Halogenated initiator

As seen in Scheme 2 (A), the most of the syntheses have been carried out with the HI/I2 and HX/ZnX2 (X = halogen) initiating systems, because these systems can effectively polymerize a large variety of vinyl ethers, including those with pendant functions, into well-defined living polymers [1]. In this way, the sequential living cationic polymerizations of two vinyl ethers are mostly "reversible i.e., both A - B and B - A polymerization sequences are operable. This is in sharp contrast to the block copolymerization of a vinyl ether with a styrene derivative or isobutylene (see below), where such reversibility often fails to work. 

Other multifunctional initiators include star polymers prepared from initiators via living radical or other living polymerizations. In particular, all of the star polymers via metal-catalyzed living polymerization, by definition, carry a halogen initiating site at the end of each arm, and thus they are potentially all initiators. Thus, star-block copolymers with three polyisobutylene-Mock-PMMA arms and four poly-(THF) -A/oc/F polystyrene or poly(THF)-Woc/c-polysty-rene-Wock-PMMA were synthesized via combination of living cationic and copper-catalyzed living radical polymerizations.381,388 Anionically synthesized star polymers of e-caprolactone and ethylene oxide have... 

The halogen initiation mechanism is not directly confirmed, however. 

Alternatively, the propagating ion may combine with an anionic fragment from the counterion [16] as in BX3.OH2 (X = halogen) initiated polymerization or terminate by alkylation or hydridation [17,18] when aluminium alkyl-alkyl halide initiating sytems are used. 

There he worked with George Glockler on the electron affinity of halogens (initially iodine and later bromine and chlorine as well) from space-charge effects—Calvin s problem was to measure the amount of energy released when a halogen atom captures an electron and for that he had first to devise the methods for doing so. 

Bis (trifluoromethyl) benzene can be used together with traces of a, aGdibromo-a, tetrafluoro-/7-xylene as halogen initiator for the production of parylene-F [20,35]. 

In terms of the atom transfer reversible activation mechanism, the most actively studied method is atom transfer radical polymerization (ATRP), which was first demonstrated in 1995 [41—43]. ATRP reactions use a halogenated initiator (e.g. alkyl halide) to start the polymerization and the halide becomes the removable controlling agent on the polymer chain endgroup. A transition metal complex is present in the formulation to mediate the removal of the halide radical from the polymer chain. The general atom transfer reversible activation scheme shown previously can be represented in more detail for ATRP by the reaction shown in Scheme 13.8. 

Thus, the initiating systems for the living polymerization consist of a halogenated initiator and a transition metal complex. The effective metal complexes for the catalysis now include various early and late transition metals as reviewed in this chapter, while the initiators are typically polyhaloalkanes, a-haloesters, (a-haloalkyl)benzenes, and sulfonyl halides. The metal-catalyzed polymerization can control the reactions of a... 

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