Quenching of Metal-Catalyzed Living Radical Polymerization with Silyl Enol Ethers

Transcription

1 Quenching of Metal-Catalyzed Living Radical Polymerization with Silyl Enol Ethers KAZUKI TOKUCHI, TSUYOSHI ANDO, MASAMI KAMIGAITO, MITSUO SAWAMOTO Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto , Japan Received 24 March 2000; accepted 20 April 2000 ABSTRACT: Various silyl enol ethers were employed as quenchers for the living radical polymerization of methyl methacrylate with the ROCl/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 initiating system. The most effective quencher was asilyl enol ether with an electrondonating phenyl group conjugated with its double bond [CH 2 AC(OSiMe 3 )(4-MeOPh) (2a)] that afforded a halogen-free polymer with a ketone terminal at a high end functionality[f n 1].Suchsilylcompoundsreactedwiththegrowingradicalgenerated from the dormant chloride terminal and the ruthenium complex to give the ketone terminal via the release of the silyl group along with the chlorine that originated from the dormant terminal. In contrast, less conjugated silyl enol ethers such as CH 2 AC(OSiMe 3 )Mewerelesseffectiveinquenchingthepolymerization.Thereactivity of the silyl compounds to the poly(methyl methacrylate) radical can be explained by the reactivity of their double bonds, namely, the monomer reactivity ratios of their model vinyl monomers without the silyloxyl groups. The lifetime of the living polymer terminal was also estimated by the quenching reaction mediated with 2a John Wiley &Sons, Inc. JPolym Sci A: Polym Chem 38: , 2000 Keywords: living polymerization; radical polymerization; methyl methacrylate; ruthenium complex; silyl enol ether; quencher; end-functionalized polymer INTRODUCTION Correspondence to: M. Sawamoto ( sawamoto@star. polym.kyoto-u.ac.jp) JournalofPolymerScience:PartA:PolymerChemistry,Vol.38, (2000) 2000 John Wiley &Sons, Inc. Living polymerization can generally control not only the molecular weight and its distribution but also the terminal groups or end functionality of the produced polymers because the process consists of only initiation and propagation without side reactions such as irreversible termination and chain-transfer reactions. 1 Inparticular, the growing terminal ( end) keeps its activity until the polymerization is intentionally terminated. Thus, the quenchers that can selectively and effectively react with the growing terminal will afford controlled -end structures. The recent developments in living radical polymerization are based on the concept that the active growing radical species is reversibly converted into thecovalentdormantspeciesandiskeptmostprobably at low concentrations to suppress side reactions. 2 Themostextensivelystudiedandmosteffective methods among them are transition-metal-catalyzed,nitroxide-mediated,andreversibleadditionfragmentation chain transfer polymerizations, which possess COhalogen, COON, and COSC(S) bonds as the dormant species, respectively. In metal-catalyzed living radical polymerization, whichwehavebeendeveloping,adormantcarbon halogen bond is reversibly activated into agrowing radical species via oxidation of the metal center: 2(e j),

