Living Radical Polymerization - American

Chapter 8

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Living Radical Polymerization with Designed Metal Complexes Masami Kamigaito, Tsuyoshi Ando, and Mitsuo Sawamoto Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan

This paper discusses recent developments in the metal-catalyzed living radical polymerization with designed metal complexes in our laboratory. The design was directed to increasing the catalytic activity and to widening the applicability to a variety of monomers by several methods. They include ruthenium(II) complexes with an electron-donating aminoindenyl ligand, a neutral and labile ethylene ligand, a hemilabile bidentate P,N-ligand, and a dinuclear iron(I) complex. These novel complexes were characterized by N M R , cyclic voltammetry, and X-ray crystallography. Most of them had a lower redox potential than the previous ruthenium(II) and iron(II) complexes and proved effective in a fast living radical polymerization.

Introduction Living or controlled radical polymerization is now achieved with a variety of initiating systems based on a common concept, i.e., reversible activation of covalent bond of the dormant species into the growing radical species (/). The systems with nitroxide, metal catalysts, and R A F T agents have been widely developing in various aspects; initiating systems that can control polymer molecular weights more precisely, kind of monomers that can be polymerized in controlled fashion, and controlled architectures of polymers that can be obtained.

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© 2003 American Chemical Society

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


103 Because of Iheir relative easiness in the procedures and their wide applicability, these polymerizations are becoming common as synthetic methods for welldefined polymers. However, there seems still much room for improvement in the radical systems from the viewpoint of rates and versatility. We first discovered nithenium-catalyzed living radical polymerization (2) of methyl methacrylate ( M M A ) with RuCb(PHi3)3 (1) and have been developing the metal-catalyzed systems (Scheme 1) by using ruthenium and other metals such as iron, nickel, rhenium, etc. or by introducing various ligands (la, If). For example, Figure 1 summarizes evolution of a series of ruthenium and iron complexes in our laboratoiy. The first ruthenium complex proved highly effective in giving well-controlled polymethaoylates with controlled molecular weights [M (number-average molecular weight) = ([Μ]ο/ρ]ο) x (molecular weight of monomer)], narrow molecular weight distributions (MWDs) (MJM < 1.1), and high chlorine end functionality (F = 1.0) up to high conversions (2-4). However, the catalytic activity was not high, and the controllability for acrylate and styrene polymerizations was moderate. We then introduced electrondonating eyclopentadiene (Cp)-based ligands into the complex to lower the redox potential or to increase the activity. The ruthenium indenyl complex (2) induced a faster living radical polymerization of methacrylates (J) while Aie pentamethylcyclopentadienyl (Cp*) complex (3) proved versatile to result in good molecular weight control for three classes of monomer^ methacrylates, acrylates, and styrenes (MJM < 1.1) (6). Another modulation for 1 is the use of a hydrophilic and ionic phosphine ligand, leading to the complex (4) with which enabled homogeneous living radical polymerization of hydrophilic monomers like2-hydroxyethyl methacrylate in methanol (7). For iron counterparts, a P P h based complex (5) was employed for M M A to show a slightly higher activity than the ruthenium (1) (8). The iron-Cp complex (6) was also effective and gave polymers of styrene and acrylate with controlled molecular weights (9-11). n

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Dorment Spocfos

Scheme J. Metal-catalyzed living radical polymerization.

This paper presents a brief overview on recent and further developments in ruthenium and iron catalysts (7-10) based on ligand or metal designs for faster and more versatile living radical polymerizations. The design is intended to increase the catalytic activity by several methods; i.e., introducing an electrondonating amino-group into the indenyl ligand (7) (12), replacing the chlorine atom in 1 with a neutral and labile ethylene ligand (8) (73), using a hemilabile bidentate i^JV-ligand (9) (14), and employing iron® in place of iron(Il) (15). These strategies would be hopeful as stated below.



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Figure 7. Evolution ofruthenium and iron catalystsfor metal-catalyzed living radical polymerization in our laboratory.

