pH-Responsive Ruthenium-Based Olefin Metathesis Catalysts

Dec 21, 2010 - S.L.B. thanks the Trent Lott National Center for the Innovation Award. E.J.V. acknowledges Grant No. MRI-0618148 for crystallographic ...
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Organometallics 2011, 30, 199–203 DOI: 10.1021/om100633f

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pH-Responsive Ruthenium-Based Olefin Metathesis Catalysts: Controlled Ring-Opening Metathesis Polymerization in Alcoholic and Aqueous Media upon Acid Addition Miles A. Dunbar,† Shawna L. Balof,† Adam N. Roberts,† Edward J. Valente,‡ and Hans-J€ org Schanz*,† †

Department of Chemistry & Biochemistry, The University of Southern Mississippi, 118 College Drive, Hattiesburg, Mississippi 39406-5043, United States, and ‡Department of Chemistry, University of Portland, 5000 N. Willamette Blvd., Portland, Oregon 97203, United States Received June 30, 2010 Summary: The new hexacoordinate catalysts (PCy3)(DMAP)2Cl2RudCH(p-C 6H4)CH2NMe 2 (7) and (PCy3)(DMAP)2Cl2RudCH(p-C6H4)N(CH3)2 (8) have been synthesized via exchange of a PCy3 ligand for two DMAP ligands from their (PCy3)2Ru precursors 4 and 5 in a one-step reaction in high yield. The catalysts promoted controlled ROMP of two cationic exo-7-oxanorbornene derivatives under homogeneous conditions in various acidic protic media, including acidic aqueous media. Very low polydispersities were accomplished in TFE/ 0.1 M HClaq. (1:1 v/v) with PDIs as low as 1.05. Ru-based olefin metathesis has emerged as a powerful technology, providing synthetic tools to access organic1 and polymeric materials.2 The catalysts’ high tolerance toward functional groups, air, and moisture makes them attractive to be used in combination of a wide range of substrates and also in a wide range of solvents. Over recent years, a strong interest has been developing for the use of these Rubased catalysts in aqueous media,3 as a nonhazardous and commercially highly attractive solvent for organic

transformations,4 as well as for applications in biological media5 or in emulsion polymerizations.6 Ring-opening metathesis polymerization (ROMP) has been established as the most common metathesis polymerization technique. There is an increasing demand for (co)polymeric materials in specialty applications with highly advanced architectures and narrow molecular weight distributions.7 Living ring-opening metathesis polymerization (LROMP) has become a powerful tool for the polymer chemist, especially since the discovery of well-defined catalysts/initiators, due to the absence of side reactions such as termination and chain transfer.1b-f Whereas multiple synthetic strategies have been reported to generate Ru-based olefin metathesis catalysts with modified NHC ligands8,9 or alkylidene moieties10 for homogeneous applications in aqueous media, only very few examples exist for ROMP in homogeneous solution in protic media, providing materials with controlled molecular weights and low polydispersity indexes (PDI < 1.5).1b,11 In the late 1990s, Grubbs et al. reported catalysts 1, bearing two water-soluble phosphine ligands.12 These catalysts were capable of polymerizing water-soluble exo-(7-oxa)norbornene derivatives in acidic aqueous media to give polyionic materials with PDIs as low as 1.24.12a However, due to their cumbersome synthetic access and the costly nature of the water-soluble phosphine ligands, catalysts 1 have not been used beyond several “proof-of-concept studies”. Emrick et al. developed thirdgeneration Grubbs-type catalysts 2 bearing two pyridine ligands tethered to a hydrophilic polyethylene glycol (PEG)

