Ring-Opening Metathesis Polymerization of a Naturally Derived

Apr 30, 2013 - Sanford , M. S.; Love , J. A.; Grubbs , R. H. J. Am. Chem. Soc. 2001 ... Love , J. A.; Morgan , J. P.; Trnka , T. M.; Grubbs , R. H. An...
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Ring-Opening Metathesis Polymerization of a Naturally Derived Macrocyclic Glycolipid Yifeng Peng,†,‡ John Decatur,§ Michael A. R. Meier,*,‡ and Richard A. Gross*,† †

Center for Biocatalysis and Bioprocessing of Macromolecules, Polytechnic Institute of NYU, Six Metrotech Center, Brooklyn, New York 11201, United States ‡ Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, Building 30.42, 76131 Karlsruhe, Germany § Department of Chemistry, NMR Center, Columbia University, 3000 Broadway Mailcode 3179 New York, New York 10027, United States S Supporting Information *

ABSTRACT: Lactonic sophorolipid (LSL) is a naturally occurring macrocyclic monomer that undergoes ring-opening metathesis polymerization (ROMP) via an entropy-driven mechanism (ED-ROMP). Typically, gel permeation chromatographic analysis of poly(LSL) showed products consist of about 70% polymer with Mn up to about 180K (Mw/Mn 1.6−1.8) coexisting with 10% of oligomer and 20% monomer. Detailed kinetic studies for LSL ROMP were performed using two classic metathesis catalysts (i.e., G2 and G3). G2 exhibited apparent first-order propagation, although its slow initiation caused subsequent events of secondary metathesis that decreased molecular weight. An induction period observed for G2 at 33 and 45 °C largely disappears at 60 °C with an increase in the apparent rate constant (kapp p ) of 11 times. G3 gave fast initiation even at 33 °C while plots of ln{[M]0/[M]t} versus reaction time for G3 show that kp continuously decreased, implying a decline in G3 catalytic activity. Plots of ln{[M]0/[M]t} versus reaction time for G2 are linear, suggesting apparent first-order kinetic behavior. From analysis of an Arrhenius plot for G2-catalyzed LSL polymerization in THF, the activation energy (Ea) of propagation is 18 ± 3 kcal/mol. By keeping [LSL] constant at 0.54 M, G2-catalyzed LSL ED-ROMP (60 °C, THF) gave a plot of Mn versus [monomer]/[initiator] ratio close to that of the theoretical curve based on a living polymerization model. Hence, despite pronounced secondary metathesis in ED-ROMP, polymerization kinetics with G2 closely resembled living behavior. The length of the induction period for G2-catalyzed polymerizations is inversely proportional to the solvent dielectric constant (εDCM > εTHF > εCHCl3). Finally, this work provides an important example of how complex structures derived from nature can be transformed into unique macromolecules.

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INTRODUCTION

The presence in SLs of multiple functional groups renders them a unique type of renewable building block for polymer synthesis. Previous work by our group demonstrated that LSL can be converted into a high molecular weight polymer (pLSL) via ring-opening metathesis polymerization (ROMP) catalyzed by Grubbs initiators. PLSL was formed in high yield with number-average molecular weights (Mn) up to 103 000.11 These polymers are semicrystalline with a glass transition and melting temperature at about 61 and 123 °C, respectively.12 The crystal phase shows ordered packing of aliphatic chain segments. Furthermore, semicrystalline pLSL displays a longrange order (d = 2.44 nm) involving sophorose groups that persists after crystal phase melting. Thus, pLSL is a relatively

The rising cost of fossil fuels and detrimental environmental impact of using petroleum-derived carbons have motivated researchers to tap into biomass as a sustainable resource.1 For example, fatty acids from plant oils were extensively used as building blocks for polymer synthesis.2−5 Lactonic sophorolipid (LSL) is the major fermentation product derived from the yeast Candida bombicola with high yield up to 200 g/L.6 As shown in Scheme 1, LSL is a macrocyclic glycolipid lactone consisting of the unique disaccharide sophorose and 17-hydroxyoleic acid. A number of studies uncovered interesting bioactive properties of natural and modified SLs, such as antibacterial, antifungal, antiviral (HIV-1), and anticancer as well as being a septic shock antagonist.7,8 Ongoing research in our laboratory is focused on enhancing the biological activity of SLs to develop new therapeutics and antimicrobial compounds by chemoenzymatic modification of native SLs.9,10 © 2013 American Chemical Society