2 4736 TOKUCHI ET AL. (1) The radical species thus generated reacts with monomer to propagate and sooner or later is converted into the dormant carbon halogen species. This equilibrium most probably shifts to the dormant species; that is, a majority of the polymer terminals exist as the stable COhalogen species. Because of the stable nature of the covalent bond, the recovered polymers possess the carbon halogen bond intact at the end. For the same reason, there have been no efficient quenching reactions that can convert the stable but potentially active covalent carbon halogen bonds into more stable bonds, such as COC bonds. The existence of such potentially active bonds may affect the stability of the products, especially at high temperatures. 8 In addition, effective quenching reactions for living radical polymerizations will lead to the synthesis of not only end-functionalized polymers at the end but also telechelic polymers by combining the functionalized initiator method that has become more popular in living radical polymerization. 2(e j),9 21 Recently, we reported that a silyl enol ether with a phenyl group effectively quenches the ruthenium-catalyzed living radical polymerization of methyl methacrylate (MMA) and completely converts the COCl terminal into a COC bond with a ketone group. 22 To our knowledge, this is the first example of in situ and quantitative radical quenching reactions that transform potentially active dormant bonds into inactive ones. From the viewpoint of end functionalization via quenching, there have been quite a few examples using ionic or radical transforming reactions of COhalogen terminals in similar living/controlled radical polymerizations with copper(i) catalysts. 20,21,23 29 More recently, allyltri-n-butylstannane and allyl alcohol proved effective in quenching the Cu(I)-mediated polymerization. 30 Interesting applications of the quenching with silyl enol ethers have been reported recently. 31 As shown in Scheme 1, our silyl-compoundmediated quenching reaction probably proceeds via the addition of the growing radical into the CAC double bond of the quencher, followed by the elimination of the silyl group and the chlorine that originated from the polymer terminal. 22,32 However, it is still unknown which silyl compounds are effective and how the reaction proceeds. In this study, we investigated the quenching of the RuCl 2 (PPh 3 ) 3 -mediated living radical polymerization in more detail by using a series of silyl enol ethers (2a 2f) and related vinyl compounds (2g and 2h) under various reaction conditions. RESULTS AND DISCUSSION Survey of Quenchers: Quenching with Silyl Enol Ethers and Related Compounds We first investigated a series of p-substituted -(trimethylsilyloxy)styrenes [2 in Scheme 1; X OMe (2a), H (2b), F (2c), or Cl (2d)] for quenching the RuCl 2 (PPh 3 ) 3 -mediated living radical polymerization of MMA with dimethyl 2-chloro- 2,4,4-trimethylglutarate [(MMA) 2 OCl; 1a; initiator] and RuCl 2 (PPh 3 ) 3 in the presence of Al(Oi Pr) 3 in toluene at 80 C. When monomer conversion reached about 60% for 24 h, silyl enol ethers (2a 2d), 5 equiv to 1, were added to the reaction mixture. As summarized in Table I, the addition of these silyl compounds substantially quenched the polymerization, where conversion increased just a few percentage points over an additional 15 h and 74% of the MMA was polymerized in the

3 METAL-CATALYZED LIVING RADICAL POLYMERIZATION 4737 Scheme 1 quencher-free system. The polymers showed unimodal and narrow molecular weight distributions (MWDs), similar to those obtained before the quencher addition (Fig. 1). Their molecular weights were unchanged as well. This shows nearly complete quenching of the polymerization with these silyl compounds. Figure 2 shows the 1 H NMR spectra of the polymers thus obtained. In addition to the large absorptions of poly(methyl methacrylate) (PMMA) repeat units, the polymers exhibited characteristic signals of the -end groups terminated with the silyl compounds, that is, the aromatic protons (e g) at 7 8 ppm and the methylene groups (d) adjacent to the aromatic carbonyl groups at ppm. The terminal ketone functionality (F n) was determined by a comparison of the number-average degrees of polymerization by 1 H NMR [DP n (NMR) (e f g)/c] and size exclusion chromatography [DP n (SEC)] based on PMMA calibration. All Table I. Quenching with -Trimethylsilyloxy Styrenes (2a 2d) a b Entry Q [Q]/[I] Time (h) Conversion (%) M n b M w/m n c F n d 2 2a b c d a b c d a [MMA] 0 /[I] 0 /[RuCl 2 (PPh 3 ) 3 ] 0 /[Al(Oi Pr) 3 ] /20/20/40 mm; toluene; 80 C. b Determined by SEC. c DP n (SEC)/DP n (NMR). d F n of the chlorine terminal.

4 4738 TOKUCHI ET AL. Figure 1. SEC curves of PMMA quenched with 2a 2d in the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [quencher]/[ end] 5.0. these polymers (entries 2 5) exhibited the functionalities nearly equal to unity (F n 1.0) regardless of the substituents. Thus, the -silyloxystyrenes proved to be effective in quenching the RuCl 2 (PPh 3 ) 3 -mediated MMA polymerization to generate ketone terminals via the elimination of the silyl groups accompanying the chlorine at the polymer terminal. However, in the presence of a smaller amount of the silyl compounds (i.e., [Q]/[ end] 1.7), the quenching efficiency depended on the substituents in the silyloxystyrenes. For example, the p- methoxy derivative (2a) gave quantitative termination even at a lower concentration (entry 6). In contrast, F n for the p-chloro quencher (2d) was clearly smaller (F n 0.65; entry 9). Apparently, the functionality depended on the electronic nature of the substituents, and an electron-donating substituent on the phenyl group facilitated the quenching reaction. This was most likely due to the electron-rich double bond, with which the electron-deficient PMMA growing radical species easily reacted. Figure 3 shows a more detailed analysis of the effects of the quencher concentrations on the Figure 2. 1 H NMR spectra of PMMA quenched with 2a 2d in the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [quencher]/ [ end] 5.0.

