Anal. Chem. 2007, 79, 2722-2727
Interpretation of Reversible Addition-Fragmentation Chain-Transfer Polymerization Mechanism by MALDI-TOF-MS Guangchang Zhou and Issifu I. Harruna*
Department of Chemistry, Clark Atlanta University, 223 James P. Brawley Drive SW, Atlanta, Georgia 30314
Several polystyrene polymers were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization of styrene, using two different RAFT agent-initiator systems, and then further characterized by NMR and SEC as well as MALDI-TOF-MS techniques. The data indicate that most of the polymer chains are terminated by the active groups (Ph-C(dS)-S-) derived from RAFT agents, and few of the polymer chains bear initiator fragments at one end. Most importantly, the structures arising from the intermediate RAFT radicals and their cross-termination adducts were detected. Also, the MALDI-TOF-MS analysis shows that the combination termination between two macromolecular radicals is minor, and the amount of dead chains is quite low. Thus, narrow molecular weight distribution is obtained. This analysis confirms the operation of the Rizzardo mechanism including the Monteiro intermediate radical termination model for the RAFT polymerization. Since Rizzardo et al. first reported the reversible additionfragmentation chain-transfer (RAFT) process in 1998,1 it has been intensively investigated because it retains the advantages of a wide range of monomer applicability and a high tolerance to impurities of conventional free-radical polymerization. In addition, it has the ability to generate polymers with controlled molecular weight and narrow molecular weight distribution as well as complex macromolecular architectures.2-7 However, to best utilize the * To whom correspondence should be addressed. Tel.: 404-880-6883. Fax: 404-880-6890. E-mail:
[email protected] or
[email protected]. (1) (a) Le, T. P.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT Int. Appl. WO 9801478 A1 980115, 1998;Chem. Abstr. 1998, 128, 115390. (b) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Le, T. P.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, G.; Moad, C. L.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559. (2) Benaglia, M.; Rizzardo, E.; Alberti, A.; Guerra, M. Macromolecules 2005, 38, 3129. (3) (a) Chong, Y. K.; Krstina, J.; Le, T. P. T.; Moad, G.; Postma, A.; Rizzardo, E.; Thang, S. H. Macromolecules 2003, 36, 2256. (b) Chiefari, J.; Mayadunne, R. T. A.; Moad, C. L.; Moad, G.; Rizzardo, E.; Postma, A.; Skidmore, M. A.; Thang, S. H. Macromolecules 2003, 36, 2273. (c) Moad, G.; Chong, Y. K.; Postma, A.; Rizzardo, E.; Tang, S. H. Polymer 2005, 46, 8458. (d) Favier, A.; Charreyre, M.-T. Macromol. Rapid Commun. 2006, 27, 653. (4) Matyjaszewski, K., Ed. Controlled/Living Radical Polymerization, Progress in ATRP, NMP, and RAFT; ACS Symposium Series 768; American Chemical Society: Washington, DC, 2000. (5) (a) Lutz, J.-F.; Neugebauer, D.; Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6986. (b) Chen, M.; Ghiggino, K. P.; Thang, S. H.; Wilson, G. J. Angew. Chem., Int. Ed. 2005, 44, 4368.
