Chelate initiator

Clapper, D.L., PL. Lamothe, J.A. Davis, and D. Epel. 1985b. Sperm motility in the horseshoe crab. V zinc removal mediates chelator initiation of motility. Jour. Exper. Zool. 236 83-91. 

The search for new initiators continues past the metallocenes. There is always the desire to produce polymers with different structures in a controlled manner hy selective choice of the initiator. New initiators might produce polymers with structures different from those obtained by the metallocenes, such as branched instead of unbranched structures or different stereoregularity or different molecular weight. Finally, commerical interests require a continuing search for new initiators that have not yet been patented. Several of the newer classes of stereoselective initiators are described below. They are referred to here by the general terms postmetallocene or chelate initiators. 

Synthesis of the previously unreported W(tbq)4 chelate initially involved a Schraup synthesis of Htbq. Previous attempts in other laboratories had failed because of the acid cleavage of the tert-hutyl group in the arsenic acid oxidation step (16). We have successfully used picric acid and 4-tgrt-butyl-2-nitrophenol as oxidants with yields of 5-10%. The procedure is similar to that used for Hmq (3,6). The W(tbq)4 chelate was prepared by the decarbonylation of W(CO)e in mesitylene heated under reflux, followed by sublimation of unreacted material and crystallization from chloroform. Chromatographic separation on silica gel with 1 1 v/v CHCI3 hexane produced an analytically pure W(tbq)4 chelate. 

By comparison to peroxides, the azo compounds are generally not susceptible to chemically induced decompositions. It was shown,however, that it is possible to accelerate the decomposition of a,a -azobisisobutyronitrile by reacting it with bis(-)-ephedrine-copper(II) chelate. The mechanism was postulated to involve reductive decyanation of azobisisobutyronitrile through coordination to the chelate. Initiations of polymerizations of vinyl chloride and styrene with a,a -azobisisobutyronitrile coupled to aluminum alkyls were investigated. Gas evolution measurements indicated some accelerated decomposition. Also, additions of large amounts of tin tetrachloride to either a,a -azobisisobutyronitrile or to dimethyl-a,a -azobisis-obutyrate increase the decomposition rates. Molar ratios of [SnCl4]/[AIBN]= 21.65 and [SnCl4]/[MAIB] = 19.53 increase the rates by factors of 4.5 and 17, respectively. Decomposition rates are also enhanced by donor solvents, like ethyl acetate or propionitrile in the presence of tin tetrachloride. ... 

Figure 18 Effects of solution and grafting method on morphological structures of cotton cellulose-polyacrylonitrile copolymers as shown by electron microphotographs of fibrous copolymer cross-sections (a) cerium(IV) chelate initiation with acrylonitrile dissolved in water (b) immersion high-energy-irradiated cellulose fiber in acrylonitrile (32%) in 80% aqueous ZnCl2 (c) immersion of cellulose fiber in solution as in (b) then high-energy irradiation (d) immersion of cellulose fiber in acrylonitrile (15%) in 75% aqueous ZnClj then high-energy irradiation (e) cyanoethylation of cellulose fiber to yield...

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