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ESR Study and Radical Observation in Transition Metal-Mediated Polymerization: Unified View of Atom Transfer Radical Polymerization Mechanism 1

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Aileen R. Wang , Shiping Zhu , and Krzysztof Matyjaszewski 1

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Department of Chemical Engineering, McMaster University, 1280 M a i n Street West, Hamilton, Ontario, L8S 4L7, Canada Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, P A 15213

Using an on-line electron spin resonance (ESR) technique, we studied the electron paramagnetic species of oxidized metal centers and carbon radicals in three representative catalyst/ligand systems of transition metal-mediated polymerization of methyl methacrylate. These systems were: (1) CuBr/Bpy, (2) RuCl (PPh )/Al(OiPr) , and (3) CuBr/N-pentyl-2-pyridylmethanimine. Ethylene glycol dimethacrylate was used as crosslinker to trigger the diffusion-controlled radical deactivation so that the radical population was accumulated to an ESR detectable level. Methacrylate radicals were observed in all the systems. Their signal hyperfine structures were typical of nine line and were identical to those observed in conventional free radical polymerization processes. There were also peculiar signals observed at low dimethacrylate levels in (2) and (3). These paramagnetic species were present without the monomer addition. The nature of the propagating center type was analyzed. This work provided a unified view for the radical mechanisms of atom transfer radical polymerization. 2

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© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction A polymerization system is considered to be living as long as there is no permanent radical termination or transfer reaction (7,2). Living polymerization provides good control over chain structure and functionality. Among the developed living processes, controlled radical polymerization enjoys further advantages including versatility of monomer type, tolerance of water and protonic impurities, and mild reaction conditions. Due to the active nature of radicals, living radical process is achieved by frequently and temporarily capping propagating radical centers so that equilibrium between the dormant and active species is established. The capping molecules can be transition metal halide (atom-transfer radical polymerization or A T R P ) (3,4), nitroxide (stable free radical polymerization or SFRP) (J), or dithioester (reversible additionfragmentation transfer polymerization or R A F T ) (6). One of the key requirements for a process to be living is the high rate of radical capping (deactivation) that prevents the centers from experiencing permanent termination. The transition metal-mediated atom transfer radical polymerization has attracted enormous attention since its discovery due to potential industrial applications in synthesizing well-defined polymer chains (see the recent review papers (7,8)). However, the mechanistic understanding of the process is still lacking and requires more effort. A T R P is considered to be an extension of atom-transfer radical addition ( A T R A ) (9,10). There are numerous evidences in supporting the proposed radical mechanism (11). However, there are also some peculiar experimental observations that lead to other suggestions (12). A n important aspect of the mechanistic study is the direct observation of radical intermediates during polymerization. For this purpose, Kajiwara et al. (13) carried out an E S R study on A T R P of styrene mediated by CuBr/4,4'-di(5nonyl)-2,2'-bipyridine (dNbpy) initiated by 1-phenylethyl bromide. However, only Cu(II) signals were observed. The reason for this lack of radical detection is the low radical concentration due to the high radical deactivation rate in: PBr + Cu(I)Br/L P* + Cu(II)Br /L where Ρ = polymer chain, L = ligand; and · = radical center. Normally, a minimal level of radical concentration >10" mol/1 is needed to be detected with a recognizable signal/noise ratio. Recently, we used an on-line E S R technique to investigate the A T R P of poly(ethylene glycol) dimethacrylate mediated by CuBr/1,1,4,7,10,10hexamethyltriethylenetetramine (HMTETA) initiated by methyl abromophenylacetate (MBP). For the first time, we observed the radical intermediates involved in an A T R P process (14). The E S R signal appeared to have a typical 9-line hyperfine structure arising from methacrylate radicals that experience a glass/solid environment in the polymer matrix (15,16). The success 2

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In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

163 was due to network formation in the system that imposed diffusion limitations to the CuBr/L complex. The diffusion-controlled radical deactivation favored radical generation. Also because of the network structure, these trapped radicals had reduced mobility, and thus avoided bimolecular termination. In a more recent paper (17), we measured the radical and Cu(II) concentration profiles in the A T R P of methyl methacrylate (MMA)/ethylene glycol dimethacrylate ( E G D M A ) with the M B P / C u B r / H M T E T A system. The work provided a quantitative explanation for the change of the equilibrium constant K = k /k during the polymerization. The polymerization process showed three definable stages. In the first stage, the Cu(II) concentration increased continuously but slowly. The methacrylate radical signal was not detectable because its concentration was below the sensitivity of the E S R machine. In the second stage, the Cu(II) concentration increased dramatically. The methacrylate radical signal started to appear and increased synchronously with the Cu(II) concentration. This autoacceleration was because the radical deactivation became diffusion-controlled. In the third stage, the Cu(II) and radical concentrations increased gradually and reached a steady state due to radical trapping in the network. As a part of continuous effort in elucidating the A T R P radical mechanisms, in this work, we carried out E S R studies on different catalyst/ligand systems for M M A / E G D M A polymerization. Three representative systems were selected. System 1 was initiated by ethyl 2-bromoisobutyrate (EBIB) mediated by CuBr/2,2'-bipyridine (3). System 2 was initiated by CC1 mediated by dichlorotris(triphenylphosphine) ruthenium (RuCl (PPh ) ) activated by aluminum isopropoxide (Al(OiPr) ) (4). System 3 was initiated by E B I B mediated by CiiBr/AT-pentyl-2-pyridylmethanimine (PPMI) (18). For comparison, we also included the M B P / C u B r / H M T E T A system. The objective of this work was to provide a unified view about the radical mechanisms for these different systems.

