Ruthenium Carborane Complexes in Atom Transfer Radical

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Chapter 8

Ruthenium Carborane Complexes in Atom Transfer Radical Polymerization Dmitry F. Grishin, Ivan D. Grishin and Igor T.Chizhevsky Research Institute of Chemistry, Nizhny Novgorod State University, Nizhny Novgorod, Russia A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia

Novel ruthenium complexes with carborane ligands were employed as efficient catalysts for controlled polymer synthesis via Atom Transfer Radical Polymerization (ATRP) mechanism. The ability of carborane ligands to stabilize high oxidation states of transition metals allows the proposed catalysts to be more active than their cyclopentadienyl counterparts. The proposed catalysts do not require additives such as aluminium alkoxides. It was shown that introduction of amine additives into the polymerization mixture leads to a dramatic increase of polymerization rate leaving polymerization controlled. The living nature of polymerization was proved via post-polymerization and synthesis of block copolymers.

Introduction Ruthenium complexes were among the first catalysts employed for controlled radical polymerization via ATRP mechanism (1, 2). One the one hand, this happened due to the fact that ATRP process originates from Kharash reaction of radical addition (3), which is catalyzed by complexes of this metal. On the other hand, this was associated with the unical properties of ruthenium atom, particularly the ability to assume different oxidation states and various coordination geometries (4-6).

© 2009 American Chemical Society

115

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, Krzysztof; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

116 Cyclopentadienyl derivatives of ruthenium were first complexes of this metal which were found to be able to catalyze ATRP (1, 7-10). Subsequently carborane (11-12) and alkylidene (13) ruthenium complexes were employed as ATRP catalysts. In our work we proposed to use some novel exo-nido- (1, 3, 7) and closoruthenacarboranes (2, 4, 5, 6) as catalysts for atom transfer radical polymerization (Figure 1).


Ph3P Ru

Ph 3P

H

H

Ph3P

PPh 3 Ru Cl

H

H H

Cl

PPh 2

(CH 2)4

Ph2P

Ru

CH 3 CH 3

H H

Cl

1

H

H

2

(CH2)4

3 (CH2)3

(CH2)2

Ph2P PPh2 H Ru Cl

Ph2P

PPh2 Ru Cl

PhP

PPh2 Ru Cl CH3 CH3

4

6

5

+

Ph3P Ph3P Cl

H Ru

H H

H

CH2

CH3 CH3

Ph2P Cl Ru

H

PPh2 PPh2

Ph2P

CH2 CH2

7

CH2

8

Figure 1. Structures of the examined ruthenium carborane complexes.

Experimental Materials Ruthenium carborane complexes 1-8, were synthesized under argon using anhydrous solvents, according to procedures described in the literature (14-16). Commercial phosphines and diphosphines (Strem Chemicals) were used. The obtained products were isolated and purified by column chromatography using silica gel Merck (230–400 mesh). MMA and styrene were dried over calcium chloride and distilled under reduced pressure before use. Amines were dried and distilled over potassium hydroxide. Other solvents were purified using standard procedures (17).


117


Polymerizations Polymerizations were carried out in glass tubes under residual pressure of monomer (1.3 Pa). The monomer and exact portions of the initiator and the catalyst were placed in glass tubes, and degassed through three freeze–pump– thaw cycles in liquid nitrogen. The polymerization kinetics was monitored under isothermal conditions by the weighing method. The tube was placed in a thermostat for a strictly specified period of time. After that, the tube was taken out and frozen in liquid nitrogen to stop polymerization. The resulting polymer was dissolved into chloroform and precipitated into petroleum ether. Samples were twice reprecipitated to be cleaned of the residual monomer, initiator, and catalysts and dried in vacuum to a constant weight. After that the degree of conversion was calculated and MWDs were measured. Instrumentation The MWDs of the obtained samples were determined by SEC on a Knauer setup with a linear column (Phenomenex, Linear 2). A RI Detector K-2301 differential refractometer and UV Detector K-2501 were used as the detectors for PMMA and polystyrene samples respectively. Chloroform was used as the eluent. The columns were calibrated using standard PMMA and polystyrene samples, obtained from Waters. The ChromGate software was implemented for interpretation of SEC data. MALDI-TOF spectra of the samples were recorded via Brucker Microflex apparatus. Samples were prepared by mixing equal volumes of polymer solution in a concentration of 10g/l in THF and matrix DCTB in a concentration of 20g/l in acetone. LiTFA was added to the mixture. 1 μl of the mixture was applied onto a stainless steel target plate.