Received: February 7, 2013 Revised: April 16, 2013 Published: April 30, 2013 3293

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Ruhland introduced a concept of turning point, which defined a critical feed concentration at which backbiting and polymerization occur at equal rates.20 Ultimately for ED-ROMP a ring/ chain equilibrium is established when the monomer concentration diminishes toward a critical point. This is described by eq 3

Scheme 1. ROMP of Lactonic Sophorolipid (LSL) and Metathesis Catalysts Used in This Study

chain(i + j) ↔ chain(i) + ring(j)

where i and j denote the number of repeat units in a particular chain or ring. Marsella et al. were among the first to demonstrate the success of ED-ROMP using the well-defined and highly efficient Grubbs first-generation catalyst.21 Their work also showed that ROMP of macrocyclic monomers was preferred relative to acyclic diene metathesis (ADMET) polymerization for the preparation of high molecular weight polymers from the corresponding α,ω-diene monomers. In addition, Tastard et al. extended the method of ED-ROMP toward much larger macrocycles and prepared polyesters and polyamides.22 Indeed, ED-ROMP appears to offer unique opportunities to prepare polymers with unconventional structures that are difficult to synthesize otherwise.16 Given that few kinetic studies have been conducted on EDROMP and the unique structure of LSL, this paper describes work to assess a number of critical reaction parameters for LSL ROMP using two classic Grubbs catalysts shown in Scheme 1.

new to the world biobased polymer synthesized by capturing the complexity of a natural glycolipid and translating that complexity to a macromolecule via ROMP. The potential use of pLSL as a bioresorbable medical material is currently being studied by us and collaborators. The development of highly efficient ruthenium catalysts has demonstrated ROMP is a powerful tool for polymer synthesis.13 Usually, ROMP is applied to monomers with high ring strain, e.g., norbornene derivatives. Thus, as shown in eq 1, the energy released upon ring-opening provides a driving force for ROMP as ΔHp ≪ 0, often leading to complete monomer consumption.14 ΔGp = ΔHp − T ΔSp

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EXPERIMENTAL SECTION

Materials. (1,3-Bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro(phenylmethylene)(tricyclohexylphosphine)ruthenium (G2) and [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(phenylmethylene)bis(3-bromopyridine)ruthenium(II) (G3) were purchased from Sigma-Aldrich and used without further purification. All other chemicals and solvents were HPLC grade and used as received. Sophorolipids were synthesized by fermentation of Candida Bombicola as previously reported,6,10 using 40 g of pure oleic acid (>99%), 100 g of glucose, 10 g of yeast extract, and 1 g of urea for 1 L of water. The crude sophorolipids were extracted with ethyl acetate from fermentation broth and dried under vacuum. Monomer Purification. LSL was separated from crude SLs by flash chromatography using chloroform and methanol (10:1). Prior to polymerization, the above purified LSL were further recrystallized from a 1:1 v/v mixture of ethyl acetate and hexane (yield = 90%). It was mentioned previously that a few percent of LSL was found to be saturated and thus would not undergo polymerization.11 1H NMR (500 MHz, DMSO-d6) δ: 1.12 (3H, d, J = 6.5 Hz, −CH3), 1.16−1.60 (20H, br, −CH2−), 2.00 (10H, br, −CHC, COCH3), 2.30 (2H, t, J = 6.7 Hz, −COOCH2−), 3.05−3.15 (2H, m, br, H4′, H2″), 3.24 (1H, t, J = 9.4 Hz, H2′), 3.35−2.49 (3H, br, H3′, H5′, H3″), 3.60 (2H, m, H17, H5″), 3.98 (2H, br, H6″), 4.06 (1H, m, br, H6′), 4.23 (1H, d, J = 11.8 Hz, H6′), 4.37 (1H, d, J = 8.1 Hz, H1′), 4.51 (1H, d, J = 7.3 Hz, H1″), 4.71 (1H, t, J = 10.4 Hz, H4″), 5.32 (2H, m, cis-CH). 13C NMR (300 MHz, DMSO-d6) δ: 20.48, 20.60, 20.97, 24.00, 24.69, 26.36, 26.67, 27.44, 27.77, 29.26, 29.68, 29.83, 33.39, 36.99, 62.18, 63.49, 69.97, 70.24, 71.21, 72.69, 79.14, 79.14, 82.32, 101.77, 103.91, 129.37, 129.86, 169.78, 170.14, 171.97. LC-MS (m/z) 711 (M + Na)+. (See 1-D and 2-D NMR spectra and detailed assignments in the Supporting Information.) General Procedure for LSL ROMP Kinetic Studies. In a coneshaped micro reaction vessel, 69 mg (0.1 mmol) of LSL monomer was dissolved in 0.1 mL of anhydrous THF equilibrated at a preselected temperature. THF (85 μL) containing 0.97 mg of G2 was preheated at same temperature and then added quickly under vigorous magnetic stirring. The vial was then immediately capped with a rubber septum without protection of inert gas. The polymerization was terminated at preselected time intervals by vigorously mixing the reaction mixture with an excess of 1:1 v/v THF and ethyl vinyl ether. An aliquot of crude nonfractionated polymer product were taken from quenched