(1) (a) Grubbs, R. H., Ed. Applications in Organic Synthesis. In Handbook of Metathesis; Wiley-VCH:, Weinheim, Germany, 2003, Vol. 2. (b) Bieniek, M.; Michrowska, A.; Usanov, D. L.; Grela, K. Chem. Eur. J. 2008, 14, 806–818. (c) Hoveyda, A. H.; Zhugralin, A. R. Nature 2007, 450, 243–251. (d) Clavier, H.; Grela, K.; Kirschning, A.; Mauduit, M.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 6786–6801. (e) Conrad, J. C.; Fogg, D. E. Curr. Org. Chem. 2006, 10, 185–202. (f) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 11360–11370. (g) Wright, D. L. Curr. Org. Chem. 1999, 3, 211–240. (2) (a) Grubbs, R. H., Ed. Applications in Polymer Synthesis. In Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3. (b) Bielawski, C. W.; Grubbs, R. H. Prog. Polym. Sci. 2007, 32, 1–29. (c) Frenzel, U.; Nuyken, O. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2895–2916. (d) Slugovc, C. Macromol. Rapid Commun. 2004, 25, 1283– 1297. (e) Scherman, O. A.; Rutenberg, I. M.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 8515–8522. (f) Bielawski, C. W.; Benitez, D.; Morita, T.; Grubbs, R. H. Macromolecules 2001, 34, 8610–8618. (g) Baughman, T. W.; Wagener, K. B. Adv. Polym. Sci. 2005, 176, 1–42. (h) Trimmel, G.; Riegler, S.; Fuchs, G.; Slugovc, C.; Stelzer, F. Adv. Polym. Sci. 2005, 176, 43–87. (i) Grubbs, R. H. Tetrahedron 2004, 60, 7117–7140. (3) (a) Burtscher, D.; Grela, K. Angew. Chem., Int. Ed. 2009, 48, 442– 454. (b) Zaman, S.; Curnow, O. J.; Abell, A. D. Aust. J. Chem. 2009, 62, 91– 100. (4) (a) Shaughnessy, K. H. Chem. Rev. 2009, 109, 643–710. (b) Mecking, S.; Held, A.; Bauers, F. M. Angew. Chem., Int. Ed. 2002, 41, 545–561. (c) Fuhrmann, H.; Dwars, T.; Oehme, G. Chem. Uns. Zeit 2003, 37, 40–50. (d) Pinault, N.; Bruce, D. W. Coord. Chem. Rev. 2003, 241, 1–25. (5) Binder, J. B.; Raines, R. T. Curr. Opin. Chem. Biol. 2008, 12, 767–773. (6) Claverie, J.; Viala, S.; Maurel, V.; Novat, C. Macromolecules 2001, 34, 382–388.

(7) (a) Webster, O. W. Science 1991, 251, 887–893. (b) Lowe, A. B.; McCormick, C. L. Prog. Polym. Sci. 2007, 32, 283–351. (c) Matyjaszewski, K.; Gnanou, Y., Leibler, L. Eds. Macromolecular Engineering. Precise Synthesis, Materials Properties, Applications; Wiley-VCH: Weinheim, Germany, 2007. (8) (a) Gallivan, J. P.; Jordan, J. P.; Grubbs, R. H. Tetrahedron Lett. 2005, 46, 2577–2580. (b) Hong, S. H.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 3508–3509. (c) Jordan, J. P.; Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 46, 5152–5155. (9) (a) Balof, S. L.; P’Pool, S. J.; Berger, N. J.; Valente, E. J.; Shiller, A. M.; Schanz, H.-J. Dalton Trans. 2008, 5791–5799. (b) Balof, S. L.; Yu, B.; Lowe, A. B.; Ling, Y.; Zhang, Y.; Schanz, H.-J. Eur. J. Inorg. Chem. 2009, 1717–1722. (10) Gulajski, L.; Michrowska, A.; Naroznik, J.; Kaczmarska, Z.; Rupnicki, L.; Grela, K. ChemSusChem 2008, 1, 1–8. (11) Matyjaszewski, K. Macromolecules 1993, 26, 1787–1788. (12) (a) Lynn, D. M.; Mohr, B.; Grubbs, R. H.; Henling, L. M.; Day, M. W. J. Am. Chem. Soc. 2000, 122, 6601–6609. (b) Lynn, D. M.; Mohr, B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 116, 1627–1628. (c) Mohr, B.; Lynn, D. M.; Grubbs, R. H. Organometallics 1996, 15, 4317–4325.

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chain.13 These catalysts were able to quantitatively polymerize a hydrophilic 7-oxanorbornene monomer in acidic and neutral aqueous solution, giving materials with PDIs as low as 1.3.