5 METAL-CATALYZED LIVING RADICAL POLYMERIZATION 4739 Figure 3. 1 H NMR spectra of PMMA quenched with 2b at different concentrations and F n of the ketone terminal plotted against concentrations of 2a and 2b in the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [quencher]/[ end] functionality with 2a and 2b. The electron-rich silyl enol ether (2a; open circles) quenched the polymerization more effectively than 2b (filled circles), even at lower [Q]/[I] ratios. With 2a equimolar to that of the living end, however, the functionality became lower than unity. Therefore, the quenchers should be added in excess over the living end for complete capping reactions to occur. We further examined two silyl enol ethers [isopropenoxytrimethylsilane (2e) and 1-(trimethylsilyloxy)cyclopentene (2f)] along with 1,1-diphenylethylene (2g) and -methylstyrene (2h). Similar to 2a 2d, the former two possessed a double bond attached to the trimethylsilyloxy group (leaving group) but without a phenyl group that could conjugate with the double bond. The latter two had phenyl groups for conjugation but no silyloxyl groups, and they did not undergo radical homopolymerization at high temperatures because of steric hindrance. In contrast to the results for 2a 2d, the polymerization of MMA proceeded even with the addition of 2e and 2f to give polymers with slightly higher molecular weights than those before quenching and with unimodal narrow MWDs (entries 2 and 3 in Table II and Fig. 4). NMR endgroup analysis, however, revealed that these silyl compounds were not attached to the polymer terminal (or incorporated into the polymer chain). Thus, the nonconjugated silyl enol ethers 2e and 2f did not quench the polymerization or even react (or polymerize) with the PMMA growing end. Table II. Quenching with Various Olefins (2e 2g) a b Entry Q [Q]/[I] Time (h) Conversion (%) M n b M w/m n c F n d 2 2e f g h a [MMA] 0 /[I] 0 /[RuCl 2 (PPh 3 ) 3 ] 0 /[Al(Oi Pr) 3 ] /20/20/40 mm; toluene; 80 C. b Determined by SEC. c DP n (SEC)/DP n (NMR). d F n of the chlorine terminal.

6 4740 TOKUCHI ET AL. Figure 4. SEC curves of PMMA quenched with 2e 2h in the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [quencher]/[ end] 5.0. These compounds were regarded as substituted propylene and cyclopentene, respectively, which are difficult to copolymerize with MMA. However, the effective quenchers, 2a 2d, were derivatives of styrene that the PMMA growing radical favored over MMA, as anticipated from the monomer reactivity ratios. p-methoxystyrene, a parent compound of the most effective quencher, 2a, is especially known to show a more alternating tendency in radical polymerization with MMA (r , r ; M 1 MMA) than styrene, a parent of 2b (r 1 0.5, r 2 0.5). 33 This may be the reason for the higher activity of 2a in the Rucatalyzed living radical polymerization of MMA. Once the PMMA end reacted with the double bond of these silyl enol ethers, the resulting radical species were converted into the ketone terminal through the elimination of the silyl group. Thus, the quencher should have a double bond that can easily react with the PMMA propagating radical species. In contrast, styrene derivatives 2g and 2h terminated the polymerization (entries 4 and 5 in Table II). However, 1 H NMR analysis of the polymers with 2g showed no absorptions of the phenyl groups that originated from 2g, whereas the chlorine terminal remained intact. This shows that 2g just retarded or virtually quenched the polymerization, most likely because of the deactivation of the complex via some interaction. Although 2g may react with the PMMA growing radical, it may be less possible from the monomer reactivity ratios (r 1 2 8, r 2 0; M 1 MMA). 33 However, the polymers treated with 2h possessed the styrene unit at the polymer terminal and also in the polymer chain. The F n value (entry 5 in Table II) is thus the sum of the number of these units. In conventional free-radical polymerization, the PMMA radical growing end favors 2h over MMA (r , r ; M 1 MMA), 33 which is probably the case for Ru-catalyzed polymerization. Under the conditions where a portion of the MMA still remained unpolymerized, the 2hended growing species could react with the remaining MMA, resulting in copolymerization before the release of the terminal hydrogen from the -methyl group. This indicates that the quencher should have effective leaving groups that can abstract the chlorine from the polymer terminal. Thus, the -silyloxystyrene-type quenchers proved to be effective because they possessed not only the reactive double bonds conjugated with the phenyl group but also silyloxyl groups that could release the silyl group along with the terminal chlorine because of the high affinity between Si and Cl. We also employed 2a and 2e for the polymerization of MMA with other transition-metal-based systems such as 1a/FeCl 2 (PPh 3 ) 2 34 and (MMA) 2 O Br(1b)/NiBr 2 (Pn Bu 3 ) 2 /Al(Oi Pr) 3 35 in toluene at 80 C. The silyl compounds, 10 equiv to the end, were thus added into the polymerization mixtures where conversion reached about 50%. The reaction conditions, such as the concentrations of metal complexes and additives, were chosen as the best conditions reported so far for each polymerization. As shown in Table III, 2a was effective in quenching the polymerizations and was superior to 2e, although F n values for 2a were slightly lower than unity. Such differences may be due to the catalytic activity for the quenching reaction, to the concentrations of radical species, or to the nature of the radical species thus formed. The lower efficiency with the nickel-based system was also ascribed to the difference in the halogens at the polymer terminal, where bromine was employed for the initiator as well as the complex. We also found that F n was around 0.7 for 2a with the (EMA)OBr/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3

