Catalytic chain transfer reviews

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

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

Catalytic Chain Transfer. A highly useful variant of chain transfer was discovered in the 1970-1980s in the Soviet Union (216). A number of reviews have been published in recent years (217-221) on this synthetic method which has acquired the nomenclature of either catalytic chain transfer (CCT) or special chain transfer (SCT). The most commonly adopted catalysts are based on low spin cobalt macrocycles, although other metal-containing complexes have also been suggested in the patent and scientific literature. Some typical catalyst structures are shown as 12 and 13. 

Several reviews devoted to CRP have been already been published, and readers may refer to proceedings from ACS Meetings on CRP [42,43], general reviews on CRP [44-48], reviews on ATRP [30,49-54], on macromolecular engineering and materials prepared by ATRP [55], on nitroxide mediated polymerization (NMP) [56-58], on catalytic chain transfer [59,60], and on reversible addition fragmentation transfer polymerization, RAFT [61]. 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

Functional oligomers with a terminal alpha-substituted acrylate group can be synthesised by catalytic free-radical chain transfer polymerisation based on cobalt II or II chelates. The apphcations of such oligomers in the design of low molec.wt., graft and block copolymer emulsions and dispersions for waterborne, two-component PU paints are reviewed. The emulsions and dispersions are shown to have composition and molec.wt. control and to exhibit... 

Since a number of the studies we shall review were concerned with the effect of synthetic chain molecules or micelles on the hydrolysis rate of nitrophenyl acetate and similar esters, it will be useful to consider briefly some characteristics of the enzymic catalysis of this process. A particularly detailed study has been carried out on the enzyme chymo-trypsin (14) and a great deal of evidence shows that the catalytic site of this enzyme contains a serine residue with an unusually reactive hydroxyl group. Denoting the chymotrypsin by Ch—OH, the interaction with the ester involves first acyl transfer to the enzyme and this is followed by acyl enzyme hydrolysis to regenerate Ch—OH ... 

Pseudokinases are a protein family that constitute approximately 10% of the human kinome (for reviews on this topic, see Ref. 51-53). These proteins are characterized by the presence of a kinase-homology domain predicted to lack enzymatic activity due to the absence of at least one of the three conserved critical catalytic motifs (1) the Val-Ala-Ile-Lys (VAIK) motif in subdomain II, in which the side-chain of Lys interacts with the a and p phosphates of ATP (2) the His-Arg-Asp (HRD) motif in subdomain Ylb, in which the aspartic acid is the catalytic residue and (3) the Asp-Phe-Gly (DFG) motif in sub-domain VII, in which the carboxylic moiety of aspartic acid binds the Mg11 ion that coordinates the p and y phosphates of ATP. Owing to their lack of intrinsic phosphoryl-transfer catalytic activity, pseudokinase domain-containing... 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

In the presence of 18-crown-6 the degree of conversion increases with increased solvent polarity, best results being obtained in DMF (Table 1) as expected, the influence of temperature is also quite noticeable. Table 1 shows that the nature of the catalyst and the type of phase transfer reaction, solid-liquid or liquid-liquid, are very important factors. Short-chain tetraalkyl ammonium salts (methyl, ethyl or propyl) have no catalytic activity, while tetrabutyl ammonium or phosphonium salts have good activities several other phase transfer catalysts were also included in this study but will not be reviewed here. Reactions with aqueous solutions of potassium acetate (Table 2) confirm that best results are obtained when a concentrated solution of the salt is used. The scale of catalytic activity for these liquid-liquid reactions is the following ... 

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