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Experimental Materials. Methyl methacrylate ( M M A ) , ethylene glycol dimethacrylate ( E G D M A ) , carbon tetrachloride (CC1 ) from Aldrich were distilled over C a H and stored at 0 °C prior to use. Ethyl 2-bromoisobutyrate (EBIB, 98%), methyl cc-bromophenylacetate ( M B P , 97%), copper bromide (CuBr, 98%), 2,2'bipyridine (Bpy, 99%), 1,1,4,7,10,10-hexamethyl-triethylenetetramine ( H M T E T A , 97%), dichlorotris(trisphenylphosphine) ruthenium (RuCl (PPh ) , 97%), aluminum isopropoxide (Al(OiPr ) , 98%), 2-pyridinecarboxaldehyde (99%), amylamine (99%), magnesium sulphate (MgS0 ) were all purchased from Aldrich and used as received, A^pentyl-2-pyridylmethanimine (PPMI) was 4

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In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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164 synthesized following the procedure reported in the literature (18). Scheme 1 shows the molecular structures for some of the chemicals used in the work. MMA

CH, CH,



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Magnetic Field (G)

Figure 6. ESR spectra observed in I0%EGDMA EBIB/CuBr/PPMI sample (a) after stored at room temperature for two months, (b) exposed the sample to air for two hours at room temperature, and (c) for another 15 min at 90 °C. ESR operation parameters are the same as in Figure 4.

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

178 imposed on the species by the network slowed down their mobility, and thus increased the radical concentration to a level detectable by ESR. If the complex molecules never left the radical centers, they would not be caught by the network. Another basic question is whether there existed some type of intermediate species such as ρ·"Χ»Μΐ Χ /Ι. or Ρ·-Μί ΧΑ (η+1)

( η + 1 )

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If these intermediates existed, were they responsible for monomer propagation? Both structures would require equimolar concentrations of P* and Cu(II) species. However, all the reported here and earlier E S R data indicate much higher concentration of M t than P*. Even i f a stable radical intermediates were present and capable of monomer insertion (this hypothesis needs to be proven), the species coexisted with the free methacrylate radicals. The A T R P propagating centers have reactivities typical for free radicals.f/Pj A T R P systems are more sensitive to solvents and/or other conditions than their conventional counterparts, because of possible reactions and interactions with catalyst/ligand molecules that alter their ability to mediate. 11

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( n + 1 )

Conclusions We carried out the E S R measurements for the A T R P of M M A / E G D M A mediated by three catalyst systems, i.e., System 1 initiated by ethyl 2bromoisobutyrate (EBIB) mediated by CuBr/2,2'-bipyridine, System 2 initiated by C C U mediated by dichlorotris(triphenylphosphine) ruthenium activated by aluminum isopropoxide, and System 3 initiated by EBIB mediated by CuBr/iVpentyl-2-pyridylmethanimine. The methacrylate radical concentration in pure M M A was lower than the limit of E S R sensitivity. The dimethacrylate was added for the purpose of network formation that triggered diffusion limitations to chain radicals as well as catalyst/ligand complex species. The fast reactions such as bimolecular radical termination and radical deactivation thus became diffusion controlled. The diffusion-controlled radical deactivation moved the equilibrium P X + Mt XJL ?' + M t ^ ' ^ i / L to the right-hand-side and consequently increased the radical concentration. Methacrylate radicals were observed in all the systems upon the network formation. Their signals showed the typical 9-line hyperfine structure that was identical to those observed in conventional free radical polymerization processes. We also observed peculiar signals in System 2 and System 3, but ruled out the possibility of these species being propagating centers, because these signals were also present without the monomer addition. The methacrylate radical appeared to be the center for monomer propagation. This work supports the radical mechanisms in the A T R P systems. {n)

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

179 Acknowledgements. We would like to thank the Natural Sciences and Engineering (NSERC) of Canada for financial support of this research, and the Ministry of Education, Science, and Technology (MEST) for P R E A award. References 1

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In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.