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Catalytic Cyclopropanation of Polybutadienes

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Catalytic Cyclopropanation of Polybutadienes Erschienen in: Journal of Polymer Science, Part A: Polymer Chemistry ; 48 (2010), 20. - S. 4439-4444 https://dx.doi.org/10.1002/pola.24231 Catalytic Cyclopropanation of Polybutadienes JUAN URBANO,1 BRIGITTE KORTHALS,2 M. MAR DI´AZ-REQUEJO,1 PEDRO J. PE´ REZ,1 STEFAN MECKING2 1Laboratorio de Cata´ lisis Homoge´ nea, Departamento de Quı´mica y Ciencia de los Materiales, Unidad Asociada al CSIC, Centro de Investigacio´ n en Quı´mica Sostenible, Campus de El Carmen s/n, Universidad de Huelva, 21007 Huelva, Spain 2Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany ABSTRACT: Catalytic cyclopropanation of commercial 1,2- or 1,4- bonyl-cyclopropene)]. Catalytic hydrogenation of residual dou- cis-polybutadiene, respectively, with ethyl diazoacetate catalyzed ble bonds of partially cyclopropanated polybutadienes provided by [TpBr3Cu(NCMe)] (TpBr3 ¼ hydrotris(3,4,5-tribromo-1-pyrazo- access to the corresponding saturated polyolefins. Thermal lyl)borate) at room temperature afforded high molecular weight properties are reported. 5 À1 (Mn > 10 mol ) side-chain or main-chain, respectively, carbox- yethyl cyclopropyl-substituted polymers with variable and con- trolled degrees of functionalization. Complete functionalization KEYWORDS: carbene addition; catalysis; functionalization of poly- of 1,4-cis-polybutadiene afforded poly[ethylene-alt-(3-ethoxycar- mers; organometallic catalysis; polar groups; polybutadienes INTRODUCTION Catalytic insertion polymerization of ethyl- polypropylene.6 Examples of catalytic post-polymerization ene and propylene is employed for the production of more reactions on saturated polyolefins are rare. The oxyfunction- than 60 million tons of polyolefins annually.1 These poly- alization of polyethylenes and polypropylenes by metal- mers are hydrocarbons, without any heteroatom-containing based catalysts can afford hydroxyl groups.7 We have functional groups, such as for example ester moieties. An recently reported8 the functionalization of saturated poly- incorporation of such polar moities is of broad interest, for butene and poly(ethylene-1 octene) by insertion of :CHCO2Et example, to increase interactions with polar surfaces, such into CAH bonds employing commercially available ethyl as metals, or to achieve stability toward hydrocarbon diazoacetate (EDA) as the carbene source, using solvents. Albeit significant advances have been achieved Br3 Br3 [Tp Cu(NCMe)] as a catalyst precursor (Tp ¼ hydro- most recently with Pd(II) catalysts,2,3 the incorporation of tris(3,4,5-tribromo-1-pyrazolyl)borate). Degrees of function- polar-substituted vinyl co-monomers H C¼¼CHX in catalytic 2 alization, which occurred on tertiary and secondary sites, insertion polymerization is challenging due to unfavorable were 4–10%. A significantly higher reactivity for post-poly- interactions of the polar group (X) with the active sites.4 Thus, ethylene-vinyl acetate copolymers are produced indus- merization reactions can be provided by double bonds in trially on a large scale by high pressure free-radical hydrocarbon polyolefins or elastomers. Polybutadiene is a polymerization, without microstructure control. Saturated particularly attractive substrate as the double bonds present high-performance nitrile-containing elastomers, prepared by in every repeat unit are suitable for a variety of transforma- free-radical copolymerization of butadiene and acrylonitrile tions, and via the polymer microstructure (1,4-cis; 1,4-trans with subsequent post-polymerization hydrogenation may and 1,2-vinyl repeat units) crystallinity and thermal serve as another example. properties can be varied. Thus, post-polymerization modifica- tion of polybutadiene by oxidation,9 epoxidation,10 hydro- An alternative to direct incorporation of polar-substituted boration,11 hydroformylation,12 and hydrosilylation13 has comonomers are post-polymerization reactions on insertion 14 polymers, however, for saturated polyolefins modifications been reported. Despite the large number of studies on 15 are challenging due to the lack of reactive moieties. Most metal-catalyzed olefin cyclopropanations, the addition of methods were developed for polypropylene and are based a carbene moiety, from diazocompounds, to such unsaturated 16 on radicals formed by the decomposition of peroxides at polymers has only been scarcely reported. We are only high temperature.5 A disadvantage is chain scission, which aware of the addition of dihalocarbenes to unsaturated accompanies these reactions due to the severe conditions. polymers upon sodium iodide-catalysed decomposition Carbenes, formed by decomposition of diazoacetate at high of (trifluoromethyl)phenyl-mercury, at high tempera- temperature, can insert into a CAH bond of polyethylene or tures.17 4439 Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-126834 (1) To a solution of 1,2-polybutadiene and catalytic amounts of the complex TpBr3Cu(NCMe) in dichloromethane, a solution of EDA in the same solvent was added