Polymer Kharasch reaction

Polymer formation during the Kharasch reaction or ATRA can occur if trapping of the radical (123), by halocarbon or metal complex respectively, is sufficiently slow such that multiple monomer additions can occur. Efficient polymer synthesis additionally requires that the trapping reaction is reversible and that both the activation and deactivation steps are facile. 

The dormant species in ATRP arises from the polymer chain being capped with a halogen atom (P -X), while in the active state the halogen is chelated to a metal complex, thus allowing monomer to add. This takes advantage of the Kharasch reaction in which halo-genated alkanes add to vinyl monomers by a free-radical reaction that is catalysed by transition-metal ions in their lower-valent state (Fischer, 2001). 

This type of approach has also been used to attach antioxidants to unsaturated polymers. The novel approach of Scott in the 1970s (Scott, 1984) using the technique employed by Watson enabled the attack of substituted allyl mercap-tans and disulfides to olefinic double bonds employing the Kharasch reactions. 

Transition metal catalysed ATRP is one of the most efficient methods to control radical polymerisation [13]. ATRP is based on the reversible formation of radicals from alkyl halides in the presence of transition metal complexes, and is a direct extension to polymers of the Kharasch reaction, ATRA, (Scheme 4). Among the plethora of catalysts (or precatalysts) described in the literature for ATRP, the copper systems developed by Matyjaszewski [3, 14] and the ruthenium complexes introduced by Sawamoto [15] play a most prominent role and set the standards in the field (Scheme 5). 

A parallel development was initiated by the first publications from Sawamoto and Matyjaszweski. They reported independently on the transition-metal-catalyzed polymerization of various vinyl monomers (14,15). The technique, which was termed atom transfer radical polymerization (ATRP), uses an activated alkyl halide as initiator, and a transition-metal complex in its lower oxidation state as the catalyst. Similar to the nitroxide-mediated polymerization, ATRP is based on the reversible termination of growing radicals. ATRP was developed as an extension of atom transfer radical addition (ATRA), the so-called Kharasch reaction (16). ATRP turned out to be a versatile technique for the controlled polymerization of styrene derivatives, acrylates, methacrylates, etc. Because of the use of activated alkyl halides as initiators, the introduction of functional endgroups in the polymer chain turned out to be easy (17-22). Although many different transition metals have been used in ATRP, by far the most frequently used metal is copper. Nitrogen-based ligands, eg substituted bipyridines (14), alkyl pyridinimine (Schiff s base) (23), and multidentate tertiary alkyl amines (24), are used to solubilize the metal salt and to adjust its redox potential in order to match the requirements for an ATRP catalyst. In conjunction with copper, the most powerful ligand at present is probably tris[2-(dimethylamino)ethyl)]amine (Mee-TREN) (25). 

Complexes 50-53 were tested under standard Kharasch reaction conditions, that is, treating carbon tetrachloride with methyl methacrylate at 85°C in the presence of the palladium catalyst. Polymers with apolydispersity of 1.8-2.0 were obtained in moderate yield (Table 22.26). Interestingly, the yield of Kharasch addition product was less than 1%. It was observed that the polymer yields are... 

The same group also reported an amphiphiHc PS-PEG resin-supported mthe-nium complex, which could catalyze the Kharasch reaction in water under heterogeneous conditions without any radical initiators (Scheme 8.29) [65]. Owing to the self-concentration of hydrophobic organic substrates inside the polymer matrix in water, it was claimed that the catalytic efficiency of the PS-PEG Ru in water was comparable to the most efficient homogeneous Ru catalysis reported thus far. 

Atom Transfer Radical Polymerisation (ATRP) was discovered independently by Wang and Matyjaszewski, and Sawamoto s group in 1995. Since then, this field has become a hot topic in synthetic polymer chemistry, with over 1000 papers published worldwide and more than 100 patent applications filed to date. ATRP is based on Kharasch chemistry overall it involves the insertion of vinyl monomers between the R-X bond of an alkyl halide-based initiator. At any given time in the reaction, most of the polymer chains are capped with halogen atoms (Cl or Br), and are therefore dormant and do not propagate see Figure 1. 

The nature of the chlorinated reagent is crucial for promoting the Kharasch addition reaction (Equation 8.11). The results showed that carbon tetrachloride could be added to various olefins in a regioselective way. Under these reaction conditions, no polymerization products were detected. In contrast, when chloroform was used as the halide source the methyl methacrylate and styrene conversions reached only 33% and 40% with the best performing system (VIII), and a significant fraction of polymers was observed [61]. 

Garcia, Reiser, and co-workers developed an easily recoverable and reusable catalytic system based on a ditopicazabis(oxazoline) ligand L40 for the enantioselective Kharasch-Sosnovsky reaction. When the reaction was complete in MeCN or BTFEP, the ligand and Cu salt were able to form insoluble coordination polymers which precipitated after extraction with hexane. This method allows up to four successive uses of the catalysts without loss of catalytic activity. 

This method is potentially the most versatile, since it can be carried out in a variety of ways and adds relatively little to the cost of the final polymer. The nitroso-ene reaction in natural rubber already referred to is one example of such a modification. However, in order to avoid the necessity to gear the modification process to the vulcanisation reaction, we have concentrated in our own work on antioxidant adduct formation to the rubber double bonds either by vinyl grafting or by the Kharasch thiol addition reaction... 

ATRP is analogous to atom transfer radical addition reactions which are well known in the field of organic chemistry as Kharasch addition reactions [83]. These methods often utilize a transition metal complex based on copper, iron, ruthenium, and nickel to abstract a halogen and produce a carbon-based radical [84, 85]. Since the first reports in 1995 of living radical polymerizations based on copper(I) for styrene and methyl methacrylate [86] and ruthenium(II) for methyl methacrylate [87], this technique has become widely utilized in polymer science. 

The above approach of mechanochemically initiated addition of reactive antioxidants on different polymers, such as rubbers and unsaturated thermoplastics such as ABS is illustrated here for thiol-containing antioxidants. For example, using thiol compounds (37) and (38) as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see reaction 7) the polymer becomes much more substantive under aggressive environments. 

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