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RAFT process, it is very important to understand how it actually works. So far, various research groups have attempted to elucidate the RAFT polymerization mechanism through different approaches including nuclear magnetic resonance (NMR), UVvisible, electron spin resonance (ESR) spectrometry, and matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) as well as computation modeling.8-11 Rizzardo12 originally proposed the mechanism of the RAFT process, which involves a series of reversible addition-fragmentation steps as shown in Scheme 1. Addition of a propagating radical Pn• to the thiocarbonylthio compound 1, known as a chain-transfer agent (CTA), gives adduct radical 2, which fragments to a polymeric thiocarbonylthio compound 3 and a new radical R•. The radical R• then reinitiates polymerization to give a new propagating radical Pm•. Subsequent addition-fragmentation steps set up an equilibrium between the propagating radicals Pn• and Pm• and the dormant polymeric thiocarbonylthio compounds 3 and 5 by way of the intermediate radical 4. Equilibration of the growing chains give rise to narrow molecular weight distribution. Throughout the polymerization, the vast majority of the polymer chains are end capped by a thiocarbonylthio group. The presence of RAFT agent 1 fragments at chain ends of the resulting polymers 3 and 5 have been confirmed by several research groups using MALDI-TOF-MS as well as NMR and UV(6) (a) Zhou, G.; Harruna, I. I. Macromolecules 2004, 37, 7132. (b) Zhou, G.; Harruna, I. I. Macromolecules 2005, 38, 4114. (c) Zhou, G.; Harruna, I. I.; Ingram, C. W. Polymer 2005, 46, 10672. (d) Zhou, G.; Harruna, I. I.; Zhou, W. L.; Aicher, W. K.; Geckeler, K. E. Chem. Eur. J. 2007, 13, 569. (7) (a) Schilli, C. M.; Zhang, M.; Rizzardo, E.; Thang, S. H.; Chong, (Bill) Y. K.; Edwards, K.; Karlsson, G.; Mu ¨ ller, A. H. E. Macromolecules 2004, 37, 7861. (b) Mayadunne, R. T. A.; Jeffery, J.; Moad, G.; Rizzardo, E. Macromolecules 2003, 36, 1505. (8) Rizzardo, E.; Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, G.; Moad, C. L.; Thang, S. H. Macromol. Symp. 1999, 143, 291. (9) (a) Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 5457. (b) Alberti, A.; Benaglia, M.; Laus, M.; Macciantelli, D.; Sparnacci, K. Macromolecules 2003, 36, 736. (10) (a) Schilli, C.; Lanzendo ¨rfer, M. G.; Mu ¨ ller, A. H. E. Macromolecules 2002, 35, 6819. (b) Favier, A.; Ladavie`re, C.; Charreyre, M.-T.; Pichot, C. Macromolecules 2004, 37, 2026. (11) Coote, M. L.; Krenske, E. H.; Izgorodina, I. Macromol. Rapid Commun. 2006, 27, 473. (12) (a) Goto, A.; Sato, K.; Tsujii, Y.; Fukuda, T.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 2001, 32, 402. (b) Moad, G.; Chiefari, J.; Chong, Y. K.; Krstina, J.; Mayadunne, R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym. Int. 2000, 49, 993. (c) Moad, G.; Mayadunne, R. T. A.; Rizzardo, E.; Skidmore, M.; Thang, S. H. ACS Symp. Ser. 2003, 854, 520. 10.1021/ac061930p CCC: $37.00
© 2007 American Chemical Society Published on Web 02/22/2007
Scheme 1. Mechanism of RAFT Polymerization