(1)

Macrocyclic olefin monomers with n > 14 are virtually free from ring strain (thus ΔHp ≈ 0) but can undergo ROMP in bulk conditions or at high concentrations via an entropy-driven mechanism, referred to as ED-ROMP.15 A few examples of EDROMP were recently highlighted,16,17 and the mechanism was reviewed by Monfette and Fogg.18 Unlike low molecular weight monomers, macrocyclic monomers have much lower translational entropy. However, upon polymerization, the formation of highly flexible polymer chains contributes to a positive gain in conformational entropy such that ΔSp > 0. When macrocyclic monomers are concentrated or in bulk, this entropic contribution is more pronounced, as shown in eq 2: |ΔH | < |T ΔS| = |T (ΔS° + R ln(M )|

(3)

(2)

where M is the monomer concentration. This is also because increased viscosity of reaction media can further diminish monomer translational entropy, leading to an even larger ΔSp upon conversion of monomer to polymer.17 Thus, formation of high molecular weight polymers from macrocylic monomers such as macrocyclic olefin esters and amides were only observed at high monomer concentration or in bulk, and cyclic oligomers were the most prevalent in dilute conditions.15−17 Because of the nature of ED-ROMP, backbiting and chain transfer are competing with propagation throughout the polymerization. These phenomena were accounted for by the Jacobson−Stockmayer (JS) theory of macrocyclization, which shows that the probability of ring closure is inversely proportional to ring size.19 Moreover, Thorn-Csányi and 3294

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reaction mixtures and analyzed by GPC. Polymer molecular weights were determined using a GPC system LC-20A from Shimadzu equipped with a SIL-20A autosampler, SDV gel 5 μm precolumn (PSS, 50 mm × 8.0 mm), PLgel 5 μm MIXED-D column (Varian, 300 mm × 7.5 mm, 10 000 Å), and a RID-10A refractive index detector in THF with a flow rate of 1 mL/min at 50 °C. All determinations of molar mass were performed relative to linear poly(methyl methacrylate) PMMA standards (Polymer Standards Service, Mp 1100−981 000 Da). The monomer retention time was identified by injecting pure LSL.

the disadvantages of G2 are its relatively slow initiation and susceptibility to cause double bond isomerization at high temperatures, e.g., above 60 °C.24 The bis-bromopyridine complex G3 is often used for living ROMP because of its fast rate of initiation even below room temperature.25 Kinetic studies for LSL ROMP were performed with 0.54 M LSL in anhydrous THF and addition of 1.1 mol % G2 or G3 while varying the reaction temperature. Furthermore, reactions were quenched at preselected reaction times and, subsequently, were analyzed by GPC to determine Mn and percent monomer conversion. Results from the above set of reaction parameters are shown in Figures 2 and 3. During each experiment, the

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RESULTS AND DISCUSSION Monitoring Polymerization Reactions. Because of the fact the double bonds of monomers and polymers were indistinguishable in 1H NMR, gel permeation chromatography (GPC) was used as the primary tool to determine monomer conversion and pLSL molecular weight as a function of the reaction parameters discussed below. A typical GPC trace of a reaction mixture obtained from LSL ED-ROMP in shown in Figure 1. The monomer conversion was defined as the

Figure 2. Polymerization kinetics of G2 and G3 at different temperatures: monomer conversion versus reaction time.

