1,4-Polybutadienes with Pendant Hydroxyl ... - ACS Publications

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1,4-Polybutadienes with Pendant Hydroxyl Functionalities by ROMP: Synthetic and Mechanistic Insights Flavien Leroux, Sagrario Pascual, Véronique Montembault, and Laurent Fontaine* Institut des Molécules et des Matériaux du Mans (IMMM), Equipe Méthodologies et Synthèse des Polymères, UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans, Cedex 9, France S Supporting Information *

ABSTRACT: The reactivity of cis-3,4-bis(hydroxymethyl)cyclobutene derivatives bearing free and protected hydroxyl groups during ring-opening metathesis polymerization (ROMP) was investigated using ruthenium-based initiators. It was found that the ROMP of cis-4-benzyloxymethyl-3-hydroxymethylcyclobutene (1) using highly reactive initiators containing N-heterocyclic carbenes as nonlabile ligands leads to well-defined polymers while cis-3,4-bis(hydroxymethyl)cyclobutene (2) was reluctant to polymerize under the same conditions. Kinetic studies were performed to assess a number of critical reaction parameters: initiator structure, solvent, and temperature. The results demonstrate that Grubbs’ second- and third-generation catalysts are the best initiators to prepare welldefined 1,4-polybutadienes containing simultaneously free and protected hydroxyl side groups with predictable molecular weights (up to 40 000 g mol−1) and narrow molecular weight distributions. Besides, low values of kp/ki (the ratio of the rate constant of propagation to the rate constant of initiation) were found for the ROMP of monomer 1 with Grubbs’ second-generation catalyst in chloroform or THF as the solvent, demonstrating a living process. The so-obtained polymers having hydroxyl side groups are an ideal platform to prepare original well-defined graft copolymers through the grafting-from strategy.

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tation transfer (RAFT) polymerization,10 click chemistry,10,11 and organocatalyzed ring-opening polymerization.12 The first study devoted to the ROMP of cyclobutenes using well-defined catalysts was performed by Grubbs and coworkers.13 Well-defined 1,4-polybutadienes with low dispersities (ĐM = 1.1) were obtained using well-defined tungstenbased (W-based) initiators in the presence of an excess of trimethylphosphine (PMe3). Well-defined polymers were also synthesized by ROMP from 3-alkylcyclobutenes using W-based and molybdenum-based (Mo-based) initiators in the presence of dimethylphenylphosphine (PPhMe2).14 The presence of a phosphine limits secondary metathesis reactions during the polymerization of cyclobutene derivatives when W-based and Mo-based initiators are used. Perrott and Novak performed ROMP of 3,4-disubstituted cyclobutenes using Mo-based catalysts.15 However, protection of the functional carboxylic acid groups was necessary to obtain well-defined polymers. The ROMP of ether, ester, carboxylic acid, amide, and hydroxylfunctionalized cyclobutenes derivatives, especially 3-monosubstituted cyclobutenes and 3,4-disubstituted cyclobutenes, without any protection of the functional groups was also studied. Concerning the ROMP of 3-monosubstituted cyclobutenes, the use of Ru-based initiators (Ru(Cl2)(PCy3)RuCH−CH CHPh2 and Grubbs’ first-generation catalyst, G1) enabled the

INTRODUCTION

Over the past decade, ring-opening metathesis polymerization (ROMP) has emerged as a powerful tool to synthesize complex macromolecular architectures such as block and graft copolymers.1,2 Thanks to ruthenium-based (Ru-based) catalysts,3 a wide range of functionalized cyclic olefins can be polymerized by ROMP.4 Norbornene and derivatives are the most common monomers studied in ROMP.1,4 Norbornene has a sufficiently high ring strain (27.2 kcal mol−1)5 to promote ROMP compared to inter- and intramolecular chain transfer reactions (secondary metathesis reactions). Furthermore, the steric hindrance around the double bonds of the growing polynorbornene chains allows to minimize the secondary metathesis reactions when high monomer conversion is reached, leading to polymers with controlled number-average molecular weights (M̅ n) and low dispersities (ĐM). ROMP of cyclobutene and functionalized derivativeswhich also present a high ring strain (30.6 kcal mol−1)6has been studied with a lesser extent compared to the ROMP of norbornene1,2,4 and oxanorbornene7 derivatives. However, this synthetic methodology constitutes the best strategy to synthesize strictly 1,4polybutadiene.8 Driven by our interest in developing efficient methodologies for the preparation of well-defined graft copolymers, our group previoulsy reported the ROMP of various cyclobutene-derived macromonomers that were prepared using various orthogonal processes such as atom transfer radical polymerization (ATRP),9 reversible addition−fragmen© XXXX American Chemical Society

