Star-shaped and Triblock Copolymers

A better approach is the use of multifunctional initiators. Application of some bifunctional initiators such as anhydrides of strong protonic acids, e.g. (FS02)20, (C1S02)20 or (CF3S02)20, already have been described in Chapter 6. 

Other bifunctional initiators are the di-l,3-dioxolan-2-ylium cation 127) dicationic living polysiloxane 119) and ditetrahydrofuranium cations 128). 

The easiest to prepare are acylium cations that may be obtained by the following 

Method a is a direct synthesis and does not lead to by-products. The disadvantage is the necessity of using acyl fluorides instead or the more easily available chlorides. This, however, may be avoided, at least for some applications, by using the corresponding chloride and preparing the salt having mixed counterions. 

Method b employs acyl chloride, but the quantitative removal of AgCl is very difficult. Method c does not suffer this disadvantage, because the gaseous NOCl can easily be removed from the reaction medium however, the starting material, i.e. the nitrosyl salt, may itself initiate the polymerization (e.g. polymerization of THF)131). Thus, this method cannot be used for the in situ formation of acyl cations, which is a frequent practice. 

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