Results and Discussion Polymerization of Methyl Methacrylate and Styrene Polymerization of methyl methacrylate (MMA) and styrene in bulk via ruthenacarboranes and carbon tetrachloride was investigated first. It was found that proposed ruthenium complexes were able to efficiently catalyze polymerization of MMA in conjunction with carbon tetrachloride as an initiator. Moreover, as follows from the data obtained (Table I), high conversion of the monomer is achieved in a number of cases. The structure of ruthenium carborane complexes (closo or exo-nido) has a substantial effect on the kinetic parameters of MMA and styrene polymerization, as well as on the molecularweight characteristics of synthesized polymers.


118 Table I. MMA and styrene polymerization in the presence of ruthenium carborane complexes (0.125 mol %) and carbon tetrachloride (0.25 mol %)


Catalyst 1 2 3 4 5 6 7 8

Time, h 45 45 9 110 200 110 80 110

ММА, Т=800С conv, Mn % 94 53500 88 36000 74 105000 83 27000 39 18000 70 42000 98 52000 71 30000

Mw/Mn 1.6 1.6 2.7 1.2 1.2 1.4 1.9 1.5

Styrene, Т=900С Time, conv, Mn Mw/Mn h % 200 98 34000 1.6 100 89 32000 1.5 120 99 31500 1.4 70 89 21000 1.6 70 91 20000 1.6 200 68 17000 1.4 100 88 44000 1.7 100 96 40000 1.4

It should be mentioned that the highest conversions of MMA, namely 94 and 98% are observed for diamagnetic exo-nido complexes 1 and 7 with PPh3 ligands at the metal center. In the presence of exo-nido–ruthenacarborane with the diphosphine ligand MMA conversion of 74% is reached after less than 9 h. It should also be noted that the polymers synthesized in the presence of exo-nido complexes have relatively high Mn and Mw, (Table I). For PMMA synthesis, the maximum molecular weights are observed in the presence of complex 3. At the same time, the polydispersity of the samples obtained using the above complex is rather high (above 2.6). When exo-nido complexes 1 and 7 are employed, the polydispersity indexes are somewhat lower (Table I); however, they are rather high for polymerization processes proceeding in the living-chain mode. Thus, the obtained data suggests that the polymerization of methyl methacrylate in the presence of ruthenium exo-nido complexes is not controllable to a high extent. The use of ruthenium closo complexes leads to polymers with markedly narrower MWDs (Table I). In particular, in the presence of ruthenium carborane complexes 4–6, the Mw/Mn parameter does not exceed 1.4. In the case of compounds 4 and 5, narrow-dispersed polymers are synthesized with Mw/Mn values of 1.23–1.25. Note that MW of the samples synthesized using closo complexes, are noticeably lower as compared to the polymers obtained in the presence of either the exo-nido complexes or conventional radical initiators (such as AIBN and peroxides) where the molecular weights of polyacrylates range from several hundred thousands to several millions. Since ruthenacarboranes with the closo structure are more promising for the synthesis of narrow-dispersed polymers, we studied the kinetics of MMA polymerization in the presence of the above mentioned catalysts and analyzed the molecular-weight characteristics of the polymers in more detail, using complex 4 as an example. The polymerization was found to proceed smoothly to high conversions. The time dependence of logarithmic initial-to-current monomer concentration ratio ln(m0/m) is linear (Figure 2, curve 1), thus indicating the absence of chain termination processes, as case inherent in polymerization proceeding in the living mode. MW of the obtained polymers increases linearly with the conversion (Figure 2). The polydispersity indexes somewhat decrease with the conversion, a fact that is also typical of controlled radical polymerization. GPC