7 METAL-CATALYZED LIVING RADICAL POLYMERIZATION 4741 Table III. Quenching of Other Metal-Mediated Living Radical Polymerization with 2a and 2e a Entry Metal Complex Q [Q]/[I] Time (h) Conversion (%) M n a M w/m n b F n 1 c FeCl 2 (PPh 3 ) c FeCl 2 (PPh 3 ) 2 2a c FeCl 2 (PPh 3 ) 2 2e d NiBr 2 (Pn Bu 3 ) e 5 d NiBr 2 (Pn Bu 3 ) 2 2a d NiBr 2 (Pn Bu 3 ) 2 2e a Determined by SEC. b DP n (SEC)/DP n (NMR). c [MMA] 0 /[1a] 0 /[FeCl 2 (PPh 3 ) 2 ] /20/10 mm; toluene; 80 C. d [MMA] 0 /[1b] 0 /[NiBr 2 (Pn Bu 3 ) 2 ] 0 /[Al(Oi Pr) 3 ] /20/80/40 mm; toluene; 80 C. e The F n could not be obtained because of difficulty in peak separation. system, where the halogen in the initiator was not chlorine but bromine. Quenching Reaction Mechanism This part is directed to the analysis of the quenching reaction with the most effective quencher, 2a, in the Ru(II)-mediated polymerization. Time courses of the quenching reaction were investigated in the 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 -mediated polymerization, as shown in Figure 5. Quencher 2a, 5.0 molar equiv to the living end, was added when conversion reached 56%. The reaction mixture was kept at 80 C for 1 9 h after the addition. All of the polymers showed almost the same M n ( ) and M w/m n ( ), independent of the standing time. However, F n depended on it and became closer to unity with time. Beyond 5 h, the functionality was almost equal to unity. This means that the reaction between 2a and the polymer terminal was neither instantaneous nor slower than the polymerization. Figure 5. Time courses of the quenching reaction with 2a of the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [2a]/[ end] 5.0. The quencher was added when the polymerization was run for 24 h.

8 4742 TOKUCHI ET AL. Figure 6. Quenching of the living radical polymerization of MMA with 1a/ RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C at different conversions of MMA. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [2a]/[ end] 5.0. The quenching reaction was run for 24 h. We also examined the quenching reaction at various monomer conversions to see how fast it occurred (Fig. 6). On the addition of 2a at a 19% MMA conversion at 7 h (filled circle at the lowest conversion), the monomer was consumed slowly to reach a 34% conversion in the following 24 h (open circle at the lowest conversion). Along with the change, the SEC curve shifted to higher molecular weights slightly (from the dotted line to the solid line). The difference between the conversions for the quencher addition and the polymer recovery became smaller as the standing time was prolonged; the same trend was found for the molecular weights. This shows that the growing polymer chain propagated more before it reacted with the quencher under the conditions where larger amounts of MMA still remained. However, once the propagating species added to the double bond of the quencher, the growing terminal did not propagate any more, resulting in the ketone terminal via the elimination of the silyl group. This is supported by the fact that there were no peaks that originated from the copolymeric structure of the 2a unit that still possessed the silyloxyl group in the polymer chain. All the F n values were around unity, independent of conversions under the conditions for Figure 6. In contrast, a conventional radical polymerization system with 2,2 -azobisisobutyronitrile (AIBN) copolymerized 2a with MMA. On the addition of 2a into the polymerization mixture when the MMA conversion reached 62% in 1.5 h, MMA was further polymerized to 72% in the following 16.5 h. The obtained polymers exhibited the signals of the phenyl (c) and methoxy (d) groups of the copolymerized unit at 6.9 ppm [Fig. 7(B)] but no signals of the ketone unit [e and f; Fig. 7(C)]. The -end groups of the polymers were the hydride and olefin (a) formed by disproportionation [similar to those obtained in the absence of 2a; Fig. 7(A)]. These indicate that the silyl enol-type initiators were specifically effective in the metalcatalyzed living radical polymerization that proceeded via the activation of carbon halogen bonds. The quenching reaction mediated by the silyl compound had some similarity to the reaction induced by addition fragmentation chain-transfer agents such as 3 in conventional radical poly-