Figure 1. ORTEP diagram of (PCy3)(DMAP)2Cl2RudCH(pC6H4)CH2NMe2 (7). Scheme 1. Synthesis of Catalysts 7 and 8 and Formation of Their Respective Fast-Initiating Counterparts in Protic Media

We have recently reported the benzylidene-functionalized catalysts 3-5, which were used for controlled ROMP of a cationic 7-oxanorbornene derivative in alcoholic and mixed alcoholic/aqueous media.14 The catalyst solubility in protic media is based on the cationic PMe3þ group or pH-responsive NMe2 groups, which are converted into cationic NMe2Hþ groups in acidic media, tethered to the reactive alkylidene moiety. The catalyst can be obtained in a straightforward, one-step exchange reaction from commercially available Grubbs catalyst with the corresponding styrene derivative, which makes their syntheses highly attractive from a commercial standpoint. However, despite their cationic nature, the hydrophobicity of the two PCy3 ligands rendered these derivatives only very slightly soluble in acidic aqueous media in the absence of the alcoholic cosolvent and, thus, no ROMP conversion could be monitored for these reactions. We now wish to report the synthesis and application of two novel pH-responsive, benzylidene-functionalized olefin metathesis catalysts bearing two 4-(dimethylamino)pyridine (DMAP) ligands and their use for ROMP reactions with water-soluble monomers in acidic alcoholic and aqueous media. In order to accomplish well-controlled ROMP in aqueous media (PDIs < 1.1), catalysts 3-5 needed to be improved with respect to their solubility in aqueous media as well as their ratio of the rates of initiation and propagation. We recently demonstrated that the metathesis activity of hexacoordinate Ru-carbene species bearing two basic N-donor ligands can be significantly increased with an acid such as H3PO4.15,16 Furthermore, it was demonstrated that this external activation was mainly a result of significantly accelerated catalyst initiation.16 In particular, for the complex (13) Samanta, D.; Kratz, K.; Zhang, X.; Emrick, T. Macromolecules 2008, 41, 530–532. (14) Roberts, A. N.; Cochran, A. C.; Rankin, D. A.; Lowe, A. L.; Schanz, H.-J. Organometallics 2007, 26, 6515–6518. (15) P’Pool, S. J.; Schanz, H.-J. J. Am. Chem. Soc. 2007, 129, 14200– 14212. (16) Dunbar, M. A.; Balof, S. L.; LaBeaud, L. J.; Yu, B.; Lowe, A. B.; Valente, E. J.; Schanz, H.-J. Chem. Eur. J. 2009, 15, 12435–12446.

(PCy3)(DMAP)2Cl2RudCHPh (6), which is ROMP-active but performed very sluggishly under nonacidic conditions, this increase in the initiation rates resulted in fast and wellcontrolled ROMP reactions under acidic conditions, and the PDIs of the resulting polymers were as low as 1.04. With these effects of acid on catalysts 4-6 in mind, we designed complexes 7 and 8, which were obtained from catalysts 4 and 5 via substitution of one PCy3 ligand versus two DMAP ligands. Their syntheses were conducted as described before for complex 6,15 and the hexacoordinate complexes could be filtered from n-heptane in near-quantitative yields (Scheme 1). Our hypothesis was that, by substituting one hydrophobic PCy3 ligand versus two basic N-donor ligands, we could accomplish both the improvement of the catalyst solubility in aqueous media and the acceleration of the initiation rates upon exhaustive ligand protonation with acid. This should result in the rapid formation of cationic, fast-initiating catalysts 70 and 80 (Scheme 1), which are likely to be stabilized via an O-donor molecule such as water, as postulated previously.12a,15 Both complexes were characterized via multinuclear NMR spectroscopy and X-ray crystallography (Figures 1 and 2). Structurally, the Ru centers in hexacoordinate complexes 7 and 8 are coordinated in a pseudo-octahedral fashion with trans chlorides and cis DMAP ligands. Unsurprisingly, the

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Scheme 2. ROMP of Monomers 9 and 10

Figure 2. ORTEP diagram of (PCy3)(DMAP)2Cl2RudCH(p-C6H4)NMe2 (8). Table 1. Selected Bond Lengths (A˚) and Angles (deg) for Complexes 7 and 8 7 RudC Ru-N

1.844(3) 2.176(2) 2.333(2) C-Ru-P 92.13(8) C-Ru-N 172.19(9) 89.07(10) C-Ru-Cl 95.93(8) 87.66(8)

8 1.890(3) 2.157(3) 2.338(3) 88.40(10) 176.42(13) 93.85(12) 95.32(11) 87.22(10)

7 Ru-P Ru-Cl

2.3823(8) 2.3929(8) 2.4040(8) Cl-Ru-Cl 174.89(3) P-Ru-N 178.59(7) 94.43(6) P-Ru-Cl 93.88(3) 89.62(3)

