Catalytic Chain Transfer CCT

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. 

A corresponding principle applies to controlled radical polymerisation performed in quite a number of modes such as nitroxide-mediated polymerisation (NMP), atom transfer radical polymerisation (ATRP), reversible addition fragmentation chain transfer (RAFT) or catalytic chain transfer (CCT) reactions. All of these variants of controlled radical polymerisation lead to well-defined architectures with the particular advantage that a much larger number of monomers are suitable and the reaction conditions are much less demanding than those of living ionic polymerisation reactions. 

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. 

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. 

There could be other potential reaction pathways for nitrogen fixation that are amenable for G-chelates. In our previous work, we studied catalytic chain transfer (CCT). CCT is based on hydrogen transfer from a free radical to a monomer (olefin) and baek according to the following Scheme (8.2)... 

Catalytic chain transfer is used to control the molecular weight of PMMA commercially. The macromonomers produced by CCT show promise for synthesis of comb polymers. 

Catalytic chain transfer in acrylate polymerizations is problematic due to the propensity of acrylates to form stable Co—C bonds between the CCT agent and the propagating radical of both the monomer and its oligomers.268,269,373 It has even been possible to observe a growing polymer chain terminated with a Co—C bond directly by MALDI.374 This bond is stronger than that in the case of styrene. These complications have a direct impact on the use of CCT in acrylic polymerizations. 

Catalytic chain transfer is a versatile tool that complements other means of polymerization. It allows the synthesis of the large variety of structured polymers shown in Figure 11. The primary outlet for CCT is to control molecular weight in free-radical polymerizations without the use of stoichiometric chain terminators (sections 1—3). All of the products can be considered to be monofunctional in that they are all terminated by unsaturation. The unsaturation... 

Cc(n) Cc for each radical (when DP is considered) CCT catalytic chain transfer... 

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... 

The majority of publications deal with the application of catalytic chain transfer (GGT) in bulk or solution, but examples in the patent literature do apply GGT to emulsion and suspension. These patents report only limited data and full details of the experimental procedures are not revealed. Molecular weight data for the final products are given, but information on coagulation, conversion, and particle size are often not provided. The use of GoBF in MMA emulsion polymerization has been reported outside the patent literature but efficient CCT was not achieved, suggesting the process may be sensitive to at least some of the reaction components. 

The active LCo complexes indicated above can be used to test this theory. Porphyrins and phthalocya-nines have an O-shaped system which has a more extended -system than that in cobalamins, but it does not provide a substantial increase in reactivity. It should be noted that the hydrogen bonds of the cobaloxime catalysts are essentially as effective as 7r-bonds in continuing the effects of delocalization around the macrocyclic ring. This effect has been noted elsewhere.142 Catalyst 11 comprises an O-shaped -system. Replacement of one jr-bond with a a-bond in the analogue 13 significantly affects the catalytic properties since both complexes retain their O-shape with -conjugation. Additional replacement of "T-bonds with o-bonds leads to a complete loss of catalytic properties as chelates 13, 20, or 21 indicate. Chelate 22, cannot be a CCT catalyst because of the absence of interaction between the two jr-systems. Chelate 34 is an exception its molecular structure is similar to 21 and 13, but it catalyzes chain transfer with a measurable rate. A possible explanation of this phenomenon will be provided in section 3.7. 

CCT of benzyl methacrylate leads to a mixture of poly(benzyl methacrylate) macromonomers from which the dimer macromonomer could be isolated.516 When the benzyl dimer is used as a RAFT chain-transfer agent, PMMA with a- and co-terminal benzyl methacrylate units is obtained. Catalytic hydrogenation of the a,co-benzyl terminal methyl methacrylate polymer results in the evolution of toluene and formation of a,co-dicarboxyl functional telechelic PMMA. 

CCTP has its origins in biochemistry where coenzyme B12 is used to conduct many free-radical reactions. Enikolopyan et al. were the first who used analogues of B12 for polymerization [257,258]. Methacrylate was polymerized by a catalyzed chain transfer using a cobalt porphyrine. AIBN was used as initiator. Two possible reaction sequences for the catalytic aspect of CCT are described in the following scheme ... 

In terms of the influence of the olefin structure, CCT activity is especially high for methacrylates and styrene, while acrylates tend to yield more efficient OMRP trapping by cobalt complexes. A study of H transfer from CpCr(CO)3H to a variety of olefins yields the rate constants and relative rates are shown in Figure 30, but no equivalent information is apparently available on the catalytically more relevant H transfer from the chain-carrying radical to the transfer agent, which is usually the rate-determining step in CCT. 

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