Results and Discussion

Electron-Donating Aminoindenyl Ruthenium Complex (7) One of the keys for increasing the catalytic activity in metal-catalyzed redox processes is to reduce the redox potential of metal complexes by introducing an electron-donating ligand, because the catalyst should give one electron to a carbon-halogen terminal on activation (Scheme 1). We thus prepared a new mthenium complex with an aminoindenyl ligand [7, Ru(2-Me N-fod)Cl(PPh3)2] and checked the catalytic activity, versatility, and reaction controllability (72). The complex 7 was synthesized from the lithium salt of iV-(lH-2-indenyl>Af,JV-dimethylamine and RuChiPPlb^, and was confirmed by elemental analysis, N M R , and X-ray crystallography (Figure 2). X-Ray analysis showed that 7 is an 18-electron complex where the aminoindenyl ligand coordinates to the mthenium center in ^-fashion as indicated by a small slip parameter ( Δ = 0.0320 Â) and hinge angle (HA = 12.6°) (76). Such a complex may release one of the phosphine ligands or may undergo slipping of the indenyl ring into a η coordination state to become active for abstraction of halogen from the dormant terminal. The bond length between the ruthenium and the phosphorous atoms was slightly longer in 7 than in 2, which suggests easier release of phosphine or 2

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Time, h Figure 3. Polymerization of MMA with 7(Φ, A) or2 (Ο, Δ) coupled with (MMA)2-ClandAl(OUPr)3 in toluene at 80 °C: [MMA] = 4.0 M; [(MMA)2-Cl]o = 40 mM; [7 or 2] -4.0 mM; [Al(Oi-Pr) ]o = 40 mM. 0

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higher activity of 7. Its high activity was also suggested by cyclic voltammetiy, where 7 had a lower redox potential than 2 (Em = 0.42 vs 0.55 V vs Ag/AgCl). The complex was employed for polymerization of M M A in conjunction with an M M A - d i m e r chloride [(CHa^CCCOaCHs^HaCiCHaXCOîCHaK:!; ( M M A ) r - C l ] as an initiator in the presence of Al(0/-Pr) . As shown in Figure 3, the new complex (7) induced a faster polymerization and gave polymers with narrower M W D s (MJM < 1.1) than the indenyl (2) does. The M increased in direct proportion to monomer conversion and agreed well with the calculated values assuming that one molecule of the initiator generates one living polymer chain. The fast polymerization accompanying with fine molecular weight control and narrow M W D s indicates that the complex 7 increases not only 3

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106 polymerization rate but also the interconversion between the dormant and the radical species. Figure 4 shows high molecular weight P M M A with (MMA)y-Cl/7/Al(OiPr>3. The polymerization was fast irrespective of a low catalyst concentration (0.80 m M ) in comparison to the initiator (8.0 mM). The A 4 increased in direct proportion to monomer conversion and agreed well with the calculated values. The M W D s were narrower (MJM < 1.1) than those of a similar M (~ 1x10 ) obtained with Cu(I) (77) although the highest M were set at lower in our case. s

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Figure 4. High molecular weight PMMA with (MMA) T~Cl/7/Al(Oi-Pr) in toluene at 80 °C: [MMA] = 8.0M; [(MMA)r-Cl]o = 8.0 mM; [7] = 0.80 mM; [Al(Oi-Pr) ]o = 40 mM. The diagonal solid line indicates the calculatedM assuming the formation of one living polymer per (MMA) 2-C1 molecule. 3

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The polymerization of styrene with 7 and ( M M A ) r - C l was also examined in the presence of Al(0/-Pr) . The molecular weights were in good agreement with the calculated values, where the M W D s were narrow even with the chloride initiator (MJM = 1.26). However, for methyl acrylate ( M A ) , the M W D s were broader (MJM = 2.33) with ( M M A ) r - C l while narrower with a bromide [(CH )2C(C02CH2CH3)Br, E M A - B r ] and an iodide [ ( Œ ^ C i C a C f t C H O I ; E M A - I ] initiator (MJM = 1.47 and 1.32, respectively). These are because of stronger secondary carbon-halogen bonds in the dormant species in polystyrene and polyaciylates, and because of weaker bond eneigy in the order of CI > B r > I. A more active ruthenium catalyst may be needed for control of aciylate polymerizations by using the most stable carbon-chlorine species. 3

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Cationic Indenyl Ruthenium Complex with Labile Ethylene Ligand (8) Another approach to high catalytic activity is to promote formation of a vacant active site in complexes The chlorine in 2 was thus substituted with a


107 labile and naîtrai ethylene ligand to be converted into a cationic 18-electron mthenium complex (8) with a weakly nucleophilic borate anion [BAr ~; A r = C6F (8a), 3,5-(CF )2C6H3 (8b)], which is often employed for olefin polymerization. The catalysts were synthesized from the reaction between 2 and lithium [LiBiCeFsM or sodium borate [NaBfrS-iCFa^CeHsM under ethylene atmosphere. The elemental analysis showed that the observed contents of caibon and hydrogen were i n good agreement with the calculated. *H N M R analysis indicated the presence of the borate anion and the ethylene that is coordinated to ruthenium, as shown in Figure 5. These results supported formation of the cationic ruthenium complexes (8). 4


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Figure 5. H NMR spectrum of 8b in toluene-d at room temperature. The signals marked with an asterisk are due to the solvent. !