8 2.3759(8) 2.4005(8) 2.4428(8) 169.11(3) 177.63(8) 90.42(7) 96.51(3) 84.15(3)

comparable ligand bond distances to the Ru center are similar in both complexes with deviations of less than 0.02 A˚, with the exception of the RudC bond, which is longer by almost 0.05 A˚ in complex 8. This is likely due to the π-donating character of the aryl-bound NMe2 group, which slightly reduces the bonding order of the RudC double bond. Also, one Ru-Cl bond is longer in complex 8 by approximately 0.04 A˚ than in complex 7. Table 1 summarizes selected bond distances and angles for complexes 7 and 8. Catalysts 7 and 8 were employed in ROMP reactions with exo-7-oxanorbornene derivatives 9 and 10 in protic media containing excess HCl as acid (Scheme 2). Monomer 9 is a quaternary ammonium salt and possesses a bromide counteranion. It has been established that the presence of bromide anions accelerates the initiation while simultaneously decelerating the propagation of the metathesis reaction with Rubased catalysts on the basis of the halide exchange at the metal center.17,18 This has been demonstrated to be beneficial for the control of the ROMP reaction.17b Monomer 10 was used as its hydrochloride salt. Three different solvent conditions were chosen: 2,2,2-trifluoroethanol (TFE)/1 M HCl (98/2 v/v), TFE/0.1 M HCl (50/50 v/v), and 0.1 M HCl. We chose TFE as the alcoholic (co)solvent because ROMP reactions in TFE-containing media have been demonstrated to be living and to proceed at fast overall rates for various (PCy3)2Ru-alkylidene catalyst systems.14,17 First, the kinetics of the conversions were determined. Under controlled (17) (a) Rankin, D.; P’Pool, S. J.; Schanz, H.-J.; Lowe, A. B. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 2113–2128. (b) Rankin, D.; Schanz, H.-J.; Lowe, A. B. Macromol. Chem. Phys. 2007, 208, 2389–2395. (18) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997, 119, 3887–3897.

living polymerization conditions, the plot of ln[1/(1 - x)] versus time is linear, following pseudo-first-order kinetics. Similar to the conversions with catalysts 4 and 5, the reactions proceeded at faster rates with lower water content of the solvent. A possible explanation is that water exhibits a slightly inhibiting effect, due to coordination to the reactive site being competitive with that of the substrate. These watercoordinated Ru-alkylidene species have been postulated previously.12a Such inhibition by a competing ligand reduces the propagation rates, and this may be beneficial for the molecular weight control.19 Thus, with increasing amounts of water a slower ROMP conversion is expected. Additionally, it was observed that the catalyst did not completely dissolve at the beginning of the ROMP reaction. In fact, a solid precipitate was noticed throughout the reaction. Hence, it cannot be said with certainty that a complete initiation of the catalyst was accomplished. This may also be a contributing factor to the slow observed conversion rates. However, the ROMP reaction achieved conversion between 25% and 79% (monomer 10 with catalyst 7) in those experiments, and this is a clear improvement compared to the (PCy3)2Ru catalysts 3-5, previously reported by our group which have not been able to afford any conversion under purely aqueous conditions.14 On the other hand, the studied aqueous ROMP reactions with catalysts 7 and 8 also reached a conversion plateau, very likely due to catalyst decomposition. Therefore, none of these reactions followed first -order kinetics. On comparison of the two catalysts in TFE and TFE/HCl (1/1 v/v), all ROMP reactions with catalyst 7 proceeded at faster rates than with catalyst 8 under identical conditions. For monomer 9 (Figure 3), only catalyst 7 when used in mostly alcoholic or alcoholic/aqueous media exhibited pseudo-first-order kinetics, with conversions >95%. In TFE/ HCl (1:1 v/v), the conversion rate is 30% slower than in TFE. The ROMP reactions with catalyst 8, in contrast, exhibit overall slower conversions and significant reaction deceleration, very likely due to catalyst decomposition after approximately 60 min. Both reactions exhibit first-order kinetics for the first 60 min, and the rate is faster in TFE by 27%. In comparison to catalyst 7, the conversions in this time period are 51% (TFE) and 43% (TFE/HCl (1/1 v/v)) slower under the same conditions with catalyst 8. The observed catalyst decomposition is likely a result of the presence of the bromide counteranion. The bromide causes accelerated catalyst initiation rates and, thus, accelerated degradation rates. (19) Bielawski, C. W.; Grubbs, R. H. Macromolecules 2001, 34, 8838– 8840.

