Reverse Atom Transfer Radical Polymerization Using AIBN or BPO as

Chapter 19

Reverse Atom Transfer Radical Polymerization Using AIBN or BPO as Initiator Wenxin Wang and Deyue Y a n

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch019

College of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, Peoples Republic of China

The reverse atom transfer radical polymerization (ATRP) of methyl acrylate can be realized with the initiating system, AIBN/CuCl /bpy, in bulk by means of a new polymerization procedure. The reverse A T R P of styrene is also accessible using BPO/CuCl /bpy as the initiating system. The initiation mechanism in reverse A T R P with B P O is different from that with A I B N due to the redox reaction between B P O and Cu generated from the reaction of radicals with Cu . The "living"/Vcontrolled radical polymerization of methyl methacrylate and styrene can be implemented using BPO/Cu Cl/bpy as the initiating system in solvents at ambient temperature (40°C). The mechanism suggested the initiation is a redox reaction, and the propagation is identical with that of A T R P . 2

2

I

II

I

In recent years, the "living'Vcontrolled radical polymerization has attracted much attention. Several approaches have been reported " , among which atom transfer radical polymerization (ATRP) was considered as the most effective way to carry out controlled radical polymerization. The transition-metal-catalyzed atom transfer radical addition, A T R A , gives a unique and efficient method for carbon-carbon bond formation in organic synthesis . Matyjaszewski ' , Teyssie , Sawamoto , and Percec have successfully introduced this approach into polymerization chemistry, and developed the novel type of 'living'/controlled radical polymerization, i.e., atom transfer radical polymerization (ATRP), in which an alkyl halide, R - X , was used as the initiator and a transition-metal species complexed with suitable ligand(s), Mt /Lx, as the catalyst. In addition, Matyjaszewski and coworker further reported that a 'living'/controlled polymerization was also accessible using a conventional radical initiator (azodiisobutyronitrile, AIBN) and a transition-metal compound at higher oxidation state (e.g.,Cu Cl ) complexed with suitable ligands (e.g., 2.2'-bipyridine, bpy) as a catalyst. The latter was called reverse A T R P . As a "classical" radical initiator (AIBN) instead of an alkyl halide was used, many polymer chemists have showed great interest in reverse A T R P . Matyjaszewski and coworker pointed out 1

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Corresponding author.

© 2000 American Chemical Society

Matyjaszewski; Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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264 that reverse A T R P of styrene and methyl acrylate can be carried out under homogeneous conditions by using alkyl substituted bipyridine ligands, such as 4,4'nonyl-2,2'-bipyridine (dNbipy). Teyssie and coworkers implemented the reverse A T R P of M M A using FeCl and A I B N as the initiating system in the presence of tripenylphosphine. Most recently, Yan et a l . developed a new procedure for reverse A T R P of methyl acrylate ( M A ) in bulk; Y a n ' and Matyjaszewski further investigated the reverse A T R P using B P O / C u X / L x or BPO/Cu X/Lx initiating system. It is interesting to narrate some results on these reverse A T R P topics. 14

3

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16

1 7

18

n

!


2

n

Reverse ATRP of MA Initiated by AIBN/Cu Cl /bpy 2

u

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The initiating system AIBN/Cu Cl /bpy developed by Matyjaszewski can initiate the "living'Vcontrolled radical polymerization of styrene in bulk, however, it does so only under the high mole ratio of A I B N to C u C l . In addition, the "living'Vcontrolled polymerization of methyl acrylate can't be implemented by the similar procedure. Fortunately, the "living'Vcontrolled polymerization of methyl acrylate can be realized with AIBN/Cu Cl /bpy initiating system by the aid of a new polymerization approach. Table I shows the characterization data of poly(methyl acrylate) generated from reverse A T R P catalyzed by AIBN/Cu Cl /bpy in bulk. The mole ratio of C u C l to A I B N is not a critical parameter for the 'living' /controlled radical polymerization using the new polymerization procedure. Little influence of the [Cu Cl ]/[AIBN] ratio on the controlled nature of the polymerization has been observed. Different ratios of C u C l to A I B N used in this work always result in wellcontrolled radical polymerizations and provide high initiation efficiency and polymers with low polydispersity (for instance, M / M = 1.36). Figure 1 shows that the measured molecular weight linearly increases with the monomer conversion, and matches the theoretical value calculated from eq. 1 for bulk polymerization of M A initiated by A I B N in the presence of 2 molar equiv. C u C l and 6 molar equiv. bpy at 100 °C. 2

