Living Radical Polymerization of Acrylates with Rhenium(V)-Based

Chapter 12

Living Radical Polymerization of Acrylates with Rhenium(V)-Based Initiating Systems: ReO I(PPh ) /Alkyl Iodide Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 1, 2015 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0760.ch012

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Hiroko Uegaki, Yuzo Kotani, Masami Kamigaito, and Mitsuo Sawamoto Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan

Rhenium(V) iododioxobis(triphenylphosphine) [ReO I(PPh ) ], a group 7 transition metal complex, induced living radical polymerizations of methyl and n-butyl acrylates in conjunction with an iodide initiator such as CH CH(Ph)I and (CH ) C(CO Et)I in the presence of Al(Oi-Pr) , where the reaction was faster than the NiBr (Pn-Bu ) -mediated The number-average molecular weights of the obtained polymers increased in direct proportion to monomer conversion, and the molecular weight distributions were relatively narrow throughout the polymerizations (M /M ~ 1.6). The polymerization most probably proceeds via the rhenium-catalyzed homolytic cleavage of the polymer C-I terminal originated from the iodide initiator, as indicated by the H NMR analysis of the polymer terminal structure and quenching study of the polymerization with a stable nitroxide radical. In contrast, a bromide initiator like CCl Br led to an uncontrolled acrylate polymerization with the rhenium(V) complex. The iodide/ReO I(PPh ) initiating system also induced a fast polymerization of methyl methacrylate, though the polymerization was not controlled. 2

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Introduction Transition metal catalysts play important roles in precision control in polymerization. They can permit control of molecular weights, molecular weight distributions (MWDs), steric structure of main chain, and sometimes sequence of monomer units in various modes of polymerizations for many monomers. Radical polymerizations is now among these metal-catalyzed precision processes, although it has been believed difficult to control because of highly reactive radical intermediates and their side reactions such as bimolecular termination (/), as with radical addition reactions for small molecule synthesis (2). Thus, the emergence of transition metal catalysts has changed the recognition of radical reactions from uncontrollable to 'Corresponding author.

196

© 2000 American Chemical Society

In Transition Metal Catalysis in Macromolecular Design; Boffa, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

197 controllable. Owing to their variable oxidation states, these complexes can generally serve as effective oxidants or reductants to generate radical species from organic precursors. For example, a ruthenium complex, RuCI (PPh ) , induces controlled radical addition reactions between C C 1 and an alkene, where one of the C - C l bonds in the former is cleaved with the aid of the oxidation of Ru(II) into Ru(IH) to generate C C 1 * , which subsequently adds to the double bond of the olefin to give a 1:1 adduct effectively along with the reduction of the Ru(HI) center (3). During the addition reaction, therefore, the ruthenium center undergoes a reversible redox reaction between the divalent and trivalent states. Such addition reactions are widely employed for inter- and intramolecular reactions for organic synthesis (2). Employing similar catalysts, we have recently developed a series of transition metal-mediated living radical polymerizations that precisely control molecular weights and molecular weight distributions (MWDs) of polymers for methacrylates, acrylates, styrenes, etc (4). The processes were initially reported for the initiating systems that consist of an organic halide like C C 1 as an initiator and a metal catalyst such as RuCl (PPh ) (5, 6). Following the incipient works, such metal-catalyzed processes vastly developed in terms of the scope of usable catalysts (metals and ligands) and monomers. Effective metals thus far reported include not only Ru (5-9) but also Cu (10-14% Fe (15-17), Mi (18-21), Rh (22-24), and Pd (25). Therein the metals complexes mediate a controlled sequence of the formation of radicals from the C-halogen terminals derived from the initiator, their addition to the monomers, and the regeneration of the analogous C-halogen terminals (eq 1). In these living processes, the equilibrium between the dormant and the active or radical species is shifted to the dormant side, and thereby the concentration of the growing radical species is most probably kept so low as to diminish bimolecular radical termination and other side reactions. 2

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 1, 2015 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0760.ch012

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R-X

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Ru(Il)

CH =C j»2 R XRu(III) ^ -Ru(H) 2

#

Ru(H) ^=fe. R

i 7 R-CH -C-X 1 R

CH -C ^2 2

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R I CH -C 2

R

#

XRu(lll)

Ru(ll) (1)