Received: April 3, 2015 Revised: May 28, 2015

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DOI: 10.1021/acs.macromol.5b00696 Macromolecules XXXX, XXX, XXX−XXX

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dichlororuthenium (G1, 97%), (1,3-bis(2,4,6-trimethylphenyl)-2imidazolidinylidene)-dichloro(phenylmethylene)(tricyclohexylphosphine)ruthenium (G2, 98%), (1,3-bis(2,4,6-trimethylphenyl)-2imidazolidinylidene)dichloro(o-isopropoxyphenylmethylene)ruthenium (HG2, 97%), deuterated chloroform (CDCl3, >99%, Eurisotop), deuterated tetrahydrofuran (THF-d8, 99.5%, Euriso-top), and ethyl vinyl ether (Acros, 99%) were used as received. Dichloromethane (DCM, HPLC grade, Fisher Chemical) was dried over a dry solvent station GT S100. 1,2-Dichloroethane (C2H4Cl2, >99%, Merck Schuchardt) was distilled from CaH2. Cyclohexane (Quaron, 99.8%) was freshly distilled before use. cis-3,4-Bis(hydroxymethyl)cyclobutene (2),24 cis-4-benzyloxymethyl-3-hydroxymethylcyclobutene (1),25 (1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)dichloro(phenylmethylene)(tricyclohexylphosphine)ruthenium (NC),26 (1,3bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro(phenylmethylene)bis(3-bromopyridine)ruthenium (G3),27 and (1,3-bis( 2, 4 , 6 - t r i m e t h y l p h e n y l ) - 2 - i m i d a z o l id in y l i d e n e ) d ic h l o r o (phenylmethylene)bis(pyridine)ruthenium (G3′)27 were synthesized according to literature procedures. General Procedure for ROMP of Hydroxyl-Functionalized Cyclobutenyl Monomers. In a typical experiment, a dry Schlenk tube was charged with the desired quantity of monomer 1 or 2 and a stir bar. The Schlenk tube was capped with a rubber septum, and the desired quantity of anhydrous solvent (C2H4Cl2, THF-d8, or CDCl3) was added via a syringe to obtain a homogeneous solution ([monomer]0 = 0.1−1 mol L−1). The monomer solution was then degassed by several freeze−pump−thaw cycles. The Schlenk tube was immersed in an oil bath preset at the desired temperature (25 or 50 °C) and was stirred under N2 for 10 min. A stock solution of initiator in degassed anhydrous C2H4Cl2, THF-d8, or CDCl3 ([I]0 = 47−190 mmol L−1) was prepared in a separate vial. The desired quantity of initiator was injected quickly into the monomer solution to initiate the polymerization (initial reaction time, t = 0). The reaction mixture was stirred from 1 to 24 h. Aliquots of reaction mixture were taken at different reaction times, and polymerizations were quenched by adding two drops of ethyl vinyl ether for 1H NMR spectroscopy analysis. The solvent of aliquots was then removed under reduced pressure for further SEC measurements to determine number-average molecular weight (M̅ RI n,SEC) and dispersity (ĐM). The final reaction mixture was then diluted in DCM and precipitated into 20 mL of stirred cyclohexane, filtered, and dried overnight under reduced pressure. The recovered polymer was then analyzed by 1H NMR spectroscopy and SEC. Typical Procedure for ROMP of cis-4-Benzyloxymethyl-3hydroxymethylcyclobutene (1) (Table 2, Run 5). Cyclobutene 1 (0.200 g, 0.980 mmol) and a stir bar were added into a dry Schlenk tube. The Schlenk tube was capped with a rubber septum, and 3.9 mL of anhydrous C2H4Cl2 was added via a syringe to obtain a homogeneous solution ([1]0 = 0.25 mol L−1). The monomer solution was then degassed by several freeze−pump−thaw cycles. The Schlenk tube was immersed in an oil bath preset at 50 °C and was stirred under N2 for 10 min. A stock solution of G3 initiator in degassed anhydrous C2H4Cl2 ([G3]0 = 47.5 mmol L−1) was prepared in a separate vial, and 205 mL of this solution was injected via a syringe into the monomer 1 solution to initiate the polymerization (initial reaction time, t = 0). Aliquots of reaction mixture were taken after 10, 20, 30, 40, 50, and 60 min of reaction. Polymerization was quenched by adding two drops of ethyl vinyl ether for 1H NMR spectroscopy analysis. The solvent of aliquots was then removed under reduced pressure for further SEC measurements to determine M̅ RI n,SEC and ĐM. The final reaction mixture was then diluted in DCM and precipitated into 20 mL of stirred cyclohexane, filtered, and dried overnight under reduced pressure. The recovered polymer was analyzed by 1H NMR spectroscopy and SEC. White-brown plastic; [1]0/[G3]0 = 100/1; conversion: 86%; M̅ RI n,SEC = 22 710 g mol−1; ĐM = 1.11. 1H NMR (200 MHz, CDCl3), δ (ppm): 7.05−7.40 (broad signal, 430H, CH2−C6H5), 5.10−5.70 (broad signal, 172H, CHCH), 4.00−4.60 (broad signal, 172H, CH2−C6H5), 2.95−3.90 (broad signal, 344H, CH2−O−CH2−C6H5 + CH2−OH), 2.00−2.95 (broad signal, 172H, CH−CH2−OH + CH−CH2−O− C6H5) (Figure S6 in Supporting Information).

