Coordination mechanism

The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

The polymerization filling was effected by the ion-coordination mechanism [17-19]. The monomers were ethylene, propylene, allene, os-butylene, butadiene. The fillers were mineral materials such as ash, graphite, silica gel, glass fibers. The ultimate aim of filler conditioning prior to polymerization is to secure, on its surface, metal complex or organometallic catalysts by either physical or chemical methods [17-19],... 

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

The theoretical problems of the coordination mechanism of the propagation reaction were considered by Cossee et al. 170,186, 187). 

It should be noted that, similarly to olefin, the insertion of carbon monoxide in the active bond in the propagation centers of polymerization catalysts also follows the coordination mechanism 175). The insertion of carbon monoxide into the active bond was not feasible when a vacant coordination site of the metal ion had been occupied by phosphine. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

FIGURE 2 Anionic, cationic, and coordination mechanisms of polymerization of e-caprolactone and related lactones. 

ICM (Internal Coordinate Mechanics) software, MolSoft LLC, http //www. molsoft.com/icm pro.html. 

The top-down approach, which defines appropriate hierarchical coordination mechanisms between the different decision levels and decision structures at each level. These structures force constraints on lower operating levels and require heuristic decision rules for each task. Although this approach reduces the size and complexity of scheduling problems, it potentially introduces coordination problems. 

Middleware This is the communication and coordination mechanism to enable components to collaborate across machine boundaries. 

Chain polymerisation involves three major steps (i.e., initiation, propagation and termination). This process of chain polymerisation can be brought about by a free radical, ionic or coordination mechanism. 

Stereoselective polymerization may proceed by ionic or coordination mechanisms. In many cases one admits that in the counterion or in the catalytic complex enantiomeric active centers exist, which give rise to predominantly (R) or (S) chains, respectively. Such centers may exist prior to polymerization or may be formed by reaction of a nonchiral precursor with the enantiomeric mixture of the monomers. Alternatively, one can think that the stereoselectivity depends mainly on the interaction between the entering monomer molecule (which is chiral) and the last unit in the chain (also chiral) according to this hypothesis, the enantiomeric excess inside each chain is generally low, because the occurrence of an accidental error brings about an inversion of the sense of stereoselection. 

When nonpolar solvents are employed, polymerization proceeds by an anionic coordination mechanism. The counterion directs isotactic placement of entering monomer units into the polymer chain. The extent of isotactic placement increases with the coordinating power of the counterion (Li > NaK. Cs). The small lithium ion has the greatest coordinating power and yields the most stereoselective polymerization. Increased reaction temperature decreases the isoselectivity. 

The activated methyl groups of 5,7-dimethyl-l,8-naphthyridin-2-one (106) can be selectively metallated, reaction with two equivalents of butyllithium giving (107), while with two equivalents of sodamide in liquid ammonia (108) is obtained. Both (107) and (108) gave appropriate products on treatment with electrophiles (81JOM(213)405>. A possible explanation of this result involves a coordination mechanism for the formation of (107) and an acid-base mechanism giving (108). 

Box 14.6 Internal Control Systems - combining coordination mechanisms for social responsibility (R. Pyburn, PhD research on ICS as part of the SASA Project)... 

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

The mechanism of the polymerization of propylene to produce isotactic structure has been studied extensively. Natta and his coworkers hav presented the generally accepted anionic-coordinate mechanism. However, there has been increasing evidence that the Natta anionic-coordinate mechanism does not operate to produce isotactic polypropylene. 

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