Catalytic Chain Transfer Process

The recent investigations concerning CCT were mainly focused on improving the catalytic system by using new cobalt complexes [302-307]. But CCT reactions usually lead to the synthesis of a large variety of structured monofunctional macromonomers terminated by a vinylic functionality [308,309]. It is important to note that CCT can be conducted with rate constants Ctr hundreds or thousands of times faster than the best mercaptans [69], which 

Recently, by using a cobalt complex with porphyrin, CCT led to a meth-acrylic acid macromonomer in water [312,313]. The use of this cobalt intermediate added to a continual reinitiation (V501 is the initiator) involves a living character, unlike conventional CCT. The transfer constant at 69 °C was evaluated to be about 4000, which is unexpectedly very high [314]. 

Addition-fragmentation and CCT are of great interest for the synthesis of macromonomers. Indeed, unlike other radical techniques, they lead to macromonomers in a one-step reaction by directly introducing the chain-end double bond. This double bond is very reactive because it is activated by 

Finally, Chiefari et al. [315-317] suggested another technique leading to the synthesis of addition-fragmentation-type macromonomers but without the use of any CTA. This method, clean, easy, and economical, involves heating a mixture of acrylate or styrene monomer in an appropriate solvent with an azo or peroxy initiator. High temperatures (typically up to 150 °C) are required. To prove the expected mechanism, the authors studied the poly(alkyl acrylate) reactions in the presence (or not) of monomers and by using different conditions. They showed the reaction does not occur without the monomer. Moreover, an increase of the temperature leads to a better yield and a decrease of the molar mass. Macromonomers have been synthesized by this technique withMn between 103 and 104 gmol 

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The results in this section indicate that chemically controlled transfer reactions, such as with DDM or with the MMA trimer, are adequately represented by the rate coefficients reported for bulk polymerization. Catalytic chain transfer processes, as with CoPhBF, are speeded up by the presence of CO2. 

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. 

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 . 

End Groups in the Isotactic Polypropylene, Deriving from the Chain Transfer Processes Depending on the Catalyst Concentration [Catalytic System a-TiCl 3-AI (CaHs) -n-Heptane)... 

Interpretaiion of the Chain Transfer Process Depending on the Amount of Titanium Compound Introduced into the Catalytic System... 

It may be that these compounds act catal3rtically on the chain transfer processes which depend on the alkylaluminum concentration. We can also consider that non-catalytic complexes containing alkylaluminum, exchange their metallorganic component with the catalytic complexes bound to the active centers of the crystalUne substrate. 

The dependence of the rate of the considered chain transfer process on the olefin pressure may be justified if we assume that such a process requires an activation state of the solid catalyst. Such an activation state would correspond to the activated intermediate complex which is formed during the polymerization process at the particular stage in which a monomeric unit bounds itself to the catalytic complex. As a result, the rate of the transfer process would depend on the rate of the chain-growing process, since the two processes (growing and chain transfer) would be considered as parallel and deriving from the same activated complex. 

The importance of the chain transfer process depending on the concentration of alkylaluminum, when compared with the one depending on the amount of a-titanium trichloride present in the catalytic system, may be easily deduced from the diagrams plotted in figures 22 and 25. From such diagrams it follows for the catalytic system considered ... 

From the above summarized data, it follows that, during the polymerization, in the presence of triethylaluminum, there is a consumption of A1 atoms and ethyl groups bound to the aluminum, because the triethylaluminum is involved in chain transfer processes without being regenerated. Therefore, the polymerization process is now thoroughly catalytic only with... 

Haddleton determined the reactivity ratios for copolymerization of MMA with BMA by classical anionic as 1.04 0.81 by alkyllithium/trialkylalu-minum initiation, 1.10 0.72 by GTP, 1.76 0.67 by ATRP, 0.98 1.26 by catalytic chain transfer, 0.75 0.98 by classical free radical, 0.93 1.22 [39]. The difference in reactivity ratios between GTP and classical anionic polymerization seems to indicate GTP is an associative process. However, Jenkins has also measured reactivity ratios for the same pair by GTP and reports different results rMMA=0.44 and rBMA=0.26 [40]. 

The catalytic chain-transfer (CCT) process displays all of the features characteristic of typical, uncatalyzed chain transfer other than taking place at a rate competitive with chain propagation. Thus, the rate of polymerization at low conversions is independent of the concentration of the cobalt porphyrin (Figure 1) while the molecular weight, Mn, decreases linearly by over 2 orders of magnitude with increasing concentration of cobalt catalyst (Figure 2). As expected for a typical polymerization, the rate of polymerization increases linearly with the square root of the concentration of the azo initiator and no polymerization occurs in the absence of the initiator. 

The structures of both chain ends indicate that the insertion of the monomer on the metal-carbon or metal-hydrogen bonds of the catalytic complexes is secondary and that /3-hydrogen abstraction is the most important chain-transfer process. 

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... 

Finally, this mechanistic section cannot possibly be concluded without referring to catalytic chain transfer (CCT) this is the most probable side reaction of CMRP,and involves hydrogen abstraction by the cobalt complex, with the release of cobalt hydride ([Co "—Hj) and imsaturated polymer chains (Equation 4.4). Although CCT is mainly used for the preparation of macromonomers, in a process referred to as catalytic chain transfer polymerization (CCTP) [25], the reaction must be minimized in CMRP. 

Coordinative chain transfer polymerization (CCTP, also called catalytic chain transfer polymerization) is typically a process that comprises a chain transfer step that must be i) reversible and ii) much faster than propagation. The growing polymer chain is exchanged between a CTA and the catalyst when attached to the CTA, it is just a dormant chain, whereas propagation takes place on the catalyst (Scheme 27.1). As a consequence, if the CTA is in excess, several macromolecular chains can be produced per catalyst molecule, and ideally (if the transfer rate is not determining), all chains will have the same length. 

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