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Molecular Description of Heterophase Polymerization
Published in Hugo Hernandez, Klaus Tauer, Heterophase Polymerization, 2021
In ionic polymerization, the growing chain contains a strong nucleophilic or electrophilic ionic end group, which is also capable of reacting with an unsaturated bond or with a cyclic compound. However, ionic polymerization is very sensitive to the presence of other ions or strongly polar molecules (such as water); therefore, it is not well suited for aqueous polymerization processes. Coordination polymerization is a special type of ionic polymerization characterized by the use of a transition metal compound (coordination initiator), which strongly interacts with the double bond of a monomer. This interaction is stereo-selective and is referred to as coordination. Due to the high electronic density of the transition metal, the molecular orbitals of the monomer are strongly perturbed, and the double bond can be easily broken. Compared to the other polymerization mechanisms, the coordination polymerization is peculiar with respect to the growth mechanism because the new monomer is inserted between the catalyst (coordination initiator) and the polymer chain. The other polymerization mechanisms are different because the new monomer is added far away from the initiator residue.
Zinc(II) complexes containing N′-aromatic group substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amines: Synthesis, characterization, and polymerizations of methyl methacrylate and rac-lactide
Published in Journal of Coordination Chemistry, 2018
Sujin Shin, Hyungwoo Cho, Hyosun Lee, Saira Nayab, Younghak Kim
Syndiotactic-enriched PMMA has been observed in all the synthesized complexes, as analyzed by 1H-NMR spectroscopy [60, 61]. The syndiotactic enchainment ranged from 0.65 to 0.74 for the current systems and was found to be higher compared to the starting materials. These mediocre stereoselectivities of Zn(II) complexes are slightly higher when compared with reported complexes [35, 42]. Thus, the resultant syndiotacticity of PMMA was not high enough to confer a mechanism of coordination polymerization for the catalytic species in the current report. It is presumed that the presence of the benzhydrylamine group in [LFZnCl2] generates a coordination sphere around the metal center to effectively accommodate any steric clashes between incoming monomer, propagating polymer chain and ligand architecture during MMA polymerization. Thus, it can be concluded that the MMA polymerization activity of Zn(II) complexes of the current study should be considered as a function of steric bulk around the metal center. Further modification in ligand architecture to improve the catalytic performance and the resultant stereo-control of the MMA polymerization is presently going on in our laboratory.
Construction of polymeric Cu(I) N-heterocyclic carbene complex utilizing terpyridine-Fe(II) as linkers: formation of an efficient and recyclable catalyst
Published in Journal of Coordination Chemistry, 2018
The field of metal-coordination-driven functional assembly has been successfully used for the construction of various supramolecular entities, with continuous emergence of exciting applications in the catalysis [1, 2]. In this regard, Ding has shown that coordination polymers incorporating multiple catalytic active sites can be synthesized via directed-assembly strategies and the reactive structures have been shown to initiate or enhance catalytic processes [3–7]. Much of the current effort is directed toward the construction of supramolecular systems that are capable of performing a variety of catalytic chemical transformations. In recent years, the use of terpyridine as a key-component to the self-assembly of supramolecular network has been comprehensively demonstrated [8]. Terpyridine-metal coordination is especially appealing because of an easy access to back-to-back coordination polymers. In this respect, Fe(II) and Ru(II) were often used as the metal nodes for coordination polymerization [9]. N-heterocyclic carbene (NHC) has been extensively studied in the field of organometallics as a ligand capable of coordinating with a wide range of transition metals [10]. Diverse studies for applications of NHC-metal complexes have been conducted and a number of polymeric materials that contain tethered N-heterocyclic carbenes have been reported [11, 12]. Over the last decade, the self-supporting strategy, which incorporates ditopic ligands and active metal centers into organometallic assemblies, represents an efficient approach to immobilize specific catalysts. It is also an effective strategy for the immobilization of NHC compounds. For example, the use of di-imidazolium salts as precursors to NHC-based coordination polymers has been explored [13–16]. Moreover, the self-assembly of pyridyl, bipyridyl and carboxylic acid-functionalized imidazolium salts with transition metals to form NHC supramolecular networks were also achieved [17–19]. In contrast, the introduction of the NHC-metal units within supramolecular networks through terpyridine-metal complexes acting as linkers is rare despite the intensive investigation of various NHC complexes [20, 21]. In this paper, we reported the synthesis and self-assembly of a polymeric Cu(I) N-heterocyclic carbene complex utilizing terpyridine-Fe(II) as linkers and its catalytic behaviors in A3-coupling reaction of alkyne, aldehyde, and amine and Huisgen 1,3-dipolar cycloaddition reaction of organic azide and terminal alkyne were comparatively studied.