Figure 1. A typical GPC trace of a reaction mixture from LSL ROMP.

percentage of monomers converted into both oligomers and polymers. It was normalized by calculating one minus the area ratio of the monomer peak to the entire area and multiplying by 100. In a typical successful polymerization, from integration of GPC curves we found that about 70% of polymer coexisted with 10% of oligomer (Mn < 3000) and 20% monomer (with observed Mn ≈ 600, close to its molecular weight 688). In addition, formation of the oligomer fraction (10%) occurred rapidly, prior to reaching 40% monomer conversion. Further increase in monomer conversion did not lead to a corresponding increase in the oligomer fraction. Moreover, the oligomer fraction is about 10% regardless of the catalyst used. By precipitating the reaction mixture into ethanol, the isolated polymer fraction for a product such as that shown in Figure 1 is about 70% (w/w). In all studies below, calculations of polymer molecular weight averages and dispersity values were determined by analyzing the polymer fraction of the reaction mixture as illustrated in Figure 1. The dispersity value (Mw/Mn) of the polymers taken from quenched reaction mixtures usually ranged between 1.6 and 1.8, regardless of the reaction condition or initiator used. This is consistent with other studies of ED-ROMP where events such as secondary metathesis, i.e., backbiting and intermolecular chain transfer, are known to occur.18,20 Polymerization Kinetics of LSL ROMP Using Grubbs Second (G2) and Third Generation (G3) Catalysts. We selected catalysts G2 and G3 (see Figure 1) for our study of polymerization kinetics. G2 is the most widely employed metathesis catalyst due to its high reactivity and high tolerance toward a wide range of functional groups.23 However, among

Figure 3. Polymerization kinetics of G2 and G3 at different temperatures: number-average molecular weight (Mn) versus reaction time.

viscosity of reaction solutions increased dramatically. Furthermore, what appeared as homogeneous viscous solutions were maintained throughout reactions without formation of a gel fraction or a precipitate. Under these conditions, high conversion (∼80%) was reached using either G2 or G3. To the best of our knowledge, the catalytic behaviors of G2 and G3 have not previously been examined together in relation to the kinetics of ED-ROMP for an unstrained macrocyclic monomer. For example, in recent publications on ED-ROMP, G2 was selected as the initiator and the molecular weight was reported after terminating the polymerization after a certain 3295

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arbitrary time.11,16 Such studies do not provide sufficient information to guide the selection of optimum polymerization conditions for LSL ROMP and other ED-ROMP systems. Results of the kinetic study herein show a critical influence of the reaction time on Mn, particularly when G2 was employed. As shown in Figure 3, the evolution of Mn with G2 exhibited a rather complicated pattern. First, Mn increased rapidly as the polymer chains propagated. Subsequently, upon reaching about 70% to 80% monomer conversion, Mn began to drop sharply for a considerable time period without a change in monomer conversion and Mw/Mn (Figure 3), until Mn eventually leveled off. In contrast, Mn values obtained using G3 steadily increased along with the monomer conversion and plateaued once the maximum conversion (∼80%) was achieved. This difference must be understood through a careful consideration of initiation of catalyst, which shall not be confused with the term “initiation of polymerization”. Grubbs and co-workers26 performed kinetic studies of G2 by reaction with ethyl vinyl ether (EVE) in THFd8 at 35 °C and found the initiation of G2 showed first-order dependence on its own concentration, regardless of the EVE concentration. Furthermore, Grubbs and co-workers26 estimated that the rate constant of catalyst initiation Ki = 0.001 s−1, that is, thalf‑life ≈ 12 min. This value was believed to be the rate constant for dissociation of phosphorus ligands and, the ratedetermining step for catalyst initiation, followed by a second step in which EVE instantly binds the metal center. Thus, an even smaller Ki is anticipated for our system since LSL should be much less reactive than EVE. In a subsequent study, Bielawski and Grubbs14 reported that, even at 50 °C, the polymerization of cyclooctadiene was completed in 1 min, although by then only 5% of G2 was initiated. The complete conversion of unreacted initiators to propagating species was observed much later through secondary metathesis, in the studied case after 15 min. Within this context, the decrease in Mn observed during G2 initiated ROMP of LSL is believed to result from slow catalyst initiation. Conceivably, at the beginning of the reaction only a small portion of catalyst was initiated to form growing polymer chains. Gradually, dormant catalyst entered the polymerization through secondary metathesis and truncated already existing polymer chains resulting in the observed drop in Mn. Decreased molecular weight did not change the fraction of oligomer (≈10%). Furthermore, since catalyst initiation increases at elevated temperature,26 the fact that the rate of Mn decrease is proportional to the temperature is consistent with our hypothesis that conversion of dormant catalyst species to initiated catalyst is responsible for an increase in secondary metathesis reactions leading to chain cleavage. Over prolonged reaction times, the molecular weight obtained by G2 at different temperature tended to converge, suggesting the full initiation of the catalyst and that an apparent equilibrium may be established. In addition, as shown Figure 2, an induction period is observed for G2 at the onset of polymerizations at 33 and 45 °C. As above, we believe this is due to slow catalyst initiation which largely disappears at 60 °C as the catalyst initiation rate increased.26 Induction was immediately followed by rapid chain propagation, which can be described by the rate equation −d[M]/dt = k p[active center][monomer]

