) < CoII(Salen*) < CoII(TMP)) did not strictly match that of Keq (CoII(Salen*) < CoII(BpyBph) < CoII(TMP)), the trend for the interplay of Keq and E1/2 can still be observed. The higher redox potential of the cobalt(II)/cobalt(III) leads to a larger equilibrium constant and thus drives the control mechanism of CMRP to degenerative transfer process. More systematic study of CMRP with varied cobalt complexes would be required to build an in-depth understanding of the correlation between different factors and the cobalt equilibrium, like what was reported for the copper ATRP system.46 7366


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Table 5. Polymerization of Different Monomers Mediated by CoII(BpyBph) with the Initiation of AIBNa entry

monomer

solvent

time (min)

convb (%)



PDIc

1 2 3 4e

NVP AN NIPAM styrene

DMF DMF DMF benzene

180 300 105 150

87.9 76.3 92.0 75.1

79.9 51.6 10.04 6.3f

68.4 28.3 72.9 54.8

1.36 1.35 1.54 1.89

General conditions: [CoII]0/[AIBN]0 = 1/10, [monomer]0 = 5.47 M at 60 °C. bConversion was detected by 1H NMR. cMn was determined by gel permeation chromatography (GPC) in DMF with poly(ethylene oxide) (PEO) calibration corrected by the Mark−Houwink equation. dMn,th = ([monomer]0/[CoII]0) × conversion × MW of monomer. ePolymerization was carried out at 90 °C. fMn was determined by THF gel permeation chromatography (GPC) with polystyrene as standard. a

Figure 12. GPC traces for polymerization under the following condition of [CoII]0/[AIBN]0/[monomer]0 = 1/10/700, [monomer]0 = 5.47 M, monomer = (a) NVP, (b) AN, (c) NIPAM, and (d) Sty. K eq

CoII(L) + R• XoooY CoIII(L)−R

with Mn equal to 15 000 and PDI equal to 1.21. The chain extension from PVAc to PNVP was carried out in DMF at 60 °C to approach 20% conversion in 40 min with Mn equal to 21 000 and PDI equal to 1.30 (Figure 13). The success in block copolymer synthesis associated with the observation of the linear increased molecular weight versus conversion and a moderate polydispersity demonstrated that the CoII(BpyBph) mediated VAc polymerization fulfilled the requirements of controlled/living radical polymerization.

(1)

Keq = [CoIII(L)−R]/([CoII(L)][R•])

Polymerization of Other Monomers. The CoII(BpyBph) was further used in the radical polymerization of acrylonitrile (AN), N-vinylpyrrolidone (NVP), N-isopropylacrylamide (NIPAM), and styrene (Sty) (Table 5). The induction period followed by a linear first-order kinetic plots was observed in the polymerization of AN and NVP (Figures SI1 and SI2), and the control of polymerization was demonstrated by the linear increased molecular weight versus conversion with smoothly shifted GPC traces (Figure 12a,b) and the relatively low polydispersity. However, the significant deviation of molecular weight, broad molecular weight distribution, and the fixed GPC traces (Figure 12c,d) indicated that CoII(BpyBph) is not a good mediator for controlled/living radical polymerization of NIPAM and Sty. Synthesis of Block Copolymer. The formation of block copolymers is one of the most important features of controlled/ living radical polymerization, and thus organocobalt(III) complexes of PVAc−CoIII(BpyBph) was used as the macroinitiators to initiate the NVP polymerization for the preparation of block copolymer of poly(vinyl acetate) and poly(Nvinylpyrrolidone) (PVAc-b-PNVP). The macroinitiator was the product of CoII(BpyBph) mediated VAc polymerization

Figure 13. GPC traces of (a) PVAc macroinitiator, Mn = 15 000, PDI = 1.21, and (b) PVAc-b-PNVP, Mn = 21 000, PDI = 1.30. PVAc-bPNVP was synthesized under the condition of [macroinitiator]0/ [NVP]0 = 1/700 in DMF solution at 60 °C with [NVP]0 = 2.92 M. 7367


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under nitrogen. The gray mixture was stirred and heated at 60 °C to start the polymerization. The monomer conversion was followed by 1 H NMR. The polymerizations of methyl acrylate, N-vinylpyrrolidone, acrylonitrile, N-isopropylacrylamide, and styrene were performed with the same procedure. Synthesis of Block Copolymer. Macroinitiators of CoIII(BpyBph)−PVAc (conversion = 29.5%, Mn = 15 000, PDI = 1.21) were prepared by general polymerization procedure as [CoII]0/ [AIBN]0/[VAc]0 = 1/10/700 with [VAc]0 = 10.68 M at 60 °C in bulk, and the unreacted monomers were removed after the polymerization by high vacuum. The flask with macroinitiators was then filled with nitrogen, followed by the addition of DMF (9 mL) and Nvinylpyrrolidone (3.6 mL, 35 mmol) solution dissolving CoII(BpyBph) (31.1 mg, 0.05 mmol). The mixture was stirred and heated at 60 °C to start the polymerization.