2

n

2

n

2

n

2

n

2

u

2

w

n

n

2

M

=

n , t h ([M] /[BPO])x(MW)xconversion

(1)

0

where (MW) denotes the molecular weight of the monomer.

Table I Characterization Data of Poly(methyl acrylate)*

[AIBN] (mol/L) 0.0155 0.0155 0.0155 0.0155

fCuCl J (mol/L) 0.031 0.062 0.093 0.124 2

[bpy]

Time

Conv

(mol/L) 0.093 0.186 0.279 0.372

(h)

(%) 93 89 92 91

12 18 31 41

MJM

n

28630 27400 28330 28020

29900 28600 29000 28700

15

1.46 1.42 1.36 1.39

*SOURCE: ReproducedfromReference . Copyright 1998 Wiley-VCH Verlag GmbH



265

Figure L Molecular weight and molecular weight distribution dependence on monomer conversion for the bulk polymerization of MA initiated by AIBN/CuC^/bpy. βίΑ]=11.1Μ, [AIBN]=0.0155M, [CuCl ]=0.031M, [bpy]=0.093M. (Reproduced from Reference 15. Copyright 1998 Wiley-VCH Verlag GmbH) 2

These experimental data indicated that the bulk polymerization of M A initiated by AIBN/Cu Cl /bpy system proceeded via a living/controlled mechanism. Furthermore, the measured molecular weight of £he resulting polymer is in a good agreement with theory up to M = 3 x l 0 , indicating the high initiation efficiency of A I B N . The molecular weight distribution of the products is rather narrow, M / M < 1.42 (Table 1), The reaction scheme is suggested as below: n

2

4

n

w

Scheme 1 Initiation and termination at lower temperature: I-I

A .

21·

+ ΐΛ

I-

I-X

+ Mt

lq'j+M Ι-Ρ

Γ

+ ΜΓχ

'

I-Pi-X + Mf


n

266 Initiation and propagation at higher temperature: I-X +

Mt

I-

n+1

+ Mt X


kiJ+M

I-Pi-X

+

Mt

P -X

+

Mt

n

+1

— I-Pr + Mt" X

ρ

η·

+

n+1

Mt X

n

It is known that C u C l is a strong and efficient inhibitor of radical polymerization under usual conditions . When the polymerization of M A proceeds at 65°C-70°C, it enables slower decomposition of A I B N which allows the dissolution of Cu Cl /bpy at the rate comparable to A I B N consumption. The oxidized transition-metal, C u C l , is a strong and efficient inhibitor of the polymerization of M A initiated by A I B N . C u C l donates the halogen atom CI to the initiating or propagating radicals, I · or I-P · , forming the reduced transition-metal species, Cu , and the dormant species, I - C l and P - C l . At lower temperature, 65 °C -70 °C, the covalent species I - C l and P - C l are rather stable, therefore the reduced transition-metal species, Cu , can not promote A T R P process as it does in the living/controlled radical polymerization initiated by R - X / M t / L x at higher temperature. After ten hours at 65°C-70°C, most of A I B N was exhausted while the reduced transition-metal species (Cu ) and the dormant species ( I - C l and P - C l ) were accumulated. During this period the inhibited reaction is predominant, and little polymer was generated. In order to carry out the fast ATRP, higher temperature is needed. When the system is heated to 100°C, however, a reversible activation of the resulting alkyl chlorides occurs in the presence of a coordinative ligand, then the polymerization of the system proceeds via the reverse A T R P mechanism. 2