of propagation. Thus, a steady state can be reasonable assumed for propagation using the expression k papp = k p[active center] = ln{[M]0 /[M]t }/t

(5)

kapp p

where was the apparent rate constant of chain propagation. Plots of ln{[M]0/[M]t} versus reaction time were linear for G2, suggesting apparent first-order kinetic behavior after the induction period (Figure 4). Accordingly, kapp values for G2 p

Figure 4. Kinetic plot of ln{[M]0/[M]t} versus reaction time at different temperature initiated by G2 and G3.

at different temperatures were determined and are listed in obtained from ROMP of LSL were Table 1. Values of kapp p understandably lower than those from ring-strained systems27 but comparable to those of monomers with low or medium −1 was reported for G2-catalyzed ring-strain; e.g., kapp p = 0.026 s ROMP of cis-cyclooctene in CD2Cl2 at 20 °C.28 Furthermore, an Arrhenius plot was constructed to estimate the activation energy (Ea) of propagation when using G2 (Figure 5). The value obtained (18 ± 3 kcal/mol) falls within the 5−19 kcal/ mol range for Ea values reported elsewhere for other ROMP polymerizations.27 The fact that Ea for ROMP of LSL falls within the upper limit of this range may be a consequence of the low monomer ring strain. For LSL polymerizations conducted using G3, no induction period was observed. This is consistent with previous studies with G3 where fast initiation was observed.25,29 However, unlike ROMP of strained monomers, LSL ROMP catalyzed by G3 did not result in a polymer with narrow dispersity. This is explained by that, thus far, intermolecular chain transfer and backbiting reactions are unavoidable. G3 exhibited a nearly identical kinetic profile at 33 and 45 °C. Presumably, complete initiation of G3 was rapid and unaffected by temperature. In the plot of ln{[M]0/[M]t} versus reaction time for G3, the rate of propagation decreased constantly, implying a decline in catalytic activity. Nevertheless, Mn values obtained using G3 were higher than those from G2 under similar conditions. Catalyst Loading. The effect of ROMP catalyst loading on pLSL molecular weight was also studied. In controlled/living ROMP, where each single active chain end is expected to propagate at a similar pace without secondary metathesis, the monomer/initiator ratio is proportional to Mn. Therefore, adjusting catalyst loading offers a means by which the molecular weight can be manipulated. However, this principle does not

(4)

where kp is the rate constant of chain propagation. Because the propagation period was significantly shorter than the time required for complete initiation of catalyst, the concentration of active centers was nearly constant within the short time frame 3296

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Table 1. Apparent Rate Constants of Chain Propagation Obtained from Figures 4 and 8 for G2a −1 kapp p (s )

33 °C THF DCM CHCl3 a

45 °C −3

60 °C −2

(6.6 ± 0.7) × 10 (9.0 ± 1.0) × 10−3 (3.9 ± 0.5) × 10−3

(2.8 ± 0.3) × 10

(7.2 ± 0.9) × 10−2

Error analysis was based on uncertainty in determining the best fit from linear regression analysis.

Figure 5. Arrhenius plot of chain propagation using G2. Ea is the activation energy.