CONCLUSION The cobalt(II) bipyridine bisphenolate complex has been demonstrated as a good mediator for the controlled/living radical polymerization of vinyl acetate, methyl acrylate, Nvinylpyrrolidone, and acrylonitrile by fulfilling the living characters of linear increased molecular weight with monomer conversion, relatively narrow molecular weight distribution, and the generation of block copolymer. The VAc polymerization was proposed to be mediated by CoII(BpyBph) via a degenerative transfer process and could be accelerated by the increasing of [AIBN]0 with no significant influence to the control efficiency. The control mechanism of MA polymerization should be the reversible termination process as demonstrated by the measurement of equilibrium constant (K eq = [Co III−R]/[Co II ][R • ]) between cobalt(II) and organocobalt(III) that equals to 8.6 × 107 M−1. A positive correlation of reduction potential of cobalt(II) complexes and equilibrium constant in MA polymerization was preliminarily observed and could be used to evaluate the control property of cobalt complexes in C/LRP.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data of each polymerization, 1H NMR spectrum of PVAc-b-PNVP block copolymer, GPC traces, UV−vis spectrum and numerical method, and CV measurement of CoII(TMP). This material is available free of charge via the Internet at http://pubs.acs.org.

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EXPERIMENTAL SECTION

Materials. Solvents were dried by refluxing at least 24 h over sodium/benzophenone (toluene, THF, ether) and CaH2 (CH2Cl2, benzene). Vinyl acetate (>99%, ACROS) was degassed by three freeze−pump−thaw cycles and distilled under reduced pressure. Methyl acrylate (>98%, ACROS), N-vinylpyrrolidone (>98%, ACROS), acrylonitrile (>99%, ACROS), and styrene (>99%, Aldrich) were passed through the basic Al2O3 to remove the inhibitor. NIsopropylacrylamide (97%, Alderich) was purified by repeated recrystallization in a mixture of toluene/hexane (70:30, v/v) and dried in a vacuum. Dimethylformamide (DMF, Alfa Aesar) and 2,2′azobis(isobutyronitrile) (AIBN, Showa) were used as received. Bromophenols and bipyridine were purchased from Aldrich and used without further purification. Deuterated solvents (Aldrich) were dried over molecular sieves. Characterization. 1H NMR spectroscopy used to identify the ligand structure, and monomer conversion was measured on a Varian Unity INOVA 400 MHz spectrometer using CDCl3, CD3OD, and DMSO-d6 as solvent at 298 K. Chemical shifts of proton spectra are reported as follow: δ 7.24 ppm (CDCl3), δ 3.31 ppm (CD3OD), and δ 2.49 ppm (DMSO-d6). The UV−vis spectrum were measured by a SHIMADZU UV-1800 instrument from 1100 to 190 nm. Gel permeation chromatography (GPC) was carried out by THF eluent using the Ultimate 3000 liquid chromatograph equipped with a 101 refractive index detector and three Shodex columns (Shodex KF-802, KF-803, and KF-804) or by DMF−LiBr eluent using the Waters 600 liquid chromatograph equipped with a 101 refractive index detector and styragel HR columns (HR2; HR5). The signal was collected by a DIONEX Shodex RI-101 refractometer (RI) detector and an UltiMate 3000 variable wavelength detector operated at 254 nm for the THF and the DMF system. The calibration was based on narrow linear poly(styrene) standard for the THF system and poly(ethylene oxide) standard for the DMF system. The molar masses of the PNVP and PAN were corrected using the Mark−Houwink equation with the parameters for PNVP (K = 6.910 × 10−5 dL g−1, a = 0.7236) and PAN (K = 3.87 × 10−6 dL g−1, a = 0.9088).47 Synthesis of Cobalt Bipyridine Bisphenolate Complex, CoII(BpyBph). The cobalt complex was synthesized according to published methods.39 To a 10 mL ethanol solution of Co(OAc)2 (1 equiv) was added a (BpyBph)H2 (1 mmol) under refluxing conditions; the solution was stirred for 2 h and filtered with methanol. CoII(BpyBph) solid was obtained in 60% yield. General Procedure for Polymerization. [CoII(BpyBph)] (55.9 mg, 0.09 mmol) and AIBN (298.6 mg, 1.8 mmol) were placed in a 50 mL Schlenk flask and degassed by three vacuum/nitrogen cycles. Dry, degassed vinyl acetate (5.9 mL, 63 mmol) was then added by syringe

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-H.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council, Taiwan (NSC 1022113-M-007-007-MY2), for support of this research. REFERENCES

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