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Dunbar et al. Table 2. Kinetic Data for ROMP of Monomers 9 and 10 with Catalysts 7 and 8 ([Ru] = 2.0 mM, 2% Catalyst Loading) in Acidic Protic Media entry

cat.

monomer

conversn, %(60 min)

krel

Solvent TFE/1 M HClaq (98/2 v/v) 1 2 3 4

7 8 7 8

96.3 78.9 82.1 79.2

9 9 10 10

1 ((0.012) 0.506 ((0.005)a 0.542 ((0.002) 0.481 ((0.015)a

Solvent TFE/0.1 M HClaq (50/50 v/v) 5 6 7 8

7 8 7 8

89.9 71.0 79.2 67.4

9 9 10 10

0.700 ((0.040) 0.400 ((0.005)a 0.489 ((0.003) 0.340 ((0.010)

Solvent 0.1 M HClaq 9 10 11 12 a

7 8 7 8

63.2 24.3 72.5 47.4

9 9 10 10

Relative rate determined for 0-60 min (first-order kinetics).

Table 3. ASEC Analysis of Polymers 11 and 12 Obtained with Catalysts 7 and 8 ([Ru] = 2.0 mM, 2% Catalyst Loading) in Acidic Protic Media after 60 min Reaction Time entry cat. monomer conversn, % 10-4Mntheory 10-4Mnexptl PDI

Figure 3. Conversion plots ln[1/(1 - x)] vs time for ROMP of monomers 9 and 10 with catalysts 7 and 8 ([Ru] = 2.0 mM, 2% catalyst loading) under various protic solvent conditions (TFE/ 1 M HCl (98/2 v/v) [TFE], TFE/0.1 M HCl (50/50 v/v) [TFE/ HCl], and 0.1 M HCl [HCl]).

For monomer 10 (Figure 4) in the absence of bromide anions, catalyst degradation is much less pronounced. With the exception of the aqueous reactions, the other solvent conditions provide first-order kinetics and, only for catalyst 8 in TFE, the reaction decelerates after 60 min (79% conversion). Catalyst 7 still was more active than catalyst 8 (13% and 44% faster under the solvent conditions), and reactions in TFE proceeded at slightly higher rates than in TFE/HCl (1/1 v/v) (7, 11% faster; 8, 41% faster). Table 2 summarizes the results of the kinetic investigations. Polymers 11 and 12 were synthesized under the various solvent conditions with catalysts 7 and 8, quenched with ethyl vinyl ether (EVE), and isolated. Due to the low catalyst solubility in 0.1 M HCl, the aqueous reactions were conducted under sonication at room temperature for 90 min in order to facilitate the solvating process of the catalyst. The reactions conducted with TFE as (co)solvent generated a homogeneous solution and were run for 60 min and quenched with EVE before significant catalyst decomposition occurred according to the kinetic investigations. An aliquot of the polymer samples was used to determine the conversion via 1H NMR spectroscopy. With sonication, the ROMP conversions in 0.1 M HCl could be increased to 51-85% for these polymerizations. The isolated polymers were investigated via aqueous size exclusion chromatographymulti angle laser light scattering (ASEC-MALLS), and the molecular weights (Mn) and polydispersity indexes (PDI = Mw/Mn) were determined. In fact, all determined molecular

Solvent TFE/0.1 M HCl (98/2 v/v) 1 2

7 8

97.1 84.4

9 9

1.80

2.82 4.01

1.17 1.17

2.95 3.99 2.41 2.92 2.10 3.02

1.25 1.11 1.05 1.31 1.20 1.08

7.63 8.91 6.82 4.96

1.24 1.40 1.41 1.36

Solvent TFE/1 M HClaq (50/50 v/v) 3 4 5 6 7 8

4 7 8 4 7 8

62.2 94.1 73.4 86.0 79.9 76.1

9 9 9 10 10 10

1.80 1.59

Solvent 0.1 M HClaq a

9 10a 11a 12a a

7 8 7 8

9 9 10 10

78.9 51.6 85.0 69.6

1.80 1.59

90 min reaction time.

weights were higher than the theoretical values of 1.80  104 (11) and 1.36  10 4 (12). This indicates that the amount of active catalyst was lower than that in theory. A possible explanation would be partial degradation of the species 70 and 80 in the acidic solution prior to the monomer addition or during the polymerization. The catalysts were agitated in the acidic medium for 30-60 s to generate a homogeneous solution or a solution with the highest possible amount of dissolved catalyst. However, several reactions with catalysts 7 and 8 in the mixed solvent TFE/HCl (1/1 v/v) resulted in Mn values only slightly larger than the theoretical values (entries 4, 5, 7, and 8, Table 2). Generally, these solvent conditions provide the highest molecular weight control. By comparison, catalyst 4, which was demonstrated to exhibit a high initiation efficiency with monomer 11 under these solvent conditions,14 also produced polymers with molecular weights higher than the theoretical values