polymerization of cyclobutenyl monomers functionalized by ether, ester, carboxylic acid, amide, and hydroxyl groups to give well-defined functionalized 1,4-polybutadiene backbones (ĐM ≤ 1.20).16−18 The ROMP of 3,4-disubstituted cyclobutenes, providing linear 1,4-polybutadienes with a high density of pendant functionalities, has been reported in a few studies.8 Highly active Mo-based initiators allowed the polymerization of cis-3,4-diether and cis-3,4-diester-functionalized cyclobutenes with tunable M̅ n and low dispersities (ĐM ≤ 1.20).15,19 It was found that G1 initiator also permits the synthesis of welldefined functionalized polymers (ĐM ≤ 1.20) by ROMP of cis3,4-diester functionalized cyclobutenes.20,21 Our group has previously reported that 3,4-diacetate-functionalized cyclobutenes with a trans configuration are less reactive toward ROMP: the trans 3,4-diacetate-functionalized cyclobutene does not polymerize with G1 initiator, and use of the more reactive Grubbs’ second generation catalyst (G2) initiator was necessary to reach a well-defined polymer (ĐM = 1.26).21 The influence of the nature of the substituents of 3,4-disubstituted cyclobutenes upon reactivity in ROMP has been highlighted in the literature. Thus, cyclobutenes substituted by electron-withdrawing groups result in a decrease of the reactivity by impoverishment of the electronic density of the cyclobutenyl double bonds.22 Similar observations were made for the ROMP of 1-substituted cyclobutenes.23 In the present work, we examined the reactivity in ROMP of 3-monohydroxy-functionalized cyclobutene and 3,4-dihydroxyfunctionalized cyclobutene with Ru-based initiators (Scheme 1). An extensive and systematic evaluation of the influence of Scheme 1. ROMP of Monomers 1 and 2 Using Ru-Based Initiators

the initiator structure, solvent, and temperature on the kinetics of polymerization and on the macromolecular characteristics of the resulting polymers was carried out. For the first time, strictly 1,4-polybutadiene backbones having a high density of pendant free and protected hydroxyl groups were synthesized with tunable M̅ n and low dispersity values. Such well-defined polymers can be used further for postpolymerization functionalization or grafting-from routes to prepare graft copolymers including, e.g., polar grafts.

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EXPERIMENTAL SECTION

General Characterization. 1H nuclear magnetic resonance (NMR) spectra were recorded on Bruker DPX (200 MHz) and Bruker Avance (400 MHz) spectrometers. Chemical shifts are reported in ppm relative to the deuterated solvent resonances. Size exclusion chromatography (SEC) measurements were carried out using a system equipped with a SpectraSYSTEM AS1000 autosampler, followed by a Guard column (Polymer Laboratories, PL gel Guard column, 50 × 7.5 mm), by two columns (Polymer Laboratories, 2 PL gel 5 MIXED-D columns, 2 × 300 × 7.5) and by a SpectraSYSTEM RI-150 detector. The instrument operated in tetrahydrofuran (THF) at a flow rate of 1.0 mL min−1 at 35 °C and was calibrated with narrow linear polystyrene (PS) standards with molecular weights ranging from 580 to 483 000 g mol−1. Materials. All chemicals were purchased from Aldrich unless otherwise noted. Benzylidene-bis(tricyclohexylphosphine)B


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Macromolecules Scheme 2. Structures of Ru-Based G1, NC, and G2 Initiators

Figure 1. 1H NMR spectra (200 MHz, CDCl3, 25 °C) of (A) monomer 1 and of (B) reaction mixture obtained from the ROMP of 1 in C2H4Cl2 at 50 °C using G2 as the initiator with [1]0/[G2]0 = 100 for a reaction time of 1 h (Table 1, run 5).

