Preparation

Polyisocyanides, also known as polyisonitriles or polyiminomethylenes, are prepared by the polymerization of isocyanides. The driving force for this polymerization reaction is the transformation of a formally divalent carbon atom in the monomer to a tetravalent carbon atom in the polymer, yielding a heat of polymerization of 81.4 (kJ mol"1) (Fig. 12.1A) [11].

One of the special characteristics of polyisocyanides is the fact that every carbon atom in the polymer backbone bears a substituent. A consequence of this architectural novelty is that the side chains experience a large steric hindrance forcing the polymer to adopt a non-planar conformation (see below). In addition to a

Fig. 12.1 (A) Schematic representation of the resonance structures of isocyanide and polyisocyanide; (B) The ''merry-go-round'' mechanism for the nickel(II)-catalyzed polymerization of isocyanides.

range of polymerization procedures available for isocyanides [12-14], the most successful methods involve the use of a Group 10 metal complex, of which the most widely applied is a Ni(II)-complex, such as Ni(acac) [15], NiCl2 and Ni(ClO4")2 [8, 11]. For the nickel-catalyzed polymerization of isocyanides a so-called ''merry-go-round'' mechanism has been proposed in which the polymerization takes place around the nickel(II) center that pre-organizes the isocyanides for polymerization (Fig. 12.1B) [16]. This mechanism explains many of the features and properties of polyisocyanides, however, detailed mechanistic studies by De-ming and Novak on Nickel catalyst 1 (Scheme 12.1) revealed that some aspects of the mechanism are more complex and that the actual catalytic species is probably nickel(I) [17-22]. Nickel catalyst 1 also revealed excellent living polymerization characteristics [17, 18], allowing the formation of block copolymers from two different isocyanides [18]. Block copolymers from polyisocyanide and another type of polymer can be prepared from allyl and amine initiator complexes such as 2 [23, 24], 3 [25-27] and 4 [28], of which the polybutadiene and the poly-

Scheme 12.1

peptide in 2 and 4, respectively, were polymerized by the same nickel center (Scheme 12.1).

An alternative type of polymerization catalyst, m-ethynediyl Pd-Pt complex 5a and m-ethynediyl Pd-Pd complex 5b (although the latter catalyst is less efficient), was discovered by Takahashi and coworkers (Scheme 12.2) [29, 30]. They found that 5a polymerizes aryl isocyanides, but not alkyl isocyanides under reflux conditions in THF. The isocyanides exclusively insert into the Pd-carbon bond, however, the platinum plays an essential role, since only a single insertion of isocya-nide was observed for mononuclear complexes 6 in the presence of an excess of isocyanide. The Pd-Pt catalyzed polymerization proved to be living in nature as was illustrated by the low polydispersity of the obtained polymers and the ability of the catalyst to form block copolymers. Even after work-up the Pd end group remains connected to the polymer and polymerization can be continued. Initiators 7 with two and three Pd-Pt m-ethynediyl units were used to synthesize multi-armed polyisocyanides (Scheme 12.2) [31, 32]. More details on the preparation of polyisocyanides can be found in a recent review by Suginome et al. [14].

Scheme 12.2
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