Chain transfer catalysis

H transfer from the a carbon of an organic radical back to a metaUoradical (the reverse of 1.18) is a key step in CCT (catalytic chain transfer), a process that can be used to control radical polymerizations. Many of the effective CCT catalysts contain inexpensive and abundant transition metals the first catalysts all contained cobalt 

Suess [64] and Schulz [65] observed that the presence of solvents during the polymerization of styrene lowered the molecular weight In the case of CCLt, Mayo suggested that the chain-carrying radical abstracted Cl from CCh (1.20), terminating the chain but leaving CCls, which added to monomer and initiated a new chain (1.21). 

Mayo defined a chain transfer constant that could be empirically determined by Equation 1.22. P is the degree of polymerization, S and M are concentrations of solvent and monomer respectively, and Pq is the degree of polymerization in the absence of solvent. Thiols are particularly effective (albeit stoichiometricaUy as in 1.20 and 1.21) reagents for chain transfer [66]. 

It is possibly to carry out chain transfer catalytically. The process is related to atom transfer radical polymerization (ATRP) [67, 68] and related living polymerizations which keep the concentration of chain-carrying radicals low. ATRP employs a halide complex (often Ru X) that is subject to facile one-electron reduction that complex reversibly donates X to the chain-carrying radical (1.23) and thereby decreases the concentration of the latter [69, 70]. 

The efficienqf of a chain transfer catalyst has traditionally been measured by its chain transfer constant (Cs), which is the ratio of the rate constant for chain transfer (feu) to that for propagation (fep). Values of Cs are generally determined from the slope of a plot of 1/DP vs. (CTA)/(M] (the Mayo method. Equation 1.27 [74]), in which DP is the degree of polymerization, CTA is the chain transfer agent, and M is monomer. 

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Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

For general reviews on catalytic chain transfer catalysis, see (a) J. P.A. Heuts,... 

The ratio between the metalloradical and the corresponding hydride during chain transfer catalysis is determined by the balance between H transfer from the chain-carrying radical to M (as in 1.25) and reinitiation by transfer from M-H to the monomer (as in 1.26). We can define a Cs(true) in terms of the apparent Cs (from the slope of a Mayo plot) by Equation 1.28, where [M ]st is the concentration at steady state. For the various Cr hydrides, with their relatively strong Cr-H bonds, the balance lies shghtly on the side of H transfer to Cr, so [Cr-H] > [Cr ],... 

There is a possibility that, during chain transfer catalysis, H transfer occurs not only to monomer but also to the vinyl-terminated oligomers generated by chain transfer. Wayland and coworkers have reported that the degree of polymerization increases with the extent of conversion during the polymerization of MMA and... 

Tang, L. (2005) Scope and mechanism of chain transfer catalysis with metalloradicals and metal hydrides. 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

In specific applications to phase transfer catalysis, Knbchel and his coworkers compared crown ethers, aminopolyethers, cryptands, octopus molecules ( krakenmole-kiile , see below) and open-chained polyether compounds. They determined yields per unit time for reactions such as that between potassium acetate and benzyl chloride in acetonitrile solution. As expected, the open-chained polyethers were inferior to their cyclic counterparts, although a surprising finding was that certain aminopolyethers were superior to the corresponding crowns. 

The nucleophilic substitution on poly(vinyl chloroformate) with phenol under phase transfer catalysis conditions has been studied. The 13c-NMR spectra of partly modified polymers have been examined in detail in the region of the tertiary carbon atoms of the main chain. The results have shown that the substitution reaction proceeds without degradation of the polymer and selectively with the chloroformate functions belonging to the different triads, isotactic sequences being the most reactive ones. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

We next focus on the use of fixed-site cofactors and coenzymes. We note that much of this coenzyme chemistry is now linked to very local two-electron chemistry (H, CH3", CH3CO-, -NH2,0 transfer) in enzymes. Additionally, one-electron changes of coenzymes, quinones, flavins and metal ions especially in membranes are used very much in very fast intermediates of twice the one-electron switches over considerable electron transfer distances. At certain points, the chains of catalysis revert to a two-electron reaction (see Figure 5.2), and the whole complex linkage of diffusion and carriers is part of energy transduction (see also proton transfer and Williams in Further Reading). There is a variety of additional coenzymes which are fixed and which we believe came later in evolution, and there are the very important metal ion cofactors which are separately considered below. 

Keywords Catalyzed olefin polymerization, Chain shuttling catalysis, Chain transfer, Olefin block copolymers, Thermoplastic elastomers... 

P. Bako, T. Novak, K. Ludanyi, B. Pete, L. Toke, G. Kegle-vich, D-Glucose-based Azacrown Ethers with a Phos-phonoalkyl Side Chain Application as Enantioselective Phase Transfer Catalysis , Tetrahedron Asymmetry 1999,10, 2373-2380. 

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