19

n

2

n

2

n

2

1

1

1

n

Reverse ATRP of Styrene Initiated by BPO/Cu Cl /bpy 2

n

The polymerization of styrene (St) with BPO/Cu Cl /bpy as the initiating system can also result in a well-controlled radical polymerization with high initiation efficiency, 90%, and the polydispersity of resulting polymer is rather low, M / M = 1.32. Figure 2 shows that the measured molecular weight linearly increases with increasing monomer conversion, and matches the theoretical value calculated from eq. 1 for bulk polymerization of styrene initiated with B P O in the presence of 2 molar equiv. Cu C l and 4 molar equiv. bpy at 110°C. Figure 3 shows the first-order kinetic plot, indicating that the concentration of the growing species remained constant. The first point in Figure 3 is beyond the experimental error from the linear plot, and the monomer conversion at this moment is apparently higher than that in a living polymerization. It seems that the polymerization occurs partially at low temperature. 2

w

11

2


n


267

Conversion (%)

Figure 2. Molecular weight and molecular weight distribution dependence on monomer conversion for the heterogeneous reverse ATRP of styrene at 110 "C in bulk. [Styrene] :[BPO]:[CuCl ]:[bpy]=100:1:2:4. (Reproducedfrom Reference 16. Copyright 1999 Wiley-VCH Verlag GmbH) 2

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t(h) Figure 3. Semilogarithmic kinetic plot for the bulk reverse ATRP of styrene at 110 V. [Styrene] :[BPO]:[CuCl ]:[bpy]=l 00:1:2:4. (Reproducedfrom Reference 16. Copyright 1999 Wiley-VCH Verlag GmbH) 2



268

J

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Figure 4. H-NMR of the polystyrene with BPO/Cu Cljbpy. Reference 16 Copyright 1999 Wiley-VCH Verlag GmbH)

(Reproducedfrom

These experimental data indicate that the polymerization of styrene initiated by the BPO/Cu Cl /bpy system proceeds via a living/controlled radical mechanism. The molecular weight is essentially proportional to the reciprocal of the concentration of BPO. Furthermore, a good correlation between the calculated molecular weight and the experimental one was found, and the polydispersity is lower than 1.39, indicating a high initiation efficiency of BPO. The structure of resulting polystyrene was studied by ^ - N M R . The ^ - N M R spectrum of PS is illustrated in Figure 4. Signals were observed at 1.2~2.1ppm, originating from the methene and methine protons of the main chain. A signal at 7.9 ppm was assigned to the aromatic protons at ortho position of the benzoyl moiety attached to the polymer head group, The broad triplets at 4.4ppm was attributed to the end group chlorine. Moreover, comparison of the integration of the signals of end group with those of methene and methine in the main chain of PS gives a molecular weight, M =12000, close to the one from G P C based on PS standards, M =11000. The reaction scheme is suggested as below: n

2

n

n

S

E

N

M

R

C

Scheme 2 0

0

0

[g^-o-o-c^ 0 ^

0 _

II

(QpC-O- +nM

(1)

-

II

0^

:-0-Μη·


(2)

269

l-O-Mn- + CuCl -bpy

+ CuClbpy


(3)

:—σ-Μη-Cl + CuCl-bpy

2

+

i-C-0"-Cu Cl-bpy

(4)

At first, a B P O molecule decomposes into two primary radicals. The Cu cation can't take electron from primary radicals due to the strong oxidation of primary radicals resulting from BPO. Secondly, after a monomer adds to a primary radical, the oxidation of the radical decreases greatly. Thirdly, C u C l can get rid of electron from this radical, forming dormant species and Cu . Finally, redox reaction between B P O and Cu takes place and produces one primary radical. This process continually repeats at lower temperature. From the analysis we can find that one mole B P O will result in one mole radicals. So the initiation mechanism of B P O /Cu Cl /bpy is somewhat different from that of AIBN/Cu Cl /bpy. After ten hours at 70 °C, most of B P O was exhausted while the reduced transition-metal species, C u , and the dormant species were accumulated. When the system is heated to 110°C, however, the polymerization under consideration proceeds via the A T R P mechanism which is the same as AIBN/Cu Cl /bpy system. n