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RESULTS AND DISCUSSION

The influence of the initiator on the ROMP of hydroxylfunctionalized cyclobutenes 1 and 2 (Scheme 1) was studied first. Ru-based initiators were selected because of their high reactivity and their tolerance toward a wide range of functional groups.1,3,4 Table 1. Characteristics of the Polymers Obtained from ROMPs of 1 and 2 in C2H4Cl2 at 50 °C after 1 h, Using G1, NC, and G2 Initiators (I) with an Initial Ratio [1 or 2]0/[I]0 = 100 run

monomer

initiator

M̅ n,calca (g mol−1)

convb (%)

1 2 3 4 5 6

1 2 1 2 1 2

G1 G1 NC NC G2 G2

20504 11504 20504 11504 20504 11504

0 0 53 0 83 0

c M̅ RI n,SEC (g mol−1)

ĐM c

10810

1.46

21560

1.13

Figure 2. SEC traces of crude polymers obtained by ROMP of 1 in C2H4Cl2 at 50 °C using NC as the initiator with [1]0/[NC]0 = 100 for reaction times of (A) 1 h (Table 1, run 3) and (B) 4 h.

Influence of the Nonlabile L1 Ligand on the ROMP of Hydroxyl-Functionalized Cyclobutenes. ROMP of 1 and 2 were first studied using G1, Nolan’s catalyst (NC), and G2 as initiators (I), which bear a different nonlabile L1 ligand (Scheme 2). The polymerizations were performed at 50 °C using 1,2dichloroethane (C2H4Cl2) as the solvent with an initial ratio of [1 or 2]0/[I]0 equal to 100 and [1 or 2]0 = 0.25 mol L−1. The monomer conversion to polymer was determined by 1H NMR analysis of the crude product, by comparing the peak areas of the cyclobutene alkene protons at δ = 5.95−6.05 ppm (labeled (a) in Figure 1B) and of the alkene protons of polymer at δ =

M̅ n,calc = ([monomer]0/[I]0) × Mmonomer + Mextr with M1 = 204 g mol−1, M2 = 114 g mol−1, and Mextr = 104 g mol−1. bThe monomer conversions were determined by comparing the peak areas of the cyclobutene alkene protons at δ = 5.95−6.05 ppm and of the alkene protons of polymer at δ = 5.00−5.70 ppm from 1H NMR spectra of the crude mixtures. cDetermined by SEC in tetrahydrofuran (THF) using a RI detector, calibrated with linear polystyrene (PS) standards. a

C


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53% conversion of 1 after 1 h of reaction (Table 1, run 3). The extension of reaction time to 4 h allowed the conversion of 1 to reach 63%. SEC analyses of the crude products obtained after reaction times of 1 and 4 h featured unsymmetrical SEC traces together with broad dispersity (ĐM ≈ 1.46) (Figure 2). This result suggested the presence of backbiting side reactions, highlighted by a spreading of SEC traces toward higher retention times. Initiator G2 bearing a 1,3-bis(2,4,6-trimethylphenyl)-2imidazolidinylidene (SIMes) as the NHC nonlabile ligand L1 has a high reactivity in ROMP.4,30 G2 is known to allow the ROMP of monomers with moderate reactivity and to provide unsaturated polymers with controlled structures31 despite its slow initiation rate32 and competing chain-transfer reactions.30,33 ROMP of 1 using G2 led to 83% conversion after 1 h of reaction (Table 1, run 5). SEC analysis of the crude product showed a symmetrical trace (ĐM = 1.13) (Figure 3). The presence of a small shoulder toward lower retention times suggests the presence of chain-transfer reactions at 83% monomer conversion. Exchange of the nonlabile ligand L1 from PCy3 (G1) to NHC carbenes (NC and G2) allowed the ROMP of 1. This result seems to be a consequence of the presence of an imidazolyl group in NC and G2 initiators that improve the stability of the Ru−olefin intermediate after dissociation of the phosphine L2 compared to Ru−PCy3. Such a behavior is consistent with the fact that imidazolyl is a stronger electron donating group and induces more steric crowding than PCy3. These results are in good agreement with kinetic studies of Grubbs and co-workers concerning the substitution of PCy3 ligand by an olefin.32 They have shown that k−1/k2 ratio (with k−1, the rate constant for the coordination of PCy3 on L1Cl2RuCHPh intermediate, and k2, the rate constant for the coordination of the olefin on L1Cl2RuCHPh) obtained with G1 initiator is higher that k−1/k2 ratio obtained for NC and G2 initiators, resulting in a loss of reactivity (Scheme 3). The higher reactivity of G2 relative to NC is due to the lack of carbene stabilization of SIMes compared to IMes (Scheme 2). NHC carbene of G2 is a poor π-donor but a strong σacceptor ligand,34 facilitating the dissociation of ligand L2 relative to NC during initiation step. As G2 initiator leads to better results in terms of monomer conversion and dispersity values, the SIMes ligand was retained as the nonlabile L1 ligand for the following study. The influence of L2 ligand was then examined. Influence of Labile Ligand L2 on the ROMP of Hydroxyl-Functionalized Cyclobutenes. The influence of the structure of G2, Hoveyda Grubbs (HG2), Grubbs 3 (G3), and Grubbs 3′ (G3′) (Scheme 4) used as the initiator on the ROMP of 1 and 2 was studied. Such initiators are substituted by a saturated NHC carbene ligand L1 and bear different labile

Figure 3. SEC trace of the crude polymer obtained by ROMP of monomer 1 in C2H4Cl2 at 50 °C, using G2 as the initiator with [1]0/ [G2]0 = 100 for a reaction time of 1 h (Table 1, run 5).