9 METAL-CATALYZED LIVING RADICAL POLYMERIZATION 4743 merization. 36 As shown in eq 2, the propagating radical species added to the CAC double bond of 3 and was followed by fragmentation, resulting in the benzyl radical, which could initiate polymerization along with the formation of a dead chain end with a CAS double bond: (2) However, the silyl compound underwent the addition of the radical species generated by Ru(II) to its CAC double bond followed by fragmentation to release the silyl group, which could easily combine the halogen that originated from the dormant terminal along with the formation of the CAO double bond. Thus, these silyl enol ethers could act as addition fragmentation irreversible termination agents in the metal-catalyzed living radical polymerization, as if they generated the silyl radical that combined the halogen that originated from the dormant polymer terminal (path A in eq 3): (3) Alternatively, however, the reaction proceeded not via a free-radical process (path A) but via a coordination process that involved a penta-coordinated silyl species and an electron-transfer reaction (path B). To confirm that the quenching reaction was catalyzed by the Ru(II) complex similarly to the polymerization, we examined the reaction between 2a and the isolated Cl-capped PMMA. We thus synthesized the Cl-capped PMMA by 1a/ RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 and isolated the polymer by preparative SEC to remove the catalyst residues (M n 4800, M w/m n 1.22, F n 0.93). The reactions between the Cl-capped polymer and 2a were carried out under four conditions: (1) without any compounds, (2) in the presence of RuCl 2 (PPh 3 ) 3 and Al(Oi Pr) 3, (3) in the presence of RuCl 2 (PPh 3 ) 3 but without Al(Oi Pr) 3, and (4) in the presence of Al(Oi Pr) 3 but without RuCl 2 (PPh 3 ) 3 (Fig. 8). All the reactions were run for 15 h in toluene at 80 C under conditions similar to those for the quenching reaction of the polymerization. All the products thus obtained exhibited almost the same molecular weights and MWDs as those of the PMMA before the reaction. The polymers obtained in the presence of the ruthenium complex showed UV absorption in the SEC curve, which agreed well with the traces monitored by refractive index [RI; Fig. 8(B,C)]. Further analysis of the products by 1 H NMR revealed the presence of the phenyl group that originated from 2a. In contrast, the polymers obtained in the absence of RuCl 2 (PPh 3 ) 3 had no such groups at the terminal. This further indicated that the quenching reaction was also induced by the ruthenium com-

10 4744 TOKUCHI ET AL. Figure 7. 1 H NMR spectra of PMMA obtained with (A) AIBN (M n 5100, M w/m n 2.03), (B) AIBN/2a (M n 4000, M w/m n 2.48), and (C) 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 /2a (M n 4000, M w/m n 1.33) in toluene at 80 C. [MMA] M; [AIBN] 0 20 mm; [2a]/[AIBN] 0 5.0; or [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [2a]/[ end] 5.0. plex [Fig. 8(A,D)]. However, F n clearly increased with the use of the aluminum compound, so the aluminum compound accelerated the quenching reaction and the polymerization. 3,37,38 Lifetime of the Living End The development of effective quenchers for living polymerization makes it possible to measure the lifetime of the living end. 39 Especially with the use of aromatic silyl enol ethers for PMMA, the obtained polymers showed characteristic signals of the aromatic protons, which could be separated well from the other peaks of PMMA. The quencher 2a was thus added to the reaction mixtures, which were kept unquenched but without monomer at 80 C for various times (14 97 days). Under these conditions, the polymerization was completed in about 7 days. The reaction with 2a was run for 24 h at 80 C. Figure 9 shows the 1 H NMR spectra of the products at various standing times. As already mentioned, the terminal group of the polymer obtained with 2a during the polymerization was the ketone terminal structure (T2) with F n 1.00 [Fig. 9(B)]; this again showed complete and quantitative conversion of the dormant Cl-capped terminal [T1 in Fig. 9(A)] into T2. Therefore, almost all the growing terminal was still alive or dormant throughout the polymerization. In contrast, the polymers obtained after the completion of the polymerization showed a terminal hydrogen (T3) 40 and an exo-olefin (T4) along with the decrease of the T2 content [Fig. 9(C)]. For example, the content of T2 decreased to 0.18 when the addition of 2a was done in 42 days after the complete MMA consumption. On further standing, T2 terminal completely disappeared [Fig. 9(D)]. The contents of these terminals were then plotted against the standing time (Fig. 10). Apparently, T2 decreased with time along with an increase of T3 and T4, which showed that the living or dormant terminals underwent side reactions at least after the monomer completion. The MWDs became broader with the formation of dead polymer chain ends. Thus, the lifetime of the growing polymer terminal in the Ru-catalyzed polymerization was finite, similar to the lifetimes in other living polymerizations. CONCLUSIONS Silyl enol ethers such as 2a with conjugated phenyl groups with an electron-donating me-