119 traces of the obtained polymers are unimodal, and the peak gradually shifts toward higher molecular weights with conversion. There is practically no difference in the polydispersity between PMMA samples prepared in the presence of the diamagnetic (18-electron) and paramagnetic (17-electron) closo complexes 4 and 5, respectively. On the contrary, steric factors have a quite noticeable effect on the chain propagation step and the macromolecular characteristics of the samples. Thus, if the phosphine groups at the metal center are linked via methylene bridges (complexes 4 and 5), favorable steric conditions for controlling the polymer chain growth probably occur that lead to the formation of polymers with MWDs much narrow than in the case of complex 2. ln(m0/m)

2

1,8

Mn*10-3

a

Mw/Mn

b

30

1

2

1

1,2

20

0,6

10

1,8 1,6

2

1,4 1,2

0

1

0 0

10

30

50 Time, h

0

20

40

60

80

100

P,%

Figure 2. (a) Time dependence of ln(m0/m) for MMA polymerization without amine additive (1) and in the presence of tert-butylamine (2); (b) dependence of Mn (1) and the polydispersity index (2) on the conversion for polyMMA samples synthesized in the presence of complex 4 and CCl4 at 80 °C. Fairly close polydispersity values of polymer samples prepared in the presence of complexes 1 and 2 having different structures may be explained by the transformation of exo-nido complex 1 into its closo isomer 2, which is known to proceed in high yield at 80 °C (14). This transformation does not occur for the more sterically hindered C,C-dimethyl-substituted exo-nido complex 7, and, in this case, a polymer with a higher polydispersity index (Mw/Mn = 1.93) is formed. This finding indicates that the propagation step and the molecular-weight characteristics of PMMA are affected by both steric and structural features of the ruthenacarborane catalysts. The stereoregularity of the obtained PMMA samples was investigated via NMR analysis. The obtained data showed that polymers are predominantly syndiotactic (rr:rm:mm=58:37:6), as in case of free radical polymerization of MMA initiated by AIBN (1). The MALDI-TOF-MS analysis of the polyMMA, obtained in linear mode shows only one series of peaks, whose interval was regular, ca. 100, the molar mass of MMA unit. It indicates the absence of irreversible chain termination processes via recombination or disproportionation. According the proposed mechanism of polymerization the absolute masses of the peaks should be equal



120 the polyMMA with a CCl3 group at the α-end, a chlorine atom at ω-end and a lithium ion from salt used for MS analysis. So, for example for macromolecule of 84 monomer units it should be 35.5*3 + 12 + 84*100 + 35.5 + 7 = 8561 But the obtained data differ from calculated ones. The spectrum of the sample is shown on the Figure 3. There is only a peak with m/z=8517. The difference between predicted and obtained values is 44. This value is very close to 42.5, which equals the mass of LiCl. So the obtained data indicate that a halogen loss occurs during laser irradiation, as it is often observed for polymers, synthesized via ATRP (18). The presence of a CCl3 group at the α-end allows to suppose that carbon tetrachloride acts as a monofunctional initiator when it is used for MMA polymerization.

Figure 3. The MALDI-TOF mass spectrum of the obtained polyMMA For styrene polymerization in the presence of all the complexes tested, the highest polymer molecular weights are observed in the case of exo-nidoruthenacarboranes (Table I). The same compounds are more promising for the attainment of maximum monomer conversion. In the presence of compounds 1 and 3, polymerization at 90 °C proceeds to a conversion close to 100% and the synthesized samples are characterized by lower polydispersity indexes (well below two) than, for instance, PMMA samples prepared in the presence of the same complexes. Polystyrene samples synthesized in the presence of closo complexes 2 and 4-6 have somewhat higher polydispersity indexes (1.37-1.62) than the PMMA


121 samples synthesized via the same catalysts. However, the above values are lower than those observed in the presence of closo-ruthenacarboranes with phosphorus- or sulfur-containing cage substituents (11).