visible techniques.10,13 Importantly, the radical adducts 2 (and 4) as intermediates in the addition-fragmentation process have also been directly observed with ESR.12a However, there is ongoing debate regarding the mechanism that causes the rate retardation phenomena observed in some RAFT polymerization systems.14 Inhibition is an initial time period without any polymerization activity, and rate retardation is a decrease in the overall rate of polymerization with increasing initial RAFT agent concentration. The reasons for the rate retardation are not clearly understood. The retardation is mostly associated with significant stabilization of the intermediate RAFT radicals 2 and 4 in addition to the Z and R groups of the CTA 1. Two opposing hypotheses have been proposed for the retardation. One is slow fragmentation in which the adduct radicals 2 and 4 are suggested to be rather stable and have long lifetimes,15 and the other is cross-termination in which the adduct radicals 2 and 4 terminate with themselves or with the propagating radicals.16 Although some evidence is provided for this RAFT mechanism, there is no comprehensive evidence to demonstrate all possible structures that can arise from the RAFT mechanism. MALDI-TOF-MS was developed in late 1980s by Karas and Hillenkamp17 and has become established as a technique for the analysis and accurate molecular weight determination of large macromolecules such as proteins, polysaccharides, nucleic acids, and synthetic polymers with high mass accuracy, extreme sensitivity, and an almost unlimited mass range and speed of analysis. MALDI is a “soft” ionization process that produces minimum fragmentation. Currently, MALDI-TOF-MS has been demonstrated as an important technique for analysis of a variety of (13) Ganachaud, F.; Monteiro, M. J.; Gilbert, R. G.; Dourges, M.-A.; Thang, S. H.; Rizzardo, E. Macromolecules 2000, 33, 6738. (14) (a) Barner-Kowollik, C.; Coote, M. L.; Davis, T. P.; Radom, L.; Vana, P. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2828. (b) Wang, A. R.; Zhu, S.; Kwak, Y.; Goto, A.; Fukuda, T.; Monteiro, M. S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2833. (15) (a) Barner-Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1353. (b) Feldermann, A.; Coote, M. L.; Stenzel, M. H.; Davis, T. P.; Barner-Kowollik, C. J. Am. Chem. Soc. 2004, 126, 15915. (16) (a) Monteiro, M. J.; de Brouwer, H. Macromolecules 2001, 34, 349. (b) de Brouwer, H. D.; Schellekens, M. A. J.; Klumperman, B.; Monteiro, M. J.; German, A. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3596. (17) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299.
synthetic polymers because it provides valuable structural information such as absolute molecular weight, repeat unit, and end group of synthetic polymers. This information, especially the endgroup information, affords deeper insight into polymers structures, reaction mechanisms, and side reactions.18-20 In this paper, to further understand the RAFT polymerization mechanism, we thus employed the MALDI-TOF-MS technique and analyzed in detail the end-group structures of several polystyrene (PS) polymers prepared by two different CTA-initiator systems, CTA1 versus TPY-AIBN, and CTA2 versus AIBN (Figure 1). EXPERIMENTAL SECTION Chemicals. 2,2′-Azobis(isobutyronitrile) (AIBN, 97%, Aldrich) was purified by recrystallization from methanol and stored in a freezer. Styrene (99%, Aldrich) was purified by washing with an aqueous solution of NaOH (5 wt %) to remove inhibitor, followed by distilled water until the washings were neutral to litmus and fractionally distilled under vacuum. CTA1 was synthesized according to the procedure described previously.6b The chaintransfer agent CTA221 and the initiator TPY-AIBN22 were synthesized according to the procedures that will be reported elsewhere. Instrumentation. 