11 METAL-CATALYZED LIVING RADICAL POLYMERIZATION 4745 thoxy group were the most effective quenchers for the living radical polymerization of MMA mediated by the ROCl/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 initiating system. The quenchers reacted with the growing radical polymer terminal, which was generated from the dormant chloride polymer terminal and the ruthenium complex, in competition with the remaining monomer to give a ketone terminal via the release of the silyl group along with the chlorine in the dormant terminal. The electron-donating aromatic substituent increased the electron density and the reactivity of its double bond, so that the electron-deficient PMMA radical species could easily react with it. Another critical factor was the silyl group, which acted as a good leaving group that abstracted the terminal chlorine because of its high halogen affinity. The quenching reaction afforded halogen-free polymers with a more stable COC bondatthe end by converting a less stable COhalogen bond, which was usually obtained by the cooling of the metal-catalyzed living radical polymerization mixture. This should lead to an end-functionalized polymer synthesis through the introduction of functional groups into the silyl enol ethers, a subject now under investigation in our group. EXPERIMENTAL Materials Figure 8. SEC curves of PMMA obtained after the reaction of 2a and Cl-capped PMMA [M n 4800, M w/ M n 1.22, F n(cl) 0.93] prepared by the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. The reaction was run with or without RuCl 2 (PPh 3 ) 3 and Al(Oi Pr) 3 in toluene at 80 C: (A) without Ru and Al, (B) with Ru and Al, (C) with Ru and without Al, and (D) with Al and without Ru. The reaction was run for 15 h. [Prepolymer] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [2a] mm. Conjugated silyl enol ethers (2a 2d) were prepared and purified as reported. 41 2e (Shin-etsu Silicon; purity 99.9%) and 2f (Tokyo Kasei; purity 97%) were used as received. 2g (Aldrich; purity 97%), 2h (Wako; purity 98%), and MMA (Tokyo Kasei; purity 99%) were dried overnight over calcium chloride and distilled twice over calcium hydride under reduced pressure before use. RuCl 2 (PPh 3 ) 3 (Merck; purity 99%) and Al(Oi Pr) 3 (Aldrich; purity 99.99%) were used as received and handled in a glove box (M. Braun) under dry ( 1.0 ppm) and oxygen-free ( 1.0 ppm) argon. Dimethyl 2-chloro-2,4,4-trimethylglutarate [(MMA) 2 OCl; 1a] 22,42 and dimethyl 2-chloro-2,4,4-trimethylglutarate [(MMA) 2 O Br, 2a; initiator] 43 were prepared as reported. Toluene (solvent) and n-octane (internal standard for gas chromatography) were dried overnight over calcium chloride, distilled twice over calcium hydride, and bubbled with dry nitrogen for more than 15 min immediately before use. Polymerization Procedures The polymerization was carried out by the syringe technique under dry nitrogen in baked glass tubes equipped with three-way stopcocks or in baked and sealed glass tubes. 3 A typical example was the following: The polymerization was initiated by the addition (via dry syringes) of toluene (5.47 ml), n-octane (0.29 ml), a solution of Al(Oi Pr) 3 (3.36 ml, 125 mm in toluene), MMA (1.12 ml), and a solution of 1a (0.26 ml, 800 mm in toluene), in that order, into RuCl 2 (PPh 3 ) 3 (201.4 mg, 0.21 mmol) in toluene at room temperature. The total volume of the reaction mixture was ml. Immediately after mixing, aliquots (1.50 ml each) of the solution were injected into baked glass tubes equipped with three-way stop-