Scheme 3. Ligand Exchange Mechanism

5.00−5.70 ppm (labeled (a′) in Figure 1B). Furthermore, SEC analysis was performed to determine number-average molecular weights (M̅ RI n,SEC) and dispersity (ĐM) values using a refractive index detector. The results are summarized in Table 1. Monomer 2, bearing two hydroxyl groups per cyclobutene unit, did not polymerized by ROMP whatever the initiator (G1, NC, or G2) used (Table 1, runs 2, 4, and 6), and despite the initiator tolerance toward protic and polar groups like alcohols.1,3,28,29 Moreover, no polymer was obtained with monomer 1 bearing only one hydroxyl group per cyclobutene unit when G1 was used as the initiator (Table 1, run 1). However, previous studies have shown that the ROMP of cyclobutene substituted with one hydroxyl group leads to polymers with low dispersities (ĐM = 1.20) using G1.16 This discrepancy observed with monomer 1 can be explained by the proximity of the electron withdrawing hydroxyl group with the double bond of cyclobutene, leading to a decrease of reactivity. Concerning 2, the density of hydroxyl groups near the double bond of cyclobutene seems to have an additional influence upon reactivity. Exchange of the nonlabile L1 from tricyclohexylphosphine (PCy3) group in G1 into a N-heterocyclic carbene (NHC) in NC and G2 induces a dramatically improved ROMP reactivity when compared to G1.30 Thus, use of NC initiator, obtained by replacement of one PCy3 ligand in G1 by one 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene (IMes) (Scheme 2), led to

Scheme 4. Structures of Ru-Based Initiators G2, HG2, G3, and G3′

D


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Figure 4. (A) Conversion of 1 versus reaction time and (B) kinetic plot of ln([M]0/[M]t) versus reaction time for the ROMPs of 1 in C2H4Cl2 at 50 °C using G2, HG2, G3, and G3′ as initiators with [1]0/[I]0 = 100.

to the limited mobility of the isopropyloxyaryl ligand linked to the growing chain when HG2 is used as the initiator while PCy3 (in G2), 3-bromopyridine (in G3), and pyridine (in G3′) ligands are free in the reaction medium (Scheme 5). The M̅ RI n,SEC of the resulting polymers increase linearly with monomer conversion for ROMPs of 1 using initiators G2, HG2, G3, and G3′ (Figure 5A). Such a behavior is in accordance with controlled/living polymerization systems. However, M̅ RI n,SEC values for polymers obtained using G3 and G3′ as initiators were higher than ones obtained using G2 and HG2 (Figure 5A) for monomer conversions higher than 55%. This result could be explained by a lower thermal stability of G3 and G3′ compared to G2 and HG2.35 The polymers obtained using G2, G3, and G3′ displayed low dispersity values (ĐM ≤ 1.16) (Figure 5A). When HG2 initiator was used, the SEC traces of the resulting polymers showed a shoulder toward higher molecular weights (Figure 5E), which is consistent with a higher concentration of active species as explained previously. The lower control on the ROMP of 1 observed using HG2 compared to G2, G3, and G3′, and the similar results obtained when G3 and G3′ were used, led us to select G2 and G3 as initiators to study the ROMP of 1 in the following studies. Influence of [Monomer]0/[Initiator]0 Ratio and Monomer Concentration on the ROMP of Hydroxyl-Functionalized Cyclobutenes. ROMP of 1 was carried out at 50 °C in C2H4Cl2 with an initial monomer concentration of 0.25 mol L−1 and with [1]0/[I]0 ratios ranging from 25/1 to 250/1 using G2 or G3 as the initiator (Table 3, runs 3−8). High conversions of 1 were obtained whatever the [1]0/[I]0 ratio. SEC traces of polymers prepared using G3 and isolated after purification are presented in Figure 6A. Resulting polymers showed a shift of SEC traces to higher molar mass values when the [1]0/[I]0 ratios increase from 25/1 to 250/1. Moreover, polymers are obtained with narrow dispersities (ĐM ≤ 1.16). Further SEC analyses of polymers synthesized showed that RI are proportional to [1]0/[I]0 ratios (Figure 6B), M̅ n,SEC indicating a good control over the ROMP of 1 using G2 and G3 as initiators. The influence of the initial monomer concentration on the ROMP of 1 was then investigated. Polymerizations were carried out in C2H4Cl2 at 50 °C using either G2 or G3 as initiators. For an initial ratio [1]0/[I]0 = 100 and an initial monomer

