Chain transfer catalytic

Enikolopyan et found that certain Co porphyrin complexes e g. 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.The scope and utility of the process has been reviewed several times,most recently by Heuts et al, Gridnev, and Gridnev and Ittcl. The latter two references provide a historical perspective of the development of the technique. 

The major applications of catalytic chain transfer are in molecular w eight control and in synthesis of macromonoincrs based on methacrylate esters. However, they have also been shown effective in poIymeri/.ations and copolymerizations of MAA, MAM, MAN, AMS, S and some other monomers. 

With acrylate esters and other monosubstituted monomers, the 

Certain cobalt(II) complexes such as the cobaloxime XXVIIa and the corresponding boron fluoride XXVIIb (L is a ligand) terminate propagating chains by a transfer reaction called 

The use of 1,2-disubstituted tetraphenylethanes is, however, of great interest because it allows for the synthesis of telechelic oligomers in a one-step reaction for monomers such as MMA which give a high amount of dispropor- 

The authors characterized the expected structure by means of 1H NMR. By performing kinetics analysis of the MMA polymerization, the authors observed an inhibition period of about 1 h, corresponding to the lifetime of the monoadduct formed. 

Trimethylsilyl-terminated PMMA was well characterized and proved the potentiality of such a method. However, it could be interesting to chemically modify the end group of PMMA, aiming at further polycondensation reactions. 

Focusing on telechelic polymers, the concept of iniferter is probably more interesting. like initer, iniferter compounds will be able to initiate and terminate the polymerization. They also function as a CTA [84], To get telechelic polymers by radical polymerization, it is necessary to use compounds with high transfer constants along with the radical initiator. Cho and Kim [85] suggested such a system, based on the use of two compounds bear- 

Allyl alcohol, acting as a transfer agent, allows the terminal hydroxyl function to be obtained. The chain transfer constant of allyl alcohol was calculated to be about 2 x 10-2 towards poly(styryl radical). The authors used different monomers (Table 11) and always got functionalities close to 2, according to gel permeation chromatography (GPC) PS standards. Results in terms of conversion were excellent (above 70%). Oligomers were obtained with PDI around 1.8. 

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Conventional - with catalytic chain transfer MALDI-TOF PMMA, copolymers 0 - 5 ... 

Macromonomers such as 66, 68 and 94 are themselves catalytic chain transfer agents (Section 6.2.3.4) and transfer to macromonomer is one mechanism for chain extension of the initially formed species. The adduct species in the case of monomeric radical adding dimer (100) may also react by chain transfer to give 101 which is inert under polymerization conditions (Scheme 6.25). Polymerizations to... 

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). 

Many Co11 porphyrins (87)110 131 and phthalocyanine complexes (102)110 have been examined for their ability to function as catalytic chain transfer agents and much mechanistic work has focused on the use of these catalysts. The more widespread application of these complexes has been limited because they often have only sparing solubility and they are highly colored. 

Various Co111 cobaioximes (90-92) have also been used as catalytic chain transfer agents.133 148"149 To be effective, the complex must be rapidly transformed into the active Co11 cobaioximes under polymerization conditions. The mechanism of catalytic chain transfer is then identical to that described above (6.2.5.1). 

Other complexes also react with propagating radicals by catalytic chain transfer.110 These include certain chromium,151 152 molybdenum152 1" and iron154 complexes. To date the complexes described appear substantially less active than the cobaloximes and are more prone to side reactions. 

Catalytic chain transfer has now been applied under a wide range of reaction conditions (solution, bulk, emulsion, suspension) and solvents (methanol, butan-2-one, water). The selection of the particular complex, the initiator, the solvent and the reaction conditions can be critical. For example ... 

End-functional polymers are also produced by copolymerizations of monosubstituted monomers with a-methylvinyl or other monomers with high transfer constants in the presence of catalytic chain transfer agents (Section 6.2.5)/11 "36 Thus, copolymerization of BA wilh as little as 2% AMS in lhe presence of eobaloximc provides PBA with AMS at the chain cnd.2j7... 

It is of interest that thermogravimetric analysis has been used as a means of determining end group purity of PM VIA macromonomers formed by catalytic chain transfer. 

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

The catalysts 153-155 shown in Table 9.7 have been used for polymerizations of acrylates and methacrylates and S. The catalyst 155 used in conjunction with an iodo compound initiator has also been employed for VAc polymerization.3"0 Catalytic chain transfer (Section 6.2.5) occurs in competition with halogen atom transfer with some catalysts. 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

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

Catalytic biosensors, 3 796-799 Catalytic chain transfer polymerization (CCTP), 20 442, 444... 

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