2

1

1

n

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n

2

1

n

2

Reverse ATRP of MMA and St Initiated by BPO/C^Cl/bpy The "living'Vcontrolled radical polymerization of M M A and St in butanone is also accessible using B P O / C u t l / b p y as the initiator. When a solution of methyl methacrylate, Cu'Cl, bpy (2 molar equiv. relative to Cu C\) and BPO, (1 molar equiv. relative to Cufcl) was heated, the solution was homogeneous and gradually became viscous. As shown in Figure 5, the molecular weight measured, M , is higher than the theoretical one during the low monomer conversion period. With increasing monomer conversion, a linear increase of number average molecular weight, M , versus monomer conversion up to 90% is observed. It is interesting that the "living'Vcontrolled radical polymerization is still valid when the number-average molecular weight of products approaches to as high as 100,000. This result indicates that B P O / C u t l / b p y acts as an efficient initiating system of "living'Vcontrolled radical polymerization. The reaction temperature in this work is rather lower comparing with those of A T R P and reverse A T R P initiated by A I B N . The molecular weight distribution of products is rather narrow ( M / M < 1 . 6 ) . A linear plot of ln([M]o/([M]) versus polymerization time (Figure 6) demonstrates that the concentration of growing radicals remains constant during propagation, and termination is not significant. These experimental data suggest that the polymerization of M M A is a "living'Vcontrolled radical polymerization process with a negligible amount of irreversible transfer and termination. l

nS E C

n S E C

w

n


270 100000

80000

60000


40000

20000

0 0

10

20

30

40

50

60

70

80

Conversion (%)

Figure 5. Molecular weight and molecular weight distribution dependence on monomer conversion in polymerization of MMA with BPO/CuCl/bpy. [MMA]=5M, [BPO] = [CuCl] = [bpy/2] = 0.005M, butanone, 40 V.

t(h)

Figure 6. Semilogarithmic kinetic plot in polymerization of MMA with BPO/CuCl/bpy. [MMA] = 3M [BPO] = [CuCl] = [bpy/2] = 0.01M, butanone, 40 V. f


271 The reaction scheme is suggested as below: Scheme 3


Initiation

Propagation 0

0 -0-MQ-CI

jQj-C-O-Mn-

+

Cu^bpy

n

+ Cu Cl-bpy

It is known that cuprous salts act as very efficient accelerators in the decomposition of peroxides ' . The coordination of the bidentate nitrogen ligand to Cu increases the solubility of the inorganic salt and facilitates the redox reaction between CuCl and B P O . First, radicals are generated through the electron transfer from Cu complexed by bpy to the peroxide (BPO), and initiate the radical polymerization of M M A . Then, Cu complexed by bpy reacts reversibly with growing radicals and gets rid of electron from these radicals, forming dormant species P - C l and Cu . Finally in the presence of a coordinating ligand a reversible activation of the resulting alkyl chlorides occurs, and the polymerization proceeds via the A T R P mechanism. During the low monomer conversion period the molecular weight measured, M , is higher than the theoretical one, M . It seems that the initiation has not completed in the earlier stage of the polymerization at 40°C. More probably, at the beginning of the polymerization a certain amount of primary radicals were generated, which start the general radical polymerization at first and then switched to A T R P due to the formation and presence of Cu . Similarly, using B P O / C u t l ^ p y (1:1:2) initiating system, the reverse atom transfer radical polymerization of styrene can also afford polymers with the predetermined molecular weight and low polydispersities (

Reverse Atom Transfer Radical Polymerization Using AIBN or BPO as

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