Scheme 5. Chemical Structure of the Growing Active Species during ROMP of 1 Using HG2 as the Initiator

L2 ligands. ROMP was performed in C2H4Cl2 at 50 °C using [1 or 2]0 equal to 0.25 mol L−1 and an initial ratio of [1 or 2]0 to [I]0 equal to 100. The results are shown in Table 2. As observed previously with G2, the use of initiators HG2, G3, and G3′ did not allow the ROMP of monomer 2 (Table 2, runs 2, 4, 6, and 8), even for high reaction times. However, high conversions were obtained for ROMP of cyclobutene 1, whatever the initiator used (85% ≤ conversion ≤ 90% after a reaction time of 1 h; see Table 2, runs 1, 3, 5, and 7). Moreover, the ln([M]0/[M]t) vs reaction time showed a linear correlation for each initiator used (Figure 4B). Such a kinetic behavior is consistent with a constant concentration of active species. The propagation apparent rate constants (kpapp) were determined from the slope of the kinetic plots (eq 1) for the ROMP of 1 in the presence of G2, HG2, G3, and G3′ initiators (Table 2). ln

[M]0 = k pappt [M]t

with k papp = k p[active species]

(1)

app

Similar values of kp were obtained when ROMP of 1 was performed with G2, G3, and G3′. Use of HG2 as the initiator led to an increase of the kpapp value compared to Grubbs’ catalysts G2, G3, and G3′ (kpapp(HG2) > kpapp (G3′) ≈ kpapp(G2) = kpapp(G3)). As the propagation rate constant (kp) is constant, such a result shows that the concentration of active species using HG2 is higher than the concentration of active species when G2, G3, or G3′ is used. It appears that the growing active species are less easily recoordinated by the isopropyloxyaryl ligand compared to the growing active species generated by the initiators G2, G3, and G3′. This is likely due E


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Figure 5. (A) M̅ RI n,SEC and ĐM versus conversion of 1 and SEC traces of crude polymers obtained at different conversions of 1 for the ROMPs of 1 in C2H4Cl2 at 50 °C using (B) G2, (C) G3, (D) G3′, and (E) HG2 as initiators (I) for an initial ratio [1]0/[I]0 = 100.

Table 2. Characteristics of the Polymers Obtained from ROMP of 1 and 2 in C2H4Cl2 at 50 °C after 1 h, Using G2, HG2, G3, and G3′ as Initiators (I) with an Initial Ratio [1 or 2]0/[I]0 = 100 run

monomer

initiator

M̅ n,calca (g mol−1)

convb (%)

c −1 M̅ RI n,SEC (g mol )

ĐM c

kpapp d (s−1)

1 2 3 4 5 6 7 8

1 2 1 2 1 2 1 2

G2 G2 HG2 HG2 G3 G3 G3′ G3′

20504 11504 20504 11504 20504 11504 20504 11504

88 0 90 0 86 0 85 0

20690

1.13

6 × 10−4

20870

1.16

9 × 10−4

22710

1.11

6 × 10−4

21560

1.13

7 × 10−4

M̅ n,calc = ([monomer]0/[I]0) × Mmonomer + Mextr with M1 = 204 g mol−1, M2 = 114 g mol−1, and Mextr = 104 g mol−1. bThe monomer conversions were determined by comparing the integrations of alkene protons of the cyclobutene at δ = 5.95−6.05 ppm and the alkene protons of polymers at δ = 5.00−5.70 ppm from 1H NMR spectra of the crude mixtures. cDetermined by SEC in THF with an RI detector, calibrated with linear PS standards. dDetermined from the slope of ln([M]0/[M]t) vs reaction time. a

concentration [1]0 = 0.1 mol L−1, incomplete conversion of 1 (≈50%) was obtained after 1 h of reaction, using either initiators G2 or G3 (Table 3, runs 1 and 2). Higher monomer conversion (>83%) together with narrow molecular weights distributions (ĐM ≤ 1.16) were obtained by increasing the initial monomer concentration up to 0.25 mol L−1, while retaining the same reaction time and the same initial [1]0/[I]0 ratio (Table 3, runs 1 and 2 vs runs 5 and 6, respectively). By using G2 and by increasing the monomer concentration for an initial [1]0/[G2]0 ratio equal to 500, ROMP of 1 led to an increase of monomer conversion from 61% to 77% and to a decrease of the dispersity after a reaction time of 5 h (Table 3, run 11 vs run 13). Similar results were observed for ROMP of 1