Figure 6. Mn obtained after 30 min using different monomer/initiator ratios for the polymerization at 60 °C. (Dotted line represents theoretical values based on a living polymerization model with 85% monomer conversion.) For G2: y = 0.58 + 9.37x, SD = 6.58, R = 0.991. For G3: y = 1.73 + 21.71x, SD = 12.77, R = 0.987.

hold in ED-ROMP, where secondary metathesis reactions including backbiting and intermolecular chain transfer prevail. Moreover, it is known that the extent of secondary metathesis might also be affected by the kinetic nature of specific catalysts. For example, it was reported that when less active ROMP catalysts such as WCl6/EtAlCl2 and WCl6/Sn(CH3)4 were used in polymerization of monomers of low ring strain, such as cyclododecene, the molecular weight behavior resembled that of polycondensations.31 For ring-closing metathesis of α,ωdienes, while certain Grubbs catalysts showed a strong tendency to follow a straightforward pathway to form ringclosed products,18 Fogg and co-workers30 recently observed that some Grubbs catalysts, including G2, showed an intrinsic kinetic bias toward formation of ADMET polymerization products, i.e., large and median rings, which were subsequently converted into smaller ring-closed products via backbiting. Although ROMP using G2 and G3 were often shown to catalyze polymerizations of ring-strained monomers by a chaingrowth type mechanism, their roles in ED-ROMP were less clear. By keeping [LSL] constant at 0.54M, LSL ED-ROMP was performed at 60 and 45 °C for G2 and G3, respectively, with catalyst feeds ranging from 0.5 to 22 mol % relative to monomer. Each reaction was quenched and subsequently analyzed after 30 min since pLSL Mn values plateaued within this time period (see Figure 3). In general, monomer conversion (typically about 85%) and the oligomer product fraction were not affected by the catalyst loading regardless of whether G2 or G3 was used (Table 1S, Supporting Information). Moreover, Figure 6 shows that for both G2 and G3 a relationship closely approximating linear was observed for Mn vs [monomer]/[initiator]. Such behavior is normally found in controlled/living polymerization systems. For G2, the plot of Mn versus [monomer]/[initiator] ratio was close to that of the theoretical curve based on a living polymerization model. In contrast, Mn values for pLSL using

G3 were much higher than that using G2 and theoretical values for the living polymerization model. The behavior of G3 implies that the number of active propagating chains is substantially less than G3 molecules initially added to the reaction. Furthermore, since a linear relationship exists for plots of Mn vs the monomer-to-initiator ratio, it then follows that the ratio of active propagating chains to total G3 in polymerizations remains constant for differing catalyst loading (see Figure 6). In contrast, the number of active propagating chains closely approximates G2 molecules initially added to the reaction. Hence, the plot of Mn vs the monomer-to-initiator ratio for G2 closely approximates that based on the living polymerization model. Furthermore, the behavior of ln{[M]0/[M]t} versus reaction time plots for G3, displayed in Figure 4, is consistent with the above discussion for G3 as well as a decline in G3 catalytic activity during polymerizations. Chain Extension. To further ascertain the catalytic roles of G2 and G3 in this polymerization system which has some characteristics found in living-controlled systems, a chain extension experiment was performed by adding LSL in two portions. First, LSL (0.54 M) was polymerized with 1.1 mol % G2 at 60 °C for 5 min to ensure complete initiation so that the molecular weight remained unchanged. Subsequently, a second portion of 0.54 M LSL in THF was added so that the catalyst loading decreased to 0.5 mol %. Over the two step monomer feeding strategy, the combined monomer conversion exceeded 80% and Mn increased from 73 000 from the first monomer portion to 131 000 after the second without a significant change in the dispersity. GPC curves corresponding to products obtained after the first and second stage of this polymerization are displayed in Figure 7a. This result, obtained via a chain extension experiment, was compared to a one-step polymer3297

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Figure 7. GPC traces of chain-extension experiments with LSL at 60 °C and monomer concentration of 0.54 M. (a) Using G2, before second batch of LSL was added: Mn = 73K, monomer conversion = 83%, Mw/Mn = 1.6; after second quantity of LSL was added: Mn = 131K, conversion = 85%, Mw/Mn = 1.7. (b) Using G3, no further polymerization was observed upon addition of a second portion of monomer.