slowly over 15 h by FIGURE 1 NMR spectra (400 MHz, CDCl3) of cyclopropanated means of a syringe pump. After completion, no EDA was 1,2-polybutadienes with variable degrees of functionalization. detected by NMR on the reaction mixture. NMR spectroscopy of the isolated polymers revealed their functionalization as well as the degree of incorporation of the carbene unit. We now report on saturated, cyclopropyl carboxylate-substi- Figure 1 shows the 1H NMR spectra of several samples with tuted polyolefins prepared by catalytic cyclopropanation of different degrees of functionalization. The CHCO2CH2CH3 polybutadienes with variable microstructures under mild fragments are unambiguously assigned to the resonance cen- conditions. tered at 4.10 ppm of the methylene protons and 1.25 ppm of the methyl group. The degree of incorporation can be RESULTS AND DISCUSSION determined from the relative integrals of the methylene protons of the CHCO2Et moiety and the olefinic resonances Functionalization of 1,2-Polybutadiene at 5.30 ppm. Early work from our group18 demonstrated that complexes TpxCu (Tpx ¼ hydrotrispyrazolyborate ligand) catalyze the Polymers with degrees of functionalization in the range from cyclopropanation of low-molecular-weight olefins with EDA 3 to 80% were obtained, depending on the EDA: polymer under mild conditions. We have now studied the catalytic double bond ratio (Table 1). A straightforward, linear corre- modification of commercially available polybutadienes by lation was observed (Fig. 2), allowing for a facile control of this approach (eq 1): polymer composition. TABLE 1 Cyclopropanation of 1,2-Polybutadienea Degree of c c Cat./EDA/ Functionalization Mn Mw Tg Tm b 3 3 d d Entry PBD [%] [10 g/mol] [10 g/mol] Mw/Mn [ C] [ C] 1 n.a. 0 100 245 2.4 À10 96 2 1/30/300 3 93 256 2.8 À485 3 1/50/300 4 98 280 2.9 À380 4 1/50/150 10 86 273 3.2 6 / 5 1/100/300 16 98 254 2.6 10 / 6 1/200/300 36 133 276 2.1 26 / 7 1/150/150 42 121 267 2.2 30 / 8 1/300/300 43 120 274 2.3 27 / 9 1/600/600 45 121 267 2.2 31 / 10 1/450/300 68 150 293 1.9 43 / 11 1/600/300 80 156 289 1.8 49 / a Br3 0.3 g (5.5 mmol of polybutadiene, 9.25 lmol Tp Cu(NCMe), 20 mL CH2Cl2; EDA added as solution in 10 mL of CH2Cl2. b Determined by 1H NMR spectroscopy on the isolated polymer. c Determined by GPC in THF at 40 C vs. linear PS standards. d Determined by DSC. 4440 Functionalization of 1,4-Polybutadiene The above methodology was studied as a means for modifi- cation of a 1,4-cis-polybutadiene elastomer (eq 2) NMR spectroscopy on the isolated polymer again allows for confirmation and quantification of cyclopropanation (Fig. 3). Resonances for the carboxylate moiety were observed in similar regions of the NMR spectra related to the aforemen- tioned 1,2-polybutadiene derivatives. FIGURE 2 Degree of functionalization (as percentage of cyclo- A completely functionalized polymer could be obtained using propanated double bonds) of 1,2-polybutadiene with the EDA: a two-fold excess of EDA (Table 2). The resulting polymer, polymer double bond molar ratio (catalyst to polymer double poly[ethylene-alt-(3-ethoxycarbonylcyclopropene)], that has bonds 1/300). not been yet reported, to our knowledge, displays molecular weight values of Mn ¼ 360,000, Mw ¼ 637,000 with Tg ¼ 25 C. Such degree of incorporation based on converted In view of that molecular weights from GPC are apparent double bonds of the polymer has only been once in the molecular weights determined vs. linear polystyrene stand- aforementioned addition of dihalogencarbene units from ards, and the hydrodynamic behaviour must be expected to mercury-based carbene precursors.17 vary with functionalization, the molecular weights overall The symmetrical nature of poly[ethylene-alt-(3-ethoxycarbo- reflect the increase of molecular weight with increasing nylcyclopropene)] and reduced structural complexity by degree of functionalization. comparison to only partially substituted products allows for Furthermore, the essentially unaltered polydispersity can be a more detailed NMR analysis (see SI for spectra). 1H/13C taken as an evidence of the lack of chain scission, a notori- chemical shifts of the ethoxy group respectively appear at ous problem of many post-polymerization reactions. GPC 1.2/14 ppm for the methyl, 4.1/61 ppm for the methylene, traces obtained with UV and RI detection are quite similar, the carbonyl group resonating at 173 ppm. For less function- indicating that modification occurs uniformly over all molec- alized groups, relative integration of these resonances and ular weights. Glass transition temperatures increase with the olefin ones provide the degree of incorporation (see increasing degree of modification, which can be related to an Fig. 3). It is worth mentioning that these metal-induced increased chain stiffness and reduced mobility by the compa- carbene additions from diazocompounds occur throughout a ratively bulky carboxyethyl cyclopropyl substituents. At the concerted mechanism,15 therefore retention of the con- same time Tm and crystallinity are reduced by comparison to figuration of the CAC double bond is expected.
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