New Features of the Mechanism of RAFT Polymerization - ACS

Chapter 1

Downloaded by UNIV OF NEBRASKA - LINCOLN on April 2, 2013 | http://pubs.acs.org Publication Date: August 13, 2009 | doi: 10.1021/bk-2009-1024.ch001

New Features of the Mechanism of RAFT Polymerization Graeme Moad,* Y.K. Chong, Roger Mulder, Ezio Rizzardo, San H. Thang, CSIRO Molecular and Health Technologies, Bag 10, Clayton South, Victoria 3169, Australia

RAFT polymerizations of styrene with azobis(isobutyronitrileα-13C) as initiator and various RAFT agents (cumyl dithiobenzoate (5), cyanoisopropyl dithiobenzoate (6), benzyl dithiobenzoate-thiocarbonyl-13C (7) or cyanoisopropyl dodecyl trithiocarbonate (8)) were followed by real time 13C NMR. While the rate of AIBN decomposition and the initial fate of the initiator-derived radicals (the initiator efficiency) were not substantially affected by the RAFT agent, the rate of polymerization was strongly dependent on both the type and concentration of RAFT agent. Polymerizations with the more active dithiobenzoate (5,6) RAFT agents and trithiocarbonate 8 display formation of a single unit styrene chain prior to any substantial formation of higher oligomers. An unexpected finding is the observation of 13C CIDNP for the ketenimine formed by cage recombination of AIBN-derived cyanoisopropyl radicals. With benzyl dithiobenzoate (7), consumption of the initial RAFT agent is slow. By-products from intermediate radical termination are also observed.

© 2009 American Chemical Society

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In Controlled/Living Radical Polymerization: Progress in RAFT, DT, NMP & OMRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction The RAFT process is a versatile method for conferring living characteristics on radical polymerizations which provides unprecedented control over molecular weight, molecular weight distribution, composition and architecture.15 It is suitable for most monomers polymerizable by radical polymerization and is robust under a wide range of reaction conditions. RAFT polymerizations of styrene were described in the first communication of RAFT polymerization in 19986 and have been the subject of many subsequent papers. The mechanism of the RAFT process is shown in Scheme 1. Ideally, since radicals are neither formed nor destroyed as a consequence of the RAFT equilibria, they should not directly affect the rate of polymerization. RAFT agents can behave as ideal chain transfer agents.7-9 Scheme 1 Mechanism of RAFT polymerization

1

2

3

3

4

3

However, retardation has been observed in some circumstances. In 2000,1 we reported that RAFT polymerizations of styrene, BA and MMA were subject to retardation when high concentrations of RAFT agent were used and that the extent, mechanism and particular manifestation of retardation were dependent on the specific RAFT agent-monomer combination used. Much has now been published on retardation in RAFT polymerization and the possible causes of a slower rate of polymerization.7 The situation with respect to control of radical polymerization with dithiobenzoate RAFT agents has been summarized by an IUPAC task group in a ‘dilemma’ paper.10 Factors that may influence the polymerization kinetics include (a) slow fragmentation of the intermediated