1H and 13C NMR spectra were recorded on a Bruker ARX400 spectrometer at 400 Hz and 100 MHz, respectively. Size exclusion chromatography (SEC) was carried out on a Viscotek SEC assembly consisting of a model P1000 pump, a model T60 dual detector, a model LR40 laser refractometer, and three mixed-bed columns (pore size,10 µm; the molecular weight range for those columns is 1000-5 000 000) from America Polymer Standards Corp. using THF as an eluent with a flow rate of 1.0 mL min-1 at ambient temperature. Polymer solutions for SEC experiments were prepared in concentrations of ∼3.0 mg/ mL. The SEC system was calibrated using a narrow polystyrene standard (Mn ) 2500, Mw ) 2630, Mp ) 2560, Mw/Mn )1.052). As routine analysis, MALDI-TOF-MS was performed on Applied Biosystems 4700 proteomics analyzer using 1,8-dihydroxy-9(10H)anthracenone (dithranol), R-cyano-4-hydroxycinnamic acid, or 2-(4-hydroxyphenylazo)benzoic acid as a matrix in the absence of cationic agents in the Mass Spectrometry Laboratory of Georgia Institute of Technology, Atlanta. Preparation of Polystyrene Samples via RAFT Polymerization. A typical procedure for bulk polymerization of styrene was as follows. Styrene (2.660 g, 25.54 mmol), TPY-AIBN (22.0 mg, 2.38 × 10-2 mmol), and CTA1 (56.0 mg, 1.18 × 10-1 mmol) were mixed to give a clear solution. The solution was transferred into an ampule and degassed through five freeze-thaw-evacuate cycles, sealed under vacuum, and heated at 65 °C for 6 h. The polymerization mixture was poured into a large excess of methanol (18) (a) Gies, A. P.; Nonidez, W. K.; Ellison, S. T. Anal. Chem. 2005, 77, 780. (b) Gies, A. P.; Nonidez, W. K.; Anthamatten, M.; Cook, R. C. Macromolecules 2004, 37, 5923. (19) (a) Chen, H.; He, M.; He, H. Anal. Chem. 2003, 75, 6531. (b) Chen, H.; He, M.; Pei, J.; Liu, B. Anal. Chem. 2002, 74, 6252. (20) (a) Williams, J. B.; Chapman, T. M.; Hercules, D. M. Anal. Chem. 2003, 75, 3092. (b) Trimpin, S.; Grimsdale, A. C.; Ra¨der, H. J.; Mu ¨ llen, K. Anal. Chem. 2002, 74, 3777. (c) Guttman, C. M.; Wetzel, S. J.; Blair, W. R.; Fanconi, B. M.; Girard, J. E.; Goldschmidt, R. J.; Wallace, W. E.; Vanderhart, D. L. Anal. Chem. 2001, 73, 1252. (21) Zhou, G.; He, J.; Harruna, I. I. J. Polym. Sci., Part A: Polym. Chem., submitted for publication. (22) Harruna, I. I., Zhou, G. Unpublished report.
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Figure 1. Structures of the RAFT agents and initiators used for RAFT polymerization of styrene. Table 1. Experimental Conditions and Characterization Data for RAFT Polymerization of Styrene with Two Different CTA-Initiator Systems sample
styrene (M)
CTA (M × 10-2)
initiator (M × 10-3)
temp (°C)
time (h)
conv (%)
Mna (theory)
Mnb (NMR)
Mnc (MALDI)
Mnd (SEC)
Mw/Mnd (SEC)
PS-1 PS-2 PS-3 PS-4 PS-5
8.73 8.73 8.73 8.73 8.73
CTA1 (4.02) CTA1 (4.02) CTA2 (4.03) CTA2 (4.03) CTA2 (4.03)
TPY-AIBN (8.15) TPY-AIBN (8.15) AIBN (8.32) AIBN (8.32) AIBN (8.32)
65 65 65 60 65
6 5 5 4 6
6.92 5.47 4.51 3.19 9.14
2040 1720 1340 1040 2390
2230 1820 1470 1100 2520
2560 2040 1320 1050 2460
2460 1930 1520 1110 2870
1.25 1.10 1.16 1.26 1.13
a The theoretical molecular weight was calculated from the following expression: M n(theory) ) ([M]i/[CTA]i)CM0 + MCTA, where [M]i and [CTA]i are the initial concentrations of the monomer and the chain-transfer agent CTA, respectively, C is the fractional conversion, and M0, MCTA are the b molecular weights of the monomer and the used RAFT agent, respectively. Determined by 1H NMR spectroscopy. c Determined by MALDITOF-MS. d Determined by SEC using THF as eluent, and molecular weights were reported as polystyrene equivalents (Mn ) 2500, Mw ) 2630, Mp ) 2560, Mw/Mn )1.052).