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Active Species As evident from eq I, the dormant-active species equilibrium depends on the nature of halogens, monomers, and metal complexes along with other factors. This in turn means that the halogens and the metal complexes should be carefully selected for a particular monomer so as to affect the reversible homolytic cleavage of the dormant carbon-halogen bonds. The halogens thus far generally employed were chlorine and bromine, and on the other hand, most of the complexes were based on late transition metals (4). Quite recently, we have reported a Re0 I(PPh ) -catalyzed living radical polymerization o f styrene in conjunction with an iodo-initiator CH CH(Ph)I (26). This system is unique in that it involves rhenium, a group 7 metal that clearly differs 2

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198 from the group 8-10 counterparts. Specifically for the styrene polymerizations (26), for example, the Re(V) complex is more active and gives narrower MWDs of polystyrene than does RuCl (PPh ) under similar conditions. In this study, we employed Re0 I(PPh ) for possible living radical polymerization of methyl and ft-butyl acrylates (MA and BA, respectively) as well as methyl methacrylate (MMA), in conjunction with an alkyl iodide or bromide as an initiator (1-5; Scheme 1). Specifically, the iodide (1 or 2)/Re0 I(PPh ) initiating system proved effective in living radical polymerization of acrylates in the presence of Al(0/-Pr) . 2

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RH

v

R

CH =CH C0 R'

CH -. ) C0 R' l-Re R C h^-CH "f Re CO;R' 2

Re

R

:Me>/

2

Bu

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N

-

v

Re7AI(0*-Pr)

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Re

R " ^ ^ ^ C H"~C H I io R'

_

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V = = S

• , CH -CH !-Re C0 R'

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Dormant Species

Active Species

CH -CH CH - 90%

Conversion, %

MW(PMMA)

Figure 3. M , MJM and SEC curves ofpoly (MA) obtained with R- l/Re0 l(PPh ) / Al(Oi~Pr) in toluene at 80 °C: [M] = 2.0 M; [R-I] = 20 mM; [ReO f(PPh )J = 10 mM; [Al(Oi-Pr)J = 40 mM. (0) 1; (O) 2; (A) 3; (D) 4; (M) 5 (see Scheme I). The diagonal dashed line indicates the calculated M„ assuming the formation of one living polymer per R-l molecule. n

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(i

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201 Living Radical Polymerization of //-Butyl Acrylate The rhenium(V) complex was also employed for B A polymerizations in conjunction with CH CH(Ph)I and Al(0/-Pr) in toluene at 80 °C. The polymerization occurred smoothly to reach 91% conversion in 20 h. As shown in Figure 4, the M increased in direct proportion to monomer conversion, similarly to those of poly(MA) obtained under similar conditions (cf. Figure 2), although the molecular weights were higher than the calculated values. The M W D s were relatively narrow throughout the polymerization and became narrower with conversion. Thus, the CH CH(Ph)I/Re0 I(PPh ) initiating system also induced living radical polymerization of B A . 3

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Conversion, %

MW(PMMA)

Figure 4. M„, MJM and SEC curves ofpoly(BA) obtained with CH CH(Ph)I/ Re0 /(PPh )/Al(Oi-Pr) in toluene at 80 °C: [M] = 2.0 M; [CH CH(Ph)I] = 20 mM; [ReO /(PPh )J = 10 mM; [Al(Oi-Pr)J = 40 mM. The diagonal dashed line indicates the calculated M„ assuming the formation of one living polymer per CH CH(Ph)I molecule. W

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Polymerization of Methyl Methaery late We then used the Re(V) complex for M M A polymerization in conjunction with C C l B r or CH CH(Ph)I in the presence of Al(0/-Pr) in toluene at 80 °C. The catalyst was also active for M M A to induce faster polymerizations than the nickel-based system under the same conditions (Figure 5). The molecular weights of the obtained polymers were higher than the calculated values and decreased with conversion. The M W D s were broad throughout the polymerizations with both initiators. Other rhenium complexes such as ReOCl (PPh ) and ReCl (PMe Ph) also resulted in uncontrolled polymerizations of M M A . 3

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Time, h

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\ 10

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Figure 5. Polymerization of MMA with R-X/Re0 l(PPh )^Al(Oi-Pr) in toluene at 80 °C: [MJ = 2.0 M; [R-X] = 20 mM; [Re0 l(PPh )J = 10 mM; [Al(Oi-Pr) ] = 40 mM R-X: (0) CH CH(Ph)I; (O) CCl Br. 2

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Quenching Experiments The effects of potential terminators [TEMPO (2,2,6,6-tetramethyl-lpiperidinoxyl), methanol, and water] were examined to clarify the nature of the Re(V)-mediated living polymerization of acrylates. Thus, the polymerization was first run without additives, where conversion reached 49% in 5.5 h and 71% in 17.5 h (open circles in Figure 6). At 49% conversion, the inhibitors (10 equiv each to the initiator) were added to the reaction mixtures. The polymerizations were not affected by methanol or water, proceeding without significant changes in rate, molecular weights, and M W D s relative to the additive-free system. In contrast, T E M P O quenched M A consumption almost completely, where the conversion remained unchanged (51%) in an additional 12 h. There were almost no effects on the molecular weights and MWDs, either. These may suggest that the polymerization proceeds via a radical mechanism.

