Sacrificial Diblock Copolymer Synthesis

Hydroxy-terminated polymers prepared in this manner were used to build more complex macromolecular architectures such as graft copolymers [50] or block copolymers [51]. Using a 1,3-dithiepine instead of a 1,3-dioxepine as sacrificial monomer, allylic thiol polymer end groups could be obtained [52]. The poly(l,3-dithiepine) block was cleaved (sacrificed) under hydrogenation conditions using Raney-Nickel. 

The use of an initiator system that already carries the desired functional group is certainly the most attractive way of introducing a functionality at the end or start, as all polymer chains formed will necessarily be end-functional. However, only very few reports on functional initiators have been reported to date. One major drawback is probably the lack of straightforward synthetic methods to functionalize commercially available catalyst systems. 

Slugovc and coworkers attached luminescent dyes to the Grubbs first-generation ruthenium initiator by reacting the latter with a number of differently substituted styrene derivatives [61]. The new functionalized benzylidene initiators prepared in this manner polymerized NBE derivatives with PDIs ranging from 1.09 to 1.4, that is, with similar control as observed for the parent catalyst. 

Another synthetically challenging approach was carried out by Hutchings and Khosravi, who attached a ruthenium carbene initiator to the chain end of a styrene end-capped poly(ethylene oxide) [63]. The resulting macro benzylidene derivative was used to polymerize different NBE derivatives to yield amphiphilic diblock copolymers. 

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In a sacrificial synthesis, cyclic monomers containing a cleavable group are incorporated into a block copolymer structure, as shown in Figure 3.7 [47]. A propagating ruthenium carbene complex is used as a macro initiator for the polymerization of the cleavable cyclic monomer to form a diblock copolymer. The polymer block composed of the cleavable monomer can be broken down into low molecular weight fragments (sacrificed), leaving just one functionality at the chain end of the first polymer block. 

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