to precipitate the resulting polymer. The polymer was purified by reprecipitation from 1,4-dioxane into a large excess of methanol and then dried under vacuum at room temperature to yield 0.240 g of a pink polymer PS-1. The conversion of the monomer styrene was determined to be 6.92% by gravimetrical method. The detailed experimental conditions and characterization data for all samples are given in Table 1. RESULTS AND DISCUSSION Five polystyrene polymers were prepared by RAFT polymerization of styrene using two different CTA-initiator systems, CTA1 versus TPY-AIBN and CTA2 versus AIBN. The molecular weight and polydispersity index of these RAFT-prepared polystyrene samples were determined by SEC and are summarized in Table 1. As shown in Figure 2 and Table 1, the molecular weight distributions of all samples are normal and narrow. The two representative RAFT-prepared polystyrene polymers, PS-1 and PS-3, were characterized by both 1H and 13C NMR techniques. Their 1H NMR spectra (Figures S1A and S2A in Supporting Information) both show the appearance of signals located at 4.60-5.20 and 7.84 ppm attributed to the proton of the methine group adjacent to the sulfur and aromatic protons of the active end group (Ph-C(dS)-S-) from the RAFT agent, CTA1 and CTA2, respectively. The existence of the active Ph-C(dS)S- group at one end of the polymer chain was further confirmed by the appearance of two signals located at 52.40-54.00 and 226.04 ppm assigned to the backbone CH carbon next to sulfur and the CdS carbon, respectively, in their 13C NMR spectra (Figures S1B and S2B). Terpyridine and bipyridine functional end groups 2724 Analytical Chemistry, Vol. 79, No. 7, April 1, 2007
Figure 2. SEC chromatograms (RI traces) of these polystyrene polymers prepared by bulk RAFT polymerization of styrene using two different CTA-initiator systems (for PS-1 and PS-2, CTA1 versus TPY-AIBN; for PS-3, PS-4, and PS-5, CTA2 versus AIBN).
derived from RAFT agents, CTA1 and CTA2, were also present in their corresponding polystyrene polymers as observed by their characteristic peaks in 1H and 13C NMR spectra. For comparison,
Figure 3. Expanded MALDI-TOF-MS spectra of the RAFT-prepared polystyrenes PS-1 (a), PS-2 (b), PS-3 (c), and PS-4 (d) and the theoretical monoisotopic distribution of each actual cluster (e for PS-l and PS-2, f for PS-3 and PS-4; regarding sample PS-5, see Figure S5 in Supporting Information).
Scheme 2. Possible Side Reaction during MALDI Analysis Process
1H
NMR and monomer conversion were also used to calculate their molecular weights; all calculation data are given in Table 1. As can be seen from Table 1, their 1H NMR-determined molecular weights are in good agreement with theoretical values. These RAFT-prepared polystyrene polymers were further analyzed by the MALDI-TOF-MS technique. Their expanded MALDI-TOF-MS spectra are shown in Figure 3 as well as Figures S3-S8 (Supporting Information). Several series peaks are observed, all corresponding to a molar mass distribution of polystyrene with 104.15 u between the peaks. For a given series and a given degree of polymerization, the multiplicity of the peak corresponds to the isotopic distribution, which is a function of the various atoms existing in the structure, including the proton. However, some unexpected polymer chain end groups were produced during the course of the MALDI-TOF-MS analysis. For example, the Ph-C(dS)-S-terminated polystyrene chains were promptly fragmented to produce vinyl end groups (eq 1 in Scheme
2). Since evidence of unsaturation is not seen in the 1H NMR data (Figures S1A and S2A), it is therefore proposed that this species is produced within the mass spectrometer. The loss of HS-C(dS)-Ph in such a manner is a known fragmentation route in MS due to the weakness of the C-S bond between the polymer chain and dithiobenzoate chain end.23 Similarly, the loss of terminal halide of the polymers generated by atom-transfer radical polymerization, which results in unsaturated chain ends, had been observed during the MALDI-TOF-MS analysis process.24 Only a careful selection of the matrix and counterion system as well as instrumental operating conditions can sometimes avoid such a fragmentation during the ionization process. (23) (a) Jiang, X.; Schonenmakers, P. J.; van Dongen, J. L. J.; Lou, X.; Lima, V.; Brokken-Zijp, J. Anal. Chem. 2003, 75, 5517. (b) Beyou, E.; Chaumont, P.; Chauvin, F.; Devaux, C.; Zydowicz, N. Macromolecules 1998, 31, 6828. (24) Barner-Kowollik, C.; Davis, T. P.; Stenzel, M. H. Polymer 2004, 45, 7791.