12 4746 TOKUCHI ET AL. Figure 9. End-capping of PMMA with 2a after monomer consumption in the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. The quenching reaction was run for 24 h. PMMA obtained before monomer consumption: (A) without 2a (1 day; M n 3800, M w/m n 1.34) and (B) with 2a (1 day; M n 4000, M w/m n 1.33). PMMA obtained after monomer consumption: (C) with 2a (42 days; M n 5900, M w/m n 1.34) and (D) with 2a (97 days; M n 6000, M w/m n 1.37). [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [2a]/[ end] 5.0. cocks, which were then placed in an oil bath kept at 80 C. After a predetermined time, a solution of quencher in toluene (0.16 ml, 0.96 M) was added into the polymerization solution. After a certain time, the polymerization solution was cooled to 78 C. Monomer conversion was determined from the concentration of residual monomer measured by gas chromatography with n-octane as an internal standard. The quenched reaction solutions were diluted with toluene (ca. 10 ml), washed with water, and evaporated to dryness to give the products, which were subsequently dried overnight under vacuum at room temperature. Measurements The MWD, M n, and M w/m n ratios of the polymers were measured by SEC in chloroform at room

13 METAL-CATALYZED LIVING RADICAL POLYMERIZATION 4747 Figure 10. Fate of the end after monomer consumption in the living radical polymerization of MMA with 1a/RuCl 2 (PPh 3 ) 3 /Al(Oi Pr) 3 in toluene at 80 C. [MMA] M; [1a] 0 20 mm; [RuCl 2 (PPh 3 ) 3 ] 0 20 mm; [Al(Oi Pr) 3 ] 0 40 mm; [2a]/[ end] 5.0. temperature on three polystyrene gel columns (Shodex K-805L 3) that were connected to a Jasco PU-980 precision pump, a Jasco RI-930 refractive index, and 970-UV ultraviolet detectors. The columns were calibrated against 11 standard PMMA samples (Polymer Laboratories; M n ,000, M w/m n ) and the monomer. 1 H NMR spectra were recorded in CDCl 3 at 25 C on a JEOL JNM-LA500 spectrometer operating at MHz. Polymers for the 1 HNMR analysis were fractionated by preparative SEC (Shodex K-2002 column) to be freed from low molecular impurities that originated from the catalysts. With appreciation, M. Sawamoto and M. Kamigaito acknowledge the support of the New Energy and Industrial Technology Development Organization under the Ministry of International Trade and Industry, Japan, through a grant for Precision Catalytic Polymerization in the project Technology for Novel High-Functional Material ( ). REFERENCES AND NOTES 1. (a) Webster, O. W. Science 1991, 251, 887; (b) Aida, T. Prog Polym Sci 1994, 19, 469; (c) Matyjaszewski, K.; Sawamoto, M. In Cationic Polymerization; Matyjaszewski, K., Ed.; Marcel Dekker: New York, 1996; Chapter 4, p 265; (d) Kennedy, J. P.; Ivàn, B. Designed Polymers by Carbocationic Macromolecular Engineering: Theory and Practice, Hanser: Munich, 1992; p 31; (e) Hirao, A.; Nakahama, S. Trends Polym Sci 1994, 2, 267; (f) Hsieh, H. L.; Quirk, R. P. Anionic Polymerization; Marcel Dekker: New York, 1996; (g) Breslow, D. R. Prog Polym Sci 1993, 18, (a) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K. Trends Polym Sci 1994, 2, 66; (b) Hawker, C. J. Acc Chem Res 1997, 30, 373; (c) Malmström, E. E.; Hawker, C. J. Macromol Chem Phys 1998, 199, 823; (d) Colombani, D. Prog Polym Sci 1997, 22, 1649; (e) Sawamoto, M.; Kamigaito, M. Trends Polym Sci 1996, 4, 371; (f) Controlled Radical Polymerization; Matyjaszewski, K. Ed.; ACS Symposium Series 685; American Chemical Society: Washington DC, 1998; (g) Sawamoto, M.; Kamigaito, M. In Synthesis of Polymers; Schlüter, A.-D., Ed.; Materials Science and Technology Series; VCH-Wiley: Weinheim, 1999; Chapter 6, p 163; (h) Sawamoto, M.; Kamigaito, M. CHEMTECH 1999, 39(6), 30; (i) Patten, T. E.; Matyjaszewski, K. Acc Chem Res 1999, 32, 895; (j) Matyjaszewski, K. Chem Eur J 1999, 5, 3095; (k) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, K.; Tam, P. T. L.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, Wang, J.-S.; Matyjaszewski, K. J Am Chem Soc 1995, 117, 5614.