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(entries 3 and 6, Table 2). Hence, catalysts 7 and 8 performed at a superior level. The highest Mn values were consistently observed for polymers obtained in 0.1 M HCl, very likely as a result of the combined slow catalyst initiation due to low solubility and significant catalyst decomposition. These polymers also exhibited the largest PDI’s (1.36-1.41). These values are surprisingly low and suggest that the polymers were still produced in a somewhat controlled manner,11 in spite of the reactions not exhibiting strict pseudo-first-order kinetics. With respect to the molecular weight distributions, the lowest PDI values were unsurprisingly observed for the reactions conducted in TFE/HCl (1/1 v/v). Most notably, catalyst 8 afforded polymer 11 with Mw/Mn = 1.05, the lowest PDI ever accomplished for a ROMP reaction conducted in a water-containing medium, whereas the PDI is slightly higher for the polymer obtained in TFE with Mw/Mn = 1.17, which is the overall faster ROMP reaction. The aqueous/alcoholic solvent mixture provided the optimal ratio between the rates of catalyst initiation and propagation and therefore the best control over the ROMP reaction in this series. As expected, the nature of the catalyst exhibited only a minor influence on the PDI. Also, polymer 12 was obtained with slightly higher PDIs than monomer 11 under the same conditions, very likely due to the absence of the bromide anion. However, catalysts 7 and 8 consistently provided polymers with PDI e 1.2 in TFE/HCl (1/1 v/v), which is very low in comparison to other reported PDI values for a ROMP reaction in a water-containing medium.12a,13 By comparison, the polymers obtained with catalyst 4 under these conditions exhibited PDIs of 1.25 and 1.31, which are clearly larger than those obtained with catalysts 7 and 8. Table 3 summarizes the properties of polymers 11 and 12. In conclusion, the pH-responsive (PCy3)(DMAP)2Cl2Ru complexes 7 and 8, bearing NMe2-functionalized benzylidene moieties, represent the first complexes to accomplish well-controlled ROMP in a water-containing medium, more specifically in TFE/0.1 M HCl (1/1 v/v), with PDIs as low as 1.05. Due to the protonation of two different basic moieties in the initial complexes with acid, the DMAP ligands and the NMe2 groups, the catalysts became soluble in protic media while simultaneously the initiation rates were dramatically

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increased, leading to favorable ratios of the rates of initiation and propagation. Polymerizations of bromide-containing monomer 10 are generally faster and produce materials with PDIs narrower than those of monomer 11; however, for slow monomer conversions noticeable catalyst degradation was also observed. In purely acidic aqueous media both catalysts exhibit limited solubility, slow overall reaction rates, and significant catalyst decomposition. As a consequence, the ROMP reactions do not follow pseudo-first-order kinetics; however, the PDIs of the resulting materials, albeit somewhat larger than those obtained with TFE as (co)solvent, are still within the range for them to be considered controlled polymerizations.

Acknowledgment. This work was supported by the University of Southern Mississippi (Dean Research Initiative and Aubrey Keith Lucas and Ella Ginn Lucas Endowment for Faculty Excellence Award for H.-J.S.) and by the NSF Material Science Research and Engineering Center (MRSEC) for Response Driven Films (Grant No. DMR-0213883, stipend for A.N.R.). S.L.B. thanks the Trent Lott National Center for the Innovation Award. E.J.V. acknowledges Grant No. MRI-0618148 for crystallographic resources. We also thank Dr. Adam E. Smith, Mr. Chris Holley, and Mr. Joel Flores from the McCormick research group at USM for conducting the ASEC-MALLS experiments and for helpful discussions of the results. H.-J.S. also acknowledges Dr. Ann Conolly (University College Dublin) for conducting the elemental microanalyses. Supporting Information Available: Text, figures, and tables giving experimental details, 1H, 13C, and 31P NMR spectra of the synthesized Ru complexes 7 and 8, and kinetic experimental data and CIF files giving crystallographic data for 7 and 8. This material is available free of charge via the Internet at http:// pubs.acs.org. Crystallographic data for structures 7 and 8 has also been deposited with the Cambridge Crystallographic Data Centre (CCDC 780426 and 780427, respectively). Copies of the data can be obtained, free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, United Kingdom (fax, 44-1223-336033; e-mail, [email protected]).