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Table 2. Assignments of Peaks in the MALDI-TOF-MS Spectra (Figure 3a-d) of the RAFT-Prepared Polystyrenes PS-1, PS-2, PS-3, and PS-4 Obtained in the Absence of Cationic Agentsa
a Calculated molar mass of the monoisotopic peak (first peak of the isotopic distribution) for the various possible structures and comparison with the experimental values (expt11 values are from samples PS-1 and PS-3, exptl2 are from samples PS-2 and PS-4; NF, not found).
In addition to the occurrence of fragmentation, the terpyridine and dithiobenzoate end groups were also oxidized during the MALDI process (eqs 2 and 3 in Scheme 2). Such an oxidization transformation has also been observed by various research groups.10b,25 Considering that the three types of side reactions originated from MALDI itself, we elucidated the four MALDI-TOFMS spectra shown in Figure 3a-d and summarized the results in Table 2. (25) Venkatesh, R.; Staal, B. B. P.; Klumperman, B.; Monteiro, M. J. Macromolecules 2004, 37, 7906.
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Based on the MALDI-TOF-MS analysis and 1H NMR data, regardless of which CTA-initiator system was used, the series peaks A1-A5, B, and A1′-A3′, B′ from the polystyrenes with the structures 5 and 3 shown in Scheme 1 always appeared. According to the relative intensities of these series peaks in the MALDI-TOF-MS spectrum, it was concluded that the polystyrene species with the structures 5 and 3 were the main components of the polystyrene product prepared by RAFT. In other words, the thiocarbonylthio groups terminated most of the polymer chains. More importantly, the weak series peak C1, C2, C1′, and
C2′ was assigned to the polystyrene with the structures 2 or 4 in Scheme 1, which were formed through the termination of the intermediate RAFT radicals by accepting a hydrogen atom. This provides powerful evidence for the existence of the intermediate RAFT radicals 2 and 4 that play key roles in RAFT polymerization. On the basis of their corresponding signal intensity in the MALDITOF-MS spectrum, the concentration of the intermediate RAFT radicals is in fact quite low during the overall polymerization. Interestingly, the series peaks D1, D2, D1′, and D2′ were assigned to the polystyrene with the structure 6 in Scheme 1, which originated from the self-combination termination of the intermediate RAFT radicals 2 and 4. The series peaks E1, E2, E1′, and E2′ were assigned to the polystyrene with the structure 7 in Scheme 1, which were formed by cross-termination between the intermediate and propagating radicals. Possibly, the process of crosstermination of the intermediate RAFT radicals is reversible and also the main cause for the rate retardation phenomena. The series peaks F and F′ were attributed to the dead polymer that was formed by the combination termination between the macromolecular radical Pm• and primary radical I• or Pn• and R•. The amount of the dead polymers is also quite low as shown by the weak signal intensity of the two series peaks F and F′ in MALDI-TOF-MS spectrum. Therefore, the narrow molecular weight distribution of the polymers prepared by RAFT is not affected. Moreover, in all cases, the experimental isotopic distribution matched well with the theoretical distribution (Figure 3). This further confirmed the
structural assignment of all series peaks present in the MALDITOF-MS spectra. CONCLUSIONS The results described herein established the identities of all possible structures that can arise from the RAFT mechanism, especially the intermediate RAFT radicals and their crosstermination adducts. This confirms the operation of the Rizzardo mechanism including the Monteiro intermediate radical termination model for the RAFT process and provides direct evidence as to the cause of the rate retardation during RAFT process. ACKNOWLEDGMENT The financial support for this research work from the Office of Naval Research (ONR Grant N00014-01-1-1042) and the Army Research Office (ARO Grant W911NF-04-0369) is gratefully acknowledged. We thank Dr. Airan Perez of the Office of Naval Research, Arlington, VA, for helpful discussions. Also, we thank Dr. David Bostwick, Georgia Institute of Technology, for MALDITOF-MS analysis. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review January 22, 2007.
October
11,
2006.
Accepted
AC061930P
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