5


11-14

15-17

18-20

(b) intermediate radical termination, (c) “missing steps”, (d) radical, inefficient reinitiation,8 (e) a reduced gel or Trommsdorf effect and effects of chain length dependent termination,9,12 (f) high C-tr (=k−β/ki)21 (g) impurities in the RAFT agent,22 (h) impurities such as oxygen in the reaction medium,2 and (i) various combinations of these effects. High level molecular orbital calculations suggest that, when dithiobenzoate RAFT agents are used, the intermediates formed (2 and/or 4) have sufficient stability such that slow fragmentation, by itself, is a potential cause of retardation.13 ESR studies show that the intermediates (2 and/or 4) are present only in very low concentrations and by implication that slow fragmentation, by itself, cannot be the cause of retardation.23-25 A recent paper by Konkolewicz et al.26 purports to suggest a possible resolution of these conflicting results. This study suggests that intermediate radical termination occurs, but only involves initiator-derived or oligomeric chains. With many rate constants unknown or uncertain, kinetic modeling, while a useful tool for excluding some possibilities, is not able to unambiguously discriminate the models for retardation. It is possible to fit the evolution of the molecular weight distribution with time using many of the above-mentioned models. Real-time 1H NMR has previously been used to study RAFT polymerization of styrene with azobis(isobutyronitrile) (AIBN) initiator and with cumyl27 or cyanoisopropyl dithiobenzoate28 as the RAFT agent and more recently polymerizations of methyl acrylate,29,30 vinyl acetate31 N-vinyl pyrrolidone31 and styrene-maleic anhydride32,33 and styrene-acrylonitrile copolymerization33 with various RAFT agents. 13C NMR was also used to study RAFT polymerization of styrene with AIBN initiator and cumyl dithiobenzoate-thiocarbonyl-13C as the RAFT agent.34 For styrene polymerization at 70 or 84 ºC with high concentrations of cumyl or cyanoisopropyl dithiobenzoate, complete conversion of the initial RAFT agent to a single unit ‘chain’ was observed prior to any significant formation of two unit or higher chains.27,28,34 This phenomenon was called ‘efficient initialization’. This outcome can be predicted by kinetic simulation based on (a) the assumption of slow fragmentation and rate constants estimated by ab initio calculations12,13 or (b) with faster fragmentation (so as not to cause retardation directly) and intermediate radical termination.35 Our kinetic modeling studies show that the observation of such ‘efficient initialization’ is not dependent on slow fragmentation or the occurrence of intermediate radical fragmentation. It is observed for the more active RAFT agents when the rate constant for the first monomer addition (ki) is rapid with respect to that for subsequent propagation steps (as is usually the case) and the RAFT agent concentration is such that 3 even for small elapsed time).



11 The evolution of the normalized concentration of the various AIBN derived products with time for experiments with 0.5 M RAFT agent is shown in Figure 3. It can be seen that the rate of disappearance of AIBN and formation of TMSN, IBN and total end groups is similar for all experiments. As mentioned above the amount of K could not be directly determined. However, the amount of K formed approximated as 1-(AIBN+TMSN+IBN(×2) + total labeled cyanoisopropyl end groups)+KB is also essentially independent of the RAFT agent and is consistent with the amount of K observed in the polymerization mixture after it had been cooled to ambient temperature. Even though the total labeled cyanoisopropyl end groups is RAFT agent independent, the fractions of oligomers of different chain lengths is strongly dependent on the particular RAFT agent. In experiments with benzyl dithiobenzoate-thiocarbonyl-13C (7) a group of resonances is seen in the region δ 70-80. These are tentatively attributed to the labeled dithioacetal carbon of “3-arm stars” such as 10-12. Calitz et al. reported products tentatively identified as 3-arm or 4 armed stars in experiments with cumyl dithiobenzoate-α-13C at longer reaction times. Kwak et al. observed formation of 9 when phenylethyl radical was generated in the presence of phenylethyl dithiobenzoate.

Ph

Ph S

n

S CN

917 Ph S n

10 n

S CN

Ph

11

12

When AIBN-α13 C is decomposed in the presence of benzyl dithiobenzoate but in the absence of monomer, signals attributable to the product 14 (Scheme 4) are observed. This product is not observed in polymerization experiments probably because the cyanoisopropyl radical is preferentially consumed by reaction with styrene under those conditions. There is also evidence for the formation of other dithioesters. Such (e.g. 17) might be formed as shown in Scheme 4. Signals associated with dithioketene acetals (e.g. 15) have not been identified. One product observed with benzyl dithiobenzoate is characterized by doublet resonances at δ 74.7 (derived from 7) and 34.3 (derived from AIBN-α13 C) with Jcc 4.2 Hz (a singlet resonance 34.3 derived from AIBN-α13 C in


12 experiment with unlabeled RAFT agent 7a). The analogous product was not observed in experiments with the other RAFT agents as evidenced by the absence of signals at δ 34.3.