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The catalysts were used for polymerization of M M A in conjunction with ( M M A ) r - C l and Al(0/-Pr) in toluene at 80 °C (13). The polymerizations with the cationic mthenium complexes (8a and 8b) proceeded faster than that with the neutral counterpart (2) (Figure 6). The obtained polymers had unimodal M W D s and M„ that increased in direct proportion to monomer conversion. These results indicate that the cationic ruthenium complexes induce a fast living radical polymerization of M M A . However, slightly broader M W D s and slightly higher M may suggest some side reactions. We further investigated copolymerization of M A and olefins such as 1hexene with the cationic (8b) and the neutral completes (2) in conjunction with an iodide initiator (ΜΑ-Γ) in the presence of Al(0/-Pr) . Both catalysts led to copolymerizations where the hexene was consumed much slower than M A . The cationic complex showed a slightly higher activity. The molecular weights of the polymers increased with consumption of monomers while the M W D s were 3

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broad (MJM = 2-3). *H and C N M R analysis showed that the copolymers had about 10% hexene, which were distributed among M A units in the copolymers. There were almost no differences in the comonomer distributions between the polymers obtained with 2 and 8b. Ethylene was also copolymerized with M A in place of 1-hexene. These results demonstrate that a olefins, much less reactive than M A , may be copolymerized via a radical mechanism which is veiy similar to that i n the metal-catalyzed living radical polymerization (Scheme 1). Further studies are now in progress. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 27, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch008

N

Figure 6. Polymerization of MMA with 8a (O) or 8b (*)or2 (A) coupled with (MMA)r-Cl andAl(Oi-Pr) in toluene at 80 °C: fMMAJo = 4.0 M; [(MMA)t-CI] = 40mM; [8a or 8b or 2] -4.0 mM; [Al(Oi-Pr) ]o = 40 mM. 3

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Ruthenium Complex with Hemilabile Bidentate P^iV-Ligand (9) We have already found that the ruthenium-catalyzed radical polymerization was accelerated on addition o f Al(Oi-Pr) (18) or amine (19, 20), where the enhancement effects were larger for the latter. Furthermore, such additives often make the M W D s much narrower. This is most probably attributed to interaction between the ruthenium complex and the additives, which may result in a new ruthenium species with a higher activity. Although some interactions between the mthenium complex and the additives were observed by N M R analysis o f the mixtures (19), isolation of the amine-coordinated ruthenium species was difficult due to relatively weak coordination of amines to ruthenium. We herein employed a chelating Ρ,ΛΓ-ligand i n the ruthenium-catalyzed living radical polymerization for rate enhancement and more fine control of the polymerization. The nitrogen group in such a ligand can strongly interact or coordinate to the ruthenium center due to the chelating effect assisted by the strong coordination of the phosphine part to the ruthenium. One of the selected Ρ,ΛΓ-ligands is 2-(#,AT-dimethylamino)benzyl diphenylphosphine (21), which 3



109 possesses triphenylphosphine and trialkylamine moieties. The former is a good ligand for the ruthenium-catalyzed living radical polymerization while the latter is effective in rate enhancement and fine control in the polymerization. The P,iV-compound was added to polymerization of M M A with ( M M A ) r - C l / 3 in toluene at 80 °C (Figure 7) (14). The compound accelerated the polymerization (filled circles in Figure 7), where the rate enhancement was larger than those with « - B U 2 N H (filled triangles) and Al(0/-Pr)3 (open triangles), despite a Iowa* concentration of the added P,JV-compound. In contrast, a similar amine without the phosphine part, ΑΓ,ΛΓ-dimethylbenzylamine, led to little rate enhancement (open squares). Further addition of triphenylphosphine to the amine rafter retarded the polymerization (filled squares). The chelating effect on the catalytic activity was thus shown clearly.

