Stereoregular polyolefins are of vast academic and industrial interest.1 A multitude of polymer microstructures are accessible today via catalyst design. A control of polymer morphology is equally important. Polyolefin particle sizes and structures have been studied intensively in the context of traditional olefin polymerization processes. However, a synthesis of defined submicrometer particles of stereoregular polyolefins remains a challenge, despite the increasing general interest in the preparation of particles and other structures on the length scale of 1 nm to 1 μm (i.e., the colloidal range). Such defined particles, that is individual nonaggregated and thus easy to handle particles could be possible by preparing them in an aqueous dispersion, that is a polymer latex. Beyond the aforementioned fundamental interest in the synthesis of small particles, it can be noted that polymer dispersions are also of practical interest, e.g. for environmentally friendly coatings and paints.2 The early transition metal Ziegler or metallocene catalysts used traditionally for stereospecific olefin polymerization are very sensitive to water.3,4 Late transition metal catalysts are much more water-stable, and they enable the synthesis of polyolefin dispersions by ethylene polymerization in aqueous systems.5,6 However, transition metal catalysts most often polymerize 1-olefins in a nonstereospecific fashion.7 As an approach to the preparation of dispersions of submicrometer particles of stereoregular polyolefins, we have investigated catalytic polymerization in emulsion proceeded by polymer-analogous modification in the dispersed particles. Catalytic polymerization of butadiene with a CS2-modified cobalt catalyst yields latices of syndiotactic 1,2-polybutadiene (1,2-PBD).8 Catalytic hydrogenation9 of such latices seemed a possible route to stereoregular poly(1-butene) dispersions (Scheme 1). To the aqueous 1,2-PBD dispersion, ruthenium(III) tris(2,4-pentanedionate) and triphenylphosphine were added, and the emulsion was submitted to hydrogen pressure. Colloidal stability is fully retained despite the high reaction temperature (vide infra). The 1H NMR spectra of the polymer (Supporting Information, Figure S1) reveals the disappearance of the olefinic protons (5.0-5.5 ppm range), demonstrating complete hydrogenation (> 99%) of the double bonds present in the starting polymer (Table 1, entry 4, 75 bar H2). No metathesis, which could be conceivable,11 is observed. Remarkably already low hydrogen pressures (5 bar) are sufficient to hydrogenate a substantial part of the double bonds of the polymer as seen by NMR and IR (65%, entry 1). At higher pressures an increase of the degree of hydrogenation is observed (entries 2-4) under otherwise identical conditions. Reasonable conversions are only observed at temperatures greater than 100 °C (entry 5). Most likely, this effect relates to the necessity of some softening of the crystalline dispersed particles to facilitate diffusion of the hydrogen substrate and possibly also the catalyst in the initially highly crystalline latex particles. At 150 °C substantial coagulation was observed. Differential scanning calorimetry (DSC) measurements were performed on the isolated polymers obtained by precipitation from the dispersions (cf. Supporting Information, Figure S4). As expected, the high Tm of the 1,2-PBD (190 °C) is reduced by hydrogenation of part of the vinyl groups (65%, entry 1). Complete hydrogenation affords a rubbery, noncrystalline polymer. No thermal transitions are observed, in accordance with the properties expected for syndiotactic poly(1-butene).12 Transmission electronic microscopy (TEM) reveals how the increase of the degree of hydrogenation of the polymer dispersion results in soft particles, by comparison to the hard crystalline particles of the starting 1,2PBD dispersion (Figure 1). Chain scission resulting in substantial molecular weight decrease is a crucial issue for any polymeranalogous modification, albeit it appears unlikely for the reaction studied. GPC reveals no evidence for chain ruptures, e.g. for a >99% hydrogenated polymer (Table 1, entry 4) Mw ) 1.2 × 105 g mol-1 was determined vs Mw ) 1.1 × 105 g mol-1 for the 1,2-PBD starting material (Figure S6). This difference is within experimental error, given that these are apparent molecular weights (determined vs polyethylene standards). One of the remarkable attributes of this catalytic polymer-analogous modification is that it proceeds in * Corresponding author. E-mail: stefan.mecking@unikonstanz.de. Table 1. Hydrogenation Resultsa
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Sep 4, 2023
Xiaobei Zhao + 6
Open Access