oligomers (Mn < 3000) were formed using either G2 or G3; the monomer conversion failed to improve over prolonged reaction times. However, long chain polymers are formed at monomer concentrations ≥0.27 M. At 0.27 M LSL, monomer conversions for G2 and G3 were 85 and 55%, respectively. The low monomer conversion (55%) in the case of G3 is likely due to its deactivation as evidenced in the chain extension experiment. Though, at 0.54 M LSL, monomer conversions for G2 and G3 were similar in value. These results suggest that there is a critical LSL concentration >0.05 and εTHF > εCHCl3). In other words, from Figure 8, Kini is fastest in DCM and slowest in CHCl3. Despite differences in pLSL solubility, polymerization in these solvents yielded similarly high monomer conversion in final products (>80%). These observations suggest that the solvent character influenced the rate of catalyst initiation. The linear fit between ln{[M]0/[M]t} and time throughout propagation suggests apparent first-order kinetics in all three solvents. From the linear region of plots in Figure 8, apparent rate constants of chain propagation (kapp p ) were determined, and values are listed in Table 1. These values also increase as the solvent dielectric constant increases. This can be rationalized as follows: higher dielectric constant solvents resulted in propagating chain ends with higher activity for monomer addition. The decline of Mn as a consequence of slow initiation of G2 was also observed in DCM and CHCl3 (Figure 3S). However, the Mn of final products in DCM and CHCl3 was somewhat lower than that obtained in THF. This difference is likely related to the lower solubility of pLSL evidenced by gel formation in DCM and CHCl3. Indeed, poor polymer solubility in ROMP is known to contribute to relatively higher rates of backbiting reactions.18

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ASSOCIATED CONTENT

* Supporting Information S

Effect of catalyst loading on ROMP of LSL, polymerization kinetics in different solvents at 33 °C using G2, methods of 2-D NMR experiments, discussion for NMR assignments, 1H and 13 C NMR spectra, 2-D NMR spectra including HSQC, HSQCTOCSY, HMBC, and COSY. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.A.G.); [email protected] (M.A.R.M.).

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Notes

The authors declare no competing financial interest.

CONCLUSIONS High monomer conversion of LSL and high molecular weight polymers were achieved under optimal ED-ROMP conditions. Two classic metathesis catalysts, G2 and G3, were compared for LSL ROMP. G2 exhibited apparent first-order propagation with slow initiation. Solvent choice had a strong effect on G2 initiation. Also, catalyst loading can be used as a means to manipulate molecular weight of pLSLs. In summary, this kinetic study of LSL ROMP was consistent with that polymerizations were entropically driven toward an equilibrium state. Earlier work using WCl6/EtAlCl2 and WCl6/Sn(CH3)4 as catalysts showed that ROMP of olefin monomers with small ring strains resemble the mechanism of polycondensation.31 However, unexpectedly, LSL ROMP catalyzed by G2 and G3 showed behavior resembling many characteristics of living systems. ROMP of highly strained monomers generally resembles a chain-growth type mechanism.31−33 Living ROMP can be achieved using fast initiating catalysts such as G3, in dilute solution and at low temperature (e.g., −20 °C).34 For a typical living system, it is desired that the rate of initiation is significantly faster than that of chain propagation and that chain transfer is eliminated. These characteristics were not found for LSL ED-ROMP. Pronounced secondary metathesis and slow initiation are both believed to attribute to Mw/Mn values that are generally between 1.6 and 1.8. However, some characteristics of living-type polymerization systems were observed for G2/G3-catalyzed LSL ED-ROMP. For G2 and G3, the ratio of active propagating chains to total catalyst in polymerizations remains constant for differing catalyst loadings resulting in plots of Mn vs monomer-to-initiator ratio that approximate linear behavior. Furthermore, for G2, the number of active propagating chains closely approximates G2 molecules initially

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ACKNOWLEDGMENTS The authors thank Professor Stanislaw Penczek, Member of Polish Academy of Science, for his helpful advice during the preparation of this manuscript. Y.P. acknowledges members of the Meier laboratory who welcomed him into their laboratory at Karlsruhe Institute of Technology (Germany). R.G. is grateful for funding received from the National Science Foundation Office of International Science and Engineering (NSF-OISE) under Award #1214469 for their financial support of this research.

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