using G3 initiator: the monomer conversion increased from 40% to 66%, and the dispersity decreased from 1.42 to 1.38 by increasing [1]0 from 0.5 to 1 mol L−1 (Table 3, run 12 vs run 14). Chain extension is thus favored in ROMP for higher monomer concentration. This behavior is explained by the fact that an increase of the monomer concentration promotes meeting between active species and monomer molecules. Influence of Solvent and Temperature on the ROMP of Hydroxyl-Functionalized Cyclobutenes. Solvents are known to strongly influence the reactivity of metathesis catalysts.29 Solvent effects during ROMP of cyclobutene 1 were thus investigated. Polymerizations were performed in deuterated tetrahydrofuran (THF-d8), deuterated chloroform F


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Macromolecules

Table 3. Characteristics of the Polymers Obtained from ROMP of 1 in C2H4Cl2 at 50 °C Using G2 and G3 as Initiators run

initiator

[1]0/[I]0a

[1]0 (mol L−1)

time (h)

convb (%)

M̅ n,calcc (g mol−1)

d −1 M̅ RI n,SEC (g mol )

ĐM d

1 2 3 4 5 6 7 8 9 10 11 12 13 14

G2 G3 G2 G3 G2 G3 G2 G3 G2 G3 G2 G3 G2 G3

100 100 25 25 100 100 250 250 250 250 500 500 500 500

0.1 0.1 0.25 0.25 0.25 0.25 0.25 0.25 0.5 0.5 0.5 0.5 1 1

1 1 0.5 0.5 1 1 2 2 1 1 5 5 5 5

52 49 >99 >99 83 86 79 75 84 81 61 40 77 66

10712 10100 5204 5204 17036 17648 40394 38354 42944 41414 62324 40904 78644 67424

11800 12830 7930 8120 20690 22710 48000 45150 48820 49560 55960 41250 68900 78800

1.25 1.19 1.17 1.16 1.16 1.11 1.23 1.14 1.33 1.24 1.62 1.42 1.54 1.38

a Monomer-to-initiator molar ratio. bThe monomer conversions were determined by comparing the integrations of alkene protons of the cyclobutene at δ = 5.95−6.05 ppm and the alkene protons of polymers at δ = 5.00−5.70 ppm from 1H NMR spectra of the crude mixtures. cM̅ n,calc = ([1]0/[I]0) × conv × M1 + Mextr with M1 = 204 g mol−1 and Mextr = 104 g mol−1. dDetermined by SEC in THF using a RI detector, calibrated with linear PS standards.

Figure 6. (A) SEC traces of polymers issued from ROMP of 1 in C2H4Cl2 at 50 °C using G3 as the initiator with [1]0/[I]0 ratio equal to 25/1, 100/ 1, and 250/1 (Table 3, runs 4, 6, and 8), and (B) M̅ RI n,SEC versus [1]0/[I]0 ratio.

Figure 7. Kinetic plots of ln([M]0/[M]t) versus reaction time for the ROMPs of 1 at 50 °C using (A) G2 and (B) G3 as initiators in THF-d8, C2H4Cl2, and CDCl3 with an initial ratio [1]0/[I]0 = 100.

G


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other factors constant. The kpapp of the ROMP using G3 as the initiator at 50 °C was increased 5-fold compared to the ROMP carried out at 25 °C (Figure 8). The resulting activation energies (Ea) for the propagation have been determined using the Arrhenius equation. The values were estimated to be 51 and 57 kJ mol−1 in THF-d8 and C2H4Cl2 solvents, respectively. Such values are close to the reported ones for the propagation during ROMP of (oxa)norbornene and derivatives (Ea = 60−79 kJ mol−1).29,37 Similar kinetic parameters could not be obtained using G2 instead of G3 as the initiator: a loss of control over the structure of polymers (ĐM ≥ 1.35) was observed using G2 in THF-d8 and C2H4Cl2 at 25 °C (Figure 9). In order to explain such a behavior, the kp/ki ratios (with kp corresponding to the propagation rate constant and ki corresponding to the initiation rate constant) were determined for ROMP of 1 using G2 as the initiator in CDCl3 and THF-d8 at 25 and at 50 °C according to a previously reported method (see Supporting Information).38 Polymerizations have been performed using low [1]0/[I]0 ratios in order to follow the disappearance of G2 by 1H NMR spectroscopy. When ROMP of 1 was performed with an initial ratio [1]0/[I]0 = 5, a multiplet appeared at δ = 17.5 ppm in the 1 H NMR spectrum of the crude mixture (Figure 10). This signal is attributed to the carbene proton of the propagating species. Comparing the integrations of the singlet RuCH of G2 at δ = 19.2 ppm against the singlet RuCH of the propagating carbene at δ = 17.5 ppm (Figure 10) allows the determination of the [I] t /[I] 0 ratio versus time and consequently of the kp/ki ratio (see eqs E1−E5 in the Supporting Information). The kp/ki values determined for the ROMP of 1 at 25 °C were found to be 10.4 and 384 when THF-d8 and CDCl3 were used as solvents, respectively. Such high kp/ki ratios values are consistent with a low initiation rate and a loss of control over the ROMP of 1 using G2 as the initiator at 25 °C. Increasing temperature to 50 °C led to a decrease of kp/ki ratios values to 2.4 in THF-d8 and to 3.5 in CDCl3 together with polymers having narrow molar mass distributions (ĐM ≤ 1.25). Such result is in good agreement with previous data reported in the literature concerning the ROMP of monomers with a moderate reactivity in the presence of initiator G2.39

