Article pubs.acs.org/Macromolecules
Mild Catalytic Functionalization of Styrene−Butadiene Rubbers Á lvaro Beltrán,‡ Bella Pilar Gómez-Emeterio,‡ Carlos Marco,§ Gary Ellis,§ M. Dolores Parellada,†,⊥ M. Mar Díaz-Requejo,‡,* Sergio Corona-Galván,†,⊥,* and Pedro J. Pérez‡,* ‡
Laboratorio de Catálisis Homogénea, Departamento de Química y Ciencia de los Materiales, Unidad Asociada al CSIC, Centro de Investigación en Química Sostenible (CIQSO), Campus de El Carmen s/n, Universidad de Huelva, 21007-Huelva, Spain § CSIC−Instituto de Ciencia y Tecnología de Polímeros, c/Juan de la Cierva 3, 28006 Madrid, Spain † Dynasol Elastómeros, P° de la Castellana 257, Planta 1a, 28046 Madrid, Spain ⊥ Centro de Tecnología de Repsol, Ctra. de Extremadura km. 18, 28931 Móstoles, Spain S Supporting Information *
ABSTRACT: Styrene−butadiene rubbers (SBR) have been modified upon catalytic addition of carbene groups (:CHCO2Et) from ethyl diazoacetate (N2CHCO2Et) under very mild conditions using copper catalysts. The modified rubbers contained 3− 5% (w/w) of carboxylate groups and displayed hydrodynamic properties very similar to those of the starting material, evidencing the lack of chain scission processes. Preliminary studies on the effect of the presence of the carboxylate functionality on some physical properties of the rubbers are reported, and the morphology of their blends with nylon-6 is also described.
■
INTRODUCTION One of the most important features of a polymer in terms of its practical use resides in the mechanical behavior.1 The degree of deformation under stress can be crucial to decide a possible application of a given material. It is well-known that this behavior depends of the presence of polar groups in the polymer chains, a feature that also affects other polymer properties such as adhesion or solvent resistance,2 and can be of particular importance in polymer blending as a strategy to improve material performance through interfacial and morphological modifications.3 However, the preparation of polymers bearing polar groups (Scheme 1) is not a routine procedure,
consists of the so-called a posteriori functionalization of the polymer (Scheme 1b), in which the polar group is added to a previously synthesized polymer. This strategy has been developed using radical processes5,6 that generated functionalized materials, albeit at the cost of carbon−carbon bond cleavage and subsequent chain scission. On the other hand, a few examples have been reported that employ a metal-based catalyst to induce the incorporation of the polar group. The hydroxylation polyolefins was first described by Hillmyer, Hartwig and co-workers7,8 and later expanded by Bae’s group,9 using a process in which the C−H units are first borylated with the aid of rhodium catalysts and the resulting boronic groups are further hydroxylated. An alternative approach has been based on the release of carbene moieties from diazocompounds, either thermally induced or catalyzed by metal complexes. The former was described by Aglietto and coworkers10 by simple thermal decomposition, although at high temperature, of ethyl diazoacetate, producing functionalized materials derived from the insertion of the :CHCO2Et units into C−H bonds of polyethylene and polypropylene. Later, our group described the same transformation but using the copperbased catalyst TpBr3Cu(NCMe) (TpBr3 = hydrotris(3,4,5tribromopyrazolyl)borate, Scheme 2) that allowed the controlled insertion of the carbene group into saturated polyolefins (Scheme 2).11 At variance with the thermal procedure, the very mild copper-catalyzed procedure precluded the appearance of chain scission processes and favored the control of the degree of incorporation of polar groups. More recently, the same methodology has been applied to polybutadienes, the modified materials containing cyclopropane rings (Scheme 2).12 As a consistent feature of this methodology, the transformation was
Scheme 1. Strategies for the Preparation of Polymer Bearing Functional Groups: (Top) Direct Polymerization of Functionalized Monomers; (Bottom) Post-Polymerization Functionalization
compared with the plethora of olefin polymerization protocols described to date.4 This process is mainly undertaken via radical-based mechanisms that suffer little or no control over the selectivity. On the other hand, when catalysts are employed to promote the polymerization reaction, a route that usually allows certain selectivity control, the tolerance of the catalyst to the polar groups is frequently low, and catalyst deactivation constitutes a major drawback of this methodology. An alternative pathway © 2012 American Chemical Society
Received: October 9, 2012 Revised: November 10, 2012 Published: November 21, 2012 9267
dx.doi.org/10.1021/ma302120a | Macromolecules 2012, 45, 9267−9274
Macromolecules
Article
in several properties of the functionalized materials have been evaluated.
