Lanthanum isopropoxide was found to serve as a novel anionic initiator for the polymerization of hexyl isocyanate affording poly(hexyl isocyanate) with very high molecular weight ((M) over bar (n)> 10(6)) under appropriate conditions. Other lanthanoid alkoxides, such as samarium, ytterbium and yttrium isopropoxides, also brought about the polymerization of hexyl isocyanate. Butyl, isobutyl, octyl and m-tolyl isocyanates also underwent the polymerization reaction to form the corresponding polymers by using lanthanum isopropoxide as initiator, while polymerizations of tert-butyl and cyclohexyl isocyanates with lanthanum isopropoxide did not occur under identical conditions.