Post-polymerization and block-copolymerization One of the main inherent advantages of controlled radical polymerization is the possibility of obtaining block-copolymers. Our experiments showed that polymers obtained in the presence of ruthenacarborane 4 after purification from monomer, catalyst and initiator could be used as macroinitiator for the synthesis of block-copolymers. Figure 4 illustrates the shift of molecular weight distribution of the macroinitiator after addition of new portions of monomer and catalysts and reinitiation of polymerization. Virtually all of the chains were extended, corroborating that there is no significant amount of “dead” chains, incapable for propagation. MWD of the obtained PMMA-block-polystyrene is slightly broader than that of the initial macroinitiator. However, such situation usually exists in case of extending a living chain with another monomer. Actually, when the same macroinitiator was used in post-polymerization of MMA, MWD of the obtained polymer became narrower than that of macroinitiator. (Figure 4). Shift of GPC trace into area of high molecular weights indicates the controlled character of polymerization of MMA in the presence of ruthenacarboranes.

2

1

4

5 log Mn

3

6

Figure 4. GPC traces of initial macroinitiator (1, Mn=18 200, Mw/Mn= 1.33), PMMA-block-polystyrene (2, Mn=85 000, Mw/Mn=1.57) and post-PMMA, (3, Mn= 207 000, Mw/Mn= 1.29) According to our experimental data and recently published articles on living radical polymerization we propose the following mechanism for the polymerization of MMA and styrene in the presence of ruthenacarboranes:


122 Run+1Clm+1Lp

RunClmLp CCl4

CCl3 CH2 CX Y Run+1Clm+1Lp

RunClmLp Cl3C CH2 CX Cl Y

Cl3C CH2 CX Y


CH2 CX Y Run+1Clm+1Lp

RunClmLp Cl3C

CH2 CX Cl Y

Cl3C

CH2 CX Y

where X – H, CH3; Y – Ph, COOCH3; -dicarbollide ligand; - propagating chain.

Increase of polymerization rate caused by amine additives The main disadvantage of controlled radical polymerization is its low rate, being at least two orders of magnitude lower than that of the conventional one. Recently it was shown that the rate of ATRP catalyzed by ruthenium cyclopentadienyl or alkylidene complexes may be increased by introduction of amines (19, 20). Our investigations show that amine additives significantly accelerate controlled polymerization of MMA catalyzed by ruthenacarboranes. In particular, both tert-butylamine and triethylamine additions were found to result in dramatic (up to two orders of magnitude) rate increase of this process. The increase of the reaction rate was observed in case of both closo and exo-nido catalytic ruthenium systems. When tert-butylamine was used as an additive, MMA was smoothly polymerized up to 99% in less than 2.5 hours (Table II). It was shown that the molecular weight of the polymer increased with monomer conversion (Figure 5), and the MWDs became narrower with the conversion. To prove the controlled nature of the polymerization of MMA in the presence of tert-butylamine, a post-polymerization was carried out. Fresh feeds of monomer, amine and ruthenium complex were added to the preliminary washed and dried polymer. A new portion of monomer polymerized smoothly in 5 hours. Use of amine additives along with exo-nido-ruthenacarboranes resulted not only in increase of polymerization rate, but also led to narrowing of MWDs. Polymerization of MMA via complexes 3 and 7 as catalysts without amine


123


additives had poor control over molecular weights and produced the polymers with a broad polydispersity. Amine addition turned out as a way of improving control over polymerization process. Narrowing of MWDs was accompanied by the decrease of molecular weights. So, using amines in conjunction with exonido-ruthenacarboranes is a convenient way for increasing their regulating ability in ATRP. 2 1 3

3

4 log Mn

5

Figure 5. GPC traces of PMMA, synthesized in the presence of CCl4, complex 4 and tert-butylamine. Conversion, %: 29 (1), 49 (2), 88 (3); Mn = 10100 (1), 13100 (2), 18100 (3); Mw/Mn = 1.4 (1), 1.4 (2), 1.3 (3). Amines introduction into the polymerization mixture led also to a slight increase of polydispersity indexes, when closo-ruthenacarboranes were used as catalysts. Polydispersity of the formed polymers remained narrow (

Ruthenium Carborane Complexes in Atom Transfer Radical

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