Scheme 4. Some Possible Side Reactions of Benzyl Dithiobenzoate

7a

13

14

15 16 17 The rate of monomer consumption with time was linear with time in all experiments indicating that a steady state of some form was established (Figure 4). For experiments with 0.5 M RAFT agent, the rate of styrene consumption increased in the series cumyl dithiobenzoate ~ benzyl dithiobenzoate < cyanoisopropyl dithiobenzoate < cyanoisopropyl trithiocarbonate. For both 0.1 M and 0.5 M RAFT agent the rates of styrene consumption were similar with cumyl and benzyl dithiobenzoates even though the rate of consumption of RAFT agent and the molecular weight of polymer formed was very different (vide infra). With both cyanoisopropyl RAFT agents the consumption of styrene is initially rapid with respect to the steady state rate.


13 1.2 8 7

0.8 ln(M /M ) o

t

5

0.4

6 7


0.0 0

400

5 1200

800

elapsed time (mins)

Figure 4. Kinetic plot showing rate of monomer consumption with time during polymerization of styrene in benzene-d6 (50% v/v) at 68.5 ºC with 0.5 M [5] (□), 0.1 M [5] (∇), 0.5 M [6] (○), 0.5 M [7] (◊), 0.1 M [7] (∆), 0.5 M [8] (×) RAFT agent and with 0.1 M azobis(isobutyronitrile). (a)

(b) m d

1351 min

d

m d

1351 min t

t

t t

246 min 5

246 min

0 min 228

0 min 227

226 ppm

225

224

32.3

32.1

31.9 ppm

31.7

31.5

Figure 5. Portions of 13C NMR spectra recorded during polymerization of styrene (4.36 M in benzene-d6) at 70 ºC with cumyl dithiobenzoate (5, 0.5 M) and AIBN-α-13C (0.1 M) showing signals attributed to (a) labeled thiocarbonyl carbons and (b) the labeled cyanoisopropyl end groups of styrene oligomers (m=1 unit chain (e.g. 19), d=2 unit chain (e.g. 20), t=3 unit chain (e.g. 21)). For details of signal assignments see Figure 6.



14

5

18

6

19

7

20

21 29.7(2) 29.6 29.45 29.4 29.0 28.9

119.8 217.8 26.7

CN 42.5

S

S

22.7

36.9

S

27.7

31.9

13.9

8 27.0

CN

26.9 31.8

29.7(2) 29.6 29.45 29.4 29.0 28.9

221.8

123.5

Ph

S

36.9

52.3

S 45.4

S

28.0

22.7 31.9

13.9

Figure 6. 13C NMR chemical shifts (benzene-d6, 70 ºC) of RAFT agents and macro RAFT agents formed with styrene. Conversion of the K to a stable byproduct KB by itself is expected to cause some retardation, since in other circumstances, K would revert to cyanoisopropyl radicals. However, this effect should be of little significance. Nonetheless, there appears to be a correlation (possibly fortuitous) between the rate of styrene consumption and the yield of the ketenimine by-product KB.


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Conclusions Real-time NMR has been used to study RAFT polymerizations of styrene with azobis(isobutyronitrile-α-13C) as initiator and various RAFT agents. The rate of AIBN decomposition and the initiator efficiency are essentially affected by the presence of RAFT agent even when high concentrations are used. However, the rate of polymerization and the polystyrene chain length distribution is strongly dependent on both the type and concentration of RAFT agent with the rate of styrene consumption increasing in the series cumyl dithiobenzoate ~ benzyl dithiobenzoate < cyanoisopropyl dithiobenzoate < cyanoisopropyl trithiocarbonate. An unexpected finding is the observation of 13C CIDNP for K formed by cage recombination of AIBN-derived cyanoisopropyl radicals and the formation of an as yet unidentified by-product (KB) from K. There appears to be a correlation between the yield of KB and the observed rate of polymerization though this may be fortuitous. With benzyl dithiobenzoate (7), consumption of the initial RAFT agent is relatively very slow and various by-products. These byproducts include direct coupling with the RAFT intermediate (intermediate radical termination) and other products possibly include some formed by the “missing step” process proposed by Buback and Vana.