Figure 7. MMA polymerization with (MMA) 2-Cl/S in the presence ofadditives [Ρ,Ν-ligand (Φ), Me2NCH CSs (Π), Me^^CMs + PPh (M), Al(Oi-Pr) (A), n-Bu NH(A)]in toluene at 80 °C: [MMA] = 4.0 M; [(MMA)t-CI]o = 40mM; [3] = 4.0 mM; [additives] = 16 (·, D, M) or 40 (A A) mM. 2

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The polymers obtained with the P, JV-compound had narrow M W D s (MJM = 1.09) and controlled molecular weights that increased in direct proportion to monomer conversion. These results indicate that the P,Af-compound containing alkylamine and triphenylphosphine moieties is effective in the RuCp*-mediated living radical polymerization. The P,JV-ligand most probably coordinates to the mthenium to modify the catalytic activity. The formation of a new complex was suggested by *H and P N M R analysis of the mixture. We thus synthesized the P,JV-chelating Cp*-ruthenium complex from the ligand and [RuCp*Cl]4, isolated it by recrystallization, and identified by elemental and N M R analyses. The H and P N M R spectra were the same as those of the in-situ formed compound above. The X-ray crystallographic analysis indicate that the Ρ,ΛΓ-ligand coordinates to the ruthenium by chelation n

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110 (Figure 8), where the bond length of Ru and Ρ is almost the same as that of R u and Ν (2.2982 vs 2.2611 Â ) irrespective oflaiger radius of Ρ than that N . This means that the phosphorous part strongly coordinates to the ruthenium and that the amine moiety is more labile. The redox potential of the iVV-chelated complex (9) was lower than the phosphine complex (3) (0.26 vs 0.46 V vs Ag/AgCl), which indicates a higher activity of 9. The complex actually induced a faster polymerization than 3, while the M W D s became slightly broader (MJMn = 122) than that obtained with a mixture of theP,AT-ligand and 3. There seem to be some additional effects of excess ligands or the triphenylphosphinecomplex on the deactivation process.

Figure 8. ORTEP diagram of 9 (thermal ellipsoids at the 50% level).

Dinuclear Iron(I) Complex (10) Catalytic activity or redox potential can be modulated by the oxidation state of a metal center in addition to ligand design. W e have recently found that dinuclear iron(I) complex (10) not only induced a faster polymerization of styrene than mononuclear iron(II) (6) (10) but also enabled polymerization of less reactive monomers like vinyl acetate to result in control of molecular weights (22). This is most probably attributed to the lower oxidation state or redox potential of the iron(I) species. The iron(I) complex, 10, was thus employed for polymerization of M A in conjunction with E M A - I in toluene at 60 °C (15). The polymerization proceeded very fast to reach about 90% conversion in 1 h (Figure 9). The high activity is in a remarkable contrast to no activity of the mononuclear version 6 under the same conditions while the latter became moderately active (93% conversion in 80 h) on addition of Al(0/-Pr) . However, the molecular weight control with Ε Μ Α - Ι Λ 0 was inferior to that with EMA-I/6/Al(Oi-Pr) probably due to formation of excess radical species and/or the slow transformation of the resulting radical species into the dormant. For controlling the polymerization, molecular iodine (I ) was added to the reaction mixture, where the additive may 3

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not only serve as a radical scavenger to trap the growing radicals but may regenerate the C - I dormant terminal more rapidly. On addition of iodine (20 mM), the polymerization with the EMA-1/10 system was slightly retarded but reached a high conversion (93%) in 5 h. The M W D s of the polymers became much narrower (MJMn = 1.28). The polymers obtained in the presence of iodine had M„ that increased in direct proportion to monomer conversion (filled circles in Figure 10) and had

Figure 9. Polymerization of MA with EMA-I/10 (O), or ΕΜΑ-1/lÛ/h (Φ), or EMA-I/6/Al(OUPr) (A) in toluene at 60 °C: [MA] = 4.0 M; fEMA-IJo = 40 mM; [10 or 6] = 40 mM; [Al(Oi-Pr) ] - 40 mM; [I ]o = 20 mM. 3

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Figure 10. Polymerization of ΜΑ (Φ, A) or DMAΑ (Ο, Δ) with EMA~I/1Ù/I in toluem at 60 °C: [MAJ = 4.0M; [EMA-IJo = 40 mM; [lu] = 40 mM; [I ] = 20 mM. 2

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112 one initiating moiety per one polymer chain. This system can be also applicable for j¥,iV-dimethylacrylamide ( D M A A ) to induce a similarly fast and controlled radical polymerization (open circles in Figure 10). The M W D s were similarly narrow (MJMn = 122). Although the working mechanism of the Fe(I) complex and the added iodine, as well as the possibility of degenerative iodide-transfer mechanism, should be further discussed, the Fe(I>based initiating system is one of the most active and effective from the viewpoint of its wide applicability to various monomers such as acrylamides and vinyl acetate.