Scheme 2. Copper-Catalyzed Carbene Insertion Functionalization of Saturated and Unsaturated Polyolefins
■
RESULTS AND DISCUSSION Functionalization of Styrene−Butadiene Rubbers with Ethyl Diazoacetate Catalyzed by TpxCu Complexes. Following our previous work, we have chosen the complex TpBr3Cu(NCMe) (1) as the precatalyst for this study. In addition, and for comparison purposes, we have employed the bulkier TpPhCu(NCMe),16 2 (TpPh = hydrotris(3-phenylpyrazolylborate)), to evaluate the effect of a smaller catalytic pocket on the course of the reaction. From a practical point of view, incorporation of carboxylate in SBR matrixes in the range 1−5% (w/w) is desirable.1 As a general procedure, a solution of ethyl diazoacetate (EDA) in cyclohexane was slowly added onto a solution of SBR with catalytic amounts of the copper complex (1 or 2) in the same solvent, all the process being carried out at room temperature. At the end of the addition time, no EDA was detected by GC, and the crude product (after isolation by precipitation with methanol) was investigated by NMR spectroscopy. Figure 1 shows the 1H NMR
carried out at room temperature under an exquisite control of the percentage of carboxylate groups incorporated. As a consequence of the aforementioned results, we decided to explore possible applications of the above strategy using styrene−butadiene rubbers (SBR, Scheme 3) as starting Scheme 3. (Top) Potential Reaction Sites for the MetalCatalyzed Carbene Insertion in a SBR Material; (Bottom) Potential Side, Non-Desired Reactions That Might Occur in This Catalytic System
Figure 1. Top: 1H NMR of a sample of carboxylated SBR (carboxylate groups indicated by arrows). Bottom: Starting SBR material. NMR spectra recorded at 20 °C in CDCl3.
materials, since the incorporation of the carboxylate moiety into the SBR chain can affect properties such as adhesive and impact resistance. 1 We selected a SBR material (see Experimental Section) that displays a number of different potential sites to react with the metallocarbene intermediate formed during the catalytic cycle (Scheme 3, Top): (i) two different, terminal (vinyl) and internal (cis or trans) double bonds that may undergo cyclopropanation; (ii) alkylic, tertiary and secondary C−H bonds suitable for carbene insertion; (iii) phenyl rings, which could be converted into cycloheptatriene moieties due to the metal-catalyzed Büchner reactions. These transformations have been previously described in our laboratory using copper-based catalysts with several monomeric olefins,13 hydrocarbons14 and arenes,15 respectively. Therefore, we are facing a quite complex catalytic system that requires the appropriate catalyst design for the selective functionalization of the substrate both from an inter- (SBR vs side reactions, Scheme 3) and an intramolecular (different reaction sites in SBR) standpoint. In spite of such difficulties, we have found that the use of copper-based catalysts in the reaction of SBR with EDA (ethyl diazoacetate, N2CHCO2Et) allows the incorporation of carboxylate groups into the SBR chains, under mild conditions, without any chain scission process. The effect of these groups
spectra of both the functionalized SBR and the starting material for comparative purposes. Two characteristic resonances centered at 4.10 (q) and 1.25 (t) ppm were assigned to the ethyl group of CO2Et, thus assessing the incorporation of the carbene:CHCO2Et into the polymer chain. 13C{1H} NMR data have provided additional as well as relevant information. Ethyl ester resonances appeared at 60.9 and 14.3 ppm. A third new resonance centered at 27.1 ppm was assigned to the methine group of C(H)CO2Et of a cyclopropane ring by means of 2D NMR. No other signals attributable to a possible incorporation of the carbene groups into C−H bonds or Ph-rings of the rubber were observed; therefore the reaction took place onto the double bonds in a selective manner. The degree of incorporation was calculated in a straightforward manner by integration of resonances in the 1 H NMR spectrum (−CO2CH2CH3 vs olefinic signals), GPC analysis using UV detection was also performed to obtain information on the changes in the polydispersity. As shown in Figure 2, similar PDI and Mn values were found in both cases and are a consequence of the lack of chain scission processes during catalysis. This feature was also observed in our 9268
dx.doi.org/10.1021/ma302120a | Macromolecules 2012, 45, 9267−9274
Macromolecules
Article
catalytic activity than complex 2. The same trend was observed when moving to 20 g of SBR, with degrees of incorporation of 4.6−5.7% (Table S1 in the Supporting Information). Stability of Modified Materials. The functionalized materials prepared as above showed a good solubility in tetrahydrofuran and their GPC analysis revealed similar polydispersities to those of the starting materials. However, after two months of storage at room temperature under the same atmosphere in which they were synthesized (nitrogen or air, Table 1), those stored under air decomposed, as inferred by the observance of loss of solubility in THF as well as changes in GPC data (see Supporting Information). Therefore, although the use an atmosphere of nitrogen or air during the functionalization process seems to have little or no effect on the short-term degradation, the exposure to air led to degradation. Those stored under nitrogen remained unaltered. To verify the above data, we have carried out two series of functionalization experiments (see Table 2) and the resulting Figure 2. GPC (THF, 40 °C, UV detection vs polystyrene standards). Starting material, SBR (bottom) and functionalized SBR (top).
