Atom transfer radical polymerization carbon—halogen bond

To promote a polymerization, the newly formed carbon-halogen bond must be capable of being reactivated and the new radical must be able to add another alkene. This was accomplished for the radical polymerizations of St and methyl acrylate (MA), which were initiated by 1-phenylethyl bromide and catalyzed by a Cu(I)/2,2 -bipyridine (bpy) complex [42,79-81]. The process was called Atom Transfer Radical Polymerization (ATRP) to reflect its origins in ATRA. A successful ATRP relies on fast initiation, where all the initiator is consumed quickly, and fast deactivation of the active species by the higher oxidation state metal. The resulting polymers are well defined and have predictable molecular weights and low polydispersities. Other reports used different initiator or catalyst systems, but obtained similar results [43,82]. Numerous examples of using ATRP to prepare well-defined polymers can now be found [44-47,49]. Scheme 4 illustrates the concepts of ATRA and ATRP. To simplify schemes 3,4 and 5, termination was omitted. 

Atom Transfer Radical Polymerization. Atom transfer radical polymerization (ATRP) (80,81) involves reversible homol5d ic cleavage of a carbon-halogen bond by a redox reaction between an organic halide (R-X) and a transition metal, such as copper(I) complexed with 2,2 -bipyridine (bpy), as illustrated in reaction 20 ... 

Fig. 2. Schematic diagram of atom-transfer radical polymerization. The resulting polymer (P -X) usually has a terminal carbon-halogen bond.

A second type of reaction that involves the formal addition of a carbon-halogen bond to a double carbon-carbon, both inter- and intramolecularly, will also be discussed. These are the atom transfer radical reactions, and also include the polymerization of some olefins such as styrene or acrylates. 

Chain transfer reactions mostly proceed by abstraction of a monovalent atom such as hydrogen or a halogen. The scission of a bond carbon - oligovalent (e.g., H) atom is of interest for the introduction of endgroups into a polymer produced in a free radical reaction. Radically induced vinyl monomer polymerization with the possibility of chain transfer to a polymer of different chemical structure present in the reaction mixture leads to graft copolymers if bond scission occurs outside the main chain, no matter whether a single atom or a grouping is abstracted. Quite a different result is obtained if a radical attack involves a bond in the main chain of the polymer, if this bond scission occurs at a monovalent atom, which must be at the chain end, there is block copolymer formation. If bond scission occurs inside the polymer backbone, either block or random copolymers are produced [63]. 

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