Experimental

General A l l experiments involving air-sensitive metal compounds were performed under a moisture- and oxygen-free argon atmosphere ( H 0 < 1 ppm; O 2 < 1 ppm) in a glove box (M.Braun Labmaster 130) or under argon or nitrogen with standard Schlenk line techniques. Toluene was dried overnight over calcium chloride and distilled from sodium/benzophenone ketyl. w-Pentane, tf-hexane, noctane, and tetralin were dried overnight over calcium chloride, and distilled twice over calcium hydride. Methylene chloride was dried overnight over calcium chloride, distilled from phosphorous pentoxide and then from calcium chloride. A l l solvents were degassed before use by freeze-thaw-pumping technique or by bubbling with dry nitrogen over 15 min. Water (Wako, distilled) was degassed under reduced pressure at 50 °C and saturated with dry nitrogen for several times. Methyl meftacrylate ( M M A ) (Tokyo Kasei, >99%), stymie (Wako, >99%), and methyl acrylate ( M A ) (Wako, >98%) were dried overnight over calcium chloride and distilled twice over calcium hydride under reduced pressure before use. RuCl (PPh )3 (1; Merck, >99%), Ru(Ind)Cl(PPh3)2 (2; Strem, >98%), Fe(Cp)I(CO) (6; Aldrich, >97%), F ^ C p ( C O ) (10; Aldrich, >99%), and Al(Oi-Pr) (Aldrich, >99.99%) were used as received. The chloride initiator [ ( C H ^ C i C ^ C H ^ f t C i C f t X C O ^ H j i C l ; ( M M A ) r - C l ] was prepared as reported (23). The bromide initiator [ ( C H ^ C i C O ^ t ^ r , E M A - B r ] (Tokyo Kasei; >98%) was distilled twice over calcium hydride under reduced pressure. The iodide initiator [(CH^CiCCfeEt)!; E M A - I ] was prepared as reported (10, 24). 2

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Synthesis of Complexes The aminoindenyl mthenium complex (7) (12) was prepared from the lithium salt of the ligand [JV-(lH-2-indenyl>iV,i\r-dimethylamine] (25, 26) and


113 RuCl (PPh )5 in toluene at 90 °C. After removal of the solvent followed by extraction with C H 2 C I 2 , the solid was washed with deionized water and nhexane for several times. The complex was re crystallized by layering of nhexane on the CH C1 solution. *H N M R and X-ray crystallography indicated that one molecule of C H 2 C I 2 was contained per one molecule of the ruthenium compound. Anal. Calcd for C 4 8 H 4 4 C I 3 N P 2 R U : C , 63.76; H , 4.90; N , 1.55; CI, 11.76. Found: C, 64.21; H , 4.96; N , 1.55, CI, 11.63. The cationic ruthenium complexes (8a and 8b) (13) were obtained from Ru(Ind)Cl(PPh )2 and lithium [LiBiCeFsVEfeO] and sodium borate [NaB[3,5(CF ) C6H ]4 2H 0], respectively, in C H 2 C I 2 at room temperature under an ethylene atmosphere. After filtration of the solution followed by removal of the solvent under vacuum, the completes were precipitated by addition of «-hexane into the CH C1 solution. Anal. Calcd for C 7 i H 4 i B F P 2 R u (8a): C, 58.90; H , 2.85. Found (8a): C, 59.04; H , 3.14. Anal. Calcd for C 9 H B F P R u (8b): C, 58.14; H , 3.27. Found (8b): C, 58.13; H , 3.21. The P,JV-chelated mthenium complex (9) (14) was synthesized from [Ru(Cp*)Cl]4 (27) and theP,AMigand (21) in toluene at 80 °C. Aft©- removal of the solvent followed by extraction with C H 2 C I 2 , the complex was reciystallized by layering of pentane on the C H 2 C I 2 solution. Anal. Calcd for C i H 7 C l N P R u : C, 62.99; H , 6.31; N , 2.37; CI, 6.00. Found: C, 62.80; H , 6.20; N , 2.29, CI, 6.00. 2