Polymer Terminal Structure Figure 7 shows the ' H N M R spectrum o f a typical sample o f the poIy(MA) obtained with C H C H ( P h ) I / R e 0 I ( P P h ) / A I ( 0 / - P r ) 3 . It shows the characteristic signals of the initiator moiety; e.g., peaks c-e were attributed to the aromatic protons of the phenylethyl group at the a-end. The number-average degree of polymerization based on the absorptions of the a-end terminal and the main-chain methyl ester was 90 [DP (NMR) = (h + /z')/3e], which is higher than the calculated value [DP (calcd) = 71] based on the initial ratio of the monomer to the initiator and conversion by gas chromatography. The difference is apparently due to some loss of low molecular weight fractions in purification of the polymers by preparative S E C , as indicated by 3

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Figure 6. Effects of additives (200 mM) on polymerization of MA with CH CH(Ph)l/ Re0 I(PPh )/Al(Oi-Pr) in toluene at 80 °C: [M] = 2.0 M; [CH CH(Ph)IJ = 20 mM; [ReO I(PPh ) ] = 10 mM; [Al(Oi-Pr) J = 40 mM. Additives: (*) TEMPO; (V) MeOH; (A) H 0; (O) none. Each additive was added to the polymerization mixture when conversion reached 49% in 5.5 h. 3

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the increase in M (SEC) from 8100 to 8800. Another cause may be a low initiation efficiency of the iodide due to some side reactions. The iodo co-end terminal was seen at 4.3 ppm (peak /), which supports the proposed reversible activation of the C - I terminal derived from CH CH(Ph)I. However, the ratio of the co- to the cc-ends was 0.63, suggesting a partial loss of the living end during the polymerization or the work up process. Along with this, there appeared some olefinic protons around 6 ppm. These observations indicate that the C - I terminal is less stable than the corresponding C - B r or C - C l terminal. n

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Conclusion This study shows that the complex based on a group 7 metal is also effective in acrylates living radical polymerization with the use of an appropriate initiator. Specifically, the rhenium(V)-complex, Re0 I(PPh ) , induced living radical polymerization of acrylates specifically in conjunction with an iodide initiator such as CH CH(Ph)I in the presence of Al(0/-Pr) to give polymers with controlled molecular weights and MWDs, similarly to the polymerization of styrene (26). In contrast, the system is not effective in controlling methacrylate polymerization. This is probably due to that such an active C-halogen terminal as C - I is not suited for the polymerizations of et-methyl-substituted monomers. Re0 I(PPh ) shows a higher catalytic activity than NiBr (Pw-Bu ) but gives broader M W D s . 2

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204 a b ( f 9 \ f / CH —CH—(-CH —CII J CH —CH—1 C=0 3

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OCH

d%Ji

e

h

OCH

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h' c, d e


f

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3 T T

/

4.4,4.2

ppm

g

7

1 ^ j l 5

a, b, f,

3.5

AJLLI

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y

F/gire 7. //iVM/? spectrum ofpoly(MA) obtained with CH CH(Ph)l/Re0 /(PPh )/ 3

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Al(Oi-Pr) in toluene at 80 °C: [MJ„ = 2.0 M; [CH CH(Ph)I] = 20 mM; [ReO I(PPh ) ] = 10 mM; [Al(Oi-Pr) ] = 40 mM. M„(NMR) = 8000; M„(SEC) = 8800. 3

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Experimental Materials M A , B A , and M M A (ail Tokyo Kasei; purity >99%) was dried overnight over calcium chloride and distilled twice over calcium hydride under reduced pressure before use. Re0 I(PPh ) (Aldrich), ReOCl (PPh ) (Aldrich), ReCl (PMe Ph) (Aldrich, purity >97%), NiBr (P/7-Bu ) (Aldrich; purity >97%) and Al(OZ-Pr) (Aldrich; purity >99.99%) were used as received and handled in a glove box under a moisture- and oxygen-free nitrogen atmosphere ( H 0

Living Radical Polymerization of Acrylates with Rhenium(V)-Based

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