Halogen chain ends, transformation

The introduction of the concept of Click chemistry, as a family of organic reactions that fulfil certain criteria drawn by Sharpless and coworkers in 2001 [194], has indeed captured the attention of synthetic chemists in the field of postpolymerization modification towards glycopolymer synthesis [32]. The most widely employed Click reaction is the CuAAC reaction. ATRP has been used extensively in conjunction with CuAAC Click chemistry. This is probably because both techniques are mediated by Cu(I). Moreover, the halogen chain ends of polymers prepared using ATRP can easily be transformed into azides to form what is known as azido-telechelic polymers. Many examples of glycopolymers prepared by the combination of Click chemistry and ATRP have been reported [32, 99]. 

Different reactions may affect the chain-end bromine atom of PS during ATRP transfer process, bimolecular terminations, or elimination reactions induced by the Cu(II) complex. The authors showed that the loss in functionality was predominantly due to /1-hydrogen elimination reactions. This result is very important for the synthesis of telechelic polymers by ATRP, because all processes (described later) are based on the halogen transformation. 

The steric environment for iron complexes with tridentate pyridine-bis(imine) ligands is similar to that for the Ni diimine complexes, except that the two metal-bound halogen atoms, which are presumably transformed into a polymerization site upon activation, are located in the plane perpendicular to the Fe-N3 plane (9, Figure 6.2), while they are in the Ni-N2 plane of the square planar Ni complexesIn the iron complex-catalyzed polymerizations, the propylene monomer is inserted in a highly regioregular 2,1 -fashion and exclusively yields 1 -propenyl chain ends. The polypropylene that is produced is prevailingly isotactic (up to 67% mmmm at —20 C 69% mm at 0 °C) irrespective... 

The halogen atoms, at the active chain ends, can be removed either by a reduction process or transformed to other useful functionalities [167], as shown for styrene and acrylate systems (Scheme 10) [168]. 

Advantages of end-group transformation include the ability to incorporate functionality incompatible with the polymerization procedure, to prepare halogen-free materials for subsequent reactive processing, to allow characterization of the initial copolymer prior to further functionalization, and an ability to prepare telechelic polymers, block copolymers, and materials that can be immobilized to surfaces, by a full range of substitution and addition chemistry. The use of a difunctional initiator allowed for the first time in a radical process preparation of functional homo-telechelic polymers with almost any desired chain end functionality (Scheme 33). ... 

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