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Polymerization Procedures Polymerization was carried out by the syringe technique under dry nitrogen in glass tubes equipped with a three-way stopcock or in baked and sealed glass vials. A typical example for the polymerization o f M M A with 7 / ( M M A ) 2 - C l / A l ( 0 / - P r ) is given below: In a 50-mL round-bottomed flask was placed 7 (14.1 mg, 0.0156 mmol), toluene (0.40 mL), «-octane (0.33 mL), M M A (1.67 mL, 1.56 mmol), a solution of Al(Oi-Pr) (1.25 mL of 125 m M in toluene, 0.156 mmol), and a solution of (MMA)2-C1 (0.25 mL of 620 m M in toluene, 0.155 mmol) at room temperature. The total volume of reaction mixture was 3.90 mL. Immediately after mixing, four aliquots (0.50 mL each) of the solutions were injected into backed glass tubes. The reaction vials were sealed and placed in an oil bath kept at 80 °C under vigorous stirring. In predetermined intervals, the polymerization was terminated by cooling the reaction mixtures to -78 °C. Monomer conversion was determined from the residual monomer concentration by gas chromatography with «-octane as an internal standard. The quenched reaction solutions were diluted with toluene (ca. 20 mL) and rigorously shaken with an absorbent [KYOWAAD-2000G-7 (Mgo.7Alo.sO1.15); Kyowa Chemical Industry] (ca. 5 g) to remove the metal-containing residues After the absoibent was separated by filtration (Whatman 113 V), the filtrate was washed with water and evaporated to dryness to give the products, which were subsequently dried overnight under vacuum at room temperature. 3

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114 Measurements The M W D , A/ , and MJM ratios of the polymers were measured by sizeexclusion chromatography (SEC) in chloroform at 40 °C on three polystyrene gel columns (Shodex K-805L χ 3) that were connected to a Jasco PU-980 precision pump and a Jasco RI-930 refractive index detector. The columns were calibrated against twelve standard poly(MMA) samples (Polymer Laboratories; M = 630-1200000; MJM = 1.06-1.22) as w d l as the monomer. *H N M R spectra were recorded in CDC1 at 25 °C on a J E O L J N M - L A 5 0 0 spectrometer, operating at 500.16 M H z . Pofymers for N M R analysis were fractionated by preparative S E C (column: Shodex Κ-2002) to be freed from low molecular impurities originated from the catalysts. Cyclic voltammograms were recorded on a Hokuto Denko HZ-3000 apparatus. Measurements were carried out under argon at 0.10 Vs" in a CH C1CH C1 solution (5.0 m M ) with w-BiiiNPFe (100 m M ) as the supporting electrolyte. A three-electrode cell, equipped with a platinum disk as a working electrode, a platinum wire as a counter electrode, and an A g / A g C l electrode as a reference, was used. X-Ray data were collected at 130 Κ for 7 and at 293 Κ for 9 with a Bruker S A M R T - C C D area detector diffractometer using the M o K a radiation (λ = 0.71073 À). Cell parameters were refined with 16868 reflections for 7 and 21848 reflections for 9. The diffraction frames were integrated on the SAINT package. The structures were solved and refined with SHELXL-97. n


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Acknowledgment This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI), Japan. W e thank Professor Tamejiro Hiyama and Dr. Masaki Shimizu at Kyoto University and Professor Kyoko Nozaki and Mr. Koji Nakano at the University of Tokyo for the X-ray crystallographic analysis.

References 1.

For recent reviews on living radical polymerization, see: (a) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689. (b) Matyjaszewski, K . ; Xia, J. Chem. Rev. 2001, 101, 2921. (c) Hawker, C. J.; Bosma, A . W.; Harth, E . Chem. Rev. 2001, 101, 3661. (d) Matyjaszewski, K., Ed.; A C S Symposium Series 768; American Chemical Society: Washington, D C , 2000. (e) Sawamoto, M.; Kamigaito, M. In Synthesis of Polymers; Schlüter, A . - D . , Ed.; Materials Science and Technology Series; W i l e y - V C H : Weinheim, Germany, 1999, Chapter 6; pp 163-194. (f) Sawamoto, M.; Kamigaito, M. CHEMTECH 1999, 29 (6), 30.


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