Experimental Nuclear magnetic resonance (NMR) spectra were obtained with a Bruker DRX500 spectrometer operating at 125.8 MHz for 13C and 500.1 MHz for 1H. Chemical shifts are reported in ppm from external tetramethylsilane. Quantitative 13C NMR spectra were obtained using an inverse-gated pulse sequence with a 30° pulse (zgig30) allowing a 20 s relaxation delay between scans and were summed over 64 scans for each data point. The following reagents were used without further purification: Benzoic-carbonyl-13C acid (Aldrich, 99 atom% 13C); dicyclohexylcarbodiimide (Aldrich 99%); Lawesson reagent (Aldrich 97%). AIBN (DuPont) was purified by crystallization from chloroform /methanol at -20ºC. Benzyl Dithiobenzoate-thiocarbonyl-13C. Dicyclohexylcarbodiimide (0.84 g, 4.1 mmol) was added in one portion to a solution of benzoic-carbonyl-13C acid (0.5 g, 4.06 mmol) and benzyl mercaptan (0.51 g, 4.06 mmol) in dichloromethane (10 mL). The resulting mixture was allowed to stir at room temperature overnight. The by-product, dicyclohexyl urea, was separated by filtration and the filtrate was concentrated to give the product, S-benzyl thiobenzoate-carbonyl-13C (0.41 g, 46.5 %) as a colorless liquid which was used directly in the next step. 13C NMR (CDCl3) δ191.3 (C=O)



16 A solution of S-benzyl thiobenzoate-carbonyl-13C (0.41 g) and Lawesson reagent (0.46 g) in toluene (5 mL) were heated at 110 °C for 45 hours during this time the initially colorless reaction mixture turned dark red. After cooling to room temperature, the reaction mixture was concentrated and chromatographed on a silica-gel with 3% ethyl acetate in n-hexane as eluent to yield benzyl dithiobenzoate-thiocarbonyl-13C (0.35 g, 75.6%). The 13C NMR showed the product to be contaminated with ca 5% of the unchanged S-benzyl thiobenzoate-carbonyl-13C. 13 C NMR (benzene-d6 , 70 ºC) δ227.1 (C=S), 145.0 (d, C1, JCC=54 Hz), 135.3 (d, benzyl C1, JCC=2.3 Hz), 131.9 (d, C4, JCC=1.3 Hz), 129.2 (s, 2×benzyl C3), 128.7 (s, benzyl C4), 128.5 (s, 2×benzyl C2) 128.1 (d, 2×C3, JCC=4.5 Hz), 126.9 (d, 2×C2, JCC=2.6 Hz) , 42.1 (d, CH2, JCC=1 Hz). AIBN-α-13C. AIBN-α-13C was available from our previous study.36,37 13C NMR (benzene-d6, 70 ºC) δ 118.3 (s, C≡N) 67.6 (s, Cq), 24.5 (d, CH3, JCC=60Hz) Polymerizations. The following experiment is typical. Benzyl dithiobenzoate-thiocarbonyl-13C (14.7 mg, 0.1 M) and AIBN (9.96 mg, 0.1 M) were weighed in a vial and styrene (0.3 mL, 273 mg, 4.36 M) and benzene-d6 (0.3 mL, 264 mg, 5.63 M) added and the solution was transferred to an NMR tube. The contents of the NMR tube were degassed through three freezeevacuate-thaw cycles and the NMR tube sealed under nitrogen. The NMR tube was then placed in the preheated probe of the NMR spectrometer and the acquisition of data commenced immediately (first data point after ~7 minutes). Details of reagent concentrations used in other experiments are provided in Table 1. Table 1. Reagents concentrations used in styrene polymerizationsa [AIBN-α-13C] M 0.1 0.1 0.1 0.1 0.1 0.1 0.1 a

RAFT agent 5 5 6 7 7 7+7a 8

[RAFT agent] M 0.1 0.5 0.5 0.1 0.1 0.1+0.4 0.5

[Styrene] M 4.36 4.36 4.36 4.36 0b 4.36 4.36

Concentrations at 22 ºC, solvent is benzene-d6. b Control experiment.


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New Features of the Mechanism of RAFT Polymerization - ACS

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