Figure 8. Kinetic plots of ln([M]0/[M]t) versus reaction time for the ROMPs of 1 at 25 and at 50 °C using G3 as the initiator in THF-d8 and C2H4Cl2 with an initial ratio [1]0/[I]0 = 100.

(CDCl3), or C2H4Cl2 at 50 °C using G2 and G3 as initiators and an initial ratio [1]0/[I]0 = 100 and [1]0 = 0.25 mol L−1. The evolution of ln([M]0/[M]t) vs reaction time for G2 and G3 in different solvents is shown in Figure 7. The linear fit between ln([M]0/[M]t) and time throughout propagation suggests an apparent first-order kinetic in all solvents. From the linear region of kinetic plots in Figure 7, kpapp were determined for G2 and G3 used as initiators in different solvents (see eq 1). Higher values of kpapp were obtained when THF-d8 is used in comparison with CDCl3 or C2H4Cl2 whatever the initiator used (Figure 7). This is attributed to the probable coordination between THF and alkylidene actives centers, leading to a higher stability of the active species.32 The kpapp value also increases with increasing dielectric constant (ε) of the solvent, with kpapp values of 2 × 10−4 s−1 and 6 × 10−4 s−1 in CDCl3 (ε = 4.53) and in C2H4Cl2 (ε = 10.36), respectively, when G2 is used as the initiator. This can be rationalized as follows: solvent with higher dielectric constant resulted in propagating chain ends with higher activity for monomer addition due to an increase of initiation rate.36 This explanation is supported by higher monomer conversion values, 71% and 81% obtained in C2H4Cl2 and THF-d8, respectively, in comparison with 34% monomer conversion obtained in CDCl3 after 0.5 h of reaction at 50 °C using G2. The kinetic parameters obtained at 50 °C have been compared to those obtained at 25 °C while maintaining the

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CONCLUSIONS The present study investigated the reactivity of a series of Rubased catalysts as initiators for the ROMP of hydroxylfunctionalized cyclobutenes. ROMP of cis-4-benzyloxymethyl-

Figure 9. SEC traces of polymers obtained by ROMP of 1 at 25 °C using G2 as initiator with an initial ratio [1]0/[G2]0 = 100 in (A) CDCl3 and in (B) THF-d8. H


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Figure 10. Superposition of 1H NMR spectra from δ = 15.0 to 19.5 ppm of the crude mixture obtained for the ROMP of 1 in THF-d8 at 25 °C using G2 with [1]0/[I]0 = 5 after 2 and 25 min of reaction.

G3′) and Amélie Durand for 1H nuclear magnetic resonance (NMR) analyses.

3-hydroxymethylcyclobutene (1) carried out in the presence of Ru-based initiators led to well-defined polymers while cis-3,4bis(hydroxymethyl)cyclobutene (2) was reluctant to polymerize under the same conditions. The initiator structure and the polymerization solvent play crucial roles in the living character of the polymerization. Kinetics studies performed using cyclobutene 1 as the monomer showed clean first-order kinetics. Use of G2 or G3 as the initiator and of C2H4Cl2 or THF as the solvent was found to be the most efficient combination to obtain 1,4-polybutadienes containing both free and protected (benzyl ether) hydroxyl functionalities as the side groups, with predictable molecular weights up to 40 000 g mol−1 and narrow molecular weight distributions. Such polymers based on a 1,4-polybutadiene backbone are able to produce innovative graft copolymers containing polar side chains by using the pendant hydroxyl groups to initiate further orthogonal polymerizations.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental procedures for the synthesis of initiators, additional NMR spectra, and kinetic data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00696.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +33 (0)2 43 83 33 30; Fax +33 (0)2 43 83 37 54 (L.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Fabien Boeda for assistance with the syntheses of ruthenium-based initiators (catalysts NC, G3, and I


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