14 4748 TOKUCHI ET AL. 5. Percec, V.; Barboiu, B. Macromolecules 1995, 28, Granel, C.; Dubois, Ph.; Jérôme, R.; Teyssié, Ph. Macromolecules 1996, 29, Haddleton, D. M.; Jasieczek, C. B.; Hannon, M. J.; Shooter, A. J. Macromolecules 1997, 30, Ellzey, K. A.; Novak, B. M. Macromolecules 1998, 31, Haddleton, D. M.; Waterson, C.; Derrick, P. J.; Jasieczek, C. B.; Shooter, A. J. Chem Commun 1997, Mecerreyes, D.; Moineau, G.; Dubois, Ph.; Jérôme, R.; Hedrick, J. L.; Hawker, C. J.; Malmström, E. E.; Trollsås, M. Angew Chem Int Ed 1998, 37, Hawker, C. J.; Hedrick, J. L.; Malmström, E. E.; Trollsås, M.; Mecerreyes, D.; Moineau, G.; Dubois, Ph.; Jérôme, R. J. Macromolecules 1998, 31, Matsuyama, M.; Kamigaito, M.; Sawamoto, M. J Polym Sci Part A: Polym Chem 1996, 34, Percec, V.; Kim, H.-J.; Barboiu, B. Macromolecules 1997, 30, Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31, Nakagawa, Y.; Matyjaszewski, K. Polym J 1998, 30, Carrot, G.; Nilborn, J.; Hedrick, J. L.; Trollsås, M. Macromolecules 1999, 32, Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, Haddleton, D. M.; Waterson, C. Macromolecules 1999, 32, Ohno, K.; Fujimoto, K.; Tsujii, Y.; Fukuda, T. Polymer 1999, 40, Malz, H.; Komber, H.; Hoigt, D.; Hopfe, I.; Piotech, J. Macromol Chem Phys 1999, 200, Coessens, V.; Matyjaszewski, K. Macromol Rapid Commun 1999, 20, Ando, T.; Kamigaito, M.; Sawamoto, M. Macromolecules 1998, 31, Coessens, V.; Nakagawa, Y.; Matyjaszewski, K. Polym Bull 1998, 240, Coessens, V.; Matyjaszewski, K. J Macromol Sci Pure Appl Chem 1999, 36, Coessens, V.; Matyjaszewski, K. J Macromol Sci Pure Appl Chem 1999, 36, Coessens, V.; Matyjaszewski, K. J Macromol Sci Pure Appl Chem 1999, 36, Coessens, V.; Matyjaszewski, K. Macromol Rapid Commun 1999, 20, Koulouri, E. G.; Kallitsis, J. K.; Hadiioannou, G. Macromolecules 1999, 32, Asgarzadeh, F.; Ourdouillie, P.; Beyou, E.; Chaumont, P. Macromolecules 1999, 32, Coessens, V.; Pyun, J.; Miller, P. J.; Gaynor, S. G.; Matyjaszewski, K. Macromol Rapid Commun 2000, 21, Percec, V.; Barboiu, B.; Bera, T. K.; Kim, H.-J.; Fréchet, J. M. J.; Grubbs, R. H. Prepr. IUPAC International Symposium on Ionic Polymerization, Kyoto, Japan, July 1999, p Kamigata, N.; Udodaira, K.; Shimizu, T. J Chem Soc Perkin Trans , Polymer Handbook, 4th ed.; Brandrup, J.; Immergut, E. H.; Grulke, E. A., Eds.; Wiley: New York, 1999; p II Ando, T.; Kamigaito, M.; Sawamoto, M. Macromolecules 1997, 30, Uegaki, H.; Kotani, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 1998, 31, (a) Colombani, D.; Chaumont, P. Acta Polym 1998, 49, 225; (b) Colombani, D.; Chaumont, P. Prog Polym Sci 1996, 21, Ando, T.; Kato, M.; Kamigaito, M.; Sawamoto, M. Macromolecules 1996, 29, Ando, T.; Kamigaito, M.; Sawamoto, M. Macromolecules, 2000, 33, Choi, W. O.; Sawamoto, M.; Higashimura, T. Macromolecules 1990, 23, Hatada, K.; Kitayama, T.; Ute, K.; Terawaki, Y.; Yanagida, T. Macromolecules 1997, 30, Fukui, H.; Sawamoto, M.; Higashimura, T. Macromolecules 1993, 26, Ando, T.; Kamigaito, M.; Sawamoto, M. Macromolecules, 2000, 33, Ando, T.; Kamigaito, M.; Sawamoto, M. Tetrahedron 1997, 53,