Abstract
Dnsity functional theory (DFT) calculations have been carried out for the highly selective cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex bearing an ancillary PNP ligand. It has been found that the chain initiation and propagation of butadiene polymerization occurs via the favorable cis-1,4-insertion route. The trans-1,4 and 1,2-insertion are unfavorable both kinetically and thermodynamically. The chain growth follows the π-allyl-insertion mechanism. The analyses of energy decomposition of transition states indicate that the likelihood of rival insertion pathways is predominantly controlled by the interaction energy of butadiene with a metal center and the deformation energy of butadiene moiety. The electronic factor of the central metal has a decisive influence on the cis- vs. trans-insertion and the regioselectivity (cis-1,4- vs. cis-1,2-insertion) is mainly determined by steric hindrance. Tetrahydrofuran (THF) coordination made monomer insertion less favorable compared with THF-free case and had more noticeable impact on the trans-monomer insertion compared with the cis case. During the chain propagation, cis-insertion of monomer facilitates THF de-coordination and the THF molecule could therefore dissociate from the central metal.
Highlights
The microstructures of polyisoprene (PIP) and polybutadiene (PBD) have significant influences on its physical and chemical properties, and for certain usage [1,2,3,4]. cis-1,4 selective polymerization of isoprene and butadiene is a very important process in the chemical industry to provide products that are among the most significant and widely used rubbers [5,6]
Based on the previous computational studies [28,31,35,48], one may propose that the reaction process for polymerization of butadiene catalyzed by rare-earth metal complexes has five possible pathways (Scheme 1): (a) 1,2-insertion of butadiene into the metal–alkyl bond to form
Presented is a computational study on the mechanism of cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex [(PNPPh )Y(CH2 SiMe3 )(THF)]+
Summary
The microstructures of polyisoprene (PIP) and polybutadiene (PBD) have significant influences on its physical and chemical properties, and for certain usage [1,2,3,4]. cis-1,4 selective polymerization of isoprene and butadiene is a very important process in the chemical industry to provide products that are among the most significant and widely used rubbers [5,6]. The microstructures of polyisoprene (PIP) and polybutadiene (PBD) have significant influences on its physical and chemical properties, and for certain usage [1,2,3,4]. Cis-1,4 selective polymerization of isoprene and butadiene is a very important process in the chemical industry to provide products that are among the most significant and widely used rubbers [5,6]. The synthesis of PIP and PBD with high cis-1,4-selectivity has attracted considerable interest. In this content, transition metal complexes have been widely used as precatalysts, which showed excellent performance in isoprene and butadiene polymerization [8,9,10]. Catalyst systems showing high both activity and stereoselectivity for isoprene and butadiene polymerization remained less. Further exploration of new and highly active catalysts for Polymers 2017, 9, 53; doi:10.3390/polym9020053 www.mdpi.com/journal/polymers
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