Polymerisation of Alkyl Isocyanates

Coordination polymerisations of alkyl isocyanates have not been widely studied, since these monomers could be polymerised via their C=N bond by using anionic initiators. However, such anionic polymerisations require low-temperature conditions [264]. It has been found recently [265] that alkyl isocyanates are capable of polymerisation in the presence of coordination catalysts at ambient temperature. By contrast, phenyl isocyanate appeared capable of coordination copolymerisation with oxirane [266]. 

Polymerisation of w-butyl isocyanate was carried out with the Pruitt-Baggett adduct Re(OR)2Cl [267] at ambient temperature, yielding polyamide, a polymer of the nylon-1 type (Table 9.3) [268]. For more detailed mechanistic studies, -butyl isocyanate was also subjected to polymerisation with a catalyst derived from the reaction of the Pruitt-Baggett adduct with /i-naphthyl-/V-( -butyl) urethane (for the sake of clarity, the catalyst association and internal complexation of the Fe atom by etheral O atoms of the OR substituent are omitted in formulae)  

The presence of the naphthyl group of the naphthyl urethane-terminated chains in the polymer obtained with the catalyst containing a naphthyl urethane group was detected by means of UV spectroscopy. Other results obtained indicate the involvement of the monomer nitrogen atom in coordination at the catalyst iron atom. Thus, the coordinating monomer then undergoes an enchainment to yield a urethane function, which may be shown schematically as follows [268]  

Polymerisation of n-butyl isocyanate was accompanied with cyclic trimer formation, which has been explained in terms of the backbiting reaction [268], 

It should be noted that, in the presence of zinc-based coordination catalysts of the EtZnX type, the cyclotrimerisation of aliphatic as well as aromatic isocyanates occurred exclusively [269]. 

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