Table 2. Study of the Stability of the Samplesa entry catalyst
previous work,11,12 at variance with other procedures for the induction of polar groups in polymer chains.5,6 Optimization and Scale Up. As mentioned above, to test the influence of the ligand in this transformation two copper complexes were selected as catalysts, TpBr3Cu(NCMe) (1) and TpPhCu(NCMe) (2). Table 1 displays the results obtained in several experiments carried out upon varying catalyst loadings, mass of rubber, and the reaction atmosphere (N2 or air). Employing the aforementioned protocol, ca. 3−5% of incorporation of the CHCO2Et group into the rubber was targeted. Both catalysts led to similar results with regard to the degree of functionalization in weight that occurred within a range of 3.1−3.9% in all cases. However, some variability was observed in their effect on the side reactions. In the case of catalyst 1, the carbene coupling reaction to give ethyl fumarate and maleate was almost negligible (Table 1, entries 3 vs 4, 5 vs 6 and 7 vs 8). On the other hand, complex 2 was less active toward the functionalization of cyclohexane (Table 1 entries 3− 4 or 5−6). Scale-up experiments were carried out with an initial amount of 10 g of the rubber. Following a similar procedure, several reaction conditions were tested (see Table S1 in the Supporting Information). In all cases, the functionalization was achieved with values within the desired range (3−4%), while the average percentage of initial EDA incorporated into the polymer depended of the catalyst employed. The reactions were conducted in air. At this scale, complex 1 displays a better
1 2 3 4 a
synthesized under
heating under N2b heating under O2b
N2 air N2 air
1 1 2 2
stable decomp stable decomp
stable decomp decomp decomp
2.5 g of polymer sample. bHeating for 4 days at 60 °C.
materials were heated in the solid state at 60 °C for 4 days, under N2 or O2 atmospheres. All samples synthesized and further heated under a nitrogen atmosphere maintained their properties. In contrast, those that were synthesized in air then heated under nitrogen suffered degradation, similar to the samples prepared in air, and heated under oxygen. However, the samples synthesized under a nitrogen atmosphere and heated under oxygen showed a catalyst dependent-behavior (Table 2, entries 1 and 3). With 1, no decomposition was observed whereas with 2 the samples showed some degradation. This is in good agreement with the already reported high resistance to oxidation of the former.16 At this stage we can conclude that degradation of functionalized polymers is due to the presence of some of the copper catalyst occluded in the polymer, particularly Cu(II) ions formed upon oxidation induced by air. Development of Copper Removal Strategies. In order to completely remove the metal impurities, we have taken advantage of the capabilities of silica gel to absorb the TpxCu complexes.17 Thus, when SiO2 was added to the reaction
Table 1. Functionalization of SBR by EDA Insertion Catalyzed by 1 and 2a entry 1 2 3 4 5 6 7 8 a
catalyst (mmol) 1 2 1 2 1 2 1 2
(0.01) (0.01) (0.01) (0.01) (0.025) (0.025) (0.025) (0.025)
SBR (g)
EDA (mmol)
N2/air atmosphere
% DEF + DEMb
% cyclohexane functionalized
% EDA incorporated
% functionalization (in weight)
2.5 2.5 5 5 2.5 2.5 5 5
1 1 2.5 2.5 1 1 2.5 2.5
N2 N2 N2 N2 air air air air
− − 3.0 8.5 − 5.0 2.0 18.9
1.0 1.0 6.0 1.5 1.0 1.5 5.3 2.1
99.0 99.0 91.0 90.0 99.0 93.5 93.2 79.0
3.4 3.4 3.9 3.8 3.4 3.1 3.9 3.3
Reaction time = 12 h, room temperature, average two runs. bDEF = diethyl fumarate; DEM = diethyl maleate. 9269
dx.doi.org/10.1021/ma302120a | Macromolecules 2012, 45, 9267−9274
Macromolecules
Article
mixture before inducing the precipitation of the polymer with methanol, studies based on atomic absorption spectroscopy (AAS) showed a decrease in the amount of occluded copper as shown in Table 3. In a second set of experiments using complex
Table 4. Functionalization of Partially Hydrogenated SBR by EDA Insertion Catalyzed by 1 and 2a
1 2 3 4b 5b 6c
Table 3. Removal of Cu by Treatment with Silica-Gela entry 1 2 3 4 5b 6b 7c 8c
catalyst (mmol) 1 2 1 2 1 2 2 2
(0.01) (0.01) (0.025) (0.025) (0.01) (0.01) (0.05) (0.05)
SBR (g)
EDA (mmol)
% Cu occluded in polymer
2.5 2.5 5 5 2.5 2.5 10 10
1 1 2.5 2.5 1 1 5 10
36 23 43 31 28 24
