External Initiation of Polymerization

Smeets and colleagues observed that Ar-Ni(PPh3)2-Cl gives only about 65% of Ar/H terminated polymers whereas about 95% Ar/H terminated polymers were observed by the addition of two equivalents of dppp ligands (addition of one 

The Nesterov group used a one-step method for the synthesis of the initiator. They reacted aryl halide with Ni(dppp 2 to obtain the initiator Ar-Ni(dppp)-Cl in reasonably good yields (45-55%) [165]. Their method not only involved the one-step synthesis of Ar-Ni(dppp)-Cl initiator, but it also prevented the undesired presence of PPhs ligand, which may reduce the efficiency of the catalyst, promote disproportionation of the initiator, and also promote undesired aryl-aryl coupling. The authors claim that this procedure yields polymers that are defect-free and with 100% regioregularity. However, the polydispersities reported varied between 1.16 and 2.00. 

1 Variation of Functional Groups That Can be Attached to the External Initiator 

Smeets et al. synthesized para-siloxyl-substituted o-tolyl bromide and para-ethynyl-trimethyl sUyl-substituted o-tolyl bromide, i.e., l-bromo-2-methyl-4-([[(t-butyl) dimethylsilyl]oxy methyl)benzene and l-hromo-2-methyl-4-[2-(trimethylsilyl)-ethynyl]benzene (Fig. 7) [168]. The oxidative additimi of these aryl halides with Ni (PPh3)4 followed by the addition of two equivalents of dppp ligand yielded 95% Ar/H-terminated polymer chains for the sUoxyl substituent and about 80% Ar/H for the ethynyltrlmethylsilyl substituent. These results, combined with Doubina s results [169] on para-substituted phosphonate esters, confirmed the importance of the presence of an ortho substituent position on the aromatic initiator, irrespective of the presence or absence of a substituent on the para position of the phenyl ring. 

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Fig. 7 Different functional groups explored for external initiation of polymerization [154, 159-161, 164, 168-170]...

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